GENERAL OUTLINE THE ANIMAL KINGDOM. LONDON : PRINTED BY SAMUEL BENTLEV, Bangor House, Shoe Lane. GENERAL OUTLINE THE ANIMAL KINGDOM, MANUAL OP COMPARATIVE ANATOMY. BY THOMAS RYMEB JONES, F.Z.S. PROFESSOR OP COMPARATIVE ANATOMY IN KING'S COLLEGE, LONDON} FULLERIAN PROFESSOR OF PHYSIOLOGY TO THE ROYAL INSTITUTION OF GREAT BRITAIN, &C. &C. ILLUSTRATED BY THREE HUNDRED AND THIRTY-SIX ENGRAVINGS. LONDON: JOHN VAN VOORST, ], PATERNOSTER ROW; M.DCCC.XLT. ., K- To RICHARD OWEN, ESQ., F.R.S. &c. &c. &c. THE FOLLOWING PAGES ABE INSCRIBED BY HIS SINCERE FRIEND, THE AUTHOR. M367753 PREFACE. THE object of the writer of the present work has been twofold ; first, to lay before the Naturalist a complete view of the organization and physiological relations of every class of living beings ; and secondly, to offer to the Ana- tomical Student a succinct account of the structure and developement of the vital organs through all the modifi- cations that they present in the long series of the animal creation. Extensive indeed is the field of study that offers itself to the zealous cultivator of Natural History, if he would step beyond the limits that not unfrequently too narrowly circumscribe his views of animated nature. Needlessly to multiply specific distinctions, or to arrange trivial groups of external forms in imaginary circles, is an easy occupa- tion to the superficial Zoologist, easier perhaps than it would be to one more deeply conversant with the anato- my and intimate composition of the creatures thus sum- marily classified; and, accordingly, it is by no means un- common in the present day to see the most strenuous supporters of this or that theory resolutely shutting their eyes against all evidence deducible from the laws of phy- siology, and stoutly maintaining that outward form is in itself enough for the purpose they have in view, namely, the establishment of some favourite principle or fancied Ylll PREFACE. analogy. Discussions of this kind have been carefully avoided in the following pages : to collect from every available source the ascertained facts connected with anatomical structure, and to arrange the grand divisions of the animal world in conformity with progressive de- velopement as we advance from humbler to more com- plex types of organization, has been the chief aim of the Author; and, if he has at all succeeded in divesting so important a subject of those technicalities which not un- frequently impede the progress of the general reader, his labour has not been thrown away. To the Physiologist little apology is necessary for the production of a work intended to exhibit at one view the leading facts of Comparative Anatomy. In this country, unfortunately, so extended a view of Nature is considered as being by no means essential to a correct intelligence of the laws of animal life, and as a branch of professional education has been hitherto completely neglected. Our illustrious countryman John Hunter entertained a dif- ferent opinion. May the fire which he first kindled amongst us, and which has since his time been kept alive by the fostering care of that College, the depository of his invaluable works, soon burst forth, and irradiate the realms of science as brightly as the great founder of Comparative Physiology foresaw that it might ! PHYSIOLOGICAL INDEX. NERVOUS SYSTEM. General classification of Animals, in accordance with the condition of ACRITA ........ NEMATONEURA ...... . Anatomy of the nervous system in Linguatula tamioides ..... Ascaris lumbricoides . . . . . . Notommata clavulata ..... Actheres percarum ...... Asterias ....... Echinus ....... Holothuria ...... Siponculus ....... HOMOGANGLIATA ....... Anatomy of the nervous system in Hirudo medicinalis . . . . . . Myriapoda ...... Insecta ....... Changes that take place in the condition of the nervous system during the metamorphosis of Insects Crustacea ....... Motor and Sensitive tracts in the nervous centres of Homogangliata HETEROGANGLIATA . . . . . . . Anatomy of the nervous system of Cirrhopoda . . . . . . . Brachiopoda ...... Tunicata . . Conchifera ...... .Gasteropoda ....... Pteropoda ...... Cephalopoda ....... Nautilus Pompilius ..... VERTEBRATA . ...... Anatomy of the nervous system of Fishes ....... Reptiles ....... Birds Mammalia SENSE OF TOUCH in Polyps in Holothuria in Siponculus in Leech ORGANS OF THE SENSES. Sense of touch in Insects in Crustacea Tentacula of Gasteropoda Pteropoda Page Sec. 6 8 6 8 99132 101135 103140 124163 132173 158199 171211 178219 183225 184227 198240 229273 270312 303349 336372 340375 351387 355391 367_400 372408 391425 415452 427468 457499 457500 484520 521557 576633 607675 691796 22 28 178219 180181 199_240 225267 274317 343378 402439 425466 X PHYSIOLOGICAL INDEX, Tactile organs of Cephalopoda Sense of touch in Reptiles in Birds in Mammalia SENSE OF TASTE in Insects ..... in Crustacea ..... in Cephalopods ..... in Fishes ..... in Reptiles ..... in Birds ..... in Mammals ..... SENSE OF SMELL in Insects . . in Crustacea ..... in Nautilus Pompilius .... in Fishes ...... in Reptiles . . in Birds ...... in Mammals ..... SENSE OF VISION. Red specks observable in Acrita . in Rotifera in Lamproglena pulchella Eyes of Leech . Simple ocelli of Insects Compound eyes of Insects . Eyes of Crustacea .... the Scallop (Pecten) . Snail .... other Gasteropoda Nautilus Pompilius Cuttle-fish . Fishes .... Reptiles . - . Birds .... . Mammalia . . . . . SENSE OF HEARING in Insects in Crustacea . in Cuttle-fishes ..... in Fishes ... in Reptiles ..... in Birds . . . in Mammalia ..... Page Sec. 460501 584645 608676 703825 275318 343_378 462503 510544 555-611 608677 673752 275319 343_378 463-504 521558 578633 608678 692800 64 92 124162 134177 199_241 276321 277321 343379 391426 402439 416452 464506 466507 522560 578634 609679 695805 275319 344380 470512 526568 580638 614-684 699819 LOCOMOTIVE ORGANS. Condition of muscular system in Acrita Coelelmintha Bryozoa Rotifera Epizoa Suckers of Star-fishes Suckers of Echini Spines of Echini Suckers of Holothuria Muscular system of Siponculus Locomotive organs of the Leech . . Earthworm Setae of Dorsibranchiate Annelidans 6 8 102139 111147 121159 132172 149190 163204 163205 173214 180221 191234 201224 213258 PHYSIOLOGICAL INDEX. Feet of Julus ..... Scolopendra . Legs of Insects ..... Mechanical structure of the feet of Insects Wings of Insects ..... Muscular system of Insects . . Spinning organs of Arachnidans Locomotive organs of Crustacea Muscles of Cirrhopoda .... Arms of Brachiopoda . Mantle of Ascidia ..... Foot of Conchifera . Byssus of Mussel and Pinna Apparatus for opening and closing the shells of Conchifera Muscular system of Snail .... Locomotive organs of Pteropoda Tentacula and suckers of Cuttle-fishes Sails (so called) of Argonaut . . Arms and float of Nautilus Pompilius . . Locomotive apparatus of Argonaut Fins and muscular system of Fishes Limbs of Reptiles . Muscular system of Reptiles Locomotion of Birds . . . Muscular system of Mammalia XI Page Sec. 225268 228272 240282 241283 246287 250292 316363 319364 356392 369402 381415 383417 384418 395_430 424465 431474 435477 437479 443485 500537 542590 555610 591657 660740 SKELETONS OF INVERTEBRATA. Horny and calcareous framework of Sponges Shells of Infusoria .... Polyparies of Polyps ...... Fungidae .... . Cortical Polyps ..... Tubiporidae ..... Sertularidae . ..... Internal plates of Velella and Porpita Cells of Bryozoa ...... Shells of Rotifera ..... Skeletons of Echinodermata. Crinoidae ...... Asteridae ...... Echinidae ...... Skeletons of Homogangliata ..... Myriapoda Insecta . Arachnida ..... Crustacea ...... Structure and growth of the shells of Mollusca : Cirrhopoda . Brachiopoda ..... Tunicata ...... Conchifera ..... Gasteropoda ...... Cephalopoda ..... First appearance of an internal skeleton (Endo-skeleton} 13 16 53 74 30 42 18 21 29 40 34 48 45 63 69 97 110146 118155 136180 147189 160202 184228 224266 238279 306351 319364' 352389 363398 369402 384419 418458 440482 439481 SKELETONS OF VERTEBRATA. a. Cuticular skeleton, or Exo-skeleton. Dermo-skeleton of Fishes Growth of hair and other epidermic appendages Horns of the Deer 506541 687788 688790 Xll PHYSIOLOGICAL INDEX. b. Osseous skeleton, or Endo- skeleton : General view of the skeleton of Vertebrata Osteology of Fishes Reptiles Birds Mammalia . Page Sec. 477518 489522 544593 592658 633710 NUTRITIVE SYSTEM. DIGESTIVE ORGANS OF ACRITA. Sponges ..... Fungiae ...... Alcyonium ..... Hydra viridis ..... Cortical Polyps .... Tubipora musica .... Sertularidae ..... Actinia ..... Polygastrica .... Acalephae ..... Hydatid Trichina ..... Taenia Distoma ..... Planaria ..... Diplozoon . . Echinorynchus .... DIGESTIVE ORGANS OF NEMATONEURA. Linguatula taenioides .... Ascaris lumbricoides . . Bryozoa ..... Rotifera ..... EPIZOA. Adheres percarum .... Lamproglena pulchella ECHINODERMATA . Asterias . Echinus ..... Holothuria ..... Siponculus ..... DIGESTIVE ORGANS OF HOMOGANGLIATA. ANNELIDA. Leech Earthworm .... Dorsibranchiata ..... Tubicola ... . MVRIAPODA. Julus ...... Scolopendra .... INSECTA. Mouths of Insects .... Alimentary canal of Insects of Arachnid a CRUSTACEA 15 17 18 21 27 37 23 29 34 46 35 48 45 64 40 55 56 79 71 99 81110 83114 84115 87118 89119 92122 95126 100134 103141 108145 122160 131171 134_177 151192 166207 174_215 180222 192235 203246 215260 221265 226269 229274 254295 260301 310356 328367 PHYSIOLOGICAL INDEX, Xlll DIGESTIVE SYSTEM OF HETEROGANGLIATA. Cirrhopoda Brachiopoda Tunicata Conchifera . Gasteropoda. Snail Mouths of Gasteropoda . Alimentary canal, &c. Pteropoda Cephalopoda DIGESTIVE SYSTEM OF VERTEBRATA. FISHES. Teeth of . Digestive apparatus of REPTILES. Teeth of ... Alimentary system of BIRDS . MAMMALIA. Teeth of ... Alimentary apparatus . , Page Sec. 356393 365399 370405 378413 395430 410445 413448 424466 445487 510544 515545 556611 561619 597664 663744 677759 RESPIRATORY AND CIRCULATORY SYSTEMS. Rotifera ........ 125164 Asterias ....... 155 195 Echinus ........ 170209 Holothuria ....... 175 216 Siponculus . . . . . . . ___ 181 223 Leech ........ 195237 Earthworm ........ 204247 Dorsibranchiate Annelidans ..... 217 262 Insects . . . . . . . . 264306 Arachnidans ....... 311358 Crustacea ........ 329 368 Cirrhopoda ....... 357394 Brachiopoda . . . . . . . 366400 Tunicata ....... 370 403 Oyster ........ 378412 Snail 397433 Gasteropoda ....... 403 440 Pteropoda ....... 427 467 Cephalopoda ........ 451 494 Fishes ........ 517553 Reptiles ........ 564626 Birds ........ 602671 Mammalia ........ 682 778 GENERATIVE SYSTEM. FIISSPAROUS GENERATION in Polygastrica in Annelida . GEMMIPAROUS GENERATION in Sponges in Fungia in Hydra viridis . 59 85 211254 16 19 19 23 25 33 XIV PHYSIOLOGICAL INDEX. GENERATIVE SYSTEM IN Page Sec. Tubipora ....... 36 49 Acalephae .... 78107 Sterelmintha ...... . 82111 Ccelelmintha. Ascaris lumbricoides ..... . 105142 Hermaphrodism of Syngamus trachealis Rotifera ..... 106144 . 127166 Epizoa ....... 133_174 Asterias ....... . 158198 Echinus ..... 172212 Holothuria ..... . 178218 Siponculus Leech . . 183226 . 200242 Earthworm 207249 Nais . 209252 Dorsibranchiate Annelidans 219263 Myriapoda ... . Insects ....... . 230275 279323 Arachnidans . . ... . 315361 Crustacea . , . . 344_381 Cirrhopoda . . . 357395 Brachiopoda Tunicata ... . 367401 . 403407 Conchifera ...... 393427 Snail . 399435 Gasteropoda 417453 Pteropoda ... Cephalopoda ...... . 428470 472513 T1- 1 Fishes ....... . 530576 Reptiles ....... 585648 . 616687 Mammalia . 706833 LYMPHATIC SYSTEM. In Fishes ....... . 517552 In Reptiles ...... In Birds ...... 563624 . 602670 In Mammalia ... 682776 URINARY SYSTEM. 520555 . 584646 In Birds . 615685 In Mammals . 704831 DEVELOPEMENT OF THE EMBRYO. In Adheres percarum Ova of Earthworm Metamorphoses of Myriapoda Tnfifcts 133175 . 209251 227271 . 288336 Crustacea 348386 . 361397 475516 . 539589 572630 . 620697 707834 . 714839 728858 Embryo of Cuttle-fish Metamorphosis of the Tadpole Changes in vascular system of Tadpole Developement of the chick in ovo Anatomy of the ovum of Ornithorynchus paradoxus Anatomy of marsupial ovum Developement of the placental fetus GENERAL INDEX. CLASSIFICATION OF THE ANIMAL KINGDOM. ACRITA. Page Sponges ........ 12 Polyps . ..... 17 Polygastrica . . * . . . .50 Acalephae ........ 64 Sterelmintha ...... 79 NEMATONEURA . .... 99 Coelelmintha ........ 99 Bryozoa . . . . . . . . 107 Rotifera . . . . . . .117 Epizoa .... ... 128 Echinoderraata . . . . . . . 135 HOMOGANGLIATA .... . . 184 Annelida . . . . . . .188 Myriapoda ........ 224 Insecta . . . . . .- . 231 Arachnida ........ 306 Crustacea .... ... . 319 HETEROGANGLIATA . . 351 Cirrhopoda . . . . . . 352 Brachiopoda ........ 362 Tunicata ........ 368 Conchifera ........ 375 Gasteropoda ....... 394 Pteropoda . . . . . . . . 423 Cephalopoda ....... 430 VERTEBRATA . .... .476 Fishes ........ 488 Reptiles ........ 537 Birds .... .... 591 Mammalia . . . . . . .632 A GENERAL OUTLINE OF THE ANIMAL KINGDOM. CHAPTER I. ON CLASSIFICATION. (1). FROM the earliest periods to the present time, the great desideratum in Zoology has been the establishment of some fun- damental system of arrangement, which, being universal in its application, should distribute the countless beings surrounding us into natural groups or divisions, such as might be subdivided into classes, orders, and genera, by obvious differences of structure in the tribes composing them, and thus enable the Zoologist at once to indicate the position which any unknown animal ought to occupy in the scale of existence, and its relations with other creatures. (2.) Aristotle, the father of our science, was the first who at- tempted a scientific division of the animal world ; * the outlines of his system were rude in proportion to the necessarily limited knowledge at his disposal, although his efforts were gigantic, and still excite our warmest admiration. This acute observer ad- mitted but two great sections, in one or other of which all known beings were included, the highest comprehending creatures pos- sessed of blood, (i. e. red blood,) corresponding to the vertebrata of modern authors ; the lowest embracing animals which in his view were exsangueous, or provided with a colourless fluid instead of blood, and corresponding to the invertebrata of more recent Zoologists. (3.) Linnaeus, like Aristotle, selected the circulatory system as * Historia Animalium. 2 ON CLASSIFICATION. the foundation of his arrangement,* dividing the animal creation into three great sections, characterized as follows : I. Animals possessed of warm red blood, and provided with a heart containing four compartments, viz. two auricles and two ventricles. Such are the mammalia and birds. II. Animals with red cold blood, their heart consisting of but one auricle and one ventricle, as he believed to be the case in reptiles and fishes. III. Animals possessed of cold white sanies instead of blood, having a heart consisting of a single cavity which he designates an auricle : under this head he includes insects and all other inverte- brate animals, to which latter he gives the general name of vermes, worms. We shall not in this place comment upon the want of anatomi- cal knowledge conspicuous in the above definitions, or the insuffi- cient data afforded by them for the purposes of Zoology. The appa- ratus of circulation, being a system of secondary importance in the animal economy, was soon found to be too variable in its arrange- ment to warrant its being made the basis of zoological classification, and a more permanent criterion was eagerly sought after to supply its place. (4.) Among the most earnest in this search was our distinguished countryman John Hunter, who, not satisfied with the results ob- tained from the adoption of any one system, seems to have tried all the more vital organs, tabulating the different groups of animals in accordance with the structure of their apparatus of digestion, of their hearts, of their organs of respiration, of their generative organs, and of their nervous system, balancing the relative im- portance of each, and sketching out with a master hand the outlines of that arrangement since adopted as the most natural and satisfactory. ( The result of the labours of this illustrious man cannot but be of dee*p interest to the zoological student, and accordingly an epitome of his ideas upon the present subject is here concisely given. The apparatus of digestion appears to be among the least efficient for the purpose of a natural division; as the separation * Systema Naturae Vindobonae, 1767. Thirteenth Edition. t Descriptive and illustrated Catalogue of the Physiological series of Comparative Anatomy, contained in the Museum of the Royal College of Surgeons in London, Vol. III. Fart I. 1835. ON CLASSIFICATION. 3 of animals into such as have a simple digestive cavity, receiving and expelling its contents by the same orifice, and such as have an aperture for the expulsion of the contents of the alimentary canal distinct from that by which food is taken into the stomach, is by no means of practical utility, although this circumstance, as we shall afterwards see, has been much insisted upon. Hunter's arrangement of the animal kingdom in conformity with the structure of the heart, was a great improvement upon that of Linnseus, founded upon the same basis. He divides in this manner all animals into five groups. I. Creatures whose hearts are divided into four cavities Mam- malia and Birds. II. Those having a heart consisting of three cavities Rep- tiles and Amphibia.* III. Animals possessing a heart with two cavities Fishes and most Mollusca. IV. Animals whose heart consists of a single cavity - Articu- lated Animals. V. Creatures in which the functions both of stomach and heart are performed by the same organ, as in Medusce. We shall pass over Hunter's sketches of arrangements founded on the respiratory and reproductive organs, as offering little satis- factory ; but the researches of this profound physiologist upon the employment of the nervous system for the purpose of zoological distribution, did much to approximate a more natural method of classification, afterwards carried out with important results. (5.) The appearance of the " Animal Kingdom distributed in accordance with its organization" of Cuvier, formed a new and im- portant era in Zoology. In this we find all creatures arranged in four great divisions, VERTEBRATA, MOLLUSCA, ARTICULATA, and R ADI AT A. These divisions, with the exception of the first, are named from the external appearance of the creatures composing them, nevertheless the three first are defined by characters exclu- sively drawn from their internal organization, the arrangement of the nervous system being essentially the primary character of dis- tinction, and have been found to be strictly natural ; whilst the last division, characterized by the appellation of R ADI AT A, in the formation of which the structure of the nervous system has * For the important discovery that the heart of the Amphibia is divided into three cavities, instead of being composed of a single auricle and ventricle, we are indebted to Professor Owen. Vide Zool. Trans. Vol. I. 4 ON CLASSIFICATION. been allowed to give place in importance to other characters of secondary weight, obviously embraces creatures of very dissimilar and incongruous formation. The VERTEBRATA are distinguished by the possession of an internal nervous centre or axis, composed of the brain and spinal cord, which is enclosed in an osseous or cartilaginous case, and placed in the median plane of the body, giving off symmetrical nerves, which are distributed to all parts of the system. This general definition indicates a large division of the animal world, which, by secondary characters drawn from the structure of their organs of respiration and circulation, is separable into mammals, birds, reptiles, amphibia, and fishes. The MOLLUSCA have a nervous system constructed upon a very different type, and do not possess any vertebral column or articu- lated skeleton. The nervous centres consist of several detached masses placed in different parts of the body, without regularity of distribution or symmetrical arrangement ; and the entire group is obviously natural, although Cuvier has ranged in it some creatures which, in the structure of their nervous system, differ essentially from those comprised in his own definition. The class of ARTICULATED ANIMALS is likewise well cha- racterized by the nervous system, which, in all the members of it, is composed of a double series of ganglia or masses of neurine, arranged in two parallel lines along the abdominal surface of the body, united by communicating cords, and from which nerves are given off to the different segments of which the body consists. But the fourth division of Cuvier, namely, that of ZOOPHYTES or RADIATED ANIMALS, is confessedly made up of the most hete- rogeneous materials, comprising animals differing in too many important points to admit of their being associated in the same group ; and the efforts of subsequent Zoologists have been mainly directed to the establishment of something like order in this chaotic assemblage. (6.) The evident relation which the perfection of the nervous system bears to that of animal structure, and the success of Cuvier in selecting this as the great point of distinction in the establish- ment of the higher divisions of the animal kingdom, necessarily led succeeding naturalists still to have recourse to this important part of the economy in making a further subdivision of the Radiata of Cuvier. In some of the radiated forms, indeed, nervous filaments are distinctly visible, and such are among the ON CLASSIFICATION. 5 more perfectly organized of the group ; these, therefore, have been classed by themselves, and designated by Mr. Owen the NEMATONEUROSE* division of the animal world ; while those which are apparently without the least trace of distinct nervous matter, have been formed by Mr. M'Leay into a group by them- selves, to which he has given the denomination of ACRITA.J- (7.) There can be no doubt that the nervous matter must be regarded as the very essence or being of all creatures, with which their sensations, volition, and capability of action are inseparably connected ; and such being the case it is a legitimate inference, that the capacities and powers of the several tribes are in im- mediate relation with the developement and perfection of this supreme part of their organization, and their entire structure must be in accordance with that of the nervous apparatus which they possess. The nature of the limbs and external members, the existence or nonexistence of certain senses, the capability of loco- motion, and the means of procuring food, must be in strict cor- respondence with the powers centred in the nervous masses of the body, or in that arrangement of nervous particles which represents or replaces them. Granting the accuracy of the above view, it is obvious, that if exactly acquainted with the structure and elaboration of the nervous apparatus in any animal, we might to a great extent pre- dicate the most important points in its economy, and form a tolerably correct estimate of its powers and general conformation. But, unfortunately, such knowledge is not always at our disposal : in the lower forms of the animal world especially, we are far from being able to avail ourselves of such a guide, and it will probably be long ere our improved means of research permit us to apply to practice the views which Physiology would lead us to adopt. The grand divisions of the animal kingdom, grounded upon the principal varieties in the arrangement of the nervous system, we shall, however, proceed to consider, leaving to future occasions those comments which a consideration of the structure of par- ticular groups will force upon our notice. , a thread ; N-upov, a nerve. t , priv.; notva, to discern. ON CLASSIFICATION. 1st Division. ACRITA* (M'Leay); Cryptoneura, (Rudolphi)-f- Protozoa,]. Oozoa. (8.) In animals belonging to this division, no nervous filaments or masses have been discovered, and the neurine or nervous matter is supposed to be diffused in a molecular condition through the body, mixed up with the gelatinous parenchyma of which they consist. Possessing no brain or central mass, to which external impressions can be transmitted, or nervous filaments calculated to conduct sensations to distant points of the system, or associate muscular movements, they are necessarily incapable of possessing those organs which are dependent upon such circumstances ; instruments of the external senses are therefore totally wanting, or their ex- istence at least is extremely doubtful ; the contractile molecules of their bodies are not as yet aggregated into muscular fibre. The alimentary apparatus consists of canals or cavities, permeating the parenchyma of the body, but without distinct walls, as in the higher divisions, where it floats in an abdominal cavity. The vascular system, where at all perceptible, consists of reticulate channels, in which the nutrient fluids move by a kind of cyclosis. Their mode of reproduction is likewise conformable to the diffused state of the nervous and muscular systems ; not only are most of them susceptible of being multiplied by mechanical division, but they generate by spontaneous fissure, as well as by gemmae, ciliated gemmules, and true ova. Many appear to be made up of a repetition of similar parts, forming compound animals of various forms, and different degrees of complexity. In this division are included 1. Sponges. 2. Polyps. 3. Polygastric animalcules. 4. Acalephse. 5. Parenchymatous Entozoa or Sterelmintha. * Horae Entomologicae, Vol. I. Part II. page 202. We adopt the term, however, according to its improved application by Mr. Owen, viz. to the exclusion of the higher organized Polyps and Entozoa, and the admission of part of the Radiata of Macleay. t Beytriige sur Anthropologie. 1812. J U^uros, first ; 2*>ov, animal. '(lov, an egg ; 2&Jv, animal, so called by Carus, because they resemble the eggs or rudiments of more perfect forms. ON CLASSIFICATION. Second Division. NEMATONEURA (Owen).* (9.) In the second division of the Radiata of Cuvier, the nervous matter is distinctly aggregated into filaments, and in some cases nuclei of neurine, which may be regarded as rudimentary nervous centres, have been noticed. It is to be lamented, however, that in this most interesting group of animals, in which we have the first developement of most of the organs subservient to the vital functions, the extreme minuteness of some genera, and the diffi- culty of distinctly observing the nervous system in the larger species, has prevented our knowledge regarding their organization, in this particular, from being of that satisfactory character which it is to be hoped it will hereafter attain to. Owing to the want or imperfect condition of the nervous centres, the nematoneura are necessarily incapable of possessing external organs of the higher senses, the general sense of touch being as yet the only one of which they are indubitably possessed ; yet in their muscular system they are much more efficiently provided than the acrite orders, as the developement of nervous threads of communi- cation renders an association of muscular actions possible ; and therefore, co-apparent with nervous filaments, we distinguish in the structure of the nematoneura distinct fasciculi of muscular fibre, and powers of locomotion of a much more perfect description. The digestive apparatus is no longer composed of canals merely excavated in the parenchyma of the body, but is provided with distinct muscular and membranous walls, and loosely attached in an abdominal cavity. The circulation of the nutritious fluid is likewise carried on in a separate system of vessels, distinct from the alimentary apparatus, yet still unprovided with a heart, or exhibiting pulsations for the forcible impulsion of the contained blood. The fissiparous mode of reproduction is no longer witnessed, an obvious consequence of the increased complexity of struc- ture, and these animals are for the most part androgynous, or capable of producing fertile ova, without the co-operation of two individuals. Among the nematoneura, therefore, we include * Cyclopaedia of Anatomy and Physiology. Article, ACRITA. 8 ON CLASSIFICATION. 1. Bryozoa, or Polyps, with ciliated arms. 2. Rotifera. 3. Epizoa. 4. Cavitary Entozoa or Ccelelmintha. 5. Echinodermata. The reader will perceive, that this division, however well sepa- rated from the preceding by physiological characters, is, in a zoological point of view, principally composed of groups detached from the members of other orders. The Bryozoa are evidently dismemberments of the family of Polyps, from which they differ in their more elaborate internal organization. The Coelelmintha are more perfect forms of the Parenchymatous Entozoa. The Roti- fera, formerly confounded with the Infusoria, exhibit manifest analogies with the articulated Crustaceans, as in fact do the Epizoa. The Echinodermata alone appear to form an isolated group, properly belonging to the division under consideration. Third Division. HOMOGANGLIATA (Owen) ; Articulata (Cu- vier)*; Annulosa (Macleay) ; Diploneura (Grant). ~f* (10.) The articulated division of the animal kingdom is charac- terized by a nervous system, much superior in developement to that possessed by the two preceding, indicated by the superior propor- tionate size which the ganglionic centres bear to the nerves which emanate from them. The presence of these central masses of neurine, admits of the possession of external senses of a higher class than could be expected among the Acrita or Nematoneura, and gives rise to a concentration of nervous power, which allows of the existence of external limbs of various kinds, and of a complex muscular system capable of great energy and power of action. The nervous centres are arranged in two parallel lines along the whole length of the body, forming a series of double ganglia or brains, belonging apparently to the individual segments of which the animal is composed. The anterior pair placed invariably in the head above the oesophagus, and consequently upon the dorsal aspect of the body, seems more immediately appropriated to the higher senses, supplying nerves to the antennae, or more special in- struments of touch, to the eyes, which now manifest much com- plexity of structure, to the auditory apparatus where such exists, * The Cirripecla are excluded from the Articulata of Cuvier. -f- The Entozoa and Rotifera are included in the Diploneura of Dr. Grant. ON CLASSIFICATION. 9 and probably to the senses of taste and smell. This dorsal or anterior pair of ganglia, which evidently is in relation with the higher functions of the economy of the creature, is brought into communication with the series of nervous centres placed along the ventral aspect, by means of filaments which embrace the oesophagus, and join the anterior pair placed beneath it ; the whole system may therefore be regarded as a series of independent brains destined to animate the segments of the body in which they are individually placed. Such a multiplication of the central organs of the nervous system, is obviously adapted to the elongated forms of the vermi- form orders, but from the want of concentration which such an arrangement implies, this type of structure is still very inferior in its character. As the articulata become more perfect in their out- ward form, the number of the brains becomes diminished, while their proportionate size increases ; and thus in the carnivorous Insects, Arachnida and Crustacea, they are all united into a few great masses, which, becoming the general centres of the entire system, admit of a perfection in their external senses, a precision in their movements, and an energy of action, of which the detached character of the ganglia in the lower tribes was incapable. (11.) This dependence of the perfection of the animal upon the concentration of the central masses of the nervous system, is strik- ingly proved by the changes perceptible in the number and arrange- ment of the ganglia, during the progress of an insect through the different stages of its existence. In the elongated body of the worm- like caterpillar, each segment possesses its appropriate pair of ganglia, and the consequence of such diffusion of its nervous apparatus, is apparent in its imperfect limbs, its rude organs of sense, its sluggish movements, and general apathy, but as it successively attains to more mature forms of existence, passing through the different me- tamorphoses which it undergoes, the nervous ganglia gradually coalesce, increase in power, as they diminish in number, until in the imago or perfect state, having arrived at the greatest concen- tration compatible with the habits of the insect, we find it endued with new and far more exalted attributes, the organs of its senses are more elaborately formed, it possesses limbs which previously it would have been utterly incapable of wielding, its movements are characterized by their activity and precision, and its instincts and capabilities proportionately enlarged and exalted. The Homogangliate division of the animal world is extremely natural, and includes the following classes : 10 ON CLASSIFICATION. 1. Cirripeda. 4. Insecta. 2. Annelida. 5. Araclmida. 3. Myriapoda. 6. Crustacea. Fourth Division. HETEROGANGLIATA (Owen) ; Mollusca (Cuvier)*; Cyclogangliata (Grant). (12.) The characters of this division are well defined, and the irre- gular and unsymmetrical forms of the bodies of most of the genera which compose it, in exact relation with the arrangement of the nervous apparatus. As in the articulata there is a large nervous mass placed above the oesophagus, which supplies the principal organs of sense, but the other ganglia are variously dispersed through the body, although always brought into communication with the supracesophageal portion by connecting filaments. Throughout all the forms, we find a distinct relation between the size and developement of the nervous centres, and the perfection of the animal, indicated by the senses and organs of motion with which it is provided. This division includes 1. Tunicata. 4. Gasteropoda. 2. Conchifera. 5. Pteropoda. 3. Brachiopoda. 6. Cephalopoda. Fifth Division. VERTEBRATA (Cuvier); Myelencephala (Owen); Spinicerebrata (Grant). (13.) The arrangement of the nervous centres in the highest or vertebrate division, indicates the greatest possible concentration and developement. The ganglionic masses assume a very great pro- portionate size when compared with the nerves which emanate from them, and are principally united into a long chain, denominated the cerebro-spinal axis or cord, which is enclosed in a cartilaginous or bony canal, occupying the dorsal region of the animal. The anterior extremity of the cerebro-spinal axis is made up of those ganglia which are more especially in relation with the principal senses and the higher powers of intelligence, forming a mass denominated, from its position in the skull which encloses it, the encephalon. It is with the increased proportionate developement of this portion, that the intelligence of the animal becomes augmented ; in the lower tribes, the cerebral masses scarcely exceed in size those * The Cirripeda are included in the Mollusca of Cuvier. ON CLASSIFICATION. 11 which form the rest of the central chain of ganglia, but as we advance from fishes towards the higher forms of the vertebrata, we observe them to preponderate more and more in bulk, until at last in man they assume that extraordinary developement adapted to the exalted position which he is destined to occupy. It is in the cerebral ganglia, therefore, that we have the representative of the supracesophageal masses of the articulated and molluscous classes, which, as we have already seen, preside especially over the senses, and correspond in their proportions with the capabilities of the tribes of animals included in those divisions. The spinal cord, as the rest of the central axis of the nervous system of vertebrata is denominated, is made up of a succession of ganglia, in communi- cation with symmetrical pairs of nerves connected with them, and which preside over the generally diffused sense of touch, and the voluntary motions of the body. But besides the cerebro-spinal sys- tem, we find in the vertebrated classes another set of nervous centres, to which nothing corresponding has been satisfactorily identified in the lower divisions ; namely, the sympathetic system, which mainly controls the involuntary movements of the body connected with the vital functions. The vertebrata are further distinguished by the possession of an internal organized skeleton, either composed of cartilage or bone, which is made up of several pieces, and serves as the general support of the frame, forming a series of levers upon which the muscles act. This last division of the animal world embraces the following classes : 1. Fishes. 4. Birds. 2. Amphibia. 5. Mammalia. 3. Reptiles. Such will be the classification which we shall adopt in the following pages ; and although, perhaps, the definitions of the five great groups may be considered by the scientific reader as some- what scanty, enough, we trust, has been said to render intelli- gible the terms which we shall hereafter have frequent occasion to employ. (14.) A question naturally presents itself in this place which re- quires consideration : May we expect, as we advance from the lower types of organization to such as are more perfect, to be led on through an unbroken and continuous series of creatures, gradually rising in importance and complexity of structure, each succeeding 12 ON CLASSIFICATION. tribe of beings presenting an advance upon the preceding, and merging insensibly into that which follows it ? A very slight investigation of this matter will convince us of the contrary. Each group, in fact, will be found to present points of relationship with several others, into all of which it passes by connecting species ; as a circle would, at different points of its circumference, touch others placed around it. This, however, will be best illustrated as we proceed. CHAPTER IT. ON SPONGES. Porifera,) Grant Amorphozoa (Blainville). (1 5.) The great circles to which we may compare the animal and vegetable kingdom, like the smaller circles to which allusion was made at the close of the last chapter, touch each other ; or, in other words, there are certain forms of organization so closely allied to both, that it is difficult to say precisely in which they ought to be included. Such are the sponges, which, although by common consent admitted into the animal series, will be found to be excluded, by almost every point of their structure, from all the definitions of an animal hitherto devised. What is an animal ? How are we to distinguish it as contrasted with a mineral or a vegetable ? The concise axiom of Linnaeus upon this subject is well known, " Stones grow ; vegetables grow and live ; animals grow, live, and feel." The capability of feeling, therefore, formed, in the opinion of Linnaeus, the great characteristic sepa- rating the animal from the vegetable kingdom ; yet, in the class before us, no indication of sensation has been witnessed ; contact, however rude, excites no movement or contraction which might indicate its being perceived ; no torture has ever elicited from them an intimation of suffering ; they have been pinched with forceps, lacerated in all directions, bored with hot irons, and attacked with the most energetic chemical stimuli, without shrinking or exhibit- ing the remotest appearance of sensibility. On the other hand, in the vegetable world we have plants which apparently feel in 1'ORIFERA. 13 this sense cf the word. The sensitive plant, for example, which droops its leaves upon the slightest touch, would have far greater claims to be considered as being an animal than the sponges of which we are speaking. The power of voluntary motion has been appealed to as exclusively belonging to the animal economy : yet, setting aside the spontane- ous movements of some vegetables, the sponge, rooted to the rock, seems absolutely incapable of this function, and the most micro- scopic scrutiny has failed to detect its existence. The best definition of an animal, as distinguished from a vege- table, which has as yet been given, is, that whereas the latter fixed in the soil by roots, or immersed perpetually in the fluid from which it derives its nourishment, absorbs by its whole surface the nutriment which it requires ; the animal, being generally in a greater or less degree capable of changing its position, is provided with an internal receptacle for food, or stomachal cavity, from which, after undergoing the process of digestion, the nutritious matter is taken up. But in the case of the sponge no such reservoir is found; and in its place we find only anastomosing canals which permeate the whole body, and convey the circumam- bient medium to all parts of the porous mass. The last circumstance which we shall allude to as specially appertaining to the animal kingdom, is derived from the chemical composition of organized bodies. Vegetables contain but a small proportion of azote in their substance, whilst in animals this element exists in considerable abundance, causing their tissues when burned to give out a peculiar odour resembling that of burned horn, and in this particular sponges differ from vegetable matter. (16.) The common sponge of commerce is, as every one knows, made up of horny, elastic fibres of great delicacy, united with each other in every possible direction, so as to form innumerable canals, which traverse its substance in all directions. To this structure the sponge owes its useful properties, the resiliency of the fibres com- posing it making them, after compression, return to their former state, and leaving the canals which they form open, to suck up surrounding fluids by capillary attraction. The dried sponge is, however, only the skeleton of the living animal : in its original state, before it was withdrawn from its native element, every filament of its substance was coated over with a thin film of glairy semifluid matter, composed of aggregated transparent 14 PORIFERA. globules, which was the living part of the sponge, secreting, as it extended itself, the horny fibres which are imbedded in it. The anastomosing filaments which compose the skeleton of such sponges, when examined under a microscope, and highly magnified, appear to be tubular, as represented in fig. 1. c. Many species, although exhibit- ing the same porous structure, have none of the elasticity of the officinal sponge, a circumstance which is due to the difference ob- servable in the composition of their skeletons or ramified frame-work. In such the living crust forms within its substance not only tenacious bands of animal matter, but great quantities of crystallized spicula, sometimes of a calca- reous, at others of a silicious nature, which are united together by the tenacity of the fibres with which they are surrounded. On destroying the softer portions of these skeletons either by the aid of a blow-pipe or by the caustic acids or alkalies, the spicula re- main, and may readily be examined under a microscope : they are then seen to have determinate forms, which are generally in rela- tion with the natural crystals of the earths of which they consist ; and as the shape of the spicula is found to be similar in all sponges of the same species, and not unfrequently peculiar to each, these minute particles become of use in the identification of these bodies.* Crystallized spicula of this description form a feature in the structure of the sponge which is common to that of many vege- tables, resembling the formations called Raphides by botanical writers. Some of the principal forms which they exhibit are de- picted in fig. 1 a b, which likewise will give the reader a general idea of the appearance of the silicious and calcareous sponges, after the destruction of their soft parts has been effected by the means above indicated. The figures d, e, f 9 and g, exhibit detached spicula of different forms highly magnified. The most * Savigny (Jules Caesar) Zoologie d'Egypte gr. fol. Paris, 1809. PORIFERA. 15 convenient method of seeing them is simply to scrape off a few particles from the incinerated sponge upon a piece of glass, which, when placed under the microscope, may be examined with ordinary powers. (17.) On placing a living sponge of small size in a watch-glass or small glass trough filled with sea-water, and watching it attentively, something like a vital action becomes apparent.* The entire surface is seen to be perforated by innumerable pores and aper- tures, some exceedingly minute, opening on every part of its peri- phery ; others of larger dimensions, placed at intervals, and gene- rally elevated upon prominent portions of the sponge. Through the smaller orifices the surrounding water is continually sucked as it were into the interior of the spongy mass, and it as constantly flows out in continuous streams through the larger openings. This con- tinual influx and efflux of the surrounding fluid is produced by an agency not yet discovered, as no contraction of the walls of the canals, or other cause to which the movement may be referred, has ever been detected ; we are as- sured, however, that it is from the currents, thus continually permeating every portion of its substance, that the general mass is nourished. The annex- ed diagram, fig. 2 a, will give the reader an idea of the most usual direction of the streams : the entering fluid rushes in at the countless pores which occu- py the body of the sponge ; but, in its progress through the canals in the interior, becomes directed into more capacious channels, communicating with the prominent larger orifices, through which it is ultimately ejected in equable and ceaseless currents. Organized particles, which ne- cessarily abound in the water of the ocean, are thus introduced into the sponge on all sides, and are probably employed as nutri- ment, whilst the superfluous or effete matter is continually cast out with the issuing streams as they rush through the fecal ori- fices. The growth of the sponge is thus provided for, the living * Dr. Grant, in the New Edinburgh Philosophical Journal, 1827. 16 POHIFERA. gelatinous portion continually accumulates, and, as it spreads in every direction, secretes and deposits, in the form peculiar to its species, the fibrous material and earthy spicula which characterise the skeleton. (18.) From this description of the structure of a sponge, it will be apparent that all parts of the mass are similarly organized : a necessary consequence will be, that each part is able to carry on, independently of the rest, those functions needful for existence. If therefore a sponge be mechanically divided into several pieces, every portion becomes a distinct animal. (19.) The multiplication of sponges, however, is effected in another manner, which is the ordinary mode of their reproduction, and forms a very interesting portion of their history.* At certain seasons of the year, if a living sponge be cut to pieces, the chan- nels in its interior are found to have their walls studded with yel- lowish gelatinous granules, developed in the living parenchyma which lines them ; these granules are the germs or gemmules from which a future race will spring ; they seem to be formed in- differently in all parts of the mass, sprouting as it were from the albuminous crust which coats the skeleton, without the appearance of any organs appropriated to their developement. As they in- crease in size, they are found to project more and more into the canals which ramify through the sponge, and to be provided with an apparatus of locomotion of a description which we shall fre- quently have occasion to mention. The gemmule assumes an ovoid form, fig. 2 B, and a large portion of its surface becomes covered with innumerable vibrating hairs or cilia, as they are de- nominated, which are of inconceivable minuteness, yet individually capable of exercising rapid movements, which produce currents in the surrounding fluid. As soon therefore as a gemmule is suffi- ciently mature, it becomes detached from the nidus where it was formed, and whirled along by the issuing streams which are ex- pelled through the fecal orifices of the parent, it escapes into the water around. Instead, however, of falling to the bottom, as so appa- rently helpless a particle of jelly might be expected to do, the cease- less vibration of the cilia upon its surface propels it rapidly along, until, being removed to a considerable distance from its original, it attaches itself to a proper object, and, losing the locomotive cilia which it at first possessed, it becomes fixed and motionless, and * Professor Grant loc. cit. ON POLYPS. developes within its substance the skeleton peculiar to its species, exhibiting by degrees the form of the individual from which it sprung. It is curious to observe the remarkable exception which sponges exhibit to the usual phenomena witnessed in the reproduc- tion of animals, the object of which is evident, as the result is admirable. The parent sponge, deprived of all power of movement, would obviously be incapable of dispersing to a distance the numerous progeny which it furnishes ; they must inevitably have accumulated in the immediate vicinity of their place of birth, without the possi- bility of their distribution to other localities. The seeds of vegetables, sometimes winged and plumed for the purpose, are blown about by the winds, or transported by various agencies to distant places ; but, in the present instance, the still waters in which sponges grow would not have served to transport their progeny elsewhere, and germs so soft and delicate could hardly be removed by other creatures. Instead therefore of being helpless at their birth, the young sponges can, by means of their cilia, row themselves about at pleasure, and enjoy for a period powers of locomotion denied to their adult state. CHAPTER III. ON POLYPS. Zoophytes of old Authors Phytozoa (Ehrenberg). (20.) IT is not surprising that many members of the extensive family upon a consideration of which we are now entering, should have been regarded by the earlier naturalists as belonging to the vegetable kingdom, with which, in outward appearance at least, numerous species have many characters in common.* Fixed in large arborescent masses to the rocks of tropical seas, or in our own climate attached to shells or other submarine sub- stances, they throw out their ramifications in a thousand beautiful and plant-like forms ; or, incrusting the rocks at the bottom of the ocean with calcareous earth separated from the water which bathes them, they silently build up reefs and shoals, justly dreaded by the navigator, and sometimes giving origin, as they rise to the surface of the sea, to islands which the lapse of ages clothes with * Tournefoit, Institutiones Rei Herbaria*, 4to. 1719. 18 ON POLYPS. luxuriant verdure, and peoples with appropriate inhabitants. Va- rious indeed are the forms which these creatures offer to the zoolo- gist ; and the classification of them, even at the present day, is a subject of much doubt and uncertainty. Without entering fur- ther into the subject of their division into groups and families than is connected with our purpose of examining the main features of their economy, we shall select some of the most marked varieties for description, commencing with the simplest and least elabo- rately formed. (21.) We have already seen that in the Sponges the living portion of the animal was composed of a gelatinous film, which, without any apparent organization, was possessed of the power of extracting nutri- ment from the water around it, of deriving from the same source ani- malized materials and earthy particles, which were deposited within its texture, and used in constructing a porous frame-work or skele- ton ; and, moreover, that the same semifluid parenchyma could de- velope from its substance germs, which became ultimately expanded into other beings resembling that from which they sprung ; we shall therefore be prepared to find, in the class upon which we are enter- ing, like results produced by equally simple means. Among the calcareous structures, derived from the tropical seas, which are usually known by the general terms of Madre- pores, Corals, &c. and which, from the beauty of their structure, form the ornaments of our cabinets, few are more common than those denominated Fun- gise and Meandrinse, animals belonging to the group Madrephyllicea of systematic zoologists. These masses consist of thin plates or laminse of various dimensions (fig. 3.) disposed in different directions in different species, but in the Fungia Agariciformis, which we have selected as an example, radiating from a common centre, and forming a circular mass resem- bling a mushroom. When living in its native element, every part of the surface of this stony skeleton was encrusted with a film of animal Fig. 3. ON POLYPS. 19 matter, dipping down into the interstices of the plates, and cover- ing the whole frame-work. In the figure, the darker portion indi- cates the living crust ; whilst from the lighter parts it has been re- moved, to show the stony skeleton itself. There are no arms or moving parts adapted to the prehension of food, and no separation of organs adapted to the performance of the vital functions has hitherto been described ; the thin membranous film apparently absorbs the materials of its support from the water of the ocean, and deposits within its substance the calcareous par- ticles which it secretes, moulding them into the form peculiar to its skeleton, which it gradually enlarges as its own extent increases. (22.) The gelatinous investment, however, gives certain dubious indications of vitality, and possesses the power of contracting itself so as to retire between the laminae of its skeleton when roughly handled, and thus conceal itself from injury. Upon the surface of the soft crust are seen a number of vesicles indicated in the figure, which were regarded formerly as rudimentary tentacula, from the circumstance of their being able to contract and vary their dimen- sions ; recent observations however lead to the belief that they are cavities filled with air, and serving an important purpose in the eco- nomy of the creature, namely, that of preventing it from being turned upside down by the occasional agitation of the ocean, as in such case the animal has been found by experiment to have no power of restoring itself to its former position, and consequently perishes : these air-vessels may therefore be looked upon as floats, which, ren- dering the upper surface more buoyant than the inferior, materially assist in preventing such an accident ; for, as it lies quite loose and unattached upon the surface of the sand, it is subject to be lifted up from its bed by any sudden roll of the sea, and deposited at a considerable distance from its former place. (23.) The reproduction of fungise is effected by the developement of sprouts or gemmae, which pullulate from the animal substance as buds issue from a plant, and remain for some time fixed to the parent by a species of foot-stalk, which sustains them until they have attained to a considerable size ; the young fungiae being up- wards of an inch in diameter before they become detached. When mature, they separate from the top of the stony peduncle which hi- therto supported them ; and at this time, the skeleton of the young fungia, when divested of its fleshy part, shows a circular opening beneath, through which the radiating plates of the upper surface 20 ON POLYPS. are visible. In a short time a deposit of calcareous matter takes place, which cicatrizes the opening, the marks of which however can be traced for a considerable period, until at length the increase of this secretion continuing with the growth of the animal, entirely obliterates all appearance of its having existed. In the earliest period of its developement, the foot-stalk by which the young is united to the parent, as well as its radiating disc, is entirely enveloped with the soft parts of the animal ; but as the upper portion spreads, and assumes its characteristic form, the pedicle is left naked, and the gelatinous coating extends only to the line where the separation afterwards takes place. (24.) It is generally supposed that the calcareous matter which forms the skeleton of these madrepores is perfectly external to the liv- ing crust which secretes it, and accordingly is absolutely inorganic, and removed from the future influence of the animal which produced it. Such a supposition appears, however, at variance with the facts above stated, and incompatible with many circumstances connected with the history of the lithophytous polyps. On trying to detach the soft envelope from the surface of the skeleton, the firmness of their adherence would render such a want of connexion improbable, they appear to be, as it were, incorporated with each other ; and besides, the separation of the fungia from the peduncle which joined it to its parent during its earlier growth, necessarily supposes a power of removing the calcareous particles after their deposition. It is therefore almost demonstrable that the earthy matter secreted by the polyp is deposited in the tissue of its substance, and still remains, in a greater or less degree, subject to absorption and removal : of this, however, we shall have fuller evidence hereafter. (25.) It is astonishing how nearly the animal and vegetable king- doms approximate each other in the lower orders of these calcareous zoophytes. Admitting the animal nature of fungia, we find calcareous skeletons, essentially similar in their chemical composition, produced by a large tribe of organic forms, long classed with the creatures we are now considering, which modern observations have clearly shown to be of vegetable nature.* These are the Corallines, (Linn.) which, although so nearly re- sembling the skeletons of polyps, that Cuvier, Lamarck, and others, scrupled not to admit them into the animal circle, have been proved * Schweigger, Anatomische Physiologische Untersuchungen liber Corallen. Berlin, ON POLYPS. by microscopical researches to possess the cellular structure apper- taining to vegetable organization, and are thus placed beyond the limits of our present investigations. (26.) We have hitherto spoken of animals which do not appa- rently possess any stomach or oral aperture, any apparatus for the purpose of the digestion or prehension of food. Before describ- ing the more complex forms of polyps, we will now select a group of that class of animals, in which the organs provided for these pur- poses are easily recognisable ; and, as the simplicity of their orga- nization will well exhibit the principal points in the physiology of the acrita,we shall detail at some length the facts known concerning them. The HYDR.E, or fresh- water polyps, are common in the ponds and clear waters of our own country ; they are generally found creeping upon confervse which float upon the surface, and may readily be pro- cured in summer for the purpose of investigating the remarkable cir- cumstances connected with their history. p^ 4 The body of one of these simple animals consists of a delicate gelatinous tube, con- tracted at one extremity, which is terminated by a minute sucker, and furnished at the op- posite end with a variable num- ber of delicate contractile fila- ments, placed around the open- ing which represents the mouth. In the Hydra viridis, (jig- 4, 1,) the species most common amongst us, the tentacular fila- ments are short, and, when elon- gated to the utmost, are not equal to the length of the body; but in the long-armed species Hydra fusca, (Jig. 4, 2,) they are much prolonged, and of extreme tenuity. If placed in a small glass tube, one side of which is flattened, these animals may readily be sub- mitted to microscopical examination, and, from their transparency, their entire structure is easily made out. When highly magnified, the whole body is seen to consist of a granular substance, generally of a greenish hue, the granules being loosely connected by a semifluid albuminous matter ; but the most minute research reveals no fur- 22 ON POLYPS. ther appearances of organization : there is no trace of muscular fibre or of nervous substance, not the slightest indication of vessels of any kind, nor any apparatus destined to the function of repro- duction ; such is the hydra, offering in every particular a good example of the acrite type of structure. The young naturalist would scarcely be prepared to see an animal of this description waging continual war with creatures much more perfectly organized than itself; endowed with consi- derable capability of locomotion ; possessed not only of a refined sense of touch, but able to appreciate the presence, and seek the influence of light ; and exhibiting moreover a tenacity of life and power of reproduction almost beyond belief : a little observation, however, will convince him that it possesses all these attributes, and enable him to share in some degree the astonishment with which Trembley, their enthusiastic discoverer, first witnessed and described them.* (27.) The hydra is not like most other polyps, fixed and station- ary; but can roam about and change its situation according to circum- stances. Its usual mode of progression is by creeping along the stems of aquatic plants, or upon the sides of the glass in which it is confined : attaching first the little tubercle at its posterior ex- tremity to the surface upon which it moves, it slowly inflects its body (fig. 4, 3), and fixing its oral tentacles, moves along in the manner of a leech, by a succession of similar actions. This method of advancing is, from the small size of the animal, necessarily slow ; and a march of a couple of inches will require several hours for its performance : but, when arrived at the surface of the water, it adopts a more speedy course ; suspending itself by the tail as by a minute float, and hanging with its mouth downwards, it rows itself about with its tentacles, or, wafted by the wind, can travel to a consider- able distance without effort. (28.) When left free, the hydrse are found to select positions most exposed to the influence of light, assembling at the surface of the ponds which they inhabit, or seeking that side of the glass in which they are confined, that is most strongly illuminated. That they are able to appreciate the presence of light is therefore indubitable ; yet with what organs do they perceive it ? We are driven to the supposition that, in this case, the sense of touch supplies to a certain extent the want of other senses, and that the hydrse are able, as * Trembley, Memoires pour servir a 1'Histoire des Polypes d'eau douce. Leyde, 1744. ON POLYPS. 23 an Italian author elegantly expresses it, 6C palpare la luce," to feel the light. (29.) The tentacles placed around the mouth are eminently sensi- tive, and the smallest particles which impinge upon those organs in their expanded state appear to excite a perception of their presence ; yet their movements, as well as those of the whole body, are extremely slow and languid : it would be difficult therefore to imagine that creatures apparently so helpless should be able to obtain other prey than such as had no power of resistance ; and we could scarcely believe, were it not a matter of continual observation, that the most active little animals, entomostraca, the larvse of insects, and even minute fishes, form their usual food. When the hydra is watching for prey, it remains expanded, (fig. 4, 1, 2,5,) its tentacula widely spread and perfectly motionless, waiting patiently till some of the countless beings which populate the stagnant waters it frequents, are brought by accident in contact with them : no sooner does an animal touch one of the filaments than its course is arrested as if by magic ; it appears instantly fixed to the almost invisible threa,d, and in spite of its utmost efforts is unable to escape ; the tentacle then slowly contracts, and others are brought in contact with the struggling prey, which thus seized is gradually dragged towards the orifice of the mouth, that opens to receive it, and slowly forced into the interior of the stomach. (30.) We are naturally led to ask, what is the nature of the action by which a passing animal is thus seized ? Trembley supposed that the filamentary arms were besmeared with an adhesive secretion like bird-lime, by which the victim became glued to the tentacle ; this however can hardly be the case, as the exercise of the power of retaining prey seems quite under the control of the hydra : when hungry, seven or eight monoculi* will be captured and swallowed in succession ; but when thus gorged with prey, or when indisposed to take food, although these animals may touch the tentacula again and again, they escape with impunity. (31.) Arrived in the stomach of the polyp, the animal which has been swallowed is still distinctly visible through the transparent body of the hydra, which seems like a delicate film spread over it : (Jig* 4, 4,) gradually the outline of the included victim becomes indistinct, and the film which covers it turbid ; the process of diges- tion has begun ; the soft parts are soon dissolved and reduced to a * Minute crustaceous animals, possessing considerable strength and agility. 24 OX POLYPS. fluid mass, and the shell or hard integument is expelled through the same aperture by which it entered the stomach. We will not even hazard a conjecture concerning the process by which digestion is effected in this case, our knowledge of animal physiology is by no means sufficiently advanced to render any attempt at explanation useful ; we will rather pass on, and enquire in what manner the nutritious parts of the food are conveyed into the system of the polyp. We have already observed that no traces of vessels of any kind have as yet been detected in the granular parenchyma of which the creature seems to be composed ; coloured globules are seen floating in a transparent fluid, which, in the Hydra viridis, are green, although in other species they assume different tints. When the food has been composed of coloured substance, as, for example, red larvae, or black planarite, the granules of the body are seen to acquire a simi- lar hue, but the fluid in which they float remains quite trans- parent ; each granule seems like a little vesicle into which the coloured matter is conveyed, and the dispersion of these globules through the body gives to the whole polyp the hue of the prey which it has devoured; sometimes the granules thus tinted are seen to be forced into the tentacula, from whence they are driven again by a sort of reflux into the body, pro- ducing a kind of circulation or rather mixing up of the granular matter which distributes it to all parts. If, after having thus digested coloured prey, the polyp is made to fast for some time, the vesicles gradually lose their deepened hue and become com- paratively transparent. The granules, therefore, would seem to be specially connected with the absorption and distribution of nutriment. (32.) Rapid as is the action of the stomach upon food introduced into it, it has no effect upon other parts of the animal when immersed in its cavity : the arms, for example, of the long-armed hydra are frequently coiled around its prey during the process of its solution, without receiving the slightest injury. This circumstance may not appear very remarkable, but it has been found that other polyps of the same species are equally able to resist the solvent action. Trembley once saw a struggle between two of these creatures which had seized upon the same animal ; both had partially succeeded in swallowing it, when the largest put an end to the dispute by swallowing its opponent as well as the subject of con- tention. Trcmbley naturally regarded so tragical a termination ON POLYPS. 25 of the affray as the end of the swallowed polyp's existence, but he was mistaken ; after the devourer and his captive had digested the prey between them, the latter was regurgitated safe and sound, and apparently no worse for the imprisonment. (33.) We will now proceed to consider the mode of reproduction of these simple animals. When mature and well supplied with food, minute gemmules or buds are seen to become developed from the common substance of the body ; they spring from no particular part, but seem to be formed upon any portion of the general sur- face. These gemmsB appear at first like delicate gelatinous tu- bercles upon the exterior of the parent polyp ; but, as they increase in size, they gradually assume a similar form, become perforated at their unattached extremity, and develope around the oral aperture the tentacula characteristic of their species. During the first period of the formation of these sprouts, they are evidently continuous with the general substance from which they arise ; and even when considerably perfected, and possessed of an internal cavity and tentacula, their stomach freely communicates with that of their parent by a distinct opening, so that food digested by the latter passes into the stomach of the young one, and serves to nourish it. As soon as the newly-formed hydra is capable of catching prey, it begins to contribute to the support of its parent ; the food which it captures passing through the aperture at its base into the body of the original polyp. At length, when the young is fully formed and ripe for independent existence, the point of union between the two becomes more and more slender, until a slight effort on the part of either is sufficient to detach them, and the process is completed. This mode of increase, when the animals are well supplied with nourishment, and the temperature is favourable, is extremely rapid ; sometimes six or seven gemmse have been observed to sprout at once from the same hydra, and, although the whole process is concluded in twenty-four hours, not unfrequently a third generation may be observed springing from the newly-formed polyps even before their separation from their parent : eighteen have in this manner been seen united into one group, so that, provided each individual when complete exhibited equal fecundity, more than a million might be produced in the course of a month from a single polyp. (34.) But perhaps the most remarkable feature in the history of the hydra is its power of being multiplied by mechanical division. If a snip be made with a fine pair of scissors in ON POLYPS. the side of one of these creatures, not only does the wound soon heal, but a young polyp sprouts from the wounded part ; if it be cut into two portions by a transverse incision, each soon deve- lopes the wanting parts of its structure ; if longitudinally di- vided, both portions soon become complete animals ; if even it be cut into several parts, every one of them will rapidly assume the form and functions of the original ; the inversion of its body, by turning it inside out, does not destroy it ; on the contrary, the ex- terior surface assumes the office of a stomachal cavity, and that which was originally internal will give birth to buds, and take upon itself all the properties of the skin. (35.) Cortical compound Polyps. From what we have said con- cerning the two preceding families of polyps, one composed of animals consisting entirely of a gelatinous crust which invests a fixed and immoveable skeleton ; the other exhibiting active and hungry creatures, provided with an internal digestive cavity, and endowed with the capability of seizing and devouring living prey, we are prepared to examine the more complex structure of compound polyps, which combine in themselves the characteristics of both families. The compound polyps consist of a mass of gelatinous matter, which indicates, by its power of contraction upon the appli- cation of stimuli, a degree of sensation ; and of a great number of hydrseform polyps, which spring from the surface of the common body, and are individually capable of seizing and digesting prey, the nutriment thus gained being appropriated to the nourishment of the general mass. The animals of this division are provided with numerous mouths and stomachs, each endowed with a power of independent action. Although essentially similar in their habits, the compound polyps present various modifications of structure, which natu- rally leads them to be grouped in distinct families. Some- times the central common mass is entirely soft and gelatinous, its surface being covered with minute cells in which the polyps are lodged; such are the Alcyonidas. Sometimes the common body secretes large quantities of calcareous matter in the same man- ner as the Fungia, which, being deposited in its interior, forms arborescent masses, presenting upon their surface multitudes of cells, generally distinguishable after the removal of the outer crust, in each of which when alive a polyp existed : these form the family of Madrepores. The central axis is not unfrequently quite solid and smooth upon the surface, offering no cells for the ON POLYPS. lodgment of the hydrseform mouths ; being sometimes composed of hard and dense calcareous substance, or else flexible and horny in its texture : such are the Corallida or family of corals, properly so called. The internal central axis is, moreover, in another family, composed of several pieces united together by the living crust which secretes them ; and such individuals, being free and unat- tached, are probably able to change their position at pleasure : these form the family of Pennatula. These groups are, however, merely modifications of the same general type of structure, although differing in certain minor points of their organization, so as to render an examination of each form needful for our purpose. (36.) Alcyonidce. This family includes several genera, known by the names of Alcyonium, Lobularia, Cydonium, &c., being charac- terized by having no solid axis developed in the interior of the com- mon body. The Cydonium Fig. 5. Mulleri (Jig. 5, 1,) will give t the reader a good idea of the general appearance of one of these compound animals. The central mass, or polypary, is entirely soft, being of a gelati- nous or rather subcartilagi- nous texture. Its density varies with the state of the animal, being more firm when the crea- ture is contracted or hardened by immersion in spirits of wine, than when alive and ex- panded. Upon cutting into it, it is found to be intersected by tough fibrous bands, and not unfrequently contains calcareous spicula dispersed through its substance ; no muscular fibre or nervous matter has ever been de- tected in its composition, and its interior is permeated by nume- rous wide canals variously disposed. The alcyonidse, therefore, may justly be looked upon as intimately related to the sponges in the structure of their common body, differing from them principally in the polyps which occupy the cells upon their surface. (37.) The polyps which fill these cells resemble so many hydra in their external configuration, from which, however, they differ in the number of tentacula surrounding the mouth. In the hydra we 28 ON POLYPS. find sometimes five, sometimes six, or more of these appendages ; but in all the cortical polyps there are eight. The tentacles, also, are not unfrequently pinnated or slightly fringed on each side, but never provided with moveable cilia. The body of the polyp, when withdrawn from its cell, is somewhat globular, and more complex in its structure than that of the hydra. In Jig. 5,2, a diagram is given, representing the Alcyonium exos, in which the following parts may be distinguished. The stomach* is considerably dilated, and terminates inferiorly in a tubular prolongation, b, which ex- tends into the substance of the common mass, into which it most probably conveys nourishment. But the main difference observ- able between the alcyonidse and the hydra consists in the possession of a reproductive organ or ovary, in which the germs of its progeny are developed. This consists of a tubular filament, c, lodged in the cell which the polyp inhabits, which opens by one extremity into the bottom of the stomach, into which the ova when mature are conveyed, and they are ultimately ejected through the mouth, a, as represented in the figure. (38.) Few objects exhibit to the naturalist a more beautiful spec- tacle than the compound animals of which we are speaking. When found upon the shore contracted and deformed, it would be diffi- cult to imagine that they were really organized beings, much less possessed of the elaborate conformation we have described ; yet, on placing one of them in a tumbler of sea-water, and watching it attentively with a magnifying glass, its true nature is gradually re- vealed : the central mass expands in all directions, exhibiting the cells upon its surface, from which in time the countless flower-like polyps are protruded, and, stretching out their arms in all directions, wait for the approach of prey. A scene like this naturally leads us to make a few observations upon some points of physiology con- nected with their economy : several questions obtrude themselves upon us, which, although applicable to the whole group of com- pound polyps, may be well discussed in this place. (39.) That there is a community of nutrition, or, in other words, that food taken and digested by the individual polyps is appropriated to the support of the general body, appears to be indisputable, and is generally admitted ; but is there a community of sensation so as to render the entire mass one animal, capable of consentaneous move- ments, or is each polyp independent of the rest in its sensations and actions ? Upon this there are different opinions : some regard- * Spix (Jean), Memoire pour servir a I'histoire de 1'Alcyonium exos. ON POLYPS. ing the whole as a single animal, each part being in communication with the rest, and thus participating in the feelings and movements of the others ; whilst some consider each polyp as a distinct crea- ture, independent of the rest. The solution of this problem is a matter of some difficulty ; but there are several facts recorded by observers, which may in some measure enlighten us upon the sub- ject. From the absolute want of nervous filaments, which might bring into communication distant points of the body, we might theoretically deny the possibility of any combina- tion of actions ; and experiment teaches us that the assumption is correct. If when one of these animals is fully expanded, transparent and soft, any point of its surface be rudely touched, the whole body does not immediately shrink, but the point only where the irrita- tion was applied appears to feel the impression ; this part shortly becomes more dense, opaque, and a depression is seen gradually to appear. If the shock be severe, and extensively diffused over the body, the contraction slowly extends to the whole mass ; the most violent local injury, indeed, seems to be totally unperceived at re- mote parts of the body : whilst a general shock, such as striking the vessel which contains the expanded polyp, produces a simultaneous contraction of the whole.* The polyps, however, exhibit much greater irritability, and their movements, from their rapidity, form a striking contrast to the languid contractions of the connecting central mass ; but that they have a community of life appears im- probable : they seem to act quite independently of each other ; when one is touched and suddenly retracts itself within its cell, it is true that those in the neighbourhood will likewise not unfre- quently retire, but this circumstance may be accounted for by the sudden movement of their neighbour ; for, as the polyps often touch each other with their tentacles, there is no cause for urging a com- munity of substance to explain it."f" (40.) Madreporidce. Were we to imagine one of the alcyonidse capable of secreting not merely the calcareous spicula which are mixed up with the softer portions of its body, but abundant quan- tities of carbonate of lime, which, being stored up in the centre of its substance, should form a dense calcareous axis encrusted with the uncalcified part of the living animal, and perforated at its sur- * Professor Grant, Lectures on Comparative Anatomy, Lancet for 1833-4, vol. ii. p. 261. t Quoy et Gaimard, Zoologie du Voyage de I'Uranie. Paris, 1834. 30 ON POLYPS. face so as to form innumerable cells or lodges containing the polyps which provide nourishment for the general mass, we should have a good general idea of the structure of the tribe of polyps which now comes beneath our notice. The shallower parts of the tropical seas contain countless forms of madrepores, known to us, unfortunately but too often, only by the earthy skeletons which the beauty of their appearance induces the mariner to bring to our shores. These calcareous masses assume more or less an arborescent appearance, spreading to a considerable extent, so as to cover the bottom of large tracts of the ocean, and not unfrequently they play an important part in pro- ducing geological changes which are continually witnessed in the regions where they are abundant. (41.) The extent of our knowledge of the animals themselves is, unfortunately, but very limited. That the entire skeleton, whatever its form, is encrusted with living substance ; that the cells contain polyps, resembling more or less those of the aleyonidse, and which provide for the nutrition of the whole, is pretty much the extent of our information concerning them : and should the scientific naturalist ever be placed in circumstances where he can more closely examine them in their living state, there is scarcely a department of science in which his labours could be more beneficially employed than in the investigation of their structure and history. (42.) That the madrepores, from the immense masses of chalky material which they accumulate in the regions inhabited by them, not unfrequently become the cause of excessive danger to the mariner, by raising the bottoms of the shallow seas which they frequent, so as to render regions once covered with deep water no longer navi- gable, or filling up by their accumulation the bays and harbours of the South Seas, is undeniable; and a knowledge of this fact justly makes the navigator cautious in passing through the localities where they most abound. Yet the imagination of authors has not seldom far exceeded the truth in detailing the circumstances connected with them. That the harbour of Tinian, so extolled in the Voyages of Lord Anson and others, is now choked up with the skeletons of madreporegynous polyps, is readily credited ; that islands are gra- dually formed, where none existed, by the agency of these creatures, is equally authenticated ; and that madrepores are found in strata much elevated above the level of the seas in the neighbourhood, is a fact attested by many voyagers. Yet when we are told of coral reefs, some hundred miles in length, entirely formed by the agency ON POLYPS, 31 of these apparently insignificant creatures, of perpendicular cliffs rising from immense depths, which are altogether the produce of their secretions, we have only to turn to the details in our posses- sion, concerning their habits and mode of increase, to assure us of the inaccuracy of such statements.* In the hot climates in which the saxigenous corals abound, they are found to frequent shallow bays and sheltered spots, where they can enjoy the full influences of light and air, un exposed to the agitation of the ocean, which, were it to beat continually upon them, would infallibly destroy their delicate substance : in such situations, the sub-marine rocks become gradually encrusted with the calcareous skeletons which they produce ; and if undisturbed, in the lapse of years, successive generations will of course deposit such large quantities of calcareous matter as to form beds of considerable thickness. That there are at the bottom of the ocean bold and precipitous cliffs, rising from a depth of 1000 or 1200 feet, their broad tops approximating the surface of the ocean, every one will admit, without having recourse to the labours of madrepores to account for their formation, although the sheltered portions of the summits of such mountain ridges afford an eligible position for their increase. In such situations, therefore, they accumulate, and slowly deposit continually increas- ing masses of earth upon the brow of these sub-marine mountains, until at last the pile approaches the surface of the sea, and even at low water remains uncovered by the waves. The further elevation of the rock, as far as the polyps are concerned in its construction, here ceases ; but a variety of causes tends gradually to heap materials upon the newly appearing island : storms, which tear up the bottom of the sea, perpetually throw to the surface sand and mud ; which becoming entangled among the madrepore, and matted together with sea-weed, forms a solid bed over which the waves have no longer any power. The circumference of the islet is perpetually augmented by the same agency: sea-weeds and vegetable sub- stances cast upon it, by their decay cover its top with vegetable mould ; and if its proximity to other land permit the united action of winds and currents to bring the germs of vegetation from neigh- bouring coasts, they take root in the fresh soil, and soon clothe with verdure a domain thus rescued from the ocean. (43.) The coasts described by Cook and Bougainville, whereon strata of coral are found much elevated above the level of the sea, are undoubtedly of volcanic origin. The bottom of the ocean, * Quoy etGaimard, Op. cit. ON POLYPS. crusted over by thick masses of madrepore, has been suddenly heaved up by one of those stupendous convulsions of nature, pro- bably produced by the sea finding its way into some sub-marine volcano ; and rocks and corals, raised from their beds by the tre- mendous explosion so produced, give birth to islands and elevated tracts of country, such as are met with in the South Seas. CORALLID.E. The Corallidse are compound polyps of appa- rently more perfect organization than those forming the last family. The polypary or central axis, which supports the external or living crust, is solid, without cells, and variously branched ; the larger species resembling shrubs of great beauty, frequently coloured with lovely hues, and studded over their whole surface with living flowers, for such the polyps which nourish them were long consi- dered even by scientific observers. The central stem of these zoophytes differs much in its composition in different families ; sometimes being of stony hardness, in other cases it is soft and flexible, resembling horn ; and not unfrequently it is formed of both kinds of material : it is however always produced by the living cortex, which secretes it in concentric layers, the external being the last deposited. The example which we shall select for special description is the Coral of commerce, Corallium rubrum, (Jig. 6.) from which we derive the material so much prized in Fi S- 6 - the manufacture of ornaments. (44.) The red coral is principally obtained in the Mediterranean. When growing at the bottom of the sea, it consists of small branched stems, en- crusted with a soft living investment, by which the central axis is secreted, and studded at intervals with polyps possessing eight fringed arms, and capable of being contracted into cells contained in the fleshy covering, but not penetrating the stem itself. The skeleton or polypary of the coral is of extreme hardness, and susceptible of a high polish ; a circumstance to which the estima- tion in which it is held is principally owing. But in other genera of this family, the central axis, instead of being con- structed of calcareous matter, is formed of concrete albumen, and resembles horn both in appearance and flexibility ; such are the ON POLYPS. Gorgonise of the Indian Ocean. In the Isis Hippuris (jig. 7, B) the central axis is alternately composed of both these substances, exhibiting calcareous masses united at intervals by a flexible mate- rial, allowing the stem to bend freely in every direction. The object of such diversity in the texture of the polypary of the Coral- hdce will be at once apparent when we consider the habits of the different species : the short and stunted trunks of Corallium, composed of hard and brittle Fig. 7. substance, are strong enough to resist injuries to which they are exposed ; but in the tall and slender stems of Gor- gonia and Isis, such brittle- ness would render them quite inadequate to occupy the si- tuations in which they are found, and the weight of the waves falling upon their branches would continually break in pieces and destroy them ; this simple modifica- tion, therefore, of the nature of the secretions with which they build up the skeleton which supports them allows, them to bend under the passing waves, and secures them from otherwise inevitable destruction. (45.) Upon making a transverse section of one of these poly- paries, (Jig. 7, A,) the solid axis is distinctly seen to be made up of layers arranged in a somewhat undulating manner around the centre, and successively deposited by the living cortex : the growth of the stem, in the harder species at least, is very slow, and several years are necessary to its maturity ; a circumstance whicFlias ren- dered it needful to impose strict laws, forbidding the Mediterranean coral-fishers to disturb too frequently the same localities, which are only visited at stated periods. (46.) The deposition of solid matter in the soft bodies of these polyps is not confined to the production of the central stem, but in many even of the Keratophyta * cretaceous particles are extensively * An old name for polyps with a horny axis, x'^xf, horn; Qvrov, a stem ; as distin- guishing them from the stony polyps, Lithophyta, *.i0 a{ , a stone ; <f>vrav. D 34 ON POLYPS. diffused through the cortex, which not unfrequently is likewise gorgeously coloured by secretions of different hues. In the Gor- goniae, a section of one of which (Gorgonia verrucosa) is repre- sented in Jig. 7, A, the earthy matter in the crust is so abundant, that, even when dried, it will retain in some measure its natural form, and exhibit the tints peculiar to the species. The structure of the individual polyps of the Corallidse, as far as we are acquainted with their history, resembles that of one of the polyps of the Alcyonidse already described ( 36) ; and the prey obtained by each, goes to the support of the general mass. Their reproduction is undoubtedly from germs developed in in- ternal filamentary ovaria, which escape either through the mouth, as in Alcyonium, or else, as Cavolini* supposed, through apertures placed between the origins of the tentacles. (47.) Pennatulidce. This family belongs likewise to the divi- sion of cortical polyps, and agrees with the two last in most points, the principal distinction consisting in the character of the internal axis which supports the body. In some species this part is reduced in fact to a ligamentous mass, interspersed with calcareous granules ; but, in the most typical forms, the skeleton consists of several pieces, capable of moving upon each other. The whole animal, in such cases, resembles a feather, the stem supporting lateral branches, upon which the polyps are arranged. From the circum- stance of these compound animals being unattached to any foreign support, they have been supposed to be capable of swimming at large in the sea, by the voluntary movements of their articulated branches, a fact strongly contested by many modern zoologists ; but, as we can say nothing from our own observation upon this subject, we must leave the question open to future investigation. Many species are eminently phosphoric. Tubiporidcc. We now have to speak of a class of polyps very different in their construction from those which have been described. Instead of encrusting an internal solid skeleton, the Tubiporidse are enclosed in a calcareous or coriaceous sheath or tube, from the ori- fice of which the polyp is protruded, when in search of prey : these are named by authors Vaginated Polyps. (48.) The Tubipora musica (jig. 8, a) is the species which has been most carefully studied, and the details connected with its or- ganization will be found of the highest importance, as affording a * Cavolini (Philippe), Memorie per servire alia storia diPolipi marini. 4to. Naples, 1785. ON POLYPS. clue to the investigation of other forms, to be mentioned hereafter.* The Tubiporse live in society, but do not appear to be organically united as the compound polyps; a group of these animals presents Fig. 9. several stages of tubes, placed one above another ; the tubes are ge- nerally straight, and nearly parallel to each other, but appear slightly to diverge, as ra- diating from a common centre ; they are separated by considerable intervals, and reciprocally support each other by horizontal laminae of the same substance as the tubes themselves, which unite them. From each tube issues a little membranous animal of a brilliant grass- green colour, the mouth being surrounded by eight tentacles, which are furnished along their edges with two or three rows of minute fleshy papillae. Within the mouth of the specimen examined by M. Lamouroux, was found an * Anatomic de Tubipore Musical, par M. Lamouroux, in the Zoology of Quoy et Gaimard, Voyage de 1'Uranie. 36 ON POLYPS. oval membranous sac, but not in sufficient preservation to be properly described. This was most probably the stomach. (49.) Around this sac, alternating with the tentacles, are eight triangular filaments, (Jig. 9 ; 1 e,) which are at first free and floating, but they soon become attached to a membrane which lines the calcareous tube ; and, gradually diminishing in size, they extend through its whole length. These filaments are analogous to the ovaries of the Corallidse and Pennatulidse ; their inner sur- face, in mature individuals, is studded with ova of different sizes attached to them by short pedicles (Jig. 9 ; 8). (50.) At the point where the ovigerous filaments reach the ten- tacles, a membrane is observable which assumes the shape of a funnel when the animal retires into its shell, and at the open end of the funnel the membrane is seen to fold outwards, and become continuous with the calcareous tube; (Jig. 9 ; 1, &;) its inner sur- face indeed is prolonged under the form of a thin pellicle over all that part of the interior of the tube which is inhabited by the polyp, terminating at a kind of diaphragm composed of the same hard substance as the tube itself. The remains of these diaphragms are found in the interior of old tubes at various distances from each other. The funnel-shaped membrane does not terminate suddenly at its point of junction with the calcareous tube ; the latter, indeed, is a continuation and product of the first, the calcareous substance being evidently deposited in this gelatinous membrane, in the same man- ner as phosphate of lime is deposited in the bones of very young subjects, changing its soft texture into hard, solid substance. The manner, therefore, in which this tube is formed, cannot be compared to the mode of formation of the shells of Serpulce or the shells of mollusca; in the latter case it is a secretion from the skin, almost an epidermic product, but in these polyparies there is a real change of soft into solid substance, which is effected gradually, but not deposited in layers. (51.) When the tube has acquired a certain height, the animal forms the calcareous horizontal plate which unites it to those around ; the still membranous upper part of the tube extends itself horizontally outwards around the aperture, (Jig. 9 ; 2, 6,) doubling itself so as to form a circular fold ; this part of the membrane is no longer irritable ; its internal surfaces unite so as not to interrupt the continuity of the tube ; carbonate of lime is gradually deposited within it, and soon a prominent partition, com- posed of two lamellae, soldered together through almost their entire ON POLYPS. 37 extent, surrounds the tubular cell. Generally many polyps of the same polypary form these partitions at the same time and upon the same plane. In this case the gelatinous margins of the folded mem- brane unite, no space is left ; and they ultimately become most inti- mately soldered together, and the solid plane or stage (fig. 8) is formed. If the animal constructs its partition against a tube already perfect and solidified, it fixes its collar to its sides, so that the point of junction is imperceptible ; but when it is quite insulated, as at &, Jig. 8, the horizontal collar is still formed, and it then assumes somewhat of an octagonal shape. The tube-forming membrane exhibits no appearance of vessels or other traces of organization. When the polyp is withdrawn within its cell, its tentacles form a cylindrical fasciculus {Jig- 9, c) ; the papillae which partially cover them being laid upon each other like the leaflets of some mimosa when asleep. The protrusion of the creature from its tube is accomplished by the contraction of the membrane, 6, inserted into its neck. (52.) How the eggs formed upon the oviferous filaments issue from the polyp, has not been ascertained : it is most probable, from their size, that they are not expelled during the life of the parent ; but that, when it dies, the eggs all come out of the tube, except one, which developes itself in the old cell ; the rest fixing themselves upon the neighbouring stage, there to form a new story of tubes. The germs, during the first period of their developement, have no organs distinguishable, not even the rudiment of a tube ; each ap- pears to consist of a simple gelatinous membrane folded upon itself, (Jig. 9 ; 4, c,) and forming upon the stage upon which it is fixed a little tubercle resembling a small Zoanthus or other naked zoophyte. This tubercle gradually elongates, and assumes the form of a polyp, provided with all its organs ; but the sac which encloses it is still gelatinous at its upper part, and membranous near the base, (Jig. 9 ; 4, 6,) where it gradually diminishes in thickness, and, becoming calcareous, gives to the animal the general appearance of its original. (53.) In Tubularia indivisa the structure of the tentacula around the mouth is different from what has been described in Tubipora mu- sica, although in the principal points of its structure the resemblance between the two is very great ; when the Tubularia is expanded, its protruded portion is seen to be furnished with two circles of arms, one placed around the opening of the mouth, the other at a consi- derable distance beneath it, (Jig. 10 ; 1,) and nearly on a level with the inferior circle a second aperture (Jig> 10 ; 1, a) is observable, ON POLYPS. Fig. 10. communicating with that portion of the body which is lodged within the tube, and resembling a second mouth. A remarkable action has been observed to take place in these parts of the polyp, producing a continual variation in their form ;* a fluid appears at intervals to be forced from the lower compartment into the space intervening be- tween the two rows of tentacula, which becomes gradually dilated into a globular form (Jig. 10 ; 2 and 3.) This distension continues for about a minute, when the upper part, contracting in turn, squeezes back the fluid which fills it into the lower compartment through the opening a, which then closes preparatory to a repeti- tion of the operation. The intervals between these actions were, in the specimen observed by Mr. Lister, very evenly eighty seconds. In Tubularia indivisa the sheath or cell, &, which en- closes the polyp, is perfectly diaphanous, allowing its contents to be readily investigated under the microscope. When thus examined, a continual circu- lation of particles was visi- ble, moving in even, steady currents in the direction of the arrows (fig. 10; 1) along slightly spiral lines represent- ed in the drawing. The par- ticles are of various sizes, some very minute, others apparently aggregations of smaller ones ; some were globular, but they had gene- rally no regular form. In fig. 3, d, a series of longitu- dinal lines are perceptible, which most probably are ovi- gerous filaments, resembling those of Tubipora musica. Actiniadce. The next family of polyps, from the fibrous character which the substance of their bodies assumes, have been named by zoologists " Fleshy Polyps.' 1 '' They differ indeed re- markably from the soft gelatiniform structures which have hitherto come under our notice, exhibiting traces of muscular fibre which are not to be mistaken. * Lister, on the structure and functions of Tubular and Cellular Polypi. Philoso- phical Transactions, 1834. ON POLYPS. Fig. 11. Fig. 12. Although the genera composing this division are exceedingly numerous, and vary much in their external characters, they will be found more or less to conform in the essential points of their organization with the subject which we have chosen as the type of this extensive tribe, and of which, being common upon our own coasts, the reader will have little difficulty in procuring specimens for examination. (54.) The body of an Actinia when moderately expanded, (fig. 11?) is a fleshy cylinder, attached by one extremity to a rock, or some other sub- marine support ; whilst the opposite end is sur- mounted by numerous tentacula, arranged in se- veral rows around the oral aperture (fig. 1) . When these tentacula are expand- ed, they give the animal the appearance of a flower, a resemblance which is rendered more striking by the beautiful colours which they not unfrequently as- sume ; and hence in all countries they have been looked upon by the vulgar as sea-flowers, and distin- guished by names indica- tive of the fancied resem- blance. Their animal na- ture is however soon rendered evident by a little attention to their habits ; when expanded at the bottom of the shallow pools of salt-water left by the retreating tide, they are seen to manifest a degree of sensibility, and power of spontaneous movement, which we should little an- ticipate from their general aspect. A cloud veiling the sun will cause their tentacles to fold, as though apprehensive of danger from the passing shadows : contact, however slight, will make 40 ON POLYPS. them shrink from the touch ; and if rudely assailed, they com- pletely contract their bodies so as to take the appearance of a hard coriaceous mass, scarcely distinguishable from the substance to which they are attached. (55.) It is in seizing and devouring their prey however that the habits of the Actiniae are best exemplified ; they will remain for hours with their arms fully expanded and motionless, waiting for some passing animal which chance may place at their disposal, and when the opportunity arrives, are little inferior to the Hydrse in their voracity or powers of destroying their victims. Their food generally consists of crabs or shell-fish, animals apparently far superior to themselves in strength and activity, but even these are easily over- powered by the sluggish yet persevering grasp of their assailant. No sooner are the tentacles touched by a passing animal than it is seized, and held with unfailing pertinacity ; the arms gradually close around it ; the mouth, placed in the centre of the disc, ex- pands to an extraordinary size ; and the creature is soon engulph- ed in the digestive bag of the Actinia, where the solution of all its soft parts is rapidly effected, and the hard undigestible remnants speedily cast out at the same orifice. The Actiniae, although exceedingly voracious, will bear long fasting :* they may be preserved alive for a whole year, or per- haps longer, in a vessel of sea- water, without any visible food ; but when food is offered, one of them will devour a crab as large as a hen's egg, or two muscles in their shells : in a day or two the shells are voided through the mouth, perfectly cleared of the soft parts which they contained. (56.) The Actiniae, like the Hydras, possess the power of chang- ing their position : they often elongate their bodies, and, remaining fixed by the base, stretch from side to side as if seeking food at a distance ; they can even change their place by gliding upon the disc which supports them, or detaching themselves entirely, and swelling themselves with water, they become nearly of the same specific gravity as the element which they inhabit, and the least agitation is sufficient to drive them elsewhere ; Reaumur even asserts that they can turn themselves so as to use their tentacles as feet, crawling upon the bottom of the sea ; but this mode of pro- gression has not been observed by subsequent naturalists : when they wish to fix themselves, they expel the water from their dis- * Encyclopaedia Londinensis, art. Actinia. ON POLYPS. tended body, and sinking to the bottom attach themselves again by the disc at their base, which forms a powerful sucker. (57.) From this sketch of the outward form and general habits of these polyps, the reader will be prepared to examine their internal economy, and the more minute details of their structure. On ex- amining attentively the external surface of the body, it is seen to be covered with a thick mucous layer resembling a soft epidermis, which extending over the tentacula, and the fold around the aper- ture of the mouth, is found to coat the surface of the stomach itself; this epidermic secretion forms in fact a deciduous tunic which the creature can throw off at intervals. On removing this, the walls of the body are seen to be made up of fasciculi of mus- cular fibres, some running perpendicularly upwards towards the tentacula ; and others, which cross the former at right angles, pass- ing transversely round the body ; the meshes formed by this in- terlacement are occupied by a multitude of granules apparently of a glandular nature, which give the integument a tuberculated aspect : these granules are not seen upon the sucking disc at the base. The tentacula are hollow tubes, composed of fibres of the same description. The stomach is a delicate folded membrane, forming a simple bag within the body ; it seems to be merely an extension of the ex- ternal tegument, some- what modified in tex- ture ; it is closed infe- riorly, the same orifice serving both for the in- troduction of food, and the expulsion of effete or indigestible matter. (58.) On making a section of the animal, as represented in jig. 13, the arrangement of these parts is distinctly seen : a being the muscular integument ; b the ten- tacula formed by the same fibrous membrane ; and c the stomach, which is apparently de- ON POLYPS. rived from it. Between the digestive sac c, and the fibrous ex- terior of the body a, is a considerable space d, divided by a great number of perpendicular fibrous partitions, /, into numerous com- partments, which however communicate freely with each other, and likewise with the interior of the tentacula, as seen at e. Every tentacle is perforated at its extremity by a minute aperture , through which the sea-water is freely admitted into these compart- ments, so as to bathe the interior of the body ; and when from alarm the animal contracts itself, the water so admitted is forcibly expelled in fine jets through the holes by which it entered. There can be no doubt that the surrounding fluid, thus copiously taken into the body, is the medium by which respiration is effected ; and every one who has been in the habit of keeping Actiniae in glass vessels for the purpose of watching their proceedings, must have noticed that as the fluid in which they are confined becomes less respirable, from the deficiency of air, the quantity taken into the body is enormous, stretching the animal until it rather resembles an inflated bladder than its original shape. (59.) It is in the compartments which are thus at the will of the creature distended with water, that we find the organs of reproduc- tion, which here assume a developement far exceeding what we have noticed in other zoophytes. On raising a portion of the mem- brane which forms the stomach, as aty, we see lodged in each par- tition an immense number of ova attached to a delicate transparent membrane, and arranged in large clusters, g. The ovigerous mem- brane which secretes these eggs is represented unravelled at h ; it is through its whole extent bathed with water admitted into the compart- ment in which it is lodged, a circumstance which provides for the re- spiration of the ova during their developement. The convoluted ovary is seen to terminate by a minute aperture near the bottom of the sto- mach k, into which when mature the young escape. The eggs found in the ovaria are round and of a yellow colour, resembling minute grains of sand : it is probable that sometimes they are hatched after their ex- pulsion, but it is likewise asserted by numerous authorities that the young are not unfrequently born alive. The manner in which the ova are extruded has been long a matter of controversy, and perhaps cannot yet be regarded as definitively ascertained. Our own dissec- tions would lead us to concur with those anatomists who describe them as escaping from the ovaria into the bottom of the stomach, whence they have been seen to escape by the mouth fully formed : it is possible, however, that they may likewise be expelled with the ON POLYPS. 43 streams of water forced by the contractions of the animal through the orifices at the extremities of the tentacula. The Abbe Dicquemare* relates several curious experiments on the multiplication of these animals by mechanical division. When transversely divided, the upper portion still stretched out its ten- tacles in search of food, which, when seized, sometimes passed through its mutilated body, but was occasionally retained and di- gested. In about two months tentacles grew from the cut ex- tremity of the other portion, which soon afterwards began to seize prey. By similar sections he even succeeded in making an animal with a mouth at each end. (60.) The entire organization of the Actinia is evidently very supe- rior to that of any animals which have been described in the preceding pages ; the muscular fasciculi, now for the first time distinctly recog- nisable, give an energy to their contractions very different from the languid movements of the gelatinous polyps. The Actinia can in- deed hardly be classed in the acrite division of the animal kingdom ; the developement of muscular fibre which it presents, presupposes the existence of nervous filaments, and we might a priori infer their existence. Spix, many years ago, described a nervous sys- tem, which he believed he had discovered, in the neighbourhood of the base, or sucking disc by which the animal attaches itself to foreign bodies ; in which situation he was led to look for it, by ob- serving that when galvanic shocks were sent through the body, convulsive movements were excited most distinctly in this part, and also from the supposition that the organ of attachment, here placed, must necessarily be the most abundantly endued with sen- sibility, j* Having raised the longitudinal muscles by a slight incision near the middle of the base or disc of attachment, he thought he perceived an interlacement formed by some pairs of nodules, disposed around the centre, which communicated by several cylindrical threads ; from each nodule two filaments ran forwards, one accompanying the lon- gitudinal fleshy fasciculi, the other penetrating to the internal longitudinal septa, which have likewise a muscular character. Suc- ceeding anatomists have, however, totally failed in their endeavours to detect the arrangement here described ; and which indeed, did it exist, would be contrary to every analogy with which we are ac- quainted. It is more probable that the nervous system consists in * Philosophical Transactions, 1773. t Spix (Jean) Annales du Museum, tome 13. 44 ON POLYPS. a delicate thread, which we are pretty well convinced we have de- tected running round the roots of the tentacles, embedded in a strong circular band of muscle which surrounds the orifice of the stomach, and acts the part of a powerful sphincter in closing the aperture. (61.) After the account which has been given of the general structure of the Actinia, the mechanism by which the tentacula are expanded and withdrawn will be easily understood : these do not, like the horns of a snail, become inverted and rolled up within the body, but owe their different states of extension entirely to the forcible injection of water into the cavities which they contain. We have seen already that the interior of each tubular arm communi- cates freely with the space which intervenes between the stomach and the external integument, a space which, at the will of the animal, is filled with sea-water drawn through the orifices seen at the extremity of each arm : when these minute orifices are closed, and the body of the creature contracted, the water, being violently forced into the tentacula, distends and erects them, as when watching for prey ; and, on the other hand, when emptied of the fluid thus injected, they shrink and collapse. This circumstance, so easily seen in the Actiniae, will probably enable us to account for similar phenomena observable in other polyps, the internal economy of which is by no means so conspicuous. (62.) The next tribe of polyps which presents itself to our notice, differs widely from the preceding families in outward form, as well as in many important features of internal structure. It would seem, indeed, to comprise animals distinguished from each other by so many important circumstances, and yet so intimately related by ex- ternal configuration, that it is difficult to separate them, or to leave them in the same group. It was imagined a few years ago, before accurate researches had been made concerning the internal structure of these zoophytes, that in all the compound species the polyps or mouths of the general mass were in their essential structure analogous to the Hydra, being simple digestive sacs, without more complication of structure than we have found those of the cortical polyps to possess. Recent investigations, however, have shown that amongst the species ranged by Cuvier under the head of Tubular Polyps, " Polypes a Tuyaux" many are exceedingly complex in their organization, possessing the outward form of the simpler kinds, but ON FOLYPS. 45 furnished with a complete digestive canal, and approximating in their general economy very superior orders of animals. These latter would appear to be distinguishable by the nature of the tenta- cles around the mouth, which, in all the families as yet examined, we have found to be smooth or merely fringed, as they are indeed in some of the tubular polyps hereafter to be noticed ; but, in the more perfect species, the arms are covered with vibratile hairs or cilia, forming important agents in securing prey : such have been separated by Ehrenberg into a distinct class, under the title of BRYOZOA, and have been recently designated by Dr. Arthur Farre, ClLIOBRACHIATE POLYPS. Further observation is necessary before the boundaries of these important divisions can be accurately laid down ; we shall neverthe- less, without entering upon a question foreign to our present sub- ject, arrange them in conformity with the analogies of their internal structure, rather than of their outward general form, and defer the consideration of the ciliobrachiate division to another place. (63.) In the unciliated tubular polyps, the common body of the animal, instead of encrusting a solid skeleton, is enclosed in a horny sheath, which it traverses like the pith of a tree, follow- ing all the ramifications of the branched stem of the polypary : to the central part are attached, at intervals, cells opening exter- nally, in which the polyps which provide nourishment for the whole are lodged. Zoophytes of this description are readily found on our own coasts, and the microscopic observer can scarcely enjoy a richer treat than the examination of them affords. In order to study them satisfactorily, it is necessary to be provided with several glass troughs, of different depths, in which the living animals immersed in their native element may be placed : in this situa- tion, if the water be carefully renewed at short intervals, they will live for some time. (64.) On examining a piece of one of these polyparies with a good glass, the tubular horny envelope is seen to be filled with granular matter ; and, on attentively watching it, globules will be seen moving in different directions, producing a sort of circulation or cyclosis very much resembling what is observable in some plants. The glo- bules thus moving do not appear to be contained in vessels, but steal in slow currents, ascending along the sides, and returning down the middle in an opposite direction, as represented by the arrows in fig- 14. 46 ON POLYPS. (65.) It has been generally stated that the living pith exuded from its surface the horny matter which, by its concretion, forms the tube or external skeleton investing the whole ; the accuracy of such a supposition, however, may well be questioned. We have already seen, in the Tubipora musica, that the calcareous tube investing that polyp was produced by the interstitial deposit of earthy matter in the membrane which formed originally its outer case. In the tribe of zoophytes which we are now speaking of, we shall find the exterior tube to be formed in a way precisely similar. On referring to the diagram, (fig. 14,) the mode of its growth will be rendered in- telligible: the soft part or living axis of the polypary is seen to be contained in two distinct layers ; the inner one composing the digestive sac of the polyp, and embracing the granular matter, which seems to be the special seat of the nutritive process ; the outer or tegumentary layer, i, after leaving the tentacula, may be traced down the sides of each polyp to the bottom of the cell, where its course is arrested by a slight partition, at which point it turns outwards, lining the interior of the cell as far as its margin, where, as in the Tubipora, it is seen to be continuous with the horny matter itself. It is this tegumentary membrane, then, which forms by its develope- ji. 14. ment the entire skele- ton : as it expands, it gives origin to the cells and branches character- istic of the species; and, from being at first quite soft and flexible, it gra- dually acquires hardness and solidity by the de- position of corneous matter in its sub- stance. The cells thus formed are inhabited by polyps analogous to those which provide nourishment for the cortical families ; but differing in the number and appearance of the tentacula, which are ON POLYPS. 47 here studded with minute tubercles, but never provided with cilia. Few objects are more admirable than these polyps, when watched with a good microscope : protruding themselves beyond the mouths of their cells, they inflect their bodies in all directions in quest of prey, waiting till some passing object impinges upon their tenta- cula, which is at once seized and conveyed into the stomach with a rapidity and dexterity almost beyond belief. The multiplication of these singular animals appears to take place in three different modes : 1st, by cuttings, as in plants ; Sndly, by off-shoots, or the formation of new branches bearing polyps ; 3dly, by gemmules capable of locomotion. (66.) The first mode strikingly resembles what is observed in the vegetable kingdom ; for as every branch of the plant-like body contains all the parts necessary to independent existence, it can hardly be a matter of surprise that any portion, separated from the rest, will continue to grow and perform the functions of the entire animal. (67.) The second mode of increase, namely, by the formation of new branches and polyps, seems more like the growth of a plant than the developement of an animal. We will consider it under two points of view : first, as regards the elongation of the stem ; secondly, as relates to the formation of fresh cells containing the nutritive polyps. On examining any growing branch, it will be found to be soft and open at the extremity, and through the terminal orifice, the soft tegumentary membrane above described as forming the tube by its conversion into hard substance is seen to protrude ; the skeleton is not therefore merely secreted by the enclosed living granular matter, but it is the investing membrane, which continually shoots upwards, and deposits hard material in its substance, as it assumes the form and spreads into the ramifications peculiar to its species, (68.) Having thus lengthened the stem to a certain distance, the next step is the formation of a cell and a new polyp, which is accom- plished in the following manner :* the newly formed branch has at first precisely the appearance and structure of the rest of the stalk of the zoophyte, (Jig. 15, 1,) being filled with granular matter, and exhibiting in its interior the circulation of globules already described, moving towards the extremity along the sides of the tube, and in an opposite course in the middle ; the end of the branch, however, before soft and rounded, soon becomes perceptibly * Lister, Philosophical Transactions, Loc. cit. ON POLYPS. dilated. After a few hours the branch is visibly longer, its extremity more swollen, and the living pith is seen partially to Fig. 15. f have separated itself from the sides of the tube, the boundaries of which become more defined and undulating (2). The growth still proceeding, the extremity is distinctly dilated into a cell, in which the soft substance seems to be swollen out, so as to give a rude outline of the bell-shaped polyp (3), but no tentacula are yet distinguishable ; a rudimentary septum is now visible stretching across the bottom of the cell, through the centre of which the granular matter, now collected into a mass occupying but a portion of the stem, is seen to pass. The polyp and cell gradually grow more defined, (4, 5, 6,) and the tentacula become distinguish- able ; the cell, moreover, is seen to be continued inwards by a mem- branous, infundibular prolongation of its margin (7), which imme- diately reminds us of the funnel-shaped membrane of Tubipora ( 50), and its office is no doubt similar. As the developement proceeds, the tentacles become more perfect (8), and the polyp at length rises from its cell to exercise the functions to which it is destined. (69.) The third mode of multiplication, or that by reproductive gemmules, seems to be specially adapted to the diffusion of the ON POLYPS. 49 species ; and as it is peculiar to zoophytes of this description, we shall dwell upon it at some length. At certain periods of the year, besides the ordinary cells which contain nutritive polyps, others are developed from different parts of the stem, which may be called female or fertile polyps, although usually simply termed the vesicles. The cells of this kind are much larger than the nutritive cells, and of very different forms ; they are moreover deciduous, falling off after the fulfilment of the office for which they are provided. They are produced in the same manner as the rest of the stem, by an extension of the tegumentary mem- brane, (fg. 14,6,) which, as it expands into the form of the cell, becomes of a horny texture ; it may be traced, however, over the opening of the cavity, where it sometimes forms a moveable oper- culum. The cell being thus formed from the expansion and subse- quent hardening of the tegumentary membrane, it remains to explain the origin of the reproductive germs which soon become developed in its interior.* These are seen to spring from the inner or nutritive layer of the polyp (a), to which they are attached by pedicles, re- garded by authors as fulfilling the office of umbilical cords during their early growth. As the germs expand, they gradually advance towards the opening of the cell, where, as they are protruded, each becomes covered with a layer derived from the tegumentary membrane ( f) which closed the orifice, and protruding externally, has very much the form and appearance of a young polyp, for which indeed it has often been mistaken. We are assured, how- ever, that this supposition is erroneous, and that the polypiform bodies are only external capsules inclosing the real germs (e), from which young polyps are to be formed.^ On tearing open one of these capsules when the included germs are ripe, the latter are seen to be rounded grains of a gelatinous appearance, covered externally with minute cilia, which, like those of the gemmules of the sponge, enable them to swim about at pleasure in search of a proper locality whereon to fix their permanent habitation. These ciliated gemmules are highly irritable, and frequently contract their bodies into different shapes during their progress through the water ; but at length, when about to fix itself, each gemmule becomes flat and circular, and assumes a radiated appearance, resembling a minute grey star, having the interstices between the rays filled with * Lcefling. Miiller's Archives, 1826. Lister, Loc. cit. t Professor Grant, Edinb. New Philosoph. Journal, 1827. Observations on the spontaneous motions of the Campanularia Dichotoma, &c. 50 POLYGASTKICA. a colourless transparent matter, which seems to harden into horn. The grey matter swells in the centre, where the rays meet, and rises perpendicularly upwards, surrounded by the transparent horny substance, so as to form the trunk of the new zoophyte. The rays first formed are obviously the fleshy central substance of the roots ; and the portion of that substance which grows perpendi- cularly upwards forms the fleshy part of the stem, from which in due time polyps become developed. CHAPTER IV. POLYGASTRICA. ANIMALCULA INFUSORIA. Auct. (70.) Previous to the discovery of the microscope, it was little sus- pected that animals existed of such minute size as totally to elude the search of unassisted vision ; much less that every drop of water in which animal or vegetable substances have been allowed to decay, swarms with numberless forms of living beings ; that countless millions inhabit every stagnant pool or running stream ; nay, that every drop of the surface of the ocean is in itself a little world, peopled by innumerable active creatures, as various in their out- ward forms as they are elaborately adapted by their internal organi- zation to the circumstances in which they live. The terms Infusoria and Animal cula, as first used by the earliest discoverers of these beings, were applied to an immense number of creatures widely differing from each other in every particular except in the minuteness of their size, which had previously concealed them from observation. The germs of embryo polyps, the larvae of insects, and all microscopic forms of being, including the won- derful tribes of living atoms which inhabit various secretions in the interior of other animals, were thus thrown together in one heteroge- neous and chaotic group, without reference to the structure, rela- tions or habits of the creatures so denominated. This motley assemblage has, however, by subsequent laborious investigations, been separated and arranged so as in some measure to enable us to acquire accurate notions concerning the animals formerly confounded under one common designation. POLYGASTRICA. 51 (71.) The character which distinguishes the class of microscopic creatures which first offers itself for consideration, is derived from the nature of the digestive apparatus with which the creatures com- posing it are provided ;* this consists of a number of internal sacs generally regarded as stomachs, which are easily distinguishable with the microscope, and form a feature in their economy so peculiar, that they are from this circumstance alone at once recognised as an exceedingly natural and well-defined group, allied with each other in the general details of their history, and exhibiting most astonishing powers, not met with in other forms of being. In order to investigate the facts which will be hereafter stated, connected with the history of these animals, the young naturalist must be provided with a good microscope, furnished with glasses capable of magnifying objects from 200 to 1000 diameters, the last will be seldom needed ; but a power of one-fourth of an inch focus will be indispensable. As some practice and dexterity is requisite in prosecuting researches of this description, a few hints relative to the best methods of procur- ing and observing animalcules will not be improper in this place. It would be needless to advert to the situations in which they are to be found ; every stream and stagnant pool contains some forms in countless numbers ; but, in order to obtain many uncommon species, a little care is necessary. The lemna or duck-weed should be skimmed from the surface of ponds which are exposed to the rays of the sun, or the green film, which not unfrequently covers stagnant waters ; and from these sources examples of most tribes may readily be collected : or else recourse may be had to infusions of various vegetable substances, of hay, chopped straw, or the leaves of plants, which, if left in open glass vessels, and fully exposed in the open air to the influence of the sun, will in a few days swarm with polygastric animals, sometimes not to be pro- cured by other means. A drop of water derived from any of these sources, if placed upon a thin plate of glass, and covered with a film of talc, will rea- dily enable the observer to examine the beings which inhabit it ; or if it be deemed advisable to insulate the larger species, they may be separated from the rest with a feather, and placed in small tubes or flat troughs in filtered water, and their developement and mode of increase watched from day to day. (72.) We shall now proceed to describe some of the most common forms which the Polygastrica thus procured exhibit. In all water * Ehrenberg. E 2 POLYGASTRICA. containing putrefying vegetable matter, innumerable moving points are visible, scarcely distinguishable except under the highest powers of the microscope, but, when magnified to the utmost, assum- ing the appearance represented at Jig. 16, 1 : these have been termed Monads; and, as they F - lg may well be supposed to be the smallest creatures in ex- istence, have been regarded as the limit of the animal world ; their minuteness, in- deed, is incalculable. Dr. Ehrenberg * has described monads which are not larger than from ToW to T^O o of a line, and which appeared to be separated from each other by intervals not greater than their diameter. Each cubic inch of the water in which they are found must contain, therefore, 800,000 millions of these animalcules, estimating them to occupy but one-fourth of its space. A single drop, brought under the field of the micro- scope, and not exceeding one cubic line in diameter, will there- fore contain 500 millions, equal to the whole number of human beings upon the surface of the globe. Well may the mind, overwhelmed with wonder at such an astounding fact, launch into visionary speculations when contemplating it ; and we are little surprised to see the fertile imagination of Buffon figuring all animal and vegetable bodies as composed of aggregations of these living particles, believing them to be the primitive materials of which organized substances are made up. (73.) The Proteus, (Am<ebaE.)Jig. 16, 2, is not frequently met with, but affords a singular example of an acrite animal. It ap- pears under a good glass to be an atom of transparent jelly, which perpetually changes its form by contractions of different parts of its body ; at one time being a roundish mass, then expanding into a linear * Ehrenberg's valuable researches concerning the Polygastrica are to be found in the Transactions of the Berlin Academy, Abhandlungen der Academic von Berlin. vols.68, 69, and 71. POLYGASTRICA. 53 figure, and again shooting out processes of its substance in various directions, so as to assume all kinds of shapes with the greatest facility. The Flask animalcule, (Enchelis,) fig. 16, 3; the Trichoda sol, fig. 16, 4 ; the Euglena viridis, fig. 16, 5 ; the Gonium pectorale, fig. 16, 6 ; the Trachelius anas, fig. 16, 7 ; the Para- mecium aurelia, fig. 16, 8 ; the Navicula, fig. 16, 9 ; the Vibrio Spirillum, fig. 16, 10 ; and the Vorticella Stentor, fig. 1 6, 11, will give the reader an idea of the most common species of these crea- tures, the structure of which we shall now proceed to investigate. (74.) With regard to their external covering, thePolygastrica may be divided into two parallel groups, in one of which the body is en- tirely soft, whilst in the other the animals are enclosed in a delicate transparent shell : the former are termed nuda, or naked ; the latter loricata, or loricated animalcules. The shells of the lori- cated division vary much in form ; sometimes being mere transparent shields covering the back, as in Euplsea Charon (Jig. 17, 4) ; at others they would seem to be capable of opening, like the bivalve shells of mollusca, as in the minute Naviculse^g. 16, 9. Delicate as these shells are, and requiring the most accurate examination, even with a good microscope, to detect their presence, we shall be surprised to find that they play an important part in nature, mak- ing up by their immense accumulation for their diminutive size. We have before us, while writing this, a specimen of pulverulent matter collected upon the shores of Lake Lettnaggsjon, two miles and a half from Urnea in Sweden, which from its extreme fineness resembles flour : this has long been known by the natives of the region where it is plentiful, under the name of Bergmehl or mountain meal, and is used by them, mixed up with flour, as an article of food ; experience having taught them that it is highly nutritive. On examination with the microscope, the Bergmehl is found to consist entirely of the shells of loricated infusoria, which, having been accumulating from age to age at the bottom of the waters in which the living animals are found, form a stratum of considera- ble thickness. Nor is this all : for, when agglomerated and mixed up with siliceous and calcareous particles, these exuviae become con- solidated by time into masses of flint and marble, in which the shape and characters of the shells are perfectly distinguishable, so that even the species of the animalcules to which they originally belonged is easily made out. (75.) The movements of the polygastrica, when seen under the POLYGASTRICA. Fig. 17. microscope, are exceedingly vivacious ; and although many of them inhabit a space not larger than the point of a needle, they swim about with great activity, avoiding each other as they pass in their rapid dance, and evidently directing their motions with wonderful precision and accuracy. Our next enquiry therefore must be con- cerning the organs of locomotion which they possess. These are of various kinds, and are arranged differently in different species. In the smallest animalcules, monads, &c. no locomotive organs have been satisfactorily detected ; yet even in some of these Mons. F. Dujardin perceived one or more filaments of extreme tenuity attached to their globular bodies, which he regards as instruments for progression. These filaments he describes as not exceeding -30 j 00 of a millimetre in diameter, and consequently requiring the utmost penetration of the microscope for their detec- tion. Tn Amteba diffluens (Jig. 16, #) organs of locomotion are formed at the pleasure of the animal, by shooting out processes from different parts of its semifluid substance, which may be used as fins or legs, as occasion requires. Some are provided with styli^ or articulated, stiff, bristle-like organs, which are moveable, and perform in some measure the office of feet, and with uncini, or little hooks, serving for at- tachment to foreign bodies ; these are seen in Eupltea Charon (Jig. 17, 4). (76.) But the most impor- tant locomotive agents are the cilia,* with which the Poly- gastrica are generally fur- nished (fig. 17; 1, 2, 3). On attentively examining most forms of these creatures, espe- cially those of comparatively large size, the body will be seen in some cases to be en- tirely covered with minute vibrating hairs, or at least furnished with such appen- dages on some part of its surface. The existence of these cilia is readily detected by a practised eye, even when using glasses * Cilium, an eye-lash. POLYGASTRICA. 55 of no very great magnifying power, by the peculiar tremulous movement which they excite in the surrounding fluid, somewhat resembling the oscillations of the atmosphere in the neighbour- hood of a heated surface ; but on applying higher magnifiers, espe- cially if the animalcule is in a languid state, the motion is seen to be produced by the action of the delicate filaments of which we are speaking. It is extremely difficult accurately to define the motion of the individual cilia ; it is most probable that each forms by its rotation a cone, the apex of which will be at the root of the organ this at least is the opinion of the best ob- servers, and the combination of such movements gives rise to cur- rents in the water, serving a variety of purposes in the economy of these minute creatures. The vibrating organs, notwithstanding their indescribable minuteness, vary considerably in size ; and it is more than probable that in those monads, and other species, in which their existence has not been detected, the apparent want of them is owing to the imperfection of our means of investigation. A few years ago, indeed, some species now distinctly proved to be covered with cilia, were looked upon as being absolutely deprived of locomotive apparatus, as the Volvox globator (Jig. 20) ; and few greater proofs can be given of the superiority of the microscopes now at our disposal, than the fact of our being able, not only to detect with facility their existence on the surface of the parent volvox, but even upon the young volvoces before their birth. (77.) The cilia, as has been already observed, are sometimes dispersed over the whole body, either arranged in parallel rows or scattered irregularly ; they are, however, most frequently only met with in the neighbourhood of the mouth, in which position they are always most evident : here they produce, by their vibration, currents in the surrounding fluid which converge to the oral aper- ture, and bring to the mouth smaller animalcules, or particles of vegetable matter, which may be floating in the neighbourhood, and thus ensure by an admirable contrivance an abundant supply of food, which without such assistance it would be almost impossible for these little creatures to obtain. (78.) We may be expected, in this place, to make a few obser- vations concerning the agency by which these numberless and almost invisible organs are made to perform their rapid move- ments. The subject is one of no little difficulty, and in the present state of our knowledge probably inexplicable. Ehrenberg indeed asserts, that round the base of every cilium is an appa- POLYGASTRICA. ratus of radiating muscular fibres, to the successive contractions of which the rotation of the cilium is owing. Such an arrangement is, to say the least, hard to be conceived, for in this case we must attribute to these acrite beings an elaboration of structure of infi- nite complexity ; and in creatures so small, how can the human mind imagine the cilia to be wielded by many millions of distinct and independent muscles, as such a supposition would infer ? Some authors attempt to get rid of the difficulty by ascribing the apparent ciliary movement to the rapid undulations of mem- branous fins ; others altogether deny its existence, asserting that the vibratory appearance is caused by the mingling of some secre- tion which exudes from the surface of the animalcule with the sur- rounding fluid, in the same manner as the union of spirit of wine and water gives rise to an oscillation of particles visible to the naked eye : to these suppositions, however, we barely allude, be- cause we are convinced that any one who with a good microscope and an unbiassed mind investigates the subject, will be con- vinced that the cilia are such as we have described above, however unable he may be to conjecture the cause of their movement. (79.) The mouth of the polygastrica is generally a simple and extremely dilatable orifice, and, with a few rare exceptions, is un- provided with any masticating organs ; yet in Nassula elegans, (Jig- 17, 1,) and a few kindred species, Ehrenberg describes a dental system of a most extraordinary description : this consists of a prominent cylinder (a), of which an enlarged view is given at , composed of numerous long teeth adapted to seize and bruise materials used as food. (80.) The digestive apparatus itself, from the peculiarity of its structure, has given the character usually employed to distinguish the entire class : it is described as consisting essentially of a number of internal sacculi, varying from four to two hundred in number in different species. These sacs are readily distinguishable without any preparation, but are rendered more conspicuous by feeding the animalcules with pure carmine or indigo, the coloured particles of which substances they eagerly swallow. In one large division, called A NEXT ERA, the sacculi or stomachs are said to arise by separate tubular pedicles from the mouth itself (./? 18, 1); whilst in others, ENTERODELA, there is supposed to be a complete intestinal canal, terminated by a mouth and anus, to which the sacculi or stomachs, as they are called, I'OLYGASTHICA. 57 Fig. 18. n are appended : sometimes the mouth and anus are lodged in the same fossa, and the intes- tinal canal forms a circle in the body (ANOPISTHIA, Ehren.), as in the Vorticella (Jig. 18, 2) : or else the mouth and anus are placed at opposite extremities of the body, through which the intestinal tube passes either in a straight course, * or exhibiting several flexuous curves in its passage. (ENANTIOTRETA and ALLOTRETA, Ehren.) (fig- 18, 3 and 4.) When neither the mouth nor anus are terminal, as in Kolpoda, (Jig. 19 ; 7, a, ,) such animals belong to the group denominated KATOTEETA by the same author. (81.) However imposing, from their completeness, the views of Ehrenberg concerning the digestive system of the polygastrica may be, and sanctioned as they are by almost general consent, we can- not pass over a subject of so much importance without expressing ourselves as being far from admitting their accuracy in all respects, and we must say that our own observations upon the structure of the polygastrica have led us to very different conclusions.* The positions of the mouth and anal aperture we are well assured, by frequent examination, to be such as are indicated by the illustrious Professor of Berlin ; but with regard to the tube named by him intestine, and the stomachs appended thereto, our most patient and long-continued efforts have failed to detect the arrangement depicted in his drawings. In the first place, as re- gards the function of the sacculi, which he looks upon as the organs in which digestion is accomplished ; in carnivorous animalcules which devour other species we might expect, were these the stomachs, that the prey would at once be conveyed into one or other of these cavities ; yet, setting aside the difficulty which must manifestly occur in lodging large animalcules in these microscopic * It may be proper to state that the microscope used in these and similar re- searches to which allusion will be made, is a compound achromatic, made by Ross of London ; and the powers employed, of -&, , ^, and $ of an inch focus. 58 POLYGASTRICA. sacs, and having recourse to the result of actual experience, we have never in a single instance seen an animalcule, when swallowed, placed in such a position, but have repeatedly traced the prey into what seemed a cavity excavated in the general parenchyma of the body. In the second place, the sacculi have no appearance of being pedunculated, and consequently in a certain degree fixed in defi- nite positions : during the last two hours we have been carefully examining some beautiful specimens of Paramecium aurelia, (Jig- 18, 4,) an animalcule which, from its size, is peculiarly adapted to the investigation of these vesicles ; and so far from their having any appearance of connection with a central canal, as represented in the figure copied from Ehrenberg, they are in con- tinual circulation, moving slowly upwards along one side of the body, and in the opposite direction down the other, changing moreover their relative positions with each other, and resembling in every respect the coloured granules which have been described ( 31,) as visible in the gelatinous parenchyma of the hydra. With respect to the central canal, (Jig. 18 ; , 3, 4,) we have not in any instance been able to detect it, or even any portion of the tube seen in the figures, much less the branches represented as leading from it to the vesicles or stomachs, as they are called. Even the circumstances attending the prehension of food would lead us to imagine a different structure ; witness for example the changes of form which Enchelis pupa undergoes when taking prey, as shown in fig. 16, 3, where it is represented in the act of devour- ing a large animalcule, almost equal to itself in bulk, and is seen to assume a perfectly different shape as it dilates its mouth to receive the victim, with which its whole body becomes gradually distended. Such a capability of taking in and digesting a prey so dispropor- tionate, would in itself go far to prove that the minute sacculi were not stomachs ; as it evidently cannot be in one of these that digestion is accomplished. (82.) Looking at the above facts as a whole, we cannot mistake the analogy which there is between the organization of the so- named Polygastrica and of the Hydra viridis ; there is the same dilatable body in which the solution of food takes place, and the same granular vesicles by which the nutritious portions are ab- sorbed : that the vesicles become coloured by the coloured food given to the animalcule, cannot be considered as a proof of their being stomachs, as in the experiments of Trembley, above nar- POLYGASTRICA. 59 rated, the granules which circulate in the body of the hydra became dyed with the juices of the animals with which it was fed, precisely in a similar manner. The reproduction of the polygastric animalcules is effected in various ways, and not unfrequently the same individual would appear to propagate in two or three different modes. (83.) The first is by external gemmules or buds, resembling those by which the hydra is multiplied which sprout like minute gelatinous tubercles from the surface of the body, and, gradually attaining the shape of their parent, develope the cilia characteristic of their species, and soon become independent beings, although they do not attain to their full growth until some time after their separation. (84.) A second mode of reproduction is witnessed in the Volvox, and others of similar conformation. In these animalcules (Jig- 19, 1,) the parent is a delicate green transparent globe, which Ffe.19. under a good microscope is seen to be entirely covered with cilia, whose action produces currents in the water, the course of which is represented by the arrows in the figure ; impelled by these cilia, the little globe makes its / j way with a revolving motion through the element which it inhabits. In the interior of the volvox, the observer readily discovers "other smaller globes of a dark green colour, which a little attention will prove to be young vol- voces, exactly resembling the larger one which contains them, and covered in like manner with vibratile cilia, by the assistance of which they swim about in the body of their parent, and seem to have ample space for their motions. At length, when the imprisoned gem- mules are ripe for exclusion, the skin of the original volvox bursts, (Jig. 19, ,) and the young ones, (Jig. 19, 8,) escaping through the fissure, enter upon a wider stage of existence : yet, even before their escape, the gemmules of a third generation are seen within their bodies, which, gradually enlarging, are destined to terminate by their birth the life of the newly liberated beings. (85.) The most usual mode of propagation however is by 60 POLYGASTRICA. spontaneous fissure, or division of the body of an adult animalcule into two or more portions, each of which is perfect in all its parts. This singular kind of generation, by which the old animalcule literally becomes converted into two or more young ones, is accomplished in various ways, which will require separate notice. In the oval forms of the polygastrica, the line of separation generally divides the body transversely into two equal portions, by a process, the different stages of which are represented in Jig. 20 ; 1, 2, 3. The body of an animalcule about to di- vide in this manner becomes at first slightly elongated, and a line more transparent than the rest of its body is seen to cross its middle por- tion : a constriction becomes gradually apparent at each extremity of the line of divi- sion, which soon grows more decided, and at length the two parts are only united by a narrow isthmus, {Jig. 19, 3,) which, getting thinner and thinner, allows a slight effort on the part of either of the now nearly distinct por- tions to tear itself from the other half, and complete the separation. In some elongated species {fig. 20, 4) the fissure is effected in a longitudinal direction, the separation gradually proceeding from the posterior to the anterior extremity of the body (fig. 20, 6) ; yet even in these the division is occasionally transverse, the newly formed creature appearing truncated at one end (fig. 19, 5) for some time after the completion of the process. (86.) The mode of generation in Convallaria, a group of which is seen at fig. 20, 11, is very curious; and from the different forms which the young assume during the progress of develope- ment much confusion has occurred, each stage of its growth having been described as the permanent appearance of a distinct species. This beautiful animalcule seems to be propagated in several ways : sometimes this is effected by external gemmules, which appear like minute points, scarcely more than y^oo of a l me * n POLYGASTIUCA. 61 diameter, upon the pedicles of the adult convallarise ; these in time become pedunculated, and, although still very small, exhibit the cilia upon the margins of their delicate cups ; in this state they were called by Schrank Vorticella monedicce. The Convallarise generally however multiply by fissure, the bell-shaped cup at the extremity of their highly irritable pedicles separating longitudinally into two ; but the progress of this division requires our particular notice, as the unpractised observer might be considerably puzzled on witnessing some of the phenomena attending it. The adult animalcule, seen with its pedicle fully extended, (Jig. 20, 9,) when it is alarmed, shrinks by throwing its stem into spiral folds (10) : in the latter figure, the bell or body of the animalcule is seen to have extended considerably in breadth, pre- paratory to its becoming divided into two distinct creatures. At 11, the commencement of its division is depicted; the separation gradually extending from the base, or ciliated extremity, to the point where the body is attached to its stem. When the division has extended thus far, (12,) the newly formed portion is seen with surprise to have become furnished with cilia at both ends, and, when finally detached, (13,) only at the opposite extremity to that on which they originally existed ; it then, freed from its pe- dicle, and thus losing the great characteristic of its species, swims about at large, exhibiting forms represented at 14, 15, 16, 17, all of which have been described as distinct species by different writers ; at last it puts forth a new stem, and, assuming the adult form, becomes fixed by its pedicle to some foreign body. (87.) This fissiparous mode of reproduction is amazingly pro- ductive, and indeed far surpasses in fertility any other with which we are acquainted, not excepting the most prolific insects or even fishes. Thus the Paramecium aurelia, if well supplied with food, has been observed to divide every twenty-four hours, so that in a fortnight, allowing the product of each division to multiply at the same rate, 16,384 animalcules would be produced from the same stock ; and in four weeks the astonishing number of 268,435,456 new beings would result from a continued repetition of the process : we shall feel but little surprise, therefore, that with such powers of increase these minute creatures soon become diffused in countless myriads through the waters adapted to their habits. (88.) The capability of spontaneous division is one of the most distinctive attributes of the acrite type of structure ; and was the organization of these animalcules as simple as it was supposed to POLVGASTR1CA. be a few years ago, when they were thought to be mere specks of living gelly, imbibing nourishment at every point of their surface, which became diffused through all parts of the homogeneous tex- ture of their bodies, such a mode of multiplication would be per- fectly intelligible, and every step of the process easily understood : but setting aside the conformation of their digestive apparatus, which, as we have before observed, is in our opinion not satisfac- torily determined, there are many circumstances attending the operation, which would indicate a power of developing new organs in the construction of every fresh individual, which must be looked upon as a very interesting feature in their history. Thus a new oral orifice, surrounded with cilia, must be formed upon the poste- rior segment of each divided animalcule, while an anal aperture is developed upon the anterior half. In Nassula elegans (Jig. 17, 1) the dental apparatus a, complex as its structure seems to be, must be formed upon a new part of the body preparatory to every sepa- ration ; and accordingly, in the plates which Ehrenberg gives of the reproduction of this animalcule, a new mouth or dental cylinder is actually seen to sprout from the hinder half of the creature before its transverse fissure is complete. These structures therefore, and others hereafter to be mentioned, must continually be called into existence at new and distant parts of the system. (89.) We have as yet only spoken of those forms of fissiparous generation in which the original animalcule divides either trans- versely or longitudinally into two portions ; yet there are instances where several new beings result from a like process. In Gonium pectorale (Jig- 16, 6) the entire animalcule seems to consist of sixteen globules enclosed in a delicate film or capsule ; which, divides both in a transverse and longitudinal direction, so as to separate into four portions, each composed of one large and three smaller globules, which, after their separation from the rest, swim freely about, and soon develope the parts and assume the appear- ance of the parent. In Gonium pulvinatum the offspring is still more numerous ; the parent resembles a square piece of delicate membrane, and, on assuming its full growth, is seen to be marked by three transverse and as many longitudinal lines, crossing each other at right angles, and dividing the original into sixteen smaller squares, which soon separate from each other, and become as many detached beings. (90.) Productive as the above-mentioned modes of increase are, it would seem that they are not the only sources of propagation POLYGASTRICA. 63 in the polygastric class of animals ; as many tribes have been observed to be produced from ova or spawn, as well as by fis- sure and gemmation. The Kolpoda cucullus {Jig. 20, 7) is one in which Ehrenberg succeeded most perfectly in detecting this kind of generation, but he has likewise observed it in many others. The ova seem to be produced in the general parenchyma of the body, without the visible existence of any organ specially destined to their formation ; and, when mature, are expelled in a delicate reticulate mass (Jig. 20, 8). Ehrenberg even describes some contractile vesicles discovered to exist in many species, which he regards, though perhaps without sufficient grounds, as being a male apparatus provided for the fertilization of the ova previous to their expulsion. In Paramecium aurelia (Jig. 17, 2) these were two in number, (a, g,) placed at the two extremities of the body, each seeming to consist of a delicate irritable central portion, from which he could see, on gently pressing the animalcule be- tween two plates of glass, eight canals issuing in a radiating manner and diverging toward all parts of the body ; these became gradually enlarged as the vesicle contracted, and, on the contrary, became narrow and disappeared as the vesicle dilated. The contractile organs were detected in twenty-two species belonging to very different families ; but the radiating canals were only seen in two, viz. Paramecium aurelia and Ophryoglena : their appearance in Nassula elegans, Stentor polymorphus, and Euplotes charon, is seen in Jig. 17 ; 1, 3, 4, b. The function of these organs Ehrenberg believes to be connected with the secretion of a fecun- dating fluid, which, being dispersed by their contraction through the body, serves to fertilize the ova. (91 .) No circulation, properly so called, has been seen in the polygastrica ; neither have vessels of any kind been satisfac- torily made out. There is however in Paramecium aurelia, as has been already mentioned, a constant sap-like movement in the gra- nular matter of the body, which is easily detected, and was described by Gruithuysen : this appearance Ehrenberg attributes to the movements of the intestine ; but as we have been quite unable to detect the arrangement which he indicates, or to reconcile the ap- parent course of the globules with the supposed direction of the alimentary tube, we are still inclined to regard the flow of particles alluded to as analogous to what has been described as existing in the stems of polyps. Neither do we find any distinct apparatus devoted to respiration in these minute beings : the cilia upon the AC ALE PH. E. surface, by the constant currents which they excite, necessarily ensure a continual supply of aerated water, which bathing the whole body exposes every part to the influence of oxygen, and Ehrenberg thinks that he has even perceived the existence of a delicate net-work of minute canals hollowed out in the periphery of some species, which, if filled with nutritive juices, might be regarded as the first rudiments of a vascular system. (92.) The nervous matter, or neurine, which we must suppose to exist in a molecular state mixed up with the tissues of the body, has never been detected in an aggregated form ; nevertheless, upon many species, when observed under good glasses, it is easy to see one or two extremely minute red or brown specks, which have been conjectured to be eyes, though probably without further reason for the supposition than the resemblance which they exhibit, in colour at least, to the visual organs of some ento- mostracous Crustacea : in some cases, these points exist only in the young animalcule prior to its birth ; thus in Eudorina elegans, an animal resembling the Volvox in its mode of gene- ration, the offspring, while confined in the body of their parent, are each seen to be furnished with a red speck, as well as a long bristle, which is exserted through the parent envelope ; but as soon as, by the rupture of the sac, the contained gemmules are set at liberty, a time when we should imagine the faculty of vision to be most useful, the red point disappears ; and, were that the only means of appreciating the presence of light, we might suppose the liberated animalcules to be deprived of the power of seeing when most capable of enjoying it. CHAPTER V. ACALEPHJE, (CuV.) (93.) The fourth class of acrite animals is scarcely inferior to that last described, either in numbers or interest. The ocean in every climate swarms with infinite multitudes of animals, which, from their minuteness and transparency, are almost as impercepti- ble to the casual observer as the infusoria themselves ; their exist- ence being only indicated by the phosphorescence of some species, which, being rendered evident on the slightest agitation, illuminates ACALEPH^E. 65 the entire surface of the sea. All however are not equally minute, some grow to a large size ; and their forms are familiar to the inha- bitants of every beach, upon which, when cast up by the waves, they lie like masses of gelly, melting as it were in the sun, inca- pable of motion and exhibiting few traces of organization, or indications of that elaborate structure which more careful examina- tion discovers them to possess. Their uncouth appearance has obtained for them various appellations by which they are fami- liarly known, as sea-gelly, sea-blubber, or gelly- fishes; whilst, from disagreeable sensations produced by handling most of them, they have been called sea-nettles, stingers, or stang-fishes. The faculty of stinging is indeed the most prominent feature in their history, so that their names in almost all languages are derived from this circumstance : they were known to the older naturalists by the title of Urtica marina ; and the word at the head of this chapter, applied by Cuvier to the entire class, and originally used by Aristotle, is of similar import (axaXrjpij, a nettle). There are few subjects which come under the observation of the physiologist more calculated to excite his astonishment than the history of these creatures. If he considers, in the first place, the composition of their bodies, what does he find ? -an animated mass of sea-water, for such in an almost literal sense they are. Let him take a medusa of any size, and lay it in a dry place ; it will be found gradually to drain away, leaving nothing behind but a small quantity of transparent cellular matter, almost as delicate as a cobweb, which apparently formed all the solid frame-work of the body, and which, in an animal weighing five or six pounds, will scarcely amount to as many grains ; and even if the water which has escaped from this cellulosity be collected and examined, it will be found to differ in no sensible degree from the element in which the creature lived. The conclusion therefore at which he naturally arrives is, that, in the medusae, the sea-water collected and deposited in the delicate cells of an almost imper- ceptible film becomes in some inscrutable manner instrumental to the exercise of the extraordinary functions with which these creatures are endowed. The Acalephse have been divided by zoologists into groups distinguished by the nature of their means of progression : in describing therefore the organs of locomotion, with which we commence their history, the reader will be made acquainted with the principal modifications of outward fonn which they exhibit. 66 ACALEPH^. (94.) Pulmonigrada. The most ordinary examples of the aealephse found in our climate, when examined in their native element, are seen to be composed of a large mushroom-shaped gelatinous disc, from the inferior surface of which various pro- cesses are pendent, some serving as tentacula, others for the pre- hension of food. In Rhizostoma (fig. 21) the central pedicle resembles in structure and function the root of a plant, being destined to absorb nourishment from the water in which the creature lives. The body of one of these medusae is specifically heavier than the water of the ocean, and would consequently sink but for some effort on the part of the animal. The agent employed to sustain it at the sur- face, and in some mea- sure to row it from place to place, is the um- brella-shaped expansion or disc, which is seen continually to perform movements of con- traction and dilatation, repeated at regular intervals about fifteen times in a minute, having some resemblance to the motions of the lungs in respiration, whence the name of the order (pulmo, the lung; gradior, I advance). By these constant movements of the disc, the medusa can strike the water with sufficient force to insure its progression in a certain direction when swimming in smooth water, but of course utterly inefficient in stemming the course of the waves, at the mercy of which these animals float. The tentacula, in such species as are provided with these organs, are likewise capable of contractile efforts, and may in some slight degree assist as agents of impulsion, although they are destined to the exercise of other functions. The locomotive disc, when cut into, seems perfectly homogeneous in its texture, nor is any fibrous appearance recognisable to which its movements could be attributed ; but in the larger species its inferior surface appears ACALEPH.E. 67 corrugated, as it were, into minute radiating plicae, which seem to contract more energetically than the other portions, and re- semble a rudimentary developement of muscular fibre. (95.) Ciliograda. In the Ciliograde acalephse, the organs of motion are of a very different description, consisting of narrow bands of vibratile cilia variously disposed upon the surface of the body, which in their motions and office resemble those of the polygastric animalcules. In the globular forms of Beroe (fig. 22) the cilia are generally arranged in eight longitudinal bands, and appear to be attached to subjacent arches of a firmer consistence than the rest of the body. They are generally quite naked, but in Pandora are lodged between folds of the skin, which will close over and completely conceal them ; their motion is extremely rapid, and sometimes only recognisable by the currents which they produce, or the iridescent hues which play along the arches. The ciliary action seems to be perfectly under the control of the animal, as it can retard or stop their motions at pleasure, sometimes arresting the play of one, two or more rows, whilst the rest continue in rapid vibration, and thus changing its course, or causing its body to revolve in any direction. In some of the Ciliograda, the loco- motive cilia are of considerable size ; and in Cydippe pileus their structure has been particularly examined by Dr. Grant.* In this animal eacli cilium, instead of being a simple filament, seems to be made up of several, arranged side by side, so as to form a flat membranous organ, not unlike the fin of a fish (fig. 22 ; 3, 4) : the individual filaments appear tubular when viewed under a powerful magnifier, and are slightly curved backwards, so that * Transact. Zoolog. Society of London, vol. i. 68 ACALEPH.E. the whole apparatus gives not a very bad representation of the paddle-wheel of a steam-boat. The cause of their movements is however as little evident in the Beroeform acalephte as in the minute Polygastrica. Under the arches which support them are vessels containing a fluid, which Dr. Grant imagines may in some manner be injected into the tubular structure, and thus cause them to become erected ; but how their rapid motions are excited, is still far from being explicable. But one of the most beautiful examples of a ciliated medusa is seen in the Girdle of Venus (Cesium Veneris) (fig. 23). Fig.23. This creature is a long, flat, gelatinous riband, the margins of which are fringed with innumerable cilia, tinted with the most lovely iridescent colours during the day, and emitting in the dark a phosphorescent light of great brilliancy : in this animal too, which sometimes attains the length of five or six feet, canals may be traced running beneath each of the ciliated margins, analogous to those which exist in the Beroe, and no doubt answering a similar purpose. (96.) Physograda. In the third division of acalephae, de- nominated by Cuvier " Acalephes Hydrostatiques," the body is supported in the water by a very peculiar organ, or set of organs, provided for the purpose. This consists of one or more bladders, capable of being filled with air at the will of the animal, which are appended to the body in various positions, so as to form floats of sufficient buoyancy to sustain the creature upon the surface of the sea when in a state of distension ; but, when partially empty, allowing it to sink, and thus escape the approach of danger. In ACALEPH.fc 0'9 Physalus, (Jig- 24,) known to sailors by the name of the Por- tuguese man-of-war, the swim- p igt 24. in ing-bladder is single, and of great proportionate size, so that when full of air it is exceed- ingly buoyant, and floats con- spicuously upon the waves. The top of this bladder bears a crest, c, of a beautiful purple colour, which, presenting a broad surface to the wind, acts as a sail, by the assistance of which the creature scuds along with some rapidity. The air-bladder is endowed with a considerable power of contrac- tion, and, when carefully exa- mined, two orifices are observ- able, one at each extremity, (a, &,) through which, upon pressure, the contained air readily escapes ; a provision which enables the creature to regulate its specific gravity at pleasure, and, when alarmed, at once to lessen its buoy- ancy by diminishing the capacity of its swimming-bladder, and to sink into the waves. The nature of the air with which the little voyager distends its float has not been accurately deter- mined ; but it is undoubtedly a secretion furnished at pleasure when at a considerable distance from the surface, although the mode of its production is still unknown. Among the diversified forms of the Hydrostatic acalephse, few are more elegant than one named by Peron Cuvieria cariso- chroma (Jig. 27). In this beautiful medusa we find the floats arranged like a string of pearls around the margin of its circular body ; which, thus supported, spreads its long and delicate fila- mentary tentacles to a considerable depth, in search of passing food, as it swims upon the tranquil bosom of the ocean. (97.) Cirrigrada. The Cirrigrade acalephse form a very remarkable family, peculiarly distinguished by the possession of an internal solid support or skeleton secreted in the substance of their soft and delicate bodies. In For pita (Jig. 25) this consists of a flat plate of semicartilaginous texture, (2,) evi- 70 ACALEPtLE. 1. Fig. 25. dently deposited in thin secondary lami- nae, which gradually increase in size as the animal advances in growth, the inferior being the largest and last formed. When examined after its removal from the body, this fragile skeleton is seen to be extremely porous or cellular ; and, the pores being filled with air, it is specifically lighter than water, a circumstance which may contribute to the buoyancy of the animal, even when alive. The lower surface of Porpita is furnished with numerous appen- dages called cirri, some of which appear to be organs of prehension, but perform also the office of oars, which in this species are the principal agents in progression ; yet in other Cirrigrada, as Velella and Rataria, besides the horizontal lamella, which forms the whole skeleton of Porpita, there is a second subcartilaginous plate rising at right angles from its upper surface, and supporting a delicate membranous expansion, which rises above the water and exposes a considerable surface to the wind, so as to form a very excellent sail. To perfect so beautiful a contrivance, in Rataria the crest is found, to contain fibrous bands, apparently of a muscular nature, by the contractions of which the sail can be lowered or elevated at pleasure. (98.) Diphyda. The last family of acalephse derives its name from the singular appearance of the creatures which compose it : each animal, in fact, seems to consist of two portions so slightly joined together, that it is by no means easy to understand the nature of the con- nection which ex- ists bet ween them; and from the per- fect transparency of their bodies, which is such that it is with great difficulty they are discoverable even in small quanti- > ACALEPH^E. 71 Fix. 27. ties of sea-water, our knowledge of their internal structure is at present extremely imperfect. The annexed figure of Diphyes cam- panulifera (^g". 26) will give the reader a general idea of their form. The two bell-shaped portions of which the creature may there be seen to consist, are constantly found united together, and seem to compose but one animal, although they might readily be conceived to be distinct creatures ; the apex of the posterior part is received into a cavity in the other portion, but the connection between the two is so slight, that, when preserved in spirits at least, the slightest touch is sufficient to tear them asunder ; their principal bond of union appears to be a delicate filament, which, arising from the anterior compartment, passes through the whole length of the posterior portion. This strange compound body, concern- ing the structure of which our knowledge is very imperfect, swims through the water with consider- able rapidity, urged forward by the alternate contractions of the two campanulate halves, which con- tinually take in and eject the cir- cumambient fluid, with sufficient force to propel the creature in an equable and uniform course. (99.) Interesting as the acalephse may justly be considered when we contemplate the singular beauty of their external configuration, and the wonderful design conspicuous in their locomotive organs, a more in- timate acquaintance with their habits and economy will be found to disclose many facts not less curious in themselves than important in a physiological point of view. In the higher animals we are accustomed to find the nutritive apparatus composed of several distinct systems ; one set of organs being destined to the prehension of food, another to digestion, a third to the absorption of the nutritious parts of the aliment, a fourth provided for its distribution to every part of the body, and a fifth destined to ensure a constant exposure of the cir- culating fluid to atmospherical influence. These vital operations 72 ACALEPH.E. are carried on in vessels specially appropriated to each ; but, in the class of animals of which we are now speaking, we find but a single ramified cavity appropriated to the performance of all these functions, and exhibiting in the greatest possible simplicity a rough outline, as it were, of systems afterwards to be more fully developed. In the Pulmonigrade acalepha we have the best illustration of this arrangement : in these the stomach or digestive cavity is ex- cavated in the centre of the disc, and is supplied with food by a mechanism which differs in different species. In Rhizostoma, (fig. 21), which receives its name from the nature of the communication between the stomach and the exterior of the body,* the organ destined to take in nourishment consists of a thick pedicle, composed of eight foliated divisions, which hang from the centre of the disc. Each of these appendages is found to contain ramifying canals, opening at one extremity by nu- merous minute apertures upon the external surface, whilst at the opposite they are collected into four large trunks communicating with the stomach ; as the Rhizostoma therefore floats upon the waves, its pendent and root-like pedicle absorbs, by the numerous oscules upon its exterior, such food as may be adapted to its nutrition, finding most probably an ample provision in the mi- croscopic creatures which so abundantly people the waters of the ocean. The materials so absorbed are conveyed through the canals in the interior of the arms into the stomachal cavity, where their solution is effected. But it is not upon this humble prey that some of the medusae feed ; many are enabled, in spite of their apparent helplessness, to seize and devour animals which might seem to be far too strong and active to fall victims to such assailants : Crustacea, worms, mollusca, and even small fishes are not unfrequently destroyed by them. In- credible as this may seem when we reflect upon the structure of these feeble beings, observation proves that they are fully competent to such enterprises. The long tentacula or filaments, with which some are provided, form fishing-lines scarcely less formidable in arresting and entangling prey than those of the Hydra ; and, in all probability, the stinging secretion which exudes from the bodies of these medusae speedily paralyzes and kills the animals which fall in their way. The mouth of these acalephse is a simple aperture leading into the gastric cavity, and sometimes surrounded with * 'Pi&, a root j frozen, a mouth. AC ALE PILE. tentacula, which probably assist in introducing the food into the stomach. In Cassiopea Borbonica, the principal agents in procuring nourishment are numerous retractile suckers, (Jig. 28, a,) terminat- ing in small violet- F/g.28. coloured discs, which are dispersed over the fleshy appendages to the under surface of the body ; the stem of each of these suck- ers is tubular, and conveys into the sto- mach nutritive mate- rials absorbed from animal substances to which they are at- tached during the process of imbibing food. (100.) The above examples will suffice to give the reader an idea of the most ordinary provisions for obtaining nourishment met with in the Pulmonigrada : we will therefore return to consider the structure of the stomach itself, and of the canals which issue from it, and convey the digested nutriment through the system. In Cassiopea Borbonica, which will serve to exemplify the general arrangement of these parts in the whole order, the stomach (Jig- 28) is a large cavity placed in the centre of the inferior surface of the disc, and is apparently divided into four compartments by a delicate cruciform membrane arising from its inner walls. Into this receptacle all the materials collected by the absorbing suck- ers are conveyed through eight large canals, and by the process of digestion become reduced to a yellowish pulpy matter, which is almost fluid, and which is the pabulum destined to nourish the whole body. From the central stomach sixteen large vessels arise, (Jig. 29, c,) which ra- diate towards the circumference 74 ACALEPH.E. of the disc, dividing and subdividing into numerous small branches, which anastomose freely with each other, and ultimately form a perfect plexus of vessels as they reach the margin of the mush- room-shaped body of the creature. The radiating vessels are moreover made to communicate together by means of a circular canal (Jig. 29, e) which runs around the entire animal, so that every provision is made for an equable diffusion of the nutritive fluid derived from the stomach through the entire system. Now, if we come physiologically to investigate the nature of this simple apparatus of converging and diverging canals, we cannot but perceive that it unites in itself the functions of the digestive, the circulatory, and the respiratory systems of higher animals : the radiating canals, which convey the nutritive juices from the stomach through the body, correspond in office with the arteries of more perfectly organized classes ; and the minute vascular ramifications in which these terminate, situated near the thin margins of the locomotive disc, as obviously perform the part of respiratory organs, in as much as the fluids which permeate them are continually exposed to the influence of the air contained in the surrounding water, the constant renewal of which is accomplished by the perpetual contractions of the disc itself. (1 01.) Before closing our description of the alimentary system of the Pulmonigrade acalephse, we must mention some accessory organs of recent discovery which are in connection with it. Eschscholtz* describes a series of elongated granular bodies, placed in little de- pressions around the margin of the disc, which seem to be of a glandular nature, and apparently communicate by means of minute tubes with the nutritious canals : these he regards as the rudiments of a biliary system. Other observers assign a similar office to a cluster of blind sacculi or caeca, which are connected in some species with the commencement of the radiating tubes ; it is, however, scarcely necessary to observe that such surmises relative to the function of minute parts are but little satisfactory. (102.) The Ciliograde acalephse, although their digestive system varies considerably in its general arrangement from what has been described in the Pulmonigrade division, will be found to exemplify in an equally perfect and perhaps more striking manner the for- mation of the vascular and respiratory systems from an extension of the nutritive canals. In the Beroeform species (Jig. 22) the * System der Acalephen. Berlin, 1829. Annales des Sciences Nat. vol. xxviii. p. 251. ACALEPH.E. 75 alimentary canal passes straight through the globular or barrel- shaped body, commencing at one extremity by two prominent and sensitive lips. No apparatus of prehension is here needful ; for, as these animals swim along by the action of their cilia, the water passes freely through this capacious channel, and brings into the stomach materials proper for food. From both extremities of the digestive cavity arise vascular canals which empty themselves into two circular vessels, one surrounding the oral, and the other the anal portions of the body : from these two rings eight double vessels arise, which run longitudinally from one pole to the other of the creature beneath each of the cartilaginous ribs upon which the cilia are placed ; and from these, others more minute arise, which are distributed in a delicate network through the sub- stance of the animal. In the Beroe, therefore, we must regard the vessels which convey the nutritive juices beneath the ciliated arches, not merely as arteries, but as organs of respiration ; for, thus placed close beneath the outer surface of the body, the water, which is perpetually made to rush over them by the ciliary move- ments, will serve to aerate the fluid contained within. The Cestum Veneris (fig. 23) is nearly allied to the Beroe in the arrangement of its nutritive apparatus, notwithstanding the difference of form observable in these Ciliograde medusae. In Ces- tum the digestive cavity, which is exceedingly short in comparison with the length of the animal, passes transversely across the body in a straight line from one side to the other, as represented in the engraving (fig. 23) ; but the details of its structure, and the nature of the vessels aris- ing from it, will be best understood by a reference to the enlarged diagram of these parts given in the annexed figure (fig. 30). The mouth (i) is a rhomboidal depression seen near the centre of the body, between the two lateral rows of locomotive cilia which extend from one end of the animal to the other. From the mouth arise two tubes, (/,y,) which terminate in a globular cavity common to both ; these would seem to constitute the digestive apparatus : and a straight and narrow tube (o), prolonged to the margin of the body opposite to that which the mouth occupies, may be regarded Fig. 30. 76 ACALEPH.E. as an intestine through which the residue of digestion is discharged. From around the oral extremity of the stomach, and from the glo- bular cavity in which the two principal canals terminate, arise ves- sels, (, t, ,) which diverge so as to form a cone at the base of which they all empty themselves into two circular canals, one surrounding the mouth, and the other encircling the anal aperture ; which pre- cisely correspond with the vascular rings already described in the Beroe : and, from these, four long vessels, or branchial arteries as they might be termed, (/?, p ; </, </,) are prolonged beneath the four ciliated margins all around the body. But, besides these four nutri- tive vessels, two others (#, x) arise from the anal ring which run inwards towards the centre of the animal, and afterwards, assuming a longitudinal direction, seem to distribute nourishment to the me- dian portions of the body. The caeca or blind tubes, (w, w,) ap- pended to the intestine, may possibly furnish some secretion use- ful in digestion, although we are perhaps scarcely warranted in saying decidedly that they are the rudiments of biliary organs.* Our information concerning the nutritive apparatus of the other orders of acalephse is very limited. In Physalus (Jig. 24) and Porpita (fig. 25), the suckers appended to the body would seem to be the organs by which food is taken into the system ; but, of the internal arrangement of the parts subservient to its digestion and distribution, little has been determined satisfactorily. (103.) Extraordinary as must appear the powers which these animals possess of seizing and dissolving other creatures, apparently so disproportioned to their strength, and the delicate tissues which compose their bodies, there are other circumstances of their history equally remarkable, which in the present state of our knowledge are still more inexplicable. If a living medusa be placed in a large vessel of fresh sea-water, it will be found to secrete an abun- dant quantity of glairy matter, which, exuding from the surface of its body, becomes diffused through the element around it so copiously, that it is difficult to conceive whence materials can be derived from which it can be elaborated. Of the origin of this fluid we are ignorant, although certain glandular-looking granules contained in the folds of the pedicle have been looked upon as connected with its production. (104.) We are equally at a loss to account for the pro- duction of the irritating secretion in which the power of stinging * Delle Chiaje, Memorie per servire alia storia degli Animali senza vertebre del regno di Napoli. 4to. 18231825. ACALEPHSE. 77 seems to reside, but it is observed that the tentacuhi seem to be more specially imbued with it than other parts of the body. Perhaps the most remarkable property of the acalephse is their phosphorescence, to which the luminosity of the ocean, an appearance especially beautiful in warm climates, is princi- pally due. We have more than once witnessed this pheno- menon in the Mediterranean, and the contemplation of it is well calculated to impress the mind with a consciousness of the profu- sion of living beings existing around us. The light is not con- stant, but only emitted when agitation of any kind disturbs the microscopic medusse which crowd the surface of the ocean : a pass- ing breeze, as it sweeps over the tranquil bosom of the sea, will call from the waves a flash of brilliancy which may be traced for miles ; the wake of a ship is marked by a long track of splendour ; the oars of your boat are raised dripping with living diamonds ; and, if a little of the water be taken up in the palm of the hand and slightly agitated, luminous points are perceptibly diffused through it, which emanate from innumerable little acalephse scarce- ly perceptible without the assistance of a microscope. All, how- ever, are not equally minute ; the Beroes, in which the cilia would seem to be most vividly phosphorescent, are of considerable size ; the Cestum Veneris, as it glides rapidly along, has the appearance of an undulating riband of flame several feet in length ; and many of the larger Pulmonigrade forms shine with such dazzling bright- ness, that they have been described by navigators as resembling " white-hot shot " visible at some depth beneath the surface. This luminousness is undoubtedly dependent upon some phospho- rescent secretion, but its nature and origin are quite unknown. (105.) The principal instruments of sensation in the acalephse are the tentacula and suckers, which, under various forms, are append- ed to different parts of the body, and which are individually capa- ble of contraction and elongation to a considerable extent. In the discophorous forms, these are frequently appended to the margin of the disc (Jig- 27) ; sometimes they are only found around the aperture of the mouth. In Porpita and Physalus they are nu- merous, and hang in clusters from the inferior surface of the body : but the most beautiful tentacular apparatus is that which is met with in the Beroe (Cydippe) pileus; this is represented in Jig. %% ; 1, a, a, and consists of two very long and delicate filaments, many times exceeding the length of the body when extended to their full length ; from these arise others of still greater tenuity, 78 ACALEPHJE. which are likewise capable of spontaneous elongation. When not in use, these organs are retracted within the body, and lodged in two membranous sheaths visible in the drawing, from which they are protruded at the pleasure of the animal, and, as they expand, gradually uncurl the spiral, secondary tentacula by movements which are singularly graceful and elegant. In Medusa aurita there are seen around the circumference of the locomotive disc certain red spots, which Ehrenberg regards as eyes, without however adducing the slightest proof that they possess any claims, derived either from their structure or function, to the name which he is pleased to give them. (106.) Most anatomists have failed to detect nervous filaments even in the largest medusae ; nevertheless Ehrenberg is inclined to believe that in some Pulmonigrade species a delicate thread, which encircles the margin of the disc, is to be regarded as nervous, as well as others, which he describes as being visible around the base of the pedicle. In the Beroe (Cydippe) pileus, (Jig. &2) Pro- fessor Grant* regards a double cord which runs around the oral extremity of the alimentary canal, of which an isolated view is given at Jig. %%, 2, as constituting the nervous system ; this ar- rangement, however, has not been confirmed by later observations, and we are inclined to think that the vascular circle which sur- rounds the mouth ( 102) of the Beroeform species has been in this case mistaken for nervous fibre. (J 07.) We know little satisfactorily concerning the mode of gene- ration in the acalephse, the opinions of authors upon this subject being in the last degree vague and contradictory. Confining our- selves to the examples which have been selected as best adapted to put the reader in possession of the principal facts known concerning the class under consideration, we find the organs usually regarded as the agents of reproduction assuming very different forms. In Cassiopea Borbonica, the parts which Delle Chiaje describes as ovaria, are four membranous tubes filled with granular matter, and placed above the stomach (Jig. 28, c) ; from each of these a canal issues, which, dividing into several smaller branches, opens by as many minute orifices into four cavities placed around the stomach, into which the sea-water is freely admitted. According to Gaede*^ and Eysenhardt,j the ovaria examined * Transactions of the Zoological Society, vol. i. f Beytrage zur Anatomic und Physiologic der Medusen. Berlin, 1816. 8vo. J Zur Anatomic und Naturgeschichte der Quallen j Rhizostoma Cuvierii. Mem. de 1' Academic Leopold des cur. de la Nature. STERELMINTHA. 79 in oilier forms of the Pulmonigrada occupy a similar position, and at certain seasons of Fig.si. the year become re- markably distended with ova ; but, from the observations of these writers, it would seem that the young medusae arc hatched in the ova- ria, and afterwards escape in a very per- fect state of deve- lopement. One of the ovaria of Medusa aurita is represented in the annexed figure, (fig. 81, 1,) taken from Ehrenberg's ela- borate plates of the anatomy of this animal, in which a, b indicate the extremities of the convoluted organs in which the germs are developed. The gemmules, when mature, are, according to this author, covered with locomotive cilia like those of sponges and polyps (). In Physalus the ova would seem to be generated by the long undulating filaments attached to the lower surface of the body, and in Beroe the ovaria are seen to form clusters around the alimentary canal ; but we are ignorant of the mode of their developement, and of the circumstances connected with the exclusion of the young. CHAPTER VI. STERELMINTHA. Parenchymatous Entozoa. (Cuv.) The ENTOZOA, as the name implies, are nourished within the bodies of other animals, from the juices of which they derive their sustenance. It may naturally be supposed that living under such 80 STERELMINTHA. circumstances, deprived of all power of locomotion, as is gene- rally a necessary consequence of the localities in which they are found, debarred from the influences of light, and absolutely de- pendent upon the fluids which bathe their bodies for nutriment, the entozoa have little occasion for that elaborate organization needful to animals living in immediate communication with exter- nal objects. We find therefore, among these creatures, some whose structure is more simple than that of any other animals, in adaptation to the circumscribed powers of which they are capable. Yet, however ap- parently insignificant some may appear from their diminutive size, they not unfrequently become seriously prejudicial to the animals in which they are found, by the prodigious numbers in which they exist, or from their growth in those organs more especially essential to life, and not a few of them from their dimensions alone some- times prove fatal. The annexed figure (Jig. 32) represents a Ligula developed in the abdominal cavity of a Fig. 32. fish. There are probably no races of animals which are not infested with one or more species of these parasites, from the microscopic infu- soria up to man himself, and sometimes several different forms are met with in the same spe- cies, to which they would appear to be peculiar, and even in some cases the entozoa would seem themselves to enclose other species para- sitically dwelling in their own bodies. Neither is their existence confined to any particular parts ; they are met with in the alimentary canal, in the liver, the kidneys, the brain, the arteries, the bronchial passages, the muscles and cellular tissue, and in fact in almost all the organs of the body. (109.) It would appear that some of the ordinary secretions of ani- mals are, when in a healthy state, naturally inhabited by innumerable active beings, scarcely equalling in bulk some of the most minute infusoria, and consequently requiring the highest magnifiers to detect even their presence. The best known of these are found in the seminal fluid, and of their size the reader may form some judg- ment by the following calculations upon this subject. Reil esti- mated the length of those found in man at the 3 0*0 o o P ar t of an inch, or at the 25,000th part of a line, and their breadth at the STERELMINTIIA. 81 thousandth part of the diameter of a hair ; and Clifton Wintring- ham, in order that our ideas concerning them should be as perfect as possible, recorded his estimate of the weight of one of these animalcules, which he supposed might be about the hundred and forty thousand millionth part of a grain !* Notwithstanding their inconceivable minuteness, however, the Zoosperms have each a definite and symmetrical figure, which is peculiar to their species, so that those taken from different animals may be recognized by their outward form. In quadrupeds they have generally the ap- pearance of minute tadpoles, with flattened globular bodies, termi- nated by long tails of extreme tenuity ; but, in fishes and inverte- brate animals, they are often without tails, sole-shaped, or even globular. Nothing of course is known concerning the internal organization of these living atoms. (110.) The Cystiform Sterelmintha, which are generally known by the name of Hydatids, are the simplest in structure ; and with these, therefore, we shall commence our enquiry into the economy of these creatures. The Ccenurus cerebralis, (Jig. 33,) one of Fig. 33. the most common, is met with in the brains of sheep, and is the cause of a mortal disease but too well known to the farmer ; it is likewise occasionally met with in other ruminating quadrupeds, and, by partially destroying the substance of the brain, soon proves fatal. This entozoon, represented in the figure of its usual size, consists of a delicate transparent bladder, the walls of which, * De Blainville, (H. M. D.) Manuel d'Actinologie. Paris, 1834. 8vo. G STERELMINTHA. Fig. 34. during the life of the creature, are visibly capable of sponta- neous contractions on the application of stimuli. To this bladder, or common body, are appended numerous heads, or rather mouths, which are individually furnished with an apparatus of hooks and suckers, (Jig. 33, 2, a, &,) calculated to fix them to the surrounding tissues, whence they derive nourishment. (111.) The Cysticerci, or common hydatids, agree in the main features of their structure with the Ccenurus, but are provided with only one head or oral orifice resembling those of Coenurus (Jig. 34; 2). These animals are found in almost all the viscera of the body ; and not un- frequently, especially in pigs, exist in great num- bers, not only in the liver, which is their most usual seat, but in the cellular texture of the muscles, and even in the eyes themselves. The human frame is not free from their ravages, and, when they abound, serious consequences frequently result from their presence. The Cysticercus crassicollis is less fre- quently met with than the ordinary hydatid (C. tenuicollis). In this animal the head is provided with a prehensile apparatus analogous to that found the last described species ; a structure which resembles pre- in cisely what we shall afterwards find in the Tcenice or tape-worms, with which these creatures are closely related in a zoological point of view. Even in external form they are allied to the cestoid worms, as may be seen in the annexed figure, in which, notwith- standing the vesicular character of the posterior part of the body, the anterior portion is distinctly divided into segments. (112.) The mode of reproduction in these entozoa resembles that of the Volvox globator. They propagate by internal gem- STERKLMINTHA. 83 mules, which grow from the membranous walls of the sac ; and which, having attained a certain growth, become detached, and are found floating in the glairy fluid contained in the interior of the parent. (113.) It is difficult even to conjecture the manner in which these parasites first obtain admission to the localities where they are found, and some zoologists have been content to allow the possi- bility of their being spontaneously generated : but the present state of our knowledge can scarcely sanction the occurrence of such developements. It seems more probable to imagine that the entozoa exist in some other form under other circumstances, but that, when introduced into the body, their eggs may be conveyed by the circulating fluids to a nidus proper for their developement, where their inordinate growth is due to the abundant supply of already animalized food placed within their reach, and the exalted temperature at which they are kept. (114.) The Trichina spiralis (fig. 35) is an entozoon hitherto only found in the human body, Fig. 35. and, although of recent dis- ii; ;i|lliuilla -M^^ co very, several cases of its oc- | currence are recorded. This minute worm is found in im- mense numbers imbedded in the cellular intervals between the muscular fibres, and in some instances all the vo- luntary muscles seem full of these creatures, exhibiting, when viewed with the naked eye, an appearance imitated in the annexed figure (fig. 35, c.) * On examining the white specks attentively under the microscope, every one of them is seen to be a flask-shaped vesicle, apparently formed of condensed cellular membrane, in which the minute animal is lodged ; and when this outer covering is ruptured, as at (a), the worm escapes. A magnified view of the entozoon is given at (6), coiled up in the position in which it is seen prior to the destruction of the sac which enclosed it. The body seems to be filled with granular * For the knowledge which we possess of the anatomy of Trichina, we are princi- pally indebted to the researches of Professor Owen and Dr. Arthur Farre ; though it was first discovered by M. Hilton, Vide Zool. Trans, vol. i. t-t O G <<> 84 STERELMIXTHA. matter, which escapes when the worm is torn asunder (d) ; but whether it possesses a true alimentary tube, is not as yet satis- factorily determined. (115.) The Tania., or tape-worms, are among the most inter- esting of the Sterelmintha, whether we consider the great size to which they sometimes attain, or the singular construction of their compound bodies. Several species of these worms infest the human body, and many other forms of them are met with in a variety of animals. They are usually found in the intestinal passages, where, being amply provided with nutritious aliment, they fre- quently grow to enormous dimensions, being not unusually twenty or thirty feet in length, and some have been met with much longer ; it is therefore manifest how prejudicial their pre- sence must prove to the health of the animals in which they reside, and we are little surprised at the emaciation and weak- ness to which they generally give rise. The T<znia solium, the species most usually met with in the human subject, at least in our own country, is that which we select for description. The body of this creature consists of a great number of segments united together in a linear series (Jig. 36) : the segments which immediately succeed to the head are very small, and so fragile that it is rarely that this part of the animal is obtained in a perfect state ; they gradually however in- crease in size towards the middle of the body. Each segment of the tape-worm might be re- garded as a distinct ani- mal, for every one of them, with the excep- tion of the smallest, or those in the vicinity of the head, is found to contain a complete ge- nerative apparatus ; yet the alimentary tubes are common to them all, those of each joint Fig. 36. STERELMINTHA. 85 freely communicating with the nutritive canals of the adjoining segments. The first joint of the Tsenia, which may be called the head, differs materially in structure from all the rest ; it is in fact converted into an apparatus by means of which the entire animal derives its nourishment. This part in the Tsenia solium, when highly magnified, is found to be somewhat of a square shape ; in the centre is seen the mouth, surrounded with a circle of minute spines, so disposed as to secure its retention in a posi- tion favourable for imbibing the chyle in which it is immersed. Around this prominent mouth are placed four suckers, which are no doubt additional provisions for the firm attachment of the head of the worm. In other Tsenise the structure of the oral segment is variously modified : thus in Tania lata the aperture of the mouth has no spines in its vicinity ; in Bothryocephalus there are only two longitudinal sucking discs ; in Floriceps these are replaced by four proboscidiform prolongations, covered with sharp recurved spines, which, being plunged into the coats of the intestine, form effectual and formidable anchors : yet the intention of all these modifications is the same, namely to retain the mouth in a position adapted to ensure an adequate supply of nutritious juices. (116.) The alimentary canal, which extends from the mouth, is a double tube, which may be traced through the whole length of the body, without any other perceptible communication with the ex- terior than the oral orifice in the centre of the head : at the com- mencement of every segment, moreover, there is a cross-canal, which communicates with the corresponding canal of the opposite side (fig. 37, a), so as to facilitate a free distribution of the nutrient fluids. In some species a delicate vascular network is perceptible in the parenchyma of the body, which may likewise be connected with the nutritive function. (117.) A distinct generative system is found in every segment of these remarkable animals ; and, judging from the number of eggs produced by each, we are at a loss to reconcile the disproportion which exists between the extreme fertility of the Tsenise, and the comparative rareness of their occurrence. The ovaria in which the eggs are produced are of great relative size, occupying the centre of each joint. In the annexed figure (fig. 37), which represents one of the segments of the Tsenia solium highly magnified, the ovigerous organ (&) is seen to consist of a central cavity, from the circumference of which radiate a great number of csecal tubes ; these at certain seasons are filled with granular ova. From 86 STEEELM1KTHA. Fig. 37. the central portion of this ramified ovary issues a wide canal or excretory duct (c), which may be traced to a prominent tu- bercle placed on the lateral margin of every segment (e), where it terminates in a mi- nute pore opening externally. This canal, which may be called the oviduct, is seen just before its termination in the external pore to be joined by a delicate tube (d), which appears as a dark line under the microscope, and derives its origin from a small bulb or vesicle, and may be regarded as most probably furnishing a secretion serving to fertilize the ova prior to their expul- sion ; such, at least, is the office generally assigned to it. Many thousands of eggs must be produced from such multiplied sources of reproduction ; and yet how are they preserved and replaced in circumstances favourable to their developement ? Fortunately it is rare to meet with more than one of these creatures at the same time, taking up a residence in the same individual ; and, in fact, the species which has been specially the subject of our description is often called, par excellence, " the solitary worm," from this circumstance. Yet what becomes of the reproductive germs furnished in such abundance ? Do they, as was the opinion of Linneus, live in a humbler form in stagnant waters and marshes, until they are casually introduced into the body of some animal, where, being supplied profusely with food and placed in a higher temperature, they attain to an exuberant developement ? Or are the germs thus numerous in proportion to the little likelihood of even a few of them finding admission to a proper nidus ? To these questions we can only reply by conjec- tures ; and, interesting as the subject is, few are more entirely involved in mystery. (118.) In the Fluke, Distoma (Fasciola, Linn.) hepaticum, we have an entozoon of more complex and perfect structure ; one of those forms, continually met with, which make the transition from STERELMINTHA. 87 Fig. 38. one class of animals to another so insensible, that the naturalist hesi- tates with which to associate it. In the Distoma, in fact, notwith- standing its intimate relationship with the Tsenioid Sterelmintha, the first rudiments of nervous filaments are apparent, and we find its whole organization approximating the nematoneurose type rather than strictly exhibiting the simple structure common to the Acrita. The Distoma is commonly found in the liver and biliary ducts of sheep, and other ruminants, deriving nourishment from the fluids in which it is immersed. The body of the creature, which is not quite an inch in length, is flattened, and resembles in some degree a minute sole or flat-fish. At its anterior extre- mity is a circular sucker or disc of attachment, by which it fastens itself to the walls of the cavity in which it dwells, as well as by means of a second sucker of similar form, placed upon the ventral surface of the body. In the annexed diagram (Jig. 38) the posterior sucker has been removed, in order more distinctly to exhibit the internal structure of the animal. The name which this entozoon bears seems to have been given to it from a supposition that it possessed two mouths, one in each sucker ; whereas the anterior or terminal disc (a) only is perforated, the other being merely an instrument of adhesion. The ali- mentary canal (b) takes its origin from the mouth as a single tube, but soon divides into two large branches, from which ramifications arise which are dispersed through the body, each ter- minating in a blind clavate extre- mity. These tubes, from being generally filled with dark bilious matter, are readily traced, even without preparation ; or they may be injected with mercury intro- duced through the mouth. Through the walls of the ventral surface of the body, two nervous filaments (c) are discoverable, crossing over the root of the anterior sucker or acetabulum, and, gradually diverging, may be observed to run in a serpentine course towards the caudal extremity, where they are lost : it would even seem that on either side of the oesophagus there is a very slight ganglion, from which 88 STERELMINTHA. other nervous filaments arise to supply the suckers, and the anterior part of the body. The organs of generation in the fluke are very voluminous, occupying with the ramifications of the alimentary tubes the whole of the interior of the animal : in the diagram they are not represented on the right side, in order that the distribution of the intestine may be better seen ; and on the left side the ali- mentary vessels are omitted, to allow the general arrangement of the sexual system to be more clearly intelligible. These animals would seem to be completely hermaphrodite, not only possessing distinct ovigerous and seminiferous canals, which open separately at the surface of the body, but even provided with external organs of impregnation, so that most probably the co-operation of two individuals is requisite for mutual fecundity. To commence with the female generative system, we find the ovaria (h) occupying the whole circumference of the body. When distended with ova, the ovigerous organ is of a yellow colour ; and, when attentively examined under the microscope, is seen to be made up of delicate branches of vesicles united by minute filaments, so as to have a racemose appearance. From these clusters of ova arise the oviferous canals, which, uniting on each side of the body into two principal trunks, discharge their contents into the large oviducts (g). The oviducts terminate in a capacious receptacle (e), usually called the uterus ; and from this a slender and convoluted tube leads to the external orifice, into which a hair (d) has been inserted. On each side of the uterus we find a large ramified organ, made up of csecal tubes, (/,) which opens into the uterine cavity, and no doubt furnishes some accessory secretion needful for the completion of the ova. The male apparatus occupies the centre of the body. The testes (A-), in which the spermatic fluid is secreted, consist of convoluted vessels of small calibre, arranged in close circular folds, and so inextricably involved, that it is difficult to get a clear idea of their arrangement ; but towards the middle of the mesian line they become more parallel, and terminate in two larger trunks (t), (one of which has been removed in the figure,) which are enclosed and hidden in the seminal vessels. These great canals, which run side by side in a longitudinal direction, become gradually much attenuated (/), and terminate in the root or capsule of the penis (m). The external male organ (n) is STEBELM1NTHA. 89 placed a little anterior to the orifice which leads to the female parts ; it is a short spiral filament, distinctly traversed by a canal, and perforate at the extremity, so as indubitably to perform the office of an instrument of intromission. (119.) The PLANARI.-E, although they do not inhabit the interior of other animals, are so nearly allied in every part of their organi- zation with, the Flukes, (Distoma,) that their history cannot be more appropriately given than in this place. The Planarise are common in ponds and other stagnant waters ; they are generally found creeping upon the stems of plants, or amongst the healthy confervse which abound in such situations, and wage perpetual war with a variety of animals inhabiting the same localities. The body of one of these minute creatures appears to be entirely gelatinous, without any trace of muscular fibre ; * yet its motions are exceedingly active, and it glides along the plane upon which it moves with a rapid and equable pace, of which the observer would scarcely expect so simple a being to be capable ; or, by means of two terminal suckers, progresses in the manner of a leech. No agglomeration of nervous fibre has hitherto been sa- tisfactorily detected in the Planarise ; nevertheless, many species possess two red specks upon the anterior part of the body, which, as in other cases, have been unhesitatingly pronounced to be eyes, although their claim to such an appellation is not only unsubstan- tiated by any proofs derivable from their structure, but com- pletely negatived by experiments, which go to prove that in the pursuit of prey no power of detecting the proximity of their food by the exercise of sight is possessed by any of them. * Duges, Annales des Sciences Nat. Fig. 39. 90 STERELMINTHA. The phenomena which have been observed connected with the multiplication of the Planarise by division are analogous to those which we have witnessed in other acrite animals ; for it has been proved, that if an individual be cut to pieces, every portion continues to live and feel, from whatever part of the body it may be taken ; and, what is not a little remarkable, each piece, even if it be the end of the tail, as soon as the first moment of pain and irrita- tion has passed, begins to move in the same direction as that in which the entire animal was advancing, as if the body was actuated throughout by the same impulse, and, moreover, every division, even if it is not more than the eighth or tenth part of the crea- ture, will become complete and perfect in all its organs. The mouth, in a few species of Planarise, is placed at the an- terior extremity of the body, but generally it is found to occupy the middle part of the ventral surface. Its structure is quite peculiar, and admirably adapted to the exigencies of the creature : it con- sists of a wide, trumpet-shaped proboscis, (fig- 39, 3 and 4,) which can be protruded at pleasure, and applied to the surface of such larvae or red-blooded worms as may come within reach, so as to suck from them the juices which they contain ; or, if the prey be small, animalcules and minute Crustacea are seized by it and con- veyed into the digestive canals. The internal organs appropriated to nutrition resemble in all essential points those of the Distoma ; they consist of a multitude of blind tubes, hollowed out in the parenchyma of the body, which, when distended with coloured substances, are sufficiently distinct. The principal trunk, (fig* 39, 1,) which communicates with the proboscidiform mouth, soon di- vides into three primary branches ; one of which runs along the median line of the body towards the anterior extremity, whilst the other two are directed backwards towards the tail. From these central canals secondary ones are given off, which permeate all parts of the body. There is no anal aperture, so that of course the residue of digestion is expelled through the mouth ; but the nature of the process by which defecation is thus effected is curious : the Planaria, slightly bending its body, is seen to pump up through its proboscis a quantity of water, with which all the branches of the alimentary ramifications are filled ; the creature then contracts, and, forcibly ejecting the contained fluid, expels with it all effete or useless matter. Besides the arborescent tubes in which digestion is accomplished, a rudimentary vascular system is distinctly visible, by which the STERELMINTHA. 91 nutritive juices are dispersed through the system. This consists of a delicate network of vessels, arising from three large trunks, one placed in the centre of the dorsal aspect, and the other two running along the sides of the animal (Jig. 39, 2). (120.) The Planarise are perfectly androgenous, as each indi- vidual possesses a distinct male and female generative system ; but they are not apparently self-impregnating, as the co-operation of two individuals has been found needful for the mutual fertilization of their ova. In every one of these animals two distinct apertures are seen to exist upon the ventral surface, at a little distance be- hind the root of the proboscis ; the anterior of which gives issue to the male organ, while the posterior leads to the oviferous or fe- male parts. In Planaria tremellaris, the penis, which during copulation is protruded from the anterior orifice, (Jig. 89,* 6,) is a white, con- tractile body, enclosed, when in a retracted state, in a small oval pouch; it is perforated by a minute canal, and receives near its root two flexuous tubes, which gradually decrease in size as they diverge from each other, until they can no longer be traced. These are the seminiferous vessels (Jig. 39, 5, a). The posterior genital orifice, which leads to the female organs, communicates with a small pouch, or uterus, as it might be termed (Jig. 39, 5, b) : into this open two lateral oviducts, which run on each side of the male ap- paratus and of the proboscis ; these are very transparent, and only recognisable under certain circumstances by the ova which they contain. In Planaria lactea the oviduct opens into the uterine cavity by a single tube, which, passing backwards, divides into two equal branches ; and both of these, again subdividing, ramify extensively among the cseca derived from the stomach. We likewise find in this species two accessory vesicles, which pour their secretions into the terminal sac. (121.) The Diplozoon paradoxum is another form, which, though it cannot strictly speaking be classed with the entozoa, is so nearly allied to Distoma in its internal structure, that its anatomy will be most conveniently examined in this place.']" This remarkable animal, as its name imports, is literally pos- sessed of two bodies, precisely resembling each other in every particular, and united by a narrow communicating band, so as to form but one animal, the nutrient canals of one division commu- * This figure represents two Planariae as they appear in the act of sexual inter- course, t Nordmann. STERELMINTHA. 40. nicating most freely with those of the opposite half. We might be led to imagine such an extraordinary arrangement as the result of some monstrous connexion of two separate creatures, did not observation show that the conformation is perfectly natural and common to all the species. Each half of the body of the Diplozoon possesses a mouth and digestive apparatus, a distinct set of vas- cular channels, in which a circulation of the nutritive juices is evident, and more- over contains a com- plete and indepen- dent generative sys- tem ; but in the annexed diagram, (fig. 40,) for the sake of clearness, these are only par- tially shown, the ali- mentary organs alone being seen upon the left portion, whilst in the opposite the organs of reproduc- tion are displayed : the reader, there- fore, will imagine similar parts to exist on both sides of the body. These animals, which are of very small size, being not more than two or three lines in length, are found attached to the gills of the bream, (Cyprinus brama,) from which they absorb nutri- ment. They are fixed in this position by two sucking acetabula, resembling those of Distoma, (6, &,) which are seen on each side of the mouths, and also by four oval membranous appendages (m, m) attached to the opposite extremities of the body, upon which likewise suckers are placed, so that at all four extremities the creature is provided with instruments of adhesion. (122.) The mouths (a, a) are two orifices of a somewhat semi- circular form, and at the lower margin of each two teeth are per- STERELMINTHA. 93 ccptible, which are either merely provisions for fixing the mouth firmly when in the act of imbibing food, or else they may act as lancets, by scarifying the surface from which nourishment is de- rived. From the outer orifice we may trace a canal which extends a little way into the body, and becomes slightly dilated ; into the bottom of this cavity a small tongue-shaped organ (d) is seen to project, having its surface perforated by a number of exceed- ingly minute holes, which indeed might be looked upon as the real mouths destined to imbibe the nutritious juices, and convey them to the stomach. The stomach, (c, c, c, c, c,) which has been partly removed on the right side of the figure, is a wide canal, extending through the whole length of both divisions of the body, and passing by a capacious cross-branch from one half to the other, so that the nutriment taken in by either mouth will pass freely to the opposite side. From these central channels great numbers of blind canals issue, resembling those of Distoma and Planaria, which ramify extensively; there is, however, no anal orifice or outlet for excrementitious matter. (123.) But, besides the ramifications of the alimentary canal, other vessels are discernible, running through the parenchyma of the Diplozoon, where nutritious fluids circulate, and which corre- spond to the vascular arrangement met with in Planaria. Of these the main trunks only are represented in the figure; the branches given off from them, which are very numerous, being for the sake of distinctness entirely omitted. Each half of the body contains four of these vessels, (/, /,) which run from one extremity to the other. In these a fluid is observed to move, running in the directions indicated by the course of the arrows in the diagram ; namely, in two of them from the head toward the posterior end of the body, and in the other two in an opposite direction. This ru- dimentary circulation must be for the purpose of more perfectly diffusing through the system the fluids which result from the process of digestion, and which are probably taken up by imme- diate osculation, between the terminations of the branches from the stomach, and the origins of the vascular system. Upon the opposite side of the figure is given a diagram of the arrangement of the generative apparatus insulated from sur- rounding parts, so as to give the reader a distinct view of the different organs composing it. (124.) As in the two last described species, we find both oviger- ous and impregnating organs constituting complete hermaphrodism, 94 STERELMINTHA. and this not on one side only of the creature, but on both ; all the parts being precisely similar in the two lateral halves. The ovarium is not distinguishable as a distinct viscus, the gems or granular-looking ova (e) being apparently diffused through the parenchyma of the body around the alimentary channels. From this situation the ova are taken up by two long oviducts, which, turning upon themselves near the mouth, are seen to per- form a long course through the anterior part of the body, until at (/) they unite, and immediately expand into a capacious intestini- form cavity, or uterus, (g), from which the eggs escape when mature through a lateral aperture (A). The male or seminiferous apparatus is quite unconnected with the female organs, and its structure is easily distinguishable. The testicle (i) is a small pear-shaped vesicle, from which a duct may be traced, which ends in a long cirrus (Ar), represented in the figure as coiled up in a spiral form ; but when unrolled it is of considerable length, and analogous both in structure and office to the male organ of Distoma. (125.) We now arrive at the most perfect type of structure found in the Parenchymatous Entozoa, which leads us by a gradual trans- ition to the more highly organized forms which are possessed of a distinct nervous apparatus. The reader will observe that in all the preceding genera the alimentary canal has consisted en- tirely of nutritive canals excavated in the substance of the body, and unprovided with any outlet distinct from the mouth adapted to the discharge of the residue of digestion. From the nature of their food, indeed, we might be led to infer the reason of such a structure ; for living, as these creatures do, upon juices already completely animalized and prepared for the purposes of nutrition, the assimilation of the materials provided for them constitutes nearly the entire process of alimentation. The same con- formity to one type has been also visible in the nature of the reproductive system ; all the species which we have as yet ex- amined, except perhaps the Planarise, having possessed indepen- dent powers of propagation, either containing no visible organs appropriated to the developement of the germs which they produce, or possessing both an ovigerous and impregnating apparatus combined in the same body. The Entozoa acantho- cephala, of which we are now about to speak, will be found still to exhibit a digestive system analogous in structure to that which exists universally among the Sterelmintha, but in the organs 8TEEELMINTHA. 95 of reproduction we find a mani- fest analogy with higher classes in- dicated in the complete separation of the sexes, which we now for the first time meet with, the ovig- erous and impregnating organs be- ing found in separate and distinct individuals. The Echinori/nchus gigas is the species which has undergone the most complete investigation,* and will serve as an example of the usual structure of the Acanthocephala. (126.) The Echinorynchi inhabit the intestinal canal of various ani- mals, to the walls of which they fasten themselves by a singular contrivance. In the animal under consideration, which is found in the intestines of the hog, the head (a, fig. 41 ; 1, 2, 3) is represented by a retractile pro- boscis, armed externally with four circlets of sharp recurved hooks, which, when plunged into the coats of the intestine, serve as secure an- chors by which the creature retains itself in a position favourable to the absorption of food. In Jig. 41, 1, 2, this aculeated proboscis is repre- sented of its natural size relative to the body of the entozoon, as it ap- pears when fully protruded ; but, when not in use, the spinous part is retracted, and concealed by the mechanism, of which an enlarged view is given at Jig. 3. When extended, the position of the organ is indicated by the dotted lines ; but in the drawing the whole or- gan is represented as drawn inwards * Cloquet, Anatomie des Vers intestinaux. Fig. 41, \rv R Paris, 1824. 96 STEKELMIXTHA. and lodged in a depression formed by the inversion of the in- tegument, so as completely to hide it within the body. This inversion is produced by the contraction of two muscular bands, (d, e,)* which arise from the inner walls of the body, and are inserted into the root of the proboscis around the oesophagus : two other muscles, (6, 6,) antagonists to the former, arise near the spines themselves ; and these, aided by the contractions of the walls of the body, are the agents by which the protrusion of the head is effected. Although the teeth or spines, which render this organ so formidable, are merely epidermic appendages, they are found to be rendered erect or depressed at the will of the creature ; and it is therefore probable that, minute as they are, they have muscular fibres connected with them serving for their independent motions : these spines, moreover, are not always confined to the head ; but in many intestinal worms are found on various parts of the body, wherever their office as instruments of attachment is by circumstances rendered needful. (127.) The digestive system of the Echinorynchus is extremely simple. The mouth is a minute pore placed at the extremity of the proboscis, which communicates with two slender canals, (f,f,) at first of great tenuity, but towards the middle of the body assum- ing something of a sacculated appearance. Towards the tail these vessels gradually diminish in size until they are no longer distinguishable ; but they have not been seen to give off any branches, or to communicate with each other. Near the origin of these nutrient tubes are two large caeca, nearly an inch in length, called lemnisci, (fig. 41, 1 and 2, d, d,) which are probably connected with the digestive function. (128.) The female Echinorynchus is, as is usually the case in Dioecious Entozoa, considerably larger than the male, as may be seen in the figure. In the former (fig- 41, 1) the ovary (c) is a capacious organ occupying the centre of the body, and extending along its entire length. When minutely examined, it is found to consist of two compartments or distinct sacs, one occupying the dorsal, the other the ventral aspect ; the two tubes being separated by a septum. The dorsal ovary commences near the tail, at g, by a cul-de-sac ; and, enlarging as it runs forward, terminates near the point c, by uniting with the ventral portion. The anterior part of the canal (b) is common to both divisions of the ovary ; and from this the ventral tube runs backwards to the posterior end of * These muscles are seen of their natural size in fig. 1 at e, e. STERELMINTHA. 97 the body, where it ends in a narrow duct, which opens externally at h. It would seem therefore that the last-mentioned opening is the only excretory passage from the ovarium ; the connection apparent in the figure, between the common sac (b) and the root of the proboscis, being merely of a ligamentous character. (129.) The generative system of the male Echinorynchus is re- presented in fig. 41, 2. The organs which secrete the fecundating fluid (y, g) are two cylindrical vesicles attached at one extremity by minute filaments to the walls of the body : from each of these arises a duct (A), and the two, uniting at t, form a common excretory canal. This canal speedily dilates into a number of sacculated receptacles in which the secretion of the testes accumulates, and from them a duct leads to the root of the penis (m). The penis or organ of intromission, when extended, protrudes through the aperture p, placed at the anal extremity of the body ; but when retracted it is folded up, and lodged in a conical sheath (o). The protrusion and retraction of this part of the male apparatus is effected by a very simple mechanism : two muscles, (/, /,) arising from the inner walls of the body, are inserted into the base of the sheath, (m,) and serve to draw it inwards ; and two others, (w, w,) inserted at the same point, but arising from the posterior ex- tremity of the animal, by their contraction force outwards the intromittent organ, an arrangement precisely corresponding with that by which the movements of the proboscis are provided for. (130.) In Distoma perlatum (fig. 42), we have another example of organization intermediate be- tween that which is most usual among the STERELMINTHA, and what we shall afterwards meet with in the more perfect entozoa. The animal in question resembles most closely in its outward form the liver-fluke of which we have already spoken, and possesses a similar suctorial apparatus. In the annexed figure (fig. 42), the oral disc only is seen, the ventral sucker having been removed for the sake of displaying the interior of the animal, as in the diagram of 98 STERELMINTHA. Distoma hepaticum already given (Jig. 38). On comparing the two we are at once struck with the superior concentration of all the systems of the body, visible in Distoma perlatum. The ali- mentary canal (Jig. 42, ) commences, as in the former example, by an aperture situated in the oral sucker ; but, instead of ramifying through the parenchyma of the body, is contained in an abdo- minal cavity, in which it floats in common with the other viscera. The oesophagus (a) is a simple flexuous tube terminating abruptly in two lateral and more capacious intestines, (b, b,) terminated by blind dilated extremities, which form the digestive apparatus. Two vascular canals (d, d) are seen on each side of the body, which ramify extensively, but of these the principal trunks only are represented. (131.) The Distoma perlatum is allied to the STERELMINTHA in the hermaphrodism of its generative organs, and the parts subservient to reproduction will be found analogous in structure and arrange- ment to what we observed to be the usual conformation in that order. The ova would seem to be produced in the parenchyma of the body, as in the fluke ; from this situation they are con- veyed by two canals (e) into a capacious receptacle (/), from which arises the tortuous oviduct (g), represented in the en- graving distended with eggs. Near its termination the oviduct is joined by two secerning vesicles having their interior appa- rently of a villous texture. These vesicles are regarded as being the testes, and are supposed to pour out an impregnat- ing secretion, by which the ova are rendered fertile as they pass out of the body. The external aperture through which the eggs are discharged is placed upon a prominent tubercle (t), which, if mutual impregnation is essential in these animals, may indeed perform the office of an intromittent instrument. CffiLELMINTHA. 99 CHAPTER VII. NEMATONEURA. CffiLELMINTHA* (Owen). Vers Intestinaux cavitaires (Cuv.) ; Nematoidea (Rudolplii). The entozoa which belong to the nematoneurose division of the animal kingdom have long been separated in zoological clas- sification from those which have been described in the last chapter, on account of the superiority of their internal organization. In the STERELMINTHA, or parenchymatous forms, we have seen the digestive process carried on in canals simply excavated in the sub- stance of the body, without any anal outlet for the discharge of superfluous matter; the nervous system either perfectly diffused through the tissues, or but obscurely visible even in the most per- fect species, and the muscular tissue, as a necessary consequence, scarcely aggregated into distinct fibres : the sexes, moreover, except in the Echinorynchi, which form the transition from the more im- perfect to the more elevated type of structure, have been invariably combined in the same individual. But we now arrive at a point in the scale of animal developement at which the nervous fibre be- comes for the first time distinctly recognisable, forming a more perfect means of intercourse, if we may be allowed the expression, between the different parts of the body ; the muscular contractions, being thus more intimately associated, assume far greater energy, and muscular fasciculi are distinguishable, arranged in precise and definite directions ; the alimentary canal is visible as a separate and distinct tube, enclosed with other viscera in an abdominal cavity; and the ovigerous and impregnating sexual organs are found to exist in different individuals. Still, however, we find no nervous centres developed, or the ganglia which exist are so extremely minute and rudimentary that in no case can we suspect the existence of organs appropriated to the higher senses ; the sensations of all the tribes composing this division of the animal world are therefore apparently limited to the generally diffused * xo7*.o, hollow 'fafuvs-ivfaf, a worm. H 100 CCELELMINTHA. Fig. 43. sense of touch and its modifications, to which the perception of taste and odours must be referred. (133.) The Linguatula teenioides (fig. 43, 1) is the first example which we shall select to illustrate the structure of the Ccelel- mintha. This ento- zoon, which is gene- rally found to inhabit the frontal sinus of quadrupeds, is about three inches in length, and as many lines in breadth, at the broadest part of its body. In external form it has some resemblance to the tape- worm, being divided into slightly im- bricated segments; but in its internal structure it is widely different, especially as relates to the arrangement of the generative organs, which, instead of being multiplied until they are nearly as numerous as the segments of the body, ( 117,) form but one continuous system. The Linguatula is invested externally with a delicate cuticle, easily separable by maceration, so as to peel off as represented in the figure.* (134.) Around the mouth (fig. 43,1, a), are several oval pits or cavities containing as many sharp, recurved hooks by which the an- terior extremity of the body is securely attached to the walls of the frontal sinus, and the mouth retained in a position adapted to secure an adequate supply of nutritive material. The mouth itself is a simple aperture, from which a short and narrow ossophagus leads to a dilated cylindrical stomachal cavity, (fig. 43, 2, a,) that forms a somewhat capacious receptacle for food ; to this succeeds a straight intestinal tube (/}> which tra- Owen, Transact. Zool. Soc. vol. i. C(ELELMINTHA. 101 verses the whole length of the body, and terminates by an anal aperture at the extremity of the tail. (135.) The nervous system of the Linguatula is distinctly de- veloped. It consists of a central ganglion, situated beneath the oeso- phagus^ from which eight pairs of nervous filaments proceed in dif- ferent directions : of these the greater number are distributed to the parts immediately around the mouth, but the posterior pair (o, o), which is by far the most considerable in size, runs backwards along the ventral aspect of the body, taking first a wavy or serpentine course, but afterwards becoming straight ; these nerves may be traced for some distance until they are gradually lost in the integuments, to which they are distributed. It will be seen that in such a condition of the nervous apparatus, we have a type of structure decidedly superior to what has been observed in any of the parenchymptous entozoa, and adapted to the situation in which the Linguatula is generally found ; a situa- tion which allows of considerable change of position, and of some selection as regards the food which it imbibes. The muscular movements, therefore, being more perfectly associated by the de- velopement of nervous filaments, exhibit a greater energy of action ; and although the nervous matter is not as yet sufficiently concen- trated to allow of the possession of organs appropriated to the higher senses, there is provision made by the developement of the rudimentary sub-oesophageal ganglion for more delicate sensibility in the neighbourhood of the mouth, adequate, no doubt, to the perception and choice of such aliment as may be best adapted to nutrition. (136.) The female Linguatula^ as is generally the case among the dioecious entozoa, is considerably larger than the male. The generative organs exhibit a peculiar arrangement, and form nu- merous convolutions in the body, which are visible through the semi-transparent integument (Jig. 43, 1). The ovary (Jig. 43, 2, g) is a narrow, minutely granulated body, running along the two anterior thirds of the dorsal aspect of the body. It terminates about half an inch from the head in two capillary tubes (c c), which pass on each side of the stomach and nervous cords, embracing them as in a ring. These two tubes unite behind the mouth into a common canal or oviduct, through which the eggs escape ; but, before their junction, each receives a duct derived from a glandular sacculus (e, e), destined no doubt to furnish some secretion essential to the completion of the ova. 102 CGELELMINTHA. The oviduct formed by the junction of the oviferous canals which embrace the oesophagus, is very narrow at its commencement, but after running backwards for some distance it dilates a little, and, becoming much convoluted, it winds around the alimentary tube in numerous and extremely complex gyrations (d). Towards the lower third of the body, the coils become less numerous and more distant from each other, and are seen to contain brown ova in scat- tered masses, until at length the oviduct assumes a course parallel to that of the intestine (e), and accompanies it to the anus, in the vicinity of which it terminates. The ova are of a firm resisting texture, and do not lose any of their form or contour by drying ; hence they may preserve their vitality for a long period under very different circumstances, and be ready to assume the actions of developement when deposited in a fit situation. (137.) In the male Linguatula, the structure of the generative apparatus is very simple. Two long convoluted tubes, which float loosely in the abdominal cavity, secrete the seminal or impreg- nating fluid ; and these tubes, which may be called the testes, ter- minate by forming a single canal or vas deferens, leading to the external organs appropriated to sexual union, which are two fili- form appendages found in the neighbourhood of the head, through which the fecundating secretion is expelled. (138.) The only other example which will be necessary to illustrate the structure of the CCELELMINTHA, is an evident approximation to the annulose type of animal organization. The Ascaris lumbri- coides indeed, as its name imports, so strongly resembles some of the annelida in its external configuration, that the zoologist who should confine his attention to outward form alone, might be tempted to imagine the affinities which unite them much stronger than a comparison of their anatomical relations would sanction. This entozoon is found in the intestines of many animals, and is endowed with some considerable capability of locomotion adapted to the circumstances under which it lives ; for in this case the worm, instead of being closely imprisoned in a circumscribed space, may traverse the entire length of the intestines in search of a con- venient locality and suitable food. (139.) In accordance with such an enlarged sphere of existence, we observe muscular fibre distinctly recognisable in the tissue which composes the walls of the body, not as yet indeed exhibiting the complete characteristics of muscle as it is found in higher CffiLELMINTHA. 103 animals, but arranged in bundles of contractile filaments, run- ning in determinate directions, and thus capable of acting with greater energy and effect in producing a variety of move- ments. In this rudimentary state, the muscular fibre does riot possess the density and firmness which it acquires when completely de- veloped ; it has, when seen under the microscope, a soft gelatinous appearance, apparently resulting from a deficiency of fibrin in its composition ; the transverse striae, usually regarded as characteristic of the muscular tissue of the more perfect animals, are not yet distinguishable, and the individual threads are short, passing over a very small space before they terminate. On examining the arrangement of these fasciculi, they are seen to be disposed in two layers, in each of which they assume a different course ; thus in the outer layer they are principally arranged in a longitudinal direction, while the inner stratum of fibres is placed transversely, affecting a spiral course, so as to encircle the viscera. From this simple structure various movements result ; by the action of the longitudinal fasciculi the whole body is shortened, by the contractions of the spiral layer an opposite effect is produced, or by the exertion of circumscribed portions of the muscular in- tegument lateral flexions of the body are effected in any given direction. These motions in the living worm are vigorous and easily excited by stimuli ; they are therefore abundantly sufficient for the purpose of progression in such situations as those in which the creature lives, and enable it to change its place in the intes- tines with facility. (140.) The nervous system of the Ascaris is strictly conformable to the nematoid type. Around the mouth or anterior part of the oesophagus, there appears to be a delicate nervous ring, probably specially connected with the association of such movements of the oral extremity as are essential to the imbibition of nourish- ment. From this oral ring proceed two long nervous filaments, (fig. 44, e, e,) one of which runs backwards along the dorsal aspect of the body, while the other occupies a similar position upon the ventral surface. The last-named filament is described by Cloquet as dividing in the female Ascaris, at the point where the termination of the organs of generation issue from the body (fig. 44, w), so as to enclose the termination of the vagina in a nervous circle. (141.) The digestive apparatus in this order of intestinal worms is 104 CCELELMINTHA. very simple. In Ascaris lumbricoides, (Jig. 44, a,) when highly magnified, is seen to be surrounded by three minute rounded tubercles ; into each of these, fasciculi, derived from the longitudinal muscles of the body, are inserted in such a manner as to cause the separation of the tuber- cles, and consequent opening of the mouth, which is again closed by a sphincter muscle provided for the purpose. To the mouth succeeds a short 03sophagus, (Jig. 44, 1 & 2, &,) which is separated by a constriction from the rest of the alimentary canal, and would seem, from the muscularity of its walls, to be an agent employed in sucking in the liquid food upon which the crea- ture lives. The true digestive ca- vity (Jig. 44, 1 & 2, c, c) is a sim- ple and extremely delicate tube, which arises from, the oesophagus, and without presenting any appear- ance indicative of separation into stomach and intestine, gradually en- larges as it proceeds backwards, until it terminates at the hinder extremity of the body by a narrow aperture (Jig. 44, 1 & 2, d.) It would seem that the food of these entozoa being already ani- malized by having undergone a pre- vious digestion, requires little further preparation ; and we are little sur- prised at finding in the generality of the Coelelmintha no accessory glan- dular apparatus appended to the di- gestive canals for the purpose Lof furnishing auxiliary secretions. In two species only have tributary the aperture of the mouth, Fig. 44. f k 1/r K I 1 CGELELMINTHA. 105 secreting organs been detected ; in one example, Gnathostoma aculeatum, (Owen,) found in the stomach of the tiger, and which is remarkable as possessing a pair of rudimentary jaws, four slen- der elongated caeca are appended to the mouth, into which they pour a fluid analogous, no doubt, to that of the salivary glands.* In a species of ascaris, found in the stomach of the dugong, Mr. Owen likewise discovered a csecal appendage opening into the ali- mentary tube at some distance from the mouth, and which, without much stretch of imagination, may be regarded as the first and sim- plest rudiment of a biliary system.')' In further prosecuting our inquiries concerning the process of nutrition in these entozoa, we must now speak of a peculiar structure first noticed by Cloquet,j and apparently intimately connected with the assimilation of nutriment. Projecting from the inner surface of the abdominal cavity, especially in the dorsal and ventral regions, there is a great number of gelatinous, spongy processes (appendices nourriciers) , which, although they have no apparent central cavity, would seem to be appended to vascular canals seen upon the lateral aspects of the body : it is probable, therefore, that their office is to absorb the nutritive juices, which exude through the delicate walls of the intestine, and convey them into the circulatory apparatus ; or they may be reservoirs for nou- rishment, analogous to the adipose tissue of higher animals. (142.) In the Ccelelmintha the sexes are separate, and the genera- tive organs, both of the male and female, exhibit great simplicity of structure. In the female Ascaris, the aperture communicating with the ovigerous apparatus is placed upon the ventral aspect of the body, a little anterior to the middle of the worm (Jig. 44, 1, ra). This opening leads into a wide canal (/), usually called the uterus ; and from the last-mentioned organ arise two long and undulating tubes, which, diminishing in size, run towards the posterio ex- tremity, where they become completely filiform, and turning back upon themselves are wound in innumerable tortuous convolutions around the posterior portion of the alimentary canal, until the termination of each becomes nearly imperceptible from its extreme tenuity. In these tubes, which when unravelled are upwards of four feet in length, the ova are formed in great numbers, and are found to advance in maturity as they approach the dilated * Owen, Proceedings of the Zoological Society, Nov. 1836. t Preparation, No. 429 A. Mus. Coll. Surg. Phys. Catalogue, p. 121. t Cloquet, Anatomic des Vers Intestinaux ; Paris, 1824. 106 CCELELMINTHA. terminal receptacle common to both oviducts (/), from which they are ultimately expelled. (143.) The male Ascaris lumbricoides is considerably smaller than the female, and the. structure of its generative system remarkably similar to what has been just described in the other sex. The testis or gland, which secretes the impregnating fluid, is a single, delicate, tubular, filament (fig. 44, 2,jf), which when unravelled is found to be nearly three feet in length, and is seen winding in close and almost inextricable folds around the middle and hinder parts of the intestine. The termination of this tube (g) may be traced to the tail or anal extremity of the worm, where it ends in a fila- mentary retractile penis (z), in which the microscope exhibits a minute receptacle wherein the seminal fluid accumulates prepara- tory to its expulsion. During copulation, the penis of the male is introduced into the vulva of the female, by which it is firmly embraced, and the different positions which the external parts occupy in the two sexes is evidently an arrangement favourable to their intercourse. (144.) There are few more striking exemplifications of that gra- dual transition by which we are led from one type of structure to another, than we meet with in tracing the progressive separation of the sexes as we advance from the monoecious to the dioecious families of the entozoa. Leaving those forms of hermaphroditism in which the male and female parts are both found in each division of the body, we find in Syngamus trachealis an animal " in which the male is organically blended by its caudal extremity with the female, immediately anterior to the slit-shaped aperture of the vulva, which is situated as usual near the anterior third of the body. By this union a kind of hermaphroditism is produced ; but the male apparatus is furnished with its own peculiar nutrient system, and an individual is constituted distinct in every respect, save in its terminal confluence with the body of the female. This condition of animal life, which was conceived by Hunter as within the circle of physiological possibilities, (see Animal (Economy, p. 46,) has hitherto been only exemplified in this single species of entozoon, the discovery and true nature of which is due to the sagacity and patient research of Dr. Charles Theodore Von Siebold."* * Cyclop, of Anat. and Phys. ; article Entozoa, by Professor Owen. Vide Siebold, in Weigmann's Archives, 1835. BRYOZOA. 107 CHAPTER VIII. BRYOZOA* (Ehrenberg) ; CJLIOBRACHIATE POLYPI (Farre). (145.) It is only within the last few years that microscopical re- searches have revealed to naturalists the real structure of a series of animals originally confounded with the simpler polyps, with which, as far as external form is concerned, they are indeed intimately related. The observations of Milne Edwards,*f* Audouin, Ehren- berg,^: and Thompson, gradually led the way to more correct and precise ideas concerning the more highly organized genera ; and Dr. Arthur Farre, || by a series of investigations, followed up with exemplary industry and perseverance, seems to have com- pleted our knowledge of the anatomical details of these creatures, in a manner which leaves few points of their economy unknown. We shall select an in- dividual, named by Dr. Farre JBowerbankia den- sa 9 as an illustration of the general structure of the BRYOZOA, partly from the complete manner in which its organization has been developed in the me- moir alluded to, and partly because we have had fre- quent opportunities of ve- rifying the accuracy of the descriptions, and the ex- treme fidelity of the draw- ings by which it is illus- trated. The animal Bowerban- kia, which is only about a line in length, inhabits a Fig. 45. sea-moss Zuov, an animal. t Annales des Sciences Naturelles, for Sept. 1828, and July 1836. $ Symbolae Physicae. Zoological Researches and Illustrations, Memoir v. ; Cork, 1830. || Philosoph. Trans. Tart 2, for 1837. 108 BRYOZOA. delicate and perfectly transparent tube of horny texture, which arises from a repent stem, common to a great many individuals, found aggregated in small patches upon the surface of Flustra foliacea, upon which they are apparently parasitic. The mouth is surrounded by ten long and slender tentacula, (Jig. 45,) which, during the expanded state of the animal, are kept quite straight and motionless, as represented in the drawing. Each tentacle is provided upon its outer aspect with a series of stiff and immoveable spines, probably serving to keep off any foreign bodies, which, by their proximity, might interfere with the ciliary movements immediately to be described. Besides the stiff spines, the tentacula are covered with an im- mense number of vibrating cilia, which at the will of the animal are thrown into most rapid movement, so as to produce strong and continuous currents in the surrounding fluid, by which particles floating in the neighbourhood are hurried along with great velocity. From the direction of the streams produced by the cilia, namely, towards the mouth, we at once perceive the utility and beauty of the contrivance which compensates to a great extent for the fixed condition of the Bryozoon ; animalcules floating in the vi- cinity no sooner come within the influence of the currents so pro- duced, than they are forced towards the mouth, which is placed at the roots of the tentacula, and, being at once seized, are imme- diately swallowed. The tentacula themselves, notwithstanding their immobility dur- ing the process of watching for prey, are highly irritable, and sensi- ble of the slightest contact. No sooner does an animalcule im- pinge upon any part of their surface, than the tentacle touched bends with extraordinary quickness, as if endeavouring to strike it towards the mouth ; and, if the object be sufficiently large to touch several at the same moment, all the tentacula simultaneously co- operate in seizing and retaining it. (146.) The existence of the cilia upon the tentacula would seem to be characteristic of the BRYOZOA, and is invariably accompanied, as far as our information extends at present, with a digestive apparatus of far more complex structure than what we have seen in the un- ciliated polyps, for in the class before us, besides the stomach, we find a .distinct intestinal tube and anal outlet. In the specimen under consideration the organization of the alimentary organs is even rendered more elaborate than is usual in the class, from the addition of a gizzard or cavity in which the food is mechanically BRYOZOA. 109 bruised before its introduction into the proper stomach. The mouth is placed in the centre of the space enclosed by the tenta- cula ; it appears to be a simple orifice, incapable of much distension, through which the particles of food brought by the ciliary action pass into a capacious oesophagus, (Jig- 45, a, 1, 2,) which, gradually contracting its dimensions, ends in a globular muscular organ to which the name of gizzard has been applied. (8) The walls of this viscus are composed of fibres which radiate from two dark points seen in the figure, and its lining membrane is covered with a great number of hard horny teeth, so disposed as to represent, under the microscope, a tesselated pavement. The contractions of the giz- zard are vigorous ; and, from the structure of its interior, its office cannot be doubtful. To the gizzard succeeds a stomach (j^^.45, #, 4,) which is studded with brown specks apparently of a glandular nature, and probably representing a biliary apparatus. The intestine leaves the stomach at its upper portion, close to the gizzard (5) ; and, running parallel with the oesophagus towards the tentacula (6), terminates at the side of the mouth (7), in such a position that excrementitious matter is at once whirled away by the ciliary currents. The whole intestinal apparatus floats freely in a visceral cavity, which contains a transpa- rent fluid, and encloses distinct muscular fasciculi, which we shall speak of in another place. The process of digestion in this minute, yet highly organized being, is well described by Dr. Farre in the memoir above mentioned. " The little animal, when in vigour, is seen projecting from its cell with the arms extended, and the cilia in full operation ; the upper part of the body being frequently turned from side to side over the edge of the cell, the extremity of which, from its peculiar flexibility, moves along with it. The particles carried to the mouth in the vortex produced by the action of the cilia, after remaining a little while in the pharynx, are swallowed by a vigorous contraction of its parietes, and carried rapidly down the oesophagus and through the cardia to the gizzard, which expands to receive them. Here they are submitted to a sort of crushing operation, the parietes of the organ contracting firmly upon them, and the two dark bodies being brought into opposition. Their residence, however, in this cavity, is only momentary, and they are immediately propelled into the true stomach below, where they become mixed up with its contents, which, during digestion, are always of a dark, rich, brown colour, being tinged with the secretion of its parietal follicles." 110 BRYOZOA. The food appears to be retained for a considerable time in the stomach, and may be frequently seen to be regurgitated into the gizzard, whence, after having been again submitted to its opera- tions, it is returned to the stomach. Here it is rolled about by the contraction of its parieties, and at its upper part is frequently sub- mitted to a rotating motion. This rotation of particles is chiefly near the pyloric orifice ; and a mass may be occasionally seen pro- jecting through the pylorus into the intestine, and rotating rapidly in the direction of the axis of the orifice. In an animal having a similar form of pylorus to this, but in which the parts were more transparent, I could distinctly see the cilia by which this rotation is effected surrounding the orifice." The granular matter, after rotating for some time at the pylorus, (a provision for preventing its too rapid escape from the stomach,) passes into the intestine, where it accumulates in little pellets, which are rapidly pushed by the contraction of the intestine to- wards the anal orifice, through which they are expelled from the body. The tube or cell inhabited by this bryozoon is of exquisite struc- ture, and the mechanism concerned in the protrusion and retraction of the animal of great simplicity and beauty. The inferior two-thirds of the cell in the species under considera- tion is hard and corneous, but perfectly transparent : the upper third, on the contrary, is flexible, and so constructed as to form a very complete operculum by which the entrance is guarded. The flexible part consists of two portions, the lower half being a simple continuation of the rest of the cell, while the upper is composed of a circle of delicate bristle-shaped processes or setse, which are ar- ranged parallel to each other around the mouth of the cell, and are prevented from separating beyond a certain distance by a membrane of excessive tenuity which connects them ; this membrane is evi- dently analogous to the infundibular termination of the cells of polyps already described. When the bryozoon retires into its abode, the setse and soft ter- mination of the cell are gradually folded inwards in the manner exhibited in the annexed figures (Jig. 46), which represent the various stages of the process. The esophagus surmounted by its tentacula descends first, whilst the integument of the upper part of the body begins to be inverted at the point where it has its insertion around the base of the tentacles (c). As the descent of the tenta- cula proceeds, the inversion of this membrane continues ; and when the extremities of the arms have reached the level of the extremi- Ill 4 321 ties of the setse, it is seen to form a complete sheath around them. The animal being thus retracted, the next part of the process is to draw in the upper portion of the cell after it. The setse are now brought together in a bundle (jig. 46, Q, ), and are gradually drawn inwards, inverting around them the rest of the flexible por- tion of the cell until they form a close fasciculus (fig. 46, 3 & 4, a), occupying the axis of the opening of the tube, and forming a com- plete protection against intrusion from without. (147.) The muscular system exhibits the earliest appearance of muscular fibre. The filaments are unconnected by cellular tissue, and have a watery transparency and smooth surface, neither do they exhibit cross markings or a linear arrangement of globules, even when examined under the highest powers of the microscope. The muscles may be divided into two sets, one serving for the retraction of the alimentary apparatus, the other acting upon the setse around the mouth of the cell, and serving for the inversion of its flexible portion. The bundles of muscular fibre which act upon the alimentary canal are two in number, and arise from near the bottom of the cell : one of these is inserted into the stomach (fig. 45, a, 8) ; the other passes upwards along the side of the oesophagus (fig. 45, a, 9), to be attached in the vicinity of the tentacula : the latter fasciculus is evidently the great agent in drawing the animal into its retreat, and in doing so it throws the alimentary canal into close sigmoid folds. The muscles which close the operculum are arranged in six distinct fasciculi ; they arise from the inner surface of the upper hard part of the cell, and act upon the upper flexible portion of the tube and upon the setse (fig. 46, d, d). The mode in which the protrusion of the tentacula is effected is BRYOZOA. not so easily explained ; it would seem that the lining membrane of the shell is furnished with circular muscular fibres, so disposed as by their action to compress the fluid contained in the visceral cavity, and thus tend to elongate the body. Dr. Farre, however, believes the alimentary canal itself to be the great agent in effecting this object, and he conceives it to possess a power of straightening itself from the flexures into which it is thrown during the retracted state of the animal. (148.) The FLUSTILE and ESCHARS are intimately allied to Bowerbankia in all the details of their structure, as we are assured by the researches of Dr. Milne Edwards concerning these singularly aggregated forms of Bryozoa.* The cells of the Flustrte and Eschara are disposed side by side upon the same plane, so as to form a common skeleton of a coriaceous or horny texture. The individual cells, which are ex- tremely minute, vary in shape in different species ; and the orifice of each is generally defended by projecting spines, or sometimes by a moveable operculuin, or lid, which closes the orifice in the contracted state of the animal. The extension of one of these skeletons is effected by the regular addition of new cells around the circumference of the Flustra, those of the margin being, of course, the most recent; and the latter are not unfrequently found inhabited by healthy animals, whilst in the older or central ones the original occupants have perished. The facts which have been observed relative to the formation of these cells possess a high degree of interest, and materially support the views already given concerning the formation of the tubes of zoophytes in general ; proving that the calcareous matter to which their hardness is owing is not a mere exudation from the surface of the animal, but is deposited in an organized tegumentary membrane, from which it can be removed with facility by means of extremely dilute muriatic acid. When so treated, a brisk effervescence is pro- duced, the cells become flexible and are easily separated from each other, but they are not altered in form, and evidently consist of a soft and thick membrane, forming a sac containing the digestive organs of the creature. In this state the opening of the cell is no longer de- fined as it was before, but the membranous cell appears continuous with the tentacular sheath. We see, therefore, that in these crea- tures the cell is an integrant part of the animal itself, not a mere * Recherches Anatomiques, Physiologiques, et Zoologiques sur les Eschares. Ann. des Sciences Nat. for 1836. BRYOZOA. H3 calcareous crust moulded upon the surface of the body, being a portion of the tegumentary membrane, which, by the molecular deposit of earthy matter in its tissue, ossifies like the cartilage of higher animals without ceasing to be the seat of nutritive move- ment. It is evident, likewise, that what is called the body of the Bryozoon constitutes, in fact, but a small portion of it, principally consisting of the digestive apparatus. As to the operculum destined to close the entrance of the tegumentary cell, it is merely a lip-like fold of the skin, the marginal portion of which acquires a horny consistence ; while, at the point where it is continuous with the general envelope, it re- mains sufficiently soft and flexible to obey the action of the mus- cles inserted into it. (149.) The tegumentary sac, deprived of its carbonate of lime, seems to be formed of a tomentous membrane, covered, especially upon its outer side, with a multitude of cylindrical filaments disposed perpendicularly to its surface, and very closely crowded together. It is in the interstices left by these fibres that the calcareous matter appears to be deposited ; for, if a transverse section be examined with a microscope, the external wall is seen not to be made up of superposed layers, but of cylinders or irregular prisms arranged perpendicularly to the axis of the body. But the above are not the only arguments adduced by Milne Edwards in confirmation of our view of the mode in which these skeletons are held in vital connection with the animal. On ex- amining the cells at different ages, it is found that they undergo material changes of form. This examination is easily made, since in many species the young spring from the sides of those first formed, and do not sepa- rate from their parents ; each skeleton, therefore, presents a long series of generations linked to each other, and in each portion of the series the relative ages of the individuals composing it are indicated by the position which they occupy. It is sufficient, therefore, to compare the cells situated at the base, those of the middle portion, those of the young branches, and those placed at the very extremi- ties of the latter. When examined in this manner, it is seen that not only does the general configuration of the cells change with age, but also that these changes are principally produced upon the external surface. For instance, in the young cells of Eschara cervicornis, the subject of these observations, the walls of which are of a stony hardness, the external surface is much inflated, so that the cells are 114 BRYOZOA. very distinct, and the borders of their apertures prominent ; but by the progress of age their appearance changes, their free surface rises so as to extend beyond the level of the borders of the cell, and defaces the deep impressions which marked their respective limits. It results that the cells cease to be distinct, and the skeleton pre- sents the appearance of a stony mass in which the apertures of the cells only are visible. It appears evident, therefore, that there is vitality in the sub- stance composing these stony walls ; and the facts above narrated appear only explicable by supposing a movement of nutrition like that which is continually going on in bone. (150.) The anatomy of these Bryozoa differs slightly from that of Bowerbankia. The crown of ciliated tentacula is inserted into the extremity of a kind of proboscis, which is itself enclosed in a cylindrical retractile sheath. From the margin of the opening of the cell arises a membrane equalling in length the contracted ten- tacles, and serving to enclose them when the animal retires into its abode. These appendages, thus retracted, are not bent upon them- selves, but perfectly straight and united into a fasciculus, the length of which is nevertheless much less than that of the same organs when expanded. By the opposite extremity to that fixed to the margin of the opening of the cell, the tentacular sheath unites with a tolerably capacious tube, the walls of which are exceedingly soft and deli- cate ; and near the point of their union we may perceive a fasciculus of fibres running downwards to be inserted upon the lateral walls of the cell : these fibres appear to be striated trans- versely, and are evidently muscular ; their use cannot be doubted : when the animal wishes to expand itself, the membranous sheath above alluded to becomes rolled outwards, everting itself like the finger of a glove as the tentacles advance. The muscular fasciculi are thus placed between the everted sheath and the alimentary canal, and by their contraction they must necessarily retract the whole within the cell. The first portion of the alimentary tube is inflated, and much wider than the rest ; it forms a kind of chamber, in which the water set in motion by the vibration of the cilia upon the tentacles appears to circulate freely. The walls of this chamber are ex- tremely delicate ; the soft membrane forming them is puckered, and appears traversed by many longitudinal canals united by minute transverse vessels ; this appearance, however, may be deceptive. BRYOZOA. 115 Beneath the first enlargement, the digestive apparatus becomes narrower, but immediately expands again, and offers at this point a certain number of filiform appendages, which appear to be free and floating in the interior of the cell. To the second cavity succeeds a narrow canal, opening into a third dilatation, generally of a spherical form. From the last-named viscus issues a kind of intestine, which soon bends upon itself and be- comes attached to an organ of a soft and membranous texture, having the appearance of a caecum, and which seems to be con- tinuous superiorly with the digestive tube ; the latter continues its progress towards the upper part of the cell, and ultimately ter- minates by a distinct anal aperture upon the upper aspect of the tentacular sheath. The operculum which closes the cell in Flustra and Eschara is moved by two muscular fasciculi inserted into the internal face of this valve by the intermedium of two filaments analogous to ten- dons : by their inferior extremity, these muscles are attached to the walls of the cell ; and when, by its own elasticity, the operculum is turned back, and the mouth of the cell thus opened, they, by their contraction, can close it like a door. (151.) A very singular form of Bryozoon is met with in fresh water, of which the Cristatella Mucedo* is an example that has undergone minute investigation. The Cristatella (Jig. 47, 3) consists of a common body or enve- lope (d), which is Fig. 47. membranous, and slightly cordiform ; its surface is tubercu- lated,and it is incapa- ble of con traction. In this outer covering several individuals are contained, but, al- though produced from one another, they are only aggregated, be- ing lodged in distinct tubular cells. The body of each animal appears to consist of a digestive canal, constricted once or twice * M. Turpin, Etude microscopique de la Cristatella Mucedo, espece de polype d'eau douce. Ann. des Sciences Nat. for 1837. Also, another memoir upon the same subject, by M. P. Gervais. Ibid. 116 BRYOZOA. in its course, and terminated by an anal orifice. When these creatures are extended, the upper part of the body protrudes from the cell ; the tentacular apparatus being supported on a kind of neck, whereon the mouth (a) is easily seen, and near it the anus. On each side of the mouth the body divides into two arms, which, when spread out, resemble a horse-shoe, being flattened and blunt ; and upon the arms are arranged about a hundred slender, transparent, and retractile tentacles, disposed on each side and upon the summit, like the barbs of a feather ; and all covered with an infinite number of cilia, whose action produces currents directed towards the mouth, hurrying in that direction organized particles contained in the water. The three individuals that thus inhabit the same general cover- ing are produced at two distinct generations ; the two lateral being the offspring of the central one, derived from it by a pro- cess of gemmation, but, when complete, they are evidently quite se- parate from and independent of their parent. (152.) From what is known concerning the propagation of the Bryozoa, it would appear that their reproduction is effected in several different ways. The most ordinary is by the developement of gemmae or buds, that sprout from the parent stem in the branched species, or, as in the Flustra and Eschara, are derived from the sides of con- tiguous cells. A. second mode of increase is by the production of ciliated gemmules capable of locomotion. These gemmules have been attentively examined by Dr. Farre in the paper above alluded to, and the nature of the ciliary action by which they are moved most satisfactorily investigated, as we shall elsewhere have occasion to notice more particularly ; but the organs wherein the reproductive gemmules are developed are as yet undescribed. The Cristatella seems to be developed in an ovum, provided with a shell of extremely singular construction. In fig. 47, 2, the investment of one of these extraordinary eggs is represented prior to the exclusion of the embryo Bryozoon, its natural size being shown in the same figure (1): the external surface is seen to be covered with numerous long processes arising perpen- dicularly from it, and each terminates in a minute double hook, adapted apparently to fix the egg upon marine plants at the sur- face of the water : but how these hooks become developed is still a mystery ; it would seem impossible that an ovum so formidably HOTIFERA. 117 armed could be expelled from the parent animal in the usual way ; we must therefore suppose that the spines grow, or become hardened at least, subsequently to the birth of the ovum. Since the discovery of this microscopic egg in a recent state, similar bodies have been detected in great numbers in a fossil condition im- bedded in flint ; a fact which, in conjunction with what has been already stated ( 74) concerning the occurrence of the shells of loricated infusoria in the same situation, tends materially to show that masses of flint are agglomerations of siliceous particles inclos- ing immense quantities of the debris of organized bodies.* That the bryozoa are very far superior to the polyps in all the details of their structure, will now be sufficiently mani- fest. The ciliated tentacula, although selected as affording the most convenient character for the guidance of the Zoologist from the constancy of their coexistence with elaborately or- ganized internal viscera, are probably only organs of secondary importance in a physiological point of view ; for the analogies between this class and that which will form the subject of our next chapter are not to be mistaken, and the transition from one to the other is so gradual, that where observation has failed in com- pletely developing the anatomy of the animals we have been considering, the facts which have been ascertained concerning the Rotiferous Animalcules, will go far towards supplying the defi- ciency. CHAPTER IX. fRoTiFERA (Ehrenberg). (153.) The class of animals that next presents itself for our consideration was, until very recently, confounded with the chaotic assemblage of minute creatures to which the name of Infusorial Animalcules was indiscriminately applied; but the information at present in our possession concerning their internal structure and general economy, while it exhibits, in a striking manner, the assi- duity of modern observers, and the perfection of our means of ex- ploring microscopic subjects, enables us satisfactorily to define the limits of this interesting group of beings, and assign to them the elevated rank in the scale of zoological classification to which, from their superior organization, they are entitled. * Tuipin, Ann. des Sciences Nat. 1837. t Rota, a wheel; fcro, / bear. 118 ROTIFERA. The character whence the class obtains its name is derived from the peculiar organs placed upon the anterior part of the body, which are subservient to locomotion, and assist in the prehension of food ; these consist of circlets of cilia variously disposed in the neighbourhood of the mouth, and having, when in action, the ap- pearance of wheels spinning round with great rapidity, so as to pro- duce strong currents in the surrounding water. Yet, notwithstand- ing this peculiar structure of the locomotive apparatus, the ROTI- FERA present very marked relations with the BRYOZOA, described in the last chapter ; and the conversion of the ciliated tentacula of the latter into the rotatory organs of the present class is effected by several intermediate forms, which would seem to indicate a closer alliance between the two than, from an examination of the more typical genera of each, we should be inclined to suspect. (J54.) The annexed engraving of the Stephanoceros Eichornii* Fig. 48. (jig- 48) exhibits an animal that would seem to be one of the connect- ing links by which this transition is accomplish- ed ; the transparent cell, and ciliated tentacula around the mouth, would indicate this creature to be a BRYOZOON ; but the tentacula are no longer the stiff and slender arms which we have seen in Bowerbankia, but are vi- sibly stunted and thick- ened at their base, thus approximating in character the cilia-bearing lobes of a Rotifer ; while the inter- nal organs, the pharynx, gizzard, and stomach, in this animal conform ex- actly to the type of structure common to the Rotifera properly so called. (155.) The body of one of the wheel animalcules is enclosed in * Ehrenberg. ROTIFERA. 119 a delicate transparent envelope of considerable consistency, often terminating at the upper extremity in wavy indentations or tooth- like processes, as in Brachionus urceolaris* (Jig. 49, c, c ). This harder integument is essentially analogous to the cell of a Bryo- zoon, but in this case is so constructed as to allow the animals to move at large in the element they inhabit, instead of being per- manently fixed to the same locality. Continuous with the free margin of the shell is a delicate membrane connecting it with the bases of the cilia-bearing lobes around the mouth, so as to allow those organs, when not in use, to be retracted within the cell by a mechanism resembling that provided in Bowerbankia for the re- traction of the tentacula. To the posterior extremity of the body is generally appended a pair of forceps composed of two moveable pieces (Jigs. 50 and 51), used as anchors or instruments of prehension; and by means of these the little creatures fix themselves to the confervse or aquatic plants amongst which they are usually found. In Brachionus urceolaris the prehensile forceps (fig. 49, o p,) is at- tached to the extremity of a long flexible tail in which the muscular fibres des- tined for its motions are distinctly visible. (156.) The cilia, whose action produces the ap- pearance of wheels turn- ing upon the anterior part of the body, are variously disposed, and from their arrangement Ehrenberg has derived the characters whereon he bases the di- vision of the class into orders. The peculiar movements excited by the vibration of these organs, was long a puzzle to the earlier microscopic observers, who, imagining them to be really wheels turning round with great velocity, were utterly unable to * The engravings of the Rotifera are all copied from Ehrenberg's papers. Abhand- lungen der Koniglichen Akademie der Wissenchaften zu Berlin, for 1833. 120 ROTIFERA. conceive what could be the nature of the connection between such appendages and the body of the animal. The apparent rotation has, however, been long proved to be an optical delusion, and to be produced by the progressive undulations of the cilia placed in the neighbourhood of the mouth. (157.) With respect to the agents employed in producing the ciliary movement in the rotifera, we are as much in ignorance as we are concerning the cause of the same phenomenon in the polygastrica. Ehrenberg describes the cilia as arising from a series of lobes as re- presented in Notommata clavulata (Jig. 51 a); these he regards as being muscular, and capable of producing by their contractions the rapid vibrations of the fibrillse attached to them. We confess, however, that such lobes, even was their existence constant, seem very clumsy instruments for effecting the purpose assigned to them, and it is not easy to conceive how the rapid and consecutive undu- lations to which the appearance of rotation is due can be produced by organs of this description. The observations of Dr. Arthur Farre* concerning the ciliary movements visible upon the gemnmles of some of the Bryozoa appear best calculated to throw light upon the nature of the action of these wonderful appendages, and to explain the cause of the apparent rotatory motion of the so-called wheels of the rotifera. The very accurate observer alluded to remarks that under high powers, the cilia have the appearance of moving in waves, in the production of each of which from a dozen to twenty cilia are concerned, the highest point of each wave being formed by a cilium extended to its full length, and the lowest point between every two waves by one folded down completely upon itself, the intervening space be- ing completed by others in every degree of extension, so as to pre- sent something of the outline of a cone. As the persistence of each cilium in any one of these positions is of the shortest possible duration, and each takes up in regular succession the action of the adjoining one, that cilium which, by being completely folded up, formed the lowest point between any two waves, in its turn by its complete extension forms the highest point of a wave ; and thus, while the cilia are alternately bending and unbending themselves, each in regular succession after the other, the waves only tra- vel onward, whilst the cilia never change their position in this di- rection, having, in fact, no lateral motion. The whole of the ciliary movements are so evidently under the control of the animal as to leave not the slightest doubt in the * Phil. Trans, for 1837. ROTIFERA. mind of the observer upon this point. The whole fringe of cilia may be instantly set in motion, and as instantaneously stopped, or their action regulated to every degree of rapidity. Sometimes one or two only of the waves are seen continuing their action, whilst the remainder are at rest ; or isolated cilia may be observed slowly bending and unbending themselves, while the others are quiescent. It is by the constant succession of these movements that the eye is seduced to follow the waves which they seem to produce, and thus the apparent rotation of the wheels is easily understood. (158.) Such being,as we conceive, the nature of the ciliary motion, we will proceed to examine the uses to which it is made subservient in the class of animals under consideration. A very slight examina- tion of one of these creatures under the microscope will show that the cilia answer a double purpose : if the Rotifer fixes itself to some stationary object by means of the anal forceps, it is precisely in the position of a Bryozoon ; and the ciliary action, by producing currents in the water all directed towards the oral orifice, ensures a copious supply of food by hurrying to the mouth whatever minute aliment may be brought within the range of the vortex thus caused ; or, on the other hand, if the animal disengages itself from the substance to which it held by its curious anchor, the wheels acting upon the principle of the paddles of a steam-boat carry it rapidly along with an equable and gliding movement. (159.) The whole ciliary apparatus when not in use is retracted within the orifice of the shell, and lodged in a kind of sheath formed for it by the inversion of the tegumentary membrane. The muscular fasciculi by which this is effected are very conspicuous ; they arise from the lining membrane of the shell, and run in distinct fasci- culi in a longitudinal direction to be inserted into the lobules whereon the cilia are arranged (^g. 50, A, h). But, besides these retractor muscles, other fasciculi of muscular fibres are seen to run transversely, (Jig. 50, z, i 9 ) crossing the for- mer at right angles : these are, most probably, the agents pro- vided for the extrusion of the wheel-like apparatus; for, aris- ing, as they do, from the inner membrane of the hard integument, they will, by their contraction, compress the fluid in which the viscera float, and, forcing it outward towards the orifice of the shell, it will, of course, push before it the wheels, so as to evert the te- gumentary membrane connecting them with the shell, by unrolling it like the finger of a glove, and thus they will cause the rotatory organs to protrude at the pleasure of the animal. ROTIFERA. We have already described the means whereby the Rotifera pro- cure a supply of food, namely, by exciting currents in the surrounding water ; the materials so obtained pass at once into a pharynx, the capacity of which would seem to vary considerably in different species : from the pharyngeal receptacle it is conveyed into a singularly constructed gizzard, to be bruised and broken down by an apparatus provided for that purpose ; thus prepared it is allowed to enter a third cavity, wherein digestion is accomplished, which may be called the stomach, and this, after becoming gradu- ally constricted in its diameter, terminates at the caudal extremity of the body. (160.) The usual arrangement of the digestive apparatus will be readily understood on reference to the annexed figures ; thus, in Ste- phanoceros Eichornii, (Jig- 48,) the pharynx (a) is very capacious, receiving readily the materials brought into it by the ciliated arms ; the gizzard (e) is a small globular viscus, containing the instruments of mastication hereafter to be noticed ; while the digestive cavity properly so called (6), which presents no per- ceptible division into stomach and intestine, extends from the gizzard to the anal aperture. In Brachionus urceolaris (Jig. 49) the pharynx or oesopha- gus (e) is less capacious ; the gizzard (/) exhibits through its transparent coats the peculiar dental organs placed within it ; and the stomach (g) is seen partially folded upon itself by the retraction of the body. We observe moreover in this animal, appended to the commencement of the stomach, two large csecal appendages (h A), which were scarcely perceptible in the last figure, and which no doubt are of a glandular nature, furnishing some fluid to be mixed up with the bruised aliment contained in the stomach, to assist in the digestive process. To these secreting caeca Ehrenberg has chosen to give the name of pancreas, but for what reason it is difficult to conjecture, since the first rudiments of a pancreas are only met with in animals far higher in the scale of animal existence ; every analogy indeed would lead us to denominate these cseca the first rudiments of a liver, by far the most important and universal of the glandular organs subservient to digestion, and in a variety of creatures we shall afterwards find it presenting equal simplicity of structure. In the Notommata centrum (Jig. 50, g, #), the cseca are merely two pouches opening into the top of the stomach, whereas in Notommata clavulata there are six of these appendages (Jig. ROTIFERA. 123 51, e, e) communicating with that enlarged portion of the digestive canal (c) which may be looked upon as the proper stomach. (161.) We must now revert to the consideration of the dental apparatus contained in the gizzard, represented in situ in (fig- 49, /), and exhi- Fig. 50. bited on a still larger scale in (fig- 50, 2). This curious masticat- ing instrument consists of three distinct pieces or teeth, which are made to work upon each other by the con- tractions of the gizzard, so as to tear in pieces or bruise all matters made to pass through the cavity containing them. The central piece (Jig. 50, 2, 1) may be com- pared to an anvil pre- senting upon its upper surface two flattened facets ; and upon these the other two teeth, that might without much stretch of fancy be compared to two hammers, act. Each of the superior teeth (fig. 50, a, a) may be described as consisting of two portions united at an angle : the larger portion, or handle as it might be called, serves for the attachment of muscles ; whilst the other part is free in the cavity of the gizzard, and works upon the facets of the anvil, the edge being apparently divided into teeth resem- bling those of a comb, and evidently adapted to bruise or tear substances submitted to their action. Such is the transparency of the whole animal, that the effect of these remarkable masti- cating organs upon the animalcules used as food is distinctly visible under a good microscope, and if the Rotifer be compressed between two pieces of glass, so as to break down the soft textures of its body, the teeth may from their hardness be procured in a detached state for minute examination. The whole apparatus described above evidently resembles very closely the kind of ROTIFER A. stomach met with in the Crustacea, to which the rotifera will be found gradually to approximate. (162.) Notwithstanding the microscopic size of the Rotifera, and the consequent difficulty of detecting the more minute details of their structure, Ehrenberg thinks he has succeeded in discovering filamentary nerves, and even nervous masses, distributed in different parts of their body ; an arrangement which not only would account for the complete association of their voluntary movements, but would, from the presence of ganglia, render these animals capable of possessing some of the local senses ; indeed Ehrenberg imagines he has discovered such to exist in the shape of red specks, to which he gives the name of eyes. The organ alluded to is a minute red spot, indicated in the figures (Jig. 49 and 50, c) ; nevertheless, no organization has been described of such a nature as to entitle us unhesitatingly to designate it an organ of vision, even if it should, as he intimates, invariably be in connection with a nervous mass, which, from examining his drawing of the arrangement of the nerves, we should have little expected to be the case. (163.) The nervous system of Notommata clavulata, as describ- ed by this indefatigable observer, is represented in fig. 51. It would seem to consist of several minute nodules, exhibiting a somewhat symmetrical arrangement, and disposed apparently in pairs ; some of these nodules, which are about ten in number, communicate with each other by delicate filaments, whilst others seem to be quite insulated from the rest. Every one who is acquainted with the difficulty of conducting microscopical observations, especially with the high powers needful in detecting structures so minute as the nerves of the Rotifera, will be exceedingly cautious in admitting the complete establishment of facts involving important physiological principles ; and we cannot help thinking that Ehrenberg has been misled by some appearances which it is impossible for the most correct observer always to guard against, in assigning to the rotifera an arrangement of the nervous system so totally different from what is met with in any other class of animals, as that represented in his figure from which our engrav- ing has been accurately copied. All our ideas of the physiology of the nerves would lead us to suspect some error. The uses of ganglia, as far as we know at present, are either to associate nerves derived from different sources, or to serve as centres for perception, or else they are for the con- centration of nervous energy. The position of the ganglia depicted in the figure as being in relation with the nervous threads would ROTIFERA. 125 scarcely seem to be consistent with either of the above offices, and therefore we cannot but regard the observations which have been hitherto recorded concerning the nervous system of the rotifera as far from being complete. (164.) In addition to the elaborate organization described above, the Prussian naturalist conceived that he had discovered a vascular apparatus, consisting of transverse vessels, (Jig. 51, w, w,) in which Fig. 51. he supposed a circulation of the nutritive fluids occurred. But the vascular character of the transverse striae visible in this position is more than doubtful, as there seems every reason to suppose that the appearance depicted in the figure is due to the existence of the transverse muscular bands whereby the extrusion of the rotatory ap- paratus is effected, analogous to those occupying a similar situation in the Bryozoa : in fig. 50, i, i, these transverse fasciculi are distinctly de- lineated, and their nature is at once evident. (1 65.) The mode in which respiration is effected in the class of animals under consi- deration has been a subject of much dispute. Some have supposed the contact of water, applied to the general surface of the body, sufficient for the aeration of the nutritious juices, especially as its constant renewal would be ensured by the ciliary movements. Bory St. Vincent, * on the contrary, regarded the rotatory cilia as real gills, resembling those of fishes ; and mistaking the move- ments of the gizzard for the contractions of a heart, conceived these animalcules to be even superior to insects in the organization of their vascular system. Ehrenberg, moreover, thinks that he has discovered an internal respiratory apparatus of a most extraordinary description. In Notommata centrura (fig. 50 ) he remarked * Diet, des Sciences Naturelles ; art. Rotifera. in. 126 ROTIFERA. seven vibrating points on one side, and six on the other, attached to two long and undulating viscera, (/, /,) which he elsewhere describes as being the testes of the animal : the above-mentioned points were never at rest, and appeared to be placed in determinate positions opposite to each other. Accurate observations, he says, have shown each to be a peculiar little organ, provided with a tail resembling that of a note in music, and to be thrown into vibration by three little vesicles or folds of their inflated extremity ; these organs floated freely in the abdominal cavity by their enlarged portion, while by their tail they were attached to the long tubular organ above referred to (figs. 49 and 50). Ehrenberg's first idea, on seeing these organs, was, that they formed a vascular system, executing movements of pulsation ; but he now considers them as internal branchiae, or organs of respira- tion, to which the external water is freely admitted in the following manner. In many species of the rotifera, we find, projecting from the neck of the animal, a horny tubular organ, called by Ehrenberg the Calcar or spur (figs. 49 d, and 50 Z>) ; this he at first considered to be the male organ of sexual excitement, but he now regards it as a syphon or tube of respiration, through which the circumambient water passes freely into the cavity of the body. He thinks, more- over, that the periodical transparency, and the alternate distension and collapse of the animal, seen to occur regularly in almost all the Rotifera, are produced by the introduction of water into the visceral cavity, and its subsequent expulsion therefrom, upon which action the fluctuations observed in the interior of the body would there- fore depend. The supposition that water is injected in this manner into the body seems to be favoured by other appear- ances ; for, when the internal cavity is thus filled, all the viscera appear isolated, so that the boundaries of each can be distinctly seen, but when the water is discharged they approximate each other, their limits become confounded, and the external membrane of the body assumes a crumpled appearance. Upon reviewing the above account of the mode of respiration in the rotifera, we must say that we consider that the office assigned to the little organs called internal branchiae is extremely proble- matical, especially as we have but the most vague intimations con- cerning the existence of a circulating system at all, much less of such a double circulation carried on in arteries and veins as the presence of such organs would infer. " I presume," says Ehrenberg, ROTIFERA. fc< that the branchiae possess a vascular system ; for, when the local contractions occur in the body of the animal, we see distinctly a certain number of filaments (vessels ?) loose and delicate." The opinions of the Professor himself concerning the nature of the organs which he describes being so indefinite, we must pause before adopting the physiological views to which their admission would lead ; more especially as, from the very fact of the whole visceral cavity being perpetually filled with aerated water, the existence of any localized organs of respiration could hardly be esteemed necessary. (166.) The last subject which we have to consider relative to the internal economy of the rotifera is, the conformation of their gene- rative apparatus, which now assumes a considerable perfection of developement. The reproductive system is composed apparently of two distinct parts : the one subservient to the formation of the ova ; the other destined either to furnish some secretion essential to the completion of the egg, or, as is more probably the case, secret- ing a fertilizing fluid by which the impregnation of the ova is effected prior to their escape from the body. The ovary, as we might term it, or female portion of the system, (jigs. 48 c, 49 7n, n 9 50 &, A*, 5iy,) is a transparent sacciform organ, in which, at some seasons, the eggs are distinctly perceptible through the pellucid coverings of the animal, as represented in the figures. The male organs, or testes,as we may call them, are two in number (Jigs. 50 /, and 50 h) ; they resemble long wavy cseca, extending nearly the whole length of the animal, and terminating near the oral extremity by closed extremities. It is to these organs that the small appendages mentioned above as organs of respiration are appended ; and, should the latter not perform the office of respiratory branchiae, they are most probably organs of secretion, such as in many other animals we shall see appended to the spermatic tubes. Both the ovigerous organ and the two seminiferous vessels terminate in a common receptacle (Jig. 51, g,) that may be named the cloaca ; this consists of a transparent vesicle endowed with great irritability, in which the fertilization of the ova is apparently effected, the eggs being here brought in contact with the secretion of the testes before they escape through the excretory passage (Jig. 51, d). The ova of the rotifera, before they are hatched, form very in- teresting objects for the microscope ; as the movements of the in- cluded young, and even the action of the cilia forming their wheel-like organs, may be distinctly seen through the exquisitely transparent investment of the egg. 128 EPIZOA. CHAPTER X. EPIZOA. (167.) Not only are the internal parts of living animals occasion- ally made the residence of creatures adapted by their organization to live under such circumstances, but there is an extensive class of beings destined to an equally parasitical life, so constructed as to be capable of attaching themselves to the external parts of other creatures, from which they suck the nourishment suited to their nature. These parasites are commonly found to infest Fishes, Crustaceans, and other inhabitants of fresh and salt water ; generally fixing themselves in positions where an abundant supply of animal juices can be readily obtained, and where, at the same time, the water in which they are immersed is perpetually renewed for the purpose of respiration. The gills of fishes, therefore, offer an eligible situa- tion for their developement, as do the branchiae of the lobster ; or they are sometimes found attached in great numbers to the interior of the mouth in various fishes, deriving from its vascular lining, or from the abundant secretions met with in such a locality, a plentiful supply of food, while they are freely exposed to the currents of water which the mode of respiration in the fish brings in contact with them. (168.) Allied, however, as these creatures are in the nature of their mode of life to the entozoa, it is easy to perceive that, from their residence upon the surface of the body, they enjoy a far greater capability of action, and a more enlarged intercourse with the ex- ternal world ; so that we are not surprised at finding them possessed of organs which in both the Sterelminthoid and Ccclelminthoid entozoa would have been entirely useless. In none of the indi- viduals of either of those classes, therefore, have we found external organs developed ; but in the Epizoa* we perceive, in a very in- teresting form, the first sproutings as it were of articulated mem- bers, which in higher classes attain their perfect developement. The least elaborately organized of these animals exhibit, indeed, exceedingly grotesque and singular shapes, resembling rather im- * E*v, upon ; wv, an animal. EPIZOA. 129 perfect embryos than mature beings ; the first buddings of external limbs in the earlier period of foetal developement imitating not very remotely the appearance of Fig. 52. the rudimentary appendages re- presented in the annexed figure* (Jig. 52). But this resem- blance is not confined merely to a fancied similarity in outward form ; it exists in the physio- logical relation that there is between the embryo and the Epizoon, and seems dependent upon that great principle which inseparably connects the perfec- tion of an animal with the cha- racter of its nervous system : the nerves of the Epizoa are simple filaments, the ganglia being indistinct or scarcely de- veloped ; and the imperfection of the limbs is a necessary consequence. In the same manner, in the earliest stages of foetal growth, when we know that the nerves are as yet but mere threads, it is interesting to observe the resemblance, even in outward appearance, between the embryo in this transitory stage of its growth, and the permanent condition of the Epizoa which we are considering. (1 69.) A great number of species of these parasites, generally de- scribed under the name of Lerneans, have been observed by authors, and it would seem indeed that each is peculiar to a particular kind of fish. The varieties observable in their outward form are, of course, exceedingly great; but the examples depicted in the figure, namely, the Lerneea gobina, found in the branchiae of Coitus Gobio and Lernaa radiata, which infests the mouth of Coryphcena rupestris, will make the reader sufficiently acquainted with their general ap- pearance and external structure. In the former parasite, of which an anterior and posterior view are given in the engraving (a, b), the appendages seen upon the head and sides of the body answer the purpose of hooks or grappling organs, whereby the creature re- tains its position ; and so firm is its hold upon the delicate covering of the gills, that, even after the death of the fish, it is not easily * Miiller (Othone Frederico) Zoologia Danica, 1788. 130 EPIZOA. detached. In the second example, (c, d,) besides the rudimentary limbs, the lower surface of the head and ventral aspect of the body (d) are covered with sharp spines calculated to increase very materially the tenacity of its hold upon the surface from which it imbibes food. The sacculi appended to the posterior part of the animal are receptacles for the eggs, and will be explained hereafter. These examples, however, are taken from the most imperfectly organized Epizoa ; but, as we ascend to more highly developed species, we shall at once see how gradually an approximation is made to the articulated outward skeleton, and jointed limbs, met with in the homogangliate forms of being, until at last the zoolo- gist remains in doubt whether the more elaborately constructed ought not to be admitted among the crustacean families, which they most resemble. (170.) The Adheres percarum (fig. 53) is one of those spe- cies most nearly allied to the ARTICULATA ; and, the details of its anatomy having been fully investigated by Nordmann,* it will serve as a good example of the type of structure which prevails throughout the class. The Actheres is found to infest the perch (Percafluviatilis), adhering firmly to the roof of the mouth, to the tongue or some- Fig. 53. times even to the eyes of that fish ; in which situations it is concealed by a brownish slimy secretion, so that its presence might easily escape the notice of a casual observer. The female, which is represent- ed in the figure, is about two lines in length; the male, which differs materially from the other sex in many points, is considerably smaller. The outer covering of the body of these little creatures is at once seen to have assumed a horny hardness ap- proximating the density of the cover- ings of the articulated classes, and in- dications are even perceptible of a division into segments : the distinct- tion, moreover, between the trunk (cephalo-thorax), to which the limbs * Mikrogvaphische Beitriige zur Naturgeschichte der witbellosen Thiere ; Berlin, 1832. EPIZOA. 131 arc appended, and the abdomen, wherein the viscera are lodged, is obvious. The rude and imperfect limbs that we have seen in the Lerneans are visibly more perfect in their entire construction ; and in the fe- male the posterior pair of these appendages is converted into a most singular instrument of attachment, by which it fixes itself to the gums of the fish. The hinder pair of extremities alluded to (Jig. 53, &, b) are, in fact, enormously developed; they curve forward after their origin from the posterior part of the trunk, and are so much extended that they project considerably beyond the head of the creature, where, becoming considerably attenuated, the two are joined together by a kind of suture, and support, upon the point where they are united, a cup-shaped organ whereby the creature fixes itself. This singular instrument, represented upon an enlarged scale &tjig. 54, 1, is of cartilaginous hardness, and resembles a little bowl, the inside of which is studded with sharp teeth, and calcu- lated not only to act as a powerful sucker, but, from the hooks within its cavity, it is capable of taking a most tenacious hold upon the lining membrane of the mouth. The other members (Jig. 53, o) are much less developed, but are nevertheless so constructed as to assist materially in fixing the Epizoon ; they are represented upon a very enlarged scale in Jig. 54, 2, where the outer pair (a, a) are seen to exhibit in the transverse lines indented upon their surface the first indication of articulated members ; and their extremities, armed with minute hooks, evidently form powerful agents for prehension. Internal to these are two other jointed organs, still more feeble in their construction, the ends of which (6, 6), being armed with three spines, will assist in effecting the same object. (171.) The Figm 54 . mouth itself (Jig. 54, 2, c) is formed upon similar prin- ciples, the exter- nal orifice being surrounded with a circle of minute recurved spines well calculated to ensure its firm ap- plication to the 132 EPIZOA. surface from which nourishment is obtained ; and, within this, rudimentary jaws furnished with strong teeth are visible, adapted, no doubt, to scarify the part upon which the mouth is placed, in order to ensure an adequate supply of food. In the male Aether es, the sucking-bowl possessed by the female does not exist ; the pre- hensile organs being merely four stout articulated extremities, armed at the end with strong prehensile hooks. As we might suppose, from the nature of the food upon which this creature lives, the alimentary system is extremely simple. The oesophagus, the course of which is represented by dotted lines in the last figure, terminates in a straight digestive canal (a), which passes through the centre of the abdomen, but no separation between sto- mach and intestine is visible : the entire tube, from the transverse constrictions visible upon its surface, has a sacculated appearance, and is perceptibly dilated towards the centre of the abdominal cavity ; after which it again diminishes in size as it approaches the anal orifice (Z>), situated at the posterior extremity of the body. Near the termination of its Fig. 55. course, the alimentary canal passes through a loop formed by transverse bands (w, w), and, moreover, seems to be retained in its position by radiating fibres apparently of a ligamentous cha- racter, but which has been de- scribed as representing a biliary apparatus. (172.) The muscular system of this animal is far more perfect in its arrangement than in the preceding classes, and the deli- cate fasciculi which move the rudimentary limbs are visible through the transparent integument (fig. 54). In the abdomen, the muscles form longitudinal and transverse bands, which intersect each other at right angles (Jig. 55, d) ; an arrangement not very different from what we have already seen in the rotiferous ani* malcules. (173.) The nervous system appears to consist principally of two long filaments (Jig- 55, c), which run beneath the alimentary ca- nal : but it is extremely probable that these communicate with some EPJZOA. 133 minute ganglia in the neighbourhood of the head ; at least, the perfect structure of the oral apparatus, and the developement of the limbs, would seem to indicate such a type of structure. (174.) The generative organs in the female Adheres consist of two parts ; the ovaria, wherein the eggs are formed, contained in the abdominal cavity (Jig. 53, d, d), and of two external append- ages, or egg-sacs (Jig. 53,/,/), which are attached to the pos- terior extremity of the body for the purpose of containing the eggs until their complete developement is accomplished ; this ar- rangement we shall again have an opportunity of examining in the entomostracous crustaceans. The internal ovaria (Jig- 55, /), when distended with ova, occupy a great part of the cavity of the abdomen, and present a racemose appearance ; but when empty, as represented upon the opposite side of the same figure (e), each is found to be a simple blind canal, with sacculated walls, opening externally by an ori- fice (g, g), through which the ova are expelled into the egg-sacs, where their developement is completed. (175.) It would seem that, even when the eggs are hatched, the excluded young are far from having attained their perfect or adult form ; but undergo, at least, two preparatory changes or metamor- phoses, during which they become possessed of external organs so totally different from those they were furnished with on leaving the egg, that it would be difficult to imagine them to be merely different states of existence through which the same animal passes. On first quitting the egg, the young Adheres is in fact by no means adapted to the parasitical life to which it is subsequently destined ; possessing no organs of prehension like those of the adult, but merely two pairs of swimming-feet, each armed with a brush of minute hairs, and calculated to propel it through the water. Be- fore, however, the first change is effected, another set of feet may be perceived through the transparent external covering, encased as it were in the first ; when these are completely formed, the original skin falls off, displaying, in addition to two new pairs of swimming-feet, three pairs adapted to prehension ; and it is only when the second set of feet is thrown off in a similar manner that the animal assumes its perfect or mature form. (176.) The affinities between the more highly organized EPIZOA and the CRUSTACEA are evidently very strong ; yet, independently of the different character of the nervous system, there is another important distinction between them, derived from their compara- 134 EPIZOA. tive anatomy. In the CRUSTACEA, the organs of circulation and respiration are well developed and easily recognisable ; but, in the class we are now considering, no parts adapted to either of those functions have hitherto been satisfactorily discovered : neverthe- less, that the EPIZOA form a gradual transition from the humbler creatures we have hitherto examined to the great division of ar- ticulated animals, must be obvious to the most superficial observer. (177.) In Lamproglena pulchella we have a still more decided approximation to the crustacean type of structure, and the rudimen- tary feet arranged in symmetrical pairs are as numerous as the segments of the body. The limbs, however, are as yet only adapted to secure a firm hold upon the structures to which this parasite attaches itself, namely, the gills of the chub (Cyprinus Jeses), in which situation it is most usually found. The two an- terior pairs (fig. 56, b, c) are far more large- Fig. 56. ly developed than those which are placed upon the posterior parts of the animal, and are apparently strengthened by a cruciform cartilaginous frame-work seen through the transparent integument. The first pair of these holding feet consists of two robust and powerful hooks, terminated by simple horny points ; whilst the second, which are likewise unciform, terminate in trifid prongs, and are evidently equally adapted to pre- hension. The four pairs of members which succeed to these are mere rudiments, and can be of little service as organs of attach- ment ; but, to make up for their imperfec- tion, we find at the posterior^ extremity of the body, between the orifices of the ovaria (g), a pair of cartilaginous suckers well cal- culated to fix this part of the animal. The muscular system is readily seen through the transparent skin : four longitudinal bands are visible (d), running from one end to the other, and, besides these, broad transverse fasciculi are discernible in the fifth and sixth segments of the body ; from the nature of the feet, however, and general struc- ture of the creature, we must imagine the existence of muscles provided for the movements of each articulated member, although, from their extreme minuteness, they escape detection. ECHINODERMATA. 135 The opening of the mouth is placed in the centre of the space bounded by the four anterior prehensile hooks ; and the alimen- tary canal is a simple tube passing straight through the body to the tail, where the anal orifice is distinguishable. The walls of the intestine have a reticulated appearance, being covered with a kind of glandular net- work, which probably constitutes a biliary apparatus. (178.) In a creature thus highly organized we may well expect to find senses of proportionate perfection, and in Lamproglena their existence is no longer doubtful. The eyes are distinctly apparent, of a reddish colour ; but as yet, as in the lowest crustaceans, united into one mass. The antennae likewise, which may be re- garded as special instruments of touch, are well developed ; and, both in number and position, resemble those which characterise the crustacean orders, to which we are thus conducted by almost imperceptible gradations. The reproductive organs are entirely similar to those of Adheres already described. Those of the female, represented in the figure, consist of sacciform ovaria, in which the ova are secreted : and from these, when mature, the eggs are expelled through two simple triangular orifices situated on each side of the anus. CHAPTER XL ECHINODERMATA.* (Cuv.) (179.) The last class of beings belonging to the Nematoneurose division of the animal world seems, upon a partial survey, to be com- pletely insulated, and distinct from all other forms of living crea- tures ; so peculiar is the external appearance, and even the internal organization of the families which compose it. The casual observer who should, for the first time, examine a star-fish or a sea-urchin, two of the most familiar examples of the ECHINODERMATA met with upon our own shores, would indeed find it a difficult task to associate them with any other class, or to imagine the affi- nities whereby they are related, either to the simpler animals we have already described, or to more perfect forms of existence hereafter to be mentioned : they would seem to stand alone in the creation, * "E%ivos , a hedgehog ; $&/*, the skin. 136 ECHJNODERMATA. without appearing to form any portion of that series of devclope- ment which we have hitherto been able to trace. But this apparent want of conformity to the general laws of developeinent vanishes on more attentive examination ; so that we may not only trace the steps by which every family of this exten- sive class merges insensibly into another, but perceive that, at the two opposite points of the circle, the ECHINODERMATA are inti- mately in relation with the POLYPS on one hand, while on the other they as obviously approximate the annulose animals, to which the most perfectly organized amongst them bear a striking resem- blance. It would be impossible within our present limits to do more than lay before the reader the most important types of structure which the Echinodermata exhibit ; it must, nevertheless, be under- stood that innumerable intermediate families connect the different genera ; so that, however dissimilar the examples we have selected for the purpose of exhibiting their general habits and economy may appear, the gradation which leads from one to another is easily traced. (180.) Crinoidce. We have already found that many tribes of polyps secrete calcareous matter in large quantities, so as to con- struct the solid skeletons or polyparies, which generally seem to be placed external to their soft and irritable bodies, but occasion- ally, as in Pennatula, within the living substance. Let us for a moment suppose a polyp supported upon a prolonged stem, and that, instead of depositing the earthy particles externally, they should be lodged in the substance of the polyp itself, so as to fill the pedicle, the body, the tentacula around the mouth and all the appendages belonging to the animal with solid pieces, of definite form ; such pieces being connected together by the soft parts, and surrounded on all sides with irritable matter, would thus form a complete internal skeleton, giving strength and support to the entire animal, and at the same time allowing flexure in every di- rection. A polyp so constituted would obviously, when dried, present an appearance similar to what is depicted in the annexed engraving (Jig- 57), representing an Encrinoid Echinoderm in its perfect condition. That animals thus allied to polyps in their outward form have in former times existed in great num- bers upon the surface of our planet is abundantly testified by the immense quantities of their remains which are met with in various calcareous strata, but their occurrence in a living state is at ECHINODEKMATA. 137 Fig. 57. present extremely rare : one minute species only has been detected in our own seas ;* while specimens of larger growth, such as that repre- sented in the engraving, deriv- ed from tropical climates, are so seldom met with, that it is fortunate that one or two ex- amples have been found to reveal to us the real structure of a race of animals once so common, but now almost com- pletely extinct. The body of the Encrinus (Jig' 57, a) (or pelvis, as the central portion of the animal is termed by geological writers,) is com- posed of numerous calcareous plates, varying in shape and arrangement, so as to become important guides to the identification of fossil species ; from this central part arise the large rays (b, b), each furnished with a double row of articulated appendages, which, as well as the arms, are, no doubt, instruments for seizing prey and conveying it to the mouth, situated in the centre of the body near the point a. This part of the animal, when found in a fossil state, from its re- semblance to a flower, has received the common name of a " lily- stone." The body above described, with the rays proceeding from it, is supported upon a long pedicle (e), composed of numerous pieces ; and, upon the sides of the stem, similarly constructed filamentary branches are fixed (d, d) at equal intervals. The skeleton of an En- crinite consists, therefore, of thousands of regularly shaped masses of calcareous earth kept together by the living and irritable flesh in which they are imbedded, and it is to the contractions of this living investment that the movements of the animal are due ; but after the death of the creature, and the consequent destruction of its soft parts, the pieces of the earthy frame-work become sepa- rated and fall asunder, forming the fossil remains called " Troeki," * Thompson (J. W.), Memoir concerning the Pentacrinus Europaeus ; Cork, 1827, 4to. 138 ECHINODEllMATA. Fig. 58. and known in the northern districts of our own island, where they are very abundant, as " St. Cuthberfs beads" Of the internal structure of the Encrinites nothing is satisfac- torily known. That they possessed a dis- tinct mouth and anal aperture is evident, from the structure of the plates of the body; but this is the extent of our information concerning them.* (181.) Asteridce. In order to convert an Encrinus into an ani- mal capable oflocomo- tion, and'able to*crawl about at the bottom of the sea, little fur- ther would be requi- site than to separate the body and arms from the fixed pedicle upon which they are sup- ported, and we should have an animal resembling in every particular the star-fishes. The Comatula, for example, (fig. 58,) one of the lowest of the asteroid Echinodermata, might be looked upon as an animal thus detached. The central part, or body, which contains the viscera, is made up of numerous calcareous pieces, having in its centre a stelliform mouth, and near this is a tubular orifice which might be regarded as an anus. Around the margin of the central disc arise five stunted arms which immediately divide into a variable number of long radiating branches, composed, like those of the En- crinus, of innumerable articulated earthy masses enveloped in a liv- ing and irritable integument. We find, moreover, issuing from the sides of every one of the prolonged rays, a double row of se- condary filaments, each containing an internal jointed skeleton, and capable of independent motion. The complicated arms of the Co- matula, therefore, are not, like those of a polyp, merely adapted to seize prey ; but, from their superior firmness, may be used as so many legs, enabling the animal to travel from place to place. * For a detailed account of the fossil Encrinites, the reader is referred to " A Natural History of the Crinoidea, or lily-shaped animals, by J.S. Miller ; 4to. Bristol, 1821. ECHINODERMATA. 139 Setting out from this point to trace the gradual developement of organization in the Echinodermata, we shall observe a progressive concentration of their entire structure. The central part, or vis- ceral cavity, so small in the Comatula when compared to the complicated rays derived from it, enlarges in its proportional di- mensions as the viscera contained within it become more perfect in their structure ; whilst, on the other hand, the radiating or po- lyp form, so visible in Encrinus and Comatula, becomes obliterated by degrees, until, at length, almost all vestiges of it are lost, or but obscurely recognisable. In the Gorgonocephalus (Jig. 59), the proportionate size of the rays when compared with that of the central disc still preponde- rates very considerably, although even here some concentration is manifest. The secondary articulated filaments appended to the rays of Comatula are F- lgt 59. no longer recognis- able, their place be- ing supplied by the continual division and subdivision of the rays themselves ; the same end, how- ever, is obtained in both cases, for the numerous jointed and flexible rays of Gorgonocephalus still form so many legs, enabling the creature to drag it- self along the bottom of the sea, or to entwine itself among the submarine plants, as well as supplying the office of tentacula in securing food. Continuing our progress towards more perfect forms of these remarkable animals, we at length arrive at genera in which the rays become divested of all elongated appendages, either in the shape of articulated lateral filaments or dichotomous ramifications. In Ophiurus, for instance (Jig. 60), the rays are long and simple, re- sembling the tails of so many serpents a circumstance from whence the name of the family is derived ; nevertheless, on each side of every ray we still trace moveable lateral spines, which, although 140 ECHINODERMATA. but mere rudiments of what we have seen in Comatula, may still assist in locomotion, or perhaps may contribute to retain the prey more firmly when seized by the arms. The rays themselves are composed of many pieces curiously imbricated and joined together by ligaments, so that they are, from their length and tenuity, ex- tremely flexible in all directions, and serve not only for legs adapt- ed to crawl upon the ground, but are occasionally serviceable as fins, able to support the animal in the water for a short distance by a kind of undulatory movement. The body, or central disc, is beautifully constructed, being made up of innumerable pieces ac- curately fitted together. The mouth occupies the centre of the ventral surface, and is surrounded by radiating furrows in which are seen minute apertures that give passage to a set of remark- able prehensile organs, to be described hereafter : these are calcu- lated to act as suckers, and so disposed as either to fix the body of the animal, or to retain food during the process of deglutition. Fig. 60. Leaving the Ophiuri, we are led through a long series of almost imperceptible gradations to animals apparently of most dissimilar structure. The star-fishes (Asterias) (Jig. 65) form the next step : ECHINODERMATA. 141 in these, from the increased size of the body, the rays are united at their origin, and become so much dilated as to contain prolongations of the viscera lodged in their interior ; an arrangement not met with in Ophiuri and other slender-rayed Asteridse. The dilatation of the central part proceeds, and in the same proportion the rays be- come obliterated ; so that at length, the asteroid shape becoming totally lost by the progressive filling up of the interspaces between the rays, we arrive ultimately at completely pentagonal forms, the sides of the pentagon being perfectly straight lines. (182.) It is extremely interesting to remark the changes which occur in the nature of the locomotive organs during these diversi- fications of external figure. We have seen that, in the lower Echinodermata possessing long and flexible rays, such organs were fully adequate to perform all movements needful for progression ; but as the mobility of these parts is diminished by their gradual curtailment, and the filling up of the spaces between them, some compensating contrivance becomes indispensably necessary, and accordingly we find an apparatus gradually developed, well cal- culated to meet the exigencies of the case. In Ophiurus we have already mentioned the existence of protrusible suckers around the opening of the mouth, well adapted, from their posi- tion, to take firm hold of food seized by the animal ; and it is by increasing the number of such organs that ample compensation is made for the loss of motion in the rays themselves. On ex- amining the lower surface of an Aster ias, even in those forms which most approximate a right-lined pentagon in their marginal contour, the number of rays will still be found to be distinctly in- dicated by as many furrows radiating from the mouth, and indicat- ing the centre of each division of the body. These ambulacral furrows, as they are termed, exhibit, when examined in a dried specimen, innumerable orifices arranged in parallel rows, through each of which, when alive, the animal could protrude a prehensile sucker, capable of being securely attached to any smooth surface. No verbal description can at all do justice to this wonderful mechanism, even leaving out of the question the means by which each individual sucker is wielded, for of this we shall speak here- after ; but let any of our readers, when opportunity offers, pick up from the beach one of these animals, the common star-fish of our coast, which, as it lies upon the sand left by the retiring waves, appears so incapable of movement, so utterly helpless and inanimate ; let him place it in a large glass jar filled with its ECHINODE11MATA. native element, and watch the admirable spectacle which it then presents : slowly he perceives its rays expand to their full stretch, hundreds of feet are gradually protruded through the ambulacral apertures, and each, apparently possessed of independent action, fixes itself to the sides of the vessel as the animal begins its march. The numerous suckers are soon all employed, fixing and detaching themselves alternately, some remaining firmly adherent while others change their position ; and thus, by an equable gliding movement, the star-fish climbs the sides of the glass in which it is confined, or the perpendicular surface of the submarine rock. But it is not only as agents in locomotion that the ambulacral suckers are used ; helpless as these creatures appear to be, they are among the most formidable tyrants of the deep, as will be readily admitted by any one who watches them in the act of de- vouring prey. When seizing its food, the rays of the Asterias are bent towards the ventral aspect so as to form a kind of cup, in the centre of which is the opening of the mouth ; the cup thus. formed will, to a certain extent, lay hold of a passing victim, but, without other means of securing it, the grasp would scarcely be very formidable to animals possessed of any strength ; armed, how- ever, as the rays have been found to be, with hundreds of tena- cious suckers, escape is almost impossible, for prey once seized is secured by every part of its surface, and, in spite of its utmost efforts, is speedily dragged into the mouth and engulphed in the capacious stomach, where its soft parts are soon dissolved. But to continue our survey of the class before us. Having ar- rived at the point at which, by the diminution of the rays and consequent extension of the central part, the body has assumed a pentagonal outline, we may now advance in an equally gradual manner to those globular species, of which the Echinus, or sea- urchin, is the type or most perfect example. (183.) Echinidce. In the Scutellce (fig. 61), we have a flat and shield-like body, in which even the angles of the margin are lost, and the whole circumference acquires a circular form ; but still the five radiating ambulacra are visible upon the centre of the disc, al- though evidently imperfectly developed when compared with those of the Asteridse above-mentioned. The nature of the integument has, in fact, become so changed in its texture, that another modi- fication of the locomotive organs is here imperatively called for, and the means of progression are therefore proportionately altered. In the Asteridse, the integuments, especially upon the dorsal as- ECHINODE11MATA. Fig. 61. 143 pcct, arc always more or less composed of a coriaceous material, or, at least, of solid pieces so articulated together as to permit of con- siderable flexibility ; but in the Echinidso the nature of the external covering is very different, for these creatures are completely en- cased in a dense calcareous shell, composed of numerous angular pieces accurately fitted together and incapable of movement. The Scutellte, moreover, bury themselves beneath the surface of the sand, a situation in which suckers would be of little use, but for which these animals are admirably adapted by a contrivance not less calculated to excite the admiration of the observer. The exterior of the shell is entirely covered with minute appendages, resembling, when seen with the naked eye, delicate hairs, but which, when ex- amined under a microscope, are found to be spines of most elaborate structure, as may be seen from the magnified view of one represent- ed in the annexed figure (Jig. 61). Innumerable as these spines are, every one of them is articulated to the shell by a kind of ball- and-socket joint, and susceptible of being moved in all directions, so that by their combined efforts the Scutella can speedily bury itself, either for the purpose of procuring food, or of eluding observation. (184.) From the flat Scutellcc^ the passage to the globose Echi- nidee is most gradual ; and a beautiful series of connecting forms, many still existing as living species, but a still greater number found only in a fossil state, demonstrate the gradual expansion of the shell, and its conversion into the spherical figure seen in the Echinus esculentus (Jig. 62). The Echinus in shape resembles an orange, 144 ECHINODERMATA. its dense calcareous crust enclosing the viscera within its cavity, while the locomotive apparatus is placed upon the external surface. The mouth is a simple orifice in the shell placed at one extremity of its axis, and through it, as represented in the figure, the points of five singular teeth project externally ; while the anal aperture occupies the opposite pole of the sphere. The instruments of locomotion occupy the entire superficies of the shell, and consist of two distinct sets of organs adapted to different uses. The first consists of a multitude of sharp purple spines, every one of which is articulated to a distinct and prominent tubercle whereon it moves. These numerous spines, therefore, which are essentially similar in their office to those we have already described in Scutella, differing only in proportionate size, are so many inflexible legs upon which the Echinus rolls itself from place to place, or by their assistance it can bury itself in the sand with the greatest facility. But these wonderfully constructed animals are by no means con- fined to this mode of progression ; for, impossible as it might appear from their outward appearance, they are able to climb rocks in search of food, and thus destroy the corallines and shell-fish upon which they principally feed. In order to effect this, we find the shell perforated with ten rows of small orifices so disposed as to form five pairs of ambulacra extending from one pole to the other : through these apertures a system of long suckers is made to pig. 52. issue, which protrud- ing, as represented in the figure (jig- 62), beyond the points of the spines, can be firmly fixed to any smooth sur- face, and, like the suckers of Asterias, become locomotive agents. (185.) Holothu- ridce. Having trac- ed the developement of the Echinodermata from the polypiform Encrinite to the globu- lar Echinus, we now shall find them perceptibly approximate an annulose or worm-like form. In the Holothuria (Jig. 70), the ECHINODERMATA. 145 Fig. 63. commencement of this change is perceptible : instead of being com- posed of hard, calcareous pieces, the integuments of the body now become soft and irritable, a few thin laminae of earthy matter around the mouth being the only vestiges of the shell and the spines, of course, are no longer met with ; the suckers, however, remain, and, when protruded through innumerable apertures dis- tributed over the surface of the body, they still form the principal instruments of progression. (186.) Fistularidx.At length, in the last division of the class, even the loco- motive suckers are lost, and the only ex- ^ ternal resemblance left between the now worm-like body and the forms above enumerated is met with in the radiating tentacula which surround the mouth. The apodous Echinodermata, " Echino- dermes sans pieds," of Cuvier have indeed been expunged from the list of radiated animals by some modern writers, but in every point of their in- ternal structure we shall find them offer too many points of similarity to permit of their expulsion from the class under consideration, although they evi- dently form the connecting link between the Radiata and the lowest families of the articulated division of the animal kingdom. The genus Fistularia (Jig. 63) strikingly exhibits approximation to the outward form of the ANNELIDA ; and the anatomy of these creatures, which we shall afterwards consider, equally indicates the affinities which unite them. (187.) We have already, when speak- ing of the general division of the Echino- dermata, put the reader in possession of all that is satisfactorily known concern- ing the structure of the Crinoid* ge- * K{/m, a lily'; tfiat, like. 146 ECHINODERMATA. nera ; our knowledge of those singular animals being entirely derived from the exterior conformation of two recent species, and from the mutilated skeletons of fossil Encrinites, which exist in such abun- dance in the limestone strata of our own country. Commencing, therefore, with the Asteridce,* we shall now enter at once upon the consideration of the anatomy of such species as have been most carefully examined, and merely notice incidentally the modifications which occur in the disposition of various organs in kindred genera. (188.) On examining a living Asterias, the outer covering of its body is found to be composed of a dense coriaceous substance, in which numerous calcareous pieces are apparently imbedded. The cori- aceous integument is generally coloured externally with lively tints, and is evidently possessed of considerable irritability, as it readily shrinks under the knife, or upon the application of various stimuli. When cut into, it has a semicartilaginous hardness, and fibrous bands, almost resembling tendon in their aspect, may be seen to radiate from the centre of the body towards the extremities of the rays. There is no doubt that the movements of the rays are effected by the contractions of this fibrous membrane ; and that, especially in the most polyp-like forms, as in Comatula and Gor- gonocephalus, the irritable skin is the principal agent in effecting locomotion. Besides the calcareous matter deposited in its interior, this outer covering of the star- fish appears to furnish several secretions of different descriptions. The colouring matter upon its surface is no doubt one of these ; as is a reddish fluid which exudes from the in- tegument of A. rubens, and is of so caustic a quality as occasion- ally to produce great irritation of the skin in persons by whom individuals of this species are incautiously handled : moreover, in A. aranciaca, the whole animal is coated with a thick mucus, so dense and filamentous that it may be raised in thin films resembling a cobweb, and might easily be taken for a cuticular covering. The exterior of the body is generally rendered rough and un- even by various structures, either imbedded in the substance of the coriaceous skin or projecting from its external surface. We have already described the articulated pieces attached to the rays of Comatula and others, which seem to be the most perfectly de- veloped forms of these cutaneous appendages. In the common star-fish of our own coast, similar spinous processes, but composed * The name of this family, and of its typical genus, is derived from ao-rng, a star. ECHINODERMATA. 147 of but one calcareous piece, are attached to the inferior margins of each ray, sometimes in several rows ; and, being still moveable, they may be useful in seizing prey, or even as assisting in progression. Upon the dorsal aspect of the body are other calcareous projec- tions, exhibiting a great variety of forms, so as to render the en- tire surface of the animal uneven and tuberculated. But the most remarkable appendages to the integument of the Asterias are minute bodies, which have been named by authors Pedicellarice, and have been looked upon by many naturalists as distinct animals, allied to polyps in structure, and living parasiti- cally upon star-fishes and other ECHINODERMATA. Each of these curious processes consists of a short stem fixed by one extremity to the skin of the Asterias, and terminating at the opposite end in two or three points resembling in some respects the prongs of a fork : the stem itself does not seem to be perforated by any canal ; but, nevertheless, the terminating points are found to be highly irritable, and quickly seize hold of any minute body placed between them. Some writers regard these bodies as organs of prehension, used under certain circumstances for fixing the animals which possess them ; but, from their small size and general appearance, they seem but ill adapted to such an office. (189.) The skeleton or calcareous framework imbedded in the skin of the Asteridse is by no means the least remarkable part of their structure : this consists of several hundred pieces variously disposed, and for the most part fitted together with great accuracy; being either firmly soldered to each other, as we have seen them to be in the formation of the calcareous box that constitutes the central portion of Ophiurus, or united by ligaments, so as to allow of a considerable degree of motion to take place between them, as in the rays of Ophiurus, Gorgonocephalus, and other asteroid forms. In the generality of star-fishes, the arrangement, and indeed the entire character of the calcareous plates, differs materially in differ- ent parts of the body; and, even in different species, considerable modifications are observable. In the coriaceous integument form- ing the dorsal parietes of the animal, the pieces in many cases seem rather to be represented by calcareous granules disseminated through the interior of the skin, or in other cases they are ar- ranged in lines anastomosing with each other in all directions, so as to represent, when the skin is dried, a rude network of solid par- ticles, upon the exterior of which the various cutaneous appendages already noticed are sustained. ECHIXODERMATA. It is, however, upon the ventral aspect of the Asterias that the skeleton assumes its most perfect developement ; the floor of every ray is made up of a continuous series of detached pieces, or verte- brae, as they are generally called, fitted to each other and united by a strong ligamentous substance, so as to form a succession of joints, upon which the flexibility of the ray depends. The pieces around the mouth constitute a strong circular framework enclosing the oral aperture, from which, as from a centre, the rest of the skeleton radiates. The joints forming the floor of the ray succeed to this ; these are partially represented in Jig. 67, where the soft parts having been removed from the ray marked &, their general arrange- ment is displayed. The vertebrae thus exposed are individually composed of several pieces, and each is articulated by oblique facets to those which precede and follow it ; a kind of union which admits of considerable motion, and provides for the flexibility of the ray, so as to render it capable of executing those movements which are requisite for the purpose of progression, or of seizing prey. The connection of the vertebras is effected in such a manner, that between each pair of calcareous plates minute orifices are left, which in the entire state of the ray are seen to be arranged in a quadruple series ; these holes give passage to the locomotive suckers, and from this circumstance have been named the am- bulacral holes, while the furrows seen upon the ventral surface into which they open are designated the ambulacra! grooves (fig- 64). (190.) The singular organs which, at the will of the animal, are protruded through the ambulacral apertures, forming the principal agents whereby, in the generality of species, locomotion is effected, next require our notice. In the annexed figure (Jig. 64) they are seen fully extended, projecting for some distance beyond the margins of the ambulacral grooves which occupy the centre of each ray, every one of them being furnished at its extremity with a sucking disc, adapted to take firm hold upon any smooth surface. The mechanism by which these suckers, or feet, as they are usually called, are extended from the body and again retracted, is very simple. That portion of each foot which is external to the shell is a muscular tube, closed at one extremity, namely, that where- unto the sucker is appended ; whilst, by the opposite, it communi- cates through the corresponding ambulacral hole with a globular contractile vesicle situated within the body of the animal. Both ECH1NODE11MATA. Fig. 64. 149 the tubular foot, and the vesicle appended to it, are endowed with a power of independent action, so that, if the vesicle contracts, the fluid within it is forced into the external tubular portion of the organ, which thus becomes distended and rendered erect ; but if, on the other hand, the muscular tube shrinks in turn, the contained fluid is forced back again into the internal vesicle, and the whole foot collapses. The arrangement referred to will be easily intel- ligible on reference to the rough diagram in the next page, which represents a longitudinal section of one of the rays of the Asterias depicted above. The internal vesicles (Jig. 65, 1, h) occupy the floor of each segment of the body, and, when viewed from above, (Jig. 67, d,) the entire series resembles strings of transparent beads placed above the rows of ambulacral apertures, through which they communicate with the tubular feet (fig- 65, 1, g). In fig. 65, 2, three of these organs are represented in different states of 150 ECHINODERMATA. extension, and their whole structure is developed. The foot, d, is shown protruded to its full extent; the vesicle, much contracted, has forced the fluid which it contained into the external tube (t), whereby it is rendered tense and prominent. The muscular coats, which invest the exterior of the protruded portion, are likewise de- picted ; the internal layer (&), immediately in contact with the membranous canal continued from the vesicle, is made up of longi- tudinal bands passing from the root of the organ towards the sucker at its extremity, while the outer layer (/) consists of cir- cular fibres, an arrangement evidently adequate to the performance of all required movements. The other portions of this diagram represent the feet in differ- ent stages of protrusion : in Jig. 65, 2, c, the vesicle being par- tially contracted, the tubular portion is seen in a medium state of distension ; and at b, the sucker is shown in a still more retracted state, the contained fluid having been completely expelled from the muscular tube, and driven back into the vesicle, which is dis- tended to the utmost. Fig. 65. The fluid that thus fills the suckers, and performs so important a part in causing all their movements, is not secreted by the vesi- cles in which it is contained, but is conveyed into them by a special vascular apparatus, (g, /, ) from which branches are given off to each tube. The nature of the fluid, however, and the ar- rangement of the vessels through which it flows, will be more properly discussed hereafter. ECHINODE11MATA. 151 (191.) The whole inner surface of the elaborately constructed box which forms the skeleton, as well as the integuments of the star-fish, is lined by a thin membrane, aptly enough called the pe- ritoneum ; for, like the serous tunic so named in higher animals, it not only spreads over the walls of the body, but is reflected there- from upon the contained viscera, so that they are completely in- vested by it, each viscus having a distinct mesenteric fold by which it is supported and retained in situ. (192.) The mouth of the Asterias occupies the centre of the lower surface of the body (j^g. 65, a). It is usually described as being a simple orifice entirely destitute of teeth, although it is not impro- bable that the osseous ring around it, and the articulated spines thereunto attached, may, to a certain extent, perform the office of a dental apparatus. The oesophagus is very muscular, and susceptible of great dila- tation, its parietes being gathered into deep longitudinal folds. The stomach (fig. 65, b) is a wide sacculated bag, occupying the central portion of the body, and, like the oesophagus, is evidently calculated to undergo considerable distension. There is no anal orifice, and consequently, as in the polyps, the indigestible parts of the food are again expelled through the mouth. The walls of the stomach, as well as those of the oesophagus, contain muscular fibres, and are further strengthened by fibrous bands, apparently of a ligamentous character, derived from the peritoneal covering which spreads over its outer surface. Ten narrow canals open by as many distinct orifices into the sides of the stomach ; each of which, after a short course, expands into a capacious caecum (fig. 65, 1, c). The whole of the digestive apparatus is displayed in Jig. 66 : every one of the five rays contains two of the caecal prolongations derived from the stomach or central bag (a) ; and in the rays marked c, d, e, these organs are represented in situ, but at /, they are seen raised from their natural position and carefully unravelled, so as to display more distinctly their complicated structure. When thus unfolded, the caeca present an arborescent appearance, the central canal being dilated into numerous lateral sacculi, from which in turn secondary pouches are given off; and in this manner innumerable ramifications are formed, so that the extent of internal surface is enormously increased, as may be seen in the ray g, in which, the upper walls of the caeca having been removed, their sacculated internal structure is rendered visible. ECHINODERMATA. (193.) With respect to the exact office of these capacious ap- pendages to the stomach, there exists some diversity of opinion. Fig. 66. It is scarcely possible that they can be at all instrumental in the digestion of food, the passages by which they communicate with the central cavity being too narrow to admit any solid substance into their interior ; the digestive process would therefore seem to be entirely accomplished in the receptacle into which the food is first introduced. But there is every evidence to prove that, although they can have little part in digestion, they are inti- mately connected with the absorption of nutriment ; and thus, although possessing no excretory orifice, they must be looked upon as strictly analogous in function to the intestinal canal of other animals : the great extent of surface which they present internally would alone lead to this supposition, even did not the nature of the material usually found in them, namely a pultaceous creamy fluid, evidently a product of digestion, abundantly confirm this view of their nature. The matter seems, however, to be put beyond a doubt by the arrangement of the vascular system connected with these organs, as the veins which ramify so extensively through their walls are here, as in other ECHINODERMATA, the only agents by which the absorption of chyle can be effected ; this will be evident when we examine the organs subservient to the circulation of the nutritious fluids. Those physiologists who have adopted a different view of the nature of the csecal appendages to the stomach, consider them to be adapted to the secretion of some fluid, and probably represent- ECHINODERMATA. 153 ing a biliary apparatus. Their enormous extent, however, would alone lead us to dissent from such a conclusion ; more especially as another organ has been pointed out to which the functions of a liver have been assigned. This is situated upon the base of the stomach (Jig. 66, Z>), and is a yellow or greenish-yellow racemose sacculus, which opens into the bottom of the digestive sac by a free aperture : the contents of this organ, moreover, resemble bile both in taste and colour.* In the slender-rayed genera, such as Ophiuru*, the csecal appendages are not met with ; but their deficiency appears to be supplied by the plicated walls of the stomach itself, the ^nu- merous folds of which resemble lateral leaflets attached to the cen- tral cavity. We are unacquainted with the precise organization of the alimentary canal in Comalula ,- but, from the orifices visible in the shell, it would appear that in this genus, as well as in some Crinoid species, the digestive tube was furnished with an anal aperture. (194.) The star-fishes, grossly considered, might be regarded as mere walking stomachs ; and the office assigned to them in the eco- nomy of nature, that of devouring all sorts of garbage and offal which would otherwise accumulate upon our shores. But, as we have already seen, their diet is by no means exclusively limited to such ma- terials, since crustaceans, shell-fish of various kinds, and even small fishes, easily fall victims to their voracity. Delle Chiaje found a human molar tooth in the stomach of an individual which he exa- mined. Neither is the size of the prey upon which they feed so diminutive as we might suppose from a mere inspection of the orifice representing the mouth ; for this is not only extremely dilatable, but, as we have found to be the case in the Actiniae, the stomach is occasionally partially inverted, in order more completely to embrace substances about to be devoured. Shell-fishes are fre- quently swallowed whole ; and a living specimen of Chama anti- quata, Lin., has been taken from the digestive cavity of an Asterias in an entire state. It appears, moreover, that it is not necessary for testaceous mollusca to be absolutely swallowed, shells and all, to enable the Asteridse to obtain possession of the enclosed animal, as they would seem to have the power of attacking large oysters, to which they are generally believed to be peculiarly destructive, and of eating them out of their shells. The ancients believed that, in order to accomplish this, the star-fish, on finding an oyster par* * Delle Chiaje, op. cit. 154 ECHINODERMATA. tially open, cunningly inserted one of its rays between the valves, and, thus gradually insinuating itself, destroyed its victim.* Mo- dern observations do not, as far as we are aware, fully bear out the above opinion of our ancestors as to the mode in which star-fishes attack oysters ; although the destruction which they cause is pretty generally acknowledged. The observations recorded by M. Eudes Deslongchamps upon this subject are however exceedingly curious.*)* As the waves had receded from the shore, so as to leave only one or two inches of water upon the sand, he saw numbers of Asterias rubens rolling in bunches, five or six being fastened together into a sort of ball by the interlacement of their rays. He examined a great number of such balls, and constantly found in the centre a Bivalve Mollusc (Mactra Stultorum, Lin.) of an inch and a half in length. The valves were invariably opened to the extent of two or three lines, and the star-fishes were always ranged with their mouths in contact with the edges of the valves. On detaching them from the shell which they thus imprisoned, he found that they had introduced between the valves large rounded vesicles with very thin walls, and filled with a transparent fluid. Each Asterias had five of these vesicles ranged around its mouth, but they were of very unequal size ; generally there were two larger than the rest, equal in size to large filberts, while the other three were not bigger than small peas. These vesicles appeared to be attached to the Asterias by short pedicles, and at the opposite end of each was a round open aperture, through which the fluid contained in the vesicle flowed out drop by drop. No sooner was the animal detached from the shell that it was thus sucking, than the vesicles collapsed and became no longer distin- guishable. The Mactra were all found to be more or less de- voured, some having only their adductor muscles left; but, however little they had been injured, all had lost the power of closing their * This maybe gathered from Aldrorando, who writes as follows : " AHi ostrea- rom bostes soot Stella; marina? moll& cntsti intectae, rer&am crodeltter, (at JElianos, Kb. ix. cap. 22, ait,) iotmicae ot hcc ipsas exedant et confidant Ratio iosidiantm qoas eis molhmter ejosmodi est. Com testacea soas patefaciant conchas, com vel refri- geratioaeegeot,Telotaliqaidpeittnensadirictam iocidat; eae, ooo de sois stve cnmbos sire radii* infra testas ostreae biaotis iosito eas clandi probibens, came implentar." Testae, lib. iiL page 497. Tbos likewise Oppian, " Sic strait insidias, sic sobdola fraodes Stella marina para L, sed mil to adjota lapillo Whiter, et pedUms scabris disjuogit Mantes." t Bulletin des Sciences de M. le Baron Ferossac, vol. *. p. 296. ECHINODERMATA. 155 valves and were apparently dead : nevertheless there was nothing to lead to the supposition that only dead shell-fishes were attacked, so that it is difficult to imagine how the delicate vesicles above de- scribed escaped injury from the closing of the valves. M. Deslong- champs thinks that probably the Asterias pours into the shell a torpifying secretion, and thus ensures the death of its victim. (195.) The absorption of the nutritious portions of the food in the Echinodermata is entirely accomplished by the veins distributed upon the coats of the digestive cavities, so that the chyle resulting from digestion is at once introduced into the vessels appropriated to circulation. In Asterias, the intestinal veins form a fine vascular network, covering the stomach and the ten digestive caeca. The venous trunks derived from all these sources unite to form a circular vessel (Jig. 67, e), which likewise receives branches derived from the ovaria and other sources. The circular vein thus formed, which seems to be the common trunk of the venous system, communicates with another vas- cular circle placed around the mouth (s), by means of a dilated 156 ECHINODERMATA. vertical tube of communication (/), which, from its muscular ap- pearance and great irritability, Tiedemann regards as being equiva- lent in function to a heart. The circle around the mouth (s) would seem to be arterial in its character ; and from it branches are derived which supply the various viscera of the body. But besides the vessels above described, apparently so disposed as to collect and distribute the nutrient fluids, there is another set of canals appropriated to the supply of the numerous vesicles connected with the locomotive suckers ( 190) ; these Tiedemann regards as being totally unconnected with the vascular system properly so called, and considers the fluid contained in them as quite of a different nature. Delle Chiaje, on the contrary, asserts that the two sets of vessels are derived from each other, and describes a peculiar apparatus connected with them as performing an important part in effecting the protrusion of the suckers. The circular vessel around the mouth, which forms the central receptacle of the vascular system, resembles a sinus analogous to those of the dura mater in man ; and is lodged in a groove between the oral circle of vertebrae and the pieces of the skeleton articu- lated therewith. Connected with the sinus above mentioned, and placed regularly in the interspaces between the rays, are several oval vesicles (Jig. 67, &, &), filled with a reddish-coloured transpa- rent fluid. These vesicles, which in dsterias aranciaca are seven- teen in number, communicate by distinct ducts with the central sinus, and are regarded by Delle Chiaje as reservoirs in which the nutritive fluids accumulate until expelled by the contraction of the vesicles. Besides the arteries above described as arising from the vascular circle around the mouth, according to the author last mentioned, vessels are given off which communicate with the am- pulls6 connected with the ambulacral suckers, apparently for the purpose of supplying to them the fluid which they contain. These vessels are seen to run along the floor of each ray, and to give off lateral branches communicating with every vesicle, as represented in the enlarged sketch (Jig. 62, 2 g) . By this arrangement it would seem that the contractile organs (Jig. 65, % e.) appended to the vascular sinus f, are in reality antagonists to the tubular structure of the feet, and serve as receptacles for fluid, which, by their con- traction, they can force into the whole system of locomotive suckers whenever the feet are brought into action. The above view of the arrangement of the vascular system of Asterias is, however, by no means universally admitted to be cor- ECHINODERMATA. 157 rect. Professor Sharpey agrees with Tiedemann in the opinion that the vessels of the feet form a system perfectly distinct from that of the blood-vessels, and even supposes that the fluid by which the ambulacra! tubes become distended is neither more nor less than pure sea-water. (196.) Before quitting this part of our subject, we must briefly mention a singular organ, apparently intimately connected with the circular vessel around the mouth, and called by Tiedemann the sand canal. This organ is represented in j#g. 67, enclosed in the same sheath as the dilated vessel (/), upon the right side of which it is placed ; it appears to communicate by one extremity with an isolated calcareous mass of a rounded figure, seen upon the exterior of the dorsal surface of the star-fish, while by its opposite extremity it opens apparently into the circular sinus which surrounds the mouth. The tube itself Dr. Sharpey describes* as being about the thickness of a surgeon's probe, and composed of rings of calcareous substance connected by a membrane, so that viewed externally, it is not unlike the windpipe of a small animal. On cutting it across, it is found to contain two convoluted laminae of the same nature as its calcareous parietes, which are rolled upon themselves in a longitudinal direction in the same manner as the inferior turbinated bones of an ox. The convoluted arrangement becomes more complete towards the upper end of the tube, where the internal laminae, as well as the external articulated portion, join the dorsal disc, appearing gradually to be- come continuous with its substance. The use of this curious organ is quite unknown, although a variety of conjectures have been hazarded upon the subject. The most probable appears to be that of Dr. Sharpey, who suggests that, should the fluid which distends the feet and the vessels connected with them be indeed sea- water, it may be introduced and perhaps again discharged through the pores of the disc by means of the calcareous tube, which will thus serve as a sort of filter to exclude impurities. (197.) The Asterias possesses no organs specially appropriated to respiration ; but the sea-water, being freely admitted into the general cavity of the body through a set of minute membranous tubes seen upon the exterior of the animal, bathes all the viscera, and consequently ensures a complete exposure of the circulating fluids to the influence of oxygen, the whole peritoneal surface per- forming the office of a respiratory apparatus. The mechanism by * Cyclopaedia of Anatomy and Physiology j art. Echinodermata. 158 ECHINODERMATA. which the surrounding element is thus drawn into the body, and the process by which its expulsion is effected, are not accurately known ; nevertheless, apparently with a view to ensure a continual circulation of aerated water through all parts of the system, the entire surface of the membrane which lines the shell, as well as that which forms the external tunic of the digestive organs, has been found to be covered with multitudes of minute cilia, destined by their ceaseless action to produce currents passing over the vascular membranes, and thus to ensure a perpetual supply of oxygenated water to every part.* But it is not only on the peritoneal surfaces that the existence of cilia has been detected, they are found to be extensively distributed over the external surface of the body, within the cavities of the tubular feet, and even over the whole internal lining of the stomach and caeca. This amazing apparatus of vibratile cilia must necessarily serve some important purpose in the economy of these creatures ; and Professor Sharpey, to whose observations upon ciliary motion phy- siology is deeply indebted, regards them as being most probably subservient to respiration. (198.) The organs belonging to the reproductive system in the Asterida exhibit the greatest possible simplicity of structure : there is no distinction of sex, neither have any parts been discovered in connection with the ovigerous organs, which can be regarded as ministering an accessory secretion. The ovaria (fig. 67, /, /) are slender cseca arranged in bunches around the oasophagus, two -dis- tinct groups being lodged at the origin of each ray. In Asterias aranciaca (fig. 67), the excretory ducts are not easily seen ; but in the twelve-rayed star-fish, especially if examined when these organs are in a gravid state, each ovary may be observed to com- municate externally by a wide aperture, which perforates the osseous circle encompassing the mouth. (Fig. 65, f.) (199.) In order to complete the history of the Asterida, we have yet to mention the nervous apparatus with which they are furnished. This consists of a simple circular cord, which runs around the mouth of the animal ; from this ring, three delicate filaments are given off opposite to each ray, one of which, according to Tiede- mann, runs along the centre of the ambulacral groove upon the under surface of the body, and gives minute twigs to the locomo- tive suckers placed on each side of its course ; the other two fila- * See the article Cilia by Dr. Sharpey, in the Cyclopaedia of Anatomy and Physiology. ECHINODERMATA. 159 ments pass into the visceral cavity, and are probably distributed to the internal organs. There are no ganglia developed on any part of this nervous apparatus ; or at least, if, as some writers assert, ganglionic enlargements are visible at the points whence the ra- diating nerves are given off, they are so extremely minute as not in any degree to merit the appellation of nervous centres. (200.) Such an arrangement can only be looked upon as serving to associate the movements performed by the various parts of the animal, for no portion of these simple nervous threads can be re- garded as being peculiarly the seat of sensation or perception. But this inference is not merely deducible from an inspection of the anatomical character of the nerves ; it is based upon actual experiment. We have frequently, when examining these animals in a living state, that is, when with their feet fully developed they were crawling upon the sides of the vessels in which they were confined, cut off with scissars successive portions of the dorsal covering of the body so as to expose the visceral cavity ; but, so far from the rest of the animal appearing to be conscious of the mutilation, not the slightest evidence of suffering was visible : the suckers placed immediately beneath the injured part were inva- riably retracted; but all the rest, even in the same ray, still continued their action, as though perfectly devoid of participation in any suffering caused by the injury inflicted. Such apathy would in- deed seem to be a necessary consequence resulting from the defi- ciency of any central seat of perception, whereunto sensations could be communicated ; nevertheless Ehrenberg insists upon the exist- ence of eyes in some species of star-fish, attributing the function of visual organs to some minute red spots visible at the extremity of each ray, behind each of which he describes the end of the long nerve which runs along the ambulacral groove as expanding into a minute bulb. We must however confess, that the proofs adduced in support of such a view of the nature of these spots, appear to us to be anything but satisfactory; and as we have already stated in the first chapter the physiological objections which may be urged against the possibility of any localised organ of sense being co- existent with a strictly nematoneurose condition of the nervous sys- tem, they need not be repeated here. The general sense of touch in the Asteridse is extremely delicate, serving not only to enable them to seize and secure prey, but even to recognise its presence at some little distance, and thus direct these animals to their food. Any person who has been in the habit of fishing with a line in the 160 ECHINODERMATA. shallow bays frequented by star-fishes, and observed how frequently a bait is taken and devoured by them, will be disposed to admit this; yet to what are we to attribute this power of perceiving exter- nal objects ? It would seem most probably due to some modifi- cation of the general sensibility of the body, allowing of the per- ception of impressions in some degree allied to the sense of smell in higher animals, and related in character to the kind of sensation by which we have already seen the Actiniae and other polyps able to appreciate the presence of light, although absolutely de- prived of visual organs. (SOI.) The ECHINI, however they may appear to differ in out- ward form from the Asteridoe, will be found to present so many points of resemblance in their general structure, that the detailed account we have given above, of the organization of the last-mentioned family, will throw considerable light upon the still more elaborately con- structed animals which now present themselves to our notice. The Echinida, as we have already observed, differ from the star-shaped Echinodermata in the nature of the integument which encloses their visceral cavity, as well as in the more or less circular or spherical form of their bodies ; so that the locomotive apparatus with which they are furnished is necessarily modified in its cha- racter and arrangement. The shell of an Echinus (Jig. 68, 1) is composed of innu- EOHINODERMATA. 161 merable pieces accurately joined together, so as to form a globular box enclosing the internal parts of the animal, but perforated at each extremity of its axis by two large openings, one of which represents the mouth, and the other the anus. The calcareous plates entering into the composition of this ex- traordinary shell may be divided into two distinct sets, which differ materially in size, as well as in the uses to which they are subser- vient. The larger pieces are recognisable in the figure by hemisphe- rical tubercles of considerable size attached to their external surface, adapted, as we shall afterwards see, to articulate with the moveable locomotive spines. Each of these larger plates has somewhat of a pentagonal form; those which are situated in the neighbourhood of the mouth and anal aperture being considerably the smallest, and every succeeding plate becoming progressively larger as they ap- proximate the central portion of the shell : the entire series of pieces in each row resembles in figure the shape of the space included between two of the lines which mark the degrees of lon- gitude on a terrestrial globe, broad at the equator, but gradually narrowing as it approaches the poles ; an arrangement, of course, rendered necessary by the spherical form of the creature. There are ten rows of these tuberculated plates ; but as they are disposed in pairs, each row of large pieces being united by a zig-zag suture with another of a similar description, there are in reality only five large segments of the shell, each supporting a double row of tubercles. The reader must not, however, conclude that the great central tubercles above mentioned are the only parts of the shell to which spines are affixed ; hundreds of smaller elevations are disseminated over the surface, to which smaller spiculae are appended, although, from their diminutive size, these are of secondary importance in locomotion. The five large double segments which thus form the greater por- tion of the calcareous shell are separated from each other by the interposition of ten rows of perforated plates, likewise disposed in pairs, and composed of much smaller pieces than those which sup- port the tubercles ; hundreds of foramina, which pierce these ambu- lacral bands, give passage to as many tubular feet or protrusible suckers, in every respect resembling those of Asterias, and dis- tended by a similar apparatus. It is impossible by any verbal description, at all commensurate with the limits of our present undertaking, adequately to explain M 162 ECHINODERMATA. the more minute contrivances visible in the disposition of every portion of these wonderfully constructed coverings : it is sufficient for our present purpose to observe that the globular crust of an Echinus is made up of several hundred polygonal pieces of different sizes, and, although presenting every variety of outline, generally approximating more or less to a pentagonal form ; that these pieces are so accurately and completely fitted to each other, that the lines which unite them are scarcely to be distinguished even upon the most minute examination ; and that from the union of so many distinct and dissimilar plates results a firm, compact, and beautiful box, similar to that represented in the figure. The first question which naturally suggests itself on examining a shell of this description, is concerning the object to be attained by such remark- able complexity ; it would appear indeed, at first sight, that a simple calcareous crust, had it been allowed to exude from the entire sur- face of the Echinus, would gradually have moulded itself upon the body of the creature, and thus have formed a globular shell with- out suture, but answering every purpose connected either with support or defence. (203.) A very little investigation, however, will suffice to show the necessity for the elaborate arrangement to which we have alluded. In the first place, as we shall immediately see, the earthy matter is not deposited upon the surface of the body, but within the soft external integument by which it is secreted ; the interior of the shell being filled with sea-water, in which the viscera are loosely suspended. But a second and more important reason for the em- ployment of so many pieces in the construction of the shell of an Echinus is to be derived from examining the mode in which the animal grows ; was it to retain the same dimensions throughout the whole period of its life, or could it at stated intervals cast off its old investment, and secrete a new and more capacious covering, as growth rendered the change necessary, a simple earthy crust would have been sufficient, without the presence of such an immense number of sutures and joinings. The calcareous plates of the Echinus, it must be remembered, are merely secreted from the soft parts, having no vital action going on within them, by which, as in the bones forming the skeletons of vertebrate animals, a con- tinual deposition of fresh particles could be effected, allowing of extension by interstitial deposit. How, therefore, could the growth of the animal be provided for ? How is the gradual expansion of the entire shell, thus composed of a dense and extravascular crust, ECHINODERMATA. 163 to be effected ; and that without ever deranging the proportions of the whole fabric, or necessitating a loosening of its parts ? No other contrivance could apparently have been adequate to the pur- pose : nevertheless, by the structure adopted, we see how admirably the growth of Echinus proceeds in all directions; for the living and vascular membrane which covers the whole external surface of the body dips down between the edges of the various calcareous pieces, and continually deposits around the margin of each, successive layers of earthy particles, which, assuming a semi-crystalline ar- rangement, progressively increase the dimensions of each individual plate. But the continual augmentation in size, which is thus going on, is attended with no change in the mathematical figure of any given piece of the skeleton ; so that, as they all increase in diameter by the unceasing deposition of earthy matter around the circumference of every plate, the spherical shell gradually expands, without in any degree altering its form or relative proportions, until it has acquired the mature dimensions belonging to its species. (204.) The tubular suckers or retractile feet, which are pro- truded at the pleasure of the animal from the countless minute apertures seen in the ten rows of ambulacral plates, are so similar in all essential points to those of Asterias already described, that little further need be said concerning their structure, or the mechanism by which their motions are effected. The tubular part of each foot communicates with the interior of the shell by two branches which pass through two apertures, and these branches in some species (as Echinus saxatilis) receive offsets from the ves- sels which run along the centre of each ambulacral groove, and convey to the feet the fluid by which their distension is effected. In Echinus esculentus the feet open into a plexus of vessels, formed in leaf-like membranes, equal in number with the feet, and disposed in double rows upon the inner surface of the ambulacral pieces,* by the intervention of which they are connected with the canals above mentioned. (205.) The tubercles upon the external surface of the shell of the Echini support a corresponding number of long spines, which, as well as the apparatus of suckers, are employed as locomotive agents. These spines vary materially in their form and propor- tionate size, and even in their internal structure and mode of * Cyclopaedia of Anat. and Phys. art. ECHINODERMATA. M 9 M /< 164 ECHINODERMATA. growth, as may be readily seen by a comparison of different species. Thus, in the flattened forms of Scutella and allied genera, they are so minute as to require the employment of a mi- croscope for their investigation ; in Echinus esculentus (Jig. 62) they are sharp, and almost of equal length over the entire surface of the animal; while in the specimen represented in the an- nexed figure (Jig. 69), the shell of which we have already Fig. 69. examined when divested of these appendages, the length of the spines which are articulated upon the large tubercular plates fully equals the transverse diameter of the body of the creature, and in some cases they are even found much more largely developed. Every spine, examined separately, is seen to be united with the tubercle upon which it is placed by an apparatus of muscular and ligamentous bands, forming a kind of ball-and-socket joint, allow- ing of a considerable extent of motion. In^/zg. 68, 2, the structure of this articulation is exhibited. The large tubercle (a) supports upon its apex a smaller rounded and polished eminence, perforated in the centre by a deep depression : the bottom of the spine, moreover, (c) is terminated by a smooth hemispherical cavity accurately fitted to the projecting tubercle, so that the two form complete articular surfaces. The bonds of union connecting the spine with the shell are of two kinds : in the first place, there is a stout ligament (a, c), extending from the little pit seen upon the centre of the tubercle, to a corresponding depression visible upon the articular surface of the spine, resembling very accurately ECHINODERMATA. 165 the round ligament found in the hip-joint, and obviously a provi- sion for the prevention of dislocation. The whole joint is moreover enclosed in a muscular capsule, composed of longitudinal fibres (b, b) arising from the circum- ference of each tubercle, and inserted all around the root of the spine : these fibres therefore, which must in fact be regarded as merely derived from the general irritable skin that clothes the shell externally, are the agents which, acting immediately upon the spine, produce all the movements of which it is capable. (206.) The next thing to be accounted for in the history of these elaborately constructed animals is the growth of the spines themselves, which, as we have already seen, are completely detached from the rest of the shell, to which they are only secured by the central ligament, and by the muscular capsule enclosing their base. To account, therefore, for the production of organs so completely insulated as the spines appear to be, especially when we consider that there is no vascular communi- cation between them and the body of the Echinus, would appear to be a matter of some difficulty; and in fact r had we not already seen in the polyps the amazing facility with which calcareous matter was secreted by the living textures of those animals, ifc would be almost impossible to conceive by what process their growth was effected. On examining one of these appendages, taken from a species in which they are largely developed, when fresh, before its parts have become dry, every portion of its surface is seen to be invested with a thin coat of soft membrane, derived from that which covers and secretes the whole shell, of which indeed the muscular capsule enclosing its articulation with the tubercle is only a thickened portion. The living covering of the spine therefore, like the crust which invests the cortical polyps, is the secreting organ provided for its growth, depositing the earthy particles separated from the waters of the ocean, layer after layer, upon its outer surface, so as to form a succession of concentric laminae, of which the outer one is always the^last formed. The calcareous matter thus deposited has more or less completely a crystallized appearance ; and on a transverse section of the organ being made, and the surface polished by grinding, the whole process of its formation is at once rendered evident. Such sections, indeed, form extremely beautiful and interesting subjects for microscopical examination, as nothing can exceed the minute accuracy and mathematical precision with which 166 ECHINODERMATA. each particle of every layer composing them appears to have been deposited in its proper place : indeed, if the zootomist would fully appreciate the minuter details connected with their organization, it is only by the employment of the microscope that he will arrive at adequate ideas concerning them ; for it is not in the num- ber and variety of the pieces entering into the composition of the skeleton of one of these animals, the extraordinary apparatus of prehensile suckers with which they are furnished, or the singular locomotive spines upon the exterior of the shell, that he will find the most remarkable features of the history of the Echini ; it is only by a minute examination of the intimate structure of each of these parts that the mechanism conspicuous throughout can be properly understood. (207.) The calcareous pieces which surround the mouth of the Echinus are not so immoveably consolidated as those composing the rest of the shell, but, on the contrary, admit of considerable move- ment, by which the prehension of food is more easily effected. The mouth itself (Jig- 68, 1) is a simple orifice, through which the points of five sharp teeth are seen to protrude. These teeth obviously perform the office of incisors, and, from their sharpness and extreme density, are well calculated to break the hard sub- stances usually employed as food. The points of such incisor teeth, although of enamel-like hardness, would nevertheless be speedily worn away by the constant attrition to which they are necessarily subjected, was there not some provision made to ensure their perpetual renewal ; like the incisor teeth of rodent quadrupeds, they are therefore continually growing, and are thus always pre- served sharp and fit for use. In order to allow of such an arrange- ment, as well as to provide for the movements of the teeth, jaws are provided, which are situated in the interior of the shell ; and these jaws, from their great complexity and unique structure, form perhaps the most admirable masticating apparatus met with in the whole animal kingdom ; we must therefore entreat the patience of our readers while we describe at some length the parts connected therewith. The entire apparatus removed from the shell is repre- sented in (Jig. 70), and consists of the following parts : There are five long teeth, (c, c,) each of which is enclosed in a triangular os- seous piece, (a, a,) that for the sake of brevity we will call the jaws. The five jaws are united to each other by various muscles, (&, &, i, i 9 ) so as to form a pentagonal pyramid, having its apex in contact with the oral orifice of the shell, while its base is con- ECHINODERMATA. 167 nected with several bony levers, by means of numerous mus- cles provided for the movements of the whole. These parts we must now proceed to describe seriatim. The teeth (fig. 71, a) resemble, at the part protruded from the mouth, long three- sided prisms, and at this point they are extremely hard and brit- tle : each tooth is fixed in a socket passing through the jaw, (fig. 71, e,) from which it projects by its opposite extremity, (fig. 71, a',) that may be called the root of the tooth, where, instead of being of glassy hardness like the point (a) which issues from the mouth, it is flexible and soft, resembling fibres of asbestos, and is covered by a membrane apparently connected with its secretion. The jaws, which thus support and partially enclose these teeth, are five in number : when examined separately, each is found to resemble in figure a triangular pyramid, the external surface (fig. 71, e,) being smooth, and presenting eminences provided for the attachment of muscles ; while the other two sides (fig. 71, b, b^ are flat, and marked with transverse grooves, so as to have the appearance of a fine file. When the five jaws are fixed together in their natural positions, they form a five- sided conical mass, aptly enough compared by Aristotle to a lantern, and frequently described by modern writers under the name of the " lantern of Aristotle." When thus fitted to each other, the two flat and striated sides of each jaw are in apposition with the corresponding surfaces of two others, so that 168 ECHINODERMATA. there are ten grinding surfaces formed, between which the food must pass preparatory to its introduction into the digestive canal. This arrangement will be easily understood by referring to a m l Jig. 71, 1, in which three of these jaws, each containing its in- cisor tooth, are represented in situ, the two others having been removed. The five curious jaws described above are fixed together by a set of muscles, (Jig. 70, &, A:,) consisting of short fibres passing between the external edges of the -contiguous segments of the lantern, and evidently capable of powerfully approximating the grinding surfaces and rubbing them upon each other. The jaws, moreover, are provided with five other osseous pieces (rf, d,) arranged in a radiating manner between the bases of the different segments, with which they are connected by ligaments, and like- wise by the pentagonal muscle (z, i,) which runs from one to the other. The above described parts complete the apparatus required for connecting the different portions of this remarkable mouth, but the movements of the whole are effected by a very complicated set of levers and muscles which must next be noticed. The levers attached to the jaws are five long and slender pro- cesses, (Jig. 71, 1 d, d,) each arising from the central extremity of one of the radiating osseous pieces, (c, c,) and arching outwards con- siderably beyond the base of the lantern, to terminate by a forked extremity. But there are likewise other processes projecting from the inner surface of the shell ; these, two of which are seen in (Jig. 70, 6, ,) are also five in number, and are placed around the orifice of the mouth : they are generally perforated in the centre, ECHINODERMATA. 169 so as to resemble so many bony arches ; and from them, as well as from the spaces which separate them, numerous muscles derive their origin. Of these muscles, ten (/>/->) arise from the spaces between the arches, two being inserted into the outer edge of the base of each jaw ; so that the effect produced by their contraction, when they all act in concert, will be to approximate the whole mass of the mouth to the oral aperture of the shell, and of course cause the points of the incisor teeth to protrude externally ; or, if they act separately, they can draw the base of the lantern in any di- rection, or cause the grinding surfaces of the jaws to work against each other. The antagonists to the muscles last mentioned are ten others, (> >) arising from the extremities of the arches themselves, and running in a radiating manner towards the apex of the lantern, so that the point of each piece or jaw receives a muscle from two of those processes. These fasciculi, from the manner in which the arches project into the cavity of the shell, will draw inwards the entire mass ; or, if they act separately upon the jaws to which they are individually fixed, they will produce movements precisely op- posite to those caused by the contractions of the muscles derived from the spaces between the bony processes ; or, if both sets should act in concert, they become the antagonists of the muscles (i,i,k, &,) which connect the jaws to each other, and by causing the separa- tion of the different pieces they necessarily enlarge, not only the opening of the mouth, but all the passage leading to the oesopha- gus through the axis of the lantern. Yet even these are not all the muscles which act upon the masticating apparatus ; ten others, (A, A,) arising in pairs from the middle of the interspaces between the arches, are connected with the bifurcated extremities of the slender curved processes, (e, e,) each of which receives a muscle from two contiguous spaces ; and, from the length of the levers upon which these muscles act, we may well conceive the force with which they will influence the motions of the whole mass of the jaws. Such is the complex structure of the mouth of Echinus escu- lentus ; a piece of mechanism not less remarkable on account of the singularity of its construction, than as exhibiting an example of the sudden developement of a dental system, of which not a vestige is visible in any other of the Echinoderm families. In others of the Echinida having the shell much depressed, the dental lantern is modified in form, and proportionately flattened, 170 ECHINODERMATA. Fig. 72. but the different parts are essentially similar to those we have described. (208.) The oesophagus (Jig. 72, d,) is continued from the termi- nation of the central canal, which traverses the axis of the lantern, and after a short course termi- nates in a much wider portion of the digestive tube, into which it opens on the lateral part of its csecal origin in a manner precisely resembling the com- munication between the large and small intestines of man. The dilated alimentary tube, (c,) which presents no separa- tion into stomach and intestine, is continued in a winding course around the interior of the shell, which it twice encircles, and, becoming slightly con- stricted, terminates at the anal orifice of the shell (z). The walls of the intestine are ex- tremely delicate ; although they may be distinctly seen to con- tain muscular fibres, and are covered with innumerable vas- cular ramifications. The external tunic of the whole canal is de- rived from the peritoneum, which lines the entire shell, invests the dental lantern, and forms sundry mesenteric folds as it is reflected upon the other viscera. (209.) The system of vessels provided for the circulation of the blood has been differently described by different authors, a circum- stance by no means surprising when we consider the great difficulty of tracing such delicate and extensively distributed canals. Ac- cording to Delle Chiaje, the course of the nutritious fluid is as follows. A large vein runs along the whole length of the intes- tine, from the anus to the oesophagus, where it terminates in a vascular ring surrounding the mouth ; into which, as in Asterias, the contractile vesicle, which he considers to be a receptacle for the nutrient fluid, and the antagonist to the tubular x feet, like- wise opens. The intestinal vein he regards as the great agent ECHINODERMATA. 171 in absorbing nourishment from the intestine, and conveying it to the vascular circle around the oesophagus, from which the arteries are given off to supply the whole body. These arte- ries are, 1st, a long vessel to the intestine, which runs along its whole length, and anastomoses freely with the branches of the intestinal vein. Sndly, Five arteries to the parts con- nected with the mouth. Srdly, Five dorsal arteries which run along the interior of the shell, between the ambulacral rows as far as the anal orifice, at which point each dorsal artery leaves the osseous box, through an aperture specially provided for its exit, and, arriving upon the outer surface of the shell, supplies the soft external membrane, and in some species may be traced back again between the rows of ambulacral suckers as far as the mouth. These dorsal arteries, like the corresponding vessels in Aste- rias, supply the vascular origins of the innumerable protractile feet. (210.) We found in the star-fish that respiration was provided for by the free admission of the external element into the interior of the body ; and in Echinus the aeration of the blood is effected in an equally simple manner. The sea-water is copiously admitted into the peritoneal cavity by a set of membranous tubes provided for the purpose ; and its due circulation over the lining membrane of the shell, as well as over the outer surfaces of the intestine and other viscera, is provided for by ciliary movements visible in all those situations, and likewise upon the vascular laminae connected with the origins of the feet.* Nevertheless, besides this diffused respiration, Delle Chiaje re- gards a series of pinnated tentacula in the neighbourhood of the mouth as being in some degree capable of performing the office of branchiae. These organs, which are protruded through a row of dis- tinct orifices placed around the oral aperture of the shell, are emi- nently vascular ; and as they present a large surface to the action of the water, and receive numerous vessels from the circular vessel which surrounds the mouth, they may no doubt very well contri- bute to the complete exposure of the blood to the influence of the surrounding medium. (211.) Little is known concerning the nervous system of the Echini : a few delicate filaments have been observed in the neigh- bourhood of the oesophagus, apparently of a nervous character, which renders it probable that a nervous ring is placed in that vici- nity, resembling that already described in Asterias ; its presence, * Dr. Sharpey, loc. cit. 172 ECHINODERMATA. however, owing to the complexity of the dental apparatus, has not been satisfactorily demonstrated, although analogy would lead us to infer the existence of such an arrangement. (21 2.) The Echini, like the star-fishes, exhibit no distinctions of sex : all are fertile, and in the structure of their reproductive organs, display, if possible, greater simplicity of arrangement than even the Asteridce above described. The ovaria are five delicate mem- branous bags, quite distinct from each other, which open exter- nally by as many delicate tubes, or oviducts, as we may term them. The apertures through which the eggs escape are easily seen upon the outer surface of the shell, placed around the anus ; and are re- cognisable not merely by their size, but from the circumstance of each perforation being placed in the middle of a distinct oval plate of the shell, distinguished by zoological writers as the ova- rian pieces. The membranous sacs in which the ova are secreted vary in size, in proportion to the maturity of the eggs contained within them, and at certain times of the year are enormously dis- tended : it is in this state that the " roe of the sea-egg," as the ovaria are commonly called, is used as an article of food ; and in some countries, especially upon the shores of the Mediterranean, they are eagerly sought after, when in season, by divers employed to procure them. (213.) Holothuridce. The name applied by naturalists to the animals composing the next family of Echinodermata is derived from a Greek word of uncertain application (oAo0oup*ov). In common language they are generally known by the appellation of " sea-cucumbers ;" and in fact, to a casual observer, the resem- blance which they bear to those productions of the vegetable kingdom, both in shape and general appearance, is sufficiently striking. The surface of these animals is kept moist by a mucus, which continually exudes through innumerable pores, and appears to be secreted by minute follicles imbedded in the substance of the skin. The integument which covers, or rather forms the body, is entirely destitute of those calcareous pieces which encase the Echini and Star-fishes ; but appears to consist of a dense fibrous cutis of considerable thickness, covered externally with a thin epidermic layer. Beneath the cutis is another tunic com- posed of strata of tendinous fibres crossing each other in the midst of a tissue of a semicartilaginous nature, which is capable of very great distension and contraction, and serves by its elasticity to retain the shape of the body. Within this dense covering are seen muscular ECHINODERMATA. 173 Fig. 73. bands running in different directions, which by their contraction give rise to the various movements of the creature ; of these muscle five strong fasciculi assume a longitu- dinal course, pass- ing along the entire length of the ani- mal from the mouth to the cloaca, and in the interspaces between these cir- cular and oblique muscles are readi- ly distinguishable. The whole of this muscular case is lined with a deli- cate membrane or peritoneum, from which processes pass inwards, to support the various viscera. (214.) But al- though the calca- reous shell of the Echinus is thus to- tally lost, the lo- comotive suckers or feet already de- scribed are still the principal agents employed in progression. In many species, as in that represented in the annexed figure, (Jig. 73,) these organs are distributed over the whole surface of the animal, and are pro- truded through countless minute orifices which perforate the in- tegument. In other cases, as in H. frondosa, they are arranged in five series, resembling the ambulacra of an Echinus ; and in some instances they are only found upon the middle of the ventral sur- face of the body, that forms a flattened disc upon which the ani- mal creeps, somewhat in the manner of a snail. The ambulacral feet themselves, represented on an enlarged scale at (c), pre- 174 ECHIXODERMATA. cisely resemble in all the details of their structure those of the Asterias, and their protrusion and retraction are effected in the same manner ; but, in addition to these organs, we find in some genera moveable hooks or spines (Jig. 73, d,) which are likewise retractile, and most probably assist in locomotion. (SI 5.) The mouth is a round aperture, as wide as a goose-quill, placed in the centre of a raised ring at the anterior extremity of the body (Jig. 73, a). Around the, oral orifice is placed a circle of tentacula, which are apparently extremely sensible, and serve per- haps not only as instruments of touch, but as prehensile organs used for the capture of prey, or for assisting in deglutition. When the sphincter muscle which closes the mouth contracts, the tentacles are withdrawn, and become no longer visible externally ; in this state, on opening the animal (Jig- 74, &,) they are found to resemble long caeca appended to the commencement of the oesophagus, which have been described by some authors as forming a salivary apparatus. The total deficiency of any external skeleton, or calcareous frame- work, precludes, of course, the possibility of the existence of any complex dental apparatus resembling the " lantern of Aristotle ;" the only vestige of the complex teeth of the Echinidse which here remains is a small circle of calcareous pieces, surrounding the opening of the mouth : these plates, from their extreme friability, have been aptly enough likened to laminae of dried paste ; they may indeed in some slight degree be efficient in bruising food taken into the mouth, but it is more probable that they merely form points of insertion to the longitudinal muscles of the body, which, thus fixed around the circumference of the oral orifice, will by their contrac- tion powerfully dilate that aperture for the purpose of taking in nourishment. The alimentary canal is of great length, but, like that of the Echinus, presents no stomachal dilatation ; from the mouth, (Jig. 74, a,) in which a bristle is placed, it descends to the anal extremity of the body, where, turning upon itself, it again mounts up towards its commencement, whence turning back again, and forming numerous convolutions, (d, d, rf,) it once more passes backwards, and, becoming restricted near its termination, opens into a large membranous cavity (e) which may be called the cloaca. Throughout the whole of this long course, the alimentary tube is surrounded with a membrane derived from the peritoneal lining of the visceral cavity, which forms delicate mesenteric folds connecting it to the walls of the body, and supporting it through ECHINODERMATA. 175 its entire length. The whole intestine is generally found distended with sand, in which may be detected the debris of corals, algse, fuci, and other marine substances. (216.) In the structure of the respiratory apparatus, the Holothu- ridse differ materially from the rest of the Echinodermata, and in fact from all other animals. In the Asterida and Echinida, the reader will remember that respiration was effected by the free admission of sea-water into the interior of the animal, which, thus penetrating to every part of the body, rendered the existence of special respiratory organs unnecessary. In the Holothuria like- wise the aeration of the circulating fluid is provided for by allow- ing the surrounding element freely to enter into the internal parts of the creature ; but in this case, instead of bathing the surfaces of the viscera, the water is confined in a peculiar system of ramifying canals, forming a structure of great beauty, and, from its singularity, extremely interesting in a physiological point of view. We have seen that the intestinal canal terminates in a membranous recep- tacle or cloaca {Jig. 74, e,) contained within the cavity of the Fig. 74. 176 ECHINODERMATA, abdomen, to the walls of which it is attached by delicate fleshy bands : this cloacal cavity communicates with the exterior 'of the body by a wide orifice twice as large as the aperture of the mouth, through which, in the figure, a bristle (f) has been passed ; it is by this hole that the water required for the purpose of respiration is taken in, and it is then forced by the muscular walls of the cloaca itself through the whole system of respiratory canals by which its distribution is effected. The organs of respiration commence at the upper part of the cloaca, near the termination of the intes- tine, by a large opening leading to a wide membranous tube, which immediately divides into two vessels (g, g,) forming the main trunks of the beautiful arborescent branchiae, which extend to the opposite extremity of the body, giving off in their course numerous lateral branches that divide and subdivide, so as to form what has been not inaptly termed the " respiratory tree," until they ulti- mately terminate in minute vesicular caeca, into which the water derived from the cloaca of course penetrates. One division of this elegant apparatus is maintained in close contact with the walls of the body by a series of delicate tendinous bands, while the other becomes applied to the convolutions of the intestines, with which it is likewise united. It is this last-mentioned division which would appear to be specially provided for the oxygenization of the nutritive fluids taken up by the intestinal veins. (217.) The circulation of the blood in the Holothuria, as in the Echinus, is still but imperfectly understood, and considerable difference of opinion upon this subject will be found in the writings of anatomists. According to Tiedemann,* innumerable small veins collect the blood and nutritive products of diges- tion from the intestine, and convey them into a large central vessel, (Jig. 74, i, i,) from whence the circulating fluid passes by other trunks (/, /,) to the respiratory tree ; hence it is re- turned by vessels (partly represented at m) to the intestinal artery (k) 9 by which it is again distributed over the intestinal parietes. Delle Chiaje gives a different account of the arrangement of the vascular system in these creatures, which he seems to have investigated with his usual untiring perseverance. According to the last-mentioned anatomist, the blood is taken up from the in- testines by a complicated system of veins, the main trunks of which * Anat. der Rohren, Holothuriej fol. 1816. ECHINODERMATA. 177 are indicated in the annexed diagram {fig. 75) by the letters c, e, p, p, </, q ; these communicate with each other not only by the intervention of numerous Fig. 75. anastomosing branches, (d, d,) but likewise by means of de- licate vascular plexuses (a) passing between them. All these veins terminate in two large venous canals (o), which convey the blood and nutri- ment absorbed from the in- testine to a vascular circle (g) 9 placed around the com- mencement of the oesopha- gus, which corresponds with the circular vessel around the mouth of the Echinus. This circle Delle Chiaje re- gards as the centre of the arterial system, in communi- cation with which is the con- tractile vesicle (/), which he looks upon as a reservoir for the nutritive fluid. From the circular vessel various arteries are given off; large branches pass into the tentacula around the mouth (z), so that these organs, besides being instruments of touch, from the extent of surface that they present, and their great vas- cularity, are most probably important auxiliaries in respiration. Five other large arteries, derived from the same source, (&, fc, /,) pass backwards to supply the integuments of the body, and also to communicate by small cross branches with the little vesicular organs connected with the locomotive suckers, which in the opinion of Delle Chiaje are distended with the same blood as that which circulates through the rest of the body. The descend- ing arteries, thus destined to supply the integument and distend the prehensile suckers, run in the centre of each of the five lon- gitudinal fasciculi of the muscular tunic of the skin as far as the cloaca, and exhibit in their distribution a remarkable exception to the usual arrangement of the arterial system, which is generally found to divide and subdivide continually into smaller and still smaller canals, but, in the case before us, there would seem to be N 178 ECHINODERMATA. no diminution in the size of the main trunks as they approach their termination ; and the cross branches given off in their course, instead of ramifying, all end in the minute ambulacral vesicles, to the injection of which they would appear to be subservient. (218.) The generative system of the Holothuria is essentially similar to that found in the Asteridse, consisting of long ovigerous caeca, without any superadded parts which might be regarded as con- tributing to the impregnation of the ova. The germs are secreted in slender ramified tubes (Jig. 74, A, h,) which are collected into one great bundle, and open externally by a common canal in the neighbourhood of the mouth, not into the oesophagus as Cuvier supposed, but upon the back of the animal. These generative cseca at certain times of the year become enormously distended, being at least thirty times larger than when not in a gravid state ; if examined at this period, they are found to contain a whitish, yellowish, or reddish fluid, in which the ova are suspended, but nothing is known concerning the mode of the expulsion of the eggs, or their subsequent developement. (219.) The special instruments of touch, the only sense allotted to these animals, are the branched tentacula around the mouth, which seem by far the most irritable parts of the body. The nervous system is so obscurely developed that even Delle Chiaje was unable to detect any traces of its existence ; nevertheless there is little doubt of the presence of nervous threads in the muscular envelope of the animal, although, from the dense tissues in which they are imbedded, it is next to impossible to display their course ; most probably, as in the Echinus and Asterias, these com- municate with a circular cord which embraces the oesophagus. No ganglia have as yet been discovered even in the Holothuria ; and consequently, although the muscular actions of the body are no doubt associated by nervous filaments, the movements of these creatures appear due rather to the inherent irritability of the muscular tissues themselves, than to be under the guidance and control of the animal. In many species, the slightest irritation applied to the surface of the body causes such powerful contrac- tions of the integument that the thin membranes of the cloaca, unable to withstand the pressure, become lacerated, and large portions of the intestine and other viscera are forced from the anal aperture. So common indeed is the occurrence of this circum- stance as to have induced the older anatomists to suppose that, by a natural instinct, the animals when seized vomited their own ECTIINODEIIMATA. 179 bowels. It is in fact extremely difficult to obtain perfect speci- mens of tlie Holothuridse, from the constant occurrence of this ac- cident : but, although annoying to the naturalist, such a pheno- menon affords the physiologist an important lesson, teaching that here, as in the lower Zoophytes, the muscular system possesses an innate contractile power, which would seem only to be destroyed by incipient putrefaction ; but so little is this contractility under command, that, once excited to an inordinate extent, it becomes totally unmanageable, even though its continuance inevitably causes the destruction of life. (220.) Fistularida. In order to complete our account of the organization of the Echinodermata, we have still to investigate the structure of the Fistularid<z ; a group which, from the external appearance of the individuals composing it, and the total absence of the tubular feet met with in other families, has been improperly separated by some modern writers from the class under consideration. Nevertheless, we shall find the position assigned to these animals by Cuvier to be in strict accordance with the character both of their outward form and internal structure ; only, instead of placing them with the lowest of the Echinoderms, they would have been more properly situated at the head of the class, as most nearly ap- proximating the Annelida in all the details of their economy. We have already given a description of the outward form of a Fistu- laria ( 186), and seen the completely annulose condition of its body, although the radiating tentacula around the mouth are evi- dently analogous to those of the Holothuria already described. We are indebted to the patient researches of Pallas and Delle Chiaje* for almost all that is known concerning the anatomical structure of these animals, and the descriptions of the Siponculus phalloides and bala- nophorus have left little to be desired by the systematic zootomist. The Siponculus inhabits shallow seas, concealing itself at the bottom in holes which it excavates in the sand. Having once located itself, it is seldom found to quit its concealment, but, re- taining its hold upon the sides of the retreat which it inhabits by dilating the posterior part of its body, it occasionally protrudes its head from the orifice, either for the purpose of procuring food, or of respiring more freely the element in which it lives. These animals are much sought after by fishermen, who employ them as baits for their hooks ; and one species, Siponculus edulis, is used in China as an article of food. * Storia e Notomia delle Animate senza Vertebre del Regno di Napoli. Napoli, 1823. N 2 180 ECHINODERMATA. (221.) The body is covered externally with a delicate cuticle, easily separable by maceration or immersion in spirit of wine ; and when thus detached it forms so loose a covering, that Linnseus, deceived by the appearance of an animal thus preserved, applied to it the name of Siponculus saccatus. The muscular investment, placed beneath the skin, is composed of strong fasciculi arranged in three distinct layers. The external stratum is disposed in circular rings, beneath which spiral fibres may be observed crossing each other at various angles ; and lastly, the inner coat is made up of about thirty powerful longitudinal bands, extending from one extremity of the body to the other. Such an arrangement is evidently sufficient for the general move- ments of the creature ; but, in order to facilitate the retraction of the tentacular apparatus around the mouth, eight additional muscles surround the oesophagus, and by their action the whole of the oral apparatus is completely inverted and drawn inwards. The tentacula around the oral orifice are the principal agents employed in seizing and swallowing food, an office to which they are peculiarly adapted by their great sensibility and power of con- traction ; but, as we have found to be generally the case among the Echinodermata, sand and fragments of shell form the great bulk of the contents of the intestine, so that it is by no means easy to state precisely the nature of the food upon which the Siponculi are nourished. (222.) The structure of the alimentary canal, and of the nutrient apparatus, conforms too accurately with what we have already seen in Holothuria to permit of a moment's hesitation concerning the relationship which exists between the apodous Echinodermata and the Holothuridse. The oesophagus (fig. 76, b) is narrow, and soon dilates into a kind of stomachal receptacle (c) ; but, although the diameter of the intestinal tube is at this point perceptibly larger than in any other part of its course, there is no other pecu- liarity to distinguish it from the rest of the intestine. In the ANNELIDA, the digestive apparatus is invariably straight, travers- ing the body from one extremity to the other, a circumstance which distinguishes them remarkably from the Echinoderms we are now considering ; for in Siponculus we find a digestive canal, six or seven times the length of the animal, within which it is folded upon itself in various distinct convolutions. Leaving the stomach, if we may so call the dilatation above alluded to, it passes down (d, J, d,) nearly to the tail, where it is reflected upon itself, and ECHINODERMATA. 181 mounts up again as far as the point where it commenced ; here it again turns back, and, once more reaching the bottom of the tegu- mentary sac, becomes a second time directed upwards, and re- ascends as far as the point (e), where the anus is situated. It is easy to account for this extreme . length of the intestine when we consider the nature of the materials used as food, and the small proportion of nutri- ment contained among the sand and broken shells which fill the digestive canal : but the re- markable position of the anal aperture is only explicable by a reference to the peculiar habits of the creature ; for living as it does in a narrow excavation bored in the sand, from which it seldom issues, had the excrements been discharged, as in Holothuria, through a terminal orifice, their accumulation at the bottom of the hole would soon expel the animal from its retreat ; but, by the arrangement adopted, it is only necessary that the anterior part of the body should be pro- truded from its concealment, and the excrementitious matter may be cast out without incon- venience. The intestine is retained in situ, and supported at all points, by innumerable tendinous bands, which arise from the interior of the muscular walls of the body, and form a kind of mesentery. (223.) In Sipomulus, the character of the circulating system is in all essential points strictly analogous to that of the other Echinoder- mata; and moreover, from the superior concentration visible in every part, we have the multiplied organs of the other families ex- hibiting so much simplicity of arrangement, that, whatever may ECHIXODERMATA. have appeared obscure or complicated in our description of Echi- nus and Holothuria will receive elucidation from the diagram- matic form in which all the organs connected with the circulation of the blood are represented in the adjoined figure. The intestinal vein (m) may be traced along the entire length of the alimentary canal ; commencing near the anal extremity of the bowel, it fol- lows all its convolutions, and receives from every part the minute vessels which ramify over the intestinal walls. These venous ramifications undoubtedly perform the office assigned to the lac- teals of higher animals, and imbibe the nutritive particles furnished by digestion, which, of course, are conveyed into the great venous trunk (m). Arrived opposite to the termination of the oesopha- gus, the intestinal vein divides into two vessels : one performing the office of a branchial artery, by conveying a part of the blood to the respiratory organs in the neighbourhood of the mouth ; the other, which we may call the aorta, distributing the remainder to all parts of the tegumentary system. The branchial vessel (M) runs from the bifurcation of the intestinal vein to the base of the oral tentacles, where it forms a vascular circle around the com- mencement of the oesophagus, analogous to that which we have seen in Holothuria ; and in connexion with this circular vessel we find the " ampulla Poliana " (A), which Delle Chiaje conceives to be here, as in other cases, a receptacle for the circulating fluid. From the vascular circle around the mouth, vessels are given off, to ramify minutely through the substance of the tentacula (a), so that these appendages may be considered as respiratory organs, like those of Holothuria. The other vessels derived from the oral circle have not been traced ; but we may conclude from analogy that arteries supplying the mouth and alimentary canal are furnished from this source. The aorta (o) is the other large vessel derived from the intes- tinal vein, and is seen to pass in a flexuous course from its origin to the posterior extremity of the body, following the median line, and giving off transverse branches on both sides opposite to every ring of the muscular integument. At the commencement of the aorta is a dilated vesicle (/), which may be looked upon as a heart (auricle, Delle Chiaje). The vesicle alluded to is of a conical form, the apex of the cone being directed towards the tail of the animal ; and, from the impossibility of making mercury pass from the aorta through this organ in the direction of the intestinal vein, it is probable that it contains an apparatus ECHINODERMATA. 183 of valves so disposed as to prevent any retrograde motion of the blood. At the termination of the aorta there appears to be a second enlargement, to which the name of ventricle has been given, and which is perhaps also capable of contraction, so as to assist in the propulsion of the circulating fluid. The blood of these animals is of a purple colour in the veins, but red in the arterial vessels. (224.) We have seen that the tentacula are, from their vascu- larity, well adapted to fulfil the office of a respiratory apparatus ; but it may be presumed that they are not the only agents by which respiration is accomplished. Upon the outer surface of the body, in the neighbourhood of the anal opening, two apertures arc visible, which lead into two long sacculi (jf, j^), the entrance being guarded by muscular fibres (g) : their texture presents transverse and longitudinal striae, and they contract spontaneously even after the animal is dead ; internally they are lined with a mucous membrane. The use of these organs is not precisely known ; Cuvier regarded them as belonging to the generative system, while Delle Chiaje looks upon them as respiratory organs, inter- mediate in structure between the arborescent tubes of Holothuria, and the respiratory vesicles which we shall afterwards find in some of the ANNELIDA. (225.) In this elevated form of the Echinodermata, so nearly allied to the Homogangliate type, we may naturally expect a more complete developement of nervous ganglia than we have yet met with in the class ; and accordingly we find, upon the an- terior part of the oesophagus, two little nervous tubercles (t), from which nervous filaments issue to be distributed to different parts of the body ; one of these in particular may be traced along the whole length of the intestine from the mouth to the anus. (226.) We are entirely ignorant concerning the mode of repro- duction in these creatures, as no generative apparatus has as yet been distinctly pointed out. Nevertheless, at certain seasons of the year, on opening the visceral cavity, it is found to be filled with a fluid of a reddish tint, in which thousands of minute white bodies resembling millet-seeds are seen to float : should these be ova, they are probably expelled through an orifice which exists in the vicinity of the tail. 184 CHAPTER XII. HOMOGANGLIATA (Owen). ARTICULATA (Cuv.) ; ANNULOSA (Mac Leay). ) The third great division of the animal kingdom includes an immense number of living beings adapted by their conforma- tion to exist under a far greater variety of circumstances than any which we have hitherto had an opportunity of examining. The feeble gelatinous bodies of the ACRITA are obviously only adapted to an aquatic life ; and accordingly they are invariably found either to inhabit the waters around us, or to be immersed in the juices of living animals upon which they subsist. The NEMATONEURA, likewise, are all of them too imperfect in their construction to ' admit of their enjoying a terrestrial existence, for, possessing no nervous centres adequate to give force and precision to their move- ments, they are utterly incapable of possessing external limbs endowed with sufficient power and activity to be efficient agents in ensuring progression upon land ; neither are any of them furnished with those organs of sense which would be indispensable for the security of creatures exposed to those innumerable accidents to which the inhabitants of a rarer element are perpetually obnoxious : the NEMATONEURA therefore are, from their organization, neces- sarily confined to a watery medium. But the type of structure met with in the HOMOGANGLIATA admits of far higher attributes, and allows the enjoyment of a more extended sphere of existence : senses become developed propor- tionate to the increased perfection of the animal ; limbs are pro- vided endowed with strength and energy commensurate with the developement of the nervous ganglia which direct and control their movements ; and instincts are manifested in relation with the increased capabilities and more exalted powers of the various classes as they gradually rise above each other in the scale of animal developement. (228.) The most obvious, though not the most constant, cha- racter which distinguishes the creatures we are now about to de- scribe, is met with in their external conformation ; they are all of them composed of a succession of rings formed by the skin or HOMOGANGLIATA. 185 outward integument, which from its hardness constitutes a kind of external skeleton, supporting the body, and giving insertion to the muscles provided for the movements of the animal. In the class CIRRHOPODA alone is this external characteristic wanting, and the Homogangliate organization masked by a tegumentary tes- taceous coat of mail, which they seem to have borrowed from the molluscous type. In the lowest forms of the ARTICULATA the body is extremely elongated, and the rings proportionately nu- merous ; the integument moreover is soft and yielding, and, as a necessary consequence, the limbs appended to the different seg- ments are feeble and imperfect : such is the structure met with in the worms, or ANNELIDANS, properly so called. As we advance, we perceive the tegumentary rings to become less numerous, and the skin of a denser and more firm texture, adapted to support the action of stronger and more powerful muscles ; the limbs likewise become more elaborately formed, their movements more free and energetic, and the instruments of sight and touch begin to assume considerable perfection of structure. This state of developement we find in the MYRIAPODA or Centipedes. In the INSECTS the concentration of the external skeleton is still more remarkable, and the integument assumes a hardness and solidity proportioned to the vigorous movements of which the limbs are now capable ; the rings or segments of the body, hitherto distinct, become more or less firmly soldered toge- ther in those parts where the greatest strength and firmness are necessary, and scarcely any traces are left to indicate their ex- istence as separate pieces ; so that, instead of exhibiting that succession of similar segments seen in the Centipede, the body is apparently divided into three distinct portions, viz. the head, which contains the organs of the senses and the parts of the mouth ; the thorax, sustaining the limbs or instruments of pro- gression ; and the abdomen, enclosing the viscera subservient to nutrition and reproduction. In the fourth division of articulated animals, namely the ARACHNIDANS or Spiders, a still greater consolidation of the external skeleton is visible ; for in them even the separation between the head and the thorax is obliterated, and it is in the abdomen only that the segments of the body are recognisable. Lastly, in the CRUSTACEANS we have various modifications of the outward skeleton adapted to the habits of the different tribes ; 186 HOMOGANGLIATA. in the least perfect species, which are all aquatic, the rings of the skeleton are perfectly distinct and separate, resembling those of the Myriapoda ; but in the stronger and more predacious tribes, the pieces of the head and thorax become solidly fixed together ; and in those forms most adapted to a terrestrial life, namely, the crabs, almost all traces of distinction between the thoracic segments are lost in the construction of the calcareous shield which covers and protects their whole body. (229.) We see therefore in the above rapid sketch of the dif- ferent classes which compose the articulated division of the animal kingdom, that, as their organization assumes greater perfection, the different segments of the external skeleton coalesce and become united together, so as to give greater strength to those parts which are more immediately connected with locomotion or the destruc- tion of prey ; let us now examine the nature of the nervous appa- ratus which characterises the HOMOGANGLIATA, and observe the relation which the outward form of the body bears to the arrange- ment of this primary system of the animal economy. In tracing the developement of animal structure, on the first appearance of any new apparatus, it is by no means unusual to find it repeated again and again in the same creature, divided as it were into distinct portions, prior to its appearance in its more highly organ- ized and perfect condition. Thus in Ccenurus cerebralis, 110, the reader will remember numerous mouths were dispersed over different parts of the simple sac composing the stomach of the animal ; in the compound Polyps, 36, innumerable digestive organs ministered to the support of one common mass ; in the Tape-worm, 117, the generative apparatus was repeated in nearly every segment of its compound body ; and, did we choose to antici- pate, other examples might be adduced, derived from the more per- fect animals, exemplifying the same fact. We shall not be sur- prised, therefore, to find that, on the first developement of a nervous system provided with ganglionic masses, these nervous centres, or brains as we might term them, are very numerous, and, instead of being united, are located in different parts of the system. In the humblest forms of the Annulosa it would seem indeed that every ring of the body contained a complete nervous apparatus, consisting of a pair of ganglia and a set of nerves destined to supply the particular segment in which they are lodged. All these different brains, belonging to the individual segments, communicate with each other by nervous filaments, so that a continuous chain is HOMOGANGLIATA. 187 formed, passing along the whole length of the body. With the exception of the anterior pair of ganglia, or that contained in the first ring, which we may call the head of the worm, the nervous centres are arranged along the ventral region of the body, that is, beneath the alimentary canal ; but the anterior pair itself is inva- riably placed upon the dorsal aspect of the animal, and communi- cates with the rest by a nervous collar which embraces the com- mencement of the oesophagus. The nervous masses placed along the belly would seem to preside specially over the movements of the segments to which they belong, and to have little to do with sensation or the perception of external objects ; whilst the anterior or cephalic pair, from the constancy of their communication with the organs of the senses, would appear peculiarly in relation with the perceptive faculties of the creature. (230.) It may be taken as a general law, that the perfection of the nervous system of any animal may be estimated by the propor- tionate size of the central ganglia connected with it, upon the developement of which both the energy of the actions of the body and the completeness of perception depend ; and, by follow- ing out this great principle, we shall be easily able to account for the progressive steps by which the Articulata become more and more perfectly organized, as we trace them in the series above in- dicated. In proportion as we have found the segments of the body to become less numerous, the appended limbs stronger, the outward skeleton more dense, and the muscular powers more ener- getic, we shall find the abdominal ganglia to dimmish in number by becoming consolidated into larger masses, increasing in size and energy in accordance with the developement of the limbs over which they preside : and in the same manner we shall observe the senses assume greater perfection of structure, and the instincts become more developed, as we find the cephalic or anterior pair of brains increasing in proportionate bulk. These observations will suffice to introduce the student to the Homogangliate division of the animal world, and to direct his at- tention to those physiological points connected with the nature of their nervous system which will be more fully laid before him in the following pages. 188 CHAPTER XIII. ANNELIDA.* Red-blooded Worms. (Cuv.) (231.) The lowest class of articulated animals comprehends an ex- tensive series of creatures generally grouped together under the com- mon name of Worms. In the outward form of their bodies many of them resemble some of the more perfect Entozoa, and we need not therefore be surprised that in ordinary language they are not unfrequently confounded together. But whatever may be the similarity in outward appearance between the more perfect intes- tinal worms, and the animals belonging to the class upon the con- sideration of which we are now entering, the examination of their anatomical structure will at once show that they differ widely from each other, and have thus been properly separated by a consi- derable interval in all the more modern systems of zoological arrangement. (232.) The principal characters which serve to distinguish the Annelida from other forms of the animal world are readily appre- ciated ; and, when once pointed out, will be found sufficient for the guidance of the most superficial observer. The body is always considerably elongated, and composed of a succession of rings or segments, which, with the exception of the first and last, scarcely differ from each other except in size. Each ring is generally found to be furnished with a set of short spines or setae, calcu- lated to assist in locomotion ; but in no instance are these animals provided with articulated legs. The first segment of the body, which may be called the head, contains the mouth, sometimes provided with a formidable apparatus of jaws ; and is also generally furnished with eyes, and variously shaped tentacula, which are apparently instruments of touch. The last segment also, which is generally the smallest, occasionally presents setiform appen- dages, and occasionally a prehensile sucker, used as an organ of progression. Their blood is remarkable for its red colour, and circulates in a double system of arteries and veins ; respiration is effected either in membranous sacculi contained within the body, or by means of arborescent tufts appended to various parts of their ex- * Annellus, a little ring. ANNELIDA. 189 ternal surface ; they are moreover almost all hermaphrodite, and generally require the congress of two individuals for mutual im- pregnation. (233.) These animals are separated by Cuvier into three distinct orders, distinguished by the nature and position of their organs of respiration ; they are as follows : ABRANCHIA. In this order there is no respiratory apparatus visible externally, but on each side of the body a series of minute apertures may be detected, whereby the surrounding medium is admitted into numerous internal delicate sacs, over which the blood-vessels are seen to ramify ; these form apparently the re- spiratory system : the sacculi themselves, and the ducts by means of which they communicate with the external apertures, are de- lineated in^g. 80, 2, ra. This order comprises two distinct tribes, that differ widely in their habits and external appearance : the first comprehends the LEECHES (Annelida suctoria), distinguished by the existence of a prehensile sucker at each extremity of the body ; while, in the second, instruments of attachment are totally wanting, the only external appendages to the body being a number of minute and almost imperceptible bristles, which project from the different segments and assist in progression : such are the EARTH-WORMS, &c. (Annelida terricola.) DORSIBRANCHIATA. In the second order the respiratory appa- ratus consists of numerous vascular tufts, a pair of which is ap- pended to the outer surface of every ring of the body, or, in some cases, only to those near the middle of the animal. The organs of locomotion, which are likewise attached to each segment, assume various forms, but are generally composed of short moveable spines, or packets of retractile bristles, probably destined to perform the office of oars. In the annexed figure, (fig. 77, 1,) which repre- sents the Leodice antennata, the general form of these animals is well seen, as is the most usual arrangement of the branchial tufts and locomotive setae, \nfig. 77, , showing an imaginary trans- verse section of one of the segments, the relative positions of the oars (c, rf, e), and of the branchial appendages (&), are likewise indicated. TUBICOLA. The two preceding orders of Annelidans are erratic ; but in the third we find creatures inhabiting a fixed and permanent residence, which encloses and defends them. This is generally an elongated tube, varying in texture in different species. Sometimes it is formed by agglutinating foreign substances, such 190 ANNELIDA. as grains of sand, small shells, or fragments of various materials > by means of a secretion which Fig. 77. exudes from the surface of the body, and hardens into a tough membranous substance, such is the case of Terebella Medusa (fig. 96). In other cases, as in the Serpula contortuplicata \ (fig- 78), the tube is ho- mogeneous in its texture, formed of calcareous matter resembling the shells of cer- tain bivalve mollusca, and ap- parently secreted in a similar manner. These tubes are ge- nerally found encrusting the surface of stones or other bo- dies which have been immersed for any length of time at the bottom of the sea ; they are closed at one end, and from the opposite extremity the head of the worm is occasionally pro- truded in search of nourish- ment. It must be evident that, in animals thus encased, the character of the respiratory ap- paratus must be considerably modified ; instead therefore of the numerous branchiae ap- pended to the segments of the body which we have found in the Dorsibranchiate order, the respiratory tufts are all at- tached to the anterior extre- mity of the creature, where they form most elegant arbo- rescent appendages, generally tinted with brilliant colours, and exhibiting, when expanded, a spectacle of great beauty. In some species, as in that repre- ANNELIDA. 191 sen ted in the annexed figure, there is a remarkable provision made for closing the entrance Fig. 78. of the tube when the animal retires within its cavity. On each side of the mouth is a fleshy filament resembling a tentacle ; but one of these, sometimes the right and sometimes the left, is found to be considerably prolonged, and expanded into a funnel-shaped opercu- lum, which accurately fits the orifice of the shell, and thus forms a kind of door, well adapted to prevent in- trusion or annoyance from external enemies. (234.) Abranchia. The common Leech (Hirudo medicinalis) affords the most interesting example of a suctorial Annelide. The outward form of one of these animals is familiar to every one, and their general habits too well known to require more than a very brief notice. The body is very extensible, and divided by a great number of transverse lines into numerous rings, extremely apparent in the contracted state of the animal, but nearly imperceptible when the body is elongated. The skin is soft, being merely a thin cuticular pellicle separable by maceration ; and the surface is lubri- cated by a copious secretion of mucus. Beneath the cuticle is a layer of coloured pigment, upon which the colours of the animal depend ; but the cutis, or true skin, is so intimately connected with the muscular integument of the body, that its existence as a distinct tunic is scarcely demonstrable. The muscular covering or walls of the body, which form a kind of contractile bag enclosing the viscera, is found, upon accurate dissection, to consist of three distinct strata of fibres running in different directions. The outer layer is composed of circular bands passing transversely ; in the second, the fibres assume a spiral arrangement, decussating each other ; while the internal layer is made up of longitudinal 192 ANNELIDA. muscles, extending from one end of the creature towards the oppo- site. Such an arrangement is evidently adequate to the produc- tion of all needful movements, and capable of giving rise to all the motions connected with the elongation, contraction, or lateral inflexions of the body used in progression. At each extremity of the animal, the muscular coat expands into a flattened fleshy disc, composed of circular and radiating fasciculi, which, when applied to a smooth surface, perform the office of suckers, and thus become important instruments of prehension. There are no vestiges of external limbs ; nevertheless, with the simple mechanism above described, the leech is able to crawl with considerable rapidity along the surface of subaquatic plants, or even to swim with much facility through the water. The first method of locomotion is accomplished by means of the terminal suckers : supposing the posterior disc to be attached, the animal elongates its body to the utmost, and then fixes the sucker placed at the opposite extremity ; this done, the hinder parts are drawn forward and again fixed, preparatory to a repetition of the process. In swimming, the whole body is elongated, and by some partial con- tractions of the muscular integument, not precisely understood, assumes the appearance of a flattened band, and in this condition the leech makes its way through the element which it inhabits, by successive undulatory movements of the body performed with much grace and elegance. (235.) The mouth of the leech is an exceedingly perfect appa- ratus, adapted not only to the destruction of those minute aquatic animals which constitute its usual food, but, as is universally known, admirably fitted to extract blood from the higher animals ; combining, in its operation, the offices both of the cupping-glass and the scarificator. The mouth is situated near the centre of the anterior sucker, so that the oral aperture is firmly applied to any surface upon which this part of the animal is fixed. Around the entrance of the oesophagus are disposed three minute cartilaginous teeth, im- bedded in a strong circle of muscular fibres (fig. 79, 1). Each tooth has somewhat of a semicircular form, and, when accurately examined with a microscope, is found to have its free margin sur- mounted with minute denticulations (fig. 79, 2), so as to resem- ble a small semicircular saw. On watching a leech attentively during the process of biting, the action of these teeth is at once evident ; for, as the skin to which the sucker is adherent is ANNELIDA. 193 rendered quite tense, the sharp serrated edges of the teeth are pressed firmly against it, and, a sawing movement being given to each cartilaginous piece by the strong contractions of the muscular fibres around the neck, these instruments soon pierce the cutis to a considerable depth, and lay open the cutaneous vessels, from which the creature sucks the fluid which its instinct prompts it to seek after with so much voracity. The position of the teeth around the opening of the mouth, as represented in the subjoined figure, (fig- 79, A,) will at once explain the cause of the tri-racliate form of the incision which a leech-bite invariably exhibits. On contemplating Fig. 79. this singular dental ap- paratus found in the / \ medicinal leech, and considering the na- ture of the food upon which it usually lives, it is difficult to avoid arriving at the conclu- sion that such a struc- ture, which is indeed only met with in one or two species, is ra- 3 A ther a provision in- tended to render these creatures subservient to the alleviation of human suffering than necessary to supply the wants of the animals themselves. In the streams and ponds which they usually in- habit, any opportunity of meeting with a supply of the blood of warm-blooded vertebrata must be of rare occurrence, so that comparatively few are ever enabled to indulge the instinct which prompts them to gorge themselves so voraciously when allowed to obtain it : neither does it appear that the blood which they swallow with so much avidity is a material properly suited to afford them nourishment ; for although it is certainly true that it will remain for a considerable time in its stomach, without becoming putrid, yet it is well known that most fre- quently the death of the leech is caused by such inordinate reple- tion, provided the greater portion of what is taken into the body is not speedily regurgitated through the mouth. (236.) The internal digestive apparatus is evidently adapted in the construction of all its parts to form a capaciovis reservoir for the 194 ANNELIDA. reception of fluids taken in by suction : the stomach indeed, with the numerous lateral appendages opening from it on each side, would seem to fill the whole body ; and, being extremely dilatable, allows the animal to distend itself to a wonderful extent, so that it is not unusual to see a leech, when filled with blood, expanded to five or six times the dimensions which it presented in an empty state. The stomach itself (Jig. 80, 1, A, i,) occupies about, two thirds of the visceral cavity ; on opening it, as represented in the figure, it is seen to be divided by delicate septa into nine or ten compartments, which communicate freely with each other. In each compartment we observe two lateral orifices leading into as many wide membranous pouches (&), which although shrunk and flaccid when in an undistended state, as they are seen in the figure, are easily filled with fluid introduced into the stomach, and are then swelled out into very capacious bags. Perhaps the sim- plest way of obtaining a correct idea of the relative sizes and general arrangement of these organs, is to make a cast of their internal cavities when in a state of distension ; this is readily effected by placing a dead leech in warm water until it is slightly heated : in this state the pipe of a small injecting syringe can be introduced into the oesophagus so as to fill the stomach and cseca with common wax injection ; and, if the body be immediately re- moved into a vessel of diluted muriatic acid, the soft parts will be speedily destroyed, leaving an exact model of the interior. It will then be seen that the lateral caeca increase gradually in size as they approximate the posterior extremity of the body, until the last pair (d) become so large as nearly to fill up the space intervening between the end of the stomach and the anal boundary of the visceral cavity. What is the exact nature of these capa- cious sacs which thus open into the stomach of the leech ? Are they prolongations of the digestive surface, or are they glandular cseca provided for the secretion of some auxiliary fluids poured into the stomach ? These are questions which admit of considerable discussion. On the one hand, there can be little doubt that, when the leech is filled with blood, the various csecal pouches become likewise distended, and they are apparently as well calculated to effect the digestion of their contents as the stomach itself. Those physiologists, however, who embrace a different opinion, support their views by referring to the structure of analogous parts found in other ANNELIDANS : in Aphrodita aculeata, for example, the representa- ANNELIDA. 195 tives of the wide pouches met with in the leech are narrow and branched tubes terminating in blind extremities, to which it is usual to assign the office of separating a biliary secretion ; and, according to this view, we may regard the caeca of the leech as the simplest rudiments of the assistant chylopoietic glands, the first pair (g, g) 9 from their proximity to the mouth, may be destined to furnish a salivary fluid, and the succeeding ones to perform the functions of biliary follicles. The small size of the intestine (e), when compared with the capacious stomach described above, is remarkable : it commences by a minute orifice from the termination of the digestive cavity, and becoming slightly enlarged passes in a straight line, lodged between the two posterior cseca, to the anus, which is an almost imperceptible aperture placed at the root of the posterior sucker ; four small and apparently glandular masses are appended to this short canal, but their nature is unknown. The entire alimentary apparatus is retained in situ by numerous membranous septa, (m, w,) passing between its outer walls and the muscular parietes of the body. (237.) It has already been mentioned, that, in the abranchiate Annelidans, the organs provided for respiration are a series of membranous pouches, communicating externally by narrow ducts or spiracles, as they might be termed, into which aerated water is freely admitted. These respiratory sacculi, in the leech, are about thirty-four in number, seventeen being visible on each side of the body : they are extremely vascular ; and in connection with every one of them there is a long glandular-looking appendage, repre- sented in the figure, (Jig. 80, 2, m,) that was looked upon until recently as being intended to furnish some important secretion, but which recent discoveries have shown to be connected with the propulsion of the blood over the walls of the breathing vesicle, in a manner to be explained immediately. It would seem, however, that the respiratory function is not exclusively carried on by the agency of the lateral sacculi : the entire surface of the body is permeated by innumerable delicate vascular ramifications ; and, from the thin- ness of the integument, it is evident that the blood which tra- verses the cutaneous net-work thus extensively distributed must be more or less completely exposed to the influence of oxygen contained in the surrounding medium ; nay, it would even appear from careful examination of the movements of the blood, as seen in the transparent bodies of some of the Hirudinidte, that a kind o % !<)(> .\\\i i u>\. ol' \icarious action occurs bet\\eeu the capillary vessels of the skin :md those of the respiratory sacs, So that \\hen the circulation pro- cceds languidly tlin>ii"Ji om- sd of \essels, it is carried on with :ler aeliyitx in the other. Fig. 80. 1 2 3 (288.) The vessels appropriated to the distribution of the circu- lating ilnid in the leech are rudely sketched iu.//ir. 80, 8. Then- is no heart, Imt the movements of the blood arc entirely dne to the contractions of the canals in which il Hows. The principal vascu- lar trunks are four in number, which, although they all communi- cate extensively with each other, perform distinct ollices in cllcct- ing the circulation; two of them being specially connected \sith the supply of the general system, while the other two seem sub scnicnt to the distribution of the blood over the respiratory saecnli. A \NI<: 1.1 1) \. 1 1)7 The two systemic trunks ( /', g) run along the mcsian line of l.lir body ; one upon the <lors.il, ;in<l llir oi.lirr upon the ventral as|>cet. The dorsal vessel (y) seems to be ;i.rl.eri;i.l in its eha- raetrr, and no doubt corresponds in function with the heart of more |>nfeet forms of the articulata ; receiving the blood from all parts of the system, as'well from the respiratory vessels as from the venous capillaries, and by successive undulatory contractions, uhieh may be observed to proceed from the tail towards the an- terior extremity, propelling it through all the arterial branches derived from it. The ventral vessel (g), on the contrary, seems to be venous, collecting the blood after its passage through I he systemic capillaries, and returning it partly into the dorsal artery from wliieli it set out, and partly to the lateral vessels for the purpose of undergoing respiration. The two lateral vessels (a, c) are appropriated to the supply of the respiratory system, and in them the blood moves in a circle quite independent of that formed by the dorsal artery and ventral vein, although they all communicate freely by means of cross branches, those passing from the lateral vessels to the dorsal being called by M. Duges* dor so- lateral , while those which join the lateral trunks to the ventral canal arc the latero-abdo- minal branches of that observer. The movement of, the blood in the lateral or respiratory system of vessels is quite distinct from that which is accomplished in the dorso- ventral or systemic trunks : sometimes it passes down the vessel marked a, from the head towards the tail, and in an opposite direction on the other side of the body ; but in a short time the movement of the cur- rents will be seen to become completely reversed, so that an un- dulatory motion, rather than a complete circulation, is kept up. By this action of the lateral canals the blood is made perpetually to pass and repass the respiratory sacculi ; and, opposite to each of these, branches are given off which form so many independent vascular circles, representing very closely the minor or pulmonary circulation of higher animals. (239.) On examining attentively one of the respiratory pouches (Jig. 81, y ), its membranous walls arc seen to be covered with very line vascular ramifications, derived from two sources: the latcro-ab- dominal vessel (d) gives off a branch (c), which is distributed upon the respiratory sacculus ; and there is another very flcxuous vas- * Annalcs dcs Sciences Nat. vo). xv. 198 ANNELIDA. cular loop (b) derived from the lateral vessel itself (a), which ter- minates by ramifying upon the vesicle f, in a similar manner. The Fig. 81. walls of the loop &, are extremely thick and highly irritable ; but, on tearing it across, the internal cavity or canal by which it is perforated is seen to be of comparatively small diameter, so that we are not surprised that, although such appendages to the respira- tory sacs were detected and well delineated by former anatomists,* their nature was unknown, and they were supposed to be glandular bodies appropriated to some undiscovered use. From the ar- rangement above described, it is evident that small circular cur- rents of blood exist, which are independent, to a certain extent, of the general circulation ; since opposite to each membranous bag a portion of the fluid contained in the lateral vessel (a) is given off through the muscular tube (&), which thus resembles a pulmonary heart, and after being distributed over the walls of the respiratory vesicle, and in this manner exposed to the influence of oxygen, the blood returns into the general circulation. (240.) The nervous system of the leech (Jig. 80, 2, k) consists of * Delle Chiaje, op. cit. Moquin Tandon, Monographic sur la famille des Iliru- dinees, 4to. Montpellier, 1827. ANNELIDA. 199 a long scries of minute ganglia joined by connecting filaments ; of these, about twenty-four are situated along the ventral surface of the body. The anterior pair, or that immediately beneath the oesophagus, is larger than the rest, forming a minute heart-shaped mass, which is united, by a delicate nervous collar embracing the gullet, with two small nodules of neurine situated upon the dorsal aspect of the mouth. The two minute ganglia last mentioned form that portion of the nervous system most intimately connected with sensation ; for, while the nervous filaments given off from the abdominal ganglia are distributed to the muscular integuments of the body, the nerves which issue from the supra-oesophageal pair supply the oral sucker, where the organs of the senses are situated. In all the homogangliata, indeed, it is exclusively from this ce- phalic pair of ganglia that the nerves appropriated to the instru- ments of the senses are derived, and we shall therefore not hesitate in the following pages to apply to this part of the nervous system of the articulata the name of brain ; considering it to be strictly analogous, in function at least, with the cerebral masses of more highly organized beings. When we regard the minute size of these, as yet rudimentary nervous centres, we cannot, however, expect to find them asso- ciated with any very perfect apparatus of -sensation. The oral sucker, indeed, seems to possess a more delicate sense of touch than the rest of the body, adapting it to examine the surface to which it is about to be fixed ; and probably the leech may enjoy in some measure perceptions corresponding with those of taste and smell. These senses have been found to exist in many of the animals we have already described ; but in the ffirudimdte we have, in addition, distinctly formed organs of vision, exhibiting, indeed, the utmost simplicity of structure, but nevertheless cor- responding in the perfection of their developement with the con- dition of the cerebral masses in relation with them. (241.) The eyes of the leech are eight or ten in number, and are easily detected by the assistance of a lens under the form of a semi- circular row of black points, situated above the mouth upon the suck- ing surface of the oral disc ; a position evidently calculated to ren- der them efficient agents in detecting the presence of food. The structure of these simple eyes, according to Professor M tiller,* does not as yet present any apparatus of transparent lenses adapted * Annales des Sciences Nat. vol. xxii. 200 ANNELIDA. to collect or concentrate the rays of light ; but each ocellus, or vi- sual speck, would seem to be merely an expansion of the terminal extremity of a nerve derived immediately from the brain, spread out beneath a kind of cornea formed by the delicate and transparent cuticle : behind this is a layer of black pigment, to which the dark colour of each ocular point is due. (242.) Leeches, like the generality of the Annelida, are herma- phrodite, every one possessing two complete systems of generative organs, one subservient to the impregnation, the other to the produc- tion of the ova; nevertheless these animals are not self-impregnating, but the congress of two individuals is essential to fecundity. Commencing with the male organs, we are not surprised to find the testes divided into numerous distinct masses, or rather repeated again and again in conformity with a law to which we have already alluded ( 229). The glands which apparently secrete the semi- nal fluid are about eighteen in number (Jig. 80, 2, e, /), arranged in pairs upon the floor of the visceral cavity. Along the external edge of each series there runs a common canal, or vas defer ens 9 which receives the secretion furnished by all the testicular masses placed upon the same side of the mesian line, and conveys it to a receptacle (d), where it accumulates. The two reservoirs, or vesicula seminales, if we may so call them, (d, d,) communicate with a muscular bulb (c) situated at the root of the penis. The penis itself (a) is frequently found protruded from the body after death ; it is a slender tubular filament, which communicates by its origin with the contractile bulb (c), and, when retracted, is lodged in a muscular sheath (5). The male apparatus is thus complete in all its parts : the fecundating secretion derived from the double row of testes is collected by the two vasa deferentia, and lodged in the receptacles (d, d) ; it is thence conveyed into the muscular cavity (c) situated at the root of the male organ of excitement, through which it is ultimately ejected. (243.) The ovigerous or female sexual organs of the leech are more simple in their structure than those which constitute the male system ; they open externally by a small orifice situated immediately behind the aperture from which the penis is protruded, the two openings being separated by the intervention of about five of the ventral rings of the body. The vulva, or external canal, leads into a pear-shaped membranous bag (Jig. 80, 2, g), which is usually, but improperly, named the uterus. Appended to the bottom of this organ is a convoluted canal (^), which communicates with two ANNELIDA. 201 round, whitish bodies ; these are the ovaria. The germs, there- fore, which are formed in the ovarian corpuscles, escape through the tortuous duct (h) into the uterus (), where they are detained for some time prior to their ultimate expulsion from the body. The exact nature of the uterine sacculus, as it is called, is imperfectly understood : some regard it as a mere receptacle wherein the se- minal fluid of the male is received and retained until the ova come in contact with it as they pass out of the body, and thus are sub- jected to its vivifying influence ; other physiologists believe that the germs escape from the ovaria in a very immature condition, and sup- pose that during their sojourn in this cavity they attain to more com- plete developement before they are ripe for exclusion ; while some writers go so far as to assert that leeches are strictly viviparous, in- asmuch as living young have been detected in the interior of this viscus : but all these suppositions are easily reconcileable with each other ; there is no doubt that the seminal liquor is depo- sited in this reservoir, during the copulation of two individuals, neither would any one dispute that the ova are collected in the same cavity before they are expelled from the body ; as to the discussion whether the young are born alive or not, or, as it is ge- nerally expressed, whether leeches are oviparous or viviparous, it is in this case merely a question of words, for in a physiological point of view it can make not the slightest difference whether the ova are expelled as such, or whether, owing to their being retained by accidental circumstances until they are hatched internally, the young leeches make their appearance in a living state. (44.) Abranchia terricola. The second division of those Anne- lidans which possess no external organs of respiration are easily dis- tinguishable from the suctorial worms by the different construction of their instruments of locomotion. They live in general beneath the surface of the ground, either perforating the soil in all direc- tions, as the Earthworms (Lumbrici), or burying themselves in the mud upon the sea-shore, where many of them, called Naides, (Nais, Lin.) live a semi-aquatic life. In conformity with such habits, their entire structure is adapted to a subterranean existence, and their bodies so organized as to enable them to burrow with facility through the dense and unyielding materials in which they are usually found. Whoever has attentively watched the opera- tions of an earthworm when busied in burying itself in the earth, must have been struck with the seeming disproportion between the laborious employment in which it is perpetually engaged, and the ANNELIDA. means provided for enabling it to overcome difficulties apparently insurmountable by any animal unless provided with limbs of extra- ordinary construction, and possessed of enormous muscular power. In the mole and the burrowing cricket we at once recognise in the immense developement of the anterior legs a provision for digging, admirably adapted to their subterranean habits, and cal- culated to throw aside with facility the earth through which they work their way ; but in the worms before us, deprived as they appear to be of all external members, feeble and sluggish even to a proverb, where are we to look for that mechanism which enables them to perforate the surface of the ground, and to make for them- selves, in the hard and trodden mould, the pathways which they traverse with such astonishing facility and quickness ? (245.) The structure of the outer fleshy integument of the earth- worm resembles in every respect that of the leech already described, both in the annular arrangement apparent externally, and the disposi- tion of the muscular strata. The suctorial discs, however, which in the leech formed such important instruments of progression, are here totally wanting ; and the annular segments of the body, as they approach the anterior extremity, become gradually dimi- nished in size, so as to terminate when the worm is fully stretched out in a fine point, near the apex of which is the opening of the mouth. But there is another circumstance in which the external anatomy of the terricolous Ann elides differs materially from what we have seen in the suctorial Abranchia : in the latter, the tegu- mentary segments were quite naked upon their outer surface ; but in the Lumbrici, of which we are now speaking, every ring, when ex- amined attentively, is found to support a series of sharp retractile spines or prickles ; these, indeed, are so minute in the earthworm, that, on passing the hand along the body from the head backwards, their presence is scarcely to be detected by the touch, but they are easily felt by rubbing the animal in the opposite direction ; a cir- cumstance which arises from their hooked form, and from their points being all turned towards the tail. These differences be- tween the external structure of the suctorial and setigerous Abran- chia, minute and trivial as they might seem to a superficial ob- server, are however all that are required to convert an aquatic animal into one adapted to a subterranean residence, as will be evident to any one who observes carefully the manner in which the earthworm bores its way through the soil in which it lives. The attenuated rings in the neighbourhood of the mouth are first insi- ANNELIDA. niiated between the particles of the earth, which, from their conical shape, they pene- trate like a sharp wedge ; in this position they are firmly retained by the numerous recurved spines appended to the different segments : the hinder parts of the body are then drawn forwards by a longitudinal con- traction of the whole animal ; a movement which not only prepares the creature for advancing further into the soil, but by swell- ing out the anterior segments forcibly di- lates the passage into which the head had been already thrust : the spines upon the hinder rings then take a firm hold upon the sides of the hole thus formed, and, prevent- ing any retrograde movement, the head is again forced forward through the yielding mould, so that, by a repetition of the pro- cess, the animal is able to advance with the greatest apparent ease through substances which it would at first seem utterly impossi- ble for so helpless a being to penetrate. (246.) The alimentary canal of the earth- worm is straight and very capacious. Its great size, indeed, is in accordance with the nature of the materials employed as food, for it is generally found distended with earth ; and, indeed, by the older physiologists these creatures were generally regarded as afford- ing proof that the nourishment of animals was not exclusively derived from animal and vegetable substances, since in this case they supposed nutriment to be obtained from matter belonging to the mineral kingdom. This supposition, however, has been long since exploded, for it is not from the earth that nourishment is afforded, but from the decaying animal and vegetable particles mixed up with the soil taken into the sto- mach ; so that the exception to the general law of nature supposed to exist in the earth- Fig. 204 ANNELIDA. worm has no foundation in truth. The whole intestinal tract of one of these animals is represented in the figure (Jig. 82) : it con- sists of a wide O3sophagus which terminates in a crop-like dilata- tion ; to this succeeds a muscular gizzard (fc), and a long sacculated intestine (/, /) which passes in a direct line to the anus. (247.) The circulation of the blood in the terricolous Annelidans has been the subject of much discussion, and until recently was but very imperfectly understood. In the earth-worm there are three principal trunks connected with the vascular pj g ^ 33. system,* the arrangement of which is repre- sented in the annexed diagram (Jig. 83). First, a dorsal vessel (a) runs along the whole length of the back in close contact with the intestine (Jig. 82, o, o), upon which it lies ; this vessel is tortuous, and exhibits constant movements of contraction and dilatation, by which the blood is propelled in continuous undulations from the tail towards the head. Two other large vessels occupy the ventral region of the body : of these, one (Jig. 83, 2>), which we shall call the ventral vessel, runs immediately beneath the alimentary tube ; while the other, which is situated close un- der the skin, and consequently beneath the ventral chain of ganglia composing the nervous system, by which it is separated from the last, may be distinguished as the sub-ganglionic vessel. These three great trunks are united by important branches, and form two distinct systems : one of which is deeply seated, being distributed to internal viscera ; the other is superficial, giving off innumerable vessels to the integuments of the body, which, by rami- fying through the skin, form an extensive vas- cular surface adapted to respiration. The ventral vessel (6), like the dorsal (), may be traced quite to the anterior extremity of the worm, where numerous small anastomosing branches unite the two trunks : but these inosculations are of little consequence in describing the circular movement of the blood ; a more impor- * M. Duges, Annales des Sciences Nat. vol. xv. ANNELIDA. 205 tant communication being established, through which the blood passes freely from one to the other, by the intervention of seven or eight pairs of large canals, situated in the immediate neighbourhood of the generative apparatus, with which indeed they are interwoven. Each of these voluminous vessels (d) is composed of a series of swellings, or rounded bead-like vesicles, endowed with consider- able contractile power ; and they form together a kind of heart of remarkable construction, which propels the blood received from the dorsal trunk into the ventral tube (b). Along the rest of the body, the communication between the dorsal and ventral trunks is repeated at each ring by canals which are much smaller than the bead-like or moniliform vessels, and have no vesicular arrangement ; they (g and e) run perpendicu- larly upwards, embracing the alimentary canal, and giving off branches at right angles, which divide into innumerable ramifi- cations so as to cover the whole intestine with a delicate vascular net-work ; these may be called the deep-seated abdomino-dorsal branches. The sub-ganglionic vessel (c) may be looked upon as arising from the termination of the dorsal vessel, with which it is evi- dently continuous at the anterior extremity of the body. At the posterior edge of every segment, a delicate branch is given off from this sub-ganglionic tube (/), which, running upwards in the same manner as those derived from the ventral trunk, joins the dorsal, and receives in its course a large anastomosing branch from the deep abdomino-dorsal canal which corresponds to it. From this system of superficial vessels arises a cutaneous net-work, analogous to that described above as covering the digestive viscera which tra- verses the skin in all directions. Let us now trace the blood in its circulation through this ela- borate system. In the dorsal vessel (a) the sanguineous fluid passes from the tail towards the head ; at the anterior extremity of the body it passes partly into the sub-ganglionic vessel (c), through the anastomosing branches, and partly into the ventral vessel (Z>), into which it is forcibly driven by the contractions of the monili- form canals. In both the ventral and sub-ganglionic trunks, there- fore, the course of the blood is necessarily from the head towards the tail ; and the circulating fluid is continually returned to the dorsal canal by the deep and superficial abdomino-dorsal vessels (e,/, g), completing the vascular circle. On reviewing the above arrangement, we immediately perceive 206 ANNELIDA. that, notwithstanding the similarity observable in the distribution of the ventral and sub-ganglionic systems of vessels, in a physio- logical point of view they are subservient to very different func- tions ; the former representing the systemic, the latter the pul- monary circulation. The blood derived from the dorsal trunk by the moniliform hearts (d) is supplied by the ventral vessel, which may be compared to an aorta, over the surface of the viscera, and the remnant of this blood, after furnishing materials for nutrition, is returned to the dorsal canal by the deep vessels e, g ; but that portion of the circulating fluid which passes from the termination of the dorsal tube into the sub-ganglionic trunk, not only serves for the nourishment of the skin and muscular integument, but at the same time is brought in contact with the air as it passes through the cutaneous net-work, and is thus, more or less, replenished with oxygen before it is again returned to the general circulation. The sub-ganglionic canal is, therefore, a kind of pulmonary artery, and the dorsal drives to the moniliform vessels a mixed fluid, composed partly of venous blood derived from the viscera, and partly of arterial derived from the superficial or sub-cutaneous system. (248.) We see, therefore, that the extensive diffusion of vascular canals immediately beneath the surface of the skin must undoubtedly contribute materially to effect those changes in the blood which are analogous to those produced by respiration in the higher ani- mals ; but it would seem that this is not the only provision made for the aeration of the circulating fluids. It is long since Willis* described the existence of a series of pores upon the back of the earthworm, which he regarded as stigmata, and had remarked that air blown into these openings is dispersed between the mus- cular integument and the intestine, so that it passes readily from one segment to another. Duges repeated these experiments with the same result, and found that the pores alluded to, instead of terminating in muciparous follicles, as they were supposed to do by many, penetrate into the interior of the body, so that air in- jected into one of them passes freely along the membranous com- partments which surround the intestine, and escapes through other neighbouring orifices. In like manner water is found to be taken into the body through the same apertures, from which it is often given out in great abundance when the animal is too rapidly dried by exposure to the sun, or irritated by external stimuli: * De Ahima Brutorum, 4to. 1672. ANNELIDA. 207 aerated water thus taken into the system, and brought immediately in contact with the deep-seated vascular net-work dispersed over the intestinal parietes, must therefore necessarily contribute to the respiratory function. Nevertheless, in addition to all this, we find in every segment of the body a pair of membranous vesicles {fig. 82, v) communicating externally by lateral orifices, apparently analogous to the respiratory vesicles of the leech ; and, in fact, by many authors they have been described as constituting the breath- ing apparatus.* Their real office, however, is but imperfectly understood ; they evidently have not the same relation with the circulatory system, which the lateral sacculi of the leech have been found to exhibit ; are they then merely secreting follicles destined to furnish a mucosity for lubricating the external surface of the body, or are they aquiferous tubes adapted to introduce water into the interior ? Future observations must determine these ques- tions. (249.) Few points connected with the history of the earthworm have given rise to so much speculation as the manner of their repro- duction. The generative organs have long been known to be lodged in the anterior part of the body, their position being indicated externally by a considerable enlargement or swelling which extends from the seventh to about the fourteenth segment, counting from that in which the mouth is situated. On opening this portion of the animal, a variable number of white masses are found attached to the sides of the crop and gizzard (fig. 82, A, ^, A), which have long, by general consent, been looked upon as forming the repro- ductive system ; some having been regarded as representing the testes, others the ovaria : yet so delicate are the connections which unite these glandular masses, and such the difficulty of tracing the ducts whereby they communicate with the exterior of the body, that the functions to which they are individu- ally appropriated have given rise to much discussion. The Lumbrici have been generally acknowledged to be hermaphrodite, that is, possessed of organs adapted both to the formation and fertilization of ova ; and it is likewise well understood that the congress of two individuals is essential to the fecundity of both, as, in the earlier summer months, the mode in which they copulate is a matter of constant observation. At such times two of these animals are found to come partially out of the ground from contiguous holes, and, applying together those segments of their * Sir E. Home. Lectures on Comp. Anat. 4 vols. 4to. 1323. 208 ANNELIDA. bodies in which the generative glands are situated, are observed to remain for a considerable time in contact, joined to each other by a quantity of frothy spume which is poured out in the neigh- bourhood of the sexual organs. No organs of intromission, how- ever, have ever been distinguished, neither until recently had the canals communicating between the sexual orifices and the testicular or ovarian masses been satisfactorily traced ; so that Sir Everard Home* was induced to believe that, in the kind of intercourse above alluded to, there was no transmission of impregnating fluid from one animal to the other, but that the excitement produced by mutual contact caused both the ovaria and testes to burst, so that the ova escaping into the cells of the body became there mingled with the spermatic secretion, and being thus fertilized the ova were hatched internally, and the young, having been retained for some time in the cells between the intestine and the skin, were ultimately ejected through apertures which were supposed to exist in the vicinity of the tail. There is, however, little doubt that what Sir E. Home conceived to be young earthworms were in reality parasitical Entozoa, and that, in the mode of their pro- pagation, the animals we are describing exhibit but little deviation from what we have already seen in the leech. (250.) According to M. Duges,-f- the arrangement of the sexual parts is represented in the diagram (fig. 84). The testicles (b) are placed in successive segments of the body from the seventh back- wards ; they vary in number in different individuals from two to seven : but whether this variety depends upon a difference of species, or is only caused by the posterior pairs becoming atrophied when not in use, is undetermined. Each testis is fixed to the bot- tom of the ring in which it is placed by a short tubular pedicle that opens externally by a very minute pore through which a milky fluid can be squeezed. The testicular vesicles of the same side of the body all com- * Lectures on Comp. Anat. vol. iii. Fig. 84. t Ann. des Sciences Nat. vol. xv. ANNELIDA. 209 municate by a common canal ; and the contained fluid, which like the seminal secretion of other animals contains animalcules, can readily be made to pass from one to another. The ovaria (c) are eight large ' Fig. 85. white masses of a granular texture, from which arise two delicate tubes or oviducts ; these have no connection with the testes, but, running back- wards, they become dilated into two small vesicles at their termination (d), and open by two apertures or vulvse seen externally upon the six- teenth segment of the body : in these 'ducts eggs have been detected as large as pins 1 heads. (251.) The eggs, when laid, are two or three lines in length. In figure 85, A, one of them enclosing a mature em- bryo is delineated ; its top is seen to be closed by a peculiar valve-like structure adapted to facilitate the escape of the worm, and opening (Jig. 85, B) to permit its egress. Another remarkable circumstance ob- servable in these eggs is, that they very generally contain double yolks, and consequently two germs, so that a couple of young ones is generally produced from each. (252.) The generative system of the Nais presents a somewhat different arrangement to that which exists in the earthworm. The swollen part of the body in which the sexual organs are placed, occu- pies a space of five or six rings, beginning at the eleventh. On each side of the eleventh segment is a minute transverse slit (Jig. 86, b) communicating with a slightly flexuous canal which terminates in a transparent pyriform pouch or vesicle. The latter con- tains a clear fluid, in which minute vermiform bodies are seen to float, and most probably represents the testis. The twelfth segment likewise exhibits two openings, each placed upon the centre of a little nipple (c), these are the orifices leading to the female portions of the sexual system. The ovaria (</, e) are composed of four large and several smaller masses of a granular character, and from them proceed long and tortuous 210 ANNELIDA. oviducts, which just before their termination at the lateral openings (c) become thick and glandular. These animals most likely co- pulate like the earthworm, and lay their eggs in a similar manner. We have al- Fig. 86. ready seen in the Lumbricus terrestris ova containing two yolks, and consequently giving birth to two animals ; but in the Nais every egg produces ten or a dozen young ones,* or, perhaps we ought rather to say, that what appears to be a single egg is in fact merely a capsule enclosing several distinct ova from which a numerous progeny arises. The manner in which these compound eggs are formed is easily understood, when we consider the structure of the oviduct described above. The granular germs escape no doubt separately from the ovaria, and remain distinct from each other as they pass along the tortuous canal which leads to the external opening ; but at length, ar- riving at the thick and glandular portion (c) of the oviferous tube, several of them become enclosed in a common investment secreted by the walls of the oviduct, and are expelled from the body with the out- ward appearance of a simple egg. (253.) Besides the ordinary mode of pro- pagation by ova, it has long been ascertained that some of the Annelida at least are re- produced by spontaneous division. Bonnet, M tiller, and Duges, all agree that this is the case with certain species of Nais ; and in Nais filiformis the process of separation has been witnessed from its com- mencement to its termination. The division was seen to occur near the middle of the body of the animal, the posterior half re- maining motionless upon the mud of the bottom of the vessel, whilst the anterior portion buried itself as usual ; after some days the truncated extremity of the hinder part was observed to become swollen, transparent, and vascular, and ultimately to assume the complete structure of the mouth of the perfect animal ; it then * Duges, loc. cit. ANNELIDA. buried itself in the mud, and no doubt there completed its de- velopement. (254,) It is very generally believed, that even the earthworm may be multiplied by mechanical sections, the separated portions repro- ducing such parts as are removed in the experiment, and again becoming perfect. Careful experiments made to ascertain how far the statements of former authors upon this subject are substantiated, prove that the assertion is not entirely without foundation, al- though by no means to the extent indicated in their writings. It would indeed be easily credited that the removal of the hinder part of the body of an earthworm would not necessarily destroy the anterior portion, since no organs absolutely essential to existence are removed by the operation, and even the course of the circu- lating fluids would not be materially interrupted by the mutilation ; but that the hinder moiety should be able to reproduce the mouth, gizzard, and stomach, the complicated apparatus of moniliform vessels and the sexual organs, contained in the anterior segments, could scarcely be deemed possible, and the assertion has been satisfactorily disproved by actual observation. On cutting an earthworm in two, the anterior portion is found in fact generally to survive ; and the wound caused by the operation, becoming gradually constricted, is soon converted into an anal orifice, render- ing the animal again complete in all parts necessary for its ex- istence. This, however, is by no means the case with the posterior portion ; for although it will exhibit, for a very long period, indica- tions of vitality, no signs of reproduction have been witnessed, and it invariably perishes. (255.) Nevertheless, although it is thus proved that the earthworm cannot be multiplied by mechanical division, it is undeniably able to reproduce small portions of its body, the removal of which does not implicate organs essential to life. In the experiments of M. Duges,* for example, it was found that four, or even eight, of the anterior rings might be cut off with impunity, although the cephalic pair of ganglia, the mouth, and a part of the oesophagus were necessarily taken away. In worms thus mutilated, after the lapse of from ten to thirty days, a conical vascular protuberance was observed to sprout from the bottom of the wound ; and, in eight or ten days later, this new part had become so far developed, that not only all the lost rings were apparent, but even the upper lip and mouth had assumed their normal form, and the animal again began to eat and bury itself "in the earth. * Loc. cit. p 2 212 ANNELIDA. (256.) Dorsibranchiata. We have gone too minutely into the anatomy of the two preceding orders of Annelidans to render an equally detailed account of the structure of the Dorsibranchiata ne- cessary ; we must therefore restrict our observations to those points in which remarkable variations from what has already been described present themselves to our notice. These worms are all inhabit- ants of the sea ; and although upon our own coasts they seldom attain to very considerable dimensions, rarely exceeding a few inches in length, in tropical climates some species are found of comparatively gigantic proportions, having their bodies composed of four or five hundred segments, and occasionally measuring four feet from one end to the other. We have already seen ( 233) that, in the more perfectly or- ganized forms of these worms, each segment of the body supports certain external, moveable appendages adapted to assist in locomo- tion, which are usually called the feet, or more properly the oars ; they present great diversity of appearance, and, from the nature and arrangement of the different parts composing them, are of material assistance to the systematic zoologist, as they afford important characters for the establishment of generic and specific differences. In the section of Leodicea antennata already given, (Jig. 77, 2,) these parts are seen in a very intelligible form, and are visibly composed of three distinct structures adapted to different uses. The first, which occupies the uppermost position, is the respiratory apparatus (b) ; in Leodicea its structure is extremely simple, being composed of a central stem from which a single series of vascular filaments is sent off, giving the organ a pectinated ap- pearance ; but in other cases the branchial tuft is far more con- siderably developed, dividing and subdividing into minute ramifi- cations, and thus offering a more considerable surface to the surrounding element. In most instances, as in Leodicea (Jig. 77, 1), these respiratory arbuscles are placed along the entire length of the body, being appended to every segment, with the exception perhaps of a few of the most anterior ; nevertheless, in some species, their distribution is more partial, and their presence is restricted to a few rings of the animal. In Arenicola piscatorum, for instance, (Jig. 87,) a worm met with abundantly upon our own coasts, and eagerly sought after as a bait by fishermen, who dig it from the holes which it excavates in the sand, the branchiae (b) are confined to the central portion of the body, where they form on each side a series ANNELIDA. 213 of bundles which are remarkable during the life of the creature for their beautiful red colour, derived from Fig. 87. the crimson blood which circulates copious- ly through them. But the organs of respiration in the Dor- sibranchiate Annelidans are not always arbo- rescent ; on the contrary, they are not un- frequently spread out into thin membra- nous lamellee, or resemble fleshy crests or vascular tubercles ; still, whatever their form, their office is the same, and the vessels spread over them, presenting an extensive surface with which the water is brought in contact, the blood is oxygenated as it passes through them. (257.) The second class of organs to be enumerated as entering into the composi- tion of the lateral appendages, are soft, fleshy, and sub-articulated processes called cirri (Jig- 77, 2, c, d) ; these are generally two in number, and belong one to the ventral and the other to the dorsal oar : their precise office is not well understood ; but as in some of the segments, especially in the neighbourhood of the head, they assume a tentacular form, they have with some pro- bability been regarded as instruments of touch. (258.) The seta (fig. 77, 2, d) are per- haps the most efficient agents in progression. These are long and stiff hairs disposed in bundles and implanted into strong muscular sheaths. Each packet of setse can be retracted within the body to a certain extent, and again protruded by the action of the tubular supports from which they arise, and, being capable of independent action, these organs must be looked upon as so many power- ful fins, well calculated to propel the creature through the element which it in- habits. ANNELIDA. Nothing can exceed the splendour of the colours which orna- ment some of these fasciculi of hairs ; they yield, indeed, in no respect to the most gorgeous tints of tropical birds or to the bril- liant decorations of insects : green, yellow, and orange, blue, purple, and scarlet, all the hues of Iris play upon them with the changing light, and shine .with a metallic effulgence only com- parable to that which adorns the breast of the humming-bird. But it is not for their dazzling beauty merely that these setse are remarkable ; they are not unfrequently important weapons of de- Fig. 88, fence, and exhibit a com- plexity of structure far be- yond anything to be met with in the hair of higher animals. In the Aphro- dite hispida, for example, (Jig. 88, A,) they are per- fect harpoons ; the point of each being provided with a double series of strong barbs, (fig- 88, B,) so that when the creature erects its bristles, much more formidable than those of the porcupine, the most determined enemy would scarcely venture to attack it. But here we cannot help observing an additional provision, rendered neces- sary by the construction of these lance-like spines. We have before noticed that the bundles of setse are all retractile, and can be drawn into the body by the muscular tube from which they spring. It would be superfluous to point out to the reader the danger which would accrue to the animal itself by the presence of such instruments imbedded in its own flesh, as by every movement of the body they would be inextricably forced into the surrounding tissues. The contrivance to obviate such an accident is as beau- tiful as it is simple. Every barbed spine is furnished with a smooth, horny sheath, (fig. 88, a, 6,) composed of two blades, between which it is lodged ; and these, closing upon the barbs when ANNELIDA. 215 they are drawn inwards, effectually protect the neighbouring soft parts from laceration. (259.) In the Aphrodite above alluded to we have an additional appendage developed from the upper part of each lateral oar, in the shape of a broad membranous scale, which, arching inwards over Fig. 89. Fig. 90. the back {Jig. 89, c), forms with its fellows a series of imbricated plates, or Elytra, as they are tech- nically named (Jig. 88, A) ; and beneath these the branchial organs are lodged. Each of the elytral scales is formed by a double mem- brane, between the laminae of which at certain seasons the eggs are found to be deposited ; a situation evidently adapted to ensure the ex- posure of the ova to the influence of the surrounding element, and thus to provide for the respiration of the embryo.* (260.) The structure of the mouth in the Dorsibranchiate Anne- lidans is very peculiar. The first portion of the alimentary canal or stomach, as it is most erroneously called by some writers, is muscular ; and certainly, when seen in a dead Annelide, it might easily be taken for a digestive cavity. Nevertheless, during life, this part of the alimentary ap- paratus is destined to a widely different office ; for it is so constructed, that at the will of the animal it can be com- pletely everted, turned inside out, and, when thus pro- truded externally, it forms a very singular proboscis, used in seizing food, and frequent- ly armed with powerful teeth of singular construction. The an- nexed figure (Jig. 90, A), representing the head of one of these * Milne Edwards, Ann. des Sciences Nat. vol. xxvii. 216 ANNELIDA. worms (Goniada a chevrons, Milne Edwards), will give a good idea of this curious organ when fully displayed; and in Jig. 90, B, the mechanism is exhibited by which its protrusion and re- traction are accomplished. The whole apparatus is there seen to consist of two muscular cylinders, placed one within the other, but continuous at their upper margin (B), or, to use a familiar illustra- tion, the proboscis may be compared to the finger of a glove partially inverted ; it is obvious that in this case, if the inner cylin- der be drawn inwards, that is, into the mouth, the whole structure becomes shortened, until at last it is entirely retracted into the oral cavity ; whereas, on the contrary, if the outer tube is made to protrude, it expands at the expense of the inner one, which is gradually drawn forwards. The internal surface of this remarkable proboscis is, moreover, variously modified in its struc- ture, so as to adapt it to the prehension of different kinds of prey. In Amphinome, for instance, the orifice of the mouth is a thick, fleshy, and callous circle {fig- 93, b, c, d), and the surface of the exserted proboscis is covered with delicate transverse rugae, evi- dently so arranged as to give tenacity to its gripe. In Goniada it supports two distinct sets of horny teeth, provided for very differ- ent uses ; one set, which is exposed when the proboscis is un- rolled to a very small extent, consists of a series of linear horny plates, (Jig. 90, A, d 9 ) and probably forms a kind of file, or ra- ther a scraper, with which the animal excavates the subterra- nean galleries in which it lives. The other set does not make its appearance till the proboscis is more completely expanded, and is evidently an instrument of pre- hension, formed by two horny hooks (Jig. 90, B, a, b) placed upon an elevated ridge near the entrance of the oesophagus, so as to take a secure hold of any vic- tim seized by this curious mouth. In Phyllodoce laminosa the teeth form a circle of semi-carti- laginous beads, encompassing the extremity of the proboscis when that organ is pushed out to its Fig.91. ANNELIDA. 217 full length (Jig. 91, 6), an arrangement well adapted to hold and perhaps to crush their prey. But the most formidable jaws are met with in some of the Nereidiform species, as in Leodicea antennata, of which a figure is given above (fig. 77). When the proboscis of one of these creatures is slightly everted, the extremities of three pairs of strong horny plates emerge from the mouth ; of these, one pair terminates by forming a powerful hooked forceps, while the others present strong denticulated margins (fig. 92, A, a, 6, c). The Fig.92. B A N nature of these teeth will be better seen by a glance at B in the same figure, where they are represented upon an enlarged scale, as they appear when detached from their connections. (261.) The alimentary canal of the Dorsibranchiate Annelidans offers little which requires special notice. It invariably passes in a direct line from the termination of the proboscis to the anal extremity of the body. In the Nereidte it is provided with numerous lateral pouches, somewhat resembling those of the leech. In Aphrodite these lateral cseca are very long, slender, and branched at their extremities, so that they have been thought by some to be secret- ing organs, representing the liver. In Arenicola we find at the termination of the oesophagus (fig. 94, f) two large csecal ap- pendages (e) of unknown office, while the rest of the tube (c) is entirely covered with minute sacculi, the walls of which are de- cidedly glandular, and secrete a fluid of a greenish-yellow colour. (262.) The course of the principal trunks of the circulating system in the Dorsibranchiata bears a general resemblance to what we have already seen in the Abranchiate order, modified, of course, by the variable position of the branchial tufts ; but with respect to the 218 ANNELIDA. minuter details connected with the arrangement of the vessels our information is but vague and unsa- tisfactory. The investigation, in- deed, is attended with considerable difficulty. The annexed figure of an elaborate 'dis- section of an Am- phinome (-4. ca- pillata)) copied from one of the beautiful draw- ings contained in the Hunterian Collection,* af- fords an example of a circulating system in which the propulsion of the blood is ef- fected entirely by vessels, without the intervention of any muscular cavities or heart. In this animal the respiratory organs are penni- form appendages * Descriptive and illustrated Catalogue of the Physiol. Series of Comp. Anat. in the Mus. Royal Coll. Sur- geons, London, vol. ii. pi. xiv. Fig. 93. ANNELIDA. placed along the back, and these external vascular tufts communi- cate with delicate plexuses of vessels situated in the interior of the body, called the branchial plexuses. In the figure the branchial plexuses of the left side only are represented (</, </, <?), and of these one marked q' has been turned aside. The blood and nutri- tious fluids derived from the whole alimentary tract are collected by the large ventral intestinal vein (w, w, n), and conveyed to the branchial plexuses through the numerous vessels (o, o, o), some of which (o', o', o') are displaced in the drawing in order that their connections may be better seen. Besides the blood and nutriment thus derived from the intestine, the branchial plexuses receive the circulating fluid from all the segments of the muscular envelope by separate veins (p 9 p), and thus the blood from all parts is brought to the gills and exposed to the influence of oxygen. After undergoing respiration, the blood is collected from the branchial plexuses by the lateral veins (r, r, r) ; from which, through communicating vessels (s, s, s), it passes into the aorta or great dorsal vessel (, , ), to be distributed through the body. From the aorta large trunks (v, v) are given off to form the intes- tinal artery (w, w), which, ramifying over the intestine, communi- cates with the intestinal vein (w, w), and thus completes the vas- cular circle.* In the JVereidce, the aorta, or dorsal vessel, runs along the whole length of the back, and in each ring offers a perceptible fusiform dilatation, so that it has a beaded appearance ; at every segment it gives off lateral branches, every one of which is furnished with a little rounded vesicle, which Delle Chiaje conceives to be a distinct heart or contractile cavity, calculated to assist in the propulsion of the contained blood. In Arenicola the arrangement of the vascular trunks seems to be very nearly similar to that found in the earthworm ; but, instead of the moniliform hearts, ( 247,) two large contractile sinuses communicate between the dorsal and ventral vessels {Jig. 94, 6, b). (263.) The reproductive organs of iheDorsibranchiateAnnelidans are, perhaps, less known than those of any other animals. Cuvier-)- * The parts indicated in the drawing by letters not referred to in the text are the following : a, a, the ventral surface of the segments of the body ; e, e, the ventral oars or packets of bristles ; /, /, the ventral cirri, or feelers ; g, the anal cirri ; ft, the anus ; i, i, k, k, the bases of the dorsal and ventral oars, with their surrounding mus- cles ; J, /, the dorsal longitudinal muscular bands ; m, m, the ventral longitudinal muscular bands. t Le9ons d'Anatomie Comparee, vol. v. p. 186. 220 ANNELIDA. J. observed in the anterior part of the body of Arenicola five grey vesicles resembling the ovaria of the earthworm ; and he was led to conclude, in conformity with the Figt 94 then generally received opinion, that the ova escaped from these vesicles into the cellular structure between the intestine and the walls of the body. It is, however, probable that the granular bodies {Jig. 94, m, m) usu- ally found in that situation are para- sitical Entozoa, as those of the earth- worm have been proved to be. ; { In .the Nereis, Delle Chiaje de- scribes the ovaria as two long and ex- ; trernely delicate caeca, occupying the , posterior half of the visceral cavity, ( and offering various constrictions and " dilatations in their course ; these caeca terminated by distinct apertures in the neighbourhood of the anus, and when gravid were found to be filled with granular ova of a greenish colour. (264.) In one species of Nereis (N. prolifera), Miiller* observed repro- duction to take place by spontaneous division ; a mode of propagation which, although common among the Naidce, had not previously been seen in any of the Dorsibranchiate families. The process of division is represented in the appended figure (fig. 95); the hinder part of the body, including about seventeen segments, is seen to be gradually separated from the ante- rior or larger portion, and, moreover, at the point of separation a new head with eyes and tentacular cirri is dis- tinctly formed. " In one case," says * Ohtho Fred. Mii.Mer, Zoologia Danica, pi. lii. fig. 6, fol. 1788. ANNELIDA. 221 Miiller, u I found a mother to which F/ s- 95 - three fetuses of different ages ap- peared in one length. The mother had thirty pedate segments ; the youngest daughter, or that nearest the mother, had eleven, but the head was not yet developed. The most remote had seventeen rings, with both head and eyes, and, moreover, the tail of the mother ; the middle one had seventeen segments, and a head. The two posterior were broken off from the mother by pressure : in the last, or oldest, was found a black sub- stance filled with white spots ; and the white spots, when squeezed from the body, were oval, each marked with a pellucid speck. Were they eggs ? If so, how were they formed in a young one still adhering to the body of its parent ? In the middle one were si- milar spots, but smaller. Were they younger eggs ?" Some curious speculations have been entertained by continental writers relative to this mode of propagation. The tail of the ori- ginal Nereis is still the tail of its offspring, and, however often the body may divide, still the same tail remains attached to the hin- der portion, so that this part of the animal may be said to enjoy a kind of immunity from death. (265.) Tubicola. Our knowledge of the last, or tubicolous di- vision of the Annelidans, is very limited ; it may, indeed, be said to be confined to an acquaintance with their external configuration, for the few unconnected accounts which are given by authors relative to their internal anatomy are so obviously based upon pure sup- position, that, perhaps, the zootomist who should enjoy favourable opportunities of inspecting the larger species in a fresh state, could hardly make a more valuable contribution to our science than by giving an account of the organization of these interesting animals. We have already described the different kinds of tubes in which these Annelidans live ( 233), and given a representation (Jig. 78) of the calcareous tube secreted by the Serpula contortuplicata : the ANNELIDA. annexed figure represents the curious habitation of the Te- rebella Medusa, constructed by cementing together minute shells and other small bo- dies. In neither case is there any muscular connection be- tween the worm and its abode, so that the creature can be readily drawn out from its residence in order to ex- amine the external appen- dages belonging to the indi- vidual segments of its body. When thus displayed (fig- 97), the modifications con- spicuous in the structure of the lateral oars are at once seen to be in relation with their circumscribed move- ments, and offer a wide con- trast to the largely developed spines, setse, and tentacular cirri, met with in the Dorsi- branchiata. In the upper part of the body, rudimentary protractile bunches of hairs are still discernible, but so feebly developed that their use must evidently be restrict- ed to the performance of those motions by which the protru- sion of the head is effected ; while upon the posterior seg- ments even these are oblite- rated, the only organs at- tached to the rings being minute foot -like processes adapted to the same office. The tentacular cirri, which were likewise distributed F/2-. 96. ANNELIDA along the entire length of the Dorsibranchiate order, are here transferred to the head, where they form long and delicate instruments of iouch, and, most probably, assist materially in distinguishing and seizing prey ; the branchiae, like- wise, are no longer met with upon the segments enclosed within the tegumentary tube, but are placed only in the immediate vicinity of the head, where they form fan-like ex- pansions, or ramified tufts, so ar- ranged as to be most freely exposed to the surrounding medium. The mouth placed at the origin of the tentacular cirri is a simple orifice closed with a valve-like flap or up- per lip, but is unprovided with any dental structure. The alimentary canal is generally a simple and somewhat capacious tube which tra- verses the axis of the body ; but in some species, as in Sabella pavo- nina, it assumes a spiral course, making close turns upon itself from the mouth to the anal aperture, which is always terminal. The cir- culating system probably resembles, in its general arrangement, that of the Dorsibranchiate worms, the course of the vessels being modified in accordance with the altered posi- tion of the branchiae ; but of this we have no certain knowledge, nei- ther are we acquainted with the na- ture of the generative apparatus, and the scattered remarks of au- thors upon this subject are to the last degree vague and unsatisfac- tory. Fig. .97. 224 CHAPTER XIV. MYRIAPODA.* (266.) The Annelidans examined in the preceding chapter, with the singular exception of the earthworm, are only adapted to an aqua- tic life ; the soft integument which forms their external skeleton and the setiform and tentacular organs appended to the numerous seg- ments of their elongated bodies, are far too feeble to support them in a less dense and buoyant element, so that when removed from their native waters they are utterly helpless and impotent. Sup- posing, however, that, as a mere matter of speculation, it was in- quired by what means animals of similar form could be rendered capable of assuming a terrestrial existence, so as to seek and obtain prey upon the surface of the earth, and thus represent upon land the Annelidans of the ocean : a little reflection would at once in- dicate the grosser changes required for the attainment of such an object. To convert the water-breathing organs of the aquatic worms into an apparatus adapted to aerial respiration would be the first requisite. The second would be to give greater density and firmness to the tegumentary skeleton, to allow of more power- ful and accurately applied muscular force, by diminishing the num- ber of segments composing the annulose covering, and also by converting the lateral oars into jointed levers of support sufficiently strong to sustain the weight of the whole body, to provide instru- ments of locomotion fitted for progression upon the ground. Yet all these changes would be inefficient without corresponding modi- fications in the character of the nervous system : the lengthened chain of small ganglia found in the aquatic worms would be quite inadequate to wield muscles of strength adapted to such altered cir- cumstances ; the small encephalic brain would be incompetent to correspond with more exalted senses, so that, as a necessary conse- quence of superior organization, the nervous centres must be all increased in their proportionate developement to adapt them to higher functions. The changes, which our supposition infers would be requisite for the conversion of an aquatic Annelidan into a land animal, are precisely those which we encounter when we turn our attention from the creatures described in the last chapter to the MYRIA- PODA, upon the consideration of which we are now entering : . * (*,v(>ux.S) ten thousand, i.e. many ; vrovs, a foot. MYRIAPODA. 225 they form the transition from the red-blooded worms to the class of insects, and are intermediate between the two in every point of their structure. (267.) The body of a Myriapod is composed of a consecutive series of segments of equal dimensions, but, unlike those of the generality of the Annelida, composed of a dense semi-calca- reous, or else of a firm coriaceous substance ; and to every segment is appended one or two pairs of articulated legs, generally termi- nated by simple points. The anterior segment or head, besides the organs belonging to the mouth, contains the instruments of sensation, consisting of simple or compound eyes, and of two long and articulated organs called antenna, generally regarded as appropriated to the sense of touch, but which probably are connected with other perceptions less intelligible to us. The air required for respiration is taken into the body through a series of minute pores or spiracles placed on each side along the entire length of the animal, and is distributed by innumerable ramifying tubes or tracheae to all parts of the system. The number of segments, and consequently of feet, increases progressively with age ; a circumstance which remarkably distin- guishes the Myriapoda from the entire class of insects, properly so called. (268.) The Myriapoda may be divided into two families, origin- ally indicated by Linnaeus : the Julidce, or millepedes ; and the Sco- lopendridtf, or centipedes ; each of which will require our notice. Julid<. The lowest division, which derives its name from the Julus, or common millepede, is most nearly allied to the Anneli- dans, both in external form, and also in the general arrangement of its different organs ; this, there- fore, we shall first examine, and select the Julus terrestris, one of the species most frequently met with, as an example of the rest. These animals (Jig- 98, A) are generally found concealed un- der stones, or beneath the bark of decaying timber, where they find Fig. 98. MYRIAPODA. subsistence by devouring decomposing animal and vegetable sub- stances. The body is long and cylindrical, composed of between forty and fifty hard and brittle rings, which, with the exception of those forming the head and tail, differ but slightly from each other. Every segment supports two pairs of minute feet, arising close to the mesian line upon the under or ventral surface ; but these feet, although distinctly articulated {fig- 98, c), are as yet extremely small in comparison with the bulk of the animal, and are evidently but mere rudiments of the jointed legs developed in more highly organized forms of homogangliate beings ; so that the movements of the Julus are very slow, and the creature seems rather to glide along the ground, supported on its numerous but almost invisible legs, than to walk. When at rest, the body is rolled up in a spiral form (j%. 98, B), the feet being concealed in the con- cavity of the spire, and thus protected from injury. (269.) The mouth resembles in structure that of the larva of some insects, and is furnished with a pair of stout horny jaws, mov- ing horizontally, and provided at their cutting edges with sharp den- ticulations, so as to render them effective instruments in dividing the fibres of rotten wood, or the roots and leaves of vegetables, which are usually employed as food ; and the alimentary canal, which is straight and very capacious, is generally found filled with materials of this description. (270.) In most points of their internal organization, the Myria- poda resemble insects ; and we should only anticipate the obser- vations which will be more conveniently made hereafter, did we enter into any minute description of their anatomy : we shall, there- fore, in this place, simply confine ourselves to the notice of those pe- culiarities which occur in the animals under consideration, by which they are distinguished from insects, and entitled to rank as a dis- tinct class. We have seen that in such of the Annelida as have been most carefully investigated, the orifices of the sexual organs are situated near the anterior part of the body, not, as is invariably the case among insects, at the caudal extremity : in this particular the JulidfE still present analogies with the red-blooded worms ; for in them the external openings of the male parts are situated im- mediately behind the base of the seventh pair of legs, and are found to be placed upon minute mammillary protuberances, which are each furnished with a sort of hooked scale, adapted to hold the female during the process of impregnation. In the female, also, the sexual orifices are advanced very far MYUIAPODA. 227 forward, being situated in the vicinity of the head, between the first and second segments ; the sexes, however, as in insects, are perfectly distinct, and the conformation of the internal organs coincides with that type of structure which is common to the insect orders. (271.) Another important distinction between these animals and insects properly so called, is met with in the mode of their growth and developement. Insects, as we shall more fully explain here- after, undergo a more or less complete change in their outward form as they advance through several preparatory stages to their mature state : during the progress of these changes, which consti- tute what is usually called the metamorphosis of insects, they are invariably unable to perpetuate their species ; and it is only in their last or perfect condition, which is ordinarily of very short duration, that the sexual organs attain their perfect developement, and are fit for reproduction. In this state all true insects have six legs, which is one of the most important characters of the class. The Myriapoda, likewise, undergo several changes of form as they ad- vance to maturity ; but these changes principally consist in the repeated acquisition of additional legs, so that in their perfect condition, instead of the limited number of six legs met with in insects, these organs have become extremely numerous. The progress of these transi- Fig. 99. tions, from their imma- ture to their fully de- veloped state, has been well observed by De Geer * and Savi ;*f and the result of their ob- servations is here given, in order that the rea- der may compare the different steps of the process with what we shall afterwards meet with in the more highly organized articulata. The eggs, (Jig. 99, * Memoires pour servir a 1'Histoire des Insectes. 7 vols. 4to. Stockholm, 1778. t Osservazione per servire alia storia di una specie di Julus communissima. Bo- logna, 1817. MYRIAPODA. A, ) which are very minute, are deposited in the earth or vege- table mould in which the Julus is usually met with. When first hatched, the young Myriapod is of course exceedingly diminu- tive ; at that period it resembles a microscopic kidney-bean, and is completely destitute of legs or other external organs. After a few days the embryo Julus changes its skin, and, throwing off its first investment, appears divided into distinct segments, and furnished with a head, a pair of simple eyes, a pair of antennae, and six jointed legs attached to the anterior rings of the body (fig- 99, B, c). Some days subsequent to its first moult, the skin is again cast, and the millepede acquiring larger dimensions is seen to possess seven pairs of ambulatory extremities, which are, however, still placed only upon the anterior segments (Jig. 99, D). When twenty-eight days old, they again throw off their outward covering, and assume, for the first time, their adult form : they then consist of twenty-two rings, and have twenty-six pairs of feet ; but, of these, only the eighteen anterior pairs are used in progression. At the fourth moult the number of legs is increased to thirty-six pairs ; and at the fifth, at which time the body be- comes composed of thirty segments, there pfg m 100. are forty-three pairs of locomotive organs. At last, in the adult state, the male has thirty-nine and the female sixty-four rings developed; but it is not until two years after this period that the sexual organs appear, and the animals become capable of repro- duction. (272.) Scolopendrida. In the second family of Myriapoda we have a very striking illustration of the manner in which the de- velopement of the nervous centres proceeds step by step with that of the external limbs. The slow-moving Julidse possess in their rudimentary feet organs adapted to their condition, and their feeble powers of locomo- tion are in relation with their vegetable diet and retiring habits. But in the pre- daceous and carnivorous Scolopendra (Jig. 100), which, although it lurks in the same hiding-places as the Julus, obtains its food by pursuing and devouring insects, MYRTAPODA. 229 far greater activity is indispensable, and accordingly we find the segments of the body, and the extremities appended to them, ex- hibiting a perfection of structure adapted to greater vivacity and more energetic movements. This is at once evident upon a mere inspection of their out- ward form ; the individual segments composing the animal are much increased in their proportionate dimensions, and, instead of being cylindrical, each division of the body is flattened and pre- sents a quadrangular outline. In order to give greater flexibility to the body, instead of the semi-crustaceous hard substance which forms the rings of the Julus, the integument is composed of a tough and horny substance, forming two firm plates, one covering the back, the other the ventral aspect of the segment, while all the lateral part is only incased in a flexible coriaceous membrane with which the individual rings are likewise joined together. Such an external skeleton is obviously calculated to give the greatest possible free- dom of motion, and thus to enable the Scolopendra to wind its way with serpent-like pliancy through the tortuous passages in which it seeks its prey. (273.) The ventral chain of ganglia belonging to the nervous system presents a series of nervous centres of dimensions proportioned to the increased bulk of the segments in which they are lodged, and thus fitted to direct the movements of more perfect limbs. The legs, therefore, as a necessary consequence, become proportionably powerful, divided into distinct joints, and provided with muscles calculated to bestow on them that activity essential to the pursuit and capture of active prey. Thus, then, by a simple concentra- tion of the nervous masses composing the abdominal chain of ganglia, we have the slow-moving and worm-like Julus, which we have seen to be, in consequence of its feebleness, restricted to live upon roots and dead substances, converted into the active and powerful Scolopendra, well able to wage successful war with the strongest of the insect tribes, and not unfrequently formidable from its size even to man himself. (274.) The mouth of the Scolopendra is a terrible instrument of destruction ; being provided not only with horny jaws resembling those of insects hereafter to be described, but with a tremendous pair of sharp and curved fangs, ending in sharp points, and per- forated near their termination by a minute aperture, through which a poisonous fluid is most probably instilled into the wound in- flicted by them. It is to this structure that the serious conse- 230 MYR1APODA. quences, which in hot climates not unfrequently result from the bite of one of these animals, must no doubt be attributed. (275.) In their internal anatomy the Scolopendrida resemble insects even more nearly than the Julus. The alimentary canal is straight and intestiniform, but of much smaller diameter than that of the vegetable-eating Myriapoda. It presents an oesophagus, and a small muscular gizzard ; but there is no perceptible division into stomach and intestine. The respiratory and circulating sys- tems, as far as they are understood, seem to correspond with what we shall afterwards find to exist in the larva of insects. In the position and arrangement of the sexual organs the Scolopen- dridse complete the transition between the Annelidans and insects, properly so called ; for, while in Julus we have found them still occupying the anterior part of the body as in the former class, in the Scolopendra they are removed to the tail. The structure of the male organs is remarkable. The testes are seven in number, and, on opening the posterior segments of the animal, they are found closely packed in parallel lines : each testis is composed of two fusiform parts precisely similar to each other, and from both ends of every one of these, which is hollow, arises a narrow duct, so that there are fourteen pairs of ducts arising from the fourteen secreting organs. The ducts all end in a common canal, which gradually becomes enlarged and tortuous, and terminates by a distinct aperture in the vicinity of the anus. Just prior to its ter- mination the common ejaculatory duct communicates with five accessory glands, four of which are intimately united until unravelled, while the fifth is a simple caecum of considerable length.* The ovarian system of the female Scolopendra is a single tube, apparently without secondary ramifications. Some Scolopendrse (S. phosphorea) emit in the dark a strong phosphorescent light ; and one species (S. electrica) is able to give a powerful electrical shock to the hand of the person who in- advertently seizes it. * Vide Cyclop, of Anat. and Phys. art. Generation, organs of. Comp. Anat. 231 CHAPTER XV. INSECTA. (276.) The word Insect has at different times been made use of in a very vague and indeterminate manner, and applied indiscrimi- nately to various articulated animals. * In the restricted sense in which we now use it, we include under this title only such of the HOMOGANGLIATA as in their perfect or mature state are recog- nisable by the following characters, by which they are distinguished from all other creatures. The body, owing to the coalescence of several of the segments which compose their external skeleton, is divided into three prin- cipal portions ; the Head, the Thorax, and the Abdomen. The Head contains the oral apparatus, and the instruments of the senses, including the antennae or feelers, which are articulated organs presenting great variety of shape, but invariably only two in number. The Thorax, formed by the union of three segments of the skeleton, supports six articulated legs, and sometimes four or two wings ; these last, however, are frequently wanting. The Abdomen is destitute of legs, and contains the viscera connected with nutrition and reproduction. (277.) But insects, before arriving at that perfect condition in which they exhibit the above-mentioned characters, undergo a series of change, both in their outward form and internal structure, which constitute what is generally termed their metamorphosis. When this is complete, as for example in the butterfly, the insect, after leaving the egg, passes through two distinct states of existence before it arrives at maturity and assumes its perfect form. The female butterfly lays eggs which when hatched produce, not but- terflies, but caterpillars, animals with elongated worm-like bodies, divided into numerous segments, and covered with a soft coriaceous integument {Jig. 105, A). The head of the caterpillar is provided with horny jaws and several minute eyes ; the legs are very short, six of them which are attached to the anterior rings being horny and pointed, while the rest of variable number appended to the posterior * The word Insect, derived from the Latin word Insecta, simply means divided into segments. 232 INSECTA. part of the body are soft and membranous. The caterpillars, or larveEj* live for some time in this condition, and frequently change their skin as they increase in size, until at length, the last skin of the larva being thrown off, the animal presents itself in quite a different form, enveloped in an oblong case, without any ex- ternal limbs, and almost incapable of the slightest motion, re- sembling rather a dead substance than a living creature ; it is then called a chrysalis, nymph, or pupa^ (Jig. 105, B). On examining attentively the external surface of this pupa, we may discern, in relief, indications of the parts of the butterfly con- cealed beneath it, but in a rudimentary condition. After some time the skin of the pupa bursts, and the imago, or perfect insect, issues forth, moist and soft, with its wings wet and crumpled ; but in a few minutes the body dries, the wings expand and become stiff, and, from being a crawler upon the ground, the creature is con- verted into a gay and active denizen of the air (Jig. 105, c). Such is the progress of the metamorphosis when complete ; but all insects do not exhibit the same phenomena. Those genera which, in their mature condition, have no wings, escape from the egg nearly under the same form as they will keep through life ; these form the Insecta Ametabola^ of authors : and even among those tribes which, when perfect, possess instruments of flight, the larva frequently differs from the complete insect only from its wanting wings, and the pupa is recognisable by being possessed of these organs in an undeveloped or rudimentary state ; an example of this is seen in the house-cricket, (Jig. 102,) in which A represents the imago ; B, the pupa ; c, the full-grown larva ; D, the young just hatched ; and E, the eggs. (271.) The extensive class of INSECTS has been variously arranged by different entomologists, and distributed into numerous orders. Among the different systems which have been given, we select the following as best calculated to render the reader acquainted with * So called by Linnaeus, because in this condition the perfect form of the insect is concealed as it were under a mask. Larva, Lat. a mask. t The two first of these names are purely fanciful ; the last is derived from pupa, a baby wrapped up in swaddling bands. J , without ; ptruZoXvi) change. $ The classification of insects here given is that of Burmeister, which we select without giving any opinion as to its relative merits compared with others adopted by different entomologists, but simply as being most convenient for our present purpose. Manual of Entomology, translated from the German of Dr. Hermann Burmeister by W. E. Shuckard, 8vo. 1836. INSECTA. 233 the transformations, as well as the principal forms, to which allu- sion will be made in subsequent pages. I. INSECTA AMETABOLA. The larva resembles the perfect insect, but is without wings. The pupae of such species as have wings in their imago state possess rudiments of those organs. The pupa runs about and eats. a. With sucking mouths composed of four fine setae lying in a sheath. 1st Order. Hemiptera* In such insects of this order as pos- sess wings, which when present are always four in number, the anterior or upper pair are generally coriaceous in their texture for one half of their extent, while the posterior portion is thin and membranous ; a circumstance from which the name of the order is derived. The Notonecta or water boatman, {fig' 101) is a Fig. 101. familiar example ; c and D represent immature, and F mature larvae. The pupa, G, H, differs little in outward form from the perfect insect E, but possesses only the rudiments of wings. /3. Having mouths furnished with jaws, or distinct mandibles and maxillae. 2nd Order. Orthoptera."^ In this order the perfect insect pos- sesses four wings, the posterior pair being the largest ; and, when at rest, these are folded both in a transverse and longitudinal * wfjuffus, half ; frtgov, a wing, t O^of, straight, 234 INSECTA. direction. The anterior wings are of a denser texture, resembling leather or parchment. To this order belongs the common house- cricket (Gryllus domesticus), of which, as well as of its eggs, larvae, and pupa, figures are here given (fig- Fig. 102. 3rd Order. Dictyotoptera* This order comprises the cock- roaches, in which the wings are four in number when they exist ; but they are generally of equal size, and never folded. II. INSECTA METABOLA. The larva is a worm either with or without legs. The pupa is quiet ; or, if it moves, it does not eat. 4th Order. Neuroptera.^ Insects having four equally large or equally long wings with reticulated nervures, and mouths pro- vided with strong lateral jaws. The most perfect examples of this order are the dragon-flies (Libellula), the largest of the insect inhabitants of our own country. The perfect insect (Jig. 103), equally remarkable for its beautiful form, powerful flight, and car- nivorous habits, is among the most formidable tyrants of its class ; while the larvae, which abound in our ditches and stagnant pools, are eminently destructive to their aquatic companions. The larva (Jig- 104, B) possesses six articulated legs ; while the pupa A, which certainly forms an exception to the general ot, reticulated ; a wng. a nerve ; <rri(>ov t a wing. 1NSECTA. 235 rule given above, is not only furnished with rudimentary wings, but is eminently rapacious, and possesses in the structure of its Fig. 103. mouth, to be described hereafter, peculiar facilities for gratifying its blood-thirsty disposition. In other orders, the wings are always unequal ; the pos- terior, and sometimes both pairs, not unfrequently being wanting. . Mouths adapted to sucking. 5th Order. Diptera* Instead of posterior wings, we find in this order pedunculated appendages called halteres orpoisers. The mouth contains a soft proboscis, and is usually armed with several setse and provided with a pair of palpi ; of such, the common house-fly affords a familiar instance. 6th Order. Lepidoptera.^ The insects belonging to the lepi- dopterous order are possessed of four wings, which are generally covered with microscopic scales, frequently exhibiting the most beautiful colours : the larvae are provided with feet and a dis- * ^iTTi^os (S/-j, wrigflv), with two wings. "T X /?, a scale 236 INSECTA. tinct head ; the mouth of the perfect insect is a long spiral proboscis. The butterflies, so conspi- cuous for their beauty, are well-known representatives of this order ; and the usual forms of these insects in the larva, pupa, and imago state are familiar to all (Jig. 105, A, B, c). /3. Mouths with distinct biting jaws. 7th Order. Hymenoptera.* Possessing four naked wings traversed by ramose nervures. Larvae generally without head or feet, but sometimes with both. Wasps, Bees, Sec. 8th Order. Coleoptera. In this last order, the ante- rior wings are converted into dense horny cases or elytra, be- neath which the posterior pair, a- dapted to flight, are folded up when the insect is at rest. The larvae possess a head, and are sometimes pro- vided with feet, but not always. The Coleopte- rous division of the insect world embraces the ex- * vprivtvos, a membrane; Fig. 104. INSECTA. 237 tensive tribe of beetles, both terricolous and aquatic ; of the former, we have an example in the common cock-chaffer (Melo- lonthci), of which a figure is here given, as well as of the different stages of its developement (fig- 106, A, B, c, D, E).* Fig. 106. Having thus introduced the reader to the chief orders com- posing the vast class of insects, our next object must be to ex- amine more in detail the principles upon which these animals are constructed, both as regards their external organization, and the nature and arrangement of their internal parts. We shall speak of them in the first place only in their perfect condition, leaving all observations relative to the metamorphosis to which they are subject for subsequent consideration. * It would be foreign to our present purpose to do more than enumerate other orders of insects which have been formed by different authors ; of these, the following are the most important. Dermaptera (Leach), ^i^x t skin; vrngov) a wing. Earwigs (Forficula). Trichoptera (Leach), 6^ rp%os, hair; rrsgav. May-flies (Phryganea). Aphaniptera (Kirby), a<pav/jj, invisible ; vrr&gov. Fleas (Pulex). Aptera, arr^aj, without wings. Wingless insects. Parasita, (Latreille). Lice (Pediculus). Thysanoura (Latreille), 0v<rav-euos, bushy-tailed. Spring-tails (Lepismenae). INSECT A. (79.) Insects, examined generally, differ from all other articu- lated beings in one remarkable circumstance they are capable of flight can maintain themselves in the air by means of wings : it is true, indeed, that some species are met with in all the orders de- scribed above, which are apterous, being destitute of such organs ; but these form exceptions to be noticed hereafter. Such a mode of progression, through so rare a medium as that of the atmosphere, necessarily demands an exercise of muscular power of the most vigorous and active description, and a correspondent strength and firmness in the skeleton upon which the muscles act. It is suffi- cient to cast a glance at the external construction of any of the Annelidans or Myriapoda, which have come under our notice, to be convinced that in such animals flight would be impossible under any circumstances : their long and flexible bodies present no point to which efficient wings could be appended ; neither is any part of their divided skeleton possessed of sufficient strength to support the action of muscles so forcible and energetic as would be indispensable to wield the instruments used in flying, or raise the body above the surface of the ground. Similar changes, therefore, to those which we found requisite in order to convert the aquatic Annelide into the terrestrial Myriapod, must be still further carried out before the animals last mentioned could be adapted to become inhabitants of the air. The number of segments composing their elongated bodies must be materially re- duced ; certain parts of the skeleton must be strengthened in order to sustain the efforts of muscles sufficiently strong to raise the weight of the animal ; and, in the last place, the nervous ganglia, by a like concentration of hitherto separated parts, must be gather- ed into masses of increased power sufficient to animate the more vigorous muscles with which they are in relation. (280.) Such changes are precisely those which are most remark- able when we compare the external appearance of a centipede with that of a winged insect : the entire number of segments, and conse- quently the proportionate length of the latter, is obviously reduced. The head is seen to be more distinct from the rest of the body, to which it is connected by a moveable joint. The three anterior seg- ments of the trunk become largely developed, and, from the density of their substance, form by far the strongest part of the skeleton, constituting what is called the thorax of the insect ; they are, moreover, generally united together, especially the two posterior, so as to be consolidated, as it were, into one piece ; and to these INSECT A. 239 rings only the organs of locomotion are appended. The remaining segments of the body are much less firm in their texture, especially in insects with hard or horny wing-covers, in which indeed they are almost of a membranous consistence, so as to increase as far as pos- sible the lightness of the animal in parts where strength is not re- quired. Here then is an annulose skeleton adapted to flight ; dense and unyielding where support is required for the attachment of the locomotive organs, but thin and flexible elsewhere. (281.) The above conditions being required in the arrangement of the pieces which compose the outward framework of the body in insects, we may easily conceive that the mode of union between the various segments above described is by no means a matter of indifference, inasmuch as very different degrees of motion are re- quired between the individual rings. In the Annelida and My- riapods a very simple kind of junction was sufficient ; for in them the segments were all united by the mere interposition of a thinner coriaceous membrane, extending between their contiguous margins ; but in insects several kinds of articulation are, met with in the construction of the trunk adapted to the mobility of different regions. The first mode of connection is effected by suture, or rather by a species of " harmony" as it is technically termed by anatomists ; two plates of the skeleton being accurately and immoveably fitted to each other, but without being decidedly fastened together by serrated edges. This kind of junction is met with in the thorax, and serves an important purpose ; for at the point of union both plates are bent inwards, and prolonged internally, so as to form numerous partitions and processes from which the muscles mov- ing the wings and legs derive extensive origins. A second means whereby the pieces of the thorax are fastened together is by syinphysis, in which a somewhat soft membrane is interposed between two plates, so as to admit of a slight degree of motion. More extensive movement is required between the pieces which compose the abdomen ; for in this region that rigidity and firm- ness which are essential in the construction of the thorax, would be highly disadvantageous, inasmuch as the abdominal viscera must be subject to constant variations in bulk, caused either by food taken into the intestines, or, in the case of the female, by the de- velopement of the eggs after impregnation. The rings of the abdomen are, therefore, united by a membrane passing from one 240 INSECTA. to another ; but so loosely, that the edges of the individual plates wrap over each other to some extent, and thus may be separated by the slightest pressure from within. But in other regions there is an absolute necessity for a mode of communication intermediate in character between the two kinds mentioned above ; having neither the firmness of the one, nor the mobility of the other. This is more especially the case in the junction between the head and the anterior segment of the thorax, and also between the last-named segment and the middle piece of the thorax, in those cases where these two parts are not joined by suture. The joint employed in this case is of very beautiful construction, resembling in some respects that formed by a ball and socket ; a conical prolongation of one segment is admitted into a smooth cavity excavated in the corresponding margin of the other, and secured in this position by muscles and an external ligament. Such an articulation is of course capable of being firmly fixed by muscular action, but at the same time admits of sufficient freedom of motion to allow rotation in all directions. (282.) The legs of insects, as we have already stated, are in- variably six in number, one pair being attached to each of the three thoracic segments. Considered separately, every leg may be seen to consist of several pieces, connected together by articula- tions of different kinds, which require our notice. The first di- vision of the leg, or that in immediate connection with the thorax, to which it is united by a kind of ball-and-socket joint, enclosed in a strong membranous capsule, and possessing very various de- grees of motion in different insects, is called the hip (coxa)', and upon this, as upon a centre, the movements of the limb are per- formed. To the extremity of the coxa a small moveable piece is at- tached, called the trochanter ; to which succeeds the thigh (femur), which is the thickest and most robust of all the divisions of the limb. The next piece, called the shank (tibia"), is occasionally of considerable length, and is connected to the last by a hinge ; to its extremity is appended the foot (tarsus), composed of a consecutive series of small segments, varying in number from five to one, the last of which is armed with claws, or other appendages, adapted to different kinds of progression. These divisions of the leg the reader will easily recognise ; they are for the most part united together by articulations so constructed as to allow simply of flexion and extension, which will be best understood by inspecting, in some large insect, the junction between the femur and the tibia, INSECTA. or the knee-joint, as we might term it. Upon the upper ex- tremity of the tibia the observer will find on each side a precise semicircular furrow, behind which is a concentrical but smaller ridge, and still further back a circular depression or fossulet. On examining the corresponding surfaces of the femur, he will detect a ridge accurately corresponding to the above-mentioned furrow ; behind this a furrow corresponding to the preceding ridge, and still further back, a minute elevation adapted to the fossulet of the tibia, wherein it is fastened by a minute but very strong ligament. Such ridges and grooves, when fitted into each other, form a joint evidently admitting of a free and hinge-like motion, while from its structure, dislocation is almost impossible. (283.) The above general description of the leg of an insect will prepare us to examine various modifications in outward form and mechanical arrangements by which these simple organs are adapted to progression under a great diversity of circumstances. When, indeed, we reflect how extensively this class of animals is distri- buted, and the variety of situations in which insects live, we are led to expect corresponding adaptations in the construction of their instruments of locomotion ; and in this our expectations will not be disappointed. In the generality of terrestrial species, the last segment of the tarsus or foot is provided with a pair of strong horny hooks, which are available for many purposes, being used either for creeping upon a moderately rough surface, for climbing or for clinging to various substances. Such simple hooks, however, would not always serve. In the case of the louse (Pediculus) for example, that is destined to climb slender and polished hairs, such prehensile organs could be of little use. The structure of the foot is therefore modified ; the tarsus in this insect terminates in a single moveable claw, which bends back upon a tooth-like process derived from the tibia, and thus forms a pair of forceps fitted to grasp the stem of the hair and secure a firm hold. Many insects, especially those of the Dipterous order, are able to ascend the smoothest perpendicular planes, or even to run with facility, suspended by their feet in an inverted position, along sub- stances which, from their polished surfaces, could afford no hold to any apparatus of forceps or booklets. In the common flies (Mus- cidcc), the exercise of this faculty is of such everyday occurrence, that, wonderful as it is, it scarcely attracts the attention of ordinary R 242 INSECT A. observers. The foot of the house-fly, nevertheless, is a very curious piece of mechanism ; for, in addition to the recurved hooks pos- sessed by other climbing species, it is furnished with a pair of minute membranous flaps (Jig. 107, c), which, under a good mi- croscope, are seen to be covered with innumerable hairs of the ut- most delicacy : these flaps, or suckers as they might be termed, adhere to any plane surface with sufficient tenacity to support the whole weight of the fly, and thus confer upon it a power of pro- gression denied to insects of ordinary construction. Fig. 107. In Bibio febrilis (fig. 107, B) the sucking discs appended to the foot are three in number, but in other respects their conforma- tion is the same. In Cymbex lutea (fig. 107, D) the arrangement of the suckers is different, one large and spoon-shaped disc being attached to the extremity of each tarsal joint. Moreover, in this case there is another singular structure, two spur-like organs project from each side of the extremity of the tibia, each being is provided with a sucking disc, while the two together form a strong prehensile forceps. In some water-beetles (Dytiscidce) the feet are armed with a still more elaborately constructed apparatus of suckers ; but in this case, as they are only met with in the male insect, they perhaps ought rather to be looked upon as a provision made for the purpose INSKCTA. 24-3 of securely holding the female during sexual union, than as being specially connected with locomotion. In the anterior legs of the male Dytiscus the three first joints of the tarsus are excessively dilated, so as to form a broad circular palette : on examining the inferior surface of this expanded portion under a microscope, it is seen to be covered with an immense num- ber of sucking-cups (Jig. 107, F), two or three being much larger than the rest, but they form collectively a wonderful instrument of adhesion. The middle pair of legs of the same beetle (Jig. 107, A) exhibit a somewhat similar structure ; but, in this case, the disc upon which the sucking apparatus is placed is much elongated, and the suckers are all of small dimensions. In the female Dytiscus (Jig* 109, c) this configuration of the tarsus is wanting, and, moreover, the surface of the back is marked with deep longitudinal grooves that do not exist in the male insect, but seem to be an additional provision for facilitating the inter- course of the sexes in these powerful aquatic beetles. (284.) Another mode of progression common among insects is by leaping, to which, from their extraordinary muscular power, these little beings are admirably adapted. The common flea, for example, (Pulex irritansy) (Jig. 110), will leap two hundred times its own length ; and many Orthoptera possess a power of vaulting through the air scarcely less wonderful, of which the cricket affords a fami- liar instance. In such insects (Jig. 102, A, B) the thighs of the posterior legs are enormously dilated, and the length of these limbs is much greater than that of the anterior pair. When dis- posed to leap, such insects bend each hind-leg, so as to bring the tibia into close contact with the thigh, which has often a longitudi- nal furrow armed on each side with a row of spines, to receive it. The leg being thus bent, they suddenly unbend it with a jerk, when, pushing against the plane of position, they spring into the air.* In many of these saltatorial tribes the tarsus is furnished with very curious appendages, either provided for the purpose of taking off the jar when the animal alights from its lofty leaps,')" or else by their elasticity they may act like firm cushions, adapted to give greater effect to the spring which raises the insect from the ground. In the magnified view of the tarsus of an Abys- * Kirby and Spence, Introduction to Entomology, 4 vols. 8vo. t Sir E. Home, Phil. Transact. 1816. 244 IXSECTA. sinian grasshopper (Jig. 107, E) the arrangement of these organs is well exhibited. (285.) The next modification in the structure of the legs is met with in such species as burrow beneath the surface of the ground, of which mode of progression the most remarkable example is seen in the mole-cricket (Gryllo-talpa vulgaris) (Jig. 108). In this creature Fig. 103. the anterior segment of the thorax, whereunto the fore-legs are ap- pended, is wonderfully enlarged, and of great strength, while the legs themselves are equally remarkable for their enormous bulk and muscularity. The tibia is excessively dilated, and terminates ob- liquely in four sharp and strong spines. The whole of the tarsus would, at a first glance, appear to be wanting ; but on inspection it is found to consist of three joints placed upon the inner side of the tibia, the two first being broad and tooth-shaped, while the last piece is very small, and armed with two hooks. The direction and motion of these hands is outwards, thus enabling the animal most effectually to remove the earth when it burrows, and by the help of such powerful instruments it is astonishing how rapidly it buries itself* (286.) Similar examples of adaptation in the mechanical structure of the legs of insects might be multiplied indefinitely ; we shall, * Kirby and Spence. Introd. to Ent. vol. ii. p. 362. INSECTA. 245 however, select but one other illustration before leaving this part of our subject, namely, the conversion of these organs into instru- ments for swimming, whereby, in aquatic insects, they become adapt- ed to act as oars. Nothing is, perhaps, better calculated to excite the admiration of the student of animated nature than the amazing results .obtained by the slightest deviations from a common type of organization ; and in examining the changes required in order to metamorphose an organ which we have already seen performing such a variety of offices into fins adapted to an aquatic life, this circumstance must strike the mind of the most heedless observer. The limbs used in swimming exhibit the same parts, the same number of joints, and almost the same shape, as those employed for creeping, climbing, leaping, and numerous other purposes ; yet how different is the function assigned to them ! In a common water beetle already referred to, the Dytiscus marginalis (Jig- 109, c), Fig-. 109. the two anterior pairs of legs, that could be of small service as in- struments of propulsion, are so small as to appear quite dispropor- tionate to the size of the insect, while the hinder pair are of great size and strength ; the last-mentioned limbs are, moreover, removed as far backwards as possible by the developement of the hinder seg- ment of the thorax, in order to approximate their origins to the centre of the body, and the individual segments composing them 246 INSECTA. are broad and compressed, so as to present an extensive surface to the water, which is still further enlarged by the presence of flat spines appended to the end of the tibia, as well as of a broad fringe of stiff hairs inserted all around the tarsus. The powerful oars thus formed can open until they form right angles with the axis of the body, and from the strength of their stroke are well adapted to the piratical habits of their possessors, who wage successful war not only with other aquatic insects and worms, but even with small fishes, the co-inhabitants of the ponds wherein they live. The same principles are carried out even more perfectly in the construction of the swimming legs of the water-boatman (Noto- necta), a kind of water-bug. The resemblance of this creature (Jig. 101, G, H) to a boat with its oars, cannot escape the most in- attentive examiner ; and the similarity is still further increased by its manner of swimming ; for, as it preys, upon insects that have been accidentally drowned by falling into the water, it usually rows itself about upon its back, because in such a position it can best watch for its victims. (287.) The wings of insects, when present, are invariably attached to the two posterior segments of the thorax, which, as we have al- ready seen, are strengthened in every possible manner, so as to afford a support of sufficient density and firmness to sustain the violent ex- ertions of the muscles inserted into the organs of flight. In the most perfectly organized families the wings are four in number, as in the Neuroptera (Jig. 103), the Hymenoptera (Jig. 129), the Orthoptera (Jig. 102), the Dictyoptera, the He- miptera (Jig. 101), the Lepidoptera (Jig. 105), and the Cole- optera (Jig. 106). In the Dipterous insects there are only two wings, which are fixed upon the central segment of the thorax ; while, in the posi- tion usually occupied by the posterior pair, we find a pair of pe- dunculated globular bodies, usually named the Halteres or poisers, as in the gnat (Culex,) (Jig. 131, F). But, in every one of the orders above enumerated, there are certain families which, throughout the whole period of their exist- ence, are never provided with wings at all ; and these by many entomologists have been formed into an order by themselves, under the name of Apterous insects. In the opinion of Burmeister,* whose classification we have adopted, such an arrangement is purely artificial, inasmuch as it must embrace insects of most * Manual of Entom. p. 623. INSECTA. 247 dissimilar kinds. In proof of this, he adduces the fact, that in the same family we not unfrequently meet with both winged and ap- terous species nearly related to each other ; and in many cases the males possess wings, while the females of the same insect are en- tirely destitute of such appendages. In such cases, the metamor- phosis is necessarily what is called incomplete, inasmuch as the organs which characterize the perfect state are not developed. Thus, in the flea (Pulex irritans) (fig. 110), the wings never become apparent, and the Fig. 110. thorax in conse- quence, even in the imago state, does not exhibit that develope- ment and con- solidation of its parts invariably met with in wing- ed genera. The flea, however, cannot on this account be looked upon as any other than the imago or complete insect, for it will be found to have undergone all the preparatory changes. The flea, when it issues from the egg, is in fact a worm-like and foot- less larva, in which condition it lives about twelve days. When about to become a pupa, it spins for itself a little silky cocoon, wherein it conceals itself, until, having thrown off its last skin, it appears in its mature form, deprived indeed of wings, that, under the circumstances in which it lives, would be useless appendages, but still with this exception corresponding in every particular with other insects in their imago state. (288.) The wings of insects differ much in texture. In the Neu- roptera, by far the most powerful fliers met with in the insect world, all four wings are of equal size, and consist of a thin mem- branous expansion of great delicacy and of a glassy appearance, supported at all points by a horny network (Jig. 103). Few things are met with in nature more admirable than these struc- tures ; they present indeed a combination of strength and lightness absolutely unequalled by anything of human invention, and as instruments of flight they far surpass the wings of birds, both in the power and precision of their movements, inasmuch as these insects can fly in all directions, backwards, or to the right or left, as well as forwards. Leeuwenhoek* narrates a remarkable instance * Leeuw. Epist. 6, Mart. 1717. INSECTA. in which lie was an eye-witness of the comparative capabilities of the Dragon-fly and the Swallow, as relates to the perfection of their flight. The bird and the insect were both confined in a mena- gerie about a hundred feet long, and apparently their powers were fairly tested. The swallow was in full pursuit, but the little crea- ture flew with such astonishing velocity, that this bird of rapid flight and ready evolution was unable to overtake and entrap it ; the insect eluding every attempt, and being generally six feet before it. " Indeed," say the authors from whom we quote the above anecdote,* " such is the power of the long wings by which the dragon-flies are distinguished, and such the force of the muscles which move them, that they seem never to be wearied with flying. I have observed one of them (Anax Imperator, Leach) sailing for hours over a piece of water, sometimes to and fro, and sometimes wheeling from side to side, and all the while chasing, capturing, and devouring the various insects that came athwart its course, or driving away its competitors, without ever seeming tired or inclined to alight." In Hymenopterous insects (Jigs. 128 and 129), the wings are much more feebly organized, but their structure is similar ; the nervures, or horny ribs, supporting the membranous expansion, are comparatively few, and in the Diptera they are still less nu- merous. In several orders the anterior pair of wings are converted into shields for the protection of the posterior ; such is the case in the Orthoptera, many of the Hemiptera, and more especially in the Coleopterous genera. In the latter, indeed, they are very dense and hard ; and, being nearly unserviceable in flight, the hinder pair are necessarily developed to such a size as to present a very ex- tensive surface (fig. 106, A), and when in repose are closely folded up beneath the elytra, and thus carefully preserved from injuries to which they would be constantly exposed without such provision for their security. (289.) The above observations relate only to the general disposi- tion and connection of the different parts of the skeleton, and loco- motive appendages connected with it; it remains for us now to speak more fully of the texture of the external integument, and those modifications which it presents, adapting it to various purposes. The hard covering of an insect, like the skin of vertebrate animals, consists of three distinct layers. The outer stratum or * Kuby and Spence, op. cit. p. 351. 1NSECTA. 249 epidermis is smooth, horny, and generally colourless, so that it forms a dense inorganic film spread over the whole surface of the body. Immediately beneath the epidermis is a soft and delicate film, the rete mucosum, which is frequently painted with the most lively hues, and gives the characteristic colouring to the species. The third and principal layer is the true skin or cutis, which is gener- ally of a leathery texture, and, especially in the elytra of beetles, of considerable thickness : this layer is abundantly supplied with nutritive juices, and in its substance the bulbs of hairs, scales, and similar appendages, to be described hereafter, are embedded and nourished. (290.) The wings are mere derivations from this common cover- ing, and are composed of two delicate films of the epidennis, stretch- ed upon a strong and net-like framework. Every membranous wing is in fact a delicate bag formed by the epidermic layer of the in- tegument, and in the recently developed insect can be distinctly proved to be such, by simply immersing the newly escaped imago in spirit of wine, which gradually insinuates itself between the still fresh and soft membranes ; and, filling the cavity enclosed be- tween them, distends the organ until it represents a transparent sacculus in which the ribs or nervures of the wing are enclosed.* This structure, however, is only to be displayed while the wings, after being withdrawn from the pupa-case, are still soft and moist, for they soon become so intimately united with the horny frame- work upon which they are extended, that they seem to form a single membranous expansion. The, ribs or nervures, whereby the two plates of the wing are thus supported, are slender hollow tubes, filled with a soft paren- chyma, in the interior of some Burmeister detected an air-vessel recognisable by the texture of its walls, and a minute nervous filament. (291.) We have still, in order to complete our descriptioirof the external anatomy of an insect, to describe certain appendages which not unfrequently clothe the exterior of the skeleton, and exhibit great diversity of appearance in different tribes. These may be divided into spines, hairs, and scales ; and, however much they may appear to be distinct structures, all these are essentially very nearly related to each other. The spines are horny processes developed from the epidermis ; and sometimes, especially in the Coleopterous order, as in some * Heusingcr, System der Hystologie, 2 Heft. Burraeister, op. cit. p. 224. 250 IN SECT A. lamellicorn beetles, exhibit considerable dimensions. These spines are sometimes bifurcated or branched ; but, whatever their shape or size they never grow from bulbs implanted in the cutis, but are mere prolongations of the exterior layer of the integument. The hairs in their mode of growth appear to resemble those of quadrupeds, inasmuch as they are secreted from roots embedded in the substance of the cutis or true skin : they are fine horny cy- linders, and frequently are found to be branched and divided like the feathers of birds ; but the manner of their formation will be more conveniently discussed hereafter. The wings of the Lepidoptera are covered with minute flat scales of various shapes, and not unfrequently tinted with the most beautiful colours ; such scales, nevertheless, are in reality only flattened hairs, into which indeed they frequently degenerate by in- sensible transitions, and, moreover, they grow from bulbs of pre- cisely similar construction. The variety of colours exhibited by the scales of a butterfly depends upon a film of pigment interposed between the two plates of transparent epidermic matter forming each ; but the gorgeous hues derived from this source must not be confounded with the iridescent tints for which they are not un- frequently remarkable, as these have a very different origin : the surface of every scale, that with the changing light reflects evanescent prismatic colours, is seen, when examined under a micro- scope, to be marked with regular parallel striae of exquisite minute- ness ; and such a surface, even when grossly imitated by human art, has been found to give rise to the brilliant appearances exhibited by polarized light. (292.) The muscular system of insects has always excited the wonder and astonishment of the naturalist, in whatever point of view he examines this part of their economy, whether he considers the perfection of their movements, the inconceivable minuteness of the parts moved, or the strength, persistence, or velocity of their contractions. Insects are proverbially of small comparative dimen- sions " minims of nature" " that wave their limber fans For wings, and smallest lineaments exact, In all the liveries decked of summer's pride j" their presence, indeed, around us, is only remarked as conferring additional life and gaiety to the landscape ; and, except when by some inordinate increase in their numbers they make up by their multitude for their diminutive size, the ravages committed by them INSECTA 251 are trifling and insignificant. Far otherwise, however, would it be, if they attained to larger growth, and still possessed the extra- ordinary power with which they are now so conspicuously gifted ; they would then, indeed, become truly the tyrants of the creation, monsters such " as fables never feigned or fear conceived," fully adequate to destroy and exterminate from the surface of the earth all that it contains of vegetable or of animal life. We have already seen that the flea or the grasshopper will spring two hundred times the length of its own body ; that the dragon-fly possesses such indomitable strength of wing, that for a day together it will sustain itself in the air, and fly with equal facility and swiftness backwards or forwards, to the right or to the left, without turning ; that the beetles are encased in a dense and hard integument, impervious to ordinary violence ; and we might add, that the wasp and the termite ant will penetrate with their jaws the hardest wood. Neither is the velocity of the movements of insects inferior to their prodigious muscular power. An anonymous writer in Nicholson's Journal," say Kirby and Spence, u calculates that in its ordinary flight the common house- fly (Musca domestica) makes with its wings about six hundred strokes, which carry it five feet every second ; but, if alarmed, he states their velocity can be increased six or seven-fold, or to thirty or thirty-five feet in the same period. In this space of time a race-horse could clear only ninety feet, which is at the rate of more than a mile in a minute. Our little fly, in her swiftest flight, will in the same space of time go more than the third of a mile. Now, compare the infinite difference of the size of the two animals (ten millions of the fly would hardly counterpoise one racer), and how wonderful will the velocity of this minute crea- ture appear ! Did the fly equal the race-horse in size, and retain its present powers in the ratio of its magnitude, it would traverse the globe with the rapidity of lightning."* Let the reader, therefore, imagine for an instant that great law of nature, which restricts the dimensions of an insect within certain bounds, dispensed with even in a single species. Suppose the wasp or the stag-beetle dilated to the bulk of a tiger or of an elephant cased in impenetrable armour furnished with jaws that would crush the solid trunk of an oak winged, and capable of flight so rapid as to render escape hopeless ; what would resist such destroyers, or how could the world support their ravages ? * Kirby and Spence, op. cit. vol.ii. p. 358. 252 1NSECTA. Such is the comparative strength of insects. Let us now pro- ceed to examine the muscles to which it is owing their structure and general arrangement. (293.) The muscles consist of bundles of delicate fibres, that arise either from the inner surface of the segments composing the skeleton, or else from the internal septa ( 281.) which project into the thorax. The fibres themselves are of a white or yellow colour ; and so loosely are they connected by cellular tissue, that they may be separated by the slightest touch. All the muscles of an insect may be arranged in two great divisions; the first including those that unite the different seg- ments of the body ; the second, those appropriated to the move- ments of the limbs, jaws, and other appendages : the former are entirely composed of fleshy fibres ; the latter are provided with tendinous insertions, by which their force is concentrated and made to act with precision upon a given point of the skeleton. The connecting muscles are generally arranged in broad parallel bands, arising from the inner surface of a given segment, and passing on to be inserted in a similar manner into another seg- ment, so that by their contraction the cavity in which they are lodged is diminished by the approximation of the different rings : these have no tendons. The locomotive muscles of course take their character from the joints of the limb upon which they act ; and, as we have already seen that these movements are generally confined to those of a hinge, the muscular fasciculi may be conveniently grouped into two great classes, the flexor muscles, that bend the joint ; and the extensors, by which it is again straightened, and brought back to its former position. This simple arrangement will be best understood by an inspection of the appended figure (fig. Ill), representing the muscles of the leg of a cockchafer (Melolontha vulgaris), as they are depicted by Strauss Durckheim.* In the thigh, for example, there are two muscles, one of which bends, the other straightens, the tibia. The flexor (fig. Ill, a) arises from the lining m'embrane of the femur, and is inserted by a ten- don into a process of the tibia in such a manner as to flex the leg upon the thigh ; while its antagonist (&), attached to a process derived from the other side of the joint, has an opposite effect, and by its contraction extends the leg. In the tibia there are like- * Considerations ge"ne"rales sur 1'Anat. comp. des Animaux Articules, auxquelles on a joint 1' Anatomic descriptive du Hanneton. 1 vol. 4to. Paris, 1828. INSECTA. 253 wise two muscles, so disposed as move the Fig. ill. entire tarsus and foot. The extensor (f) of the tarsus is the smallest ; it arises from the lower half of the interior of the tibia, and is inserted into the margin of the first joint of the tarsus : but the flexor of the foot (c), aris- ing from the upper half of the cavity of the tibia, ends in a delicate tendon, which passes through all the tarsal segments, to be fixed to the flexor tendon of the claw-joint upon which it acts ; and, as it traverses the penultimate joint, it receives the fibres of an accessory muscle (d). The extensor of the claw (e) is likewise placed in the penultimate tarsal segment, and strikingly exhibits, by its small comparative size, the feebleness of its action, when compared with the flexors of the same joint. It would be superfluous to describe more in detail the disposition of individual muscles, as the above example will abundantly suffice to give the reader an idea of the general ar- rangement of the muscular system, not in in- sects only, but in all the ARTICULATA provided with jointed extremities. (294.) The substances employed as food by insects are various, in proportion to the extensive distribution of the class. Some de- vour the leaves of vegetables, or feed upon grasses and succulent plants ; others destroy timber, and the bark or roots of trees ; while some, more delicately organized, are content to extract the juices of the expanding buds, or sip the honeyed fluids from the flowers. Many tribes are carnivorous in their habits, armed with various weapons of destruction, and carry on a perpetual warfare with their own or other species ; and again there are countless swarms ap- pointed in their various spheres to attack all dead and putrefying materials, and thus to assist in the removal of substances which, by their accumulation, might prove a constant source of annoyance and mischief. Such differences in the nature of their food demand of course corresponding diversity in the construction of the in- struments employed for procuring nourishment, and accordingly we find in the structure of the mouths of these little beings innu- 254 INSECTA. merable modifications adapting them to different offices. The mouths of all creatures are constructed upon purely mechanical principles ; and in few classes of the animal world have we more beautiful illustrations of design and contrivance than in that before us : -jaws armed with strong and penetrating hooks for seizing and securing active and struggling prey, sharp and powerful shears for clipping and dividing the softer parts of vegetables, saws, files, and augers for excavating and boring the harder parts of plants, lancets for piercing the skin of living animals, siphons and sucking tubes for imbibing fluid nutriment ; all these, in a thousand forms, are met with in the insect world, and thus provide them with the means of obtaining food adapted to their habits, and even of constructing for themselves edifices of inimitable workmanship. (295.) Parts of the mouth. The mouths of insects may be di- vided into two great classes, those which are adapted for biting, forming what is called a perfect or mandibulate mouth ; and those which are so constructed as only to be employed in sucking, consti- tuting the suctorial or haustellate mouth. It is in the former of these divisions that all the parts composing the oral apparatus are most completely developed . we shall therefore commence by describ- ing the different pieces of which a perfect mouth consists, viz. an upper and an under lip, and four horny jaws. We select the dragon-fly (fig 112, A) as an example. The upper lip (labrum, Fig, 112. B) is a somewhat convex corneous plate, placed transversely across the upper margin of the cavity wherein the jaws are lodged, so that, when the mouth is shut, it folds down to meet the under lip (la- INSECTA. 55 bium), and these two pieces more or less completely conceal the proper jaws, which are lodged between them. The upper pair of jaws (mandibulfs) are two hard and powerful hooks (c), placed immediately beneath the upper lip, and so ar- ticulated with the cheeks that they move horizontally, opening and shutting like the blades of a pair of scissors. Their concave edge is armed with strong denticulations of various kinds, sometimes fur- nished with cutting edges, that, like sharp shears will clip and di- vide the hardest animal and vegetable substances ; sometimes they form sharp and pointed fangs, adapted to seize and pierce their victims ; and not unfrequently they constitute a series of grinding surfaces, disposed, like the molar teeth of quadrupeds, to triturate and bruise the materials used as food. The variety of uses to which these mandibles can be turned is indeed amazing. In the car- nivorous beetles, their hooked points, more formidable than the teeth of the tiger, penetrate with ease the mailed covering of their stoutest congeners ; and in the dragon-fly they are scarcely less formidable weapons of destruction. In the locust tribes these organs are equally efficient agents in cutting and masticating leaves and vegetable matters adapted to their appetites ; while in the wasps and bees they form the instruments with which these insects build their admirable edifices, and, to use the words of a popular author, supply the place of trowels, spades, pick-axes, saws, scissors, and knives, as the necessity of the case may require. Beneath the mandibles is situated another pair of jaws, of similar construction, but generally smaller and less powerful; these are called the maxilla (F). The lower lip, or labium (E), which closes the mouth inferiorly, consists of two distinct portions, usually described as separate organs, the chin (mentum), that really forms the inferior border of the mouth ; and a membranaceous or somewhat fleshy organ, repos- ing upon the chin internally, and called the tongue (lingua) of the insect (D). All these parts enter into the composition of the perfect mouth of an insect, and, from the numerous varieties that occur in their shape and proportions, they become important guides to the ento- mologist in the determination and distribution of species. For more minute details concerning them, the reader is necessarily re- ferred to authors who have devoted their attention specially to this subject ; we must not, however, omit to mention certain appen- dages or auxiliary instruments inserted upon the maxilla and the labium, usually named the palpi, or feelers, and most probably 256 INSECTA. constituting special organs of touch, adapted to facilitate the appre- hension and to examine the nature of the food. The maxillary feelers (palpi maxillares) are attached to the external margin of the maxillae by the intervention of a small scale and very pliant hinge, and consist of several (sometimes six) distinct but ex- tremely minute pieces articulated with each other. The labial feelers (palpi labiates) are inserted into the labium close to the tongue, or occasionally upon the chin (mentum) itself. The joints in the labial palpi are generally fewer than in the maxillary, but in other respects their structure and office appear to be the same. In the suctorial orders of insects we have the mouth adapted to the imbibition of fluid nutriment, and consequently constructed upon very opposite principles ; yet, notwithstanding the apparent want of resemblance, it has been satisfactorily demonstrated by Sa- vigny* that the parts composing a suctorial mouth are fundament- ally the same as those met with in the mouth of mandibulate in- sects, but transformed in such a manner as to form a totally differ- ent apparatus. According to the distinguished authors of the u Introduction to Entomology,"')' there are five kinds of imperfect mouth adapted to suction, each of which will require a separate notice. (29 6.) The first is met with among the Hemiptera, and is to perforate the stalks and buds of vegetables, in order to procure the juices which they contain ; or in some bugs it is employed to puncture the integument of living animals for a similar purpose. This kind of mouth is exhibited in fig. 113 : first, there is a long jointed sheath (d), 1J3 which is in fact the lower lip (labium), con- siderably elongated, and composed of three or four parts articulated together; second- ly, there is a small conical scale covering the base of the sheath last mentioned, and re- presenting the upper lip ; and between these are four slender and rigid bristles or lancets (scalpella) (c) that, when not in use, are lodged in a groove upon the upper surface of the sheath so as to be concealed from view. These lancets are, in reality, only the man- * Savigny (Jules Cesar), Memoires sur les animauxsans vertebres, 8vo. Paris, 1816. t Kirby and Spence, vol. iii. p. 463. INSECTA. 257 diblcs and maxillae strangely altered in their form and excessively lengthened, so as not merely to become efficient piercing instru- ments, but so disposed as to form by their union a suctorious tube, through which animal or vegetable fluids may be imbibed. This kind of mouth, when not employed, is usually laid under the thorax be- tween the legs, in which position it is easily seen in most Hemi- ptera : in some families, as, for example, in the plant-lice (Aphides), it is of extraordinary length ; thus, in the aphis of the oak it is three times as long as the whole body of the insect, projecting posteriorly like a tail, and in the fir-aphis it is still longer. (297.) The second kind of mouth is that met with among the Diptera, and from its construction in some tribes we may well under- stand how they are enabled to become so seriously annoying. The gnat and the mosquito furnish sufficiently well-known examples of the formidable apparatus in question, which, in the horse-fly (Taba- nus), seems to attain its maximum of developement. The oral organs of the Diptera are composed of a sheath or proboscis, that represents the lower lip of the mandibulate insects ; it is sometimes coriaceous or horny in its texture, or in other cases, as in the com- mon flesh-fly, soft and muscular, and folds up when at rest in such a manner as to form two angles, representing the letter Z. At the base of this sheath or proboscis there is a small upper lip, between which and the sheath are lodged the setae, knives or lancets, which form such terrible instruments for cutting or piercing the skin of their victims. These cutting parts vary in "number from one to five : when they are all present, the upper pair (cultelli^ or knives) represent the mandibles of a perfect mouth, the two lower ones (scalpella, the lancets) are the maxillae, the fifth or middle piece (glossarium) is the tongue, and between them all is the oral opening. The strength of the above piercing instruments varies greatly ; in the ghat they are finer than a hair, very sharp and barbed occasionally on one side ; while in the horse-fly they are flat, like the blades of a lancet or penknife : occasionally they are so constructed as to form a tube by their union, through which the liquid aliment is sucked up and conveyed into the stomach. (298.) The mouth of the flea, although described by Kirby and Spence as forming a distinct type of structure, differs very little from that of the Diptera described above, as will be at once evi- dent on inspecting the accompanying figure, reduced from a beau- tiful drawing by Mr. W. Lins Aldous. 258 In this insect the piercing organs are two sharp and razor-like instruments (Jig- 114, d, d), placed on each side of the elongated tongue (e), and enclosed in a sheath (c, c), probably formed by pieces representing the mandibles of mandibulate insects. Two palpi or feelers (0, a), and a pair of triangular plates (6, i), com- plete this remarkable apparatus. (299.) Another kind of mouth adapted to suction, and which seems to differ more widely from the perfect form than any we have as yet examined, is that which we meet with in moths and butterflies. This singular organ is adapted to pump up the nectareous juices from the cups of flowers, and is necessarily of considerable length, in order to enable the insect to reach the recesses wherein the honeyed * Head of the flea, as represented by the Solar microscope in Canada balsam ; dedi- cated by permission to the President and Members of the Entomological Society, by W.Lins Aldous. INSECTA. 259 stores are lodged. When unfolded, the apparatus in question repre- sents a long double whip-lash (Jig. 115, a, 5, c, d), and, if carefully examined under the micro- scope. each division is found Fig. 115. to be made up of innumer- able rings connected toge- ther, and moved by a dou- ble layer of spiral muscular fibres, that wind in oppo- site directions around its walls. When not in use, the proboscis is coiled up and lodged beneath the head ; but when uncurled its structure is readily ex- amined. Each of the two long filaments composing this trunk, which, in fact, are the representatives of the maxilla: excessively lengthened, is then seen to be tubular ; and, when they are placed in contact, it is found that their edges lock together by means of minute teeth, so as to form a central canal leading to the orifice of the mouth. It is through this central tube, formed by the union of the two lengthened maxillae, that fluids are imbibed. Burmeister, however, asserts that the cavities contained in each divi- sion likewise communicate with the commencement of the oasophagus, so that the Lepidoptera have, as it were, two mouths, or rather two separate methods of imbibing nourishment ; one through the com- mon canal formed by the junction of the whip-like jaws, the other through the cavities of the filiform maxillse themselves : such an arrangement, however, which would be quite anomalous, may rea- sonably be doubted. In this mouth, therefore, all the parts, except the maxillse, would seem at first sight to be wanting ; they may, nevertheless, be detected upon a very careful examination, and rudiments of the upper lip, of the mandibles, of the lower lip, as well as of the labial and maxillary palpi, be distinctly demonstrated. (300.) The last kind of mouth to which we shall advert, is that met with in the louse tribe (Pediculi) ; but, from the extreme mi- nuteness of the parts composing it, the details of its structure are but imperfectly known. It seems to consist of a slender external tube, wherein a sharp sucker, armed with barbs adapted to fix it securely during the act of sucking, is lodged ; when feeding, the 260 INSECTA. barbed piercer is denuded and plunged into the skin, where it" is retained until a sufficient supply of nourishment has been obtained. (301.) Inviting as the subject is, we are compelled by the strictly general character of our investigations to abstain from entering upon further details concerning the mouths of perfect insects, and conse- quently to omit noticing innumerable secondary modifications in the mechanical structure of the oral organs of these little animals. When we turn our attention to the consideration of their internal viscera, connected with the preparation and digestion of so many different materials, we may well expect to find equal variety of conformation ; and, in fact, the course, dimensions, and relative proportions of the alimentary canal will be seen to be different to a greater or less extent in almost every species. Considered as a whole, the internal digestive apparatus of insects must be regarded as a delicate membranous tube, in which the digestion of the sub- stances used as food is accomplished, partly by mechanical and partly by chemical agents : for the former purpose, gizzard-like muscular cavities are not unfrequently provided ; and, to fulfil the second, various fluids are poured into the canal in different parts of its course : the arrangement of the cavities, and the nature of the secreting vessels, however, will be modified in conformity with the necessities of the case, and certain parts will be found to exist, or to be deficient, as circumstances may require : it would be absurd, therefore, to attempt to describe particular examples ; our observa- tions must be of general application, and such as will enable the reader to assign its proper functions to any organ which may pre- sent itself to his notice. The first part of the digestive appara- tus is disposed in the same manner in all insects, and is a slender canal, arising from the mouth and passing straight through the tho- rax into the cavity of the abdomen ; this portion represents the oesophagus (fig* 116, a, a; 117, o). t The stomach and intestine succeed to this, and, if the body of the insect be very thin, their course also passes nearly in a direct line to the tail ; but in those families which have the abdomen thick and largely developed, espe- cially if herbivorous, the intestine becomes much elongated, and winds upon itself in various convolutions : nevertheless, however tortuous the canal may be, its windings are never sustained by any mesentery or peritoneal investment ; the air-tubes, that, as we shall afterwards see, permeate the body in all directions, form a sufficient bond of connection, and one which is better adapted to the wants of these animals. INSECTA. We m-ust now examine more minutely the different portions of which the alimentary canal may consist, premising at the same time that the structures mentioned do not invariably exist together, as sometimes one part, and sometimes another, may be entirely wanting, or only found in a very rudimentary condition. They are the Crop, the Gizzard, the Stomach, the Small Intestine, and the Large Intestine. (302.) The Crop, or Sucking- Stomach, as it is called by some writers, is only met with in Hymenoptera, Lepidoptera, and Di- ptera, insects which have no gizzard.* In bees, wasps, and other Hymenoptera, it is a simple bladder-like distension of the oesophagus (Jig. 116, b) ; in butterflies and moths it forms a distinct bag, that opens into the side of the Fig. 116. gullet (Jig. 117, v, v) ; while in the Diptera it is a detached ve- sicle, appended to the oesophagus by the intervention of a long thin duct. This organ, which in bees is usually called the honey blad- der, is regarded by Burmeister, who founds the opinion upon the result of experiments made by Treviranus upon living insects, as being not merely a receptacle for food resembling the craw of birds, as Ramdohr j- and Meckel con- sider it, but as being a sucking instrument for imbibing liquids, by becoming distended, as he expresses it, and thus, by the rare- faction of the air contained within it, facilitating the rise of the fluids in the proboscis and oesophagus. It must, however, be confessed that there is something very anomalous in the idea of a delicate bag having the power of distending itself ; its muscular walls might indeed contract, but that a thin sacculus should forci- bly expand itself would be a fact new to physiology. (303.) The Gizzard is found in insects which possess mandibles, and live upon solid animal or vegetable substances. It is a small round cavity with very strong muscular parietes, situated just above the stomach properly so called, and, like the gizzard of granivorous birds, is employed for the comminution of the food preparatory to * Burmeister, op. cit. p. 125. Treviranus, Vermischte Schriften. t Ramdohr, iiber die Verdauungswerkzeuge der Insecten. Halle, 1811. 262 INSECTA. its introduction into the digestive stomach. In order to effect this, it is lined internally with a dense cuticular membrane, and occa- sionally studded with hard plates of horn or strong hooked teeth, adapted to crush or tear in pieces whatever is submitted to their action. When bruised in the gizzard, the food passes on into the proper stomach, which is generally a long intes Uniform organ (Jig. 116, d, d), extending from the crop or gizzard to the point where the biliary vessels discharge themselves into the intestine. The size and shape of this organ will vary of course with the nature of the food. Thus, in the butterfly (Jig. 117, &), which scarcely eats at all, or sparingly sips the honey from the flowers, it is very mi- nute ; but, in insects which live upon coarse and indigestible mate- rials, it is proportionately elongated and capacious. (304.) The stomach generally ends in the Small Intestine (Jig. 116, e ; 117, *), but this is occasionally entirely wanting, so that the stomach seems to terminate immediately in the colon or large intes- tine, which is the terminal portion of the alimentary canal : when much developed, the small intestine is sometimes divided by a con- striction into two parts, to which the names of Duodenum and Ilium have been applied by entomological writers. The colon (fig. Fig. in. 116,y; 117, k) is separated from the small intestine by a distinct valve; and, in con- nection with its commence- ment, a wide blind sacculus or caecum is often met with. (305.) We may now no- tice the secern ing organs that pour fluids in to different parts of the digestive apparatus ; beginning with those which open into the oesophagus in the vicinity of the mouth, and examining them in the order of their occurrence as we proceed backwards. The first are the salivary vessels, which terminate in the neighbourhood of the mouth itself, into which they seem to pour a secretion analogous to INSECTA. 263 saliva. These glands are principally met with in suctorial insects, but not unfrequently among the mandibulate orders. Their form varies ; but they are generally simple slender tubes, that float loosely among the juices of the body, from which they separate the salivary fluid. There are, for the most part, only two of these organs (fig. 117, s, s) ; but in fleas (Pulex), and bugs (Cimex), there are four, and in a water-bug (Nepa), there are six such vessels, all of which open into the cavity of the mouth. The fluid supplied by the salivary glands is usually merely intended to facilitate deglutition ; but there are cases in which the saliva is ex- cessively acrid and irritating, acting as a kind of poison when in- fused into a puncture made by the mouth : this is especially re- markable in many bugs and gnats, and is the chief cause of the pain and inflammation frequently occasioned by their bite. Besides the proper salivary vessels, there are other glands, or rather caeca, which open into the stomach itself, occasionally cover- ing that organ over its entire surface, as is the case in some water- beetles (Hydrophilus) ; these, no doubt, secrete a fluid subser- vient to digestion ; but whether of a peculiar description, or allied to saliva in its properties, is unknown. The third kind of auxiliary vessels connected with the intestinal canal of insects, is supposed to furnish a secretion analogous to the bile of other animals, and consequently to represent the liver. The bile-vessels (fig. 116, A, h; fig. 117,#, g) are generally four, six, or eight in number, but occasionally much more numerous ; they are usually of great length, but exceedingly slender, and wind around the intestine in all directions. When unravelled, they are found to terminate, in the neighbourhood of the pylorus (fig. \ 17, A, w), close to the commencement of the intestine, at which point the secretion produced by them is mixed with the food after it has undergone the process of digestion. Appended to the termination of the alimentary tube, close to its anal extremity, other vessels are met with in some insects that are looked upon by authors as being allied in function to the kid- neys of higher animals ; but apparently this opinion rests upon very doubtful grounds. They indubitably furnish some secretion, the use of which is perhaps connected with defecation ; but that it is of the same character as the fluid separated by the renal organs of vertebrata may well be called in question, as no such parts are distinctly recognisable until we arrive at much more elevated forms of life than the insects we are now considering. There is, how- 264 1NSECTA. ever, another reason for rejecting the opinion that these accessory vessels secrete urine, and that is, that they are only met with in a few beetles and some species of Orthoptera ; a circumstance that alone would be sufficient to disprove such supposition. In the vertebrate animals, as the reader is well aware, the nu- tritious products of digestion are taken up by a system of absorb- ing vessels, that ramify extensively over the coats of the intestine, and the nutriment is thus conveyed into the mass of the circu- lating fluid by ducts appropriated specially to this office ; in animals of less perfect structure than these, such as the Mollusca, the veins themselves absorb the nutritive materials. But in insects, in which we find neither absorbents nor veins, a different arrange- ment is necessary ; and, in the little creatures before us, nutrition appears to be carried on by the simple transudation of the chyle through the coats of the intestine, so that it escapes into the general cavity of the abdomen, where, as we shall see when we examine the arrangement of their circulating organs, it is im- mediately mixed up with the blood. This transudation has in- deed been actually witnessed by Ramdohr and Rengger,* and even analyzed by the last-mentioned physiologist, who found it to consist almost entirely of albumen. (306.) The respiratory organs of the INSECT A, as well as their circulatory apparatus, are constructed upon peculiar principles, and are evidently in relation with the capability of flying, which distin- guishes these minute yet exquisitely constructed articulated animals. Any localized instruments for breathing, whether assuming the shape of branchiae or lungs, would materially have added to the weight of the body, and moreover have rendered necessary an elaborate ap- paratus of arteries and veins for conveying the blood to and fro for the purpose of purifying it by securing its exposure to the in- fluence of air. By the plan adopted, however, all these organs are dispensed with ; and the organs of respiration, so far from increasing the weight of the animal, actually diminish its specific gravity to the greatest possible extent. The blood, in fact, in insects is not brought to any given spot to be exposed to oxygen, but the air is conveyed through every part of the system by innumerable tubes provided for that purpose, and thus all the complicated parts usually required to form a vascular system are rendered unnecessary. These observations, however, only apply to the insect in its perfect * Physiologische Untersuchungen iiber den thierischen Haushalt der Insekten- 8vo. 1817. 1NSECTA. 265 state; for in tlie larva and pupa condition, where flight is not possible, various additional organs, frequently of considerable bulk, are provided, that we shall speak of in another place. If we examine the external skeleton of any large insect, a beetle for ex- ample, we shall find between the individual segments of the body minute apertures or pores (spiracles) through which the air is freely admitted ; these openings, ten in number, on each side of the body, are situated in the soft membrane interposed between the different rings, and not in the rings themselves, a provision for the purpose of allowing their orifices to be opened or closed at pleasure, instead of being rigid and motionless. The margin of the spiracle is frequently encompassed by thick horny lips, which may be approximated by muscles provided for the purpose, so that the opening can be shut at pleasure, in order to exclude any extraneous substances that might otherwise obtain admission : in many insects indeed, especially in beetles which crawl upon the dusty ground, an additional provision is necessary to prevent the entrance of foreign matter, and in such cases the spiracles are seen to be covered with a dense investment of minute and stiff hairs, so disposed as to form a sieve of exquisite fineness ; a beautiful contrivance, by which the air is filtered, as it were, before it is allowed to pass into the breathing-tubes, and thus freed from all prejudicial particles. From every spiracle is derived a set of extremely delicate tubes (trachea), that pass internally, and become divided and subdi- vided to an indefinite extent, penetrating to every part of the body, and ramifying through all the viscera, so that air is thus supplied to the entire system. Upon more minutely inspecting these air-tubes, they are found to assume various forms in different parts of the body, being sometimes simple tubes of exquisite deli- cacy ; in other cases they present a beaded or vesicular structure, and in many insects they are dilated at intervals into capacious cells or receptacles, wherein air is retained in great abundance. The figure in the following page (Jig. 118), taken from Strauss Durckheim's elaborate work upon the anatomy of the cockchafer, will illustrate this arrangement. The spiracles, situated at the points respectively marked by the letters a, c, e?, e,jf, g, h, i, open into two wide air- trunks, disposed longitudinally along the whole length of the body : from these, innumerable secondary branches are given off, many of them being seen to dilate into oval vesicles, from which smaller tracheae proceed ; while others, without any vesicular en- largement, plunge at once into different textures, and supply the 266 INSECTA. viscera and internal organs. The muscular system, the legs, the wings, the alimentary canal, and even the brain itself, are permeated in all directions by these air- conducting tubes, and thus the oxygen penetrates to every corner of the body. (307.) There is one Fig. lie. circumstance connected with the tracheae, which is specially deserving of admiration, whether we consider the obvious design of the contriv- ance, or the remarkable beauty of the struc- ture employed. It is evident that the sides of canals, so slender and delicate as the tracheae of insects, would in- evitably collapse and fall together, so as to obstruct the passage of the air they are destined to convey ; and the only plan which would seem cal- culated to obviate this would appear to be, to make their walls stiff and inflexible. In- flexibility and stiffness, however, would never do in this case, where the vessels in question have to be distributed in countless ramifica- tions through so many --:->, soft and distensible viscera ; and the problem, therefore, is, how to maintain them permanently open, in spite of external pres- sure, and still preserve the perfect pliancy and softness of their walls. The mode in which this is effected is as follows : Be- tween the two thin layers of which each air-vessel consists, an INSECTA. 267 elastic spiral thread is interposed {Jig. 119, a), so as to form by its revolutions a firm cylinder of sufficient strength to insure the calibre of the vessel from being diminished, but not at all inter- fering with its flexibility, or obstructing its movements ; and this fibre, delicate as it is, may be traced with the microscope, even through the utmost ramifications of the tracheae, a character whereby these tubes may be readily distinguished. (308.) We must now consider Fi 119> the mechanism by which air is per- petually drawn into the body of the insect, and again expelled. If the abdomen of a living insect be carefully watched, it will be found continually performing movements of expansion and contraction that succeed each other at regular in- tervals, varying in frequency, in different species, from twenty to fifty or sixty in a minute,* but occurring more rapidly when the insect is in a state of activity than when at rest. At each expansion of the abdomen, therefore, air is sucked in through all the spiracles, and rushes to every part of the o/^ body ; but, when the abdomen contracts, it is forcibly expelled through the same openings. Burmeister even supposes that the humming noises produced by many insects during their flight, must be referred to the vi- bration caused by the air streaming rapidly in and out of the spiracular orifices. Insects which live in water are obliged, at short intervals, to come to the surface to breathe, at which time they take in a sufficient quantity of air to last them during the period of their immersion ; but if the spiracles are closed by any acci- dent, or by the simple application of any greasy fluid to the ex- terior of their body, speedy death, produced by suffocation, is the inevitable result. (309.) A moment's reflection upon the facts above stated, con- cerning the respiration of insects, will suggest other interesting views connected with the physiology of these little creatures. It is evident, * Sorg, Disquisitio Phys. circa Resp. Insectorum et Verminum. 268 1NSECTA. in the first place, that their blood is all arterial ; they can have no occasion for veins, as they have no venous blood, the whole of the circulating fluid being continually oxygenized as its principles be- come deteriorated. The perfection of their muscular power, their great strength and indomitable activity, are likewise intimately related to the completeness of their respiration ; so that the vital energies of the muscular system are developed to the utmost, en- dowing them with that vigorous flight and strength of limb which we have already seen them to possess. It must likewise become apparent, that, as the blood is freely exposed to the influence of oxygen in every portion of the insect to which the air-tubes reach, one great necessity for the existence of a circulatory apparatus is entirely done away with, and as we have observed before, all those parts of the vascular system required in other animals for the pro- pulsion of the vitiated blood through pulmonary or branchial organs, are no longer requisite ; so that, by dispensing with the compli- cated structures usually provided for this purpose, the body is considerably lightened. The circulation of the nutritive fluids is in fact limited to their free diffusion amongst all the internal viscera, and is effected in the following manner: If we ex- amine the back of a silkworm, or of any transparent larva, a long pulsating tube is seen running beneath the skin of the back, from one end of the body to the other ; its contractions may readily be watched ; they are found to begin at the posterior extremity, and are gradually continued forwards, so that the vessel presents a continual undulatory movement, by which the fluid contained in its interior is pushed from the tail towards the head. This dorsal vessel, which may be so well observed in the thin-skinned larva, exists likewise in the perfect insect, although from the opacity of the integument, its movement is no longer apparent, except by the vivisection of the animal. (310.) This dorsal vessel, or heart as we shall call it for the sake of brevity, is organized in a very singular manner ; for, instead of being a closed viscus, it communicates most freely, through several wide lateral apertures, with the cavity of the abdomen, and from thence derives the blood with which it is filled. The dorsal vessel is widest in the abdominal region ; but is continued, nevertheless, through the thorax into the head, where it terminates as a simple or furcate tube, that is not closed, but open at the extremity. The structure of this remarkable heart has been fully investi- gated by Strauss Durckhcim,* and is extremely curious ; it con- * Op. cit INSECT A. 269 sists, in the cockchafer, of eight distinct compartments, sepa- rated from each other by as many valves formed by productions from the lining membrane, and so disposed that the blood passes freely from the hinder chambers into those which are placed more anteriorly, but is prevented from returning in the opposite di- rection. Each compartment of the dorsal vessel communicates by two wide slits, likewise guarded by valves, with the cavity of the belly, so that fluids derived from thence will readily pass into the different chambers, but cannot .again escape through the same channel. The arrangement of these valves will, however, be best understood by reference to the accompanying Fig. 120. figure (Jig. 120), representing a magnified view of the interior of a portion of the heart of the cockchafer, as depicted by the celebrated en- tomotomist above alluded to. The organ has been divided longitudinally, so that one half only is represented in the figure upon a very large scale. The compartments (a, a, a) are distinctly composed of circular muscular fibres ; the large valves (d, d) separate the individual chambers, allowing the blood to pass in one direction only, viz. towards the head ; while the openings (c), likewise closed by semilunar membranous valves, admit blood from the cavity of the abdomen, but effectually prevent its return. (311.) Let us now consider the movements of the circulating fluids produced by the contractions of this apparatus. The chyle or nutritive material extracted by the food, eludes, as we have already seen, by a species of percolation through the walls of the intestine, and escapes into the cavity of the abdomen, where it is mixed up with the mass of the blood, which is not contained in any system of vessels, but bathes the surfaces of the viscera immersed in it. When any compartment of the heart relaxes, the blood rushes into it from the abdomen, through the lateral valvular apertures ; and as it cannot re- turn through that opening on account of the valves (c) that guard the entrance, nor escape into the posterior divisions of the heart by reason of the valves (c?), the contraction of the dorsal vessel necessarily forces it on towards the head. When it arrives there, it of course issues from the perforated termination of the 270 INSECTA. heart, but does not appear to be received by any vessels, and therefore becomes again diffused through the body. The diffused character of the circulation met with in insects may easily be made a matter of observation in many of the transparent aquatic larvae that are readily to be met with. When any of the limbs of these larvae are examined under a powerful microscope, continual cur- rents of minute globules are everywhere distinguishable, moving slowly in little streams ; some passing in one direction, others in the opposite : but that these streams are not contained in vascular canals is quite obvious, from the continual changes which occur in the course of the globules ; their movements, indeed, rather re- semble those of the sap in chara, and other transparent vegetables, in which the circulation of that fluid is visible under a microscope. The organs appropriated to furnish the different secretions met with in the economy of insects, are modified in their structure to correspond with the character of the circulation, and are invariably simple tubes or vesicles of various forms immersed in the fluids of the body, from which they separate their peculiar products. The poisonous saliva of bugs, and the innoxious salivary fluid of other insects ; the bile and auxiliary secretions subservient to digestion ; the venom which arms the sting of the wasp, and the silky en- velope of the caterpillar, are all derived from the same source, and in some mysterious manner elaborated from the blood by variously formed vessels : but of this we have already given many examples, and others will present themselves in the following pages. (312.) In the nervous system of the INSECTA, we have many interesting illustrations of that gradual concentration of the parts composing it, and consequently of increased proportionate deve- lopement of the nervous centres, corresponding with the more active movements and higher faculties by which the class before us is so remarkably distinguished from those forms of articulated animals that we have hitherto had an opportunity of examining. The su- pra-O2Sophageal ganglion, or brain, assumes a preponderance of size in relation to more perfect organs of sense, and to instincts of more exalted character ; the chain of ganglia placed along the floor of the abdomen, is composed of a few large masses of sufficient power to animate the strong and energetic muscles of the limbs ; and, moreover, anatomists have detected the existence of an addi- tional nervous apparatus, apparently representing the sympathetic system of vertebrate animals, which is distributed to the viscera ap- propriated to digestion : each of these divisions will therefore re- quire a separate notice. INSECTA. 271 The brain, or encephalic ganglion (Jig. 121, 1), is a nervous mass of considerable size placed above the gullet ; it consists es- Fig. 121. sentially of two ganglia united into one mass, and from it all the nerves appropriated to the special instruments of the senses are de- rived, so that it may naturally be regarded as the chief seat of sens- ation and intelligence. The nerves originating from this common sensorium are seen upon an enlarged scale in Jig. 122 : they are the optic (Jig- 122, a), supplying the eyes, and the antennal (Jig. 122; e), which run to the special instruments of touch, or antenna',, 272 INSECTA. organs of a very singular character that we shall examine more mi- nutely hereafter. Two other cords of variable length {Jig- 122, g, g) are given off from the inferior aspect of the brain, and serve to connect it with the anterior ganglion of the ventral chain (j%. 122, A), to which some writers have thought proper to give the name of cerebellum, though upon what grounds it is difficult to conjecture ; the mass last mentioned gives off various nerves to supply the parts connected with the mandibles, maxilla, and other organs of the mouth. The rest of the ventral chain of ganglia forms a continuous series (fig. 121, 2, 3, 4, 5, 6, 7, 8) of nervous centres arranged in pairs, and united to each other by double cords of communication, but they vary much in number and relative magnitude in different families. Those situated in the thorax are usually of the greatest proportion- ate size, inasmuch as they furnish the nerves that supply the mus- cles of the wings and legs ; the succeeding ganglia give branches to the abdominal segments ; and the last, which is commonly of consi- derable bulk, supplies the sexual organs and the extremity of the colon. (313.) It is the general opinion of modern physiologists that the intimate composition of the nervous apparatus described above is by no means so simple as it appears to ordinary observation ; and, since the experiments of Sir Charles Bell and Majendie demonstrated the existence of distinct columns or tracts in the spinal axis of ver- tebrate animals, various anatomists have endeavoured to show that corresponding parts may be pointed out in the ventral chain of ar- ticulated animals. There can, indeed, be no doubt that this por- tion of the nervous system of an insect corresponds in every parti- cular with the medulla spinalis ; and if, in the one case, the nerves which preside over the general muscular movements arise from a different column to that whence the nerves that correspond with the periphery of the body originate, while those which regulate the motions of respiration emanate from a distinct tract, we might rea- sonably suppose a similar arrangement to exist in the structure of the nervous system we are now examining. It has, in fact, been well ascertained that the nerves given off to the muscular system of the Homogangliata are not derived from the ganglionic masses them- selves, but from the cords which connect them together, while the nerves distributed to the integument and external parts of the body communicate immediately with the ganglia. These different modes of origin give presumptive evidence that at least two distinct tracts INSECTA. 273 exist in the central axis of insects ; but, from the extreme minute- ness of the different parts, it is not easy satisfactorily to demonstrate them separately. In the larger ARTICULATA, however, as for example in the CRUSTACEANS, two distinct columns of nervous matter are readily detected: it will, therefore, be more convenient to defer the investigation of this interesting subject until we have an opportunity of describing these parts upon an enlarged scale ; enough has been said at present to enable the reader to compare the nervous axis of an insect with that of a lobster, and draw correct conclusions from the comparison. (314.) The last division of the nervous apparatus, which we have already mentioned as being the representative of the sympathetic system, consists of two portions ; one corresponding, in distribution at least, with the nervus vagus of VERTEBRATA, while the other represents, apparently, the sympathetic ganglia. The nervus vagus , as we shall call it, and which has been named by Swammerdam* and Cuvier the recurrent nerve, arises (Jig* 122, i, b) by two roots from the opposite extremities of pi ff , 122. the brain close to the origins of the antennal nerves. The nervous cords thus derived soon unite to form a minute central ganglion (Jig- 122, i), from which proceeds a single nerve (fig. 122, /, *), that runs with the gullet (/) be- neath the brain, and spreads in deli- cate ramifications upon the oesopha- gus as far as the muscular stomach (Jig. 121, 9, 9), or to the gizzard, when that organ exists. (315.) The Sympathetic system, properly so called, consists of four small ganglia (Jig. 121, c, c, /, /), the two anterior of which communicate with the brain, and with each other by means of connecting filaments. These ganglia are closely applied to the commencement of the resophagus, and supply it with minute nerves. (316.) Various are the conjectures entertained by different au- thors concerning the senses possessed by the members of the insect world, and^ the organs subservient thereunto. The possession of certain sources of perception has been alternately granted and denied, * Biblia Naturaj, INSECTA. the nature of their sensations has been a fruitful subject of inquiry, and some physiologists have even gone so far as to deny the corre- spondence of the impressions derived by insects through the medium of their senses with those which we ourselves receive. It would lead us far out of our course did we even advert to the multiplicity of opinions and conjectures promulgated from various sources rela- tive to these inquiries, and, perhaps, with little addition to our real knowledge. It is true that we cannot deny the possibility of the existence of other modes of sensation than those familiar to us ; but it is likewise evident that, as we can never have the most remote conceptions concerning their nature, speculations respecting them are calculated to lead to very unsatisfactory conclusions. We must from necessity take our own senses as the standard of comparison, limiting our inquiries to examine how far insects possess means of intercourse with the external world similar to those which we enjoy, and, when we find certain faculties to exist, to investigate the struc- ture of the organs by which they are exercised. (317.) The sense of touch is indubitably bestowed upon all in- sects ; and, to judge from the perfection of the edifices which they build, and the precision of their usual operations, this must be ex- tremely delicate. It is sufficient, however, to look at the external construction of the skeletons of ARTICULATA, to perceive that the hard and insensible integument spread over the entire surface of their bodies is but little calculated to receive tactile impressions. The antennae, or feelers as they are popularly called, have been very generally regarded as being peculiarly instruments of touch ; and whoever watches the proceedings of an insect in which these appendages are largely developed, will, we apprehend, easily con- vince himself that they are employed to investigate surrounding objects by contact, Strauss Durckheim regards the feet as being specially appropriated to the sense of feeling, but this opinion seems quite inadmissible. Burmeister places the exercise of touch exclusively in the palpi attached to the maxillae and labium, and observes that in the larger insects, such as the predatory beetles, the grasshoppers, humble-bees, and many others, the apex of the palpus is dilated into a white transparent and distended bladder, which, after the death of the insect, dries up, and is no longer visible. This bladder he looks upon as the true seat of the sense in question, and remarks that the main nerve of the maxillae and of the tongue spreads to it, and distributes itself upon its superior surface in minute ramifications. INSECTA. 275 (318.) Whether taste exists in insects as a distinct sense may admit of dispute ; the tongue, already described, seems but little adapted to appreciate savours, and, seeing this, it is obvious that all opinions assigning the function of tasting to other parts are purely conjectural. (319.) Many insects are certainly capable of perceiving odours ; of this we have continual proof in the flesh-fly and other species, that are evidently guided to their food, or select the position in which to deposit their eggs, by smell ; but where the olfactory apparatus is lodged is still a matter of doubt. The antenna and the palpi have each had the power of smelling assigned to them, but without much plausibility. The respiratory stigmata have been pointed out as performing the office of examining the air admitted for the purpose of breathing ; yet other authors, with equal probability, look upon the ultimate ramifications of the trachese as forming one extensive nose. The interior of the mouth has been indicated by Treviranus ; * while Kirby and Spence find in the Necrophori, and other insects remarkable for acuteness of smell, an organ in close connection with the mouth, to which they attribute the perception of odoriferous particles : this is a cavity situated in the upper lip, containing a pair of circular pulpy cushions covered by a membrane transversely striated or gathered into delicate folds. (320.) We are scarcely better informed concerning the organs of hearing, but that insects are capable of perceiving sounds is proved by the fact of many tribes being capable of producing audible noises by which they communicate. There seems, indeed, to be little doubt that the auditory apparatus is in some way or other con- nected with the antennae. Some have supposed that these slender and jointed organs, supplied, as they are, with large nerves, are themselves capable of appreciating sonorous vibrations. Bur- meister j- thinks that, as in crabs and lobsters, it is at the base of the antenna that the ear is situated, and observes that if we examine the insertion of these appendages we shall detect there a soft articulating membrane which lies exposed, and is rendered tense by the movements of the antenna, this he looks upon as representing the drum of the ear, and conceives that it is so placed as to receive impressions of sound, increased by the vibratory movements communicated to the antennas by the sonorous undula- tions of the atmosphere. * Vermischte Schriften, vol. ii. t Op. cit. p. 296. T 2 276 INSECTA. In some moths, Treviranus* has discovered structures which would seem to be indubitably real auditory organs. He found in front of the base of each antenna a thin membranous drum, behind which, large nerves, derived from those supplied to the antennse, spread themselves out ; but this apparatus has not been detected in other insects. (321.) The eyes of insects are of two kinds, simple and com- pound ; the former being insulated visual specks, while the latter consist of agglomerations of numerous distinct eyes, united so as to form most elaborate and complex instruments of sight. Some insects, as the Dictyotoptera and Thysanoura, only possess simple eyes ; others, as for example the Coleoptera, have only compound eyes ; but in general both kinds exist together. In the Sir ex gigas (Jig. 128), for instance, besides the large hemispherical organs of sight, situated at the sides of the head, three simple spots are seen upon the vertex, which are likewise appropriated to vision. The structure of the eyes has been most minutely investigated by several distinguished entomotomists, and the labours of Marcel de Serres,j- Joh. M tiller, J Strauss Durckheim, and Duges,|| have done much to dispel the mistaken notions entertained by preceding anatomists. The simple eyes consist of a minute, smooth, convex, transpa- rent cornea, in close contact with which is a small globular lens ; behind this lens is placed the representative of the vitreous humour, upon which a nervous filament spreads out, so as to form a retina : the whole is enclosed in a layer of brown, red, or black pigment, which, bending round the anterior surface of the eye, forms a dis- tinct-coloured iris and pupillary aperture. Such an arrangement evidently resembles what is met with in higher animals, and is remarkable for its simplicity ; but it is far otherwise with the com- pound eyes of insects, for these are constructed upon principles so elaborate and complex, that we feel little surprise at the amaze- ment expressed by early writers who examined them, although their ideas concerning their real structure came far short of the truth. * G. R. Treviranus, Annalen der Wetterau. Qesel. f. d. Ges. Naturk. vol. i. 1809. t Mem. sur les Yeux composes, et les Yeux lisses des Insectes. Montpel. 8vo. 1813. t Zur Vergleichenden Physiologie des Gesichtssinnes, 8vo. 1826. Annales des Sciences Nat. torn, xviii. || Ibid. torn. xx. INSECTA. 277 A The compound eyes of insects are two in number, situated on the lateral aspects of the head, the form of each being more or less hemispherical. When examined with a microscope, their surface is seen to be divided into a multitude of hexagonal facets, between which, minute hairs are generally conspicuous. The number of facets or cornese, for such in fact they are, varies in different genera : thus, in the ant (Formica) there are 50 ; in the common house-fly (Musca domestica), 4000 ; in some dragon-flies (Libel- Ma), upwards of 12,000. In butterflies (Papilio) 17,355 have been counted, and some Coleoptera (Mordella) possess the as- tonishing number of 25,088 distinct cornese. But in order to appreciate the wonderful organization of these remarkable organs of sight, it is necessary to examine their internal structure : every cornea is then found to belong to a distinct eye, provided with a perfect nervous apparatus, and exhibiting its pe- culiar lens, iris, and pupil ; thus being completely entitled to be considered a distinct instrument of vision. By attentively examining & 123 the annexed figure, repre- senting a section of the eye of the cockchafer (Melolon- tha), as displayed by Strauss Durckheim, the whole struc- ture of the organ will be readily understood. The optic nerve (Jig. 123, a), derived immediately from the supra-cesophageal mass of nervous matter, swells soon after . its origin into a rounded ganglion, nearly half as large as the brain it- self. From the periphery of the ganglion so formed arise a considerable number of secondary nerves (6), which are very short, and soon come in contact with a layer of pigment (d) ; that in the cockchafer is of a brilliant red colour, and is placed concentrically with the convex outer surface of the eye. Behind this membrane, called by Strauss the common choroid, the second- ary optic nerves (b) unite to form a membranous expansion of nervous matter (c) which may be denominated the general retina. 278 INSECTA. From the nervous expansion so formed arise the proper optic nerves (e), appropriated to the individual eyes or ocelli, as we shall term them. These nervous filaments are as numerous as the facets of the cornea, and traverse the common choroid to radiate to- wards the individual eyes whereunto they are respectively destined, and the structure of which we must now proceed to examine. In fig. 123, B, a portion of the circumference of the compound eye is represented upon a very large scale, in order to show the construction of the hexagonal ocelli that enter into its composi- tion. Each cornea (i) is a double convex lens, adapted by its shape to bring to a focus the rays passing through it. Behind every lens so constituted is placed an hexaedral transparent prism (A), which from its office may be compared to the vitreous hu- mour of the human eye ; and it is upon the posterior extremity of these prisms that the proper optic nerves (jig- 1 23, A, e) spread themselves out, so as to form so many distinct retinae. When we reflect upon the extreme minuteness of the parts above alluded to, we may well expect slight discrepancies to occur between the ac- counts given of them by different anatomists. Strauss Durckheim represents every optic nerve as terminating in a minute pyriform bulb (fig. 123, B,/), and points out a dark layer of pigment (g), which forms a choroid tunic proper to each ocellus ; while, accord- ing to Muller and Duges, the vitreous humours (h) are conical, and terminate posteriorly in a sharp point, upon which the terminal expansion of the optic nerve spreads out without any pyriform enlargement : they likewise deny the existence of the proper choroid (g) in the situation indicated by Strauss, but find a black pigment situated immediately behind the cornea, that at first sight would appear to be continuous over the whole surface of the eye. Even Cuvier seems at one time to have adopted this opinion ; Muller, however, found that, upon carefully removing the internal structures of the organ, leaving the pigment untouched, the dark varnish in question, although very thick at the lines of union of the different facets, where it is continuous with a choroid that separates the individual ocelli, yet towards the centre of each facet it becomes exceedingly thin, and at the very centre is quite want- ing, so that a minute perforation or pupil is thus left, through which the rays of light enter. The existence of the secondary optic nerves (b) and common retina (c) is likewise disputed by Muller and Duges, who consider the proper optic nerves to arise immediately from the surface of the brain. INSECTA. 279 With regard to the wonderfully complex structure of these organs, Strauss Durckheim suggests, that, the eyes of insects being fixed, nature has made up for their want of mobility by their number, and by turning them in all directions ; so that it might be said that these little animals have a distinct eye for every object. But here we are naturally tempted to inquire, whether insects see at the same time distinctly with every one of these eyes, or if they distinguish with one eye only. Upon this point Strauss Durck- heim observes, that, if they saw clearly with all, the great number of images would necessarily produce confusion, and would prevent creatures so organized from paying special attention to any deter- minate point. It is probable, therefore, that one ocellus only is at any given time placed in circumstances precisely adapted to the complete examination of an object, the animal seeing things imper- fectly with the rest, in the same manner as we see objects situated nearer to us or further off than that upon which we fix our atten- tion ; so that, according to this supposition, insects would see very distinctly with one eye only, exactly as we see confusedly an ex- tensive landscape, although we only distinguish a small part of it. (323.) In all insects the sexes are quite distinct, and the genera- tive apparatus, both of the male and female, consists of various se- creting organs with their excretory ducts : in the male, such glands furnish the impregnating secretions ; and, in the female, give origin to the ova, and pro- vide the covering wherein the eggs are enveloped. e (324.) Commenc- ing with a descrip- tion of the male or- gans, we find in the cockchafer various parts represented in the accompanying figure, taken from the admirable work of Strauss already so often quoted. The testicles of Melolon- tha (fig. 124, a, a) are six in number on each side of the Fig. 124. 280 INSECTA. body ; but, in the engraving, those of one side only are delineated. Every testis consists of a vesicular organ, hollow internally, which, being immersed in the juices of the insect, separates therefrom the seminal fluid. Six ducts (5, b, b) may be called Vasa deferentia, and convey the spermatic liquor into a common canal (c, c), of considerable length and much convoluted. Although slender at its commencement, this tube ultimately expands into a wider portion (d), wherein, no doubt, the semen accumulates, and which has been called by authors the vesica seminalis. The canal (d) terminates by joining the corresponding duct from the opposite side (d?) to form a common tube (g), but just at the point of junction they are joined by two long auxiliary vessels (y, f) that have been named sperm-vessels, gluten-vessels, and gum-vessels, by different authors, but which appear to be appropriated to the production of some fluid, perhaps analogous to the prostatic fluid of mammalia, whereby the bulk of the seminal liquor is increased in order to facilitate its expulsion. Each of these auxiliary vessels consists of two parts, a long and much con- voluted portion (e, e, e), forming the secreting organ ; and a dila- tation (f), that must be looked upon as a reservoir for the fluid elaborated. The common canal (g) receives all these secretions ; it is at first enclosed in a kind of sheath (A), but, soon becom- ing muscular, it dilates into a strong contractile canal (g, z), called the ductus ejaculatorius, which is continued to the extremity of the penis. The intromittent organ itself is composed of two parts ; a pro- trusible corneous tube (/, /), and an external horny sheath (n, n), in which the former is usually concealed and protected. (325.) Great variety, of course, exists in the number, form, and general arrangement of all the parts alluded to in the above descrip- tion, when examined in different insects.* In the hive-bee, for example, the testes (Jig. 125, a) are only two in number, and are simple oval vesicles ; the vasa defer entia (b, b) are short ; and the seminal receptacles (c) form membranous sacculi. The aux- iliary secreting organs (d), although placed in the same position as in Melolontha, are represented by capacious cseca ; while the common excretory duct (e) swells into a strong and muscular bag * For more ample details relative to the various forms of the testis in insects, the reader is referred to the Cyclop, of Anat. and Phys. ; art. GENERATION, ORGANS INSECTA. 281 ( f ), which constitutes the ejaculatory F *8 apparatus. Still, however, it is easy to see that, although diversified in appearance, the parts here found are essentially similar to those met with in the cockchafer, and represent respectively the same organs. (326.) The female apparatus of reproduc- tion presents a general correspondence, both in form and arrangement, with the sexual parts of the male insect. The ovaria are simple secreting sacculi, or elongated tubes, in which the germs or ova are produced, instead of the seminal liquor ; and the excretory canals, or egg-passages, with the organs appended to them, although appro- priated to different functions, strikingly re- semble the organs met with in the other sex. In the female of Melolontha the ovaria are long tubes, form- ing two distinct fasciculi, symmetrically situated on the two sides of the body. At their commencement (Jig> 126, w, u) the ovi- gerous tubes are slender, and the ova which they contain at this point are in a very rudimentary state of developement ; they ge- nerally dilate, however (, , t, i), and, as they expand, the ova are seen to attain larger dimensions. Near its termination each ova- rian tube assumes a granulated texture (s 9 *), and they all ulti- mately open into the corresponding excretory canal (r, r). All the ovarian tubes of one side are united into a bundle, by a ligament (v 9 #), which Joh. Miiller* traced to the dorsal vessel, and believed to be a vascular canal adapted to bring blood imme- diately into the tubes wherein the ova are formed ; but no satis- factory evidence has been adduced in proof of the existence of such an extraordinary communication, and the thread in question is most probably a mere ligamentous connection. (327.) Taking the higher animals as a standard of comparison, we may suppose the formation of the eggs in these tubes to be accom- plished in the following manner : In the upper part of the tube (u) is formed the yolk, enclosed in its peculiar membrane, and provided with that wonderful germ from which after impregnation the future being is to be developed ; as the yolk slowly descends to the more * Nova Acta Phys. Med, n. c. vol. xii. part ii. INSECTA. dilated parts of the canal *"* 126 - (, ), it becomes clothed with the albumen which constitutes the white of the egg ; and ultimately, before quitting the nidus of its formation, receives from the granular termi- nation of the ovary its J last integument or shell. Thus completed, it passes into the excretory canal (r, r) ; and this, meeting the corresponding tube derived from the ovaries of the opposite side, joins it to form the common oviduct {I) through which the egg is conducted out of the body. (328.) But we must now advert to certain ap- pendages connected with the common oviduct. These are of two kinds ; the gluten-secretors and the spermalheca. The gluten-secretors (Jig- 126, p, p ) are glandular caeca opening into the common egg-canal, and are apparently destined to furnish a glutinous fluid with which the eggs become invested before they are expelled from the body and thus they are fre- quently united into long chains and variously shaped masses ; or else the adhesive varnish thus secreted serves to glue the ova in situations favourable to the developement of the embryo. The other organ, or spermatheca (Jig. 126, w, o), has a widely different office, being a receptacle provided to receive the seminal secretion of the male during copulation : it is always situated upon the upper aspect of the oviduct, into which it opens by a small ori- fice surrounded by a thickened margin or sphincter, embracing the neck of the bag, and so disposed as either to retain the enclosed fluid,, or to allow it to escape into the oviduct. That this organ really does receive and retain the seminal liquor is proved by the presence of seminal animalcules in its contents ; but the matter has been placed INSECTA, 283 beyond a doubt by the experiment of John Hunter,* who actually succeeded in fecundating the eggs of an unimpregnated female, by applying to them a little of the fluid contained in its cavity : but that the reader may comprehend fully the reason of such an arrange- ment, it is necessary to consider the circumstances under which insects propagate. In most animals, sexual union may be repeated several times during the life of individuals, but, in insects, intercourse between the sexes is permitted to take place but once ; and this solitary congress must suffice for the impregnation of all the ova, however numerous, and however imperfect may be the developement of some of them at the time when the embrace occurs. Let us take the hive-bee as an example ; in the females of this insect the ovigerous tubes (fig. 127, a, a) are excessively numerous, and the eggs produced in them may amount to between 20,000 and 30,000 : these eggs, of course, arrive at maturity in succession, and not all at once ; so that at the moment when the queen-bee meets her selected mate, perhaps the majority of the ova are not in a sufficiently mature condition to be rendered fertile. Never- theless, the meeting of the sexes cannot be repeated ; for no sooner has copulation taken place than the favoured male dies, and by a simultaneous butchery all the other males, or drones as they are commonly designated, are F,v. 127. destroyed by the working inhabit- ants of the hive. The quantity of the fecundating liquor, there- fore, supplied by one connection, must serve to fertilize all the eggs produced during the lifetime of the queen-bee ; and for this purpose it is stored up in the spermatheca (Jig. 127, c), so that, how numer- ous soever may be the eggs formed, they are all vivified as they pass out through the oviducts (5, e), and thus come in contact with the orifice of the reservoir of semen. In Meloe variegatus (Jig. 121) the ovaria (d) consist of wide and capacious sacs, covered externally with innumerable glandiform vesicles, opening into the cavity of the ovary (e). The gluten-secretor (h) and the spermatheca (g) are seen as in Melolontha, appended to the common oviduct (f) ; but the sperma- * Home's Lectures on Comp. Anat. vol. iii. p. 370. 284 1NSECTA. theca has a small accessory vesicle (i) connected with it, not found in the former examples. (329.) In many insects, especially of the Hymenopterous order, the generative apparatus is terminated externally by peculiar instru- ments provided for the purpose of introducing the eggs into a proper situation. This is particularly remarkable in the Ichneu- mons, which deposit their ova in living caterpillars ; and in the saw-flies (Tenthredo), whose eggs are insinuated into the sub- stance of the leaves, or even of the branches of trees. To describe all the contrivances employed for this purpose would lead us far beyond our prescribed limits : one example of an organ of this description must suffice. In the Sir ex gigas (Jig. 128) the ovipositor consists appa- Fig. 128. rently of three pieces of considerable length, seen in the figure to project from the inferior margin of the abdomen. Of these pieces, two form a sheath enclosing a third, called the terebra, or borer, which in the Tenthredo contains two saws of extremely beautiful construc- tion, as we learn from an account of them given by Professor Peck, and quoted by Kirby and Spence :* the original description, which it would be unpardonable to abbreviate, is as follows : " This in- strument," says Professor Peck, " is a very curious object ; and, in order to describe it, it will be proper to compare it with the tenon- saw used by cabinet-makers, which, being made of a very thin plate of steel, is fitted with a back to prevent its bending. The back is a piece * Introd. to Entom. vol. iv. p. 161 . 1NSECTA. 285 of iron, in which a narrow and deep groove is cut to receive the plate, which is fixed : the saw of the Tenthredo is also furnished with a back, but the groove is in the plate, and receives a prominent ridge of the back, which is not fixed (to the saw), but permits the saw to slide forward and backward as it is thrown out and retracted. The saw of artificers is single, but that of the Tenthredo is double, and consists of two distinct saws with their backs : the insect, in using them, first throws out one, and while it is returning pushes forward the other ; this alternate motion is continued till the inci- sion is effected, when the two saws, receding from each other, con- duct the egg between them into its place." (830.) With respect to the number of eggs laid by insects it varies in different species ; the flea, for example, lays about twelve, and many Diptera and Coleoptera average perhaps fifty : but others are far more prolific ; among moths, for example, the silkworm pro- duces 500, and some from 1000 to 000 : the wasp ( Vespa vulgaris) deposits 3000 ; the ant (Formica), from 4000 to 5000. The queen-bee is said by Burmeister to lay from 5000 to 6000 ; but Kirby and Spence consider that in one season the number may amount to 40,000 or 50,000, or more. Yet, surprising as this latter statement may appear, the fecundity of the queen-bee is far inferior to that of the white-ant (Termes fatalis) ; for the female of this insect extrudes from her enormous matrix innumerable eggs at the rate of sixty in a minute, which gives 3600 in an hour, 86,400 in a day, and 2,419,200 in a lunar month : how long the process of oviposition continues in the termite is unknown ; but, if it were prolonged through the entire year, the amazing number of 211 ,449,600 eggs would proceed from one individual ; setting, how- ever, the number as low as possible, it will exceed that produced by any known animal in the creation. (331.) The Aphides, or plant- lice, furnish a remarkable instance of fecundity. In these insects it has been satisfactorily ascertained by Bonnet, Lyonnet, and Reaumur, that a single sexual intercourse is sufficient to impregnate not only the female parent, but all her progeny down to the ninth generation ! The original insect still continues to lay when the ninth family of her descendants is capa- ble of reproduction ; and Reaumur estimated that even at the fifth generation, a single Aphis might be the great-great-grandmother of 5,904,000,000 young ones. (332.) Innumerable are the means employed by nature to keep the balance between the increase and destruction of the insect tribes, INSECTA. and countless enemies are provided for the purpose of checking their inordinate accumulation. Fig. 129. A (333.) Among the most remarkable provisions for preventing su- perabundant fertility, is that law which compels the most prolific insects to live in large societies, and permits but one female out of a multitude to lay eggs. As an example of this, we may take the hive-bees,* so remarkable for their elevated instincts and industri- ous habits. A swarm of bees consists, first of females, whose sex- ual organs remain permanently in an undeveloped condition, usu- ally called the Workers (Jig- 129, A) ; secondly, of perfect males or drones (c) ; and thirdly, of a solitary fertile female, called the Queen (B), which gives birth to all the progeny of the hive ; and thus, instead of 20,000 or 30,000 eggs being furnished by every one of as many females, one female only is permitted to be instrumental in perpetuating the species. (334.) The termite ants likewise, were it not for a similar restric- tion, would soon, by their overwhelming increase, depopulate whole regions of the earth, and render the countries in which they are met with absolutely uninhabitable by their extreme voracity. A community of termites is said to consist of five different members, namely, winged males and females (Jig. 130, A) ; apterous neu- ters, or soldiers, which have large heads furnished with strong pro- jecting mandibles (B) ; uriwinged pupse, having a smaller head, and the rudiments of wings only (c) ; and, lastly, of similarly formed larvte, or workers (D), differing from the latter only in wanting the rudiments of wings. The following is a brief history of the establishment and growth of a colony of these insects, as narrated by Burin eister.f At the termination of the hot season, the young * For ample details concerning the habits of these interesting creatures, the reader is referred to Dr. Bevan's work on the Honey -Bee, its Natural History, Physiology, and Management, vol. I, 12mo. Lond. t Op. cit. p. 535. INSECTA. 287 males and females disclosed in a nest quit it, and appear upon the surface of the earth, where they swarm in innumerable hosts and pair. The busied workers then convey a chosen male and a female back into the dwelling, and imprison them in the central royal cell, the entrances to which they decrease and guard ; through these apertures the imprisoned pair then receive the nutriment they require. The male now, as amongst all other insects, speedily dies after the im- pregnation of the female has been effected ; but the female from this period begins to swell enormously from the developement of her countless eggs, and, by the time she is ready to commence laying, her abdomen is about 1 500 or 000 times larger than all the rest of her body (fig. 130, E). During the period of this swelling the Fig. 130. - workers remove the walls of the royal apartment, uniting the nearest cells to it, so that, in proportion to the increase of the body of the queen, the size of the abode she inhabits is also increased. She 288 INSECTA. now commences laying eggs, and, during the process, the abdomen exhibits a continual undulatory motion, produced by the peristaltic movement of the egg-ducts ; while the workers convey away the eggs as they are laid, and deposit them in the distant rearing-cells of their wonderful habitation. The reader will be able to form some idea of the relative proportions and outward appearance of the edifices erected by these comparatively minute beings by the group of their citadels represented in the back-ground of the figure ; but to describe them more minutely would lead us into details unconnected with our subject.* (335.) The eggs of these little animals vary much in shape and external configuration ; so that, from the beauty of their forms and exquisite sculpture, some of them are interesting objects for the microscope. (336.) We have already spoken concerning the metamorphosis which insects undergo during the progress of their developement from the form under which they first leave the egg to their mature con- dition, when they become fertile, and, in most instances, acquire those instruments of flight so generally characteristic of their perfect state. Before entering upon a more minute inquiry concerning the physiological principles upon which the important changes in question depend, and the phenomena attending the process, it will be advisable to cite a few more examples illustrative of the most interesting varieties of metamorphosis signalized by authors. Fa- bricius distinguishes five different kinds of metamorphosis, and has applied a different name to each. The first class comprises all insects of which the larva is a mag- got entirely deprived of legs, that after having changed its skin, or moulted, a certain number of times, becomes, previous to its last change, incased in an oval horny sheath, or pupa-case, whereon not the least trace of the limbs of the mature insect is to be detected ; such pupse are absolutely without the power of motion, and are distinguished by the name of coarctate : examples of this sort of metamorphosis are met with in the common house-flies (Muscida), and the forms of their larvae and pupse are familiar to every one. Of the second kind, technically named obtected, the Lepido- ptera furnish well-known instances. The changes which occur in the developement of the silkworm, represented in the annexed figure (Jig- 131), may readily be witnessed. In such insects the full-grown caterpillar, having enclosed itself in a silken ball, throws * Vide Smeathman, Phil. Trans, vol.lxxi. 1781. INSECTA. 289 off its last skin, and becomes a quiescent pupa ; but while in tins state the position of the rudiments of the wings and other appen- Fi,J3l. dages of the perfect insect is strongly indicated upon the exterior of the chrysalis (A), though these parts are still closely wrapped up in the external covering. (337.) The third form of metamorphosis, called incomplete, is seen in the Hymenoptera, and in many Coleopterous insects. The maggot, in such tribes as exhibit this kind of change, is sometimes a simple worm deprived of feet or other external organs, or in other species these parts exist in a very imperfect condition ; in the pupa, however, the form of the legs and antennae is perfectly distinct, and even the wings may be seen as rudiments projecting from the thorax. This kind of chrysalis we have seen in the cockchafer (fig. 106, B), in which the grub (c) possessed feebly developed legs ; and in the hive-bee, although the larva (Jig. 132, a, c, d, e,f) has no legs or exterior appendages, in the pupa (b) all the limbs of the perfect bee are recognised with the utmost facility. Yet all these organs are still enclosed in distinct cases (thecte), to each of which names have been applied by entomological writers ; and it is only on throwing off the integument which thus imprisons the mature in- sect, that the bee makes its appearance in a capacity to begin its active and industrious existence in the winged state. u 290 INSECTA. Those insects whose larva only differs from the imago in not being possessed of wings (Jig. 102), Fabricius regarded as under- going a semi-complete metamorpho- pig. 132. sis ; and when the perfect insect did not acquire wings at all, but pre- cisely resembled the pupa, he called the latter complete. (338.) But there are innumer- able examples of metamorphosis which will not conform to any of the above definitions, and in some of them the phenomena exhibited are not a little remarkable. We have already mentioned the changes which the dragon-fly undergoes (figs. 103, 104), and have seen that in this case there is no very striking resemblance between the pupa and the adult creature, but, on the contrary, that very won- derful changes occur during the last stage of the metamorphosis. The pupa lives in water ; and, besides six jointed legs adapted to climb the stems of subaquatic plants in search of prey, is pos- sessed of a very peculiar locomotive apparatus, whereby it can propel itself through the element which it inhabits. Appended to the posterior extremity of the abdomen we find three or five leaf- like appendages, which the creature continually opens and closes, and at the same time takes in a quantity of water, sufficient to fill the muscular termination of the rectum, which is expanded for the purpose ; this water is, at intervals, forcibly expelled, mingled with bubbles of air, and thus effects the propulsion of the animal by a mechanism which human ingenuity has imperfectly attempted to imitate, But the contrivance above mentioned is also made subservient to respiration ; for, from the observations of Cuvier,* it appears that the interior of the rectum exhibits to the naked eye twelve longi- tudinal lines of black spots arranged in pairs ; and these, when ex- amined under the microscope, are found to be composed of little conical tubes, from which branches go off to join the principal longitudinal tracheae that distribute air through the body. Another remarkable peculiarity is met with in the structure of * M6m. de la SociSte d'Histoire Nat. p. 48. INSECTA. 291 the mouth of these aquatic larvae, for the oral apparatus here forms an instrument of prehension adapted to seize prey at a distance, and constitutes, in fact, a kind of projectile forceps of a very curious construction. Let the reader contrast the following de- scription with that already given of the oral organs of the dragon- fly ( 295), and observe the remarkable difference : a Conceive," say Kirby and Spence,* " your under lip to be horny instead of fleshy, and to be elongated perpendicularly downwards, so as to wrap over your chin and extend to its bottom ; that this elonga- tion is then expanded into a triangular convex plate attached to it by a joint, so as to bend upwards again, and fold over the face as high as the nose, concealing not only the chin and the first-men- tioned elongation, but the mouth and part of the cheeks : conceive, moreover, that to the end of this last-mentioned plate are fixed two other convex ones, so broad as to cover the whole nose and temples ; that these can open at pleasure, transversely, like a pair of jaws, so as to expose the nose and mouth, and that their inner edges, where they meet, are cut into numerous sharp teeth or spines, or armed with one or more long and sharp claws : you will then have as accurate an idea as my powers of description can give of the strange conformation of the lip in the larvae in question, which conceals the mouth and face precisely as I have supposed a similar construction of your lip would do yours. You will probably admit that your own visage would present an appearance not very engaging while concealed by such a mask : but it would strike still more awe into the spectators were they to see you first open the two upper jaw-like plates, which would project from each temple like the blinders of a horse ; and next, having by means of the joint at your chin let down the whole apparatus, and un- covered your face, employ them in seizing any food that presented itself, and conveying it to your mouth. Yet this procedure is that adopted by the larvae provided with this strange organ. While it is at rest, it applies close to and covers the face. When the insects would make use of it, they unfold it like an arm, catch the prey at which they aim by means of the mandibu- liform plates (Jig. 101), and then partly refold it so as to hold the prey to the mouth in a convenient position for the operation of the two pairs of jaws with which they are provided." (339.) The metamorphoses of the gnat (Culex) are not less interesting. The female deposits her eggs upon the surface of the * Introd. to Entom. vol. iii. p. 126. INSECTA. water, in which her offspring are destined to pass the earlier pe- riods of their existence, gluing the ova together at the moment of their extrusion, so as to unite them into a boat-like mass (Jig. 133, A) of such beautiful construction that the little bark swims secure from injury, even during the roughest weather. The in- dividual eggs are of a conical form (fig. 133, B, a, 6, c), and are closed at their inferior extremity by a kind of lid (rf), provided to give egress to the mature embryo. The larva (c), represented upon a magnified scale at E, bears not the slightest resemblance to the perfect insect, and is provided with a singular modification of the respiratory apparatus adapted to its habits. The head is large, and carries two ciliated organs (g, g), which by their movements bring food towards the mouth ; the thorax is even larger than the head, and is furnished with fin-like bunches of minute hairs, as likewise are the segments of the abdomen. To the extremity of the tail is appended a group of moveable leaflets or fins, so disposed that by their action they sustain the larva at the top of the water, where it generally remains suspended with its head downwards. Such a position would obviously render respiration impossible, was there not a corresponding arrangement of the breathing organs to allow of free communication with the air. For this purpose, the respiratory trachese are found to be connected with a tube appended to the antepenultimate segment of the abdomen, the perforated extremity of which, being raised above the water, procures from the atmosphere the oxygen re- quired for respiration. After several moults, the larva, having attained its full growth, enters the pupa state, and in this con- dition still remains an inhabitant of the water, and occupies a position near the surface. A remarkable change, however, is visible in all parts of its structure : the head and thorax (Jig. 133, D) are consolidated into one large mass, under which the lineaments of the mature insect may be detected ; while the tail still con- tinues to be the agent employed in natation. The condition of the respiratory organs is, moreover, completely altered : the tube fixed upon the antepenultimate segment of the larva has totally disappeared, and, instead of it, we find two tubes appended to the back of the thorax ; these, although they perform the same office as the anal pipe of the larva, are thus displaced, in order to cor- respond with the altered position in which the animal now swims ; the back of the thorax, and not the tail, being nearest to the surface, as represented in the drawing (D). The necessity for 293 this change of posture, and consequent removal of the apparatus for taking in air from one part of the body to another, will be at once obvious when we consider the circumstances under which the perfect insect, having completed its developement, emerges from its pupa investments and enters upon an aerial existence. The problem to be solved is, how shall the mature gnat escape from the water without being wetted ? and, when we consider that neither the larva nor the pupa possesses instruments of lo- comotion capable of enabling it to leave its native element by crawling on shore, the difficulties attending the change appear almost insurmountable. It is evident that, while swimming in the position in which the larva floats (.fig- 133, c), the last Fig. 133. change could not by possibility be accomplished, as the bursting of the integument would at once admit the water to the sub- merged gnat, and drown it at the moment of its birth ; but by the new arrangement the metamorphosis is easily effected, and that in a manner so beautiful, that it is hard to say which is most admirable, the simplicity of the contrivance, or the perfection with which the object is accomplished. No sooner has the en- cased imago become fitted for its escape, than the pupa, rendered more buoyant, raises its back above the surface : the protruded portion of the pupa-case soon dries, and gradually begins to split in a longitudinal direction, so as to form by its expansion a boat 294 INSECTA. wherein the gnat swims upon the top of its native pond ; and sus- tained in this frail bark, formed by its late skin, it gradually ex- Fig. 134. tricates its legs and wings from their coverings, and is kept per- fectly dry until the expansion of its instruments of flight enables INSECTA. 295 it to soar into the air and quit for ever the raft so singularly pro- vided for its use. (340.) Having thus become acquainted with the various con- ditions under which insects arrive at maturity, and the principal forms that they exhibit during the different stages of the meta- morphosis, the reader will be prepared to investigate more mi- nutely the changes in progress during the process, and the gradual developement of the organs which successively make their appear- ance. On examining the viscera of a Caterpillar, they are found scarcely at all to resemble those of the butterfly or moth, into which a larva of this description is ultimately matured. The jaws (Jig. 136, i, 6), widely different both in structure and office from the proboscis which represents them in the perfect insect (Jig. 115), are strong and horny shears adapted to cut the leaves of vegetables and other coarse materials used as food ; the oesophagus (j%. 134, g, h) is strong, muscular, and capa- cious ; and the stomach (A, i), in capacity corresponding with the extraordinary voracity exhibited by the larva, passes insen- sibly into a wide intestine (i, w), the line of separation being only indicated by the entrance of the biliary vessels (k) that wind in numerous convolutions around the posterior half of the alimen- tary canal. It is sufficient to contrast this arrangement of the digestive organs with what we have already described in the but- terfly (Jig. 117), to appreciate the amazing dissimilarity: it would be difficult indeed to imagine, did not anatomy convince us of the fact, that the digestive apparatus of the imago, with its slender oesophagus, dilated crop, short sacculated stomach, long and con- voluted small intestine, and capacious colon, was derived from a gradual modification of such viscera as those we have just been considering. The salivary glands of the caterpillar (Jig. 134, q, r) are large cylindrical caeca, and their ducts (p) pour into the mouth an abundance of saliva proportioned to the coarse nature of the materials used as food. The sides of the body are traversed by the wide longitudinal tracheae, a, 6, c, that communicate on the one hand with the lateral spiracles, and on the other give off at regular intervals the air-tubes (d, e, e, e, e), which ramify most minutely over all the viscera, and convey the atmospheric air throughout the entire system. Besides the above organs, there are other viscera, which, al- though of considerable importance to the caterpillar, would be 296 INSECTA. utterly useless to the imago, and consequently are more or less completely wanting in the mature state. The whole body of the larva is filled with a peculiar fatty tissue (Jig. 134,/,/,/) called by entomologists the rete, epiploon, or fat-mass. This material, found in great abundance in mature and well-fed larvse, consists of an oily or greasy substance enveloped in a most delicate cellulosity, and seems to correspond to the fat of higher animals, like which it is indubitably a product of digestion, and a repository of superabundant nourishment, stored up, no doubt, for the sustenance of the animal during its helpless con- dition in the dormant or pupa state serving like the fat of hiber- nating quadrupeds, for food during the confinement of the imago. (341 .) But the most re- Fig. 135. markable peculiarity of the larvse under consideration, is the presence of an appa- ratus employed for produc- ing a tenacious thread of extreme delicacy, appro- priated by different species to various purposes. In many cases (fig. 105), it is made subservient to lo- comotion ; and by its assist- ance, as by a rope, the larva can suspend itself from any object, or let it- self down from one branch to another in search of food. The most import- ant uses however to which this thread is applied are connected with the concealment and pro- tection of the quiescent and defenceless pupa ; either furnishing the means of suspending the chrysalis in a place of safety* (fig. 135), or, as is the case with the silk-worm (fig. 131), supplying the material with which the caterpillar encases itself preparatory to * For a most amusing account of the manner in which some chrysalides manage with- out any external limbs to suspend themselves by the tail in a position of security, the rea- der is referred to Kirby and Spence, vol. iii. page 207. The figure above given illustrates the different steps attending the process. The larva, A, having spun some loose silk, and fixed it upon the under side of a leaf or other suitable object, suspends itself therefrom C INSECTA. 297 throwing off the last skin of the larva. The thread of the last- named insect, the silk-worm, is of great tenacity ; and, notwith- standing its fineness, may be wound off from the cocoon in a con- tinuous thread, forming the important article of commerce, silk. (342.) Nothing can be more simple than the apparatus provided in caterpillars for the production of this valuable commodity : Placed on each side of the intestine are two long and tortuous se- creting cseca (Jig. 134, v, x 9 y)> that separate from the surrounding juices of the body a tenacious viscid fluid which is liquid silk. The viscid secretion thus formed is in the silk-worm of a golden yellow colour, and is conveyed by the excretory ducts of the secerning organs (t>, z) to the labium or under-lip, where the ducts terminate at the base of a tubular instrument, the fusulus or spinnaret^ through which the silk is drawn (Jig. 136, c). The fusulus of the silk-worm, represented in the Fi s- 13 6. annexed figure upon an enlarged scale, is a simple nipple-shaped pro- minence, perforated at its extre- mity, and surrounded by four rudi- mentary palpi. When about to spin, the larva, by placing the ex- tremity of its spinnaret in contact with some neighbouring object, al- lows a minute drop of the glutin- ous secretion to exude from its ex- tremity, which, of course, adheres to the surface upon which it is placed : the head of the silk-worm being then slowly withdrawn, the fluid silk is drawn out in a delicate thread through the aperture of the spinnaret, its thickness being regulated by the size of the orifice, and, immediately hardening by the evaporation of its fluid parts, forms a filament of silk which can be prolonged at the pleasure of the animal until the contents of its silk reservoirs are completely exhausted. (343.) Such is the structure of the larva of a Lepidopterous insect, by its hind-legs. The skin of the caterpillar then gradually splits down the back (B, c), and is slowly pushed upwards towards the tail of the chrysalis. The pupa now lays hold of the old skin, nipping it between the rings of the abdomen, and hanging in this posture inserts the apex of the tail, which is covered with hooks for the purpose, into the silk previously deposited, and thus remains fixed in safety (D.) 298 INSECTA. and the arrangement of its internal viscera, when arrived at maturity, has been already described. We have yet, however, to mention the series of phenomena observable during the progress of its growth, and the mode of its expansion from the minute size that it exhibits on leaving the egg to the full dimensions which it ulti- mately acquires. In order fully to understand the circumstances connected with this part of our subject, it is necessary to premise that the outer integument of most larvae is of a dense corneous tex- ture, coriaceous in some parts, but quite hard and horny in others. In the second place, it is but very slightly extensible ; and more- over, as is always the case with epidermic structures, is not per- meated by any vascular apparatus, and consequently is absolutely incapable of growth when once formed. This epidermis encases every portion of the larva; the body, the legs, the antennae, the jaws, and all external organs are closely invested with a cuticular en- velope, such as, from its want of extensibility, would form an insu- perable obstacle to developement was there not some extraordinary provision made to meet the necessity of the case. The plan adopt- ed is to cast off at intervals the old cuticle by a process termed moulting ; an operation which is repeated several times during the life of the insect in its larva condition, and is accomplished in the following manner : The caterpillar becomes for a few days slug- gish and inactive, leaves off eating, and endeavours to conceal it- self from observation. The skin, or more properly the cuticle, becomes loosened from the subjacent tissues, and soon a rent ap- pears upon the back of the animal, which gradually enlarges in a longitudinal direction, and the imprisoned insect, after a long series of efforts, at length succeeds in extricating itself from its old cover- ing, and appears in a new skin of larger dimensions than the one it replaces, which however in all other particulars it closely resembles. With the old epidermis the larva throws off all external appendages to the cuticle : the horny coverings of the jaws, the cornese of the eyes, the cases of the claws are all removed ; and many writers have even found attached to the exuviae an epidermic pellicle that had formed a lining to the rectum, and delicate prolongations of the cuticle derived from the interior of the larger ramifications of the air-tubes. Absurd, indeed, have been the explanations given by various writers of the nature of the process under consideration. Swammerdam and Bonnet, nay, even our own illustrious entomo- logists Kirby and Spence, believed that even at the birth of the ca- terpillar all these skins existed ready formed one beneath the other, INSECT A. 299 and that the most external being removed at intervals displayed in succession the skins placed underneath. Surely the advocates of this extraordinary theory could scarcely have reflected upon the real object of the moults in question namely, to provide a succession of larger coverings proportioned to the continually increasing bulk of the larva, when they advocated this strange doctrine, alike at vari- ance with observation and sound physiological principles : the epi- dermis and all cuticular structures are mere secretions from the sub- jacent cutis or true skin ; and it can be no more necessary to suppose the pre-existence of so many skins in order to explain the moults of a larva, than to imagine that because, when in our own persons the cuticle is removed by the application of a blister, a new layer of epi- dermis is again and again produced, man should possess as many skins one beneath the other. Nothing, in fact, can be more simple and free from the miraculous than the whole process : at certain pe- riods, when the old cuticle becomes too small for the rapidly enlarg- ing dimensions of the insect, it becomes gradually loosened and se- parated from the vascular and living skin or cutis by which it was originally secreted, and, a new secretion of corneous matter taking place, a fresh and more extensive layer of cuticle is slowly formed, and then the old, dry, and dead epidermis being quite detached, is split by the exertions of the larva, and the newly secreted layer placed beneath it appears ; when the old skin is at length completely thrown off, the newly formed one soon hardens by exposure, and the re-clothed caterpillar assumes again its former activity and habits. (344.) Neither is the change from the larva to the pupa or chry- salis less easily explained, although regarded by our forefathers as being so mysterious and astonishing a phenomenon. According to the hypothesis above alluded to, after removing three or four skins in the embryo larva, the anatomist ought to have arrived at the totally different pupa-case ready formed, and only waiting for the removal of the coats above it to exhibit its characteristic form. Leaving however such visionary notions, let us examine the real nature of this portion of the metamorphosis. The reader will bear in mind, that, whatever the form of the exterior or epidermic crust, it is merely a dead and extra-vascular secretion, unchange- able when once deposited. But the living skin or cutis, beneath it, is, during the whole process of the metamorphosis, undergoing great and important changes, increasing in size only, during the larva condition ; but, when perfectly organized, developing itself at different points, and expanding into variously shaped organs 300 INSECTA. which did not previously exist. In the dragon-fly, for example (Jig- 104), when the cutis had become expanded to its mature larva condition, it secreted from its surface the external epidermic crust which gives form to the larva, B ; this outward integument remains, of course, unchanged when once formed, and retains the same appearance during the whole period of the existence of the insect in its larva state : but underneath this cuticle, and con- sequently concealed from observation, the growth of the living dermis still goes on, and important organs begin to appear, which had no existence when the last larva-investment was secreted. The wings have sprouted as it were from the shoulders, and already have attained to a certain growth ; the old integument of the larva becomes useless, and a new one is wanted; the process already described is repeated, the old cuticle becomes detached from the surface of the body, and the cutis begins to secrete for it- self a new covering moulded upon its own shape : the newly form- ed wings, therefore, and other newly developed processes of the dermis, secrete horny coverings for themselves in the same manner as other parts of the surface of the body ; and thus, when the in- sect leaves its old skin, and once more escapes from confinement, it presents to view the wing-cases which distinguish the pupa. Whatever may be the form of the pupa, its covering is secreted in a similar way ; it is the living and vascular skin which, though concealed, continually grows more perfect in its parts, and the cases secreted by it at distant intervals correspond in shape with the different phases of its developement. (345.) After having attained the pupa state, the last steps of the process are completed, and the dermic system becomes fully de- veloped in all its parts. The oral apparatus attains its perfect con- dition ; the wonderfully elaborate structure of the eyes is com- pleted ; the antennae assume their full developement ; the legs en- closed in those of the pupa attain their mature form ; and the wings, which have been continually growing, although concealed in the wing-cases of the pupa, acquire their ultimate size : the per- fect insect is ready for liberation, and, enclosed in its last covering, creeps out of the water in which it has so long resided to enter upon a new state of existence. Fixing itself upon some plant in the neighbourhood of its birth-place, the imprisoned dragon-fly splits its pupa-case along the back (jig. 137, A), and slowly extricates its head and body ; draws its wings from their coverings, and its legs from those of the pupa as from cast-off boots ; and at INSECTA. 301 length (fig. 137, B), getting its body from its now useless cover- ing, it becomes entirely free. The wings, before soft and crumpled, slowly expand (Jig. 137, c) ; the nervures harden, the extended membranes dry, and Fig. 137. in a short time the winged tyrant of the insect world (fig. 103) commences his aerial career. (346.) A strong argument in favour of the above views concerning the pro- duction of successive skins from the der- mis, is derived from the phenomena at- tending the cure of wounds in insects. If a perfect insect be wounded, the wound is never heal- ed at all ; and, if a larva or pupa is similarly injured, the wound remains un- cicatrised until the next moult, when the newly formed in- tegument is found to exhibit no traces of the injury : the secreted and extra- vascular cuticle can not cicatrise ; but the living and vascular dermis is not only able to repair injuries in- flicted upon itself, but, in secreting the next investment, to obliter- ate all indications of their occurrence. (347.) The changes above described are produced by the pro- gressive developement of the dermic or tegumentary system ; the parts of which, as we have already seen, becoming strengthened and 302 INSECTA. consolidated by degrees, ultimately acquire that density of struc- ture which the external skeleton of the insect exhibits in its perfect or imago state. But, while this extraordinary metamor- phosis is going on externally, other changes not less important are in progress in the interior of the body. The size of the alimentary canal, and the shape, proportionate dimensions, and general arrangement of the different parts composing it, are se- cretly and imperceptibly undergoing variations in accordance with the altered necessities of the animal. We have already seen a conspicuous example of this in Lepidopterous insects, 340 ; and, in other orders, equally striking instances might easily be selected. One of the most remarkable is met with in many Hymenoptera^ as, for example, in bees (Apis), wasps (Vespa), and ant-lions (Formica- leo), as well as in most of the Ichneumonidtz. In all these genera, the larva being concealed in a close cell during its developement, under circumstances which would render the evacu- ation of excreinentitious matter an obvious inconvenience, both the larva and pupa (Jig. 132) are entirely without either intes- tinal canal or anal orifice : what little excrement is produced by the digestion of the highly nutritive substances wherewith these larvae are fed being collected in a blind cavity or caecum placed behind the stomach, until the accomplishment of the last change ; at which period the insect, liberated from its confinement, becomes provided with a pervious intestine, and able to get rid of feculent matter. The fat-mass ( 340), which at the close of the larva state has reached its maximum of developement, is gradually absorbed du- ring the concealment of the insect in its pupa-case, its nutritive portions being no doubt appropriated to the nourishment of the pupa ; so that in the mature insect the fatty material has almost entirely disappeared, nothing being left in its place but the dense cellular web in which the fat had been deposited. The silk-secreting apparatus of such genera as possess the means of spinning a silken thread is peculiar to the larvae ; and, after the commencement of the pupa state, no traces of its previous exist- ence are to be detected. (348.) But, while the above-mentioned organs disappear, others become developed ; and the perfect insect is found to possess vis- cera, for which a skilful anatomist might seek in vain in the earlier stages of its existence. The generative system appears, at first, to be absolutely wanting in the larva ; but Herold,* after much * Entwickelungsgeschichte der Schmetterlinge, 1815, 4to. INSECTA. 303 patient investigation, succeeded in detecting the undeveloped ru- diments of the future sexual organs both of the male and female. It is during the maturation of the pupa that these important parts expand ; and, before the disclosure of the imago, they are found to have attained their complete proportions, so as to be ready to per- form their functions as soon as the expansion of the wings endows the insect with means of locomotion sufficiently perfect to ensure the due dispersion of the species. (349.) It is in the nervous system, however, that the most in- teresting phenomena are observable ; and in the lessons afforded by watching the correspondence between the state of the animal during the several phases of its existence and the developement of the ner- vous ganglia, the physiologist cannot fail to recognise those great and general principles upon which our arrangement of the animal creation is based. In the worm-like larva the ganglia are numerous but of small dimensions ; too feeble to be capable of animating powerful limbs, or of appreciating impressions from the organs of the higher senses : the animal is, in fact, precisely in the condition of an ANNELID AN, which it would seem to represent. External limbs are therefore absolutely wanting in many larvae ; in others they are represented by short and stunted appendages ; and even in the most perfect, or hexapod larvae, they are feeble instruments in comparison with those of the mature imago. The senses exhibit equal imperfection ; and eyes are either entirely wanting, or are mere ocelli, simple specks, exhibiting the lowest possible organization of a visual apparatus. But, as the growth of the larva goes on, a change in the arrangement of the nervous system is perpetually in progress. The series of nervous cords connecting the different pairs of ventral ganglia in the larva (Jig. 138, A) become flexuous as the insect attains the pupa state ; the whole chain becomes shorter ; the brain, or encephalic ganglion, increases in its proportionate dimen- sions ; and, moreover, several ganglia, originally distinct, coalesce, and form larger and more powerful masses (Jig. 138, B). This co- alescence of the ganglia, which takes place more especially in the thoracic region, is evidently a preparation for the concentration of greater power and activity in this part of the body ; and although in inactive chrysalides this change is not as yet visible by its effects, in the active forms even the pupa is distinguished from the larva by a considerable increase of vigour and energy in its movements. In the imago the concentration of the nervous centres is carried to that extent which is adapted to the necessities of the mature state ; 304 INSECTA. their number is still further reduced (Jig. 138, c); their size, in the thorax especially, considerably increased ; and the brain, now ar- rived at its maximum of developement, is furnished with the won- derful apparatus of eyes and other instruments of the senses, which heretofore would have been absolutely useless, but now, with the expansion of the brain, have become suited to the more ex- alted faculties of the insect. Fig. 138. B C A Many insects are capable of producing audible sounds ; and some- times the noises they make are exceedingly shrill, and may be heard at some distance. Such sounds originate from various causes in different tribes, and it is not always easy to detect the mode of their production. In many beetles they are caused by rubbing different parts of their dense integument against each other, and the chirping of several Orthoptera seems to have a similar origin ; the acute note that these insects utter is apparently produced by friction, the edges of their hard pergamentaceous wings being INSECTA. 305 Fig. 139. either scraped against each other, or against the long and serrated edges of their thighs. The buzzing and humming noises heard during the flight of many genera results from the forcible expul- sion of the air as it streams through the respiratory spiracles, whose orifices Burmeister imagines are furnished with vibratory lami- nae, to the rapid movements of which the noise may be due. In the genera Gri/llus and Cicada among the Orthoptera, however, there is a peculiar apparatus specially provided for the production of the loud chirping to which such insects give utterance. Upon the first segment of the abdomen, covered by a broad moveable plate (fig. 139 ), there is a large aperture, wherein a tense plicated membrane is observable. This membrane is acted upon internally by certain muscles able to throw it into rapid vibration, and thus give rise to the sound in question. (350.) One other point connected with this interesting class of animals requires brief notice. Many insects are endowed with the faculty of emitting phosphorescent light, which is in some species exceedingly brilliant. The Elateridse among beetles are pre-eminently lu- minous, and in them the light seems to be principally given out by two oval spaces upon the thorax, which in the dead insect are of a greenish hue ; during life, some species (Elater noctilucus) are so strongly phospho- rescent as to enable a person to read a book by passing the animal over the lines. The Lampyri emit a light of great brilliancy ; and in Italy, during the summer nights, the groves, illuminated by their incessant scintillations, exhibit a scene equally strange and beautiful. Such insects appear to have a power of obscuring or exhibiting their light at pleasure ; but the nature of the luminous secretion, if such it be, upon which their luminosity depends, has as yet escaped detection.* * An interesting account of this subject is to be found in the article LUMINOUSNESS, ANIMAL, by Dr. Coldstream, in the Cyclopaedia of Anatomy and Physiology. 306 CHAPTER XVI. ARACHNIDA.* (351.) THE Arachnidans long confounded with INSECTS, and described as such even by recent entomologists, are distinguished by characters of so much importance from the animals described in the last chapter, that the necessity of considering them as a distinct class is now no longer a matter of speculation. In IN- SECTS, the external skeleton presents three principal divisions, the head, the thorax, and the abdomen : but in the spider tribes, the blood-thirsty destroyers of the insect-world, the separation of the head from the thorax, which, by increasing the flexibility, ne- cessarily diminishes the strength of the skeleton, is no longer admis- sible ; and the process of concentration being carried a step fur- ther, the head and thorax coalesce, leaving only two divisions of the body recognizable externally, viz. the cep halo-thorax and the ab- domen. Insects in their mature forms were found to be invariably furnished with only six legs, but in the adult Arachnidans eight of these limbs are developed. These characters in themselves would be sufficient to discriminate between the two orders ; but when to these we add, that in the Arachnidans the eyes are invariably smooth, the antennae of insects represented by organs of a totally different description, that the sexual apertures are either situated beneath the thorax, or at the base of the abdomen, and, moreover, that in the greater number of Arachnidans, respiration is carried on in localized lungs (pulmonibranchia), instead of by tracheae as in insects, we need not enlarge further in the present place upon the propriety of ranking the Arachnida as a separate class. These animals may be grouped under three principal divisions ; the first of which is evidently an intermediate type of organization, com- bining many of the characters of the Insecta with the external limbs and palpi of proper Arachnida. (352.) The ARACHNIDA TRACHEAREA, in fact, breathe by means of tracheae resembling those of insects, which are so ar- ranged as to convey air to every part of the system ; and we may therefore suppose that their circulatory apparatus, as well as their a spider. ARACHNIDA. 307 secerning organs, conform more or less to the type of structure met with in the class last described. The Mites (Acaridat) belong to this division, and form a very numerous family, which is exten- sively distributed. Some are parasitic in their habits, infesting the bodies of insects ; and one, the itch-insect (Acarus Scabiei), is found occasionally upon the human skin. Many live in cheese and other provisions, where they multiply prodigiously ; and not a few inhabit leaves, or are found under stones, or beneath the bark of trees. Some (Hydrachna) are aquatic ; but unfortunately in all, from their extremely minute size, the investigation of their internal viscera presents so many difficulties, that but little is satisfactorily known concerning their anatomy : even the pseudo- Scorpionida, which are of larger growth, and, although still breathing by tracheae, approximate most closely to the outward form of the next group, hive been very imperfectly examined. The rest of the Arachnidans breathe by means of lungs, or, as they are more properly designated, pulmonary branchiae; and consequently, in contradistinction to the last-mentioned, are called by zoologists ARACHNIDA PULMONARIA : such are the Scor- pions and Spiders. (Fig. 140.) The PEDIPALPI, forming the second division, are at once re- cognised by the peculiarity of their external configuration. Their palpi, the representatives apparently of the maxillary palpi of insects, are exceedingly strong, and furnished at their extremity with a prehensile forceps ; the hinder part of the body, correspond- ing with the abdomen of insects, is much prolonged, and composed 308 ARACHNIDA. of numerous articulated segments, terminated in the scorpion tribe by a sharp unciform sting (fig> 140), armed with a venomous se- cretion. The third section embraces the AIIANEID^E, or Spiders, distin- guished by having the abdomen short and globular, and furnished, moreover, near its posterior termination with spinnerets, by means of which these animals manufacture silken filaments applicable to a great number of purposes, and especially employed in constructing what is usually named the spider's web. The maxillary palpi in the females are simple, and more or less resemble feet ; but in the males they often form a remarkable apparatus, to be described in another place : the jaws are also armed with sharp and hooked fangs, and perforated near their points for the emission of a poisonous secretion provided for the destruction of their prey. (353.) Beginning with the first cfivision, we shall now proceed to place before the reader such facts as have been ascertained, con- nected with the anatomical structure of the class under considera- tion. In the Acaridse, or Mites, the skin of the entire body is so soft that any annulose structure is scarcely distinguishable; the division, however, into cep halo-thorax and abdomen is sufficiently evident. The eyes are minute black points, never exceeding four in number and resembling the ocelli of insects. Eight feeble legs are articulated with the thorax, properly so called. The mouth seems adapted to suction, and the jaws form a piercing instrument barbed at the extremity. The structure of the respiratory stig- mata or spiracles would seem to differ very considerably from those of insects. According to Dr. Auduoin, in the species which he examined (Ixodes Erinacei),* each spiracle resembles a spherical tubercle perforated by an infinite number of small holes, in the centre of which may be remarked a larger circular plate ; and it is through these numerous foramina that the air enters the body, and gets into the tracheae. (354.) The Pulmonary Arachnidans, both of the pedipalp and spinning divisions, are strictly carnivorous in their habits, living upon the juices of the insects they destroy; and we may consequent- ly expect, in the construction of their alimentary apparatus, a sim- plicity proportioned to the facility with which highly nutritive food composed of already anhnalized materials is capable of being assi- milated. The mouth varies somewhat in its conformation, and, if we compare the pieces composing it with those that we have found * Cyclop, of Anat. and Phys. art. ARACHNIDA. ARACHNIDA. 309 manclibulate insects to possess, we shall have good reason for surprise in noticing the strange uses to which some parts of the oral appara- tus are converted. In scorpions (fig. 140), the apparent repre- sentatives of the mandibles of an insect are transformed into a pair of small forceps, each being provided with a moveable claw ; these therefore form of themselves prehensile organs adapted to seize prey, and hold it in contact with the mouth. But it is in the maxilla that we find the most extraordinary metamorphosis ; for the maxil- lary palpi, so small in insects, are found to be developed to such prodigious dimensions, that they far surpass in size and strength any of the ambulatory extremities, and, from their resemblance to the claws of Crustaceans, have given the character from which the name of the division is derived.* Each of these formidable organs is terminated by a strong pair of pincers, and thus the maxillary palpi become converted into potent instruments either for attack or defence. The representative of the labium of an insect in the Arachnidans has no palpi connected with it. (355.) In spiders the organization of the mouth is altogether dif- ferent. The mandibles (fig. 142, o, o) are each terminated with a moveable fang (c), which ends in a sharp point, and is perforated near its extremity by a minute orifice, from which, when the spider bites, a venomous fluid of great potency is instilled into the wound in- flicted ; such, indeed, is the malignity of this poisonous secretion that its effects in destroying the life of a wounded insect are al- most instantaneous, and in some of the large American species even small birds fall victims to its virulence. The organ in which the poison is elaborated is represented in the figure above referred to : it is a long and slender bag, from which an attenuated duct may be traced through the body of the mandible as far as the perforated extremity of the fang. The palpi connected with the maxillse of the spider are termi- nated in the female by a simple hook ; but in the males of many species they exhibit a conformation slightly resembling the forceps of the scorpion, although provided for a very different purpose. When closed (fig. 141, B), the terminal part of the palpus pre- sents a club-like dilatation, which, however, on close inspection will be found to consist of several pieces (fig. 141, A, a,,c, </, e), connected with each other by articulations, and capable of being opened out in the manner represented in the figure. This strange instrument was formerly imagined to be the penis of the male spi- * Pes, a foot ; palpus, a feeler. 310 ARACHNIDA. der, and was thought to contain the terminations of the seminal ducts : the supposition, however, has Fi &' been proved to be erroneous, for the palpus is imperforate, and the sexual apertures of the male are situated else- where, but the organ in question is nevertheless apparently used in the process of impregnation, in a manner to be explained hereafter. (356.) Both in scorpions and spiders the alimentary canal is ex- ceedingly narrow, presenting scarcely any of those dilatations met with in the digestive organs of insects. This is a natural consequence of the nature of their food ; for, as they live entirely upon animal juices sucked from the bodies of their victims, there could be little necessity for the presence of capacious receptacles for nutritious matter, or for any reservoirs for the accumulation of effete ma- terial. In the Scorpionidce there is no stomachal dilatation what- ever : a straight intestine passes directly from the mouth to the anus, situated at the extremity of the abdomen ; and the insertion of the biliary vessels forms the only distinction between its ven- tricular and intestinal divisions. Five delicate caeca are derived from each side of the ventricu- lar portion, and plunge into the centre of a fatty substance in which the alimentary canal is em- bedded. In Spiders, likewise, caeca are appended to the com- mencement of the digestive ap- paratus, and a slight enlargement (Jig. 142, b) may be said to repre- sent the stomach, from which a slender intestine (g) is continued to the anus. As in the scorpion, a large quantity of fat (A) sur- rounds the nutrient organs, and fills up a great proportion of the cavity of the abdomen. Like the fat-mass of the larvae of insects, this substance must, no Fig. 142. ARACHNIDA. 311 doubt, be regarded as a reservoir of nutriment ; and when the ha- bits of these animals are considered, the precarious supply of food, and the frequent necessity for long-protracted fasts, when a scar- city of insects deprives them of their accustomed prey, such a provision is evidently essential to their preservation. (357.) One peculiarity connected with the arrangement of the chylo-poietic viscera of the spider is the manner in which the biliary organs terminate in the intestine ; for instead of entering in the usual position, namely, close to the termination of the stomach, they seem to pour their secretion into the rectum immediately in the vicinity of the anus. At this point, a kind of sacculus (figs. 142 and 143, f) joins the intestine, into Fig. 143. which the branched tubes (fig. 143^ o, o ; fig. 142, s) empty themselves. This cir- cumstance has long been a subject of in- teresting inquiry to the comparative phy- siologist. If the fluid secreted by these tubes be really bile, in what manner does it accomplish those purposes usually supposed to be effected by the biliary secretion ? It would seem to be, in this case, merely an excrementitious produc- tion. Are the caeca appended to the stomach biliary organs ? If so, the apparatus in question may be of totally distinct character, and its product only furnished to be expelled from the system. In conformity with the last supposi- tion, many antaomists have been induced to regard these vessels as -being analogous to the urinary secernents of more highly organized animals, and have not scrupled to apply to them the appellation of renal vessels : but this hasty application of names we have already animadverted upon as being highly prejudicial to the interests of science ; and in this instance, as in many others, to wait for the results of future investigations is far more advisable than rashly to assign a definite function to a part, the real nature of which is a matter of speculation. (358.) The respiratory system of the Pulmonary Arachnidans is constructed upon very peculiar principles, being neither composed of gills adapted to breathe water, nor lungs like those of other air-breath- ing animals, but presenting a combination of the characters of both. The pulmo-branchite are, in fact, hollow viscera resembling bags ; the walls of which are so folded and arranged in laminae, that a 312 ARACHNJDA. considerable surface is presented to the influence of oxygen. It is, indeed, highly probable that these organs are intermediate in function as well as in structure between an aquatic and air-breath- ing respiratory apparatus ; for, as both the pedipalp and spinning Arachnidans frequent moist situations, the dampness of the atmo- sphere may be favourable to the due action of the air upon the circulating fluids of these creatures. Each pulmo-branchia opens externally by a distinct orifice, resembling the spiracle of an in- sect, and is closed in a similar manner by moveable horny lips. In the scorpion (fig. 140) the spiracles are eight in number, placed upon the ventral aspect of the body; and just in front of the first pair of breathing-holes are two remarkable organs repre- sented in the figure, resembling a pair of combs, which are appa- rently adapted to keep the spiracular orifices free from dirt, and thus prevent any obstructions to the free ingress and egress of the air. In the Araneidse^ the form and arrangement of the spiracles is somewhat different : according to Treviranus, there are four pairs on each side of the cephalo-thorax, situated immediately above the insertions of the legs ; and in addition to these there is one pair constantly found on the under surface of the abdomen, and four pairs of smaller apertures of less importance on its upper part. In order to understand the manner in which respiration takes place in pulmo-branchice of the structure above described, it is necessary to suppose the existence of a vascular apparatus, by means of which the circulating fluid is continually spread over the laminae of the respiratory sacculi, and afterwards returned to the circulation in a purified condition. It is true, that, owing to the extreme difficulty of tracing vessels of such small dimensions, the ' continuity of the entire system is rather an inference deducible from agen eral review of the facts ascertained, than absolutely a matter of demonstration. We will, therefore, briefly lay before the reader the data upon which physiologists found the opinions entertained at the present day relative to the means whereby the circulation of Arachnidans is accomplished. (359.) According to Treviranus, spiders are provided with a long contractile vessel, which runs along the mesial line of the back, and resembles in form the dorsal vessel of insects, although in struc- ture it is widely different. In insects, it will be remembered, the dorsal vessel communicated freely with the abdominal cavity by numerous valvular apertures, and neither arteries nor veins were ARACHNIDA. necessary for diffusing the blood through the system ; but in the Pulmonary Arachnidans numerous vascular trunks are given off from both sides of the dorsal heart, and are dispersed in all direc- tions. All the branches proceeding from the sides of the dorsal vessel are presumed to be of an arterial character, with the excep- tion of a few large canals situated near the junction of the anterior and middle thirds of its length, and these are supposed to be veins* (branchio- cardiac vessels) destined to return the aerated blood from the pulmo-branchite into the general circulation. Whoever watches the movements of the blood in one of the limbs of these creatures, will perceive that under the microscope its motion bears little resemblance to that observable in the foot of a frog, or in animals possessed of an arterial and venous system completely developed. So irregular, indeed, is the course of the globules, that it would be difficult to conceive them to be confined in vessels at all ; the whole appearance resembles rather the dif- fused circulation seen in the larva of an insect, than that of a crea- ture possessing vascular canals arranged in definite directions. The only probable way of accounting for such a phenomenon is by supposing that, in this first sketch of a vascular system, if we may be pardoned the expression, the veins are mere sinuses or wide cavities formed in the interstices of the muscles, through - which the blood slowly finds a passage. From a review of the above-mentioned facts we are at liberty to deduce the following conclusions relative to the circulation of Arachnidans : The pulmo-branchite being apparently the only organs of respiration, the blood must be perpetually brought to these structures from all parts of the system, to receive the influences of oxygen, and again distributed through the body : such a circulation could only be accomplished in circumscribed channels ; some destined to propel it through all parts ; others to collect it after its distribu- tion, and bring it to the respiratory organs ; and a third set to return it in a renovated condition to the heart. The circuit of the blood may therefore be presumed to be completed in one or other of the following modes. The dorsal vessel, or heart, by its con- traction drives the blood through numerous arterial canals to the periphery of the system : the blood so distributed gradually finds its way into capacious sinuses, through which it flows to the branchial organs, and from hence it re-enters the heart by the branchio-cardiac vessels above referred to : or else the action of * Dr. Audouin, Cyclop, of Anat. and Phys. art. AIIACHNIDA. 314 ARACHNIDA. the heart drives a portion of the circulating fluid into the pulmo- branchifE by the same effort which supplies the rest of the system, and the blood so impelled to the respiratory organs becomes, after being purified, again mixed up with the contents of the veins which return it to the heart. (360.) In the nervous system of spiders we observe that pro- gressive concentration of the nervous centres, which we have traced through the lower forms of the HOMOGANGLIATA, carried to the utmost extent. Spiders are appointed destroyers of insects, with which they maintain cruel and unremitting warfare. That the destroyer should be more powerful than the victim, is essen- tial to its position ; that it should excel its prey in cunning and sagacity, is likewise a necessary consequence ; and by following out the same principles, which have already been so often insisted upon, concerning the inseparable connexion that exists between the perfection of an animal and the centralization of its nervous gan- glia, we find in the class before us an additional confirmation of this law. In scorpions, indeed, the nervous masses composing the ventral chain of ganglia are still widely separated, especially those situated in the segments of the pig. 144. tail : in the cephalo- thorax they are of proportionately larger dimen- sions ; and, moreover, exhibit this remarkable peculiarity, that, instead of being united by two cords of communication, there are three inter- ganglionic nerves connecting each di- vision. It is in spiders that the con- centration of the nervous system reaches its climax ; for in them we find the whole series of ganglia, en- cephalic, thoracic, and abdominal, ag- gregated together, and fused, as it were, into one great central brain, from whence nerves radiate to all parts of the body. The extent to which centralization is here carried will be at once appreciated by refer- ence to the annexed figure (Jig- 144) : the encephalic masses ja, a, whence the optic nerves distributed to the ocelli are de- rived, are in close contact with the anterior part of a large ARACHNIDA. 315 ganglion, c, that represents all the abdominal ganglia collected into one mass ; and from the posterior part of this, nerves, w, w, destined to supply the parts contained in the abdomen, derive their origin. The thoracic ganglia, e, e, are fusiform, and placed on each side of the mass c, with which they are apparently amalgamated at one extremity, while from the opposite they give off the nerves appropriated to the legs. The ocelli or eyes of Arachnidans have been minutely investi- gated by M tiller,* and seem to present a type of structure very far superior to that of insects. In the Scorpion this distinguished anato- mist succeeded in detecting most of the parts which enter into the construction of the eye of a vertebrate animal ; and, moreover, a great similarity in their arrangement. The cornea, a globular lens, the aqueous and vitreous humours, the retina and choroid were all found nearly in their usual relative positions ; so that the sense of vision in these animals must be extremely perfect. (361 .) The sexual organs of the male and female Fig. 145. Arachnidans exhibit very great simplicity in their structure. The testes, or secreting vessels of the male spider, are two long cseca (Jig. 145, 6), lodg- ed in the abdomen, and terminating by simple orifices at the ventral surface. No external in- tromittent organ is perceptible ; and it was on this account that the peculiar apparatus above re- ferred to, situated at the extremity of the maxil- lary palpus, was so long considered as giving pas- sage to the impregnating secretion. The singular instrument already described ( 355), would seem, indeed, to be in some manner really subservient to the fecundating process ; being used most probably as an exciting agent preparatory to the intercourse between the sexes. (362.) The ovigerous system of the female is equally devoid of complication, and, like the male testes, consists of two elongated membranous sacculi, in which the eggs are formed and brought to maturity. The impregnation of the ova is evidently effected by the simple juxta-position of the external orifices of the two sexes : yet such is the ferocity of the female spider, that the accomplish- ment of this is by no means without risk to her paramour; for the former being far superior to the male, both in size and strength (Jig. 146, A, B), would infallibly devour him, either before or after * Annales des Sciences Nat. tom.xvii. 316 ARACHNIDA. the consummation of his purpose, did he not exercise the most guarded caution and circumspection in making his advances. Fig. 146. (363.) One peculiar characteristic of the Araneida is the posses- sion of a spinning apparatus, whereby the threads composing their web are manufactured. The instruments employed for this purpose F iff. 147. are situated near the posterior extremity of the abdomen, and consist externally of four spinnerets, and twopalpiform organs (fig. 147 A, B). Each spinner 'et, when highly magnified, is found to be perforated at its extremity by innumerable orifices of extreme minuteness (^/zg.147, c), through which the filaments are drawn ; so that, unlike the silk of the caterpillar, the thread of the spider, delicate as it is, is composed of hundreds of smaller cords, some- ARACHNIDA. 317 times woven together by zig-zag lines, and thus exhibiting a structure of exquisite and most elaborate composition. The fluid silk, which, when it is drawn through the microscopic apertures of the spinneret, affords the material whereof the web is con- structed, is secreted in a set of glands represented in the sub- joined engraving (Jig. 148). The secerning extremities of the glandular tubes are composed of branched cseca (&), whence arise long and tortuous Fi 148 ducts (a, a, a), that become dilat- ed in their course into reservoirs for the secreted fluid, and terminate by several canals at the base of the external spinning tubuli. Various are the purposes to which the dif- ferent species of the Araneidse convert the delicate threads thus produced. Some construct for themselves silken tubes or cells, in which to conceal themselves from pursuit, and from this retreat they issue to hunt for prey in the vicinity of their abode ; others strew their fila- ments about at random, apparently to entangle passing insects ; many make nets composed of regular meshes, and spread them out in favourable situations to entrap their victims (Jig. 146) ; while a few species, enveloping their eggs in bags of curious construction, carry them about attached to their bodies, and defend them with the utmost courage and pertinacity : even in water these webs are turned to* many singular uses ; and ropes, nets, and even diving- bells are at the disposal of aquatic species furnished with this extraordinary spinning machinery. A few only of the most remarkable applications of this de- licate material can be noticed in this place. The mason-spiders (Mi/gale) excavate for themselves subterranean caverns, in which these marauders lurk secure from detection, even by the most watchful foe : nor could any robber's den, which ever existed in the wild regions of romance, boast more sure concealment from pursuit, or immunity from observation. The construction of these 318 ARACHNIDA. singular abodes has long excited the admiration of the naturalist : a deep pit is first dug by the spider, often to the depth of one or two feet, which, being carefully lined throughout with silken tapestry, affords a warm and ample lodging ; the entrance to this excavation is carefully guarded by a lid or door, which moves upon a hinge, and accurately closes the mouth of the pit. In order to form the door in question, the Mygale first spins a web which exactly covers the mouth of the hole, but which is attached to the margin of the aperture by one point only of its circum- ference, this point of course forming the hinge. The spider then proceeds to lay upon the web a thin layer of the soil collected in the neighbourhood of her dwelling, which she fastens with another layer of silk ; layer after layer is thus laid on, until at length the door acquires sufficient strength and thickness : when perfected, the concealment afforded is complete ; for, as the outer layer of the lid is formed of earth precisely similar to that which surrounds the hole, the strictest search will scarcely reveal to the most practised eye the retreat so singularly defended. Another spider (Clotho Durandii) constructs a dwelling equally artificial and ingenious, a kind of tent in which it lives and rears its young. This tent is composed of several superposed sheets of the finest taffeta, and its contour presents seven or eight prominent angles, which are fixed to the surface of the ground by silken cords. The young Clotho at first lays down only two sheets thus secured, between which she hides herself ; but, as she grows older, she con- tinually lays down additional coverings, until the period when she begins to lay her eggs, at which time she constructs an apartment, soft, downy, and warm, specially devoted to their reception. The exterior sheet of the tent is purposely dirtied for the purpose of con- cealment ; but within, everything is beautifully clean and white. The most admirable part of the contrivance, however, is the per- fect safety afforded to the young when the parent leaves her tent in search of food ; some of the superposed sheets are fastened toge- ther at their edges, others are simply laid upon each other, and, as- the parent herself alone possesses the secret which enables her to raise those layers by which entrance is to be obtained, no other animal can find its way into her impenetrable abode. 319 CHAPTER XVII. CRUSTACEA. INSECTS and Arachnidans are air-breathing animals ; and, even in such species of these two extensive classes as inhabit fresh water, respiration is strictly aerial. No insects or spiders are marine ; and consequently the waters of the ocean would be utterly un tenanted by corresponding forms of Articulata, was there not a class of beings belonging to this great division of the animal world so organized as to be capable of respiring a watery medium, and thus adapted to a residence in the recesses of the deep. Examined on a large scale, the Crustaceans, upon the considera- tion of which we are now entering, are marine creatures : many species, it is true, are found abundantly in the lakes and ponds around us, but these form rather exceptions to the general rule ; and we may fairly regard this extensive group of beings as the aquatic representatives of the insects and spiders, with which they form a collateral series. (364.) The tegumentary system of the CRUSTACEA corresponds in its essential structure with that of insects, and consists of a vascular dermis, a coloured pigment, and a cuticular secreted layer which forms the external shell or skeleton : the latter, or epidermic covering, however, differs materially in texture from that of other Articulata, inasmuch as it contains calcareous matter in considerable abundance, and thus acquires in the larger species great density and hardness. Figt 149 As regards the mechanical arrangement of the skeleton, we shall find the same general laws in operation as we have ob- served throughout all the annulose orders, a continual centralization and progressive coalescence of the different rings or ele- ments composing the external integument, and a strict correspondence between the degree to which this consolidation is car- ried and the state of the nervous system within. In the lowest forms of the Crustacea CRUSTACEA. we have in fact a repetition of the condition of the skeleton met with in the Myriapoda, or in the larva state of many in- sects ; the whole body being composed of a series of similar segments, to which are appended external articulated members of the simplest construction (Jig. 149). The number of rings or segments composing the body varies in different species ; but such variation would seem, from the inter- esting researches of Milne Edwards and Audouin, concerning the real organization of articulated tegumentary skeletons, to be rather apparent than real, inasmuch as the discoveries of these distinguished naturalists go far to prove that, whatever the state of consolidation in which the integument is found, the same number of elements or rings may be proved to have originally existed before, by their union, they became no longer distinguishable as separate seg- ments. The normal number of these elements Milne Edwards considers to be twenty-one, seven of which enter into the composition of the head, seven belong to the thorax, and as many appertain to the ab- dominal region of the body. To illustrate this important doctrine let us select a few examples, in order to show the manner in which the progressive coalescence of the segments is effected. In Talitra (Jig. 150) the cephalic elements are completely united, their existence being Fi S- 15 * only indicated by the several pairs of appendages ; one pair, of course, belonging to each ring. The first ring of the cephalic region, in this in- stance, has no external articu- lated member ; but in higher orders the eyes are supported upon long peduncles connected with this element of the skeleton, that may be regarded as the represen- tatives of those limbs which take different names in different regions of the body. The second and third rings support jointed organs here called antennae ; while the several pairs of jaws appertaining to the mouth indicate the existence of so many elements united toge- ther in the composition of the head. The seven segments of the thorax are still distinct, and each supports a pair of jointed organs, which, being used in locomotion, are called legs ; the abdominal elements, likewise, are equally free, CRUSTACEA. 321 and have natatory extremities developed from the five posterior rings. In the lobster (Astacus Marinus) we find not only the cephalic segments anchylosed together, but those of the thorax also ; and al- though the lines of demarcation between them are still recognisable upon the ventral aspect of the body, superiorly the entire thorax and head are consolidated into one great shield (cephalo-thorax) , the abdominal segments only remaining distinct and moveable. In the Crabs the centralization of the external skeleton is carried to still greater lengths, so as to enable this tribe of Crustaceans to become more or less capable of leaving their native element, and walking upon the shores of the sea, or even in some instances of leading a terrestrial existence, as in the case of the land- crab of the West India islands. The abdominal segments, however, still re- main free, though proportionately of very small dimensions ; and, being no longer useful in swimming, the abdomen is folded beneath the enormously developed thoracic portion of the body. In the King-Crab (Limulus Polyphemus; Jig. 151) even the divisions of the abdomen are obli- Fig. 151. terated, the whole body being co- vered by two enormous shields, and the tail prolonged into a formidable serrated spine, of such density and sharpness that in the hands of sa- vages it becomes a dreadful weapon, and is used to point their spears either for the chase or war. The reader will at once perceive the strict parallelism that may be traced between the changes which occur during* the metamorphosis of insects, and those observable as we thus advance from the lowest to the most highly organized Crusta- cean genera; and even the steps whereby we pass from the Anneli- dan to the Myriapod, and from thence to the Insect, the Scorpion, and the Spider, seem to be re- peated as we thus review the progressive developement of the class before us. Having thus found that the annuli, or rings, which compose the Y 322 CRUSTACEA. annulose skeleton may be detected even in the most compactly formed CRUSTACEA, it remains for us to inquire, in the next place, what are the principal modifications observable in the arti- culated appendages developed from the individual segments. This inquiry is one of considerable interest, inasmuch as it goes to prove that, however dissimilar in outward form, or even in function, the limbs of Crustaceans are mere developements of the same elements, which, as they remain in a rudimentary condition or assume larger dimensions, become converted into instruments of sensation, legs, jaws, or fins, as the circumstances of the case may render needful. In the lower, or more completely annulose forms (figs. 14$ and 152), these members are pretty equally developed from all the seg- ments of the body, and are subservient to locomotion, being gene- rally terminated by prehensile hooks, or provided with fin-like ex- pansions ; but, as we advance to the more perfect genera, the limbs assume such various appearances, and become convertible to so many distinct uses, that they are no longer to be recognised as consisting of similar elements, modified only in their forms and relative pro- portions. To notice Fig. 152. all the varieties which occur in the extensive class before us, would be to weary the reader with tedious and unne- cessary details ; we shall therefore select the De- capod* division of these animals, as abundantly sufficient for the illustration of this part of our subject. This division, which includes the most highly organ- ized forms, has been divided by writers into three extensive fami- lies, the Macroura, or swimming Decapods ; the Anomoura, which inhabit the empty shells of Mollusca ; and the Brachyura, or short- tailed species, of which the crab is a familiar specimen. If we take the common lobster as an example of the first of these groups, we shall find that there are five pairs of articulated limbs placed upon each side of the mouth, which are evidently adapted to assist in seizing and conveying into the stomach substances used as food. These singular organs, which, although entitled to be considered as jaws so far as their use would indicate the name belonging to them, * So called from the circumstance of their having five pairs of limbs so largely de- veloped as to become ambulatory or prehensile organs. CRUSTACEA. 323 arc no less obviously merely modifications of articulated feet ; and the term foot-jaws has now, by common consent, become the appel- lation by which they are distinguished. The pair of legs which succeeds to the remarkable members last referred to, is appropriated to widely different offices. The organs in question are developed to a size far surpassing that attained by any of the other limbs and are endowed with proportionate strength. Each of these robust extremities is terminated by a pair of strong pincers (chelee) ; but the two are found to differ in their structure, and are appropriated to distinct uses. That of one side of the body has the opposed edges of its terminal forceps provided with large blunt tubercles, while the opposite claw is armed with small sharp teeth. One, in fact, is used as an anchor, by which the lob- ster holds fast by some submarine fixed object, and thus prevents itself from being tossed about in an agitated sea ; the other is ap- parently a cutting instrument for tearing or dividing prey. To the chela succeed four pairs of slender legs, scarcely at all serviceable for the purposes of locomotion ; but, the two ante- rior being terminated by feeble forceps, they become auxiliary instruments of prehension. The articulated appendages belonging to all the abdominal segments are so rudimentary that they are no longer recognisable as assistants in progression ; and it is at once evident, when we examine the manner in which the Macroura use their tails in swimming, that the developement of large organs in this position would materially impede the progress of animals pre- senting such a construction : the false feet, as these organs are called, are therefore merely available as a means of fixing the ova which the female lobster carries about with her attached be- neath her abdomen. The tail is the great agent of locomotion in all the Macroura or large-tailed Decapods, and for this purpose it is terminated by a fin formed of broad calcareous lamellae, so arranged, that while they will close together during the extension of the tail, and thus present the least possible surface to the water, they are brought out to their full expansion by the down-stroke of the abdomen ; and such is the impulse thus given, that, as we are credibly informed, a lobster will dart itself backwards to a dis- tance of eighteen or twenty feet by one sweep of this remarkable locomotive instrument. If we now pass on to the consideration of the Anomourous De- CRUSTACEA. capods, we find that the external organs above enumerated, although existing in precisely similar situations, are so far modified in their construction and relative proportions as to become suited to a mode of life widely different from that led by the members of the last division. The Anomoura^ as their name imports, have tails of very unusual conformation : instead of being en- cased in a hard coat of mail as in the Macroura, the hinder part of the body is soft and coriaceous, possessing only a few detached calcareous pieces, analogous it is true to those found in the lobster, but strangely altered in structure. These animals (Jig- 153), usually known by the name of Soldier-Crabs or Hermit-Crabs, frequent level and sandy shores, Fig. 153. and, from their defenceless condition, are obliged to resort to artificial protection. This they do by selecting an empty tur- binated shell of proportionate size, deserted by some gasteropod mollusc, into which they insinuate their tail ; and, retreating CRUSTACEA. 325 within the recesses of their selected abode, obtain a secure retreat, which they drag after them wherever they go, until, by growing larger, they are compelled to leave it in search of a more capacious lodging. The wonderful adaptation of all the limbs to a residence in such a dwelling cannot fail to strike the most incurious ob- server. The chela, or large claws, differ remarkably in size ; so that, when the animal retires into its concealment, the smaller one may be entirely withdrawn, while the larger closes and guards the orifice. The two succeeding pairs of legs, unlike those of the lobster, are of great size and strength ; and, instead of being terminated by pincers, end in strong pointed levers, whereby the animal can not only crawl, but drag after it its heavy habit- ation. Behind these locomotive legs are two feeble pairs, barely strong enough to enable the soldier-crab to shift his position in the shell he has chosen ; and the false feet attached to the abdomen are even still more rudimentary in their deyelopement. But the most singularly altered portion of the skeleton is the fin of the tail, which here becomes transformed into a kind of holding ap- paratus, by which the creature retains a firm grasp upon the bottom of his residence. Fig. 154. In the Brachyura,) or Crabs, we have at once, in the concentra- tion observable in all parts of the skeleton, an indication of its CRUSTACEA. being formed for progression on land, or, at least, for creeping at the bottom of the sea. The tail, the great instrument of loco- motion in the lobster, is here reduced to a rudiment, and the fin at its extremity entirely obliterated ; the chela still continue to be the most powerfully developed of the extremities ; while the legs, the principal locomotive agents, are either terminated by simple points, as in those species which are most decidedly ter- restrial in their habits, or else, in the swimming crabs, the pos- terior pair become expanded into flattened oars useful in nata- tion (fig. 154). (365.) From the extreme hardness and unyielding character of the tegumentary skeleton in Crustaceans, a person unacquainted with the history of these animals would be at a loss to conceive the manner in which their growth could be effected. In insects we have seen that all increase of size occurs prior to the attainment of the perfect condition, and expansion is provided for by the moults or changes of" skin which take place during the develope- ment of the larva ; but the Crustacean, having acquired its mature form, still continues to grow, and that until it acquires in many instances a size far larger than that which any insect is permitted to arrive at. The plan adopted in the case before us, whereby growth is permitted, is attended with many extraordinary phenomena. At certain intervals the entire shell is cast off, leaving the body for the time unfettered indeed as regards the capability of expansion, but comparatively helpless and impotent, until such time as a new shell becomes secreted by the dermis, and by hardening assumes the form and efficiency of its predecessor. We are indebted to Reaumur,* who watched the process in the Cray-fish (Astacusfluviatilis)^ for what little is known con- cerning the mode in which the change of shell is effected. In the animal above mentioned, towards the commencement of autumn, the approaching moult is indicated by the retirement of the cray- fish into some secluded position, where it remains for some time without eating. While in this condition, the old shell becomes gradually detached from the surface of the body, and a new and soft cuticle is formed underneath it, accurately representing of course all the parts of the old covering which is to be removed ; but as yet little calcareous matter is deposited in the newly formed integument. The creature now becomes violently agitated, and * Me"m. de la Acad. des Sciences, 1718. CRUSTACEA. 327 by various contortions of its body seems to be employed in loosening thoroughly every part of its worn-out covering from all connection with the recently secreted investment. This being accomplished, it remains to extricate itself from its imprisonment ; an operation of some difficulty ; and, when the nature of the ar- mour to be removed is considered, we may well conceive that not a little exertion will be required before its completion. As soon as the old case of the cephalo-thorax has become quite detached from the cutis by the interposition of the newly formed epidermic layer, it is thrown off in one piece after great and violent exertion ; the legs are then withdrawn from their cases after much strug- gling ; and, to complete the process, the tail is ultimately by long- continued efforts extricated from its calcareous covering, and the entire coat of mail which previously defended the body is discarded and left upon the sand. The phenomena which attend this reno- vation of the external skeleton are so unimaginable, that it is really extraordinary how little is accurately known concerning the nature of the operation. The first question which presents itself is, how are the limbs liberated from their confinement ? for, won- derful as it may appear, the joints even of the massive chela of the lobster do not separate from each other, but, notwithstanding the great size of some of the segments of the claw, and the slender dimensions of the joints that connect the different pieces, the cast-off skeleton of the limb presents exactly the same appearance as if it still encased the living member. The only way of ex- plaining the circumstance, is to suppose that the individual pieces of the skeleton, as well as the soft articulations connecting them, split in a longitudinal direction, and that, after the abstraction of the limb, the fissured parts close again with so much accuracy that even the traces of the division are imperceptible. But this is not the only part of the process which is calculated to excite our astonishment : the internal calcareous septa from which the mus- cles derive their origins, and the tendons whereby they are inserted into the moveable portions of the outer shell, are likewise stated to be found attached to the exuvise ; even the singular dental apparatus situated in the stomach, of which we shall speak here- after, is cast off and re-formed ! And yet, how is all this accom- plished ? how do such parts become detached ? how are they renewed ? We apprehend that more puzzling questions than these can scarcely be propounded to the physiologist, nor could more interesting subjects of inquiry be pointed out to those CRUSTACEA. whose opportunities enable them to prosecute researches connected with their elucidation.* (366.) The structure of the articulations which unite the differ- ent segments of the skeletons of the Articulata, and the general ar- rangement of their muscular system, have already been described ; and, in the class before us, these parts of their economy offer no peculiarities worthy of special notice. (367.) Throughout all the Crustacean families the alimentary canal exhibits great simplicity of arrangement, and consists of a short but capacious resophagus, a stomachal dilatation or cavity in which is contained a singular masticatory apparatus, and a straight and * Since writing the above, I have been fortunate in procuring a very good specimen of Astacusfluviatilis, obtained soon after casting its shell, and also its newly cast-off covering, both of which are in excellent preservation. The following is a description of the appearances observed in each : All the pieces of the exuvium are connected together by the old articulations, and accurately represent the external form of the complete animal j the carapace, or dorsal shield of the cephalo-thorax, alone being de- tached, having been thrown off in one piece. The pedicles of the eyes and external corneae, as well as the antennae, remain in situ, the corresponding parts having been drawn out from them as the finger from a glove ; and no fissure of the shell or rupture of the ligaments connecting the joints is anywhere visible in these portions of the skele- ton. The auditory tubercles, and the membrane stretched over the orifice of the ear, occupy the same position as in the living cray-fish. The jaws, foot-jaws, and ambu- latory feet retain their original connections, with the exception of the right chela, which had been thrown off before the moult began ; and the segments of the abdomen, false feet, and tail-fin exactly resembled those of the perfect creature; even the internal processes derived from the thoracic segments (apodemata) rather seemed to have had the flesh most carefully picked out from among them, than to have been cast away from a living animal : but perhaps the most curious circumstance observable was, that attached to the base of each leg was the skin which had formerly covered the branchial tufts, and which, when floated in water, spread out into accurate representations of those exquisitely delicate organs. No fissure was perceptible in any of the articulations of the small claws j but in the chela each segment was split in the neighbourhood of the joints, and the articulating ligaments ruptured. The lining membrane of the stomach was found in the thorax, having the stomachal teeth connected with it ; from its position, it would seem that the animal had dropped it into the place where it lay before the extrication of its limbs was quite accomplished. The internal tendons were all attached to the moveable joint of each pair of forceps, both in the chela and in the two anterior pairs of smaller ambulatory legs. On examining the animal, which had extricated itself from the exuvium described above, the shell was found soft and flexible, but contained a sufficiency of calcareous matter to give it some firmness, especially in the claws. The tendons of the forceps were still perfectly membranous, presenting a very decided contrast when compared with the old ones affixed to the discarded shell. The stump of the lost chela had not as yet begun to sprout, and the extremity was covered by a soft black membrane. The jaws were quite hard and calcified, as likewise were the teeth coutained in the sto- mach. CRUSTACEA. 329 simple intestinal tube, which passes in a direct line from the sto- mach to the last segment of the abdomen, where it terminates. The description of these parts, as they exist in the lobster, will give the reader a sufficiently correct idea of their general disposition and structure ; nor are we acquainted with any class of animals in which so little variety in the conformation of this portion of the system is to be met with. The oesophagus is covered at its origin by the several pairs of foot-jaws already alluded to ; the most internal of which forms a decided cutting apparatus, resembling a pair of strong shears, while the rest are only instruments of prehension, or, perhaps, of sensation also. From the mouth, the oesophagus runs directly up- wards to the stomach, which is a considerable viscus (Jig. 157, a), a large portion of it being situated in that region of the cephalo- thorax which we should be tempted to consider as the head of the animal. The pyloric extremity of the stomach is strengthened with a curious frame-work of calcareous pieces imbedded in its walls, and so disposed as to support three large teeth placed near the orifice of the pylorus ; and, being moved by strong muscles, teeth so disposed, no doubt, form an efficient apparatus for bruis- ing the food before it is admitted into the intestine. The intestine itself (, b, I) runs in a direct course to the tail, imbedded between the two great lateral muscular masses that move the abdominal segments ; and terminates upon the ventral surface of the central lamella of the terminal fin in a rounded ori- fice closed by a sphincter muscle. The liver (c, c, c), one half of which has been removed in the engraving, consists of two large symmetrical masses, enclosing be- tween them the pyloric portion of the stomach, and a third part of the length of the intestine. When unravelled, the minute struc- ture of the liver exhibits an immense assemblage of secerning caeca agglomerated into clusters, from each of which a duct emanates, and the continued union of the ducts so formed ultimately gives origin to the common hepatic canal (d), which pours the bile de- rived from that division of the liver to which it belongs into the intestine at a very short distance from its commencement at the pylorus. A little below the insertion of the two bile-ducts, a so- litary long and slender caecum enters the intestine, but the nature of the secretion furnished by this organ is unknown. (368.) Before tracing the course of the circulation in the Crus- tacea, it will be necessary to consider the character of the apparatus - i nri i 332 CRUSTACEA. The branchial chambers are in free communication with the ex- ternal medium by means of two large apertures, through one of which the water enters, while it as constantly flows out through the other. The afferent canal is generally a wide slit that allows the water freely to penetrate to the interior of the branchial cavity; but the passage whereby the respired fluid escapes after passing over the branchiae is provided with a valvular apparatus so disposed as to produce a continual current in the water contained in the chamber, and thus, by insuring its perpetual agitation, effectually provides for its constant renewal. The mechanism is as follows : The aperture by which the water issues is in the neighbourhood of the mouth, and is closed by a broad semi-membranous plate (flabel- lum) derived from the root of the second pair of foot-jaws ; so that every motion of these foot-jaws impresses a corresponding move- ment upon the valve-like flabellum, and in this manner urges on the passage of the water out of the cavity in which the branchiae are lodged. But there are other means whereby the action of the limbs is made to assist in the perfection of the respiratory process. Thus, in the lobster, the third pair of foot-jaws, and each pair of ambula- tory legs, except the last, supports a flabelliform plate (Jig. 159, n); the movements of which must likewise keep the fluid respired in a state of agitation, and moreover, by gently squeezing and com- pressing the respiratory tufts, powerfully contribute to the per- fect renovation of the water in contact with the surfaces of the branchi*. In the crab genera the arrangement is slightly mo- dified, for here there are three flabella derived ex- clusively from the roots of the foot-jaws (Jig. 156, b, c, d) : of these, two are im- bedded among the bran- chiae ; while the third, as represented in the figure, extends in a crescentic form over the external surface of the whole series of those organs. The end answered in this case is obviously the same as that accom- CRUSTACEA. plished in the lobster, in a different, and, perhaps, more efficient manner. Fig. 157. (370.) In the lowest Crustacea the heart is a long dor- sal vessel, not very dissimilar in form and disposition from that of insects; but of course giving off arteries for the distri- bution of the blood, and receiving veins through which the blood, having ac- complished its cir- cuit, is returned. In tie Decapoda the organ becomes more centralized, and in the lobster (Jig. 157, e) the heart is found to be an oval viscus, situated in the mesial line of the body, beneath the posterior part of the cephalo-tho- rax; it is composed of strong muscular bands, and contains a single cavity of considerable size. The contractions of this heart are very vigorous, and may readily be witnessed by raising the super- jacent shell in the living animal. Several large arte- 334 CRUSTACEA. ries are derived form the above-mentioned simple heart. A consider- able trunk (Jig. 157, g,) Fig. 158. goes from its anterior extremity to supply the eyes, antennae, stomach, and neighbouring or- gans : another, the he- patic (i), which is sometimes double, sup- plies the two lobes of the liver : a third large vessel (A) supplies the abdominal or caudal re- gion : and a fourth, the sternal , derived from the posterior apex of the heart, bends down to the ventral aspect of the body, where it di- vides ; the posterior di- vision (/, /) supplying the lower parts of the abdomen, while the an- terior and larger divi- sion (m) gives off branches to the legs and foot-jaws (w, n, n, w); it likewise furnishes other vessels (o, o, o, o) which are distribut- ed through the bran- chiae. The venous system is made up of large and delicate sinuses that communicate freely with each other, and receive the blood from all parts of the body. Those of the dorsal re- gion are represented in the annexed figure: (.fig. 158), a large CRUSTACEA. 335 venous sinus (a) occupies the cephalic region, and covers the sto- mach ; another cavity (b) lies immediately above the heart ; and a series of smaller chambers (c, c, c, c) are situated above the muscles of the caudal region. These cavities, notwithstanding their appa- rent extent, are very shallow ; so that, upon a transverse section, their dimensions are by no means so great as a superficial view would indicate. The sinus (Z>), or that placed immediately over the heart, communicates with that viscus by short trunks, the termi- nations of which in the heart are guarded by valves (fig. 157, /,/,/) so disposed as to allow the blood to pass from the sinus into the heart, but prevent its return in an opposite direction. (371.) Such is the apparatus provided in the lobster for the cir- culation of the blood. Our next inquiry must be concerning the course that it pursues during its circuit through the body. Messrs. Audouin and Milne Edwards,* after very minutely examining this subject, came to the conclusion that the heart is purely of a systemic character, being only instrumental in propel- ling the blood through the body, but having nothing to do with the branchial circulation ; they conceived that the circulating fluid, having been collected in the venous sinuses, was brought to the roots of the branchiae, over which it was distributed by venous tubes, and then returned to the heart by vessels which they call branchio-cardiac to recommence the same course. The appended figures, however, which are accurately copied from engravings of the Hunterian drawings in the collection of the Royal College of Surgeons,-)- wou ^ seem to gi ye great reason to doubt the accuracy of the conclusions arrived at by the eminent naturalists referred to ; and to show that the heart, instead of being purely systemic, is partly branchial, and impels the blood, not through the body only, but also to the respiratory organs. This view of the subject, which we are disposed to consider as the most correct, is exhibited in the diagram annexed. Setting out from the heart, we find that the blood goes to all parts of the body through the different arterial trunks, and by the great sternal artery (Jig. 157, k) is con- veyed to the legs, foot-jaws, and false feet. But from this same artery (m), vessels, o, o, o, o, are furnished to the branchiae. The branchial arteries so derived (Jig. 159, g) subdivide into secondary * Recherches Anatomiques et Physiologiques sur la Circulation dans les Crustaces. Annales des Sciences Nat. tora.ii. t Catalogue of the Physiological Series of Comparative Anatomy contained in the Museum of the Royal College of Surgeons ; vol. ii. 336 CRUSTACEA. trunks (h, h, h), which ramify through the individual branchise, and supply all their appended filaments. Having undergone exposure to the respired medium, the blood is again collected from the branchiae by branchial veins (&, k, k) represented on the opposite side of the body, and conveyed by the large vessel, /, to the dorsal sinus (Jig. 158, 5), where, being mixed up with the general mass of blood contained in the sinus, the circulating fluid is admitted into the heart through the valvular orifices (d, d), to recommence the same track. (372.) As might be anticipated from an examination of the ex- ternal configuration of the different families comprised in the exten- sive class we are now considering, the nervous system is found to pass through all those gradations of developement which we have found gradually to present themselves as we have traced the Homo- gangliata from the lowest to the most highly organized types of structure. In the most imperfect Crustacea, indeed, we find a simplicity of arrangement greater than any hitherto pointed out even in the humblest Annelida ; a disposition of parts which the- oretically might have been expected to exist, but has only been distinctly recognised in the class before us. We have all along spoken of the nervous centres of the Arti- culata as arranged in symmetrical pairs, although in no example CRUSTACEA. 337 which lias yet occurred to our notice have we been able strictly to point out the accuracy of such a view of the subject. The two lateral masses of the supra-cesophageal ganglion are found united into one brain in the humblest forms of annulose animals, and even in the ganglia forming the ventral series, although we might pre- sume each to be composed of two symmetrical halves, the divisions are most frequently so intimately blended, that their distinctness is not susceptible of anatomical demonstration. In some of the Crustacea, however, among those species which have the segments of their external skeleton most perfectly separate and distinct, the nervous system is found to present itself in such a condition that the division into lateral halves is perfectly evident ; and from this condition their progressive coalescence may be traced step by step until we arrive at a state of concentration as remarkable as that already noticed in the most elevated of the Arachnidans. It is to Milne Edwards and Audouin that we are indebted for the interesting particulars connected with this part of our sub- ject ; and the results of their investigations are of such great physiological importance,* that the following condensed ac- count of their labours cannot be omitted in this place. In Talitrus every pair of ganglia consists of two separate nuclei of nervous substance, united by a transverse band so disposed as to bring them into communication with each other, while an anterior and posterior nervous filament derived from each unites it with the preceding and following ganglia of the same side of the body ; even the encephalic mass is composed of two lateral portions united by a cord passing between them : all these pairs of ganglia, thir- teen in number, corresponding with the number of the segments of the body, are exact counterparts of each other both in size and figure, so that none seems to preponderate in energy over the rest ; but the anterior or encephalic pair alone communicates with the eyes and antennae, the only organs of the senses as yet discernible. In Oniscus Asellus a concentration of the elements composing the nervous system above described is discernible, and this is found to be indicated by incipient approximation, which takes place in two directions, one longitudinal, the other acting transversely. In the first place, the entire number of pairs of ganglia is reduced to ten, three pairs having become obliterated by coalescence ; and, moreover, while the central portions still consist of two lateral * Recherches Anatomiques sur le Systeme Nerveux des Crustacea. Annales des Sciences Nat. torn. xiv. z '338 CRUSTACEA. masses each, the first and last pairs are united into single ganglia. "As we rise to higher forms the coalescence still proceeds : all the pairs of ganglia soon become united in a transverse direction, and gradually the whole chain becomes shorter by the confusion of several pairs into larger and more powerful masses. In the Crab, which, from its terrestrial habits, holds a position among the Crustacea equivalent to that which Spiders occupy among other Articulata, this centralization is carried to the utmost extent ; and all the abdominal and thoracic ganglia become agglo- merated into one great centre, from which nerves radiate to the parts of the mouth and instruments of locomotion (Jig. 160). (373.) But this change pig. 160. in the condition of the nervous system is not only observ- able as we proceed from species to spe- cies, as they rise higher in the scale of developement ; si- milar phenomena are met with in watch- ing the progress of any individual be- longing to the more perfect families, as it advances from the embryo to its ma- ture condition. Thus in the Cray-fish (Astacus fluviatilis), Rathke* observed, that, when first perceptible, the nervous system consisted of eleven pairs of ganglia, perfectly distinct from each other, and situated on each side of the mesial line of the body. The six first pairs then unite transversely, so as to form as many single masses, from which the nerves of the mandibles and foot- jaws emanate ; while the five posterior, from which the nerves of the ambulatory extremities are given off, remain separate. Such is the state at birth, or on leaving the egg ; but further changes occur before the Cray-fish arrives at maturity. The four anterior gan- glia, which supply nerves to the mandibles and foot-jaws, are, by degrees, all consolidated into one mass, and the fifth and sixth likewise coalesce, while the other pairs continue permanently dis- * Untersuchungen liber die Bildung des Flusskrebses in the Annales des Sciences Nat. torn. xx. CRUSTACEA. 339 tinct. The reader will at once recognise the resemblance be- tween these changes and those already described as taking place during the progress of evolution in the caterpillar: the same great law is, in fact, in operation in both cases, and the same results are obtained from the completion of the process.* From a review of the above facts, Milne Edwards and Au- douin arrived at the following conclusions : 1st. That the ner- vous system of Crustacea consists uniformly of medullary nuclei (ganglions), the normal number of which is the same as that of the segments or rings of the body. #. That all the modifications encountered, whether at different periods of the developement or in different species of the series, depend especially on the more or less complete approximation of these nuclei, and to an arrest of developement in some of their number. 3. That approximation takes place from the sides towards the mesian line, as well as in a longitudinal direction. Fig. 161. (374.) In the Crab the distribution of the nerves is briefly as follows : The encephalic mass, or brain, which still occupies its * For a minute account of the arrangement of the nervous system in these animals, the reader is referred to the Cyclop, of Anat. and Phys. art. CRUSTACEA ; by Dr. Milne Edwards. z 2 340 CRUSTACEA. position above the oesophagus, and joins the abdominal centre by two long cords of connection (j%. 161), gives off nerves to the eyes and muscles connected with them, as well as to the antennae and neighbouring parts. Near the centre of each division of the nervous collar that surrounds the oesophagus is a ganglionic enlargement, from which arises a nerve that runs to the mandibles, and also a very import- ant branch, apparently the representative of the nervus vagus of insects. This, after ramifying largely upon the coats of the sto- mach, joins that of the opposite side ; and, assuming a ganglionic structure, is ultimately lost upon the intestine. The nerves of the extremities, derived from the central abdo- minal ganglion, are represented in the preceding figure (fig. 161), which requires no explanation.* (375.) We have already ($ 313), when describing the nervous system of insects, hinted at the probable existence in the HOMO- GANGLIATA of distinct tracts of nervous matter in the composition of the central chain of ganglia, and in the filaments whereby they are connected with each other : reasoning therefore from analogy, it seems fair to presume that, if this be the case, such tracts corre- spond with the sensitive and motor columns which have been dis- tinctly proved to exist in the spinal axis of vertebrate animals. It is to Mr. Newport that we are indebted for the first indication of this interesting fact ;^ and the accuracy of his observations is readily demonstrable by a careful examination of the ganglionic chain of the lobster and other large Crustacean species. Each ganglionic enlargement is, upon close inspection, clearly seen to consist of two portions ; first of a mass of cineritious nervous sub- stance forming the inferior aspect of the ganglion, and of a cord of medullary or fibrous matter which passes over the dorsal or superior aspect, and appears to be distinct from the grey substance over which it passes : supposing, therefore, the longitudinal chain to consist of anterior and posterior fasciculi, as in the medulla spinalis, we have the anterior columns communicating with grey substance, while the posterior are unconnected therewith, but are continued over the ganglion instead of becoming amalgamated with its substance. Another fact, which favours Mr. Newport's view of this subject, is derived from an examination of the manner in which the nerves given off from the central axis take their origin ; * Vide Swan ; Comparative Anat. of the Nervous System. London, 4to. t Phil. Transact. 1834. CRUSTACEA. 341 for some of them undoubtedly proceed from the cineritious portion of the ganglionic swelling, while others, derived from the upper column, not only have no connection with the grey matter, but arise at some distance from the ganglion (fig. 138) : judging, therefore, by the laws at present established in physiology, there seems reason to suppose that the anterior or rather inferior fasciculi are connected with sensation, while the superior constitute the motor tract. The reader who is conversant with human physiology will at once perceive that this arrangement is precisely the reverse of that met with in man and other VERTEBRATA : and this consi- deration, apparently of little importance, has given rise to a va- riety of curious speculations ; some anatomists having even gone so far as to assert that all the organs of articulated animals are in reality placed in a similar inverted position. (376.) A more interesting inquiry connected with this part of our subject is, concerning the extent to whrch the ARTICULATA are susceptible of pain. Is it really true in philosophy, as it has become a standing axiom in poetry, that " the poor beetle, that we tread upon, In corporal sufferance feels a pang as great As when a giant dies" ? This is a question upon which modern discoveries in science entitle us to offer an opinion, and the result of the investigation would seem to afford more enlarged views relative to the benefi- cence displayed in the construction of animals than the assertion of the poet would lead us to anticipate. Pain, " Nature's kind harbinger of mischief," is only inflicted for wise and important purposes, either to give warning of the existence of disease, or as a powerful stimulus prompting to escape from danger. Acute perceptions of pain could scarcely, therefore, be supposed to exist in animals deprived of all power of remedying the one or of avoiding the other. In man the power of feeling pain indubitably is placed exclusively in the brain ; and, if communication be cut off between this organ and any part of the body, pain is no longer felt, whatever mutilations may be inflicted. The medulla spinalis, which, as we shall see hereafter, corre- sponds to the ventral chain of ganglia in articulated animals, can perceive external impressions and originate motions, but not feel pain ; hence we may justly conclude that in the Homogangliata, likewise, the supra-oesophageal ganglia, the representatives of the 342 CRUSTACEA. brain, and the sole correspondents with the instruments of the higher senses, are alone capable of appreciating sensations of a painful character. Thus, then, we arrive at a very important con- clusion, namely, that the perception of pain depends upon the developement of the encephalic masses ; and consequently, that, as this part of the nervous system becomes more perfect, the power of feeling painful impressions increases in the same ratio : or, in other words, that, inasmuch as the strength, activity, and intel- ligence of an animal, by which it can escape from pain, depends upon the perfection of the brain, so does the perception of torture depend upon the condition of the same organ. How far the feel- ing of pain is acutely developed in the animals we are now consi- dering is deducible from every-day observation. The fly seized by the leg will leave its limb behind, and alight with apparent unconcern to regale upon the nearest sweets within^ its reach : the caterpillar enjoys, to all appearance, a tranquil existence while the larvae of the Ichneumon, hatched in its body, devour its very viscera : and in the Crustacea before us, of so little import- ance is the loss of a leg, that the lobster will throw off its claws if alarmed by the report of a cannon. (377.) The singular power of breaking off their own limbs, alluded to in the last paragraph, is possessed by many Crustacea, and is a very indispensable provision in their economy. We have already found the blood-vessels of these animals to be of a delicate structure ; and, the veins being wide sinuses whose walls possess little contractility, the fracture of a limb would inevitably produce an abundant and speedily fatal haemorrhage was there not some con- trivance to remedy the otherwise unavoidable results of such a catas- trophe. Should the claw of a lobster, for example, be accidentally damaged by accidents to which creatures encased in such brittle armour must be perpetually exposed, the animal at once breaks off the injured member at a particular part, namely, at a point in the second piece from the body ; and by this operation, which seems to produce no pain, the bleeding is effectually staunched. But the most remarkable part of the phenomenon remains to be noticed : after this extraordinary amputation has been effected, another leg begins to sprout from the stump, which soon grows to be an efficient substitute for the lost extremity, and gradually, though slowly, acquires the pristine form and dimensions of its pre- decessor. A beautiful example of this curious mode of reprodu- cing a lost organ is preserved in the Museum of Comparative Ana- CRUSTACEA. 343 tomy in King's College, London, in which the new limb (one of the cheliferous claws) has already attained the form of the old chela, but still remains soft and uncovered by calcareous integument. The pro- cess of reproduction is as follows : The broken extremity of the second joint skins over, and presents a smooth vascular membrane, at first flat, but soon becoming conical as the limb begins to grow. As the growth advances, the shape of the new member becomes apparent, and constrictions appear, indicating the position of the articulation ; but the whole remains unprotected by any hard cover- ing until the next change of shell, after which it appears in a pro- per case, being, however, still considerably smaller than the cor- responding claw on the opposite side of the body, although equally perfect in all its parts. (378.) The observations made in a former chapter relative to the organs by which the senses of touch, taste, and smell are exercised in insects, are equally applicable to the animals composing the class before us ; for in the Crustacea, although we are compelled to admit the possession of the above faculties, we are utterly ignorant of the mode in which they are exercised, and therefore it would be only an unprofitable waste of time to enter at any length into a discussion from which no satisfactory conclusions are, in the pre- sent state of our knowledge, to be deduced. (379.) The eyes of Crustaceans are of three kinds, simple, ag- glomerated, and compound. The simple eyes (ocelli, stemmata) resemble those of spiders, and, like them, are said to consist of a cornea, a spherical lens, a gelatin- ous vitreous humour, a retina and deeply-coloured choroid, all occu- pying their usual relative positions. These eyes never exceed two or three in number. In the agglomerated eyes, such as those of Daphnia (Jig. 155), the organ seems to be composed of a number of simple eyes placed behind one common cornea ; such eyes are moveable, and, in the animal depicted in the figure, the muscles acting upon the visual apparatus, which in this case is single, are arranged so as to form a cone the base of which is formed by the eye and may be distinctly seen under a good microscope. The compound eyes appear to be constructed upon the same prin- ciples as those of insects. The cornese are extremely numerous and generally hexagonal ; but sometimes, as in the lobster, they are square. The vitreous humours equal the cornese in number, and behind each of these a distinct retina would seem to be expanded. 344 CRUSTACEA. The compound eyes of Crustaceans have not, however, as yet been examined with the same patient diligence as those of the cock- chaffer ; so that, as relates to their minute anatomy, much is still left to conjecture and uncertainty. One peculiarity connected with these organs is, that in the two highest orders of Crustacea, hence called Podophthalmia, the eyes are placed at the extremity of moveable pedicles articulated with the first cephalic ring of the ex- ternal skeleton, and thus they may be turned in various directions without moving the whole body at the same time. This provision was not required in insects, owing to the mobility of the head in those animals; but is absolutely indispensable in the case before us, where, the head and thorax being consolidated into one mass, the extent of vision commanded by sessile eyes would have been ex- ceedingly limited, and inadequate to the security of creatures ex- posed to such innumerable enemies. (380.) It is in the higher Crustacea that we, for the first time, indubitably find a distinct auditory apparatus ; and, from the simplicity which the organ of hearing presents in this its earliest appearance, an inquiry concerning its structure becomes of great physiological interest. In the lobster the ears are situated upon the under surface of the basal joints of the second pair of antennae. On looking carefully in this situation the student will find a pro- minent tubercle formed by the shell, the top of which is perforated by a small circular opening covered with a tense membrane. Be- hind this orifice is placed a minute vesicle filled with fluid, upon which a delicate branch of the antennary nerve is distributed. This constitutes the whole apparatus : the vibration of the water strikes upon the external membrane, the water in the sacculus participates in the tremor, and the expanded nerve conveys to the brain the sensation thus produced. In the Brachyura, or crabs, the membrane covering the external orifice of the ear is converted into a moveable calcareous lamella, from which, in some genera, a furcate process is continued inter- nally ; so that the whole, when removed by maceration, has no very distant resemblance to the stapes of the human ear, and, like it, seems to be acted upon by muscular fasciculi, so disposed as to regulate the tension of the vibratile membrane, and thus adapt it to receive impressions of variable intensity. (381.) One of the first circumstances calculated to attract the notice of the anatomist who turns his attention to the structure of the generative system both in male and female Crustacea, is the CRUSTACEA. 345 complete separation which exists between the organs belonging to the two sides of the body; for not Fig. 162. only are the internal secret- ing viscera for the most part perfectly distinct from each other, but even the external sexual orifices are equally se- parate and unconnected. (382.) Beginning with the parts observable in the male, we will take the cray-fish ( As- tacusjluviatilis)as a standard of comparison, and briefly notice the principal variations from the type of structure, observable in that species, met with in other genera. In the cray-fish and also in the lobster, the secerning organs or testes, when exa- mined in situ, are found to occupy the dorsal region of the thorax, lying upon the posterior part of the stomach. Examined superficially, the testes would seem to form but one mass consisting of three lobes (fig. 162, a, a, b) ; but, on investi- gating the minute structure of the organ, it is found to be made up of very delicate -secreting tubes that give origin to two excretory ducts (c, c). After numerous convolutions, which form a kind of epididymis (c?), each duct, becoming slightly dilated, terminates by a distinct orifice (/), seen upon the basal articulations of the last pair of ambulatory legs. There is no intromittent apparatus visible ; but, according to Milne Edwards,* the extremity of the excretory duct, by undergoing a kind of tumefaction, may be protruded exter- nally, so as to become efficient in directing the course of the fecun- dating fluid. In crabs the mass of the testis is exceedingly large, but in its essential structure similar to that of the cray-fish, and the external opening of its excretory duct is found to occupy the same situation : in some genera, however, instead of being placed upon the first joint of the last pair of legs, the orifices of the male organs are found upon the abdominal surface of the last thoracic ring itself. * Cyclop, of Anat. and Phys. art. CRUSTACFA. 346 CRUSTACEA. (383.) The female generative organs of Crustacea very accurately resemble those of the male ; and in the unimpregnated condition it is not always easy, from a superficial survey of the internal viscera, to determine the sex. In Astacus Fig. 163. Jluviatilis, the ovaria (Jig. 163, a) occupy a position analogous to that of the male testis, and a simple canal derived from each side (6, c) conducts the eggs to the external apertures found upon the first joint of the third pair of legs. In crabs an important addition is made to the female generative system : -prior to the termination of each oviduct it is found to com- municate with a wide sacculus, the function of which is apparently ana- logous to that of the spermatheca of insects ( 328), inasmuch as it seems to form a receptacle for the fecundating secretion of the male, in which the seminal fluid re- mains ready to impregnate the ova as they successively pass its orifice during their expulsion from the body. It is not precisely known in what manner copulation is effected by these animals ; neither, indeed, is it positively ascertained in many species whether the ova are impregnated prior to their expulsion or afterwards, although the latter supposition seems by far the most probable. (384.) The eggs are almost invariably carried about by the female until they are hatched, and in order to effect this various means are provided. In the Decapoda they are fastened by a stringy secretion to the false feet under the abdomen, and a female crab may generally be readily distinguished from a male of the same species by the greater proportionate size of this part of their body. In Asellus, a small Crustacean very common in stagnant water, the male may be observed during the breeding season to carry the female about with him for many days ; after which her eggs are found impregnated, and enclosed in a membranous sac placed under the thorax, from which when the young are hatched they escape through a longitudinal fissure provided for the purpose. In many genera, broad laminse, or scaly plates, are found upon the under surface of the body, beneath which the eggs are lodged. CRUSTACEA. 347 The more minute Crustacea, or Entomostraca, as they are called by zoologists, in their mode of reproduction, offer several remarkable variations from what has been described above ; and a brief account of their most interesting peculiarities is therefore still wanting to complete this part of our subject. These little creatures, in fact, seem to form a transition between the class we are now considering and the Epizoa, which many of them re- semble so nearly that they are still confounded together by many authors. The female Entomostraca frequently carry their ova in two transparent sacculi attached to the hinder part of the body, and it is in these egg-bags that the oviducts terminate ; so that the ova, as they are formed, are expelled into the singular re- ceptacles thus provided. Without such a provision, indeed, it would be difficult to conceive how the ova could possibly remain attached to the parent, as they far surpass in their aggregate bulk the size of her entire body, and could not, therefore, by any con- trivance be developed internally without bursting the crustaceous covering that invests the mother. J urine,* Ramdohr,*)" and other authors, have carefully watched the generative process in several genera, and brought to light many important and curious facts connected therewith. In Cyclops, a species to be met with in every ditch, the impregnation of the ova is undoubtedly effected in the body of the parent, and the eggs when formed are expelled into two oval sacs placed on each side of the tail, which Jurine calls external ovaries. The number of eggs contained in these sacs gradually increases, and they exhibit a brown or deep red colour, until a short period before the growth of the embryo is completed, when they become more transparent. In about ten days the eggs are hatched and the young escape ; but such is the prodigious fertility of these little beings, that a single female will, in the course of three months, produce ten successive families, each con- sisting of from thirty to forty young ones. In the genus Apus, another plan is resorted to for the protec- tion of the ova: the eleventh pair of legs, called by Schcefer\. " womb-legs," have their first joints expanded into two circular valves, which shut together like a bivalve shell, and thus form a receptacle in which the eggs are contained until they arrive at maturity. * Histoire des Monocles. 1 vol. 4to. Gen. 1820. f- Materiaux pour 1'Histoire de quelques Monocles A Demands. 4to. 1805, $ Apus pisciformis, insecti aquatic! species noviter delectae. 4to. Ratisbonne, 1757. 348 CRUSTACEA. In Daphnia (Jig. 155) the ovariaare easily distinguished through the exquisitely transparent shell, especially when in a gravid state ; and the eggs after extrusion are lodged in a cavity situated be- tween the shell and the exterior of the body, where they remain until the embryo attains its full growth. (385.) One fact connected with the reproduction of the Entomos- traca is so remarkable, that, had we not already had an instance of the occurrence of a similar phenomenon in the insect world (Aphides), the enunciation of it would cause no little surprise to the reader ; and, had its reality been less firmly substantiated by the concur- rent testimony of numerous observers who have witnessed it in many different genera (Cyclops, Daphnia, &c.), it might still be admitted with suspicion. In the genera above mentioned it has been ascertained by careful experiments that a single intercourse between the sexes is sufficient to render fertile the eggs of several (at least six, according to Jurine) distinct and successive gene- rations. Some authors have supposed, from the circumstance of all the individuals which have been met with belonging to some genera being females, that some of these little beings were hermaphrodite, or self-impregnating ; but such an opinion rests on very doubtful grounds, especially as there seems good reason to believe that in many instances the forms of the male and female of the same species are so different that they might easily be mistaken for totally distinct animals. (386.) The last point which we have to notice, in connection with the history of the Crustacea, is, the progress of their developement from the embryo condition to their mature state. This is a sub- ject which has given rise to considerable discussion, especially as relates to the changes which occur during the growth of the more highly organized forms ; some authors contending that they leave the egg complete in all their parts, and presenting their adult con- figuration, while others assert that they undergo changes so import- ant as only to be comparable with the metamorphosis of insects. Among the Entomostraca such changes have been again and again witnessed, and the appearances observed during their growth carefully recorded. From these observations very important results have been obtained, inasmuch as many forms previously described as distinct species have been found to be merely the same animal in different stages of developement. In Cyclops, for example, the newly hatched embryo possesses only four legs, and its body CRUSTACEA. 349 is round, having as yet no appearance of caudal appendages ; of young animals in this condition Muller had formed a distinct genus (Amymonc) :* in about a fortnight they get another pair of legs, and form the genus Nauplius of the same author. They then change their skin for the first time, and present the form of the adult, but with antennae and feet smaller and more slender than in the perfectly mature state. After two other changes of skin they become capable of reproduction. Many of the Entomostraca, as for example Daphnia, do not seem to undergo material alterations of form, but simply moult at certain intervals, throwing off their old integument and acquiring a new covering. Nevertheless, even in the Decapoda it is pretty certain that great metamorphoses take place in the external ap- pearance of the young animals, though many contradictory opi- nions concerning their nature are entertained by naturalists. Much confusion, indeed, still exists connected with this important subject. Cavolini long since announced that the embryo of Can- cer depressus exhibited at birth a singular and uncouth appear- ance, of which he gave a very tolerable representation;")" and Mr. Thompson, in a late number of the Philosophical Transactions, has rendered it certain that even in the developement of the com- mon crab, so different is the out- ward form of the newly-hatched embryo from that of the adult, that the former has been describ- ed as a distinct species, and even grouped among the ENTOMOS- TRACA, under the name of Zoea pelagica. On leaving the egg, according to the author alluded to, the young crab presents a cu- rious and grotesque figure (Jig. 164) : its body is hemispherical, and its back prolonged upwards into a horn-like appendage ; the feet are scarcely visible, with the exception of the two last pairs, which are ciliated like those of a Branchiopod, and formed for swimming. The tail is longer than Fig. 164. * Latreille, Regne Animal, vol. iv. t Sulla Generazione del Pesci edei Granchi. 4to. Naples, 1787. 350 CRUSTACEA. Fig. 165. the body, possesses no false feet ; and the ter- minal joint is crescent- shaped, and covered with long spines. The eyes are very large, and a long beak projects from the lower surface of the head. In a more advanced stage of growth the creature assumes a to- tally different shape, (Jig- 165,) under which form it has been known to naturalists by the name of Megalopa. The eyes become pe- dunculated, the cepha- lo-thorax rounded, the tail flat and provided with false feet, and the chelae and ambulatory extremities well developed. A subsequent moult gives it the appearance of a perfect crab ; and then only does the abdomen become folded under the thorax, and the normal form of the species recognisable (fig. 166), Fig. 166. 351 CHAPTER XVIII. HETEROGANGLIATA* (Owen) ; MOLLUSCA. (Cuv.) (387.) THE term MOLLUSCA, employed by Cuvierto designate the fourth grand division of the animal world, is obviously derived from a very unimportant circumstance of their organization, which the tribes included in it possess in common with innumerable forms both of Acrite and Nematoneurose beings, whose soft bodies are unsupported by any internal or tegumentary framework of sufficient density to merit the name of a skeleton. Subsequent anatomists have therefore, however unwillingly, been compelled to substitute another name for that given by the illustrious French zoologist to this extensive class, the boundaries and relations of which, as at present admitted, remaining precisely as they were first established by his patient and unwearied investigations relative to the anatomi- cal structure of the animals comprised within its limits. It is to the arrangement of the nervous system that we must again have recourse in order to discover a distinctive appella- tion ; nor in this shall we be disappointed, for here we at once find a character peculiar to this great section of animated nature, and generally applicable to the various classes composing it. All the Mollusca present nervous ganglia, which, in the more highly organ- ized forms, attain considerable developement and consequent per- fection ; but these nervous centres, instead of being arranged in a longitudinal series of symmetrical pairs, are variously distributed in different parts of the body ; an arrangement exactly correspondent to the want of symmetry observable both in the external configuration of these creatures, and in the anatomical disposition of their internal viscera. Still, however, one large ganglionic mass occupies a posi- tion above the oesophagus, and it is with this that the nerves of the existing senses invariably communicate ; so that we are naturally induced to regard this as the sentient brain, corresponding with the supra-eesophageal ganglion of the ABTICULATA both in position and office. The other ganglia vary considerably both in number and in situation, but, wherever placed, they all communicate with the supra-cesophageal mass ; while the branches derived from them are distributed to the viscera, or to the locomotive organs. (388.) Various are the forms, and widely different the relative s, dissimilar ; yyyX/v, a ganglion. 352 CIRRHOPODA. perfection of the Mollusca, as regards their endowments and capabili- ties. Some, as the Barnacles (CIRRHOPOUA), fixed to the surface of various submarine bodies, either immoveably or by the intervention of a flexible pedicle, entirely deprived of organs connected with the higher senses, and unable to change their position, are content to cast out at intervals their ciliated arms, which form a net of Na- ture's own contrivance, and thus entrap such passing prey as suits their appetite. Others, equally incapable' of locomotion, but fur- nished with arms of different construction, (BRACHIOPODA,) catch their food by similar efforts. The TUNICATA, enclosed in coria- ceous bags, are firmly rooted to the rocks ; or, aggregated into singular compound masses, float at the mercy of the waves. The CONCHIFERA inhabit bivalve shells ; while the GASTEROPOD or- ders, likewise defended in most cases by a shelly covering, creep upon a broad and fleshy ventral disc, and, thus endowed with a locomotive apparatus, exhibit senses of proportionate perfection. The PTE- ROPODA swim in myriads through the sea, supported on two fleshy fins ; while the CEPHALOPOD MOLLUSCA, the most active and highly organized of this large and important division of animated nature, furnished with both eyes and ears, and armed with formidable means of destroying prey, become tyrants of the deep, and gradu- ally conduct us to the most exalted type of animal existence. These different sections, which constitute, in fact, so many dis- tinct classes into which the HETEROGANGLTATA have been divided by zoologists, we shall now proceed to examine seriatim ; beginning, as heretofore, with the most imperfectly organized, and gradually tracing the developement of superior attributes and more exalted faculties as the nervous centres attain greater magnitude and con- centration. CHAPTER XIX. ClRRHOPOUA.* (389.) HOWEVER distinct in outward appearance, and even in their internal economy, the creatures composing the primary divisions of animated nature may seem to be when superficially examined, closer investigation invariably reveals to the zoologist gradations of structure connecting most dissimilar types of organization, and lead- * xtppof, a cirrus ; -rov;, -rolog, a foot. CIRRHOPODA. 353 ing so insensibly from one to another, that the precise boundary- line which separates them is not always easily defined. The CIR- RHOPODS, or Barnacles, upon the consideration of which we are now entering, present a remarkable exemplification of this import- ant fact ; and are found to be so strictly intermediate, both in external configuration, and even in their anatomical construction, between the HOMOGANGLIATA, which have recently occupied our attention, and the great class of beings that next presents itself for investigation, that these animals might, with almost equal pro- priety, be located either among the Articulated or Molluscous tribes of Invertebrata ; and it will not be surprising, if, after read- ing the details connected with their structure, some naturalists should prefer to regard them as belonging to the former rather than to the latter division. The CIRRHOPODA, indeed, present a strange combination of articulated limbs, united with many of the external characters of a Mollusk, as will be at once evident from the examination of any species of Barnacle whether sessile or pe- dunculated. We select a common form, Pentalasmis vitrea, as an example of the kind last mentioned. The animal in question is enclosed in a shell resembling in some respects that of the com- mon mussel, but composed of five distinct pieces, united together by a dense intervening membrane : of these, four pieces are lateral, and disposed in pairs ; while a fifth, which is single, is interposed between the posterior edges of the two valves, so as to unite them along the whole length of the back. Along the anterior margin the valves are only partially connected by membrane, so that a long fissure is left through which the articulated extremities may be protruded. In place of the hinge that joins the two shells of the mussel, we find the tough coriaceous membrane that unites the dif- ferent shelly pieces of the integument of Pentalasmis, prolonged into a cylindrical pedicle (Jig. 169, /), which is in some species many inches in length, and, being attached by its extremity to any submarine body, fixes the animal permanently to the same locality. The external layer of this pedicle is coriaceous or almost corneous in its appearance, being evidently an epidermic structure ; but, in- ternally, the tube is lined with a layer of strong muscular fibres arranged longitudinally (Jig. 169, m, n), which, by their contrac- tion, are no doubt able to bend the flexible stem in any given direction, and thus confer upon the animal a limited power of changing its position when necessary. On removing one half of the shelly covering, as in Jig. 167, <z, a, we expose the body of the 354 CIRRHOPODA. Cirrhopod, and discern the following particulars. The lower por- tion of the body, which encloses the principal viscera (6, Z>), is soft and much dilated, especi- Fig. 167. ally towards the dorsal re- gion ; this part of the ani- mal is covered with a de- licate membrane, beneath which is a layer of whitish granular substance. The mouth (g) is seen upon the ventral aspect, situated immediately at the inferior extremity of that longi- tudinal fissure in the man- tle through which the arms are protruded : the oral aperture appears to be raised upon a prominent tubercle, and, when atten- tively examined, is found to be provided with a ru- dimentary apparatus of jaws, presenting a distinct lip furnished with minute palpi, and three pairs of mandibles, of which the two external are horny and serrated, while the third remains permanently soft and membranous. Immediately behind the mouth we find on each side certain pyramidal fleshy appendages (d, d, d), resembling, as Hunter expressed it, a minute star-fish, which no doubt constitute the branchial or respiratory organs. Commencing above the mouth, we further notice on each side six pairs of articulated and flexible arms, or cirrhi (Jig. 167, c, c), each being composed of a series of semi-corneous pieces, and exhibiting at each joint long and stiff hairs. Every pair of cirrhi arises from a single prominent stem ; and those most distant from the mouth being the longest and most extensile, the whole appara- tus, consisting of twenty-four cirrhi, forms, when protruded from the body, a kind of net of exquisite contrivance, in which passing particles of nourishment are easily entangled, and thus conveyed to the mouth. Lastly, on separating the cirrhiferous pedicles, we find, terminating the body, and forming, as it were, a kind of tail, a long, soft, and flexible organ (Jig. 169, &), the extremity of which is CIRRHOPODA. 355 perforated by a minute aperture ; but the real nature of this instru- ment we shall examine by and by. (390.) On reviewing this general description of the external con- struction of Pentalasmis, the reader cannot but be struck with the singular combination of characters which it exhibits. Judging from its shell alone, its right to be considered as a Mollusk would seem to be at once demonstrable, for, in fact, most conchologists agree in claiming these animals as belonging to their own de- partment ; and yet, if after removing the shell we compare the animal with a Crustacean, its alliance with that class is equally evident. Suppose the body (Jig. 167, &, b) to Tepresent the thoracic portion of a Crustacean slightly bent upon itself, and enclosed in an extensively developed thorax ;* the valves of the shell would represent this thorax, which would be divided into five pieces ; the first pair of cirrhi arising from the body would then represent the true feet of a Crustacean ; the branchiae would occupy the same position in both ; the rest of the body of the Barnacle, namely, that which supports the five other pairs of feet, would represent the tail of the Crustacean, and the ciliated, nata- tory feet, generally connected with that part of the external skele- ton : even the mouth, as the author referred to might have added, with its triple series of jaws, is more nearly allied in structure to that of the Crustaceans Ffc.168. than to anything we shall meet with in the structure of the oral organs of true Mollusca. (391.) But the affinity which unites the Cirrhopoda to the Homogangliata is not merely exemplified in the analo- gies that can be pointed out between the external configuration of Pentalas- mis and some Crustacean forms ; the nervous system even, as we might be led to anticipate from the symmetrical arrangement of the articulated cirrhi, still exhibits the Homogangliate con- dition, and, besides the supra-cesopha- geal masses, forms a longitudinal chain of double ganglia arranged along the * Cuvier, MSmoire sur les Animaux des Anatifes et des Balanes, et sur leur Anatomic, p. 6. 2 A2 356 CIRRHOPODA. ventral surface of the body, from which the nerves supplying the cir- rhiferous arms take their origins. Four small tubercles (jig. 168),* placed transversely above the oesophagus, represent the brain, and give origin to four principal nerves (/?/,//), which are distribut- ed to the muscles and viscera, for in such a situation organs of sense would evidently be useless. Two lateral cords, derived from the above, surround the oesophagus, from each of which a nerve (o, o) is given off. Below the oesophagus the nervous collar terminates in a pair of ganglia (A), that gives origin to the nerves supplied to the first pair of arms ; and then succeeds a parallel series of double ganglia (i, A:,*/, m), exactly resembling those of articulated animals, from which nerves emanate that are destined to the cirrhi and sur- rounding parts. (392.) The muscular system of Pentalasmis is partly appropriat- ed to the movements of the shell, and partly to the general motions of the body. The shell is closed by a single transverse fasciculus of muscular fibres, whereof a section is seen at e,j#g\ 167, placed immediately beneath that fissure in the mantle through which the arms are protruded ; it passes directly across from one valve to the other, and approximates them by its contraction. A large muscle, whose origin is seen 'm jig. 167, f, arises from the interior of the mantle, and, as its fibres diverge, spreads over the entire mass of the viscera ; this will evidently draw the body forward, and cause the protrusion of the tentacula, while various muscular slips derived from it scarcely need further description, being destined to move the numerous arms with their jointed cirrhi and the fleshy tubular prolongation (jig' 169, A:) already noticed. (393.) The food devoured by the Cirrhopoda would seem to con- sist of various minute animals, such as small Mollusks and micros- copic Crustacea, caught in the water around them by a mechanism at once simple and elegant. Any one who watches the movements of a living Cirrhopod will perceive that its arms, with their appended cirrhi, are in perpetual movement, being alternately thrown out and retracted with great rapidity ; and that, when fully expanded, the plumose and flexible stems form an exquisitely beautiful apparatus, admirably adapted to entangle any nutritious molecules, or minute living creatures, that may happen to be present in the circum- scribed space over which this singular casting-net is thrown, and * Cuvier, Ice. cit. CIRRHOPODA. 357 drag tli em down into the vicinity of the mouth, where, being seized by the jaws, they are crushed and prepared for digestion. No sense but that of touch is required for the success of this singular mode of fishing ; and the delicacy with which the tentacula perceive the slightest contact of a foreign body, shows that they are eminently sensible to tactile impressions. As regards the digestive organs, we have already described the prominent mouth (Jig- 169, b) 9 with its horny palpiferous lip and three pairs of lateral jaws. The oesophagus (Jig. 169, c) is short, and firm in its texture ; it receives the excretory ducts of two salivary glands of considerable size (fig- 168, d, d), and soon terminates in a capacious stomachal receptacle, the walls of which are deeply sacculated and surrounded by a mass of glandular caeca (Jig. 169, d) that represent the liver, and pour their secretion through numerous wide apertures into the cavity of the stomach itself. The intes- tine (e,/) is a simple tube, and runs along the dorsal aspect of the animal, wide at its commencement, but gradually tapering towards its anal extremity ; it terminates at the root of the tubular prolongation (k) by a narrow orifice, into which a small bristle (g) has been inserted. (394.) Little is satisfactorily known relative to the arrangement of the blood-vessels and course of the circulation in these animals. Poli imagined that he had discovered a contractile dorsal vessel, intimating that he had perceived its pulsations in the vicinity of the anal extremity of the body ; and, although his observations upon this subject have not been confirmed by subsequent investigations, analogy would lead us to anticipate the existence of the heart in the position indicated by the indefatigable Neapolitan zootomist. The lateral appendages (Jig. 167, d, d, d) are most probably proper branchial organs, but, perhaps, not exclusively the instru- ments of respiration ; since the numerous cirrhi no doubt co-ope- rate in exposing the blood to the action of the surrounding medium, a function to which they are well-adapted by their struc- ture and incessant movements; especially, as each cirrhus is seen under the microscope to be traversed throughout its whole length by two large vascular trunks, one apparently arterial, and the other of a venous character. (395.) With respect to the organization of the reproductive sys- tem in these creatures, the most discordant opinions are expressed by different writers ; no two authors agreeing either concerning the names or offices which ought to be assigned to different parts of 358 CIRRHOPODA. the generative apparatus. It must therefore be our endeavour, in considering this part of their economy, to separate as far as practicable all conjecture and hypothetical reasoning from the simple facts which anatomy has placed at our disposal, and leave disputed questions to be solved by careful experiment and re- search. According to the dissection of John Hunter, the internal generative apparatus is double, occupying both sides of the ali- mentary canal. Covering the liver (Jig. 169, d), there is found a vascular substance, which Fig. 169. the above-named illustri- ous anatomist regarded as probably constituting the tubular parts of the testi- cle, from which a tortuous canal with very thick walls (vas deferens) runs up- wards, along the side of the intestine to the root of the fleshy prolongation A:, at which point it is joined by the correspond- ing tube from the oppo- site side of the body. The common canal thus formed is extremely slen- der, and passes in a flexu- ous manner through the whole length of the tubu- lar organ (&), named by Hunter, apparently for the sake of brevity, the penis, to terminate by a minute orifice at its extremity. Yet, notwithstanding the name applied to the termination of the sexual canals, Hunter was well convinced that the Cirripeds were hermaphrodites ; as he ex- pressly says,* " It is most probable that all Barnacles are of both sexes and self-impregnators ; for I could never find two kinds of parts, so as to be able to say, or even suppose, the one was a female, the other male." Cuvier found the vascular mass, considered by Hunter as being the tubular portion of the testis, to be composed of granules which * Descriptive and illustrated Catalogue of the Physical Series of Comp. Anat. in the Mus. of the Royal Coll. of Surgeons in London, vol. i. p. 259. CIRRHOPODA. 359 he deemed to be ova ; and conceived tlic delicate white vessel seen to ramify through the ovarian mass, as represented in the figure, to be the oviduct whereby the eggs were taken up and conveyed into the thick and glandular canal A, from the walls of which he imagined that a fecundating liquor might be secreted for the impregnation of the ova in transitu. He, therefore, re- garded the proboscidiform tube, Ar, as an ovipositor, whereby the ova derived from both sides of the body are expelled. Before scattering them abroad, as Cuvier noticed, the animal retains them for a considerable length of time concealed between the body and the mantle, where they form two or three irregularly shaped layers. When the eggs are found in this situation, he observed that the ovaria were empty and the testicles much less tumid, circumstances which indicate the season of oviposition to be at an end. In opposition to the views entertained by Cuvier concerning the generative process in the class before us, various continental writers consider the true ovary to be contained in the cavity of the tubular fleshy pedicle, which in Pentalasmis serves to fix the body to the substance whereunto it is attached. This, indeed, at certain periods, is found to be filled with oval granular bodies of regular shape, which are apparently real ova diffused through the loose cellulosity enclosed within it ; and these ova, being found in different states of maturity, are apparently secreted in the pedicle itself, although some authors contend that, having been formed and impregnated in the manner indicated by Cuvier, they are conveyed into this situation by the ovipositor, as upon this assumption the prolonged organ (fig- 169, k) would be named. Other anatomists, again, regard the instrument last mentioned as being a real penis, and suggest that from its length it might even be introduced into the peduncular cavity itself, and thus effect the impregnation of the ova contained therein. The observations of Mr. Thompson* relative to the progress of the ova after their escape from the pedicle, throw much addi- tional light upon this portion of our subject. " In the whole tribe of Cirripeds," says this industrious naturalist, " the ova, after their expulsion from the ovarium, appear to be conveyed by the ovipositor into the cellular texture of the pedicle, just beneath the body of the animal, which they fill to the distance of about an inch. When first placed in this position, they seem to be * Phil. Trans, for 1835, page 356. 360 CIRRHOPODA. amorphous, and inseparable from the pulpy substance in which they are imbedded ; but, as they approach to maturity, they become of an oval shape, pointed at both ends, and are easily detached. Sir Everard Home has given a very good representation of them at this stage of their progress, in his Lectures on Comparative Anatomy, from the elegant pencil of Mr. Bauer." " During the stay of the ova in the pedicle, they render this part more opaque and of a bluish tint ; the ova themselves, and the cellular texture in which they are surrounded, being of a pale or azure blue colour. It is difficult to conceive in what manner the ova are extricated from the situation above indicated ; but it is certainly not by the means suggested by Sir E. Home in the above-mentioned lecture, viz. by piercing outwards through the membranes of the pedicle, for the ova are subsequently found forming a pair of leaf-like expansions, placed between either side of the body of the animal and the lining membrane of the shells. These leaves have each a separate attachment at the sides of the animal to the septum which divides the cavity occupied by the animal from that of the pedicle : they are at first comparatively small, have a rounded outline, and possess the same bluish colour which the ova had in the pedicle ; but, as the ova advance in progress, these leaves extend in every dimension, and lap over each other on the back, passing through various lighter shades of colour into pale pink, and finally, when ready to hatch, become nearly white. These leaves appear to be composed of a layer of ova, irregularly placed and imbedded in a kind of parenchy- matous texture, out of which they readily fall, when about to hatch, on its substance being torn asunder ; indeed, it appears at length to become so tender as to fall entirely away, so that, after the period of gestation is passed, no vestige of these leafy conceptacles is to be found." (3.96.) In the second form of CIRRHOPODA (Balani), the animals, instead of being appended to foreign substances by elastic and flexible pedicles, are sessile ; the shelly investment of the body being in immediate contact with the rock, or other submarine body, to which the Barnacle adheres. The soft tube of Penta- lasmis is, in this case, represented by a strong testaceous cone composed of various pieces accurately joined together, and generally closed inferiorly by a calcareous plate ; while the representatives of the valves of the pedunculated species form a singular opercu- lum, which is moved by special muscles, and accurately shuts the CIRRHOPODA. 361 entrance of the shell when the animal retires into its abode. In their general structure, however, the Balaniform Cirrhopods accord with the description above given ; and, from the similarity of their habits and economy, a more elaborate account of the peculiari- ties which they exhibit would be superfluous in this place. (397.) One of the most remarkable circumstances connected with the history of the CIRRHOPODA, is the recently discovered fact of their undergoing a distinct metamorphosis ; so that, in the earliest periods of their existence, instead of being rooted by means of a pedicle or otherwise, the newly hatched young are endowed with lo- comotive organs, calculated to enable them to swim freely about, and giving them rather the appearance of Entomostracous Crustacea, than of animals of their own class. This singular fact was first announced by Mr. J. V. Thompson, of Cork ;* and its correctness has since been admitted by various anatomists who have devoted their attention to this subject. Mr. Thompson's first observations were made upon minute animals, which, although at first actually taken for Crustaceans, turned out to be the young fry of Balanus pusillus ; and the following is that gentleman's account of their appearance and subsequent change. The young Cirrhopod is a small translucent animal one-tenth of an inch long, of a somewhat elliptic form, but very slightly compressed laterally, and of a brown- ish tint. When in a state of repose, it resembles a very minute mussel, and lies upon one of its sides at the bottom of the vessel of sea-water in which it is placed ; at this time, all the members of the animal are withdrawn within the shell, which appears to be composed of two valves, united by a hinge along the upper part of the back, and capable of opening from one end to the other along the front, to give occasional exit to the limbs. The limbs are of two descriptions : viz. anteriorly, a large and very strong pair pro- vided with a cup-like sucker and hooks, serving solely to attach the animal to rocks, stones, &c. ; and posteriorly, six pairs of na- tatory members, so articulated as to act in concert, and to give a very forcible stroke to the water, causing the animal, when swim- ming, to advance by a succession of bounds after the same manner as the water-flea (Daphnia) and other Monoculi, but particularly Cyclops, whose swimming-feet are extremely analogous. The tail, which is usually bent up under the belly, is short, composed of two joints, and terminates in four setae, forming an instrument of progression. The animal, moreover, is furnished with large * Zoological Researches, 4th Memoir, 1830. 362 BRACHIOPODA. pedunculated eyes. After keeping several of the above for some days in sea- water, they threw off their exuvia, and, becoming firmly adherent to the bottom of the vessel, were changed into young Barnacles ; and the peculiarly formed shells with their opercula were soon distinctly formed, while the movements of the cirrhi, although as yet imperfect, were visible. As the shell becomes more complete, the eyes gradually disappear, the arms become perfectly ciliated, and an animal originally natatory and locomotive, and provided with a distinct organ of sight, becomes permanently and immoveably fixed, and its optic apparatus obliterated. Similar results were obtained by watching the developement of the pedunculated type of Cirripeds* (Lepades), many of which were proved in their earliest form to resemble different kinds of Monoculi, and to be possessed of the capability of locomotion. CHAPTER XX. BRACHIOPODA-J" (Cuv.) ; PALLIOBRANCHIATAJ (Owen). (398.) THE next class of Mollusca which presents itself for our consideration was named by Cuvier on account of the remarkable character of the organs by means of which the animals composing it procure the food destined to their support. These instruments consist of two long spiral arms placed on each side of the mouth, that in many species can be unrolled to a considerable length, and protruded to some distance, in search of aliment. The above character, however, taken by itself, would scarcely warrant us in considering the creatures before us as forming a separate class of Mollusca ; but when, in addition to this remarkable feature in their organization, we find that they possess a respiratory appa- ratus peculiar to themselves, and differ widely from all other bi- valves in almost every part of their structure, we feel little hesita- tion in continuing to regard them as distinct, and devoting the present chapter to an investigation of their anatomy. * Phil. Trans, for 1835, page 355. j- BgctX'*" 1 ) an arm ; jrayj, xobot, afoot. $ Pallium, a mantle ; branchiae, gills. This name, originally proposed by Mons. de Blainville, notwithstanding his belief that the spiral arms were the organs of respira- tion, has since been proved by the researches of Professor Owen to be strictly appro- priate to the class. JJRACHIOPODA. 363 The BRACHIOPODA inhabit bivalve shells, and for the most part are suspended by a fleshy tubular pedicle, resembling that of the Cirrhopods, to various submarine bodies. Such, at least, is the case in Lingula and Terebratula ; Fig. 170. but in the third genus belonging to this class, namely, Orbicula, the pedicle is // .AV.^fe, \ wanting, the lower valve of the shell be- ing fixed immediately to the rock where- unto the animal is attached. On separating the testaceous valves, the body of the Brachiopod is found to be enclosed between two delicate mem- branes, which exactly line the shell ; and to these membranes, as in the case of other Mollusks, the name of mantle has by common consent been appropriated. The mantle itself is thin and semi-transparent ; but its margins are thickened, and fringed with delicate cilia, the uses of which will shortly become evident. When the two lobes of the mantle are widely divaricated, as in Lingula (fig. 170), we perceive the prominent orifice of the mouth (b) placed deeply between them : on each side of the mouth are the two fleshy fringed arms, which in this case can be protruded to a distance out of the shell, and, as Cuvier* supposes, may act as oars, and thus enable the animal slightly to alter the position of its body, or else, as they are most pro- bably delicate organs of touch, they may perform the office of highly sensible tentacula. In Terebratula psittacea the arms are enormously developed, fringed upon their outer margins, and quite free except at their origins : when completely contracted, they are disposed in six or seven spiral folds, and, when unfolded, they extend beyond the shell twice its longitudinal diameter. The mechanism by which they are unfolded is described by Professor Owen~[* as being extremely simple and beautiful. The principal stem of each arm is hollow from one end to the other, and contains a fluid, which, being acted upon by the spirally disposed muscles forming the parietes of the canal, is forcibly injected towards the extremity of the arm, and the organ is thus expanded and protruded outwards. * M6moire sur 1* Animal de la Lingule. f Transactions of the Zoological Society, vol. i. 364 BRACHIOPODA. In Terebratula Chilensis, on the contrary, the movements of the arms are extremely limited, and they can no longer be protrud- ed from the shell as in the preceding species ; being connected throughout their whole length with a peculiar complex testaceous apparatus attached to the internal surface of the imperforate valve of the shell (Jig- 171, B), the arrangement and uses of which are thus described in the memoir above-mentioned. The principal part of the internal framework alluded to consists of a slender flat- tened, calcareous loop (y, f), the extremities of which are attached to the lateral elevated ridges of the hinge : the crura of the loop di- verge, but again approximate each other as they advance for a greater or less distance towards the opposite margin of the valve; the loop then suddenly turns towards the imperforate valve, and is bent back Fig. 171. upon itself for a greater or less extent in different species. The loop, besides being fixed by its origins, or crura, is commonly attached to two processes (</, d) going off at right angles from the sides, or formed by a bifurcation of the extremity of a central process (c), which is continued forwards from the hinge, but it is sometimes entirely free except at its origins. The arches of the loop are so slender, that, notwithstanding their calcareous nature, they possess a slight degree of elasticity, and yield a little to pres- sure. The interspace between the two folds of the calcareous loop is filled up by a strong but extensile membrane, which binds them together, and forms a protecting wall to the viscera ; the space be- tween the bifurcated processes in T. Chilensis is also similarly occu- pied by a strong aponeurosis. In this species the muscular stem of each arm is attached to the outer sides of the loop and the inter- vening membrane. They commence at the pointed processes at the origin of the loop, advance along the lower portion, turn round upon the upper one, are continued along it till they reach the BRACHIOPODA. 365 transverse connecting bar, where they again advance forwards, and terminate by making a half-spiral twist in front of the mouth. One use assignable to the spiral arms of the BRACHIOPODA is no doubt connected with the opening of the shell, which, in species provided with muscular and retractile organs of this descrip- tion, is mainly effected by their forcible protrusion. In Terebra- tula Chilensis, however, and other species in which the arms are not extensile, Mr. Owen conceives that the elaborate internal framework above described answers a similar purpose ; observing, that the muscular stem, by means of its attachment to the calcareous loop, has the power of acting upon that part to the extent its elasti- city admits of, which is sufficient to produce such a degree of convex- ity in the reflected portion of the loop as to cause it to press upon the perforated valve and separate it slightly from the opposite one.* (399.) The most obvious function, nevertheless, attributable to the tentacular organs of the animals composing this class is connected with the procurement of food ; for, being utterly deprived of pre- hensile instruments, without some adequate contrivance these help- less creatures, imprisoned in their testaceous covering, and fixed immovably in one locality, would be utterly unable to obtain the nourishment necessary for their support. The provision for this purpose is found in the arms, whether they be extensible or attach- ed to calcareous loops ; for these organs, being covered by cilia, pro- duce powerful currents in the sur- rounding medium, which, being di- rected towards the mouth as to a focus, hurry into the oral aperture whatever nutritive particles may chance to be in the vicinity. The mouth itself is a simple orifice with prominent fleshy lips (Jig. 170, I), but unprovided with any dental ap- paratus. The alimentary canal in Lingula is a long and convoluted tube, but without a perceptible sto- machal dilatation ; in Terebratula, A however, there is a large oval stomach (fig. 172, A, d), into which * Innumerable shells of extinct species of Brachiopoda occur in a fossil state ; and in many of them (Spirifera, &c.) an internal framework, analogous in some respects to that described in Terebratula Chilensis, is discernible. Fi 366 BRACHIOPODA. numerous ducts derived from the hepatic follicles open by large orifices. The structure of the liver in these animals is displayed by Professor Owen in the memoir from which the annexed figures are taken, and the simplicity of its organization affords an interest- ing lesson to the physiologist. The hepatic organ (Jig- 172, a, c) consists essentially of numerous secerning caeca (Jig. 172, B), as yet easily separable from each other ; over which the visceral blood-ves- sels ramify, and bring to the secreting sacculi the circulating fluid from which the bile is elaborated. (400.) The greatest peculiarity observable in the structure of the Brachiopoda is seen in the arrangement of the respiratory system ; for these animals, instead of possessing proper branchial organs as is the case with all other Mollusca, have the mantle itself converted into a respiratory surface, and traversed by the ramifications of large blood-vessels, which form an elaborate arbor- escence spreading through its texture, so that it is obviously well adapted to perform the office assigned to it ; more especially as its circumference is thickly studded with vibratile cilia, disposed in such a manner that by their ceaseless movements they impel continued supplies of aerated water Fig. 173. over the whole of this vas- cular membrane. The lobe of the mantle which lines the perforate valve of Terebratula Chilensis (fig> 173, c) contains four large longitudinal venous trunks (m, m), and two others of similar dimen- sions are seen in the op- posite lobe a. These veins take their origin by innumerable radicles from a circular canal of great delicacy which encompasses the entire circumference of the mantle (d) ; and it is in this canal that Mr. Owen supposes the branchial arteries that may be seen to accompany the veins above described terminate. The four veins which are placed in the per- forated lobe of the mantle form two trunks near the visceral mass ; and these, joining those of the opposite lobe, terminate in two dis- tinct contractile cavities, or hearts, seen near the exterior margin of the liver. The arms of the Brachiopoda, notwithstanding their gill-like structure, seem to have nothing to do with the renovation BRACHIOPODA. 367 of the circulating fluids, since the cilia which fringe the margin of the central stem (fig. 173, k, k) present, under the microscope, a horny texture, instead of being of a vascular character, and the muscular stem itself contains no blood-vessels of sufficient size to indicate that the brachia are at all efficient as respiratory organs. The course of the circulation has not been actually demonstrated, but from analogy there is no room to doubt that the two hearts are systemic, receiving the purified blood from the lobes of the mantle, and distributing it through the body. The nervous system of the Brachiopoda is but imperfectly known. Cuvier conceived the brain of Lingula to be represented by some small ganglia visible near the mouth (Jig. 170, a), but was unable to follow the nerves ; and Professor Owen, in dissecting Or- bicula, detected two small ganglia on each side of the oesophagus. (401.) The muscular system in the class before us differs very materially from that exhibited by any other bivalve Mollusca. In Terebratulct) two pairs of muscles arise from each valve :* those of the imperforate valve arise at a distance from each other ; the anterior pair (Jig. 173, /*, f) come off fleshy just behind the middle of the valve (fig- 171, B, g, g) ; they soon diminish to thin shining tendons, which converge and unite below the stomach ; they then again separate, and pass through the foramen of the per- forate valve to be inserted into the pedicle. The posterior pair are very short, and wholly carneous : they arise from the lateral depressions in the base of the central portion of the hinge (Jig- 171, B, A), and are inserted into the pedicle. The muscles of the perforated valve arise close together, so as to leave only a single muscular impression on each side (fig. 171, A, c) ; the anterior pair soon diminish to slender tendons, and are inserted into the base of the imperforate valve ; the pos- terior pass exclusively into the pedicle. The pedicle itself consists of a peculiar tendinous-looking struc- ture, enveloped in a tubular prolongation derived from the mantle. Little is known concerning the reproduction of the Brachiopoda. The ova, when present, have invariably been found lodged be- tween the layers of the two lobes of the mantle ; a position analo- gous to that in which we have already seen them deposited in the Cirripeds ( 395) preparatory to their expulsion. No internal generative system has as yet been detected ; but, notwithstanding this, we are by no means prepared to assume, as some writers do, * Professor Owen, loc. cit. 368 TUNICATA. that the ova are formed by the mantle itself in the localities where they are generally met with. Future investigations, conducted under more favourable circumstances, will no doubt reveal the existence of some internal ovarian nidus, in which the eggs are first developed, and from whence they are subsequently removed to the branchial membranes ; as we shall find hereafter to be the usual arrangement in other forms of bivalve Mollusca. CHAPTER XXI. TUNICATA.* THE singular class of Mollusca to which the name at the head of this chapter has been applied, is at once distinguished by the remarkable character afforded in the texture of the external invest- ment of the body. In their general organization the Tunicata are very nearly allied to the ordinary inhabitants of bivalve shells, with which, both in the structure and arrangement of their viscera, they correspond in many particulars ; but, instead of being enclosed in any calcareous covering, a strong flexible cartilaginous or coria- ceous integument forms a kind of bag encasing their entire body, and only presenting two comparatively narrow orifices, through which a communication with the exterior is maintained. Various are the forms under which these animals present them- selves to the eye of the naturalist ; but the enumeration of them will be more conveniently entered upon hereafter. We shall, therefore, at once lay before the reader the principal points con- nected with the structure and habits of an Ascidia belonging to one of the most perfectly organized families ; and, after examining this attentively, our descriptions of allied genera will be rendered more simple and intelligible. The Ascidians are abundantly met with upon the shores of the ocean, especially at certain seasons of the year. In their natural condition they are found fixed to the surfaces of rocks, sea- weed, or other submarine bodies ; frequently, indeed, they are glued together in bunches, but in this case in- dividuals are simply agglomerated without organic union. In- capable of locomotion, and deprived of any external organs of sense, few animals seem more helpless or apathetic than these apparently * Tunicatus, clad in a tunic. TUNICATA. 369 shapeless beings ; and the anatomist is surprised to find how re- markably the beauty and delicacy pig. 174t of their interior contrasts with their rude external appearance. In the species selected for special description (Phallusia mgra),the external envelope (jig. 174, a, a, a) is soft and gelatinous in its texture, fixed at its base to a piece of coral (/), and exhibiting at its opposite extremity two ori- fices (A, y), placed upon pro- minent portions of the body. Through the most elevated of these orifices (h) the water re- quired for respiration, and the materials used as food, are taken in ; while the other (f) gives egress to the ova and excremen- titious matter. The soft outer covering is permeated by blood- vessels which ramify extensively in it ; it is moreover covered ex- ternally with an epidermic layer, and lined within by a serous vas- cular membrane, which, in the neighbourhood of the two orifices, is reflected from it on to the body of the animal lodged inside. The creature hangs loosely in its outer covering, to which it is only connected at the two apertures by means of the reflection of the peritoneal membrane above men- tioned. (402.) On removing a portion of the exterior tunic, that in reality represents the shells of a bivalve Mollusk, the soft parts of the Ascidian are displayed. The body is seen to be covered with a muscular investment (the mantle) (Jig. 174, b, b, c), composed of longitudinal, circular, and oblique fibres, which cross each other in various directions, so as to compress by their contraction the viscera contained within ; and this so forcibly, that, when alarmed, the animal can expel the water from its branchial sac, immediately to 370 TUNICATA. be described, in a thin continuous stream, sometimes projected to a distance of many inches. (403.) Respiration is effected in an apparatus of very peculiar contrivance ; to the examination of which we must now request the attention of the student. A considerable portion of the interior of the body is occupied by a circumscribed cavity, that opens externally by the orifice h ; into this bag a bristle has been intro- duced, in the dissection represented in the figure (Jig. 174) : its walls are seen to be composed of a thin but very vascular mem- brane (dj d, d), that has been partially turned back, so as to dis- play the interior of the respiratory sac. The membrane (Jig. 174, d, d, d ; Jig. 175, e), when examined with a microscope, is found to be covered with a magnificent network of blood-vessels, formed by innumerable canals uniting with each other at right angles ; and moreover, when seen in a living state, its surface is dis- covered to be densely studded with vibratile cilia, whose rapid action constantly diffuses fresh supplies of water over the whole vascular membrane. The respiratory cavity has but one orifice for the admission of water (Jig. 175, a) ; and this is guarded by a fringe of delicate and highly sensible tentacula (fig. 175, b) ; so that the water, as it is drawn into the body, having necessarily to pass these tactile organs, any foreign substances which it might contain of a prejudicial character are at once detected and denied admission. All the vascular ramifications spread over the lining membrane of the branchial chamber, are connected with two sets of large vessels ; one of which, receiving the blood from the body, disperses it over the spacious respiratory surface ; while the other, collecting it after it has undergone exposure to the respired medium, conveys it in a pure state to the heart. (404.) The heart itself presents the simplest possible form ; be- ing generally a delicate elongated contractile tube, receiving at one extremity the blood derived from the numerous vessels that ramify over the interior of the branchial sac, whilst at the opposite end it becomes gradually attenuated into the aorta, through which it impels the circulating fluid, and disperses it through the system. The heart, above described, is extremely thin and transparent, and is lodged in a distinct pericardium, which separates it from the other viscera. (405.) When we consider the fixed and immoveable condi- tion of an Ascidian, and its absolute deprivation of all prehen- sile instruments adapted to seize prey, it is by no means evident, TUNICATA. 371 at first sight, how it is able to subsist, or secure a supply of nour- ishment adequate to its support ; neither is the structure of the mouth itself, or the strange position which it occupies, at all calcu- lated to lessen the surprise of the naturalist who enters upon the consideration of this part of their economy. The mouth, in fact, is a simple orifice, quite destitute of lips or other extensible parts, and situated, not at the exterior of the body, but at the very bottom of the respiratory sac (Jig. 174 and Jig. 175, g). It is obvious, then, that, whatever materials are used as aliment, they must be brought into the body with the water required for respi- ration ; but, even when thus introduced into the branchial cavity, the process by which they are conveyed to the mouth and swal- lowed still requires explanation. We have before noticed that the interior of the branchial chamber is covered with multitudes of vibratile and closely set cilia, well described by Mr. Lister ;* which, by their motion, cause currents in the water. When these are in full activity, observes that gentleman in the paper referred to, the effect upon the eye is that of delicately toothed oval wheels revolving continually in a direction ascending on the right, and descending on the left of each oval, as viewed from with- out ; but the cilia themselves are very much closer than the apparent teeth ; and the illusion seems to be caused by a fanning motion given to them in regular and quick succession, which will produce the appearance of waves, and each wave answers here to a tooth. Whatever little substances, alive or inanimate, the current of water brings into the branchial sac, if not rejected as unsuitable, lodge somewhere on the respiratory surface, along which each particle travels horizontally with a steady, slow, course to the front of the cavity, where it reaches a down- ward stream of similar materials ; and they proceed together, receiving acces- sions from both sides, and enter at last the 03sophagus placed at the bot- * Phil. Trans, for 1834. page 378. Fig. 175. 372 TUNICATA. torn (fig- 175, g), which carries them, without any effort of swal- lowing, towards the stomach. (406.) The oesophagus (Jig- 175, h) is short, and internally gathered into longitudinal folds. The stomach (i) is simple, mode- rately dilated, and has its walls perforated by several orifices, through which the biliary secretion enters its cavity. The liver is a glandular mass intimately adherent to the exterior of the stomach and the intestinal canal (Jig. 174, e, e), of variable length and more or less convoluted in different species, after one or two folds, termi- nates in the rectum, which, emerging from the peritoneal invest- ment covering the intestine, has its extremity loosely floating in the cavity communicating with the second orifice (f) : into the latter a bristle is introduced in the figure, having its extremity inserted into the anal extremity of the digestive tube. Excrementitious matter, therefore, when discharged from the rectum, escapes from the body through the common excretory aperture generally situated upon the least elevated protuberance of the outer covering.* It would seem that the food of Ascidians consists of very minute particles of organized matter ; for, although small Crustacea and other animal remains have been occasionally met with in the branchial cham- ber, nothing of this nature has been observed in the stomach itself, and, as must be obvious to the reader, the oral aperture seems but little adapted to the deglutition of bulky substances. (407.) The reproductive system in these humble forms of Mol- lusca presents the utmost simplicity of parts ; being composed of an .ovarian nidus, in which the germs of their progeny are ela- borated, and a duct, through which their expulsion is accomplished. Nothing resembling a male apparatus has been satisfactorily indi- cated ; and consequently, if in this form of hermaphrodism the provision of an impregnating fluid be really indispensable to the fertility of the ova, we must suppose it to be furnished by the walls of the egg-passages themselves. The ovary is a whitish glandular mass embedded with the liver among the folds of the intestine : its position injtfg. 174 is indicated by the letter m ; and at o, Jig. 175, it is seen separated from the surrounding struc- tures. The oviduct, which is occasionally very tortuous, accom- panies the rectum, and terminates near the anal aperture {Jig- 174, m, Jig. 175, o), so that the ova ultimately escape through the common excretory orifice. (408.) Deprived as these animals are of any of the higher organs * Cuvier ; M6moire sur les Ascidies, p. 14. TUNICATA. 373 of sense, and almost cut off from all relation with the external world, we can look for no very great developement of the nervous cen- tres. There is one ganglion, however, lodged in the substance of the mantle, distinctly recognizable, situate in the space between the branchial and excretory openings, from which large nerves are given off; but of other details connected with the nervous system of the TUNICATA little has been made out. (409.) Many forms of Tunicated Mollusca are met with abun- dantly in the seas of tropical latitudes, which, although allied to Ascidians in the main points of their economy, present certain peculiarities of structure that require brief notice in this place. These, grouped by authors under the general name of Salpa, are many of them so transparent that their presence in a quantity of sea-water is not easily detected ; and their viscera, if coloured, are readily distinguishable through their translucent integument, which in texture seems to be intermediate between cartilage and jelly. The body is oblong, and open at both extremities, the pos- terior opening being very wide, and furnished with a crescentic valve so disposed that water is freely drawn into the interior through this aperture, but cannot again be expelled by the same channel ; so that, being forced by the contractions of the body in powerful gushes from the opposite end, it not only supplies the material for respiration, but impels the delicate animal through the water in a backward direction. The branchial chamber of Ascidia is con- sequently in this case represented by a wide membranous canal, which traverses the body from end to end ; but, instead of the network of vessels lining the respiratory sac of Ascidians, a sin- gular kind of branchial organ is placed within it. This consists of a long vascular riband attached by both its extremities to the walls of the canal through which the water rushes, and of course, being freely exposed to the influence of the surrounding medium, the blood contained in this curious branchial apparatus is perpe- tually renovated, and afterwards distributed, by a heart resembling that met with in the genus last described, to all parts of the body. The viscera, which occupy comparatively a very small space, are lodged in a distinct compartment between the membranous respiratory channel and the external gelatinous investment, or soft shell, as we might properly term it. The mouth is a simple aperture, situated near the upper extremity of the branchial organ ; and probably, as in Ascidia, ciliary currents rushing over the re- spiratory surface bring into it a sufficient supply of nutritive mole- 374 TUNICATA. cules : the stomach is capacious, and covered with parallel rows of large white filaments, that seemingly represent the liver ; and the alimentary canal, which is perfectly simple, runs to the pos- terior extremity of the animal, and terminates there by a wide opening.* Two oblong bodies, each consisting of a granular substance, are seen upon the ventral surface of the body lodged between the external and internal membranes, which no doubt are the ovaria, and form a reproductive system as devoid of com- plication as that of the sessile Ascidians. (410.) A very remarkable feature in the history of these animals is, that many species are found swimming together in long chains, apparently adhering to each other by little suckers, but without organic connexion ; and, what is still more strange, it would appear, from the observations of M. de Chamisso,'j" that such aggregated animals give birth to insulated individuals of very different ap- pearance, which in their turn reproduce concatenated forms re- sembling their progenitors, so that the alternate generations are quite dissimilar both in conformation and habits. The last families of TUNICATA which we have to notice, would seem to constitute a connecting link between the MOLLUSCA and the BRYOZOA, which latter in many points of their anatomy they much resemble. These animals generally are exceedingly minute, and individually present an organization analogous to that of Ascidians. At first it would appear that they are detached from each other, and, like Salpa, are endowed with a power of locomotion ; but subsequently they become aggregated in groups, either incrusting foreign bodies, or else, uniting together to form a mass of definite shape, they seem to enjoy to a certain extent a com- munity of action. They are arranged by Cuvier^: in three princi- pal groups, distinguished by the following characters. In the first (Botryllus), the little bodies of the individual animals are ovoid ; but they fix themselves upon the exterior of sea-weed or other substances in regular bunches, consisting of ten or twelve, arranged like the rays of a star around a common centre. The branchial orifices in such are all placed around the circumference of the star, while the excretory apertures open into a common cavity in the * For excellent drawings, representing the anatomy of various Salpae, the reader is referred to the Descriptive and Illustrated Catalogue of the Physiol. Series of Comp. Anat. contained in the Mus. of the Royal Coll. of Surgeons, London, vol. i. plates 6 and 7. f Dissert, de Salp&, Berlin, 1830. t Regne Animal, vol. iii. p. 168. Bar^t/f, bunch of grapes. CONCHIFERA. 375 centre. If tlie external orifice is irritated, the animal to which it belongs alone contracts ; but, if the centre be touched, they all shrink at once. In Pyrosoma,* the second family, the animals are aggregated together in great numbers, so as to form a hollow cylinder, open at one end but closed at the opposite, which swims in the sea by the combined contractions and dilatations of all the individuals composing it. The branchial sacs here open upon the exterior of the cylinder, while the anal orifices are in its internal cavity. Thus, a Pyrosoma might be described as consisting of a great number of stars of Botrylli piled one above the other, the whole mass remaining free and capable of locomotion. Many of these moving aggregations of Tunicata emit in the dark a most brilliant phosphorescent light, whence the derivation of the name by which they are distinguished. In all other forms of these aggregated Mollusca, which are desig- nated by the general name of Polyclinum^ as in ordinary Ascidi- ans, the anus and branchial orifices are approximated, and placed at the same extremity of the body. They are all fixed; some spreading like fleshy crusts over submarine substances, others forming conical or globular masses, or occasionally so grouped as to produce an expanded disc resembling a flower or an Actinia ; but, whatever the general arrangement of the common mass, it is composed of numerous associated individuals, every one of them corresponding more or less closely as regards their internal structure with the de- scription above given of the organization of Salpse and Ascidians. CHAPTER XXII. CONCHIFERA (Lamarck) ; ACEPHALES TEST ACES (Cuv.) THE great majority of Mollusks which inhabit bivalve shells con- stitute a very numerous and extensive class, distinguished by certain characters possessed by them in common. Encased in dense and massive coverings of such construction as to preclude the possi- bility of their .maintaining more than a very imperfect intercourse * ri/, -9s t jire ; ffuftai, a body. -f- vol.vs, many ; xXivn, a bed. 376 CONCHIFERA. with the external world, and deprived even of the means of com- munication with each other, we might naturally expect their or- ganization to correspond in its general feebleness with the cir- cumscribed means of enjoyment and limited capabilities of loco- motion allotted to them. Numerous species, indeed, are from the period of their birth firmly fixed to the rock which gives them support, by a calcareous exudation that cements their shells to its surface, as is familiarly exemplified in the case of the common Oyster ; or else, as the Mussels, anchor themselves securely and immoveably by unyielding cables of their own construction. The Scallop, unattached, but scarcely better adapted for changing its position, rudely flaps together the valves of its expanded shell, and thus by repeated jerks succeeds in effecting a retrogressive movement ; while the Cockles, destined to burrow in the sand, are furnished with a tongue-like foot, by which they dig the holes wherein they lie concealed, and crawl, or even leap about, upon the shore. Many, as the Pholades, penetrate the solid rocks and stones, and excavate therein the caverns that they inhabit ; or, in the case of the Teredo, with dangerous industry bore into the bottoms of ships or submerged wood of any description, and silently destroy by their insidious ravages the piers or dikes which human labour has erected. (411.) Following our usual custom, we shall select for examina- tion one of the most simply organized bivalves for the purpose of illustrating the general structure which characterizes the class ; and in the common Scallop (Pecten Jacobtea) we have a species well adapted to exhibit the principal features of their economy. On separating the two valves of the shell in the animal before us, we at once perceive that each is lined internally with a thin and semitransparent membrane (fig- 176, a, A), which, like the shells, encloses the body of the Mollusk in the same way that the leaves of a book are contained between its covers. The circumference of these outer membranes, which form the mantle, is, in this case, quite free and unconnected, except in the immediate vicinity of the hinge that unites the two valves. The borders of the mantle are thickened, and surrounded with a delicate fringe of retractile filaments ; they moreover present a decided glandular appearance, and secrete colouring matter of various tints, similar to those seen upon the exterior of the shell : the glandular margins of the mantle form in fact the apparatus by which the extension of the shell is effected, and by them its outer CONCHIFERA. 377 layer is secreted, and in many cases painted with gorgeous hues, as will be explained more at large hereafter. Fig. 176. Between the lobes of the mantle are seen the branchiae (b, g) 9 always consisting of four delicate leaves, composed of radiating fibres of exquisite structure, and generally attached to the circum- ference of the body by their fixed extremities, but elsewhere per- fectly free, so as to float loosely in the water, which finds free admission to them. The mouth (/) is situated between the two inner laminae of the branchiae, in a kind of hood formed by the union of the gills at their origin ; it is a simple orifice, without any kind of dental apparatus, but bordered by four thin and mem- branous lips (k) placed on each side of the aperture. The valves, which are opened by the elasticity of a compressible ligament interposed between them at the hinge, are closed by the contraction of a powerful muscle (c), which passes directly from one to the other, and around this adductor muscle the viscera of the body are disposed : the stomach, liver, and generative system are imbedded in the mass, d, e,y ; the convolutions of the intestine may be traced occasionally (w, o) ; and the termination of the rectum, m, is visible externally, situated upon that side of the adductor muscle which is opposite to the mouth. In the neighbourhood of the oral aperture is placed a retractile fleshy 378 CONCHIFERA. organ (i), which, although in Pecten it exhibits very rudimentary dimensions, expands in other species to such a size as richly to merit the name of foot usually applied to it. (412.) Whoever for a moment reflects upon the arrangement of the branchial apparatus, and the position of the oral orifice, consist- ing, as it does, of a simple aperture unprovided with any prehensile organs, must perceive that there are two circumstances connected with the economy of a conchiferous Mollusk, and those not of secondary importance, by no means easily accounted for. It is, in the first place, absolutely essential to the existence of these animals that the element in immediate contact with the respiratory surfaces should be renewed as rapidly as it becomes deteriorated, or suffocation would inevitably be the speedy result of an in- adequate supply of fresh and aerated water ; to secure which, especially when the valves of the shell are closed, no adequate provision seems to exist. Secondly, it is natural to enquire, how is food conveyed into the mouth ? for in an animal, itself fixed and motionless, and at the same time, as in the case of the creature we are now considering, quite deprived of any means of seizing prey, or even of protruding any part of its body beyond the margins of its abode in search of provision, it is not easy to imagine by what procedure a due supply of nutriment is secured. Wonderful, indeed, is the elaborate mechanism employed to effect the double purpose of renewing the respired fluid, and feeding the helpless inhabitant of these shells. Every filament of the branchial fringe, examined under a powerful microscope, is found to be covered with countless cilia in constant vibration, causing by their united efforts powerful and rapid currents, which, sweep- ing over the entire surface of the gills, hurry towards the mouth whatever floating animalcules or nutritious particles may be brought within the limits of their action, and thus bring streams of nu- tritive molecules to the very aperture through which they are conveyed into the stomach, the lips and labial fringes acting as sentinels to admit or refuse entrance as the matter supplied be of a wholesome or pernicious character. So energetic, indeed, is the ciliary movement over the entire extent of the branchial organs, that, if any portion of the gills be cut off with a pair of scissors, it immediately swims away, and continues to row itself in a given direction as long as the cilia upon its surface continue their mysterious movements. (413.) Our next investigations must be concerning the internal CONCHIFERA. 379 anatomy of tlie CONCHIFEROUS MOLLUSCA. In the Oyster, the general disposition of the body resembles that of the Pecten described above ; and the mouth, enclosed between two pairs of delicate lips, occupies a similar position at the termination of the branchial lamellae. In this well-known Mollusk the (esophagus is extremely short, so that the mouth appears to open at once into the stomachal cavity (Jig. 177, a), which is imbedded in the substance of the liver (d), ; the biliary secretion being poured into the stomach itself through several large orifices represented in the figure. A very peculiar arrangement exists in the stomachs of many genera, the digestive cavity being prolonged in one di- rection, so as to form a lengthened caecum, or blind sacculus, wherein is lodged a cartilaginous styliform body, the use of which it is not easy to conjecture, although its office is no doubt con- nected in some way or other with the preparation of the food. Fig. 177. The liver is propor- tionately of large di- mensions, and is at once recognized by its greenish, or, in some cases, dark cho- colate colour ; it is entirely separable in- to masses of secern- ing follicles loosely connected together by a delicate cellu- losity. The intes- tine varies consider- ably in extent, and, as a necessary con- sequence, in the ar- rangement and num- ber of its convolutions. In the Oyster it is comparatively short, bending twice upon itself, and winding around the stomach and adductor muscle (6, c, d,f) ; its termination (g) projecting between the folds of the mantle upon the opposite side of the body to that where the mouth is situated, and so disposed that excrementitious matter is cast out beyond the influence of the ciliary currents. In Pecten we have already noticed that it performs sundry gyrations through the visceral mass, as well as about the muscle that closes 380 CONCHIFERA. the shell (Jig. 176, o, w, m) ; while in the cockle tribes it even penetrates the base of the foot, and winds extensively through its muscular substance (fig. 182). In the greater number of the Conchifera, but not in the Oyster tribe, there is a very remarkable circumstance connected with the course of the intestine, the object of which is involved in obscurity ; the rectum, at some distance from its termination, passes right through the centre of the ventricle of the heart, its coats being tightly embraced by the muscular parietes of that viscus. (414.) The position of the branchiae in the Ostracean family has been already described ; it now remains, therefore, to notice their inti- mate structure, and the arrangement of the vessels connected with respiration and the circulation of the blood. The branchial fringes are of course essentially vascular in their composition ; being, in fact, made up of innumerable delicate parallel vessels enclosed in cellular tissue of extreme delicacy, and exposing a very extensive surface to the influence of the respired medium. The countless branchial canals through which the blood is thus distributed termi- nate in large vessels enclosed in the stems to which the fixed extremi- ties of the vascular fringe are attached (fig. 178,/, g, A, i) ; these communicate extensively with each other, and, ultimately uniting in two principal trunks (e, &), pour the purified blood derived from the whole branchial apparatus into the auricle of the heart. The heart in the Oyster Figf 178> (fig. 177, n-> o) is situat- ed in a cavity between the folds of the intestine and the adductor muscle ; in which position, from the dark purple colour which it exhibits, it is at once dis- tinguished. It consists, in the species we are more particularly describing, of two distinct chambers, an auricle and a ventricle. The auricular cavity (fig. 178, b), the walls of which are extremely thin, and com- posed of most delicate fas- ciculi of muscular fibres, re- CONCHIFERA. 381 ceives the blood from the respiratory apparatus, and by its con- traction transmits it through two intermediate canals (c) into the more muscular ventricle (d), whence it is propelled through the body by the ramifications of the arterial system (w, o, p). The above description of the circulatory apparatus as it exists in the Oyster is applicable in all essential points to every family of conchiferous Mollusca ; but there are important modifications in the structure of the heart and arrangement of the blood-vessels, met with in different genera, which now demand our attention. Most generally, in consequence of the broad and dilated form of the animals, instead of a single auricle, such as the Oyster has, there are two auricular cavities, one appropriated to each pair of branchial lamellae, and placed symmetrically on the two sides of an elongated fusiform ventricle, into which both the auricles empty themselves, still the course of the blood is similar to what we have described above. A still greater modification is found to exist in those species most remarkable for their breadth. In Area, for example, there are not only two auricles, but two ventricles likewise, placed upon the opposite sides of the body ; that is, there is a distinct heart appropriated to each pair of gills, each receiving the blood from the branchiae to which it belongs, and propelling it through vessels common to both hearts, to all parts of the system. (415.) We must now, before entering upon the description of other families of Conchifera, examine the character of the locomotive apparatus with which those possessed of the power of moving about are furnished. The instrument employed for this purpose is a fleshy organ appended to the anterior part of the body, called the foot ; but of this apparatus, for obvious reasons, no vestige is met with in the fixed and immoveable Oyster, and even in the Scallop we have seen only a rudiment of such an appendage. When largely developed, as in Mactra (figs. 179, 180), the foot forms a very important part of the animal, and becomes useful for various and widely different purposes. In structure it almost exactly resembles the tongue of a quadruped, being entirely made up of layers of muscles crossing each other at various angles ; the ex- ternal layers being circular or oblique in their disposition, while the internal strata are disposed longitudinally. In the Cockle tribe (Cardium) this organ attains to a very great size, and on inspect- ing the figure given in a subsequent page, representing a dissection of the foot of Cardium rusticum (Jig. 182), the complexity of CONCHIFERA. its muscular structure will be at once evident, and the disposition of the several layers composing it more easily understood than from the most elaborate verbal description. (416.) Diverse are the uses to which the foot may be turned. It is generally used for burrowing in the sand or soft mud ; and, by its constant and worm-like action, those species in which it is largely developed can bury themselves with facility, and make their way beneath the sand with a dexterity not a little remark- able. Perhaps, the most efficient burro wers met with upon our own shores are the Razor-shells (*$WewzW<z), in which family the fleshy foot attains to enormous proportions ; and the rapidity of their movements beneath the soil will be best appreciated by those who may have watched the manner in which the fishermen effect their capture. The Solen excavates for itself a very deep hole in the sand, bor- ing its way by means of its foot to a depth of some feet ; and re- mains concealed in this retreat, usually occupying a position within a few inches from the surface. The fisherman, armed with a slen- der iron rod, furnished with a barbed head, resembling a harpoon, treads carefully backwards over the beach left bare by the retreat- ing tide, and finds the holes in which Solen lodges, by watching the little jet of water thrown out by the animal, when, being alarm- ed by the shaking of the sand, it contracts its body. Guided by the orifice through which the water is thrown, he plunges his rod into the sand, and generally succeeds in piercing the animal with the barbed extremity, and dragging it from its concealment ; but, should he fail in his first attempt, he well knows that to try again would be unavailing, for the animal instantly works its way down to such a distance as to render pursuit hopeless. But, however efficient, as a means of burrowing, the foot may be, it can be turned to other purposes. The Pholades, for, example, by some means, either of a mechanical or chemical nature, not as yet precisely determined, excavate the solid rocks, and form therein chambers, in which they pass their lives. In such genera, the foot, which would be useless as a boring instrument, by being simply transformed into a broad and flat disc, becomes a powerful sucker, whereby the Pholas fixes itself to the walls of its apart- ment in any convenient situation. In many of the Cockle tribe we find the foot converted into an instrument of locomotion, of a very singular description, en- abling the cardiaceous Conchifera to leap by bounds we should CONCHIFERA. 883 scarcely expect animals so unwieldy to be capable of executing. For this purpose the end of the foot is bent, and placed firmly against the plane of support in the position represented in Jig. 181 ; when thus fixed, a sudden spring-like action of the muscles of the foot throws the cockle into the air, and, by a repe- tition of these exertions, the creature can skip about with surpris- ing agility. (417.) But the most extraordinary office assigned to the foot in the class under consideration, is the manufacture of horny threads, whereby, as by so many anchors, the Mollusca thus provided fix themselves securely to foreign bodies, and that so firmly, that extraordinary violence is requisite to wrench such animals from the place where they have fixed their cables. The marine Mussel is a well-known example of a byssiferous Mollusk, and from this species, therefore, we shall draw our description of the organs by which the tough filaments referred to are secreted. The foot in the Mussel is of small dimensions, being useless as an instrument of progression. By its inferior aspect it gives attachment to the horny threads of the byssus, which are individu- ally about half an inch in length, or as long as the foot itself, by which, in fact, they are formed, in a manner quite peculiar to certain families of Conchifera ; no other animals presenting a se- creting apparatus at all analogous, either in structure or office, to that with which these creatures are provided. The manner in which the manufacture of the byssus is accomplished is as fol- lows : A deep groove runs along the under surface of the foot, at the bottom of which thin horny filaments are formed by an exudation of a peculiar substance, that soon hardens and assumes the requisite tenacity and firmness. While still soft, the Mussel, by means of its foot, applies the extremity of the filament, which is dilated into a kind of little sucker, to the foreign substance whereunto it wishes to adhere, and fastens it securely. Having accomplished this, the foot is retracted ; and the thread, of course, being drawn out of the furrow where it was secreted, is added to the bundle of byssus previously existing, all of which owed its origin to a similar process. Sometimes, instead of the numerous thin filaments met with in the Mussel, the byssus consists of a single, thick, horny stem ; while in other cases, as, for example, in Pinna, the threads are so numerous, soft, and delicate, that they are not unfrequently spun like silk, and manufactured into gloves and other small articles 384 CONCHIFERA. of dress, not unfrequently met with in the cabinets of concho- logists. (418.) Taking a more general view of the Conchiferous Mollusca than we have hitherto done, we shall now proceed to consider the me- chanism for opening and closing the valves of the shell in which they reside ; an operation effected in a very simple and elegant manner. The shells are connected posteriorly by means of a hinge differently constructed in different species. In the Oyster we have an instance of the most simple kind of junction. In these Mol- lusca a mass of elastic ligament, composed of perpendicular and parallel fibres, is interposed between the posterior edges of the shell, and so disposed, that by closing the shell the ligamentous mass is forcibly compressed while at the same time its resiliancy is such, that, immediately the compressing power is withdrawn, it expands, and thus forms a simple spring calculated to keep the valves apart, and cause their separation to a greater or less extent. The antagonist to this elastic force is the adductor muscle (jig. 176, c), a fleshy mass of very great strength, the fibres of which pass directly from one valve to the opposite. The adductor muscle, although in this case single, consists of two portions of dif- ferent texture (Jig. 177, /, m) ; so that it would appear to be formed by two muscles closely approximated, so as to compose a single power- ful mass adapted to keep the valves in contact with a force propor- tioned to its massive size. All those species having a single muscular mass, such as the Oyster and Pecten, have been grouped together by conchologists under the general name MONOMYARIA, while another and more numerous division DIMYARIA, is characterized by having two adductor muscles distinct and widely removed from each other. The Mussel tribe and many others are examples of this arrangement which is represented in subsequent figures. Simple as the structure of the hinge is in the Ostracea, in other Bivalves it frequently exhibits far greater complexity, and the op- posed valves present prominent elevations and deep fossse which lock into each other, and thus form a very secure articulation of great strength and solidity. In such cases the arrangement of the elastic ligament for opening the valves is slightly modified, being placed externally instead of within the shell, but its action in antagonizing the adductor muscles is still equally efficacious. (419.) We must, in the next place, solicit the attention of the reader to a very important subject connected with the economy of this CONCHIFERA. 385 class of Mollusks, viz. the growth and formation of their shells. Infinitely diversified are the forms presented by their testaceous valves, and equally various the colours which not unfrequently adorn their external surfaces. Some exhibit a beauty and delicacy of sculpture of a most exquisite character ; others, covered with large spines, or festoons of calcareous plates, puzzle the beholder to comprehend how the growth of such parts, in the situations which they occupy, can be effected with so much regularity of arrange- ment. The shells themselves are absolutely deprived of vitality, permeated by no vessels, and as incapable of expansion by any internal power as the rocks to which they are not uncommonly attached ; so that the young naturalist is necessarily at a loss to conceive either the mode of their formation, or the origin of all the gaudy tints and external decorations that render them the ornaments of our cabinets. The simple apparatus by means of which shells are constructed is the external membranous layer that invests the body of the mollusk, the mantle, as it has been termed ; and, whatever the form of the shell, it owes its origin entirely to this delicate organ. In order to simplify as much as possible our description of the process whereby the shell is formed, it will be necessary to consider it under two points of view : first, as relates to the en- largement of the valves in length and breadth ; and secondly, as regards their increase in thickness, very different parts of the mantle being employed in the attainment of these two ends. It is the circumference, or thickened margin of the mantle, alone, which provides for the increase of the shell in superficial extent. On examining this part (Jig. 176, hfjig. 177, e), it is found to be of a glandular character, and moreover not unfrequently pro- vided with a delicate and highly sensitive fringe of minute ten- tacula. Considered more attentively, it is seen to contain in its sub- stance patches of different colours, corresponding both in tint and relative position with those that decorate the exterior of the shell. When the animal is engaged in increasing the dimensions of its abode, the margin of the mantle is protruded, and firmly ad- herent all round to the circumference of the valve with which it corresponds. Thus circumstanced, it secretes calcareous matter, and deposits it in a soft state upon the extreme edge of the shell, where the secretion hardens and becomes converted into a layer of solid testaceous substance. At intervals this process is repeated, and every newly-formed layer enlarges the diameter of the valve. The 386 CONCHIFERA. concentric strata thus deposited remain distinguishable externally, and thus the lines of growth marking the progressive increase of size may easily be traced (fig> 179). It appears that at certain times the deposition of calcareous sub- stance from the fringed circumference of the mantle is much more abundant than at others : in this case ridges are formed at distinct intervals ; or, if the border of the mantle at such periods shoots out beyond its usual position, broad plates of shell, or spines of differ- ent lengths, are secreted, which, remaining permanent, indicate, by the interspaces separating successively deposited growths of this description, the periodical stimulus to increased action that caused their formation. (420.) Whatever thickness the shell may subsequently attain, the external surface is thus exclusively composed of layers de- posited in succession by the margin of the mantle ; and, seeing that this is the case, nothing is more easy than to understand how the colours seen upon the exterior of the shell are deposited, and assume that definite arrangement characteristic of the species. We have already said that the border of the mantle contains, in its substance, coloured spots : these, when minutely examined, are found to be of a glandular character, and to owe their peculiar colours to a pigment secreted by themselves ; the pigment so fur- nished being therefore mixed up with the calcareous matter at the time of its deposition, coloured lines are formed upon the exterior of the shell wherever these glandular organs exist. If the deposi- tion of colour from the glands be kept up without remission during the enlargement of the shell, the lines upon its surface are continu- ous and unbroken; but if the pigment be furnished only at intervals, spots or coloured patches of regular form, and gradually increasing in size with the growth of the mantle, recur in a longitudinal series wherever the paint-secreting glands are met with. (421.) The carbonate of lime, for such is the earth whereof the shells of bivalves are principally composed, is, at the moment of its deposition, embedded in a viscid secretion that forms a kind of cement ; and on dissolving the shell in a dilute acid, the animal material thus produced remains in the shape of a delicate cellu- losity, in the interstices of which the chalky particles had been entangled. If the proportion of the above-mentioned secretion be abundant, it not unfrequently, by hardening on the exterior of the shell, constitutes what has been very inaptly termed its epidermis, representing a comparatively soft external skin of semicorneous CONCHIFERA. 387 texture. If exceedingly thick, the epidermic layer thus formed becomes loose and shaggy, giving the shell a hirsute appearance ; but, both in its structure and origin, such pilose investment has no claim to be considered analogous to the hair of animals possessing an epidermis properly so called. While the margin of the mantle is thus the sole agent in en- larging the circumference of the shell, its growth in thickness is accomplished by a secretion of a kind of calcareous varnish, derived from the external surface of the mantle generally ; which, being deposited layer by layer over the whole interior of the previously existing shell, progressively adds to its weight and solidity. There is, moreover, a remarkable difference between the character of the material secreted by the marginal fringe, and that furnished by the general surface of the pallial membrane ; the former we have found to be more or less coloured by glands appointed for the purpose, situated in the circumference of the mantle ; but as these glands do not exist elsewhere, no colouring matter is ever mixed with the layers that increase the thickness of the shell, so that the latter always re- main of a delicate white hue, and form the well-known iridescent ma- terial usually distinguished by the name of nacre, or mother of pearl. (422.) Local irritation of various kinds is found to stimulate the mantle to increased action, so as to cause the pearly matter to be secreted more abundantly at the part irritated. Thus there are various minute boring annelidans that, in the exercise of their usual habits, perforate the shells of oysters, and penetrate even to the soft parts of their bodies. Stimulated by the presence of these intruders, the mantle beneath the place attacked secretes nacre in inordinate quantities to repair the injured portion of the shell, and prominent nuclei are soon formed, which, enlarging by the addition of continually added layers of nacreous matter, become so many pearls adherent to the interior of the shelly valves. Or pearls may owe their origin to another cause : It not unfre- quently happens that sharp angular substances, such as grains of sand or fragments of stone, are conveyed between the valves, and become embedded in the delicate tissue of the mantle. Thus irri- tated, the mantle throws out copiously the peculiar iridescent mate- rial which it secretes, and with it coats over the cause of annoyance, wrapping it in numerous concentric laminae of nacre, and thus form- ing the detached and globular pearls so valuable in commerce. (423.) One other circumstance connected with the growth of bivalve shells requires explanation. From the earliest appearance 388 CONCHIFEllA. of the shelly valves until the period when the included mollusks arrive at their mature size, the adductor muscle or muscles have been of necessity perpetually changing their position, advancing gradually forward as the enlargement of the shells was accomplished, so as to maintain in the adult precisely the same relative situations as they originally did in the young and as yet minute animal. Taking the Oyster for an example, it is quite obvious that the adductor muscle, which at first was connected with the thin and minute lamellae forming the earliest shell, has, during the entire growth of the animal, become further removed from the hinge, and transferred from layer to layer as the shell increased in thickness, till it arrives at the position occupied by it in connection with the last-formed stratum that lines the interior of the ponderous valves of the full-grown oyster. The manner in which this progressive advance of the adductor muscle is effected is not at first easily accounted for, seeing that it is always fixed and firmly adherent at all points of its attachment. In order to understand the circum- stances connected with its apparent removal, it is necessary to premise that a thin layer of the mantle itself is interposed between the extremities of the muscle and the inner surface of the shell, forming the bond of connection between the two, and, like the rest of the pallia! membrane, assisting in increasing the thickness of the shell by adding layers of nacre to its inner surface. Particle after particle is laid on by a kind of interstitial deposit between the mantle and the extremity of the adductor muscle, but so gradually, that the firm attachment between the muscle and the shell is not at all interfered with ; and as the animal grows the transference of the muscle from layer to layer is thus slowly and imperceptibly effected. (424.) We have, as yet, limited ourselves almost exclusively to a description of the simplest forms of CONCHIFERA, namely, those belonging to the Ostracean family, which, being generally inca- pable of locomotion, are deprived of a foot, and are recognisable by having the two lobes of the mantle unconnected w^th each other around their entire circumference. On turning our attention to the organization of the mantle in other families, we find that in them it no longer offers the same simple arrangement ; but, the two lobes becoming gradually more and more completely united along their edges, the bodies of the mollusks are by de- grees enclosed by the pallial membranes, and seem, as it were, sacculated ; moreover, sometimes the mantle is prolonged into CONCHIFERA. 389 membranous tubes of considerable length called syphons, through which the water is conveyed to the gills, and excrementitious mat- ters expelled from the body. In the Mussels (Mytilacea) the edges of the mantle are partially joined so as to present two aper- tures, through one of which the foot is protruded, while the other, the smaller of the two, gives issue to the excrement. A third family (Camacea) has the circumference of the two divisions of the mantle still more intimately united, leaving three distinct fissures, one for the passage of the foot, another for the entrance of water to the Fig. 179. branchise, and a third for the ejection of matter from the rectum. Of these, some are of gigantic dimensions, and fix themselves by a strong byssus. One species, indeed, (Tridacne gigas,) is so enormous in its size, that its shells alone not unfrequently weigh upwards of two hundred pounds, and hatchets are employed to chop its thick and tendinous cables from the rock to which it holds. The Cockle family (Cardiacea) is recognised by having the mantle open anteriorly, but prolonged at one extremity into two tubes, one of which admits the water for respiration, while the other discharges effete matter. In the Cockle (Cardium) the tubes are short, and scarcely reach beyond the shell (Jig. 181, a) ; but in other genera, as, for example, Mactra (Jig' 179, &, c), they are of such length, that, when extended, they protrude to a considerable distance. We at once perceive the use of the tubular arrangement of the mantle here referred to, when we reflect upon the already mentioned habits of this extensive division of the Conchifera, and consider how, by means of their largely developed foot, they burrow into the sand or mud of the shore. Had their mantle been open, 390 CONCHIFERA. Fig. 180. like that of the oyster, respiration would have been impossible under the circumstances in which they live ; but, by the modifi- cation of structure thus provided, their tubes being prolonged to the mouth of the excavation wherein they reside, water is freely admitted to the branchiae through one of the passages so formed, and ex- crement ejected through the other (fig. 180). Whoever watches these syphoniferous bivalves in a living state will readily ap- preciate the importance of the pallial prolongations forming this tubular appa- ratus ; especially if minute floating particles are placed in the water wherein they are confined. It will then be perceived that powerful currents are perpetually rushing through the extre- mities of each syphon, caused by the rapid action of cilia placed within ; and the streams thus produced not only form a provision for constantly changing the water in which the branchiae (fig. 180, g) are immersed, but forcibly convey floating molecules to the aper- ture of the mouth, which is situated in the position indicated in the figure by the letter h, and thus supply abundance of nutritive materials that could, apparently, in animals so destitute of prehen- sile organs, have been procured by no other contrivance.* The last family of this class includes those species which, like the Pholas and Teredo, bore in stone or wood ; or, like the Solen, pene- trate deeply into the sand. In such, the mantle is prolonged into terminal tubes of great length, and their shells remain always open * The parts represented in the above figure (fig. 180) which are not particularly pointed out in the text are, the anterior adductor muscle, c ; the posterior adductor muscle, d ; the elastic ligament of the hinge, e ; and the largely developed foot,/. CONCHIFERA. 391 at the extremities ; these constitute the division to which Cuvier has applied the name " Enfermes," on account of the very com- plete union of the two sides of the mantle ; and from such forms of CONCHIFERA the transition to the TUNICATA, described in the last chapter, is by no means difficult. (425.) In animals circumstanced as the CONCHIFERA, it would be vain to expect any high developement of the nervous system, or senses of an elevated character : nevertheless, a few small ganglia are percep- tible in different parts, and nervous threads of extreme tenuity are seen to arise from them, and to be distributed in various directions. One pair of ganglia is, in the Dimyaria, easily distinguished, occupying the ordinary position of the brain, namely above the oeso- phagus. Hence is derived a supply of nerves to the sensitive labial appendages, to the oral orifice, and other neighbouring parts. Two other ganglionic masses, of larger size than the brains properly so called, are placed near the posterior retractor muscle ; and a fifth small ganglion, in those species provided with syphons, is found in the vicinity of the breathing-tube, the muscular walls of which receive nerves from this source. Fig. 181. In the Mono- myaria the nervous centres are still more feebly deve- loped, and the pos- terior ganglia pro- portionately smaller than those found in species possessed of two adductor mus- cles. (426.) No or- gans of sense, other than those already noticed, are met within any of the Conchifera, except in one remarkable instance. In the Scallops (Pecten) the edges of the mantle are studded with numerous pearl-like points, interspersed 392 CONCHIFERA. among the retractile tentacula placed around its circumference. These, which are represented in the figure of Pecten already given (j#g.l76), are considered by Poli* to be so many distinct eyes thus singu- fig, 132. larly situated ; and, from the circumstance of their being fur- nished with so many organs of vision, he applied the name of Argus to the Mollusca possessing them. Should the bril- liant specks in question be really ocelli, they cer- tainly are placed in the only posi- tion where they could have been efficient as instru- ments of sight, inasmuch as the margin of the mantle is, in such animals, the only portion of the body capable of being protruded beyond the boun- daries of the shell to a sufficient dis- tance to allow the creature to peep into the world around it. (427.) All the CONCHIFERA are hermaphrodite as relates to the organization of their generative ap- * Poll, Testacea utriusque Sicilian, eorumque Historia et Anatome, 3 vols. fol. CONCHIFEBA. paratus ; or perhaps it would be more strictly in accordance with what is known concerning their mode of reproduction to say that they are all females ; no organ that can be regarded as belong- ing to a male system having, as yet, been pointed out.* The ovary, which in fact is the only viscus distinguishable as being connected with the propagation of these animals, is generally a wide glandular sacculus, occupying a considerable portion of the visceral mass. In the Oyster it is, when full of spawn, largely spread through the body ; and if at such seasons its delicate walls are ruptured, countless ova of microscopic dimensions escape from the lacerated part. In Pecten the ovary is very conspicuous from the brilliant colour of the eggs contained in its interior ; it con- stitutes the greater part of the bulk of that prominent tongue-like organ which projects between the branchiae (fig. 176,/) : or, in genera where the foot is very largely developed, as in Cardium rusticum, a great part of the base of that organ is hollowed out into a capacious cavity, enclosed by its muscular walls, wherein the delicate folds of the ovarium (Jig. 182, a) are partially em- bedded, together with a portion of the intestinal canal (c). (428.) The course of the oviduct has not as yet been satisfac- torily traced, and, consequently, the precise passage by which the eggs are excluded is still a matter of discussion. There is, how- ever, one very remarkable arrangement observable connected with the reproduction of conchiferous Mollusca, the object of which is sufficiently evident. When we consider the position of the ovary in these bivalves, placed as it is in the substance of the body, and reflect upon the immense numbers of eggs to which they give birth, for thousands of ova are generated by every one of these prolific beings, we perceive that, without some special provision, the imprisoned animals would, when gravid, be seriously inconvenienced and exposed to continual danger, as the inordinate enlargement of the ovary would preclude the possibility of bringing the valves of the shell in con- tact with each other. In order to obviate the difficulty referred to, the ova are expelled from the ovarian nidus in an immature * At a late meeting of the Zoological Society, a communication from M. Rudolph Wagner was laid before the meeting, from which it would appear that that gentleman has satisfied himself that in many of the lower classes of animals hitherto regarded as being Monoecious, as for example, in many tribes of Polyps, Acalephae, Tunicata, Conchifera, and Gasteropoda, in some individuals the organ generally looked upon as being an ovary, contains Spermatozoa, or Seminal Animalcules j and thus there is reason to suppose, that in such species a difference of sex exists, and that there are males which supply a fecundating secretion. 394 GASTEROPODA. condition, and complete their growth in a situation where, being diffused over a larger surface, the shells may be closely approxi- mated ; and, moreover, the eggs and their contained offspring are by this contrivance freely exposed to the influence of the medium around, so as to allow a kind of respiration to be enjoyed by the unhatched young. The situation chosen is the branchial fringes, over which the imperfect spawn, or spat, as it is technically termed, is found widely spread towards the close of gestation, still retained beneath the shelter of the shell of the parent, and thus preserved from destruction ; but at the same time, being in such a position freely washed by the ciliary currents, the respiration of the included embryo is adequately provided for. CHAPTER XXIII. GASTEROPODA.* (Cuv.) (429.) EXTENSIVELY distributed over the surface of the land, or inhabiting the waters either fresh or salt, there exists a very nu- merous body of Mollusca, differing widely among themselves in construction and habits, but distinguished by a peculiar locomotive apparatus common to the entire class, by means of which they are able to fix themselves to plane surfaces, and to move from place to place by a slow and gliding motion. The slug, the snail, the limpet, and the welk, afford familiar examples of their general form and external appearance ; but species of different kinds are so common in every situation, that it would be wasting the time of the reader to dwell at any considerable length upon their ordi- nary configuration and usual mode of progression. The bodies of the GASTEROPODA are frequently entirely soft, and devoid of other covering than a thick and slimy skin ; but more generally they are protected by a shell of very diverse form and shape, into which they can retire for protection. Feeble and languid as are the sluggish movements of these creatures, they nevertheless present to the eye of the anatomist a type of or- ganization considerably superior to any that we have had an oppor- tunity of considering in such forms of the HETEROGANGLIATA as have been described in the preceding chapters. From the supe- riority of their mode of progression, it is evident that they are adapted to enjoy a less limited intercourse with external objects than even the most highly gifted of the burrowing CONCHIFERA ; * yaffT*i, the belly ; wawj, a foot. GASTEROPODA. 395 and accordingly we find in them a nervous system exhibiting a more complete developement, senses of a higher character, and, in the organization of their internal viscera, a complexity of parts such as has not heretofore fallen under our notice, every indica- tion, in fact, that they are animals of a higher grade and more elaborate structure. The GASTEROPODA, for instance, exhibit a distinct head, in which is lodged a supra-oesophageal ganglion of large proportionate size ; and upon the head are found retractile instruments of sensation of peculiar structure, and not unfrequently perfectly formed organs of vision. Let us, however, select one species for particular description ; and, after having become acquainted with the details of its ana- tomy, we shall be better prepared to examine such modifications of the various organs ; as are found in other orders destined to exist under different circumstances. (430.) The common Snails (Helix) are well known as far as relates to their external appearance ; and, insignificant as they might be thought by those unacquainted with their habits, they not un- frequently become formidable pests to the horticulturist, from the ravages caused by their voracity. On examining a snail more attentively we find its body partially enclosed in a thick muscular envelope composed of transverse and longitudinal fibres, which, being unsupported by any skeleton, allows the shape of the animal to vary at pleasure, as it is shortened or elongated by the con- tractions of the muscles composing it. The foot, or ventral disc is equally composed of an interlacement of muscular fibres ; and not only forms an extensive sucker, but, by the successive action of various portions of its substance, a slow and gliding progressive motion is produced. From the head of the snail when its body is expanded, as when in the act of seeking food, four tentacula are protruded, (Jig. 195, c, a) which, besides being exquisitely sensitive organs of touch, carry at the extremities of the superior pair two minute but per- fect eyes. When the creature is at rest, the tentacula as well as the eyes are retracted into the visceral cavity by a mechanism hereafter to be noticed. A large proportion of the viscera is en- closed in a turbinated calcareous shell, of sufficient capacity to allow the whole body of the animal to be withdrawn from observa- tion and lodged in its interior. The mouth is situated upon the under-part of the head, and, when widely opened, exhibits a cutting instrument of singular 3.96 GASTEROPODA. contrivance. Attached to the upper part of the muscular cavity that contains the oral apparatus, there is a broad horny plate, the lower edge of which is free, very sharp, and slightly curved, form- ing in fact a knife (Jig. 195, y), admirably adapted to divide the leaves and soft parts of vegetables when they are pressed by the action of the lips against its cutting edge. The floor of the mouth is provided with a small cartilaginous tongue, covered with delicate transverse striae, and so disposed that by its movements it is well calculated to assist in propelling the food into the cesophagus. In many species of Gasteropoda the tongue is indeed even still more efficient as an agent in deglu- tition, being studded all over with minute and recurved hooks, evi- dently intended to take a firmer hold of the substances swallowed. (431.) The cesophagus (jig* 183, e) is continued from the muscular cavity (c') that encloses the dental plate, and soon dilates into a wide stomachal receptacle, v 9 r, the posterior portion of which is when in situ imbedded among the viscera contained in the shell ; but in the figure all these parts are unfolded and separated from each other. At the termination of the stomach, biliary vessels (c) are inserted, and the intestine commences ; the latter being a simple tube (a, e) intervolved among the masses of the liver, nearly of equal diameter throughout, and presenting inter- nally neither valves nor any other remarkable appearance. Ex- ternally the intestine is intimately connected with the lobes of the liver among which it lies imbedded, by means of a delicate cellu- losity and vascular twigs passing from one to the other. The anal aperture (o), when undisturbed by dissection, is placed upon the right side of the neck, in the immediate vicinity of the orifice (fig. 195, e ) that leads into the respiratory cavity. (432.) Two sets of auxiliary glands are subservient to diges- tion, the salivary and the hepatic, both of which are of consider- able size. The salivary glands are semi-transparent and of a whitish colour ; they form two irregular broad ribands, which extend along the sides of the stomach (v), spreading out so as to embrace a con- siderable portion of its extent, and they are occasionally joined together by intercommunicating processes. Two ducts, one de- rived from each gland, run along the sides of the cesophagus, and open into that canal close to the mouth. The liver is of large proportionate dimensions, and is made up of four lobes (b, d) of a dark brown colour, and composed of an GASTEROPODA. 397 infinite number of minute lobules, every one of winch produces a biliary vessel ; and these, joining continually with each other, form four large hepatic ducts, one proper to every lobe of the liver. The four hepatic ducts ultimately unite into one great central vessel (c), that opens into the alimentary canal in the immediate vicinity of the pyloric extremity of the stomach. (433.) The genus of Gasteropoda to which the Snail belongs is composed of air-breathing animals, and we must accordingly ex- pect to find these mollusca provided with a respiratory system specially adapted to the mode of life to which they are destined. The mechanism adopted is as follows : A capacious chamber, of a somewhat triangular form, is found placed beneath the dorsal sur- face of the body, and separated from the visceral cavity by a broad muscular septum forming its floor. Into this chamber a wide orifice (j%. 195, e ), placed upon the right side of the body near the margin of the shell, allows the atmospheric air to enter. The roof of the respiratory cavity is covered with a most intricate arborescence of blood-vessels rudely sketched in Jig. 183, Ar, in which the blood is freely exposed to the air therein contained ; while the muscular floor, performing alternate movements analogous to those of the human diaphragm, continually draws in and expels the air, so as to ensure its constant renewal. The manner in which respiration is effected, and the general disposition of the cir- culatory apparatus, is therefore briefly this : The blood derived from all parts of the body is brought to the respiratory chamber by large veins provided for the purpose ; arrived there, it is dis- persed through the countless ramifications of delicate vessels spread over the entire roof of the breathing cavity, and thus becomes ex- posed to the purifying influence of oxygen. The renovated blood is then re-collected by the large pulmonary vein (k) ; and being conveyed to the heart, which is composed of a single auricle (h) that communicates with a strong ventricular cavity (g), it is propelled through the entire arterial system derived from the aorta (f). (434.) The whole of that part of the body of the snail which is not permanently covered by the shell is defended by a thick skin, the surface of which is irregularly furrowed, and continually moist- ened by a viscid secretion that exudes from glands apparently imbedded in the substance of the integument ; and the tenacious slime so furnished, if the creature be irritated, is poured forth in astonishing abundance. Nevertheless, besides the slimy material thus copiously supplied 398 GASTEROPODA. by the tegumentary glands, there is in the interior of the animal a special apparatus apparently destined to furnish a viscid fluid of a similar character. The gland alluded to, called by Cuvier,* par excellence, " the secerning organ of the viscosity," is in the snail a triangular viscus (Jig. 183, ) placed in immediate con- tiguity with the pericardium. On opening it, it is found to be filled with an infinite number of very thin laminae that adhere to the walls of its cavity by one of their edges, and become joined to Fig. 183. each other as if by communicating branches. The excretory duct of this slime-secretor, which, we may observe, is found to exist in many other genera of Gasteropods, accompanies the rectum to its * Histoire des Mollusques ; M6moire sur la Limace et le Colima^on. GASTEROPODA. 399 termination, where it opens externally in the immediate vicinity of the orifice leading into the respiratory chamber. (435.) Before we enter upon a description of the somewhat complex generative system of a Snail, it will be proper to advert to one or two remarkable circumstances connected with the procrea- tion of these singular animals. We must first premise that every individual is hermaphrodite, and, moreover, presents a kind of her- maphrodism of the most perfect and complete description, possess- ing elaborately constructed male and female organs, which are dis- tinct and separate from each other ; but, nevertheless, the coopera- tion of two individuals is essential to the mutual impregnation of both. The manner in which they copulate is not a little curious ; their union being accompanied by preparatory blandishments of a very extraordinary kind, that to a spectator would seem rather like a combat between mortal foes, than the tender advances of two lovers. After sundry caresses between the parties, during which they exhibit an animation quite foreign to them at other times, one of the snails unfolds from the right side of its neck, where the generative orifice is situated, a wide sacculus, which, by becoming everted, displays a sharp dagger-like spiculum or dart attached to its walls. Having bared this singular weapon, it endeavours, if possible, to strike it into some exposed part of the body of its paramour ; who, on the other hand, uses every precaution to avoid the blow, by speedily retreating into its shell. But, at length having received the love-inspiring wound, the smitten snail pre- pares to retaliate, and in turn uses every effort to puncture its assailant in a similar manner. The darts are generally broken off in this encounter ; and either fall to the ground, or else remain fixed in the wounds they have inflicted. After these preparatory stimu- lations, the snails proceed to more effective advances. The sac of the dart is withdrawn into the body, and another sacculus is by a like process protruded from the common generative aperture. Upon the last-named organ two orifices are seen, one of which leads to the female generative system ; while from the other a long and whip-like penis is slowly unfolded, being gradually everted like the finger of a glove, until it attains the length of an inch or more ; and then each of the two snails, by inserting its penis into the female aperture of the other, impregnates its partner, and is itself impregnated at the same time. Such is the peculiar manner in which the amours of snails are conducted : let us now examine the internal viscera connected with the process. 400 GASTEROPODA. (436.) The sac of the dart first requires our attention. This viscus, when unin verted, for it must be turned inside out in order to expose the we t apon within it, is a thick muscular bag (Jig. 183, a?) ; and, on opening it, it is found to contain the dart attached to a nipple-like protuberance at the bottom of the sac. The dart itself is four-sided ; and as it grows by the constant addition of cal- careous particles deposited at its base from the surface of the vascular protuberance to which it is fixed, so, if broken off, it is speedily reproduced in a similar manner. (437.) The male part of the generative system is composed of a testicle, vas deferens, and the whip-like penis above described. The testicle is considered by Cuvier* to consist of two distinct portions: one, a soft whitish oval mass (Jig. 183, p) ; while the other is elongated, thin and granular (y) 9 being imbedded among the convolutions of the oviduct (z*?). The vas defer ens forms the excretory duct of both these portions, and terminates in the side of the penis ; its orifice becoming of course external when that organ is protruded by evolution. The intromittent organ itself, as seen when lodged within the body of the snail, consists of two parts, a muscular bag which forms its body (b') 9 and a long whip-like portion z ; the latter is hollow, but not perforated. The reader will now have little difficulty in understanding how this remarkable apparatus is protruded. The generative sac, common to both the male and female organs, first becomes inverted ; the body of the penis (>') then undergoes inversion in a similar manner, so that the orifice of the vas deferens appears externally ; and lastly, the long appendage to the penis, z 9 being likewise turned inside out by the action of the muscles that compose its walls, completes this strange- ly constructed instrument. Its subsequent retraction into the visceral cavity is effected partly by the assistance of a special re- tractor muscle (a), which acts upon the body of the penis, but principally by the same contractility that accomplished its evo- lution. (438.) The female system next demands our notice ; and this will be found to present for our investigation an ovary and lengthy oviduct, to which are appended certain auxiliary organs, namely, the spermatheca and the multifid vesicles. The ovary (Jig. 183, s) is found situated in the inmost recesses of the shell, and partially imbedded in the substance of one of the lobes of the liver. From the ovary a long oviduct (g) is de- * Loc. cit. GASTEROPODA. 401 rived, which is at first thin and slender, but, soon becoming wider and more capacious (M), it gradually expands into an extremely convoluted intestiniform viscus, to which the name of uterus has been improperly given, and ultimately terminates in a canal de- rived from the spermatheca, to be described hereafter. It is during their passage through this enormous oviduct that the eggs attain their full growth preparatory to their expulsion from the body. Another viscus, called by Cuvier simply " the bladder," is, from the constancy of its occurrence, evidently an organ of importance ; and there seems to be little room to doubt that it is intended to be a receptacle for the seminal fluid, analogous in function to the copulatory pouches we have already met with in Insects and some Crustacea. The reservoir in question, which we have called sper- matheca (Jig- 183, t), is in the snail placed above the stomach; and the canal derived from it accompanies the sacculated oviduct, which it ultimately joins near its termination, in such a manner that the ova must pass the orifice of its duct as they are expelled from the body. It must nevertheless be confessed that the office here assigned to the "bladder" is rather probable than positively established ; for in the Slug, so nearly allied to the snail in its general organization, the excretory duct of this organ opens into the common generative sac by an aperture distinct from that which leads into the oviduct, although even here the two are closely approximated. Cuvier suggests that perhaps it may furnish some material useful in forming an envelope for the ova, but experiments are still wanting upon this subject. There is still another set of organs connected with the canal by which the eggs escape from the oviduct of the snail ; and these, although peculiar to the genus we are examining, no doubt furnish a secretion of importance to their economy. They are called the multifid vesicles (fig. 183,^), and are composed of a series of branched cseca derived from two excretory ducts by which a milky fluid, secreted by the caeca, is poured into the egg-passage prior to its termination. (439.) Although it will be convenient to speak in more general terms concerning the nervous system of the GASTEROPODA than the examination of a particular species would permit, we deem it necessary, before closing our description of the snail, to describe with some minuteness the senses possessed by these terrestrial mol- lusks, and more especially the extraordinary mechanism provided for withdrawing the most important instruments of sensation into 2 D 402 GASTEROPODA. the interior of the body when they are not in actual employ- ment. The only senses that we can expect to meet with in animals deprived of either an external or internal skeleton, are those of taste, smell, vision, and touch ; any auditory apparatus being of course deficient. The sense of taste, judging from the structure of their tongue, must be extremely obtuse ; and, although these creatures are evi- dently possessed of smell, it is not easy to point out where their olfactory apparatus is placed. The eyes, however, are now found to present a perfection of structure correspondent with the enlarged brain, and occupy a singular position, being situated at the ex- tremities of the two superior tentacula appended to the head ; while the inferior pair, adapted, as it would seem, more exclusively Jty.184. to the perception of tactile impressions, are deprived of visual organs. Both the up- per and lower tentacula are retractile, and can be com- pletely inverted so as to be withdrawn into the in- terior of the body. To effect the inversion by which this end is attained, the plan represented in the accompa- nying figure is had recourse to. Each tentacle is a hol- low flexible cylinder, the walls of which are muscular, and composed of circular fibres. When partially retracted, as in the tentacle marked c in the figure (fig- 184), the extremity of the organ is drawn inwards, and two cylinders are thus formed, one within the other : if the outer cylinder is elongated, as in pro- truding the tentacle, it is at the expense of the inner one ; and, on the contrary, the inner cylinder, when the organ is retracted, is lengthened as the other becomes shorter. To evert the tentacle the contraction of the circular muscles that form its walls is suffi- cient, as they can gradually unroll the whole by squeezing out, as it were, the inner portion ; but to effect its inversion a spe- cial retractor muscle is required, which is represented in the ten- tacle indicated in the figure by the letter b. This muscle (g) GASTEROPODA. 403 arises from the general muscular mass composing the foot and re- tractile apparatus provided for drawing the snail into its shell : the long slip of muscular fibres so derived, accompanied by the optic nerve (/), traverses the interior of the cylindrical tentacle quite to its extremity, where it is attached ; and thus, as the reader will easily conceive, is quite competent to cause its inversion. The lower feeler (d) is represented in the figure as partly retracted by the action of its appropriate muscle k ; while the corresponding one (a), being completely turned inside out, is fully withdrawn and securely packed among the viscera. One circumstance connected with the contrivance above describ- ed cannot but excite attention ; and this is the peculiar arrange- ment of the tentacular nerves, whereby they are adapted to changes of position so extensive: the optic nerve (jf), for ex- ample, must not be stretched even when the eye-bearing tentacula are protruded to the uttermost ; and in order to provide for this, when the feelers are not extended, the nerves become thrown into close folds (A), and lodged within the cavity of the body. (440.) From the above somewhat lengthened account of the ana- tomy of the snail, the reader will at least have been able to become acquainted with the general features of an organization which is more or less common to all the members of the extensive class under consideration. We must now, however, enter upon a more enlarged survey of the GASTEROPODA, and divide them into such groups as will facilitate our further investigations concerning their structure and habits. The most convenient character by which the different orders composing the class are distinguished has been found to be derived from the nature and arrangement of the respiratory apparatus, which of course varies both in construction and position, according to the circum- stances under which particular tribes or families are destined to exist. We have already found that terrestrial species, such as the snail, breathe air, which is alternately drawn into and expelled from a cavity lined with a vascular net-work ; and these, from the resemblance between such a mode of breathing and that of animals possessed of proper lungs, have been formed into an order distinguished by the name of PULMOBRANOHIATA. Ne- vertheless, all the pulmobranchiate Gasteropoda are not terres- trial ; our fresh waters abound with various species that respire air by a similar contrivance, and are consequently obliged, in 2 D 2 404 GASTEROPODA. order to breathe, to come continually to the surface of the shal- low pools wherein they are found. The Planorbis and Limnceus are examples of this mode of respiration ; and are met with in every ditch, where they voraciously devour the subaquatic vege- tables upon which they feed. (441.) It is at once evident that in marine Gasteropods another mode of aerating the blood must be resorted to, and branchiae of some description or other pig. 185. substituted for a pulmo- nary cavity. The branchiae given for this purpose are variously constructed ; sometimes appearing as extensively branched and arborescent appen- dages to the skin, or else they form broad and thin lamellae at- tached to the exterior of the body ; but more frequently the respi- ratory apparatus con- sists of vascular fila- ments arranged in a pec- tinated manner along a central stem : whatever their form, however, their office is the same, namely, to present a sufficient surface to the surrounding medium, in order adequately to expose the blood that circulates abundantly through them to the influence of oxygen. It is from the position and arrangement of the branchial organs that the branchiferous Gasteropoda have been classified by zoolo- gists. Thus in the second order, called from this circumstance NUDIBRANCHIATA, they are naked and placed upon some part of the back ; sometimes, as in Tritonia, extending along its entire length; but at others, as for example in Doris (Jig- 185), they are confined to its posterior part, and form a circle around the anal orifice of exquisite beauty, and not inaptly comparable to a flower in appearance and disposition. GASTEROPODA. 405 In the INFEROBRANCHIATA the branchiae resemble two long rows of leaflets, placed on the two sides of the body, under a projecting edge formed by the mantle. The TECTIBRANCHIATA have respiratory organs upon one side of the body only, and concealed by a flap derived from the mantle. Such, for instance, is the case with Pleurobranchus and Aplysia ; in the former of which the elegant branchial fringe is situated in a deep sulcus between the edge of the mantle and the prominent margin of the foot (Jig. 186, d). But by far the most numerous order of the marine Gasteropoda, (PECTINIBRANCHIATA,) which, in fact, includes all the inhabit- ants of spiral univalve sea-shells, have their branchiae placed inter- nally in a capacious cavity, whereinto the water is freely admitted (Jig. 196, a). This cavity is situated in the last or widest turn of Fig. 186. the shell, and communicates with the exterior of the body by a very wide slit, to which in some genera a long syphon (Jig. 196, /), formed by a fold of the mantle or general covering of the animal, conducts the respired fluid. The branchiae themselves, as the name of the order indicates, are pectinated and form a single, double, or triple series of gills suspended from the roof of the branchial chamber, answering the same intention as the pulmonary net-work of the snail, but deriving their supply of air from the water, in which they are perpetually immersed. In the figure referred to, representing a species of Pterocera, the position of the branchial chamber is seen through the shell and mantle, which the reader must suppose to be transparent; and the branchial organ (a), in this case single, is likewise represented in situ, suspended from the roof of the cavity that contains it. In Jig. 198, the roof of the respiratory cavity (x) has been 406 GASTEROPODA. reflected, and the three rows of branchial fringes (n) suspended therefrom are well seen. A sixth order of Gasteropods has been formed by Cuvier, under the name of TUBULIBRANCHIATA, remarkable from the shape of their shells, which are long and irregular tubes usually fixed to foreign bodies, but still they have the earliest formed portion twisted into a few spiral curves. To this order belongs Vermetus (Jig. 187), the shells of which, agglomerated into masses, might be taken for pi gf 137, those of certain Serpu- I<E. As locomotion is here out of the ques- tion, owing to the im- moveable condition of the habitations of such genera, the foot would seem at first to be al- together deficient, but upon close inspection it is found to be convert- ed into a fleshy organ that bends forward and projects beyond the head, where its extre- mity expands into a disc furnished with a small operculum ; so that, when the animal retires into its abode, a lid is formed adapted to close the aperture, and thus prevent intrusion and annoyance from without. Nevertheless, even in these the bran- chiae are pectiniform, forming a single row attached to the roof of a branchial chamber. The SCUTIBRANCHIATA likewise have pectinated gills dis- posed in a special cavity, but their shells are very wide, and scarcely ever turbinated ; a circumstance which, combined with other features of their economy, renders it convenient to consider them as forming an order by themselves. An eighth division of this extensive class takes the name of CYCLOBRANCHIATA, because the branchiae form a fringe around the body of the animal, between the edge of the body and the foot (fig. 194, c ; fig. 197, a). Lastly, a distinct order has been established to embrace certain families in which the foot is so much compressed as to constitute GASTEROPODA. 407 a vertical muscular lamella, that presents merely a remnant of tlic ventral sucker, so characteristic of the entire class, and \vhich can only be serviceable in performing the office of a fin used in swim- ming ; hence these mollusks have been called HETEROPODA. Their branchise are placed upon the back (Jig- 188, d), and resemble small detached tufts. The form of these heteropod Gas- teropoda the reader will gather from an inspection of the accom- panying figure, representing a species of Pterotrachea ; but the details connected with their anatomy therein delineated, will be explained hereafter. Fig. 188. (442.) It would be useless to weary the student by describing the course of the blood-vessels in all the orders we have just enume- rated ; their distribution necessarily varies with the changes ob- servable in the position of the branchise ; still, whatever the situation of the respiratory organs, the general course of the circulation is the same, and essentially similar to what has been already described in the snail : one or two examples will therefore answer our pur- pose. In the Pectimbranchiata, as for instance in Buccinum (fig. 193), the heart (r, s), enveloped in a distinct pericardium, is placed at the posterior extremity of the branchial chamber, and consists, as in all the GASTEROPODA, of two cavities, a thin membranous auricle, and a more muscular and powerful ventricle. It receives the blood from the organs of respiration by a large branchial vein (Jig. 193, q), that communicates with the auricle (s). The con- traction of the auricle forces the circulating fluid into the ventricle 408 GASTEROPODA. (r), which, in turn, drives it into the aortic or arterial system of vessels. The aorta, in the case before us, divides into two princi- pal trunks ; of which one (TO) is directed forwards to supply the foot and anterior part of the body, while the other (t) winds among the mass of viscera contained in the shell, to which it distributes its ramifications. The blood thus dispersed through the system is taken up by the commencements of the veins, to be re-convey- ed to the branchiae, there to begin again the circuit we have described. When the branchiae are external, and largely distributed over the surface of the body, as for instance in Tritonia, the purified blood is brought from the branchiae to the heart by capacious veins which run beneath each branchial fringe and collect it from the numerous respiratory tufts ; or if, as in Doris (Jig* 185), the branchiae en- circle the anus, a large circular vein placed at the base of the branchial apparatus receives the blood and pours it into the auricle. In all cases, however, the course of the blood is essentially the same, and the heart is systemic. (443.) In Aplysia, one of the tectibranchiate Gasteropods, the branchiae (Jig. 189, a, b) consist of delicate lamellae minutely subdivided ; and the vessel (c), which brings the blood derived from all parts of the body to be distributed over the extensive surface thus formed, presents a structure of no ordinary interest to the physiologist.* At some distance before it arrives at the respiratory organs it divides into two main branches, and the coats of each vessel so formed appear to be made up of transverse and oblique muscular bands that cross each other in all directions, so as to leave between them very perceptible apertures, through which injections of any kind readily escape into the abdominal cavity, and, of course, fluids derived from the abdomen as easily penetrate into the interior of the veins. At some points, indeed, these veins seem absolutely confounded with the visceral cavity ; a few muscular bands widely separated from each other, and not at all interrupting a free communication, being alone interposed. The result of Cuvier's anxious researches concerning this remarkable feature in the or- ganization of these Mollusca led him to the following important conclusions, which are no doubt extensively applicable to the GAS- TEROPODA generally: 1. That in Aplysia there are no other vessels appointed to convey the blood to the branchiae than the two above described. 2. That all the veins of the body terminate in Cuv. Memoire sur le genre Aplysia. GASTEROPODA. 409 these two canals. Now as tlieir communication with the abdomi- nal cavity is evident and palpable, whether we call them vena cava, or cavities analogous to a right ventricle, or branchial arteries, for it is evident that they fulfil the functions of these three organs, the inevitable conclusion is, that fluids poured into the abdominal cavity can become directly mixed with the mass of the blood and thus conveyed to the branchiae, and that the veins perform the office of absorbent vessels. This extensive communication is undoubtedly a first step to- wards the establishment of that, still more complete, which nature has established in insects, where, as we have seen, there are not even distinct vessels of any kind appointed for taking up the nutri- tive fluid. From these facts Cuvier concludes that no proper absorbent system exists in the Mollusca, still less in animals inferior to them in the scale of creation. The vein ^.189. appointed to convey the re- novated blood from the bran- chiae to the heart, when slit open (./zg.189, d), exhibits the orifices of the smaller vessels derived from the respiratory laminae arrang- ed in circles. The auricle of the heart is made up of reticulated fibres (e), and when laid open it is seen to be separated from the more muscular ventricle (g) by a valve (/), whereby any retrograde movement of the blood is prevented. (444.) Such is the construction of the heart in a great majority of the GASTEROPODA ; but in a few of the lowest orders, namely, those most nearly allied to the CONCHIFERA, slight modifications are met with. Thus in Chiton (Jig. 197), so remarkable from the singularity of its shelly covering, the heart is situated in the middle of the posterior region of the back, and is furnished with two auricles, one appropriated to each lateral series of branchiae ; 410 GASTEROPODA. and, what is still more remarkable, each auricle would seem to com- municate with the ventricle by two distinct orifices. In Haliotis, Fissurelld) and others of the Scutibranchiate and Cyclobranchiate orders, the resemblance to the arrangement generally met with among the CONCHIFERA is even more striking ; for in such genera not only are there two distinct auricles, but the ventricle embraces the rectum, so that, when superficially examined, it seems to be perforated for the passage of the intestine. In Pterotrachea (Jig. 188), the branchiae (e) are placed upon the back, and the blood derived from the tufts composing the branchial apparatus is received into a two-chambered heart (e), whence it is distributed to the body through the aorta, which is at first double, but, after surrounding the visceral sac and supplying the viscera, the two vessels unite to form one large trunk (w), which traverses the body as far as the head. (445.) The digestive system of the Gasteropoda, as we might be led to expect from the numerous and widely different forms of the animals belonging to the class under consideration, presents endless diversity of structure; and, did we not strictly refrain from noticing any but the most important modifications, it would be easy to overwhelm the most patient reader with accumulated details. The mouth we shall consider as exhibiting four distinct types of organization ; one of which, namely that met with in the Snail and the generality of pulmonated Gasteropoda, has been already described ( 430). The second form of mouth, that for instance of Pleurobranchus (fig. 186, a, and of Pterotrachea, fig. 188, >), consists of a simple muscular proboscis, or fleshy tube, which is capable of considerable elongation and contraction : such an oral apparatus is entirely de- void of teeth or any cutting instrument, but is, nevertheless, fully able to seize and force into the stomach such materials as are used for food. (446.) A third kind of mouth, by no means so frequently met with as the last, is not a little extraordinary, and forms a more efficient cutting instrument than even that of the snail. We shall offer, as an example of this remarkable organ, that of the Tritonia ffombergii, represented in the annexed figure (fig- 190), whereof Cuvier gives the following graphic description.* In this animal the mouth forms a large, oval, and fleshy mass enclosing the jaws and * M6moire sur le Tritonia. GASTEROrODA. 411 their muscles, as well as a tongue covered with spines, and its opening is guarded by two fleshy lips. The jaws form the basis of all this apparatus : their substance is horny ; pi gt 190. their colour a yellowish brown ; and their form, very extraordinary for an organ of this kind, cannot be better described than by comparing them to the shears used in shearing sheep. They differ, however, in the following par- ticulars : instead of playing upon a common spring, the two blades are found to work upon a joint, and, instead of being flat, they are slightly curved. These two blades are very sharp, and there is nothing that has life that they cannot cut when the animal causes the cutting edges to glide over each other. For this purpose muscles of great strength are provided, the fibres of which are transverse ; and their office is to approximate the two blades, that are again separated by the natural elasticity of the articulation whereby they are united at one extremity. The aliment, once cut by the jaws, is immediately seized by the papillae of the tongue ; which, being sharp and directed backwards, continually drag, by a kind of peristaltic movement, the alimentary materials into the oesophagus. (447.) The fourth and most complicated form of the mouth is found in the pectinibranchiate Gasteropods, and with its assistance these animals can bore through the hardest shells in search of food ; making a hole as round and smooth as if it had been made by a drill of human contrivance. It is from Cuvier we again borrow the subjoined description of this unique apparatus.* The proboscis of Buccinum is organized with marvellous arti- fice : it is not simply provided, like that of the elephant, with the means of flexion and extension, joined with a limited power of contraction and elongation ; but it can be entirely retracted into the body by drawing itself into itself in such a manner that half of it which forms its base contains and encloses the half nearest its point ; and it can protrude itself from its sheath thus formed, by unrolling itself like the finger of a glove, or like the horns of the garden snail, only it is never completely retracted, but always re- mains more or less folded upon itself. It may be represented as being composed of two flexible cylin- * M6moire sur le grand Buccin (Buccinum undatum), et sur son Anatomic. GASTEROPODA. ders, one contained within the other, as shown in the annexed figure (Jig. 191), the upper edges (z, z) of the two cylinders being con- tinuous in such a manner, that, by drawing Fig. 191. out the inner cylinder (i, 6), it becomes elongated at the expense of the other, and, on pushing it in again, it becomes shorter, while the outer cylinder (k) is lengthened by adding to its upper margin. The reader must now imagine a multitude of longitudinal muscles (d, d), all very much divided at both their extremities, and attach- ed by one end to the parietes of the body, whilst by the opposite they are fixed to the interior of the inner cylinder of the proboscis (b) along its entire length, and as far as its extremity. It is evident that the action of these muscles will retract this cylinder, and consequently the entire proboscis, into the body. When thus retracted, a great part of the inner surface of the internal cylinder (b) will necessarily become a portion of the ex- ternal surface of the outer cylinder (k) ; and the contrary when the proboscis is protruded. It is in consequence of this that the in- sertions of the muscles (d, d) vary in position. The protrusion of this proboscis is effected by the action of the intrinsic circular muscles that form its walls. When the proboscis is extended, the retractor muscles (d, d), if they do not act all together, serve to bend it in any direction, thus becoming the antagonists to each other. In the internal cylinder are contained the tongue, with all its apparatus (e, e) ; the salivary ducts (/), and the greater part of the esophagus (g) : but the principal use of the proboscis is to apply the end of the tongue to the surface of bodies that the Buc- cinum wishes to erode and suck. The tongue itself (e) is a car- tilaginous membrane armed with hooked and very sharp spines. It is sustained by two long cartilages, the extremities of which form two lips (c), that can be separated or approximated ; or the cartilages can be made to move upon each other by the mass of muscles in which they are imbedded. When these cartilages move, the spines that cover the tongue are alternately depressed and elevated ; and by a repetition of similar movements, aided per- GASTEROPODA. 413 haps by some solvent quality in the saliva, the hardest shells are soon perforated by this singular file. (448.) The salivary glands are lodged in the visceral cavity, and are composed of numerous secerning cseca enclosed in a mem- branous capsule (Jig. 192, A, A:) : their ducts (g, e) 9 which are ne- cessarily as long as the proboscis when extended to the utmost, open by two apertures placed at the sides of the spinous tongue (b). The oesophagus (fig. 191, g, g) runs along the centre of the proboscis throughout its entire length, and, when that organ is pro- truded, becomes nearly straight ; but, when the proboscis is drawn in, the oesophagus is folded upon itself among the viscera. Just at the commencement of Fig. 192. the stomach there is a small crop (Jig. 192, /), and the stomach itself is single, with- out anything in its texture re- quiring special notice ; its lin- ing membrane being soft, and gathered into longitudinal folds (0- Equally simple is the alimen- tary apparatus of the Hetero- poda. In these the stomach (jig. 188, f) is a mere dilata- tion of an intestiniform tube. The intestine is not lodged in the general cavity of the body ; but, with the mass of the liver, is contained in a kind of bag attached to the back of these singularly formed animals, and in some genera, as for example Carinaria, defended by a de- licate transparent shell, which in appearance offers a miniature resemblance of that of the Argonaut. It is in this visceral sac that the heart and generative apparatus are likewise generally en- closed ; but in many forms of the Heteropoda, both the append- ed sacculus and shell are wanting, in which case the viscera are of course lodged in the general cavity of the body. (449.) But although in Buccinum, Pterotrachea, and kindred 414 GASTEROPODA. genera, the stomach is thus devoid of complication, it is by no means unfrequently found to be provided with a powerful crushing apparatus, that forms a strong gizzard adapted to bruise, cut, or tear the food introduced into it. In Scyllaa, for example, this gizzard, situated at the entrance to the stomach, contains twelve horny cutting blades disposed around its interior, and arranged in a longitudinal direction ; their sharp edges, therefore, meeting in the centre, efficiently divide whatever passes between them to- wards the proper digestive stomach. In Aplysia there is first a capacious crop, then a strong gizzard studded internally with pyramidal blunt teeth, and to this succeeds a third cavity armed with sharp pointed hooks attached to one side of its walls, and so disposed as to form a kind of carding machine by which the food is still more effectually torn to pieces. Various modifications in the form and structure of these sto- machal teeth are met with in the different genera of the GASTE- ROPODA that possess such an apparatus; but whatever their shape, size, number, or position, the office assigned to them i& the same. (450.) The liver is proportionately of very large size in the Mollusca we are now describing. Its composition is similar in all ; being made up of bunches of secreting follicles united by the branches of their excretory ducts, and kept together by means of a delicate cellulosity and the ramifications of blood-vessels. We have already described the hepatic viscera of the snail ; and the liver of Buccinum, unravelled so as to show its intimate structure, is represented in the preceding figure (j^g. 192, n, o, p), which requires no additional explanation. But if the structure of the liver is similar in all the Gasteropod Mollusca, the manner in which the bile is poured into the intes- tine varies remarkably. The most ordinary position of the orifices of the hepatic ducts is at the termination of the stomach, in the vicinity of the pylorus ; as is the case in the majority of other animals : but many exceptions to this rule are met with in the class before us. In Scyllcea the bile is poured into the oesophagus just before it terminates in the gizzard. In many genera the biliary canals open into the stomach itself; and in one remarkable genus, Onchidium, there are three distinct livers, each provided with its proper excre- tory duct ; and, what is still more anomalous, these three glands, which in every particular strictly resemble each other, unless per- GASTEROPODA. 415 haps in size, pour the secretion that they furnish into three dif- ferent situations : the first into the esophagus, the second into the oesophagus likewise, and the third into the gizzard, which forms the first of three stomachal cavities. In Doris, a figure of which is given above, a still more extra- ordinary arrangement is met with. One set of ducts derived from the liver penetrate the stomach, and pour the bile into that cavity ; while another large canal, equally given off from the liver, terminates at the exterior of the body by an orifice situated in the vicinity of the anus (fig- 185) ; and thus a part of the bile secreted would seem to be expelled from the system as excrementitious matter, a fact of no ordinary importance to the physiologist, as it would itself go far to prove that the function of the liver is not merely limited to the supply of a secretion of importance in the digestion of food, but that it powerfully co-operates with the respiratory system in purifying the circulating fluids by decarbonizing the blood. (451.) Other secretions, apparently of an excrementitious cha- racter, are furnished by many Gasteropods. Thus, in Aplysia a glandular mass is imbedded in the opercular flap that protects the gills ; from which, at the pleasure of the animal, a reddish liquor is made to exude in sufficient abundance to obscure the water around it, and thus conceal it from pursuit. Another gland furnishes an acrid limpid fluid, that distils from an orifice near the oviduct ; but the use of this last secretion is as yet unknown. (45#.) The scattered condition of the nervous ganglia, charac- teristic of the HETEROGANGLIATA, is well exhibited in the pec- tinibranchiate Gasteropods ; more especially as it not unfrequently happens that the ganglionic centres themselves are of an orange or reddish colour, while the nerves derived from them present their usual appearance. In Buccinum the brain still occupies its usual position above the oesophagus (Jig. 193, d), and gives off nerves to the organs of sensation, and largfc twigs (c, c) to the eminently sensitive pro- boscis. A large nervous mass placed beneath the oesophagus (*) is connected with the former by several communicating nerves, that embrace the oesophageal tube. Other ganglia, of smaller size (&, /, w),are distributed in distant parts of the body, and supply the vis- cera to which they are contiguous ; whilst they are connected among themselves, and with the brain, by nervous cords passing from one to another. 416 GASTEROPODA. In Pterotrachea Fi ff- 193. the same dispersion of the central gan- glia of the nervous system is equally evident. The brain and nervous collar around the oesopha- gus occupy their usual situation, and give nerves to the tentacles, eyes, and parts around the mouth ; while four smaller ganglia (Jig. 188, f) are placed in the imme- diate vicinity of the foot, to which, and to the neighbour- ing viscera, they distribute their branches. But in the most elevated Gastero- pods the ganglia assume greater con- centration, and the brain exhibits much larger dimensions as compared with the size of the body. Thus in the snail (Jig. 184) we find only two great nervous masses : the brain (7), a large ganglion placed above the oesophagus, and supplying the nerves connected with sensation ; and an equally large suboesophageal mass (m) 9 whence proceed nerves to all the viscera and locomotive organs. Here, therefore, we have another example of the great law that we have already so often illustrated the diminution in number, and the increase in size, of the nervous centres as we rise from lower to more exalted types of animal organization. The tentacula (Jig. 193,/,/) in the marine GASTEROPODA are generally not retractile, and the eyes are frequently situated at the outer side of the base of each tentacle, instead of at their apex, as in the figure referred to ; but, with these exceptions, we can add GASTEROPODA. 417 nothing to what has been said concerning the senses of these Mol- lusca in the description of the snail, already given as an example of the general structure of the entire class. (453.) We now approach an inquiry of much interest as con- cerns the economy of the animals before us ; namely, the varied forms of their organs of reproduction, and the character of the gene- rative system belonging to each order. This investigation, however, is one of no ordinary difficulty ; for so numerous are the modifica- tions of structure observable in almost every genus, that, were we not strictly to confine ourselves to the study of the most prominent and important features of this portion of their history, the patience of the student would be severely put to the test in following us through all the details connected with so extensive a subject. (454.) The three lowest orders of the Gasteropoda are still, in many particulars, more or less allied to the CONCHIFERA ; but more especially this is the case in the organization of their gene- rative apparatus. The Cyclobranchiata, Scutibranchiata, and Tubulibranchiata, like the inhabitants of bivalve shells, are all hermaphrodite and self-impregnating.* A large granular ovary is in all these orders imbedded in the mass of the liver, and from this a duct leads to an external orifice situated in the vicinity of the anus : if impregnation is in such animals essential to fecun- dity, the fertilizing secretion must be furnished by the glandular walls of the oviduct, as no male organs have as yet been disco- vered. (455.) The Pectinibranchiata, on the contrary, are all dioeci- ous ; the sexes being distinct, and intercourse between the male and female necessary for the impregnation of the latter. The male is generally at once distinguished by the penis appended to the right side of the neck (Jig. 193, g), an organ which is fre- quently of enormous proportions ; so large, indeed, that, it being impossible that it should be retracted into the body, it is generally simply folded back into the branchial chamber. The testicle is imbedded in the mass of the liver, and lodged in the inmost recesses of the shell. It gives origin to a long and very tortuous vas de- ferens, which is at first extremely slender, but on emerging from the mass of the viscera it becomes thicker, running along the right side of the body until it enters the penis, and, having made many * The announcement of the discovery of Spermatozoa in individuals belonging to these orders, mentioned in a former page, will, perhaps, materially modify the opinions of physiologists upon this point. 418 GASTEROPODA. zig-zag folds, it reaches the extremity of that organ, where it ter- minates by a small orifice. Equally simple is the structure of the generative system in the females of the PECTINIBRANCHIATE Gasteropods. A large ovary occupies the same position as the testis of the male, and shares with the liver the interior of the windings of the shell. The oviduct generally follows the same course as the vas deferens of the other sex, and is provided with thick and glandular walls. The eggs, which are very numerous, are arranged in long gelatinous ribands, and, after extrusion, are glued in various ways to the surface of rocks, sea-weed, or even to the shells of other Mollusca. Some- times in the siphoniferous tribes, as for example in the common welk (Buccinum), the ova are enclosed in tough coriaceous capsules secreted by a glandular organ in the vicinity of the oviduct. These capsules contain several eggs a-piece, and are joined together in large bunches, such as the waves continually cast up upon every beach. (456.) The HETEROPOD Gasteropoda are hermaphrodite. In Pterotrachea the female organs consist of a distinct ovary, uterus, spermatheca, and an auxiliary gland, all lodged in the visceral sac- culus appended to the back. The ovary (Jig. 188, p) is of con- siderable, size, and gives origin to a slender oviduct, which near its termination communicates with the receptacle for the ova, called the uterus (g). The spermatheca (r) joins the canal leading from the uterine cavity to the exterior of the body, which likewise receives the secretion of two small glandular sacs (k) apparently destined to furnish some investment to the eggs prior to their expulsion. The male parts are situated in the general cavity of the body, quite apart from the female apparatus. The testicles seem to be represented by two wavy cseca (Jig- 188, <), which terminate at the root of a small intromittent organ (s) placed at a short distance behind the opening of the vulva. (457.) All the TECTIBRANCHIATA, INFEROBRANCHIATA, NUDIBRANCHIATA, and the PULMONATED GASTEROPODS are hermaphrodite, having both a male and female generative apparatus arranged upon the same principles as those of the snail, which have already been described at length ; and to enumerate the variations which occur in the relative position and organization of different parts of the reproductive system in all the genera com- posing these extensive orders would scarcely answer any useful pur- pose, even were it practicable within the limits of this work. (458.) Many families of Gasteropoda, as for example the Nu- GASTEROPODA. 419 DIBRANCHIATA (Jig. 185), are absolutely deprived of any shelly defence, the investment of their bodies being entirely soft and contractile. In others, as the slug (Limax), a thin calcareous plate is imbedded in the substance of their muscular covering. This little shell is contained in a cavity within the mantle, and is quite loose and unattached to the walls of the cell wherein it is lodged. The mode of its formation and growth is exceedingly simple, and from its very simplicity is well calculated to illustrate the formation of shells of more complex character. The floor of the cavity containing the calcareous plate is vascular, and secretes cretaceous particles mixed up with a viscid animal secretion. The material thus furnished in a semi-fluid state is applied like a layer of varnish to the lower surface of the shell already formed by the same process ; and the added layer, soon hardening, increases the thickness of the original plate, while at the same time, as a necessary consequence of the progressive extension of the secreting membrane, which enlarges with the growth of the slug, each successive lamina of shell is larger than that which preceded it. Thus the extension of the shell in diameter, as well as its increase in thickness, is easily explained. In these internal shells, however, there is no colouring matter ; so that they are uniformly white, and present the same tex- ture throughout. (459.) As external shells are generally painted upon their outer surface with colours of different kinds variously disposed, in such the process of growth is somewhat more complicated, and in every essential particular resembles that already described, whereby the shells of the CONCHIFERA are extended in size and thickness. We choose, as an illustration of the manner in which the exter- nal shells of univalves are manufactured, one of the least complex forms, as being best adapted to elucidate this part of our subject. The Patella, or common limpet, is covered with a simple conical shell that extends over the whole of the dorsal surface of the mol- lusk. The testaceous shield that thus protects these animals is generally variegated externally with sundry markings of diverse colours, while within it is lined with a smooth and white nacre. On making a perpendicular section of one of these Gasteropods, the entire mechanism by which such shells are constructed and painted is at once rendered intelligible. The whole of the back of the animal covered by the shell is invested with a membranous mantle, like that of a conchiferous mollusk ; but different parts of this mantle are appointed to different offices. The extension of the 420 GASTEROPODA. Fig. 194. shell is entirely effected by the margin of the mantle (fig. 194, 6), which is thick, vascular, and studded with glands appointed to secrete the colouring material that paints the exterior. This thickened fringe of the mantle is firmly glued to the circumference of the opening of the shelly cone : the earthy matter produced by it is added, layer by layer, to the edge of the shell ; and, wherever coloured glands are situated, this earthy secretion is coloured with a corresponding pigment : in this manner is the shell gradually enlarged, and every additional stratum of calcareous deposit is thus painted at the moment of its formation. The growth of the shell in thickness is a subsequent process. After the formation of the outer layer (g) by the edge of the mantle, the ge- neral surface of the pallial membrane (a) adds fresh la- minae of pearly matter (/) to the whole interior of the testaceous shield, and it is by the accumulation of such colourless depositions that the thickening of the entire fabric is provided for. (460.) When the manner in which the limpet constructs its habitation is understood, the formation of a turbinated or spiral shell is explained with the utmost facility. On extracting a snail from its abode, all that portion of its body which was covered by the shell is seen to be invested with a thin mantle (fig. 195, a) precisely ana- logous to that of the limpet : from this pal- lial membrane the nacreous lining of the shell exudes. But around the aperture the mantle swells into a thick glandular collar (), cor- respondent in function with the margin of the mantle in Patella, Fig. 195. GASTEROPODA. 421 and in like manner provided with glands adapted to furnish colour- ing matter. From the collar, therefore, those layers are secreted by which the extension of the shell is accomplished ; and, as the deposit is in this case far more abundant in one direction than in another, the shell, as it expands, assumes more or less com- pletely a spiral shape. Wherever glands for secreting coloured pigment exist, corresponding bands or coloured patches are pro- duced as the layers of growth are formed, and the exterior of the shell is thus painted with the tints peculiar to the species. (461.) In many marine Gasteropods, spines and various external processes are found projecting from the outer surface of the shell, the production of which depends upon the shape of the margin of the mantle. Let the reader imagine one of these ornamented shells to be transparent, so as to permit the contained animal to be de- lineated in situ, as in the annexed sketch of Pterocera (Jig. 196) ; Fig. 196. and the collar, which forms the layers of growth, will be found to exhibit fringes or processes precisely resembling those upon the shell itself. But it is only at intervals that, as the growth of the mollusk proceeds, these pallial appendages encase themselves in a calcareous covering, every such interval being distinctly indicated upon the exterior of the shell by the spaces between the successive rows of spinous projections that mark the terminations of so many distinct periods in its formation ; so that the number of ridges or rows of spines is, of course, correspondent with the age of the creature within. (462.) Several of the Pectinibranchiate genera are provided with 422 GASTEROPODA. a very complete defence against the assaults of foes that might attack them while they are concealed in their habitations, and, in such a posture, necessarily helpless and incapable of resistance. The provision for their protection is sufficiently simple : attached to the posterior extremity of the body, which is the part last drawn into its abode, is a broad horny or calcareous plate (fig- 196, g), called the operculum ; this is of variable dimensions in different species, but always in shape accurately corresponding with the contour of the mouth of the shell. By this elegant contrivance a door is closely fitted to the aperture of its retreat whenever the mollusk retracts itself within its citadel; and, thus defended, it may safely defy external violence of any ordinary description. (463.) A most remarkable exception to the usual univalve con- dition of the shells in the GASTEROPODA, is observable in one solitary genus belonging to the Cyclobranchiate order. In Chi- ton (fig. 197) we find, instead of a turbinated or shield-like Fig. 197. covering formed of one piece, a kind of armour composed of several distinct plates, arranged in a longitudinal series along the centre of the back, and overlapping each other like the tiles of a house. In these curious animals the whole back is invested with a PTEROFODA. dense leatliery mantle of an oval form, and considerably more extensive than the cavity containing the viscera. Where not covered by the calcareous laminae, the exterior of the mantle forms a broad edge variously sculptured in different species : but along its central part the shelly plates, generally eight in number, are partially imbedded in its substance ; being, no doubt, secreted by the surface whereunto they are attached. These mollusks, not- withstanding the singularity of their covering, which almost re- minds us of the armour of many ARTICULATA, in their internal anatomy conform exactly to the type of structure common to the Gasteropod orders, and offer no peculiarities of organization worthy of special notice. CHAPTER XXIV. PTEROPODA* (Cuv.) (464.) NEARLY allied to the Gasteropods in their internal or- ganization, but differing from them remarkably in the character and position of their locomotive apparatus, are the PTEROFODA ; a class of mollusks of small dimensions, but met with in astonish- ing quantities, at certain seasons, in various parts of the ocean. So numberless indeed are these little beings in those regions where they are common, that the surface of the sea seems literally alive with their gambolings ; and thus the store of provisions necessary to render the waters of the ocean habitable to animals of higher grade in the scale of life is still further increased. The great character that distinguishes the members of the class upon the investigation of which we are now entering, is derived from the structure of their organs of locomotion. These are only adapted for swimming, and consist of two broad and fleshy expansions, attached like a pair of wings to the sides of the neck, and forming moveable fins ; enabling the little beings to dance merrily among the foamy waves, now sinking, and again rising to the surface, until some passing whale, opening its enormous jaws, engulphs multitudes of such tiny vic- tims, and hence derives the materials for its subsistence. (465.) Several distinct genera of Pteropoda have been esta- a wing ; vovs, vobos, a foot. 424 PTEROPODA. blished by zoologists, and some important modifications have been detected in their organization ; although, in all of them, the lateral alee form the instruments of progression. The Clio borealis, anatomized by Cuvier,* and more recently and completely investigated by Professor Eschricht of Copen- hagen,'!' is one of the species best known, as well as most abun- dantly met with ; it is, therefore, by a description of this Ptero- pod that we shall proceed to introduce the reader to the general facts connected with the history of the animals under consideration. The body of the Clio is about an inch in length, of an oblong shape, and terminating posteriorly in a point ; while at the oppo- site extremity there is a little head supported upon a short neck, and furnished with delicate retractile tentacles, apparently instru- ments of touch. The locomotive organs, as the name of the class imports, consist of two delicate wing-like appendages (Jtg> 198, a, a) attached to the two sides of the neck ; by means of which, as by a pair of broad fins, the Pteropod rows itself about with facility. But the two aliform membranes, although externally they appear separate instruments, are, as we are assured by the observations of Professor Eschricht, but one organ ; being made up entirely of muscular fasciculi, which pass right through the neck, and spread out on each side in the substance of the wing, forming an appara- tus exactly comparable to the double-paddled oar with which the Greenlander so dexterously steers his kajac, or canoe, through the very seas inhabited by the little Clio we are describing. (466.) The head of one of these animals is surmounted by va- Fig. 198. A B C * Memoire sur le Clio borealis. f- Anatomische Untersuchungen iiber die Clione Borealis, von D. F. Eschricht. Kopenhagen, 1838, 4to. PTEROPOUA. 425 rious organs appropriated to different offices, and some of them not a little remarkable from the amazing complication of structure which they exhibit. On each side of the oral opening are three conical appendages (^/zg.198, c, s), that to a superficial examiner might ap- pear to be mere fleshy tentacula, but, in reality, they are instruments of prehension of unparalleled beauty and astonishing construction. Each of these six appendages, when examined attentively, is seen to be of a reddish tint ; and this colour under the microscope is found to be dependent upon the presence of numerous minute isolated red points distributed over its surface. When still fur- ther magnified, these detached points are evidently distinct organs, placed with great regularity so as to give a speckled appearance to the whole of the conical appendage ; and their number at a rough guess may be estimated at about three thousand. Every one of these minute specks is in fact, when more closely examined, a transpa- rent cylinder, resembling the cell of a Polyp, and containing within its cavity about twenty pedunculated discs, which may be protruded from the orifice of their sheath {fig. 199, c), and form so many prehensile suckers adapted to seize and hold minute prey. Thus, therefore, there will be (3000 x 20 x 6) 360,000 of these micro- scopic suckers upon the head of one Clio ; an apparatus for prehen- sion perhaps unparalleled in the creation. When not in use, the appendages referred to are withdrawn, and concealed by two hood-like fleshy expansions, which, meeting each other in the mesial line, completely cover and protect the whole of this delicate mechanism, as represented in fig. 198, A. Still, however, even when the hoods are drawn over the parts they are intended to defend, the Clio is not left without tactile organs wherewith to examine external objects ; for each valve of the hood is perforated near its centre, and, through the apertures so formed, two slender filiform tentacula {fig- 198, c, &), somewhat resembling the feelers of a snail, are protruded at the will of the animal ; and by means of these it is informed of the presence of food, and instructed when to uncover the elaborately organized suctorial apparatus destined to seize it and convey it into the mouth. The mouth itself is described by Cuvier as being a simple trian- gular opening, resembling the wound inflicted by a trocar ; and in the solitary specimen at his disposal he did not succeed in detect- ing any dental structures. Eschricht, however, with superior oppor- tunities, was more successful in displaying the oral organs ; and found the Clio to possess jaws of very singular conformation, and a 426 PTEJIOPODA. tongue covered, as in many other Mollusca, with sharp horny spines. One of the jaws removed from the body, and magnified twenty- eight diameters, is represented in the subjoined figure (Jig. 199, A). It consists of a series of sharp horny teeth of unequal length, fixed to the sides of a lateral pedicle in such a manner that their points are all nearly at the same level. The teeth them- selves have a golden metallic lustre, and, when examined in the sunshine under water by means of a lens, are especially beautiful objects. The basis to which they are fixed is apparently of a fleshy character, and if smashed by being squeezed between two plates of glass, and then placed under the microscope, it would seem to be made up of a multitude of regularly disposed fibres that cross each other in two principal directions. The jaws thus constructed are placed on each side of the mouth, contained in two hollow curved cylinders, the walls of which are muscular ; and, if one of these muscular capsules be snipped by means of a pair of very fine scissors, the strangely-formed jaw, with its teeth, is found lodged within it. The manner in which the Clio uses these dental organs is ob- vious from their anatomical position. The curved muscular cylin- ders, by the contraction of their walls, force out the teeth, so that they then project from the mouth, and are ready to seize and drag into the oral orifice whatever food presents itself. Once conveyed by the jaws into the interior of the mouth, the prey seized is taken hold of by the tongue ; the free extremity and Fig. 199. PTEROPODA. 427 upper surface of which is seen, when highly magnified, to be cover- ed with regular rows of spiny booklets, all directed backwards, and evidently intended to assist in deglutition (Jig. 199, B). The structure of the alimentary canal is extremely simple. The oesophagus (Jig. 200, t) gradually dilates into a wide stomachal cavity that is surrounded on all sides by the mass of the liver ; while the intestine (u), in which the stomach terminates, mounting towards the left side of the neck, ends by an external anal orifice. Two long and slender salivary glands (w) are placed at the sides of the oesophagus, and furnish a secretion that is poured into the mouth. The precise character of the bile-ducts has not been satis- factorily determined in Clio ; but in Pneumodermon, another Pteropod very nearly allied to the genus we are describing, the stomach itself, which is enveloped on all sides by the liver, receives the biliary secretion through a multitude of minute pores. (467.) With respect to the real nature of the respiratory appa- ratus in C/zo, much doubt exists. Cuvier regarded the aliform fins as being subservient to respiration, as well as forming locomotive organs ; and observes, that the surfaces of these appendages seen with the microscope present a network of vessels so regular, so close, and so delicate, that it is not possible to doubt but that they are intended to perform the functions of a respiratory apparatus, and states, moreover, that their connection with the internal vessels and the heart confirms this view of the nature of these membranes. Eschricht, on the contrary, denies altogether the existence of any such vascular ramifications as Cuvier describes ; asserting that the appearance alluded to is entirely produced by the spreading out of the muscular fibres above mentioned, and that the only vessels visible in the alar processes are a few arterial branches derived from the aorta. We are still, therefore, in ignorance as to the respiratory organs of Clio ; the heart, however, is very apparent : it is composed of a single auricle and ventricle, enclosed in a pericardium (Jig. 200, m), and gives off at one extremity a large vessel (w), which Cuvier regarded as a pulmonary vein, but which Eschricht has proved to be the aorta, inasmuch as he has traced its branches to the liver and the other internal viscera of the body. (468.) The nervous system of this mollusk is easily distinguish- ed, not only on account of the large proportionate size of the ganglia, but from the circumstance of the nerves being of a pale red colour. The ganglia form a ring (Jig. 200, y) placed around the cesopha- 428 PTEROPODA. gus near the middle of the neck. There are eight large and two smaller ganglionic masses closely aggregated in this situation, and from these sources all the nerves of the body are given off. (469.) From the large dimensions of the nervous centres we may be prepared to expect senses of correspondent perfection of structure. We have already mentioned the sensitive tentacula protruded from the hood-like covers that protect the oral apparatus ; but, in addi- tion to these, organs of vision are provided, apparently of a very complete character. These eyes are two in number, and are placed on the back of the neck. Each eye has the form of a somewhat bent cylinder, having its two extremities rounded off. The anterior end of the cylinder is the transparent cornea ; and when the eye is removed from the body of the animal, and examined under the microscope by transmitted light, sundry parts may be detected in its interior, sufficient, indeed, to indicate the existence of a choroid membrane, a vitreous humour, and a distinct lens, occupying the ordinary positions of these parts of the visual apparatus. (470.) The generative system of Clio resembles in all essential particulars that of the most highly organized Gasteropoda ; and, as in them, is composed of a complete set of male organs as well as of ovigerous viscera. According to the views which Cuvier was led to entertain from the dissection of a single specimen, he supposed that the ovary (fig. 200, ri) gave off a slender oviduct (o) terminating in a thick glandular canal, the testicle (k) ; which, beginning by a csecal prolongation, and gradually diminishing in diameter until it became attenuated into a slender vas deferens (p), ultimately emptied itself into a small round sac (q) situated in one side of the neck where it communicated with the exterior. Close to the sac (q) the illustrious French anatomist pointed out another vesicle (r), which he compared to the bladder (spermatheca) of Gasteropod Mollusks. The more complete researches of Professor Eschricht have, however, rendered considerable modifications of the above description requisite ; inasmuch as that gentleman has succeeded not only in detecting a testis quite distinct from the oviferous canal, but also a very complete intromittent apparatus. The testis, in fact, in a fresh specimen is so large as to occupy a great portion of the visceral cavity ; and, no doubt, in the individual examined by Cuvier, which had been kept in spirits of wine, it formed a large portion of the mass (Jig. 200, i), which he thought to be entirely made up of the liver. The duct from this testis communicates with the receptacle (</), so that the glandular canal (k) must be regarded CEPHALOPODA. 429 as a part of the oviduct analogous to what has been called the ute- rus in the snail. Figt 2 oo. Another important discovery, for which science is indebted to the Danish Professor, is, that the Clio pos- sesses a long and sin- gularly - formed penis (fig. 198, c, A), lodged, when retracted, in the interior of the head of the Pteropod ; but which, together with the bladder (g), in which it was contained, can be extruded from the right side of the neck to such an extent that it nearly equals in length the whole body of the little creature. The mass formed by the viscera occupies but a small space in the general cavity of the body. The external investment of the visceral sac is a thin semi-transparent skin (f) of soft texture ; and within this is a second covering (g), thicker than the first, and exhibiting very distinct muscular fibres, principally distributed in a longitu- dinal direction, so that their action would seem to shorten the animal and make its shape more spherical. What fills up the space that intervenes between the muscular tunic and the viscera is as yet undetermined ; but Cuvier, in the memoir above referred to, suggests that it may possibly contain air, which, as it should be compressed or allowed to expand, would form a kind of swimming-bladder, and allow the animal to mount to the surface, or sink into the recesses of the sea, with little effort or exertion of muscular power. (471.) The other genera included in this class agree in their general form, and in the arrangement of their digestive and repro- ductive organs, with Clio above described ; but present a few im- portant modifications in the disposition of their branchiae, and other minor circumstances. In Hyalaa the mantle contains a shell composed of two unequal plates ; one of which is dorsal, and the other ventral : and the 430 CEPHALOPODA. branchiae, which are here distinctly recognisable, form a circle of of vascular leaflets enclosed in a cavity of the mantle situated between the divisions of the shell, and so disposed that the water has free admission to them through the two lateral fissures of its testaceous defence. In Pneumodermon, again, the branchise occupy a totally different situation ; the branchial leaflets being arranged in semicircular lines upon the posterior extremity of the animal : but such modifications of a general type of structure are of more interest to the zoologist than to the physiological reader. CHAPTER XXV. CEPHALOPODA* (Cuv.) (472.) WE now arrive at the highest order of Mollusca, com- posed of animals distinguished by most strange and paradoxical characters, and exhibiting forms so uncouth that the young zoolo- gist, who for the first time encounters one of these creatures, may well be startled at the anomalous appearance presented by beings so remote in their external construction from everything with which he has been familiar. Let him conceive an animal whose body is a closed bag con- taining the viscera connected with digestion, circulation, and repro- duction, furnished with a head and staring eyes ; that upon the head are supported numerous and complex organs of locomotion, used as feet or instruments of prehension ; moreover, that in the centre of the locomotive apparatus, thus singularly situated, is a strong and sharp horny beak resembling that of a parrot ; and he will rudely picture to himself a Cephalopod, such as we are now about to describe. (473.) The Octopus vulgaris, or common Poulpe, represent- ed in the next figure, will serve as an example calculated to prove, we apprehend, that the above is no exaggerated statement ; and, should the student unexpectedly observe an animal of this kind walking towards him upon the beach in the position there de- lineated, his curiosity would doubtless be excited to learn some- thing of its habits and economy. * xs<|)a>.>j, the head ; -rovs, wo^oj, the foot. CEPHALOPODA. 431 Yet not only can the Poulpc walk in the manner exhibited in the subjoined figure {Jig- 201), but it is well able to*swim, if occa- sion require, the broad fleshy expansion that connects the bases of its eight legs being fully adequate to enable it to adopt such a mode of progression ; for, by vigorous flappings of this exten- sive organ, the animal actively impels itself through the water in a backward direction, and shoots along witlf wonderful facility. (474.) The feet or tentacula appended to the head are not, how- 201. ever, exclusively destined to effect locomotion ; they are used, if required, as agents in seizing prey ; and of so terrible a charac- ter, that, armed with these formidable organs, the Poulpe be- comes one of the most destructive inhabitants of the sea; for neither superior strength nor activity, nor even defensive armour, is sufficient to save its victims from the ruthless ferocity of such a foe. A hundred and twenty pairs of suckers, more perfect and efficacious than the cupping-glasses of human contrivance, crowd 432 CEPHALOPODA. the lower surface of every one of the eight flexible arms. If the Poulpe but touch its prey, it is enough : once a few of these tena- cious suckers get firm hold, the swiftness of the fish is unavailing, as it is soon trammelled on all sides by the firmly holding tenta- cula, and dragged to the mouth of its destroyer ; the shell of the lobster or of the crab is a vain protection, for the hard and crooked beak of the Cephalopod easily breaks to pieces the frail armour ; and even man himself, while bathing, has been entwined by the strong arms of gigantic species, and struggled in vain against a grasp so pertinacious. (475.) In the genus Octopus the arms are only eight in num- ber, and nearly of equal length ; but to the Calamaries (Loligo) and other genera an additional pair is given, which, being pro- longed considerably beyond the rest, are not merely useful for seiz- ing prey at a distance, but become convertible to other purposes, and may be employed as cables whereby the Cephalopods so fur- nished ride securely at anchor in a tempestuous sea ; the suckers being placed upon an expanded disc situated {Jig. 214) at the extremity of the elongated tentacula, and thus rendered capable of taking firm hold of the surface of a rock or other fit support. The posterior extremity of the body is in such forms generally provided with two broad muscular and fin-like expansions {Jig. 214), evidently adapted to assist in sculling the animal along. Wonderful as are the provisions above described for insuring food and safety to these formidable inhabitants of the sea, it is only by an attentive examination of the individual suckers, so numerously distributed over the tentacula, that the reader will fully appreciate the mechanism we are so inadequately describing. Machines of human construction admit of being variously esti- mated, as they are found to be more or less adapted to accomplish the object of the contriver : but in estimating the works of the DEITY all degrees of comparison are merged in the superlative ; everything is best, completest, perfect. Examine any one of these thousand suckers, it is an admirably arranged pneumatic apparatus, an air-pump. The adhesive disc {Jig. 202, A) is composed of a muscular membrane, its circum- ference being thick and fleshy, and in many species supported by a cartilaginous circlet, so that it can be applied most accurately to any foreign body. In the centre of the fleshy membrane is an aperture leading into a deep cavity (6), at the bottom of which is placed a prominent piston (c), that may be retracted by muscular CEPHALOPODA. Fig. 202. fibres provided for the pur- pose. No sooner, therefore, is the circumference of the disc placed in close and air- tight contact with the sur- face of an object, than the muscular piston is strongly- drawn inwards ; and, a va- cuum being thus produced, the adhesion of the sucker is rendered as firm as mechan- ism could make it. (476.) Yet even this ela- borate and wonderful system of prehensile organs would seem, in some cases, to be insufficient for the purposes of nature. In the powerful and rapacious Onychoteuthis (fig. 203 ) the cupping- glasses which arm the ex- tremities of their long pair of muscular arms are rendered still more formidable ; for from the centre of each sucking-cup projects a strong and sharp hook, which is plunged by the action of the sucker deeply into the flesh of struggling or slippery prey, and thus a firm and most efficient hold upon the seized victim is secured. Nor is this all that claims our admiration in the organization of the arms of Onychoteuthis : at the base of each fleshy expansion that sup- ports the tenacious and fanged suckers above described is a small group of simple adhesive discs, by the assistance of which the two arms can be locked together (fig. 203, A), and thus be made to co-operate in dragging to the mouth such powerful or refractory prey as, singly, the arms might be unable to subdue ; an arrange- ment which has been rudely imitated in the construction of the obstetric forceps.* * Cyclop, of Anat. and Physiol. art. CEPHALOPODA. 2 F 434 CEPHALOPODA, Ff^.203. A B (477.) The Argonaut constitutes another family of the CEPHA- LOPODA, and is remarkable as being the inhabitant of a shell of exquisite beauty, familiarly known as that of the Paper-Nautilus ; a shell which from remote antiquity has been decorated with all the ornaments of fiction, and celebrated alike by Poetry and her sister Arts. It was, indeed, to this CEPHALOPOD, that the ancients assigned the honour of having first suggested to mankind the possibility of traversing the sea in ships ; and nothing could be more elegant than the little barque in which the Argonaut was supposed to skim CEPHALOPODA Fig. 204. 435 over the waves, hoisting little sails to the breeze, and steering its course by the assistance of oars provided for the purpose. The figure annexed (fig. 204), given by Poll in his magnificent work already referred to,* was in perfect accordance with the gene- rally received opinion ; and on such respectable authority we are not surprised to find Cuvier assenting to and sanctioning the statement, that, when the sea is calm, fleets of these little sailors might be seen navigating its surface, employing six of their tentacula or arms instead of oars, and at the same time spreading out two, which are broadly expanded for the purpose, instead of sails. Should the waves become agitated, or danger threaten, the Argo- naut, as we are told, draws in his arms, lowers his sail, and, set- tling to the bottom of his shell, disappears beneath the waters. * Testacea utriusque Siciliw. 436 CEPHALOPODA. It is a thankless office to dispel the pleasant dreams of imagina- tion ; yet such becomes our disagreeable duty upon this occasion. M. Sander Rang, in a recently published memoir upon this sub- ject,* has, from actual observation, apparently established the fol- lowing facts : 1 st, That the belief, more or less generally enter- tained since the time of Aristotle, respecting the skilful manoeuvres of the Poulpe of the Argonaut in progressing by the help of sails and oars on the surface of the water, is erroneous. 2nd, The arms which are expanded into membranes have no other function than that of enveloping the shell in which the animal lives, and that for a determinate object to be explained hereafter. 3rd, The Poulpe, with its shell, progresses in the open sea in the same manner as other Cephalopods. And lastly, That, when at the bottom of the ocean, the Argonaut, covered with its shell, creeps upon an infundibuli- form disc, formed by the junction of the arms at their base, and presenting (alas !) the appearance of a Gasteropod mollusk. (478.) It is not a little remarkable that the same animal should, even in these days, be the subject of the extremes of credulity and scepticism ; yet such has been the case with the Argonaut. While zoologists were contented to allow the creature in question the reputation of being an active and skilful navigator, it has been very generally stigmatised as a pirate, which, having forcibly possessed itself of the shell of another animal, lived therein, and made use of it for its own purposes. It was in vain to urge, in opposition to this calumny, that the Argonaut was never found in any other shell than the beautiful one represented in the preceding figure ; that no other creature had been pointed out as the real fabricator of its abode ; that, whatever the size of the Poulpe, it occupied a re- sidence precisely corresponding in dimensions with those of the possessor. The apparent want of resemblance between the out- ward form of the animal {fig' 206) and that of its fragile covering, together with the absence of any muscular connection between the two, were looked upon as furnishing sufficient evidence of its para- sitical habits. The recent observations of Madame Jeannette Power, to be noticed more at length hereafter, and those of M. Sander Rang, above alluded to, have, however, completely settled the so long agitated question ; and, the Argonaut having been watched carefully from the state in which it leaves the egg until it arrives at maturity, the manner in which it forms and repairs its frail shell is now satisfactorily understood. * Guerin's Magasin de Zoologie, translated into the Magazine of Natural History, vol. iii. New Series, p. 521. CEPHALOPODA. 437 . (479.) A still more interesting group of CEPHALOPODS, and one which in former periods of the world has been extensively disseminated, inhabited chambered shells ; but of all the varied forms of these creatures, whose remains are so abundantly met with in a fossil state, and known by the names of Ammonites, Belem- nttes, Nummulites, Sec. two species only have been found to be at present in existence, the Spirula, an animal as yet imperfectly known ; and the Nautilus Pompilius, of which the only specimen obtained in modern times* has been the subject of a monograph by Professor Owen, who has most completely investigated its gene- ral organization and relations with other families of the Cephalo- poda. The shell of the Pearly Nautilus (N. Pompilius) is extreme- ly common, and may be met with in every conchological collection, Fig. 205. notwithstanding the extreme rarity of the mollusk that inhabits it ; a circumstance, perhaps, to be explained by the fact that the * For this invaluable addition to zoological knowledge science is indebted to George Bennet, Esq. who obtained the living animal near the island of Erromanga, New Hebrides. " It was found in Marekini bay, floating on the surface of the water not far distant from the ship, and resembling, as the sailors expressed it, a dead tortoise- shell cat in the water. It was captured, but not before the upper part of the shell had been broken by the boat-hook in the eagerness to take it, as the animal was sink- ing when caught." Mr. Rennet's Journal. 438 CEPHALOPODA- living animal dwells in deep water, and when it comes to the surface is so vigilant against surprise, that at the slightest alarm it sinks to the bottom. On making a section of the shell its cavity is found to be partitioned off by numerous shelly septa into various chambers (Jig. 205, s, s), in the last of which the body of the animal is situated. A long tube, or siphuncle (A, A), partly cal- careous and partly membraneous, passes through all the compart- ments quite to the end of the series. The membranous siphuncle is continued into the animal, and terminates in a cavity contained within its body, hereafter to be described, which is in free com- munication with the exterior. Various conjectures have been indulged in concerning the end answered by the camerated condition of the shell in these MOLLUSCA. Dr. Hooke* suggested the idea that the chambers might be filled with air generated by the Nautilus, and thus made so buoyant that the specific gravity of the animal and its shell should correspond with that of the surrounding medium, and that, acting in the same manner as the swimming-bladder of a fish, the creature would float or sink, as the air in its shell was alternately compressed and rarefied. Should this supposition be correct, it would seem probable, as Dr. Buckland has pointed out, that the simple retraction of the head, by injecting water from the chamber within its body (pericardium) into the membranous siphuncle, would cause the needful condensation of the air con- tained in this singular float, and allow the Nautilus to sink to the bottom ; while the protrusion of its arms, by taking off the pressure, and thus allowing of the expansion of the confined air, would give every needful degree of buoyancy, even sufficient to permit the mollusk to rise like a balloon to the top of the sea. The body of this Cephalopod is covered with a thin mantle (a, a), of which a large fold (b) is reflected on the exterior of the shell. It is securely fixed to its residence by two lateral muscles, the insertion of one of which is seen at g. A large coriaceous hood (n) covers the head, and, when the creature retreats into its habitation, closes the entrance like a door ; while through the infun- dibulum (i) the ova and excrementitious matters are expelled from the body. The most remarkable feature, however, exhibited in the external conformation of Nautilus, is the conversion of the sucker-bearing arms of other Cephalopods into an elaborate appara- tus of tentacular organs appended to the head (o, o) ; but these, as well as the eye (wi), will be more minutely described as we proceed. * Philosophical Experiments and Observations, 8vo. 1726. CEPHALOPODA. 439 (480.) Turning our attention to the anatomical structure of the CEPHALOPODA, we find that in all of them the exterior of the body is entirely formed by an intricate interlacement of muscular fibres. The sac that contains the viscera, itself muscular, is united to the head by strong and largely-developed fasciculi ; the funnel (fig. 206, a) through which, as through a fleshy pipe, the products of excretion, as well as the eggs or seminal fluid, are ejected, is form- ed of a tissue similarly endowed with contractility ; while the arms are composed externally of muscles disposed in various directions, and moreover have their central portion occupied by strong bands, which traverse them longitudinally from end to end, so that they are thus gifted with all needful powers of motion, and may be shortened, elongated, or bent in any direction at pleasure. In those species which, like Loligopsis (fig. SI 4), or Onycho- teuthis (Jig. 203), have fins appended to the sides of the visceral sac, these organs likewise are made up of muscular substance ; and, being thus converted into broad moveable paddles, they also form efficient locomotive agents. (481.) One important circumstance observable in the class be- fore us must not be forgotten in connection with this portion of the history of the Cephalopods. We may remind the student, that in the vertebrate division of animated nature, to which these creatures immediately lead us, the locomotive system is support- ed by an internal vascular and living skeleton, composed either of cartilage, as is the case in the most imperfect vertebrated genera, or, in the more highly organized forms, of bones articulated with each other, and possessing within themselves the means of growth and renovation derived from the blood which permeates them in every part. The reader will remember that, in all the classes that have offered themselves to our notice, we have not hitherto observed anything at all comparable to an internal osseous framework such as man possesses ; dead, extra vascular shells, formed by successive depositions of layers of calcareous material, or jointed cuticular ar- mour equally incapable of growth, having as yet represented the ske- leton, and formed the only levers upon which the muscular system could act in producing the movements connected with locomotion. Having, however, already had abundant opportunities of seeing how gradually nature proceeds in effecting the developement of a new series of organs, we might naturally be led to expect in the creatures before us some faint indications, at least, of our approach to animals possessed of an internal bony framework, and our ex- 440 CEPHALOPODA. pectations in this particular will be found on investigation to be well-grounded. It is, in fact, in the CEPHALOPODA, the highest of the molluscous classes, that the rudiments of an osseous sys- tem for the first time make their appearance ; not, indeed, as yet composed of perfect bone, but formed of cartilaginous pieces, some being so disposed as to protect the ganglionic mass above the oesophagus, which now from its size well deserves the name of brain, whilst others serve to afford bases of attachment to the mus- cular system in different regions of the body. The most important piece met with in the cartilaginous skeleton of the Cuttle-fish encloses and defends the brain, and therefore is most appropriately called the cranial cartilage, being the corre- spondent both in position and office with the cranium of a vertebrate animal. This rudimentary cranium (fig. 215) embraces the oeso- phagus with a cartilaginous ring, encases the brain, affords passage to the optic nerves, and gives off orbital plates for the protection of the eyes. This cartilage likewise gives a firm origin to the muscles of the locomotive tentacula appended to the head, and, moreover, contains within its substance an auditory apparatus, presenting the earliest condition of an organ of hearing such as is met with in the vertebrate division of the animal kingdom ; in every respect, there- fore, it claims to be considered as the first appearance of a skull. Another broad cartilage is imbedded among the muscles at the base of the funnel ; and two distinct plates situated in the lateral fins of such species as possess appendages of that description offer, un- doubtedly, the rudiments of those portions of the skeleton that sustain the locomotive limbs of quadrupeds. (482.) But while we thus see in the CEPHALOPODA the earliest form of an internal osseous skeleton, we cannot be surprised to find these mollusks still retaining, at the same time, the tegumentary calcareous shell or epidermic skeleton of inferior animals. On slitting up the mantle of a Calamary (Loligo) along the me- sial line of the back, it is found to contain a large cavity, wherein is lodged a long plate of horn, called the gladius, which in shape might be not inaptly compared to the head of a Roman spear. This enclosed horny substance, notwithstanding the dissimilarity of texture, is, in fact, strictly analogous to the enclosed shell of the Slug, described in a former page ; and its growth is effected in the same manner, namely, by an exudation of corneous material from the floor of the chamber that contains it, and this horny secretion, hardening as it is deposited layer by layer, adds to the dimensions CEPHALOPODA. 441 of the gladius as the growth of the animal proceeds. Several of these plates may be produced in succession, and in old individuals it is not uncommon to find two or three enclosed in the same cavity, and placed one behind the other ; that nearest the visceral aspect of the chamber being the most recently formed. These rudimentary shells have no connection whatever with the soft parts of the Calamary, to which, in fact, they are so little adherent that they fall out as soon as the sac wherein they are secreted is laid open. In the Cuttle-fish (Sepia officinalis} the dorsal plate (os Sepia) is found in the same situation as the gladius of the Calamary, from which, however, it differs remarkably both in texture and compo- sition. The cuttle-bone, with the appearance of which every one is familiar, is principally composed of calcareous substance ; and, were we to judge of its weight from its bulk, would seem calculated materially to interfere with the movements of an aquatic animal, destined to swim about, and consequently needing whatever assist- ance might be derived from lightness and buoyancy. Did a crea- ture so apparently destitute of natatory organs possess a swim- ming-bladder like that of a fish, to assist in supporting it in the water, we should conceive such an apparatus to be far more adapted to its predatory habits than a shell so bulky as that which it is destined to carry. (488.) We have, however, already seen in the case of the Nauti- lus that it would be by no means impracticable to convert a shell into a float nearly equalling a swimming-bladder in efficiency ; and on more accurate examination it becomes obvious that even in the bone of the Cuttle we have a provision of a similar nature, though the end arrived at is obtained in a very different manner. On making a section of a cuttle-bone, it will be found to be composed of numerous stages of very thin calcareous plates placed at some distance above each other, and kept apart by the interposition of mil- lions of microscopic pillars. Thus organized, the shell in question becomes sufficiently light to float in water ; and consequently, from its buoyancy, no doubt assists, instead of impeding, the movements of the mollusk. This admirable float, like the horny gladius of Loligo, is lodged in a membranous capsule, and enclosed in the back of the Sepia, having no connection whatever with the sides of the cavity wherein it is placed, but so loose that it readily falls out on opening the sac. (484.) The cuttle-bone is formed in the same manner as other 442 CEPHALOPODA. Fig. 206. shells, by the continued addition of calcareous laminae secreted by that side of the containing capsule which is interposed between the shell and the abdominal viscera ; and these layers, being successively added to the ventral surface of the shell, thus gradually increase its bulk as the Cuttle-fish advances to maturity. Neither in the mode of its growth nor in its texture, therefore, does the os Sepice resem- ble bone, properly so called ; it receives neither vessels nor nerves, but is in all respects a dermal secretion, imbedded in the mantle, and formed in the same manner as the dorsal plate of the Slug. (485.) We now come to consider the long disput- ed question relative to the nature of the shell of the Argonaut. The Poulpe that inhabits the elegant abode repre- sented in a preceding fi- gure ( Jig. 204), when removed from its testaceous covering, has the general form of an Octopus. Its body (Jig. 206) is en- closed in an ovoid mus- cular sac (d), and the head is surmounted by eight long sucker-bearing arms, of which six (e,/)taper gradually from their origins to their extremities, while the other two, formerly regarded as sails, and which we shall con- d CEPHALOPODA. 443 tinue to designate by their ordinary name, vela, expand into broad membranes (b). M. Sander Rang, who, during a residence at Algiers, had ample opportunity of studying the living Argonaut, ascertained that in the figure copied from Poli, which we have given in a preceding page, the animal is placed in its shell in a reversed position ; and that, when alive, the creature is always found with its veliferous arms turned towards the spire of its shell, instead of in the opposite direction, as represented in the drawing referred to. Moreover, the vela, instead of forming sails, are invariably tightly spread out over the external surface of the shell (Jig. 207), which they cover and entirely conceal from view. With its veliferous arms thus firmly embracing its abode, the Argonaut has two modes of progression. It can certainly raise itself from the bottom, and sport about at the surface of the water ; but this is simply effected by the ordinary means used by Calamaries and Cephalopods in general, namely, by admitting the sea-water into its body and then ejecting it in forcible streams from its funnel, so as to produce a retrograde motion, which is sometimes very rapid. Its usual movements are, however, confined to crawling at the bottom with its head downwards ; and in this way it creeps, carrying its shell upon its back. The reader will obtain a better idea of the real appearance of the Argonaut in its shell by inspecting the annexed copy of M. Ranges figure than from any verbal description, and we borrow that gentleman's own account of its general appearance.* The membranous portions of the expanded arms, dilated beyond any- thing we could have pictured to ourselves while knowing the animal merely by specimens preserved in spirits of wine, are spread over the two lateral surfaces of the shell in such a manner as to cover it completely from the base of the hard edge to the anterior extremity of the edge of the opening, and consequently the keel. The application of these membranes is direct, and with- out any puckering or irregularity whatever : the lower part of the two large arms being completely stretched, so as to form a kind of bridge over the cavity left between the back of the mollusk and the retreating portion of the spire. When the mollusk contracts itself, it frequently draws in more or less completely its large arms * For more ample details upon this subject, the reader is referred to an excellent translation of M. Rang's paper contained in Mr. Charlesworth's Magazine of Natural History. New Series, vol. iii. 444 CEPHALOPODA. and their membranes, so as partially to uncover the shell in front, as is represented in the figure (Jig- 207). Fig. 207. There is little doubt that the vela of the Argonaut, which thus envelope its abode, are the organs employed in constructing the brittle fabric, and the agents whereby fractures and wounds in the shell are repaired and filled up. The positive experiments of Madame Power* leave no doubts upon the subject ; for not only did that lady, by rearing young Argonauts from the egg, watch the first appearance and earliest growth of the shell, but, by breaking the testaceous covering of adult specimens, she found that they could readily repair the damage inflicted. Being desirous of observing the manner in which this operation was accomplished, the lady to whom science is indebted for these interesting researches examined an individual on the day after its shell had been intentionally broken, and found that the aperture was already covered by a thin glutinous lamella, which, although as yet as delicate as a cobweb, united the margins of the fracture. The next day the lamina had become thickened to a certain degree and more opaque ; till at length, at the end of ten or twelve days, the new piece had become quite calcareous. Madame Power is likewise certain that, while in the act of mend- ing the fractures, the Argonaut applied its vela to the exterior of the shell, and wrinkled them upon it ; whence they may naturally be regarded as being the source from which the glutinous secretion that finally became hardened into shell proceeded. * Magazine of Natural History, April 1839. Observations on the Poulpe of the Argonaut, by Madame Jeannelte Power. CEPHALOPODA. 445 (486.) In order to understand the manner in which the remark- ably constructed camerated shells, such as those of Nautilus, are produced, it is not necessary to imagine any deviation from the simple mode of procedure adopted in all the cases we have as yet considered. The continual elongation of the spiral cone is, as is evident from the lines of growth visible upon its outer surface, effected by the addition of successive layers to the margin of the aperture of the last-formed chamber, wherein the animal resides ; and as the production of the calcareous secretion whereby the shell is enlarged is most rapidly effected upon that side of the body where the funnel (Jig. 205, i) is situated, the gradually expand- ing shell naturally revolves around an eccentric axis. While the growth of the shell continues, the animal is constantly advancing forwards, and thus leaves the first-formed portions of the shell un- occupied. At intervals, as the Nautilus thus removes itself fur- ther and further from the bottom of its abode, that portion of its mantle which covers the general surface of its visceral sac (fig. 205, d) secretes floors of shelly substance behind it ; and thus the septa, s, 5, are formed whereby the shell is separated into chambers, every chamber having in turn been occupied by the body of the Nautilus. The gradual prolongation of the fleshy siphon (h) is easily understood, because it naturally increases in length with the growth of the animal : but how the two muscles (fig. 205, g), that fix the body to the shell, progressively advance their points of attachment as the shell enlarges, is not so readily explained ; nei- ther are we prepared to account satisfactorily for the accomplish- ment of this part of the process. (487.) It has been already stated that in all Cephalopods the aperture of the mouth is situated in the centre of the disc formed by the union of the origins of the feet (Jigs. 210, 214). The oral orifice is generally surrounded by a broad circular lip (Jig. 208, A, a), which being not unfrequently fringed or papillose, there is little doubt of its possessing sufficient sensibility to render it of material assistance in manducation. The circular lip partially conceals a pair of strong horny man- dibles, not unlike the beak of a parrot, but differing in this parti- cular, that in the Cephalopod the upper mandible is the shorter of the two, and is overlapped by the lower jaw. - The mandibles detached from the soft part are represented in fig. 208, B, a, b. There is likewise another important difference between the struc- ture of the beak of the Cuttle-fish and that of the bird, inasmuch 446 CEPHALOPODA. as in the former there is no bony support to the horny jaws, and consequently some other means of sustaining them must be had recourse to. We accordingly find the place of the jaw-bones sup- plied by a fibro-cartilaginous substance (fig. 209, c) that fills the interior of each mandible, and thus gives it sufficient solidity for all required purposes. Externally, the jaws are imbedded to a considerable depth in a strong mass Fig. 208. of muscle (fig. 08, &), composed of several layers of fibres variously disposed, so as to open or close the jaws with a degree of force propor- tioned to their large size. Here, therefore, is an apparatus fully ade- quate to co-operate with the elabo- rately constructed prehensile arms whereby these predatory animals seize their prey ; and a victim once involved in the tenacious grasp of the tentacula, and dragged to this powerful beak, can have but little chance of resisting means of destruc- tion so formidable as those granted to the Cephalopoda. The mandibles of Nautilus Pom- pilius, instead of being entirely composed of horn, as is invariably the case in these genera that, being provided with tentacula armed with suckers, are thus capable of seizing active and slippery animals, would seem to be rather calculated to break to pieces the testaceous coverings of Mollusca or the armour of the Crustacea. They possess indeed the shape of the jaws al- ready described, but are blunt at their extremities (Jig. 10, w, o), and thickened by a covering of a dense calcareous substance ; so that they appear manifestly adapted to crush hard substances, rather than to cut or lacerate the tender bodies of fishes.* The jaws of the Nautilus, like those of the Octopus above described, are embedded in a powerful mass of muscles (p) whereby they are opened and shut with great force, and are also provided with a * Owen. Memoir on the Pearly Nautilus ; London, 1832, 4to. CEPHALOPODA. 447 distinct muscular apparatus destined to protrude them when in use, and again to retract the whole mass of the mouth deeply into the body when unemployed. The mechanism provided for the pro- trusion of the mandibles is a strong semicircular muscle (Jig- 210, r, r), which firmly embraces the base of the oral apparatus, and by its contraction pushes it outwards among the labial tentacula (A, k) ; while, on the other hand, four retractor muscles, the upper pair of which are represented in the figure referred to (<?, <?), arise from the extremities of the cranial cartilage, and, running forwards to be inserted into the oral mass, are the agents whereby the whole is again withdrawn and thus concealed from view. (488.) The tongue of the CEPHALOPODA, as in the Mollusca described in the two last chapters, is an exceedingly important in- strument, and from its construction would here seem to be an organ of taste, as well as a necessary assistant in deglutition. Tn the annexed figure, representing a vertical section of the beak of a very large Onychoteuthis, the shape and disposition of the differ- ent parts of the tongue are well seen. The substance of the Fig. 209. a tongue itself is fleshy (fig. 209, e, i), and its movements are principally performed by the action of its own intrinsic muscular 448 CEPHALOPODA. fibres : its surface is divided into several lobes (/, g, A), partially invested with a delicate and papillose membrane ; but a large portion of tlie organ is covered with sharp recurved horny hook- lets, so disposed that, with their assistance, the morsels of food taken int# the mouth are seized and dragged backwards by a kind of peristaltic motion to the commencement of the ffisophagus (z). The necessity of the provision thus made for enabling the Cepha- lopods to swallow the substances upon which they feed, must be at once apparent ; for, seeing that the walls of the mouth are formed entirely by the hard and inflexible horny beak, it is difficult to conceive how deglutition could have been accomplished by any other contrivance. (489.) Four salivary glands pour a copious supply of saliva into the oral chamber : of these, two, situated on the sides of the root of the tongue, give off distinct ducts, which terminate near the com- mencement of the O3sophagus ; while the other pair, generally larger than the superior, is lodged in the visceral sac on each side of the upper part of the crop. The inferior salivary glands each furnish an excretory canal ; but their two ducts soon unite into a single tube (wi), which, with the oesophagus, passes through the ring formed by the cranial cartilage, and, piercing the fleshy mass of the mouth, opens in the neighbourhood of the spiny portion of the tongue, so that the secretion furnished at this point serves to moisten the aliment as it is taken up by the lingual hooks to be swallowed. In Onychoteuthis two salivary glands (fig* 09, &) are situated at the root of the tongue, and their ducts are pointed out in the drawing by pins introduced into their orifices. (490.) The alimentary canal presents the same general struc- ture in all the Cephalopod families. The oesophagus (fig. 208, A, d',jig. 10, s), derived from the posterior part of the fleshy mass of the mouth, passes through a ring formed in the cranial cartilage ; or else, as in Nautiluses partially embraced by processes derived therefrom. It soon dilates into a capacious crop (fig- 210, ), the walls of which are glandular; and, being lined with a mucous membrane that is gathered into longitudinal plicae, this organ readily admits of considerable dilatation. From the crop, a short passage (u) leads into a strong muscular gizzard (v) resembling that of a granivorous bird, and lined in the same manner by a thick coriaceous cuticular layer : in this gizzard, therefore, the food is gradually bruised and reduced to a pultaceous magma. CEPHALOPODA. 449 (491.) At a little distance from the gizzard there is in the Nautilus, appended to the side of the intestine, a globular viscus (fig. 10, 3/), which is hollow, and its cavity communicates freely with the intestinal canal. The interior of this organ Professor Owen found to be occupied by broad parallel laminae, puckered transversely so as to offer a great extent of surface ; and, when examined under a lens, their structure was seen to be follicular, and F. 210. evidently fitted for secretion. The bile is poured into this cavity at the extremity farthest from the intestine, by a duct large enough to admit a common probe. 450 CEPHALOPODA. In other genera this laminated viscus is represented by a csecal appendage to the intestine, placed precisely in the same situation ; and, on opening it, its internal surface is found to be increased by a spiral lamella that winds closely upon itself from one end to the other. In such cases it is near the apex of the spire that the bile is received from the liver, so that in all essential particulars this spiriform viscus is precisely analogous to the laminated cavity of the Nautilus. There can be little doubt that this apparatus re- presents a capacious duodenum, and that it is by the extensive surface afforded in its interior that the nutritious portions of the food are separated ; as neither the gizzard nor the intestine itself present an organization adapted to such a purpose. With respect to its other uses Professor Owen remarks, that its reception of the biliary secretion renders it in some measure analogous to a gall-bladder ; but most probably its chief office is to pour into the commencement of the intestinal canal a fluid which is neces- sary for the completion of digestion, so that, like the pyloric ap- pendages of fishes, it might be considered to be the representative of a pancreas. The remainder of the intestine is a simple tube, which, after one or two turns upon itself, mounts up to the base of the funnel, into which it opens ; and thus allows the excrement to be ejected to a distance from the body. (492.) The liver (fig- 210, z) is of very great bulk when com- pared with the rest of the digestive apparatus. In Nautilus it is divided into four distinct lobes, which are themselves made up of numerous lobules of an angular form, each being invested with a very delicate capsule. On removing the capsule every lobule is seen to be composed of numerous acini, which with a needle may be readily separated into clusters connected by the ramifications of their excretory duct. In other genera, such as Octopus, wherein these acini have been minutely examined, they have proved to be delicate cells or secerning cseca wherein the bile is elaborated. The excretory canals derived from all the lobules of the liver unite by repeated anastomoses, and thus form two main trunks, which ultimately join, and pour the biliary secretion into the lami- nated or pancreatic cavity (y). In the Cephalopods, as in all the Mollusca, the bile is separated from arterial blood supplied by large vessels derived immediately from the aorta ; no system of veins analogous to the vena porta of higher animals being as yet developed. CEPHALOPODA. 451 In the Dibranchiatc genera the liver is either undivided or pre- sents only two lobes, but in other respects its composition and minute structure is similar to that of the Nautilus t (493.) In all the CEPHALOPODA, with the exception of the Nautilus Pompilius, there is an orifice in the immediate vicinity of the anus, through which a coloured secretion, generally of a deep brown or intense black colour, can be poured in astonishing abundance, and, becoming rapidly diffused through the surround- ing water, a means of defence is thus provided ; for no sooner does danger threaten, or a foe appear in the vicinity of the Cuttle- fish, than this ink is copiously ejected, and the element around rendered so opaque and cloudy, that the Cephalopod remains completely concealed from its pursuer, and not unfrequently en- sures its escape by this simple artifice. The organ wherein the inky secretion is elaborated, is a capacious pouch variously situated in different genera. In Octopus it is enclosed in the mass of the liver; in Loligo it is located in the immediate vicinity of the anus ; and in Sepia (Jig- 21 1, q) the ink-bag is lodged near the bottom of the visceral sac. On opening it and carefully washing away by copious ablution the ink within, the cavity of the ink-bag is seen to be filled up with a spongy cellulosity; wherein the black- ing material had been entangled ; and from this cellular chamber a duct leads to the outward orifice, through which the dark secretion is ejected at the will of the animal, and squirted from the extremity of the funnel. (494.) The CEPHALOPODA breathe by means of branchiae, and possess a complex and elaborate circulatory system, organized upon very extraordinary principles, to the consideration of which we now invite the attention of the reader. The branchiae (Jig. 211, g, g) in all the genera now known to exist, with the exception of the Nautilus, are two in number, one situated on each side of the body ; but in the Nautilus Pompilius there are four branchial organs, two on each side : and hence Pro- fessor Owen has divided the class into two great orders, under the names of Dibranchiata and Tetrabranckiata , the former em- bracing all the ordinary genera, while the latter is, as far as we know, only represented in modern times by the Pearly Nautilus, depicted in a preceding figure. In both the Dibranchiate and Tetr 'abranchiate orders, each bran- chia consists of a broad central stem, to which is appended a series of vascular lamellae seen in the figure given below (Jig. 211, g) ; O r. Q Xi G << 452 CEPHALOPODA. by this arrangement a very extensive surface is obtained, over which the blood is diffused for the purpose of respiration. The respiratory apparatus is lodged within the visceral sac, but sepa- rated from the other viscera by a membranous septum (Jig. 211, t) ; so that a distinct chamber is formed to contain the branchiae, where- unto the water is freely admitted ; the surrounding element being alternately drawn into the branchial cavity by the action of its muscular walls, through a valvular aperture provided for the pur- pose, and again expelled in powerful streams through the orifice of the funnel. Such, indeed, is the force with which the water is ejaculated through the funnel, that it not only serves to expel from the body excrementitious matter derived from the termination of the rectum (Jig. 211, s), which opens into the respiratory cavity, but becomes one of the ordinary agents in locomotion. This mode of progression, although in fact common to most of the Cepha- lopod tribes, is remarkably exemplified in the Argonaut, which, instead of navigating the surface of the sea, as has been already stated, simply darts itself from place to place by sudden and oft- repeated jets thus violently spouted forth ; while with its arms stretched out and closely approximated, and its vela tightly ex- panded over the outward surface of its delicate shell, it shoots backwards like an arrow through the water. (495.) Separated from the chamber in which the branchiae are lodged, by the. membranous partition already mentioned (Jig. 11, ), and likewise distinct from the peritoneum containing the viscera, is a considerable cavity, divided by a membranous partition into two compartments, wherein are placed the great trunks of the venous system (d, d). These chambers, named by Cuvier* the " great venous cavities," are very remarkable ; in as much as, although they contain the vena cava, which here pre- sent a truly anomalous structure, they are lined with a mucous membrane derived from the branchial chamber, with which they are in free communication, and from whence the external element has free admission to their interior. It is in this " great venous cavity" called by Professor Owen the " pericardium" that, in the Pearly Nautilus, the syphon which traverses the partitions of its camerated shell (Jig. 205) terminates ; and the reader will now perceive by what mechanism water received from the branchial chamber may, in that animal, be injected into its partitioned shell for the purpose already referred to (479). * M6moire sur le Poulpe. CEPHALOPODA. 453 (496.) In the "great venous cavities" or "pericardium" thus formed, are lodged the principal venous trunks (fig. 211, rf, d), whereunto the blood derived from all parts of the body is brought by capacious vessels (6, c, c) that may be called the vena cavte. The great central receptacles of the venous blood (d, d), whilst they are contained in the pericardium, (or rather project into its interior, being partially covered with the mucous membrane that lines its walls,) are enveloped by a mass of spongy appendages of a most remarkable and peculiar description. These spongy masses Fig. 211. 454 CEPHALOPODA. are of a yellow colour, and, when squeezed, they give out an opaque yellowish mucosity ; * but the most interesting circumstance con- nected with these bodies is, that they communicate by large and patulous apertures with the interior of the veins to which they are adherent. The short canals derived from these apertures are themselves pierced by very numerous orifices, and so on succes- sively, until each of the spongy bodies referred to is permeated internally by a multitude of short vessels leading one into another, and ultimately into the vein itself. Cuvier supposes, that, seeing it is impossible that these vessels should not be filled with blood, they might themselves be considered as veins; but then their extent, when compared with the very small arteries of the spongy bodies, forbids us to believe that they have no other office than that of bringing back into the general current of the venous circulation blood derived from these arterial ramifications. He suggests, therefore, that they more probably form diverticula, in which the venous blood may become diffused in order to receive, through the intervention of their spongy walls, the influence of the surrounding medium, so that in this way they may be rendered subservient to respiration ; or else it is possible that the orifices in the veins are the openings of excretory canals derived from these appendages, through which they may pour into the vein some substance derived from the water in which they float. Lastly, it is conjectured that they may be emunctories, through which some principle separated from the blood is discharged from the body through the pores upon their surface ; a supposition rendered more probable, seeing the abundant mucous secretion that may be extracted from them by pressure. " However this may be," observes Cuvier, " it is cer- tain that the communication between these bodies and the exterior is very open, for, on blowing into or injecting the vein, the air or injection passes very readily into the cavity that the vein tra- verses ; and, on the other hand, on inflating the cavity from the branchial chamber, it often happens that the vein becomes filled with air."" Mayer-[- not only adopts the last of the above-mentioned sug- gestions relative to the nature of these spongy appendages to the great veins of the CEPHALOPODA, but ventures to bring forward an opinion that they perform the office of the kidneys of higher animals, and separate from the blood a fluid analogous to the * Cuvier, M6moire sur le Poulpe, p. 18. f- Analekten fur Vergleichenden Anatomic, 4to. 1835. CEPHALOPODA. 455 urinary secretion ; so that, according to this view, the anatomist referred to does not scruple to designate the chamber called by Professor Owen " the pericardium " as a urinary bladder, and to the two orifices leading from thence to the cavity in which the branchiae are lodged he would assign the name of urethra. Pro- fessor Owen has suggested that, in addition to their subserviency to secretion, these appendages to the veins of Cephalopods may be provisions for enabling their sanguiferous system to accommo- date itself to those vicissitudes of pressure to which it must be constantly subjected, and that they bear a relation to the power possessed by these animals of descending to great depths in the ocean, thus answering the same purpose as the capacious auricle, and the large venous sinuses that terminate in the heart of fishes. According to this view, these follicles relieve the vascular system, by affording a temporary receptacle for the blood when- ever it accumulates in the vessels, owing to a partial impediment to its course through the respiratory organs, serving in this man- ner to regulate the quantity of blood sent to the branchiae.* (497.) In Nautilus Professor Owen found, in addition to the spungoid appendages connected with the veins, lodged in what he denominates the " pericardium" that the great trunk of the vena cava itself presents a structure precisely analogous to what has been already described when speaking of the venous system of Aptysia among the GASTEROPODA (^ 443), namely, a free communication between the interior of the vein and the cavity of the peritoneum. ~f* The vein is of a flattened form, being included between a strong membrane on the lower or ventral aspect, and a layer of transverse muscular fibres which decussate each other on the upper or dorsal aspect. The adhesion of the coats of the vein to the muscular fibres is very strong, and these fibres form in consequence part of the parietes of the vein itself throughout its whole course. But there are several small intervals left between the muscular fasciculi and corresponding round apertures both in the vein and in the peritoneum, so that the latter membrane at these points seems to be continuous with the lining membrane of the vena cava. The distinguished anatomist referred to counted as many as fifteen of these openings, and most of them were sufficiently large to admit the head of an eye-probe. Here, therefore, as in Aplysia, there are direct communications between the interior of the vena cava and the great serous cavity of the abdomen ; and, moreover, in both in- * Mem. on Nautilus Pomp. p. 34. f Mem. on the Pearly Nautilus, p. 72. 456 CEPHALOPODA. stances, from the peculiar muscular structure of the vein at the part where these orifices occur, their use appears to depend on, or to be in connection with, a power of regulating their diameters.* (498.) The blood derived from the great venous receptacles (d, d) is at once conveyed to the branchiae, and distributed through all the lamellae (g, g) which enter into the composition of the respiratory apparatus. Two distinct hearts, one placed on each side of the body, are interposed between the branchiae and the great trunks of the venous system ; serving by their action forcibly to drive the blood through the ramifications of the branchial arteries. These lateral hearts (Jig. 211, e, e) are of a blackish colour, and their walls moderately thick : internally, their cavities are filled with intercommunicating cells, and, moreover, a strong mitral valve is placed at the orifice through which they receive blood from the veins, as well as smaller valvules at the origin of the branchial arteries ; the latter enter the principal stem of the branchiae, and, running beneath the ligament (/)? divide and subdivide, so as to be dispersed over all the branchial leaflets. In Sepia there is appended to each lateral heart a fleshy appen- dage (TTI, w), which, however, is not met with in the generality of Dibranchiate Cephalopods. These bodies are attached to the hearts by narrow pedicles, and Professor Owen considers them to be rudiments of the additional pair of branchiae met with in the Pearly Nautilus. In Nautilus Pompilius the hearts just mentioned do not exist ; doubtless, because the greater extent of surface afforded by the four branchiae of this Cephalopod renders the presence of extra- ordinary agents for impelling the blood through them, in order to ensure efficient respiration, unnecessary. After undergoing exposure to the surrounding medium in the extensive ramifications of the branchial arteries, the purified blood is returned to the organs belonging to the systemic circulation. In Sepia it is first received from the branchiae by two dilated sinuses (z, t), which might almost be regarded as systemic auricles ; and from these it passes into a strong muscular cavity (/r), which corresponds in function with the left ventricle of the human heart, and by its pulsations forcibly propels the blood through all the arterial ramifications of the vascular system. Two aortae, one de- rived from each of its extremities, arise from the systemic ventricle, the commencement of each being guarded by strong valves so dis- * Opuscit. p. 30. . . :>'.* CEPHALOPODA. 457 posed as to prevent all reflux towards this central heart ; and thus the circuit of the blood, accomplished in this complicated system of blood-vessels, is completed. In Nautilus the lateral sinuses (w, n) are wanting, and the systemic ventricle is of a square shape ; but in other respects the course of the circulation is the same as is above described. (499.) In the nervous system of the CEPHALOPODA we may naturally expect to find not only a superiority in the developement of the nervous centres, as compared with the condition of these important masses in the lower Mollusca, but some indications at least of an approximation to that arrangement so eminently cha- racteristic of the vertebrate division of the animal world, to the confines of which we are now gradually approaching ; more espe- cially as in the activity of the movements of these creatures, and in the increased perfection of their senses, we have abundant evidence of the elevated position assigned to them, when contrasted with other mollusks of less carnivorous and rapacious habits. The nervous ganglia from whence the muscles and viscera derive their supply are still numerous and widely scattered ; but their size is considerable, and proportioned to the importance of the organs over which they preside. It is to the encephalic portions of the nervous system, however, that we must principally turn our atten- tion if we would rightly estimate this part of their economy ; and these, we at once perceive, have in the class before us attained to such magnitude and importance that they no longer dubiously emulate the brain of a fish, with which it is not difficult to com- pare them. In a Cephalopod, the encephalon for so we now may truly call it is enclosed, as has been already noticed, in a distinct cartila- ginous skull, which embraces it on all sides, and defends it from injury. The capacity of the cranial cavity is however more than sufficient to contain the brain ; and, as is the case in fishes, the interspace is filled up with a semigelatinous substance. The brain, however, still forms a ring through which the oesophagus passes ; so that we might with propriety preserve the terms supra- oesophageal and infra-oesophageal ganglia, were these parts not now become so intimately united to each other that they seem fused into a single mass (Jig. 215, a, &), from different portions of which, nerves, serving very different offices, take their origin. (500.) In Nautilus the nervous system has been most minutely and critically examined ; and the important deductions to which the 458 CEPHALOPODA. researches of Professor Owen, relative to the analogies that may be traced between the encephalon of these creatures and the brain of higher animals, have served to attach an interest to the study of this part of the economy of the CEPHALOPODA, which has scarcely as yet been sufficiently appreciated by physiologists. In the Nautilus Pompilius, the supra-cesophageal ganglion of the GASTEROPODA is represented by a thick round cord of nervous matter (Jig. 212, N), which is in communication with two nervous collars (3, 4) that surround the oesophagus, and likewise with two large ganglia (2) from which the optic nerves take their origin ; but in the Cuttle-fish the same portion of the nervous system (Jig. 215, a) is much more largely developed, and presents a gan- glionic mass of considerable size. If we inquire the reason of this want of correspondence in magnitude presented by the same organ in these two cases, we must necessarily examine the relations in which this part of the brain stands with other circumstances in the economy of the two animals in question ; and we perceive, as Professor Owen has most satisfactorily demonstrated,* that the brain is here developed in accordance wLh the relative complexity of the organ of vision, and also with the perfection of the loco- motive faculties possessed by the Cephalopods under consideration. With the exception of sundry small twigs given off to the mouth and pharynx, the optic nerves (figs. 212, 2; 215, e) are the only ones derived from this part of the encephalon, and, as we shall afterwards see, both the simply constructed eye of the Nautilus and the complicated visual organs of the Sepia are correspondent to the developement of the supra-cesophageal brain ; so that conse- quently the latter may, with every show of reason, be looked upon as the representative of the optic lobes found in the encephalon of fishes,']" and the analogues of the bigeminal bodies in the brains of the higher Vertebrata. The ganglia connected with the inferior aspect of the supra- cesophageal mass form two distinct collars embracing the oeso- phagus, an arrangement of which we have already met with an example in Clio borealis among the Pteropod Mollusca. The anterior ring of nervous substance, which no doubt ought rather to be considered as an agglomeration of ganglia than as a simple * Descriptive and Illustrated Catalogue of the Physiological series of Comp. Anal, contained in the Museum of the Royal College of Surgeons in London, vol. iii. part i. p. 187. f- Cyclopaedia of Anat. and Physiol. ; art. CEPHALOPODA. CEPHALOPODA. 459 ganglionic mass, in Nautilus gives off nerves, 1st, to the ophthalmic tentacles (fig. 212, 5*) ; 2ndly, to the digital tentacles (6). Srdly, there arises from near the ventral aspect of the ganglionic collar a pair of nerves (7), each of which soon dilates into a large ganglion (8), from whence are derived the nerves of the internal labial tentacles (9), and also other gangliform nerves (10), distri- buted to what Professor Owen regards as the olfactory apparatus. Lastly, the anterior collar gives off nerves (11) which penetrate the muscular integument and supply the infundibulum. In the Dibranchiate Cephalopods the nerves derived from that portion of the brain that may be regarded as analogous to the an- terior collar of Nautilus, supply the locomotive sucker-bearing arms, the labial apparatus, and also the auditory organs (fig. 215, c, d) ; but the latter have not been found to exist in Nautilus Pompilius. There is no possibility of doubting that the above nerves, dis- tributed as they are to the complex sensitive tentacula connected with the head and parts of the mouth, represent the fifth pair of Vertebrata ; their general distribution and semiganglionic cha- racter being, ccteris paribus, precisely similar : so that those por- tions of the brain of vertebrate animals from whence the tri- facial and auditory nerves originate, may reasonably be compared with the anterior sub-cesophageal collar of the Cephalopoda. The posterior sub-cesophageal ganglionic ring (fig. 212, 4), may be compared to the medulla oblongata of quadrupeds ; in Nautilus it gives origin, 1st, to numerous nerves (13) which, after a short course, plunge into the muscular parietes of the body to which they are distributed : 2ndly, to two large cords (14), which terminate by becoming gangliform (15), and supply the branchial apparatus and the viscera ; thus representing the par vagum in their distribution, and in like manner communicating with branches ap- parently corresponding with the sympathetic nerves that are spread out over the heart and ramifications of the vascular system. Lastly, slender nerves (17), allied to the sympathetic, accompany the vena cava into the abdomen. (501.) Such being the arrangement of the principal nervous ganglia, and the general distribution of the nerves, we must now turn our attention to the instruments of sensation possessed by these comparatively highly gifted animals ; and these, as we shall soon perceive, are in all respects correspondent in the perfection of their structure with the exalted condition of the brain. 460 CEPHALOPODA. The sense of touch, as might naturally be expected, resides principally in the tentacula, or feet, as they are generally termed, placed around the mouth, and forming, as we have already seen, instruments of locomotion as well as prehensile organs. In the Dibranchiate Cephalopods these tentacula are armed with the tena- cious suckers described in a former page ; but in the Nautilus they are so peculiar both in structure and office, that a more elaborate description of them becomes requisite in this place, for which of course we are necessarily indebted to the same source from whence we have derived all our information relative to this extraordinary animal. The head of Nautilus (fig. 205) is of a conical form, and of a much denser texture than the analogous part in the Dibranchiate Cephalopods : it is excavated in such a manner as to form a re- ceptacle or sheath, into which the mouth and its more immediate appendages can be wholly retracted, and so completely concealed as to require the aid of dissection before they can be submitted to examination. The orifice of this great oral sheath is anterior, its superior parietes being formed by a thick triangular hood (fig* 205, n) with a wrinkled and papillose exterior ; while the sides give off numerous conical and triedral processes (o, o, o) : the in- ferior portion of the cone is thin, smooth, and concave, and rests upon the funnel (z). From the disposition of the hood, and the tough coriaceous texture of its substance, it is evident that this part is calculated to perform the office of an operculum by closing the aperture of the shell when the body of the animal is retracted. The lateral processes (o, o, o) are thirty-eight in number, nine- teen on either side, irregularly disposed one upon another, and all converging towards the oral sheath ; but, as the hood itself con- sists apparently of two very broad digitations conjoined along the mesial line, twenty pairs of these lateral appendages may be enu- merated. There is not the slightest appearance of acetabula, or suckers, upon any of these cephalic appendages ; but their exterior surface is more or less rugose : each is traversed longitudinally by a canal, in which is lodged an annulated cirrus or tentacle (Jig. 205,^. 212), which is about a line in diameter, and from two inches to two inches and a half in length. In the specimen ex- amined, a few of these cirri were protruded from their sheaths to the extent of half an inch, but the rest were completely retracted so as not to be visible externally ; and, on laying open some of the canals, the extremities of several were found as far as a quarter of CEPHALOPODA. 461 an inch from the apertures, so that they appear to possess con- siderable projectile and retractile powers. To the above forty tentacula must be added four others of a different construction, which project immediately beneath the mar- gin of the hood, like antennae, one before and one behind each eye (Jig. 212, r). These tentacles would seem at first sight to be constructed upon the same principles as the last ; but, on examining 462 CEPHALOPODA. them attentively, they are found to be composed of a number of flattened circular discs appended to a lateral stem. Yet even all these organs of touch form but a small part of the tactile apparatus of the Nautilus Pompilius ; for the mouth, lodged within the oral sheath, is surrounded with a series of tentacula even more nume- rous than those appended to the exterior of the head. Around the circular lip (Jig- SIS, m) which encloses the beak (w, o), are situ- ated four labial processes (g 9 g, i, i) : each of these processes is pierced by twelve canals, the orifices of which are disposed in a single but rather irregular series along their anterior margin ; and every one of these canals contains a tentacle similar to, but rather smaller than, those of the external digitations (h, A, A-, A;), although their structure is precisely similar. These cirri, like the former, receive large nerves ; those supplying the external labial tentacles being derived immediately from the brain (Jig. SIS, 6, 6), while those distributed to the internal labial tentacles proceed from a large ganglion (8) that is in communication with the cesophageal ring through the intervention of a considerable nervous trunk (7). (502.) In the Dibranchiate Cephalopods none of the above- described cirriferous processes are found to exist ; but there is every evidence that the prehensile arms, and most probably the indivi- dual suckers appended to them, are highly sensitive to tactile im- pressions. Every one of the arms receives a large nerve, derived from the same portion of the oesophageal collar as that which gives origin to the tentacular nerves of Nautilus, which traverses its whole length, lodged in the same canal as the great artery of the limb (Jig- SOS). During this course the nerve becomes slightly dilated at short distances, and gives off from each enlargement numerous small nervous twigs which penetrate into the fleshy substance of the foot. Immediately after entering the arm and producing the dila- tations above alluded to, every nerve furnishes two large branches, one from each side, which traverse the fleshy substance connecting the bases of the arms, to unite with the nerves of the two conti- guous arms, so that all the nerves of the feet are connected near their origins by a nervous zone ;* an arrangement intended, no doubt, to associate the movements of the organs to which these nerves are appropriated. (503.) There is little doubt, from the character of the soft and papillose membrane which forms a considerable portion of the sur- face of the tongue, that in both the Nautilus and in the Dibran- * Cuvier, M6moire sur le Poulpe, p. 36. CEPHALOPODA. 463 chiate Cephalopoda the sense of taste is sufficiently acute ; far supe- rior, indeed, to what is enjoyed by any of the Gasteropod Mollusca, and possibly even excelling that conferred upon fishes, and others of the lowest Vertebrata that obtain their food under circumstances such as render mastication impossible, and the perception of savours a superfluous boon. (504.) That the Cephalopoda are provided with a delicate sense of smell, and attracted by odorous substances, is a fact established by the concurrent testimony of many authors, although in the most highly organized genera nothing analogous to an olfactory apparatus has as yet been pointed out : nevertheless, in Nautilus, Professor Owen discovered a structure which he regards, with every show of probability, as being a distinct organ of passive smell, exhibiting the same type of structure that is met with in the nose of fishes ; and, from the circumstance of its being the first appearance of an organ specially appropriated to the perception of odours, well deserving the attention of the physiologist. We may here premise, that the exercise of this function in creatures continually immersed in water must depend upon conditions widely differing from those which confer the power of smelling upon air-breathing animals. In the latter, the odorant particles, wafted by the breeze to a distance and drawn in by the breath, are made to pass, by the act of inspiration, over the nasal passages ; and, being thus examined with a minute- ness of appreciation proportionate to the extent of the olfactory membrane, give intimations of the existence of distant bodies scarcely inferior to those obtained from sight and sound. But, in an aquatic medium, information derived from this sense must be restricted within far narrower limits ; inasmuch as the dissemination of odoriferous particles must necessarily be extremely slow, and the power of perceiving their presence comparatively of little import- ance, seeing that the extent to which it can be exercised is so materially circumscribed. Smell, in aquatic animals, is therefore apparently reduced to a mere perception of the casual qualities of the surrounding element, without any power of inhaling odours from a distance. Simple contact between a sufficiently extensive sentient surface, and the water in which it is immediately immersed, is all that is requisite in the case before us ; and if an organ can be pointed out, constructed in such a manner as to adapt it to fulfil the above intention, there can be little hesitation in assigning to it the office of an olfactory apparatus. (505.) In Nautilus, the part indicated by Professor Owen* as * Loc. cit. p. 41. 464 CEPHALOPODA. appropriated to the sense of smell, consists of a series of soft membranous laminae (Jig. 210, I ; fig. 212, g) compactly arranged in the longitudinal direction, and situated at the entry of the mouth, between the internal labial processes. These laminae are twenty in number, and are from one to two lines in breadth, and from four to five in length, but they diminish in this respect to- wards the sides. They are supplied by nerves (Jig. 212, 10) from the small ganglions (8) which are connected to the ventral extremi- ties of the anterior sub-cesophageal ganglia, and from whence the nerves of the internal labial tentacula are likewise given off. (506.) The structure of the eyes in the two divisions of the Cephalopoda differs remarkably, and in both is so entirely dissimi- lar from the usual organization met with in other classes of animals, that we must invite the special attention of the reader to this por- tion of their economy. In the TETRABRANCHIATA, of which the Nautilus is the only example hitherto satisfactorily investigated, according to Professor Owen's observations* the eye appears to be reduced to the simplest condition that an organ of vision can assume without departing altogether from the type which prevails throughout the higher classes ; for although the light is admitted by a single orifice into a globular cavity, or camera obscura, and a nerve of ample size is appropriated to receive the impression, yet the parts which re- gulate the admission, and modify the direction of the impinging rays, were, in the specimen examined, entirely deficient. In this structure of the eye, observes Professor Owen,*f- the Nautilus ap- proximates the Gasteropods, numerous genera of which, and espe- cially the PECTINIBRANCHIATA of Cuvier, present examples analo- gous in simplicity of structure, and in a pedicellate mode of support and attachment to the head. Moreover, as the Pearly Nautilus, like the latter group of mollusks, is also attached to a heavy shell, and participates with them in the deprivation of the ordinary lo- comotive instruments of the Cephalopods, the anatomist whose remarks we quote hence deduces the more immediate principle of their reciprocal inferiority with respect to their visual organ, observ- ing that it would little avail an animal to discern distant objects, when it could neither overtake them if necessary for food, nor avoid them if inimical to its existence. The eyes of Nautilus (Jig. 205, m) are not contained in orbits, but are attached each by a pedicle to the side of the head, im- * Mem. on Nautilus, p. 39, et seq. f Op. cit. p. 51. CEPHALOPODA. 465 mediately below the posterior lobes of the hood. The ball of the eye is about eight lines in diameter ; and, although contracted and wrinkled in the specimen examined, it appeared to have been natu- rally of a globular form, rather flattened anteriorly. The pupil was a circular aperture, less than a line in diameter, situated in the centre of the anterior surface of the eye. This small size of the pupil in Nautilus, which contrasts so remarkably with the magnitude of that aperture in the Dibranchiate Cephalopods, Professor Owen suggests is most probably dependent on the great degree of mobility conferred upon the eye of the Nautilus, in consequence of its attachment to a muscular pedicle which enables it to be brought to bear with ease in a variety of directions ; whilst, in the higher Cephalopoda, corresponding motions of the head and body, on account of the more fixed condition of the eye in them, would have been perpetually required, had not the range of vision been ex- tended to the utmost by enlarging the pupillary aperture. The principal tunic of the eye is a tough exterior membrane or sclerotic {Jig- 212), thickest posteriorly, where it is continued from the pedicle, and becoming gradually thinner to the margins of the pupil. The optic nerves, after leaving the optic ganglions (2), traverse the centre of the ocular pedicles, and, entering the eye, spread out into a tough pulpy mass which extends as far forwards as the semidiameter of the globe. This nervous tissue, as well as the whole interior of the cavity, is covered with a black pigment which is apparently interposed between the impinging rays of light and the sentient membrane. The contents of the eye-ball, of what- ever nature they had been, had escaped by the pupil. If the eye had ever contained a crystalline lens, that body must have been very small ; as otherwise, from the well-known effect of ardent spirits in coagulating it, it would have been readily perceived. What adds, however, to the probability of this eye being destitute of a crystalline humour is the total absence of ciliary plicae, or any structure analogous to them. In some parts of the cavity a mem- brane could be distinguished which had enveloped the fluid contents of the eye ; but it had entirely disappeared at the pupil, which had in consequence freely admitted the preserving liquid into the in- terior of the globe. However much is still left to be ascertained by future observa- tions, we learn from the above able exposition of the appearances detected on examining the solitary example of a visual organ of this description hitherto met with, that the eye of the Nautilus exhibits 9 TT <V II 466 CEPHALOPODA. every indication of inferiority of construction when compared with that of the Dibranchiate tribes. Encased in no orbital cavity, and consequently unprovided with any other muscular apparatus than the fleshy pedicle whereby it is connected with the head ; unpro- tected by eyelids and devoid of lachrymal appendages ; without either transparent cornea, aqueous humour, iris, or crystalline lens ; and, moreover, coated internally with a dark pigment, apparently situated in front of the nervous expansion which represents the retina, instead of behind it in the usual position of the choroid tunic, all these are facts calculated to arrest the attention of the physiologist, and excite the surprise of every observer who studies on a large scale this part of the animal economy. (507.) The eyes of the Dibranchiate Cephalopoda are not less remarkable in their construction than those of the Nautilus, and from their greater complexity will require a more elaborate descrip- tion. In order to simplify the details connected with this portion of our subject as much as possible, we shall describe separately, as forming distinct parts of the ocular apparatus met with in the com- mon Cuttle-fish (Sepia cfficinalis), first, the orbit ; secondly, the globe of the eye ; thirdly, the chamber of the optic ganglion ; and fourthly, the muscles of the visual organ. (508.) The orbit differs from that of all other classes of ani- mals, inasmuch as it is a cavity circumscribed on all sides and covering even the front of the eye.* The bottom of the orbital cavity is cartilaginous, being partially formed by a process derived from the cranial cartilage ; but elsewhere it is made up of the com- mon fleshy integument of the body (fig. 13, d, d, e) : becoming gradually attenuated, the skin (b) passes over the anterior portion of the eye, where, being transparent (/), it represents the cornea, although it has no connection with the eye-ball itself. Beneath the cornea the integument again becomes opaque, and forms a thickened fold (a), which might be considered as the rudiment of an under eyelid. The orbit, therefore, forms a complete capsule, enclosing the whole of the apparatus of vision. (509.) The globe of the eye fills up the anterior part of the orbital chamber, and is remarkable from having no cornea properly so called ; so that, on raising the transparent skin (/) which forms the exterior wall of the orbit and supplies the place of the cornea, the * Descriptive and Illustrated Catalogue of the Physiological series of Comparative Anatomy contained in the Museum of the Royal College of Surgeons, London, vol. iii. part i. plate 52. CEPHALOPODA. 467 Fig. 213. prominent surface of the crystalline lens (o) is found quite naked beneath it ; neither an aqueous humour, nor an iris properly so called, being present. The outer coat of the eye (g, g) represents the scle- rotic tunic in man: it is tough, fibrous, and of a silvery lustre ; perforated anteriorly by a large round aper- ture representing that which con- tains the cornea in the human eye, and pierced pos- teriorly by nu- merous foramina, through which the multitudinous branches derived from the optic ganglion (k) enter. The second tunic is usually regarded as the retina, occupying a singular situation and presenting a very anomalous structure. No choroid intervenes between this retina and the sclerotic, as is the case in the eye of man ; but numerous nervous branches given from the optic ganglion (&), having penetrated into the interior of the eye through the cribriform sclerotic, immediately expand into a thick nervous membrane which lines the sclerotic tunic, and is continued forward to a deep groove in the substance of the crystal- line lens, wherein it is implanted so as to form a kind of ciliary zone (m), which is slightly plicated, and obviously assists in keeping the lens in situ. Between the retina and the vitreous humour is interposed a thick layer of black pigment, which, being thus strangely situated, has very naturally puzzled all physiological inquirers, inasmuch as it would apparently form an insurmountable barrier between the rays of light and the retinal membrane. The researches of Professor Owen would seem, however, to have removed the difficulty pre- sented by this hitherto incomprehensible and anomalous arrange- ment ; as he has succeeded in discovering, in addition to the thick post-pigmental nervous expansion, a delicate lamella in front of the pigmentum nigrum, correspondent in position at least with the 468 CEPHALOPODA. retina of vertebrate animals. " In the eyes of different Sepice which we had immersed in alcohol preparatory to dissection, we have, however, invariably found between the pigment and the hya- loid coat a distinct layer of opaque white pulpy matter, of sufficient consistence to be detached in large flakes, and easily preserved and demonstrated in preparations. We confess, however, that we can discover no connection between this layer and the thick nervous expansion behind the pigment ; but, nevertheless, we cannot but regard it as being composed of the fine pulpy matter of the optic nerve, and as constituting a true pree-pigmental retina." * It has been already stated that there are no chambers of aqueous humour ; and we are but little surprised that, in animals destined to see objects contained in water, the existence of a refracting medium scarcely at all differing in density from the surrounding element should be dispensed with. To compensate, however, for this deficiency, the crystalline, as is the case in all the aquatic Vertebrata, is of short focus and great power ; being, in fact, not merely, as it is generally described, a double convex lens, which is the usual shape of this important piece of the optic apparatus, but exhibiting that form of a simple magnifier most approved of by opticians as being best adapted to ensure a large field of view. Whoever is conversant with the principles upon which the well-known " Coddington lens " is constructed, will have little difficulty in appreciating the advantages derived by intro- ducing a precisely similar instrument in the eye of the Cuttle-fish. The Coddington lens is a sphere of glass divided into two portions by a deeply cut circular groove, which is filled up with opaque matter. The lens of the Cuttle-fish is in like manner divided into two parts of unequal size (o, o') by a circular indentation, wherein the post-pigmental retina with its coat of dark varnish (m) is fixed, and thus a picture of the most perfect character is ensured. The crystalline penetrates deeply into the vitreous humour : the latter, enclosed in a delicate hyaloid membrane, fills up, as in man, the posterior part of the eye-ball ; while the small space that intervenes between the posterior surface of the crystalline and the back of the ocular chamber sufficiently attests the shortness of the focus of so powerful a lens. (510.) The posterior portion of the orbital capsule is occupied by a large cavity quite distinct from the globe of the eye, although its walls are derivations from the sclerotic tunic, wherein is lodged * Cyclopaedia of Anatomy and Physiology, art. CEPHALOPODA. CEPHALOPODA. 469 the great ganglion of the optic nerve (&) imbedded in a mass of soft white substance. This supplementary chamber is formed by a separation of the sclerotic into two layers ; of which one, already Fig. 214. described ('), forms the posterior boundary of the eye-ball, while the other (h) passing backwards circumscribes the cavity in ques- tion. On entering the compartment thus formed the optic nerve (/>) dilates into a large reniform ganglion, almost equal in size to the brain itself ; and from the periphery of the optic ganglion arise the 470 CEPHALOPODA. numerous nervous filaments, which, after perforating the poste- rior part of the globe of the eye, expand into the post-pigmental retina. Between the globe of the eye (g) and the cornea (/) is a capacious serous cavity, which extends to a considerable distance towards the posterior part of the orbital chamber ; and holds the same relation to the visual apparatus, and the cavity in which it is lodged, as the serous lining of the human pericardium does to the heart, and the fibrous capsule in which that viscus is lodged, evi- dently forming an arrangement for facilitating the movements of the eye. The serous membrane which lines this cavity, after in- vesting the inner surface of the cornea and the interior of the orbit, is reflected upon the outer surface of the sclerotic tunic of the eye, which it likewise covers ; and moreover, at the front of the eye- ball, enters the aperture which in the eye of a vertebrate animal would be occupied by the cornea, lines the chamber corresponding with that of the aqueous humour, and passes over even the anterior surface of the crystalline. This serous membrane Cuvier very improperly named the " conjunctiva ;" but, as Professor Owen has suggested,* it is evidently rather analogous to the membrane of the aqueous humour, here excessively developed in consequence of the want of a cornea in the sclerotic aperture. This serous cavity is not, however, a completely closed sac ; but, as is frequently the case with the serous membranes of fishes and reptiles, is in com- munication with the surrounding medium, through the interven- tion of a minute orifice visible in the transparent tegumentary cornea. (511.) Four muscular slips are appropriated for the movements of this remarkable eye, and serve to direct the axis of the organ so as to ensure distinct vision : they arise principally from the orbital prolongations of the cranial cartilage, and are inserted into the sclerotic tunic. (512.) It is always interesting to the physiologist to observe the earliest appearance of a new system of organs, and witness the gradual developement of additional parts, becoming more and more complicated as we advance from humbler to more elevated grades of the animal creation. The progressive steps by which the auditory apparatus of the Vertebrata attains to that elaborate organization met with in the structure of the human ear are not a little curious. In the simplest aquatic fonns the central portion * Cyclop, of Anat. and Phys. loc. cit. p. 552. CEPHALOPODA. 471 of the internal ear alone exists, imbedded in the as yet cartilagi- nous cranium. Gradually, as in fishes, semicircular canals, pro- longed from the central part, increase the auditory surface, but still have no communication with the exterior of the body. In reptiles and birds destined to perceive sonorous impressions in an aerial medium, a tympanic cavity and drum are superadded ; and lastly, in the Mammiferous orders, external appendages for collect- ing and conveying sound to the parts within, complete the most complex and perfect form of the acoustic instrument. As far as is yet known, the Tetrabranchiate Cephalopods have no distinct organ of hearing ; but in the Dibranchiata an ear lodged in an internal cranium for the first time presents itself to our notice, and at the same time exhibits the lowest possible condi- tion of a localized apparatus adapted to receive sounds. In the anterior and broadest part of the cartilaginous cranium,* where its walls are thickest and most dense, are excavated two nearly spherical cavities (Jig. SI 5, rf), which in themselves constitute Fig. 215. the osseous labyrinth of both ears. A vesicle or membranous sac- culus (c), likewise nearly of a spherical form, is suspended in the centre of each of these cartilaginous cells by a great number of filaments that are probably minute vessels. The two auditory nerves derived from the encephalon enter these cavities through special canals ; and each, dividing into two or three branches, spreads out over the vesicle to which it is destined. The auditory vesicle itself is filled with a transparent glairy fluid ; and contains, attached * Cuv. MSmoire sur le Poulpe, p. 41. 472 CEPHALOPODA. Fig. 216. to its posterior part, a minute otolithe (1, 2, 3), a calcareous body of variable shape in different genera, the oscillations of which doubtless increase the impulses whereupon the production of sound depends. Such is the simplest form of an ear ; and if the reader will com- pare the organ above described with that possessed by the highest Articulata, as, for example, the lobster ($ 380), the similarity of the arrangement will be at once manifest. (513.) All the CEPHALOPODA are dio3cious, and the structure of the sexual organs both of the males and females is remarkable, inasmuch as it is peculiar to the class. In the females, the ovarian receptacle is lodged at the bottom of the visceral sac (Jig. 211, p, p), enclosed in a distinct peritoneal pouch. The ovary itself is a large bag, the walls of which are tolerably thick ; and, on opening it, it is found to contain a bunch of vesicular bo- dies, attached by short vascular pe- dicles to a cir- cumscribed por- tion of its internal surface (Jig. 216, a). These vesi- cles, the ovisacs or calyces, as they are called by com- parative anato- mists, are, in fact, the nidi wherein the ova are se- creted ; and, if examined shortly before ovi position commences, every one of them is seen to contain an ovum in a more or less advanced stage of deve- lopement. In this condition the walls of the ovisacs are thick and spongy; and their lining membrane, which constitutes the vas- d CEPHALOPODA. 473 cular surface that really secretes the egg, presents a beautiful reti- culate appearance. If the contained ova be examined when nearly ripe for exclusion, each is found to be composed of a yolk or vitellus enclosed in a delicate vitelline membrane, and covered externally by a thicker investment the chorion. When the ovum has attained complete maturity, the ovisac enclosing it becomes gradually thinned by absorption, and ultimately bursts ; allowing the egg, now complete with the exception of its shell, to escape into the general cavity of the ovarium (c). The oviduct (e) communicates immediately with the interior of the ovarium by a wide orifice, the dimensions of which are proportioned to the size of the mature ova. It is generally single ; but in some genera, as Loligo and the Octopoda, the canal derived from the ovary soon divides into two(d, e). The walls of the oviferous duct are thin and membranous until near the external outlet, where they suddenly become thick and glandular, and, in many genera, surrounded with a very large laminated gland (/), through the centre of which the eggs have to pass before they issue from the body. It is the gland last mentioned that secretes the external horny covering of the egg ; a defence which seems to be deposited in successive layers upon the outer surface of the pre- viously existing chorion, and, when completed, forms a thick flex- ible case made up of concentric lamellae of a dark-coloured corneous substance. (514.) After extrusion the ova of the different families of Ce- phalopoda are found agglutinated and fastened together into masses of very diverse appearance. The eggs of the common Cuttle-fish, frequently found upon the shore, are not inaptly compared by those ignorant of their real nature to a bunch of black grapes ; to which indeed they bear no very distant resemblance, being generally ag- gregated in large clusters, and fastened by long pedicles either to each other or to some foreign body. The Argonaut carries its eggs, which are comparatively of small size, securely lodged in the recesses of its shell ; while the ova of the Calamary, encased in numerous long gelatinous cylinders that conjointly contain many hundreds of eggs, are fixed to various submarine substances, and thus protected from casualties. The form and arrangement of these bunches are no doubt dependent upon the peculiar character of the terminal gland found in the oviduct of the parent, whereby the last covering to the ova is furnished. (515.) Cuvier remarks that the male Poulpes must be less nu- 474 CEPHALOPODA. merously met with than the female, as among the numerous speci- mens dissected by him scarcely one fifth were of the former sex. The various parts of the male generative apparatus are remark- ably similar both in structure and arrangement to the corresponding portions of the sexual organs of the female. The testicle strikingly resembles the ovary both in its outward form and internal arrange- ment : like that viscus, it consists of a capacious membranous sac (Jig. 217, b) ; and, on opening this, there is found attached to a small portion of its inner surface a large bundle of branched cseca (a), in which no doubt the seminal fluid is elaborated. These strangely disposed seminiferous cseca have apparently no proper excretory ducts ; but the impregnating fluid secreted by them is, as it would seem, poured into the general cavity of the sac, exactly in the same manner as the ova do in the other sex, and, being allowed to escape from this reservoir through a wide orifice (c), it enters the vas deferens. The canal last mentioned (d) is long, slender, and very tortuous, but after many convolutions it enters a wider canal (e), called by Cuvier vesicula se- Fi g* 217 - minalis, the interior of which is divided by imperfect septa ; and, its texture be- ing apparently mus- cular, this part of the excretory appa- ratus may possibly by its contractions expel the spermatic fluid from the body. On issuing from the seminal vesicle, the semen passes the ex- tremity of an oblong gland (/), which Cuvier denominates the prostate : its structure is compact and granular, and it seems to be destined to furnish some ac- CEPHALOPODA. 475 cessory fluid subservient to impregnation. Having passed the prostate, the ejaculatory duct communicates with a large muscular sacculus (g), the contents of which are very extraordinary. This sacculus is in fact filled with innumerable white filaments, each about half an inch in length, arranged parallel to each other, and disposed with much regularity. There are three or four rows of them, one above another, entirely filling the sac ; and they are maintained in situ by a delicate spiral membrane, but are quite unconnected with the sac itself. The filaments when taken out, even long after the death of the Cephalopod, exhibit, when mois- tened, various contortions, and by some have been regarded as En- tozoa ; but their real nature is entirely unknown, although from the time of Needham,* their first discoverer, to the present day, various speculations and conjectures have been entertained con- cerning them. From the pouch of Needham a short canal leads to the penis (A), a short, hollow, muscular tube, through which the fecundating fluid is expelled. It is most probable that the ova of the female are impregnated by the aspersion of the male fluid either during their extrusion, as in frogs, or after they are deposited, as is the case in the generality of fishes ; but this part of the economy of the Cephalopoda is still involved in obscurity. (516.) Although we mean to defer any minute account of the deve- lopement of the embryo in ovo until an examination of the eggs of oviparous Vertebrata shall afford more ample materials for eluci- dating this important subject, it will be as well in this place briefly to notice the condition of the young Cephalopods previous to their escape from the egg, wherein the first part of their growth is accomplished. Before the egg is hatched, the foetal Cuttle-fish already presents all the organs essential to its support and pre- * Needham. An Account of some new Microscopical Discoveries, 8vo. 1745- 476 VERTEBRATA. servation : the tentacula upon the head, the eyes, the respiratory apparatus, and even the ink-bag, which in the earlier stages of growth were quite undistinguishable in the germ of the future being (fig- &18, 1), slowly make their appearance ; and, even before birth, the little creature presents most of the peculiarities which characterize the species to which it belongs. But the most pro- minent feature that strikes the attention of the physiologist is the remarkable position of the duct communicating between the yolk of the egg, the great reservoir of nourishment provided by nature for the support of the foetus whilst retained in the egg, and the alimentary canal of the as yet imperfect Sepia. This commu- nication, whicli in vertebrate animals is invariably effected through an opening in the walls of the abdomen, whereby the vitelline duct penetrates to the alimentary canal, here occupies a very unusual situation ; being inserted into the head, through which it pene- trates, by an aperture situated in the front of the mouth, to the oesophagus, where it terminates (Jig- &18, 3). Leaving the Cephalopod Mollusca, we must bid adieu to the fourth grand division of the animal kingdom, and proceed in the next chapter to introduce the reader to beings organized according to a different type, embracing the most highly-gifted and intelligent occupants of the planet to which we belong. CHAPTER XXVI. VERTEBRATA. (517.) THE fifth division of the animal kingdom is composed of four great classes of animals, closely allied to each other in the grand features of their organization, and possessing in common a general type of structure clearly recognizable in every member of the exten- sive series, although of course modified in accordance with the endless diversity of circumstances under which particular races are destined to exist. The immeasurable realms of the ocean, the rivers, lakes, and streams, the fens and marshy places of the VERTEBRATA. 477 earth, the frozen precincts of the poles, and the torrid regions of the equator, have all appropriate occupants, more favoured as regards their capacities for enjoyment, and more largely endowed with strength and intelligence, than any which have hitherto occu- pied our attention, and gradually rising higher and higher in their attributes, until they conduct us at last to Man himself. FISHES, restricted by their organization to an aquatic life, are connected by amphibious beings, that present almost imperceptible gradations of developement, with terrestrial and air-breathing REPTILES : these, progressively attaining greater perfection of structure and increased powers, slowly conduct us to the active, hot-blooded BIRDS, fitted by their strength, and by the vigour of their movements, to an aerial existence. From the feathered tribes of Vertebrata, the transition to the still more intelligent and highly- endowed MAM- MALIA is effected with equal facility ; so that the anatomist finds, to his astonishment, that throughout this division of animated nature, composed of creatures widely differing among themselves in form and habits, an unbroken series of beings is distinctly traceable. (518.) The first grand character that distinguishes the vertebrate classes, is the possession of an internal jointed skeleton, which is not, as in the preceding classes, extravascular and incapable of increase, except by the successive deposition of calcareous laminae applied to its external surface; but endowed with vitality, nou- rished by blood-vessels and supplied with nerves, capable of growth, and undergoing a perpetual renovation by the removal and replacement of the substances that enter into its compo- sition. In the lowest tribes of aquatic Vertebrata the texture of the internal framework of the body is permanently cartilaginous, and it continues through life in a flexible and consequently feeble condition ; but as greater strength becomes needful, in order to sustain more active and forcible movements, calcareous particles are found to be deposited in the interstices of the carti- laginous substance, and, in proportion as these accumulate, addi- tional firmness is bestowed upon the skeleton, until it assumes, at length, hardness and solidity proportioned to the quantity of the contained earthy matter, and becomes converted into perfect bone. (519.) Phenomena precisely similar are observable in tracing the formation and developement of the osseous system, even in those 478 VERTEBRATA. genera possessed, when arrived at maturity, of the most com- pletely organized skeletons. In the very young animal the bones consist exclusively of cartilage ; but as growth proceeds, earth becomes deposited by the blood-vessels in the as yet soft and flexible pieces of the skeleton, until by degrees they acquire density and strength as the animal advances towards its adult condition. The complete skeleton of a vertebrate animal may be considered as being composed of several sets of bones employed for very different purposes ; consisting of a central portion, the basis and support of the rest, and of various appendages derived from or connected with the central part. The centre of the whole os- seous fabric is generally made up of a series of distinct pieces arranged along the axis of the body, and this part of the skeleton is invariably present ; but the superadded appendages, being em- ployed in different animals for various and distinct purposes, pre- sent the greatest possible diversity of form, and are many of them wanting in any given genus : "so that a really complete skeleton, that is, a skeleton made up of all the pieces or elements which might, philosophically speaking, enter into its composition, does not exist in nature ; inasmuch as it is owing to the deficiency of some portions, and the developement of others in particular races, that we must ascribe all the endless diversity of form and me- chanism so conspicuously met with in this division of the animal world. Nevertheless, although there is no such a thing in Creation as a fully developed skeleton, it will be necessary, in order to prepare the student for the contemplation of the numerous modifications met with in this portion of the animal economy, hereafter to be described, briefly to enumerate the component parts which might theoretically be supposed to enter into the construction of the framework of an animal ; and thus by comparison he will be enabled, as we proceed, to appreciate more readily the variations from a general type apparent throughout the vertebrate classes. It may, likewise, be as well thus early to caution the anatomist who has confined his studies to the contemplation of the human body, against taking the skeleton of Man as a standard whereby to direct his judgment; for Man, so highly raised by his intelligence and mental powers above all other beings, is, so to speak, a monstrosity in the creation ; and, so far from finding in the human frame the means of elucidating the laws VEKTEBRATA. 479 of animal organization, it is found to have been constructed upon principles the most aberrant and remote from those which an extensive investigation of the lower animals has revealed to the physiologist. (520.) A skeleton, described generally, is made up of the fol- lowing portions : 1st. of a chain of bones, placed in a longitudinal series along the mesial line of the back, and more or less firmly articulated with each other, so as to permit certain degrees of flexure. These bones, examined individually, present various additional parts destined to very different ends : some defend the central axis of the nervous system from external violence ; others, when present, guard and enclose the main blood-vessels ; and the rest, either acting as prominent levers, serve to give insertion to the muscles which move the spine, or afford additional security to the articulations between the vertebral pieces. Those vertebrae which defend the posterior portions of the nervous axis, usually called the spinal core/, constitute the spine ; while those enclosing the anterior extremity of the nervous axis, which, for reasons hereafter to be explained, becomes dilated into large masses forming collectively the brain, is by the human anatomist distin- guished as the cranium or skull. Secondly, we find appended to the cranial or cephalic portion of the spine, a set of bones disposed symmetrically, and forming the framework of the face : these bones, it is true, have by many Continental writers been regarded as constituting additional ver- tebrae, the parts of which are still recognizable, although amazingly modified in shape, so as to enclose the different cavities wherein the senses of vision and smell, as well as the organs of mastication, are situated. We shall not, however, waste the time of the stu- dent by considering in this place the as yet unsettled and vague opinions of transcendental anatomists upon this subject ; it is sufficient for our present purpose to indicate the facial bones as appendages to the cranial vertebrae, avoiding for the present fur- ther discussion concerning them. Another most important addition to the central axis of the skeleton is obtained by the provision of lateral prolongations, derived from the transverse processes of the vertebrae, which form a series of arches largely developed at certain points, so as more or less completely to embrace the principal viscera, and give extensive attachment to muscles serving for the movements of the body. 480 VERTEBRATA. The first set of arches is appended to the lateral portions of the cranial vertebrae, and the bones thus derived enter largely into the composition of the respiratory apparatus. In Man this important portion of the skeleton is reduced to a mere rudiment, distinguished by the name of the os hyoides ; and in the human subject its relations and connexions with the surrounding parts are so obscurely visible, that the student is scarcely prepared to witness the magnitude and importance of the hyoid framework in other classes, or the amazing metamorphoses which, as we shall afterwards see, it undergoes. Behind the hyoid apparatus, other arches are attached to the transverse processes of the spinal vertebrae, called ribs ; and the study of these appendages to the spine is one of the most in- teresting points in the whole range of osteology. In Fishes, wherein respiration is effected entirely by the movements of largely developed hyoid bones, the ribs are mere immoveable derivations from the transverse processes of the vertebrae, and serve exclusively for the attachment of muscles. In Reptiles, respiration is still accomplished by the os hyoides ; and the ribs, thus performing a secondary office, become convertible to different uses, and assume various forms and proportions. In the Amphibious Reptiles, the most nearly approximated to Fishes, they either do not exist at all, as being needed neither for respiration nor locomotion, or they are represented by minute and almost imperceptible rudiments ap- pended to the extremities of the transverse processes of the verte- bras. In Serpents the ribs are wanted for locomotion, and are accordingly developed from the head nearly to the tail, forming a series of strong arches, articulated at one extremity with the ver- tebral column by a very complete joint ; but at the opposite ex- tremity they are loose and unconnected. In proportion, however, as the hyoid bones, with the larynx, of which they form an impor- tant part, become converted into a vocal apparatus, as they gradu- ally do, the ribs assuming more complete developement in a certain region of the spine, and, being augmented by the addition of a sternal apparatus, form a complete thoracic cavity, and thus become the basis of those movements of the body which in hot blooded animals are subservient to respiration. The next additions required to complete the skeleton, are two pairs of locomotive limbs, representing the legs and arms of Man. Infinitely diversified as are these members both in form and office, they are, when philosophically considered, found to be constructed VEKTEBltATA. 481 after tlie same type. Both the anterior and posterior limbs, when fully organized, consist of similar parts, most of which are met with in the limbs of the human skeleton. Three bones constitute the shoulder, called respectively the Scapula, the Clavicle, and the Coracoid bone. Three bones in like manner sustain the hinder extremity, the Ilium, the Ischium, and the Pubis ; and these evi- dently represent individually the corresponding pieces found in the shoulder, but differently named. The formation of the limbs is likewise strictly parallel ; a single bone articulates with the osseous framework of the shoulder, or of the hip, called in one case the Humerus, in the other the Femur : two bones form the arm, the Radius and Ulna ; and two likewise enter into the composition of the leg, the Tibia and Fibula : the hand and foot are each supported by a double series of small bones, forming the Carpus of the one and the Tarsus of the other ; and in like manner con- sist of similar pieces, five in number, called the Metacarpal or Metatarsal bones, and of the Phalanges, or joints of the fingers and toes. A perfect or typical skeleton must therefore be supposed to consist of all the before-named portions, namely, 1. the cranial and spinal vertebrae ; 2. the face ; 3. an elaborately-formed hyoid framework ; 4. the ribs ; 5. a sternal system of bones, con- stituting, in conjunction with some of the ribs, a thorax ; and Gthly, of four locomotive extremities, made up of the parts above enumerated as entering into their composition. Seldom, indeed, is it that the student will find even the majority of these portions of the osseous apparatus coexistent in the same skeleton ; but, whatever forms of animals may hereafter present themselves for investigation, let the above description be taken as a general standard of comparison, and let all variations from it be considered as modifications of one grand and general type. We must, however, proceed one step further in this our prepa- ratory analysis of the skeleton ; and, instead of regarding the indi- vidual pieces of the osseous framework of an adult animal as so many simple bones, be prepared to find them resolvable into several distinct^rts or elements, all or only a part of which may be developed in any given portion of the osseous system. In order to simplify as much as possible this important subject, we will select first, what is generally considered as a single bone, one of the most complex vertebra of a fish for instance, and examine its real composition. 2 i 482 VEHTEBRATA. This bone (Jig. 219) is found to consist of a central portion (a), and of sundry processes derived therefrom, some of which the younger student of human anatomy would at once be able to call by their appropriate names : to the body of the bone (a) he finds appended the arch (6) which encloses the spinal cord, surmounted by its spinous process (c), and with equal facility he recognises in the lateral processes (d, d) the analogues of the transverse pro- cesses of the human spine ; but here his knowledge fails him, inas- much as he finds another arch (e) formed beneath the body of the bone, and moreover an inferior spinous process (g), neither of which have any representatives in the human body. It is evident, therefore, that the human vertebrae are imperfectly developed bones, and do not possess all the parts or elements met with in the corresponding portion of the skeleton of a fish. The question, therefore, to be solved is this, how many elements exist in the most perfect vertebra known ? and this being once satisfactorily settled, it is easy to detect the deficiencies of such as are less completely developed. Taking the example F/g.219. above given as a speci- men of a fully formed vertebra, it has been found to be divisible in- to the following pieces, all or only a part of which may be present in other vertebrae, even belonging to the same skeleton ; and these parts are represented detached from each other in the diagram which accompanies the figure (Jig. 219, B). They are 1st. the centre or body of the bone ; 2dly. two elements (6, >), which embrace the spinal marrow ; 3dly. the superior spinous process (c) ; 4thly. the two transverse processes (d) ; othly. two elements forming the inferior arch, and enclosing the principal blood-vessels (e) ; and 6thly. an inferior spinous process (g). With this key before us, we are able with the utmost ease to comprehend the structure of any form of vertebra that may offer itself. Thus, in different regions of the back of the same fish, the composition of the vertebrae is totally different ; near the tail VERTEBRATA. 483 the vertebrae consist of the body (a), the superior arch (b) and spinous process (c), and the inferior arch ( e) and spinous pro- cess (g). In the neighbourhood of the head, however, neither the inferior arch nor spinous process are at all developed ; but the transverse processes, which were deficient in the former case, are here of great size and strength. It is obvious, therefore, that the form of a vertebra may be modified to any extent, by the simple arrest of the developement of certain elements, and the disproportionate expansion of others, until at length it becomes scarcely recognisable as constituting the same piece of the ske- leton. Who would be prepared to expect, for example, that the occi- pital bone of the human head was merely a modification of a few of the elements of the fish's vertebra above described enormously expanded, in order to become adapted to altered circumstances ? And yet how simple is the transition ! By removing the inferior arch (e) and spinous process (g), and slightly reducing the pro- portionate length of the transverse processes (W), we arrive at the form of a human vertebra, which exhibits precisely similar elements : enlarge the arches (6, b) that surround the spinal axis of the nervous system, increase the size of the superior spinous element (c), and we have the occipital bone of a fish : and from hence, through a few intermediate links, we arrive almost imper- ceptibly at the occipital bone of the human cranium ; the main differences being that the body is in Man divided into two lateral halves, while the superior arches (b) become spread out so as adequately to defend the prodigiously developed masses of the brain, to which in the human body they correspond. One other illustration of this interesting subject. What bones compose a completely formed thorax ? In man we find, as every tyro knows, 1st. the dorsal vertebra; 2dly. the ribs, with their cartilages ; and 3dly. the sternum. But it is not in man that we must expect a perfectly developed thoracic framework ; it is in the birds that are destined to rise in the air by the assistance of their proportionately powerful thoracic extremities. If therefore we examine the thorax of a bird, we find it composed of pieces which in man are absolutely wanting: we see 1st. the vertebra; #dly. the dorsal ribs, firmly articulated on each side both with their bodies and transverse processes ; 3dly. the sternal ribs, extending from the ribs last mentioned to the sternum ; and, lastly, the sternum, itself composed, as we shall afterwards see, of various 2 i 2 484 VERTEBRATA. elements not found in the human body. If we prosecute our sur- vey a little further, we shall find this portion of the skeleton offer- ing the greatest possible variety as regards the presence or absence of the elements above enumerated : thus in the Frog we have vertebrae and sternum, but no ribs ; in the Serpent, vertebrae and dorsal ribs, but no sternum or sternal ribs ; in Man the sternal ribs are represented by the costal cartilages ; and thus a thorax of every required description is constructed by adding or taking away, expanding or contracting certain elements, all of which a typical skeleton might be supposed to contain developed in a medium condition. The nervous system of the Vertebrata is by far more complex and elaborately organized than that of any of the four preceding divisions of the animal world ; and consists, in fact, of several dis- tinct systems differently disposed and appropriated to different offices. Certain largely developed ganglia situated in the cavity of the cranium, generally considered by themselves on account of their disproportionate size when compared with the other nervous centres, are commonly grouped together under one common de- signation, and form what is called the brain or encephalon : these masses, however, as we shall hereafter see, preside over various and widely different functions ; and with them perception, volition, and intelligence are essentially connected. Continued from the brain, and lodged in a canal formed by the superior arches of the vertebral column, is a long chain of gan- gl ionic centres, so intimately united that they appear confused into a long medullary cord usually denominated the spinal marrow (medulla spinalis). The spinal medulla in reality consists of two double series or columns, composed of symmetrical and parallel ganglia ; one pair of columns, the anterior, presiding over those muscular movements which are under the control of the will, while the posterior are destined to receive impressions derived from the exterior of the body : these columns, therefore, are denominated respectively the motor and sensitive tracts of the spinal cord. From the lateral aspects of the medulla spinalis are derived at intervals symmetrical pairs of nerves, which escape from the spinal canal by appropriate orifices situated between the different bones of the vertebral column, and are distributed to the voluntary mus- cles and integument of the two sides of the body. The spinal nerves, however, are not so simple in their conipo- VERTEBRATA. 485 sition as they were considered to be by the older anatomists : each of them has, in fact, been found to arise from the spinal cord by two distinct roots, one derived from the anterior, the other from the posterior column of the corresponding side ; so that each nerve is evidently made up of two distinct sets of filaments, one set communicating with the motor, the other with the sensitive tracts ; and thus every nerve derived from the spinal cord is a compound structure, being composed of filaments distinct in office, although enclosed in the same sheath, some being connected with the mus- cular movements, the others with sensation. But in addition to the cerebro-spinal ganglia and the symmetrically arranged nerves emanating therefrom, that are distributed to the organs of sensa- tion and movement, there exists in the Vertebrata a distinct system of nervous centres lodged among the viscera, appropriated to the performance of the automatic functions, and presiding over those involuntary movements of the body upon which depend the ope- rations connected with nutrition. These ganglia are variously distributed, being situated in the head, the neck, the thorax, and the abdomen ; and from them arise large plexuses of nerves, des- tined to supply the organs belonging to digestion, circulation, and secretion ; thus forming extensive ramifications, formerly distin- guished by the name of the sympathetic nerve, but now more pro- perly considered as a distinct system presiding over organic life, as the former is connected with the phenomena of animal life. With the increased developement of the nervous system in the vertebrate classes we find the organs of the senses assume a pro- portionate perfection of structure and regularity of arrangement. The auditory apparatus, of which we have seen only rudiments in the lower animals, gradually becomes more and more elaborately organized : the eyes, now invariably two in number* are lodged in cavities formed for their reception by the osseous framework of the face ; and exhibit, in the simplicity of their structure, a higher type of organization than any we have hitherto examined. Organs of smell, also double, but of very variable construction, are likewise constantly present. The tongue becomes slowly adapted to ap- preciate and discriminate savours; and the sense of touch, the most generally diffused of all, is especially conferred upon organs of different kinds peculiarly adapted to exercise this faculty. Thus with increased intelligence higher capabilities of enjoyment are allotted, and sagacity developes itself in proportion as the nervous centres expand. But there are minor points, characteristic of the 486 VERTEBRATA. vertebrate division of the animal world, which must not be omitted in this preparatory survey of their organization. Their organs of digestion and nutrition are constructed according to a different type, and upon a more enlarged plan than in any of the classes enumerated in the preceding chapter ; and parts are superadded to the digestive apparatus which in lower tribes had no existence. In addition to the usual subsidiary glands, namely, the salivary and the hepatic, a third secretion is poured into the intestine along with the bile derived from the pancreas, a viscus which we have not as yet met with. Throughout all the MOLLUSCA we have found the bile secreted by the liver to be separated from arterial blood, as are the other secretions of the body ; but in the VERTEBRATA it is from venous blood that the bile is formed, and in consequence an elaborate system of vessels is provided, distinct from the general circulation, by which a large supply of deoxygenized blood is con- veyed to and distributed through the liver, constituting what is termed by anatomists the system of the vena portte : nay, more, in connexion with this arrangement we find another remarkable viscus make its appearance, the spleen; from which venous blood is copiously supplied to the portal vein, and added to that derived from other sources. A still more important and interesting circumstance, which strikes the anatomist on comparing the Vertebrata with lower forms of existence, is the sudden appearance of an entirely new sys- tem of vessels, destined to absorb from the intestines the nutritious products of the digestive process, and to convey them, as well as fluids derived from other parts of the body, directly into the veins, there to be mixed with the mass of the circulating blood. These vessels, of which no traces have been detected in any of the INVER- TEBRATA, ara called lymphatics and lacteals, but their structure and distribution will occupy our attention hereafter. The blood of all the VERTEBRATA is red, and is composed of microscopic globules of variable form and dimensions in different animals. In the class of Fishes, owing to the as yet imperfect con- dition of the respiratory apparatus, the temperature of the body is scarcely higher than that of the surrounding medium ; and, even in Reptiles, such is the languid condition of the circulation, and the incomplete manner in which the blood is exposed to the renovating influence of the oxygen derived from the atmosphere, that the standard of animal heat is still extremely slow. But in the higher classes, the Birds and Mammalia, owing to the total separation of VERTEBRATA. 487 the systemic and pulmonary circulation, the effect of respiration is increased to the utmost ; and, pure arterial blood being thus abun- dantly distributed through all parts, heat is more rapidly generated, the warmth of the body becomes considerably increased, and such animals are permanently maintained at an invariable temperature, considerably higher than that of the medium in which they live. Hence the distinction generally made between the hot-blooded and cold-blooded Vertebrata. The variations in the temperature of the blood, above alluded to, are, moreover, the cause of other important differences observable in the clothing, habits, and instincts of these creatures. To retain a high degree of animal heat necessarily requires a warm and thick covering of some non-conducting material ; and consequently in the hair, wool, and feathers of the warm-blooded tribes we at once recognise the provision made by Nature for preventing an undue ex- penditure of the caloric generated in the body. Such investments, however, would be but ill adapted to the inhabitants of a watery medium ; and consequently the fish destined to an aquatic life, or the amphibious reptile doomed to frequent the mud and slime upon the shores, are deprived of such incumbrances, and clothed in a scaly or slippery covering more fitted to their habits, and equally in accordance with the diminished temperature of their blood. Still more remarkable is the effect of a mere exaltation of animal heat upon the instincts and affections of the different races of the Vertebrata. The fishes, absolutely unable to assist in the matura- tion of their offspring, are content to cast their spawn into the water, and remain utterly careless of the progeny to be derived from it. The reptile, equally incapable of appreciating the pleasures connected with maternal care, is content to leave her eggs exposed to the genial warmth of the sun until the included young escape. But no sooner does the vital heat of the parent become sufficient for the purposes designed by Nature, than all the sympathies of parental fondness become developed, all the delights connected with paternity and maternity are superadded to other enjoyments ; and the bird, as she patiently performs the business of incubation, or tenderly watches over her newly hatched brood, derives a pleasure -from the performance of the duties imposed upon her, second only to that enjoyed by the mammiferous mother, who from her own breast supplies the nutriment prepared for the sup- port of her infant progeny. 488 CHAPTER XXVII. PISCES FISHES. (521.) To whatever portion of the animal world we turn our attention, we find the lowest and least perfectly organized tribes to be inhabitants of the water. To dwell upon the land ne- cessarily demands no inconsiderable share of strength and activity, limbs sufficiently strong to support the weight of the body, mus- cles possessed of great power and energy of action, acute and vigilant organs of sense, and, moreover, intelligence and cunning proportioned to the dangers or necessities connected with a terres- trial existence. The inhabitant of the waters, on the contrary, although less highly gifted, may be fully competent to enjoy the position it is destined to occupy. Being constantly buoyed up on all sides by a dense element, it is easily supported at any required altitude without much muscular effort ; but feeble limbs are needed to guide its path through the water, and slight impulses suffice to impel it forward. Thus, therefore, in Fishes we are prepared to expect a priori, that, as far as strength and compactness of struc- ture are concerned, they will be found inferior to other Vertebrata. We are likewise justified in anticipating that in intelligence, and in the relative perfection of their senses, fishes should be less highly endowed than the other vertebrate classes. Plunged in the immeasurable depths of the ocean, whereunto no sound can ever penetrate, dwellers in the realms of eternal silence, where even the roar of the storm is lost, vivid and distinct perceptions of sound can be little needed. Surrounded by a turbid element, through which the rays of light with difficulty make their way, the sphere of vision must necessarily be extremely limited. Immersed in a fluid but little adapted to distribute odorous particles, a refined sense of smell would be a useless provision. Taste, if it exists at all, must be blunted to the utmost, 'from the circum- stances under which fishes seize and swallow prey ; and even the sense of touch, in animals encased in scales and deprived of pre- hensile limbs, can only be exercised in a vague and imperfect manner. PISCES FISHES. 489 With such inferiority in their powers of communication with the external world, and with faculties so circumscribed, we might justly infer that, as relates to their intellectual powers, fishes hold a position equally debased and degraded. Destitute of the means of social intercourse, deprived of all sympathy even with indivi- duals of their own species, friendless and mateless, the fish is denied even the privileges of sexual attachment ; the female for the most part ejects her countless eggs into the sea, heedless of the male that blindly fecundates them as she is careless of the progeny to which they give birth : thus, to pursue and destroy their prey constitutes their chief enjoyment during life, and to be devoured at last is the great end of their existence. (522.) We shall commence our account of the anatomy of fishes by an examination of the internal skeleton which forms the frame- work of their bodies. The reader has already seen in the CEPHA- LOPODA the first appearance of an osseous system in the carti- laginous pieces described in the last chapter, and will neces- sarily expect that between the rudimental condition which cha- racterizes the cephalic ring of the Cuttle-fish, and the complete and perfect skeleton of the fish, various gradations of developement will occur as we advance progressively from lower to more elevated forms of the finny race. Nor in this will he be deceived. The lowest tribes of fish possess a skeleton but little superior in its organization to that of the Cephalopod : in the Myxine and Lamprey the cranium is still cartilaginous ; and even the spinal column, not yet divided into vertebrae, resembles a cartilaginous cord extending from the head to the tail. Even in the Sturgeon, the Skate, and the Shark, the skeleton is but very partially ossi- fied ; and thus we are gradually and almost imperceptibly conducted to the strong and bony framework of the typical fishes. (528.) Even in tracing the modifications observable in the construction of the vertebral column, we have a beautiful illus- tration of the progressive advances of ossification in this the central portion of the osseous system. The spine of the Lam- prey, although at first sight apparently entirely soft and cartila- ginous, presents already in the arches which compose the spinal canal, and in the soft cord that represents the bodies of the ver- tebrse, slight indications of an incipient division into distinct pieces : rings of ossific matter are distinguishable, encircling at intervals the soft spinal cartilage upon which they perceptibly encroach, so that on making a longitudinal section of the cord it 490 PISCES FISHES. offers an appearance sketched in the adjoined figure (fig. 220, A). In a more advanced form of a fish's skeleton, as for example in the Sturgeon, these ossified rings are P j , , & ., Fig. wo. found to have enlarged consider- ably, and penetrate still more * - - *- - * i deeply into the cartilaginous mass (^g.220, B). As the bony rings thus developed approximate the centre, it becomes more and more evident that they represent the bodies of so many vertebrae ; but even in the majority of fishes the central part remains permanently unossified ; so that a cartilaginous axis traverses the vertebral co- lumn from one end to the other (Jig. 220, c), and it is not usual to find the central aperture perfectly obliterated, as delineated in the fourth sketch, D. (524.) Fishes, being continually resident in an element nearly of the same specific gravity as their own bodies, require little firm- ness or solidity in the construction of their spinal column : a free and unfettered power of flexion in certain directions so as to permit an ample sweep of their expanded tail, which forms the principal agent in propelling them forwards, is far more essential to their habits. Thus the cartilaginous spine of the feeble Lam- prey is sufficient for all needful purposes ; and even in the most perfectly ossified fishes, from the manner in which the vertebrae are united to each other, the greatest possible flexibility is ensured. The body of each vertebra presents two conical cups, the apices of which are nearly or quite continuous ; the margin of each cup- like depression is united by elastic ligament to the corresponding margin of the contiguous vertebra, and thus between the bodies of each pair of vertebrae a wide cavity is formed (D), which is filled up with a semi-gelatinous substance; so that, by this beautiful con- trivance, the mobility of the whole chain is abundantly provided for. (525.) There are only two kinds of vertebrae recognizable in the skeleton of a fish, viz. the abdominal and the caudal. The abdominal vertebrae support the ribs, for in these animals the ribs do not constitute a thorax, or contain any of the viscera called thoracic in the human body : they extend from the head to the com- mencement of the tail, and are at once re- cognizable by the nature of the elements which enter in- to their com- position ; each vertebra being provided with a superior arch (fig. 219, b), through which passes the spi- nal cord, a su- perior spinous process (c), and two trans- verse processes (d), to the ex- tremities of which the ribs are generally attached. The caudal verte- brae are com- posed, as we have already seen, of differ- ent elements: the transverse processes ei- ther do not exist, or are very feebly de- PISOES FISHES. Fff.221. 491 492 PISCES FISHES. veloped ; but beneath the body an inferior arch is formed, and from this an inferior spinous process, equalling the superior in length, is prolonged in the opposite direction (Jig- 221, b). (526.) As the vertebrae approach the tail, they become some- what modified in structure to support the caudal fin ; their spines become shorter and thicker, the canals formed by their superior and inferior arches smaller or nearly obliterated, and at length the spines become, as it were, soldered to each other, and to the interspinous bones hereafter to be noticed ; so that they form a broad vertical plate, to the posterior margins of which the rays of the tail-fin are articulated (Jig. 221. 70). (527.) The ribs of fishes are slender bones, appended either to the extremities of each transverse process of the abdominal vertebrae, or else to the body of the vertebra itself : every rib is connected with but one vertebra, and that only at a single point. They do not, as we have already said, form a thoracic cavity ; but enclose the abdomen, and are embedded among the lateral muscles of the trunk, to which they give support. From each rib arises a long styliform process (78), which, inclining backwards, is likewise plunged among the muscular fasciculi ; and in some fishes, such as the Herring and Carp tribes, similar appendages are derived from the bodies of the vertebrae themselves, so that the bones of such fishes appear to be extraordinarily numerous. On the other hand, many tribes have but the rudiments of ribs ; and in some, as for example in the Skate, they are altogether wanting. (528.) No sternum, properly so called, exists in fishes ; but the extremities of the ribs are sometimes connected with ossified plates belonging to the tegumentary system, which cover the abdo- men, and which by some authors have been regarded as a sternal apparatus. (529.) We have now to request the attention of the reader to certain supplementary organs which are peculiar to the class before us. These consist in sundry appendages to both the superior and inferior spinous processes of the vertebrae, which are generally pro- longed into fins situated along the mesial line of the body. These azygos fins, which must be by no means confounded with the pairs of fins that represent the arms and legs, are very variable in their position, and in many cases are altogether wanting. When fully developed, one of them is situated along the mesian line of the back, and in the Perch (Jig- 221) this dor sal Jin is separated into two distinct portions (75) : another, denominated the caudal PISCES FISHES. 493 Jin, forms the tail ; and a third, likewise situated in the median line at a short distance behind the anal orifice, is called the anal Jin from that circumstance. These fins present two sets of bones : the interspinous bones, which form the basis to which they are affixed ; and (he Jin-rays. The interspinous bones (Jig. 21. 74) form a series of strong dagger-like bones, deeply implanted in the flesh along the mesial line of the body, between the two great masses of lateral muscles : their points generally penetrate to a little distance between the spinous processes of the Vertebrse, to which they are connected by a ligamentous attachment ; whilst to their opposite extremity, which may be compared to the hilt of the dagger, the corresponding fin- rays are affixed by a beautiful articulation. There is generally only one interspinous bone affixed to a vertebral spinous process, but in the Flat-fishes (Pleuronectida) there are two ; and, more- over, in that remarkable family, the, inferior spinous process of the first caudal vertebra, which j as we have already seen, is of enormous size, frequently has not fewer than six or seven interspi- nous bones appended to its extremity. Each interspinous bone consists of two pieces united by a suture ; one portion representing the blade, the other the handle of the dagger, to which we have compared it. The fin-rays of fishes are of two kinds, being either solid and apparently composed of one strong piece, like those which sup- port the anterior half of the dorsal fin of the Perch (Jig. 75), in which case they are called spinous rays ; or else they are composed of several slender stems derived from one common root, every one of which is made up of numerous pieces : these, which bear the name of soft rays, are found in the posterior portions both of the dorsal and anal fin of the perch, and are invariably met with in the tail of all fishes possessed of a caudal fin. This difference in the structure of the fin-rays, trivial as it might appear, is a circum- stance to which much importance is attached by icthyologists, who hence derive the means of separating osseous fishes into two great groups, the Acanthopterygii, or such as possess spinous rays in the composition of their dorsal fin ; and the Malacopterygii, in which all the fin-rays are soft. Every fin-ray, whether spinous or soft, is in reality made up of two lateral halves placed side by side : in the soft rays these are easily separable ; but in the spinous rays they are firmly united along the median line, so as to represent but one bone. 494* PISCES FISHES. The articulation between every fin-ray and the corresponding interspinous bone forms a hinge-joint, so as to allow of the eleva- tion or depression of the fin. The structure of this joint is very beautiful, the two lateral halves of the ray separate so as to form two branches, which firmly embrace the sides of the head of the interspinous bone, and terminate in little prominent tubercles, which are received into corresponding lateral depressions in the bone to which the ray is attached. Sometimes, indeed, the head of the interspinous bone is completely perforated, and then the two branches of the fin-ray passing through the opening become firmly united with each other, forming a kind of joint which is peculiar to fishes, and exactly resembles the mode of union between two links of a chain. This structure is beautifully exhibited in the articulation of the elongated rays attached to the head of Lophius piscatorius.* (530.) The composition of the skull of fishes is one of the most difficult studies connected with their history ; nevertheless, it is a subject of very considerable importance, and has recently occupied the attention of the most celebrated Continental anato- mists. It is not by any means our intention to engage our readers in discussing all the conflicting and, sometimes, visionary opinions entertained by different authors relative to the exact homology of the individual bones forming this part of the skeleton ; and we shall, therefore, content ourselves by placing before them, divested as far as possible of superfluous argumentation, Cuvier's*!* masterly analysis of the labours of the principal enquirers concerning this intricate piece of anatomy, taking the Perch as a standard of com- parison.^: The head of a fish may be conveniently divided, for the pur- pose of description, into several distinct regions, each of which will require separate notice. (531.) The Cranium, which forms the central portion of the skull, contains the brain and auditory apparatus, and constitutes the basis whereunto the other parts are connected. It is remark- able from the number of distinct pieces of which it consists, inas- much as in fishes the elements, or ossific centres, of which the * Vide YarrelPs History of British Fishes ; vol. i. p. 271. 8vo. t Cuvier et Valenciennes, Histoire des Poissons. 4to. vol. i. $ Those who would enter more fully into the discussions relative to the essential composition of the skull, are referred to the works of Geoffroy St. Hilaire, Spix, Rosenthal, Meckel, Bakker, Bojanus, and Oken, the great disputants upon this subject. PISCES FISHES. 495 cranial bones of higher animals are composed, remain here per- manently separated, overlapping each other so as to form squamous sutures ; but never becoming fused together, as the elements of the human skull invariably do at a very early period. No fewer than twenty-six bones enter into the composition of the cranium we are now considering ; to which, as is now generally allowed, the following names are applicable. The Frontal bones are each divided into three portions, called respectively the Principal frontal (1),* the Anterior frontal (2), and the Posterior frontal (4). Between the anterior frontal bones is the Ethmoid, a simple vertical lamella, which is often merely a cartilaginous plate. The middle of the base of the cranium is made up of two bones : the Basitar (Jig. %$. 5), a portion of the occipital forming the body of the occipital vert'ebra ; and the body of the Sphenoid (6), a distinct bone, which is prolonged anteriorly into a lengthened process, which serves as the base of the membranous septum be- tween the orbits. The Parietal bones (7) are placed behind the posterior frontal, but they do not generally touch each other, being separated by an interposed bone called the Interparietal (8) . The Occipital bone is made up of five portions, namely, two External Occipitals (9), two Lateral Occipitals (10), and the Basilar bone (5), already noticed, by which the head is articulated with the first vertebra of the spine. Two detached bones, which represent the great or temporal ala of the Sphenoid, fill up the space between the body of the Sphe- noid and the posterior frontal. Two other pairs of bones, which are elements of the temporal bone in man, likewise assist in forming the cranium : these are called the M astoid bones (12), and the Petrous bones (13). A single bone, analogous to the anterior portion of the body of the human Sphenoid, and which, as will be fully evident here- after, is essentially distinct from the posterior portion, bears the name of the Anterior Sphenoid, while the orbital ala of the Sphenoid are found in the two bones marked 14. These, therefore, together with the representative of the Vomer (16), complete the cranial portion of the skull ; no fewer than six azygos and twenty pairs of bones entering into its composition. * In order lo simplify the subject as much as possible, and prevent unnecessary repetition, the reader will observe that, throughout all the figures connected with the osteology of the Vertebrata, corresponding bones are indicated by the same numbers. 496 PISCES FISHES. (532.) Bones composing the upper jaw. The upper jaw con- sists of two pairs of bones, which, from the looseness of their connexion with the other bones of the face, are endowed with con- siderable mobility. The Intermaxillary bones (17) form the greater part of the margin of the jaw, and are attached by a moveable articulation to the anterior extremity of the vomer. These bones are armed with numerous sharp teeth. The Maxillary bones (18) are moveably articulated with the last, and generally are in like manner furnished with teeth. In some cases they are divided into two or three pieces. Bones of the face. The bones of the face in fishes are very numerous ; but, as they are of little importance to the osteologist, a bare enumeration of them will answer our present purpose, and enable the student to recognize them with facility. We have first the Nasal bones (20) ; then a chain of bones of variable size and number (19), so disposed as to form the lower boundary of the orbit, and hence named Sub-orbital bones. Behind these, again, a similar chain of ossicles is not unfrequently met with, arching over the temporal fossa ; and these, which are apparently peculiar to fishes, are named the Supra-temporal (21). Ptery go-palatine and temporal system of bones. Upon each side of the head is situated a somewhat complex apparatus con- PISCES FISHES. 497 nected on the one hand with the articulation of the lower jaw, and on the other with the opercula or gill-covers. These bones are seven in number on each side. The Palatine (22) are easily recognizable, forming part of the roof of the mouth, and generally armed with teeth. Two bones are connected with the posterior edge of each palate bone : one, situated externally, becomes in reptiles a very impor- tant element, it is called the Transverse bone (24) ; the second (25) is named the Internal Pterygoid. The other pieces belonging to this part of the skeleton are not a little interesting on account of their remarkable arrangement ; and, perhaps, the anatomical student will be somewhat startled at the position which some of them occupy. In the first place, the squamous portions of the temporal, instead of entering into the formation of the cranium, are here slightly displaced, and, although still called the Temporal bones (23), are articulated by a hinge- joint with the posterior frontal and mastoid bones, and thus form a moveable basis to which the opercular apparatus is at- tached. Connected with the Temporal we have the broad and flat piece (27) which is the Tympanic bone, and to these the pieces forming the opercula are appended. Lastly, supporting the lower jaw we find the Jugal bones ; and connecting these with the rest of the temporal apparatus are two small ossicles (31), which complete this portion of the ske- leton. The seven bones above enumerated are almost immoveably con- nected with each other by the interposition of cartilage between their edges, a mode of articulation distinguished by the name of synchondrosis ; but the whole apparatus moves readily upon the two hinges, one formed by the articulation of the palate bone with the maxillary and vomer, and the other by the joint which unites the temporal bone to the posterior frontal. This movement, by opening the gill-covers, enlarges the cavity of the mouth when the fish wishes to take in the water necessary for respiration ; or else, by acting in a contrary direction, again expels it. (533.) Opercular bones. The great flap, which in osseous fishes closes the gill openings externally, is composed of four pieces, to which the following names have been given. The Pra- operculum (30) is attached to the posterior edge or angle of the palato-temporal apparatus last described, and its borders often 2 K 498 PISCES FISHES. present spines and indentations, which, being visible externally, are of much importance to the icthyologist, as they afford a good character of distinction between allied genera. The second piece (28), which from its size is called par excellence the Operculum, together with the Sub-operculum (32) and the Inter-operculum (33), form a flap which covers the gill-opening like a great valve, opening and shutting continually to give exit to the water used in respiration. (534.) Lower Jaw. The lower jaw of fishes consists of two lateral halves united by a symphysis in the mesian line, each branch being articulated with the jugal bone of its corresponding side. Each division is separable by maceration into four or even five pieces : viz. the Dental (34), which supports the teeth; the Articular ($5), bearing the articulating facet ; the Angular (36), forming the angle of the jaw ; and a fourth, placed upon the inner surface of the articular, called the Opercular, because it corre- sponds with a bone met with in the lower jaw of reptiles, to which the same name has been applied. The fifth, when present, is very small and unimportant. aides and Bran- chiostegous Rays. The Os Hy- oides of a fish is situated as in other vertebrate animals ; it is composed of two branches, each made up of seve- ral pieces (37, 38, 39, 40), and is always suspend- ed from the tem- poral by means of two small ossi- cles (59), which, as they represent the styloid pro- cess of man, are called the Styloid bones. PISCES FISHES. 499 Between the two branches of the os hyoides is placed a single central piece (42), which becomes of great importance in reptiles and birds, and upon this is the bone which supports the tongue, or the Lingual bone (41). The great fissure that exists on each side between the head and shoulder of an osseous fish, wherein the gills are situated, is not closed merely by the opercular bones, but likewise by a broad membranous expansion called the Branchiostegous membrane, which is adherent to the os hyoides, and assists in forming the great valve of the operculum. This membrane is supported by a series of slender bones derived from the external margin of each branch of the os hyoides, and these are named from their office the .Branchiostegous Rays (43). (536.) Branchial apparatus. Fishes breathe by taking water into their mouths, and forcing it out again through the apertures situated upon each side of the neck ; it is thus made to pass be- tween their gills, which form a series of pectiniform vascular fringes supported upon a system of bones called the Branchial arches. The branchial arches, which are generally four in number on each side, are attached by one extremity to an intermediate chain of bones (53, 54, 55) situated in the mesial line behind the os hyoides, whilst by their opposite extremity they are connected by ligaments to the under surface of the cranium. Every branchial arch consists of several pieces (57, 58, 59, 60, 61), so joined together by ligaments that the whole is perfectly flexible, and their edges are studded with little osseous plates, generally armed with teeth, and so disposed as to prevent food taken into the mouth from being forced out through the branchial fissures with the issuing streams of water ; so that, in reality, these pieces fulfil in their way the same office as the epiglottis of Mam- malia. (537.) Pharyngeal bones.' -The last parts found to enter into the composition of this portion of a fish's skeleton, are called from their position the Pharyngeal bones. They are placed imme- diately behind the branchial apparatus, and form a second set of masticatory organs, generally even more efficient than the jaws themselves, being for the most part provided with very strong teeth. In the Perch there are eight of these bones situated just at the entrance to the oesophagus, two inferior (56), and six above 500 PISCES FISHES. (62) ; their office and efficiency as organs of mastication must be obvious to the most superficial observer. Upon reviewing the general disposition of the skeleton in one of the osseous fishes, it is at once apparent that the great instru- ment of locomotion is the tail, which by extensive and vigorous lateral movements sculls the body rapidly along through the yielding element in which these creatures live. In the construc- tion of the caudal extremity of the skeleton, every precaution has evidently been taken to convert this part of the body into a broad and expanded oar, possessed of the utmost possible flexibility in the lateral direction. No pelvis, therefore, trammels the move- ments of the spine, neither do any transverse processes limit the extent of flexion from side to side ; while, on the contrary, the extraordinary developement of the spinous processes both above and below, and more especially the vertical caudal fin, give an extent of surface proportioned to the wants of the animal. The dorsal and anal fins, situated upon the mesian plane, steady, and perhaps in some measure direct, the movements of the body ; while the arms and legs, or rather the pectoral and ventral fins, which are in this case of secondary importance as locomotive instruments, exhibit a very rudimentary condition, and are but feeble agents in progression. The posterior extremities, or ventral fins, are even less efficient than the pectoral in this respect ; and their position is found to vary remarkably in different orders. In the Perch these organs are, as we have seen, attached to the bony framework of the shoulders. In the Carp tribe (Cyprinidse) they are removed far back towards the commencement of the tail, and the bones supporting them are merely embedded in the muscles of the abdomen. In the Cod (Gadidse) the legs are absolutely in front of the arms, being sus- pended under the throat ; and in the Anguilliform fishes, the Eel for instance, the ventral extremities are altogether wanting. (538.) Such being the imperfect developement of the usual locomotive organs, we are quite prepared to expect a corresponding modification in the disposition and efficiency of different parts of the muscular system. When we compare the muscles of a fish with those of any of the higher Vertebrata, the contrast is indeed very striking. Delicate muscles (Jig. 224) are provided for the erection or de- pression of the different rays sustaining the dorsal and ventral fins, and thus the fins themselves are expanded or folded up at pleasure. PISCES FISHES. 501 Similar fasciculi spread out or approximate the rays of the tail, increasing or contracting at will the extent of surface presented by that organ. The muscles of the pectoral and ventral limbs are small in proportion to the feebleness of these extremities ; the muscles of the trunk alone constitute the great bulk of the body, and form the efficient agents in progression. Fig. 224. These great lateral masses commence at the back of the head, where they take an extensive attachment to the largely developed cranium : from this point backwards, they fill up the entire space intervening between the skin and the vertebral column, with both of which they are intimately connected, reaching even to the origin of the tail fin. The whole force of these powerful muscles is evidently exerted in bending the spine from side to side, and in effecting those vigorous lateral movements of the tail whereby the fish is propelled through its liquid element. We need, therefore, feel little surprise at the strength with which this part of the body of fishes is not unfrequently endowed, or at the velocity of their movement ; at seeing how easily their speed outstrips our fleetest ships ; how the Flying-fish (Exocetus), urged on by fear, darts like an arrow to a distance through the air; or how the Salmon, in obedience to an imperious instinct, defies even the thundering cataract to stop its course towards the locality where it is in- structed by Nature to deposit its eggs. (539.) There are sundry tribes of fishes, which, being destined to remain at the bottom of the sea, present certain peculiarities of structure, whereby they are not only distinguished from all others 502 PISCES FISHES. of the class, but form most remarkable exceptions to the general law in accordance with which the Vertebrata are organized. The animals presenting this anomalous configuration are the PlcurontctuLe, or Flat-fishes, as they are generally termed, which when at rest lie quietly upon the ground, where, from the colour of the upper part of their bodies, they are scarcely distinguish- able. To an ordinary observer the Pleuronectidse would seem to have their bodies flattened and spread out horizontally, so that, while resting upon their broad and expanded bellies, their eyes, situated upon the back of the head, are thus disposed for the pur- pose of watching what passes in the water above them ; and this, the vulgarly received opinion, is considerably strengthened by the fact, that what is usually called the belly is white and colourless, while the back is darkly coloured and sometimes even richly varie- gated. The very name used in scientific language to distinguish this extensive family (Pleuronectes*) is calculated to propagate the error ; and few imagine that, in applying the terms back and belly to the upper and under surfaces of a Plaice or a Turbot, they are adopting a phraseology quite inadmissible in an anatomical point of view. On examining the skeleton of a Flat-fish, we at once see that what we supposed to be the dorsal and ventral regions are in reality the two sides, which are thus strangely different in colour ; and that the great peculiarity of their structure is the want of symmetry between the lateral halves of the body, arising from the anomalous circumstance that both the eyes are placed upon the same side of the head. Their cranium, indeed, is composed of the same bones as that of an ordinary fish, but the two lateral halves are not equally developed ; and the result is such a distortion of the whole framework of the face, that both the orbits are transferred to the same side of the mesial line of the back. The position of the pectoral and ventral fins slightly participates in this want of symmetry, but in other respects the skeleton (Jig. 225) precisely corresponds with that of the generality of osseous fishes. The superior and inferior spinous processes of the vertebrae are amazingly developed, and the interspinous bones (74) of inordi- nate length, so that the vertical diameter of the body is dispro- portionately increased, and the animal is obliged to swim and rest upon one side. The dorsal Jin (75) runs along the whole length of the back ; the anal Jin (a) reaches from the large spines that form the posterior boundary of the abdom'en to the tail, which latter holds the same position as in other tribes ; so that the reader a, the side ; vjjx'w, a fin. PISCES FISHES. 503 will have little diffi- Fig- 225. culty in comparing the different pieces of the skeleton of the Flounder (Pleuro- nectes flesus) with the corresponding bones of the Perch already described. (540.) The ske- letons of the Car- tilaginous Fishes (Chondropterygii *) will require a dis- tinct notice, inas- much as they pre- sent very remarkable peculiarities of no inconsiderable inter- est. In the Sharks, Skates, and other genera belonging to this important divi- sion of the great class we are now con- sidering, the interior of the bones remains permanently cartila- ginous, but the ske- leton is in some re- gions encrusted, as it were, with osseous granules. No centres of ossification, from which radiating fibres of bony matter progressively extend themselves, as is the case in the osseous fishes, are ever de- veloped ; and consequently the skull, although it presents exter- nally the same regions, eminences, and apertures that are usually met with, is never divided into separate bones, but is formed of a single mass of cartilage, in which no sutures or lines of division are ever distinguishable. The face is likewise much more simple in its structure ; for, instead of the numerous pieces composing the palato-temporal region of the Perch (j 532), two bones only are met with, one of * ^avSga;, cartilage ; x-Ttpuyiav, a fin. 504 PISCES FISHES. which, the palatine, performs the office of an upper jaw and supports the teeth, while the other connects the lower jaw with the cranium. The lower jaw itself, moreover, consists of but one piece on each side, to which the teeth are at- tached. From the peculiar con- formation of the respiratory apparatus, which will be ex- plained hereafter, there is no occasion for any opercular flap ; this, therefore, is not present : nevertheless, the hy- oid and branchial arches re- semble pretty much those of osseous fishes; only the latter are situated further back- wards, being placed quite be- hind the skull, under the commencement of the spine. The bones of the shoulder are represented by a strong cartilaginous zone, which in Sharks is quite unconnected with the vertebral column, but in the Skates (Raia) it is fixed to two large lateral PISCES FISHES. 505 apophyses derived from the spine (Jig. 226). The zone, represent- ing the scapulary apparatus, consists of a single piece, which surrounds the body, and on each side supports the bones of the fore-arm. The enormously developed pectoral fin is composed of the carpus, amaz- ingly augmented in size, and of the no less remarkable hand which in the Skate is made up of an immense number of fingers or rays, and forms by itself nearly half the circumference of the body. The pelvis, or cartilaginous framework that supports the hinder extremities, i. e. the ventral fins, is a single transverse piece of cartilage quite detached from the rest of the skeleton : it expands on each side into a broad plate, to which the fin, the representative of the foot of higher animals, is appended, and likewise in the male it gives attachment to additional organs called claspers, the use of which will be explained in another place. The anterior portion of the spine in the Skate is not as yet divided into distinct pieces ; and, even in the posterior part, the number of vertebral arches is twice as great as that of the separate bodies of the vertebrse. In all the Chondropterygii the ribs are mere rudiments, and in some cases can scarcely be said to exist at all. The Sturgeons (Sturionidai) form a kind of connecting link between the osseous and cartilaginous fishes, and in them a large swimming-bladder exists, from which is obtained the va- luable material called isinglass : but in the Sharks and Rays this organ is not found ; consequently, especially in the tribe last men- tioned, it is xmly by means of the vigorous flappings of their enor- mous hands that these ground-fishes are able to raise themselves from the bottom. The disposition and relative importance of different parts of the muscular system, is, therefore, necessarily changed to meet these altered circumstances : the muscles of the trunk, which in osseous fishes formed the great agents in loco- motion, become now of secondary importance ; while those of the pectoral fins, so feebly developed in the Perch, are massive and powerful in proportion to the unwieldy size of the anterior extre- mities. Another peculiarity in the skeleton of the Chondro- pterygii is observable in the construction of the caudal fin, which even in the Sturgeon and the Shark, notwithstanding the import- ance which this organ still maintains in those genera as an instru- ment of locomotion, begins to differ very remarkably from the tail of an osseous fish. It is true that it still exhibits great expansion in a vertical direction, and to a superficial observer, if examined 506 PISCES FISHES. without dissection, might seem to be constructed on the same prin- ciples ; but, on examining the skeleton of one of these cartilaginous fishes, it will be found that the vertebral column is continued uninterruptedly into the upper half of the generally furcate tail ; whilst the lower division of the caudal fin is entirely made up of supplementary rays, appended to the inferior aspect of the caudal vertebrae. Possessing this form of the tail the transition is by no means abrupt from these highly organized fishes to the Saurian Reptiles, with which, as we shall afterwards see, they exhibit many remarkable affinities. (541.) If in the highest HETEROGANGLIATA we found, that in addition to the tegumentary skeleton, or shelly covering, so exten- sively met with among the Mollusca, the first appearances of an internal osseous system became recognizable, we are not on that account to imagine that, as soon as bones become developed inter- nally, the cuticular secretions hitherto denominated shell at once disappear, but, on the contrary, must be prepared to expect that in some form or other calcareous armour deposited by the skin should still be met with. In fishes the coexistence of an internal and of an external skeleton is undeniable ; and having already described the former, which has been aptly enough called the endoskeleton, it remains for us in the next place to examine the latter or exoske- leton, which, as we shall soon perceive, forms no unimportant part of the anatomy of the class under consideration. The most usual form of the cuticular covering of fishes is that of imbricated scales, with which the whole exterior of the body is compactly encased, as in a suit of armour. Such an investment is admirably adapted to their habits and economy. The dense and corneous texture of the scales, impermeable to water, defends their soft bodies from maceration, while from their smooth polished exte- rior and beautiful arrangement they ensure the least possible resist- ance from the surrounding medium as the fish glides along. (54.) Examined separately, each scale is found to be partially embedded in a minute fold of the living and vascular cutis, to which its under surface is adherent. Every scale is, in fact, made up of superimposed laminae of horny matter secreted by the cutis, pre- cisely in the same way as the shelly covering of a mollusk, and by maceration the different layers may readily be separated, the smallest and most superficial being of course the first formed, while the largest and most recent are those nearest to the surface of the living skin : as far as relates to the mode of growth, therefore, there PISCES FISHES. 507 is the strictest analogy between the scale of a fish and shell. Va- rious are the forms under which these scales present themselves to the icthyologist : sometimes, as in the Eel, they are thinly scat- tered over the surface of a thick and slimy cutis, more generally they form a close and compact imbricated mail ; in the Pipe- fishes (Syngnathidee) the whole body is covered with a strong armour composed of broad and thick calcareous plates ; and in the Coffin-fishes (OstracionidtE) the integument is converted into a strong box made up of polygonal pieces anchylosed together, so that the tail and fins alone remain moveable. The Sturgeon is covered with broad shield-like plates. The skin of the Sharks is densely studded with minute sharp spines of almost crystalline hardness ; and in many Skates, as in the Thorn- back, similar cuticular appendages, but of more considerable dimensions, are distributed over the back and tail, forming very efficient defensive weapons. But cutaneous spines, although while in a rudimentary condi- tion they are obviously mere extraordinary developements of scales, may occasionally become of sufficient size and importance to make them convertible to various unexpected uses ; and when thus exaggerated in their dimensions, and appropriated to distinct offices, they assume so much of the character of true bone, that it is no longer easy to demonstrate their real nature, more especially as they then become in many cases really articulated by means of very perfect joints with different pieces of the endoskeleton properly so called. Let us examine this important subject with a little attention, and we shall soon perceive how closely the endoskeleton and the exoskeleton may become connected, not to say interchangeable, with each other. There is no possibility of mistaking the spines and tubercles upon the back of a common Skate for anything but cuti- cular appendages secreted in the same manner as scales from the surface of a vascular pulp ; but in the Fire Flaire (Trygon pasti- naca), where, instead of the scattered hooks of the former species, we find a single sharp and serrated spine projecting like a bayonet from the upper surface of the root of the tail, the analogy between this formidable and bone-like organ and an epidermic structure becomes apparently more remote, and, did we not know that the fish possessing such a weapon had no ossified bones internally, we might be tempted to regard this appendage as a process derived from the endoskeleton. 508 PISCES FISHES. The spines of the common Stickleback (Gasterosteus) are indu- bitable derivations from the cuticle ; but here they become fixed by moveable articulations to the sides of the body, and are raised or depressed by means of muscles inserted into their bases. Ad- vancing one step further, we find in Silurus the first ray of the pectoral fin, enormously developed and forming a strong serrated weapon of a very formidable description, which, although both in shape and structure exactly comparable to the spine upon the tail of the Fire Flaire, are nevertheless connected by most beautiful and perfect joints with the bones of the shoulder, so that they might easily be regarded as forming pieces of the endoskeleton, did not their peculiar structure indicate their real nature. We thus arrive at the important conclusion, that different por- tions of the exoskeleton become approximated in character to those of the endoskeleton, or in truth really convertible into true bone ; and, with this fact before us, it becomes easy to understand the nature of various parts of the skeleton of a fish, which upon any other supposition would be not a little puzzling to the comparative osteologist. The nature of the rays of the dorsal and anal fin of the Perch, for example, together with the interspinous bones upon which they are sustained, is quite unintelligible if they are regarded as belong- ing to the endoskeleton ; and no dismemberments of the osseous system as yet imagined, or supposed subdivisions of the vertebrae into a greater number of elemental pieces than we have enume- rated, has been able to solve the difficulty; but, if they are regarded as ossified derivations from the exoskeleton^ all difficulties at once vanish. Again, the opercular bones (28, 30, 32, 33) forming the gill- covers of an osseous fish have been a fruitful source of discus- sion, and M. Geoffroy St. Hilaire* was reduced to the necessity of recognizing in these broad plates the ossicles of the human ear, which, after dwindling to a rudiment in the descending scale of ver- tebrate animals, suddenly reappeared in a new and exaggerated form. " J'ai peu vu dans la srie des etres de ces resurrections d'organes se remontrant subitement dans une classe apr&s avoir dis- paru dans une ou deux de celles qui la precede dans Techelle," are the impressive words of Cuvier upon a similar occasion ; and it is certainly far more simple to imagine the epidermic plates * Philosophie Anatomique des pieces osseuses des organes respiratoires. 8vo. Paris, 1818. PISCES FISHES. 509 of the Sturgeon ossified and converted into bone, than to be com- pelled to have recourse to the bold speculations of the French anatomist regarding the real nature of these opercular portions of a fish's skeleton.* (543.) In connection with the locomotive organs we must here notice one of the most elegant contrivances met with in the whole range of animated nature, by which the generality of fishes are enabled to ascend towards the surface, or to sink to any re- quired depth without exertion. The apparatus given for this purpose is called the swimming- bladder, and consists of a reservoir of air (Jig. 227, p) placed beneath the spine; in which position it is firmly bound down by the peritoneum. The outer coat of this bladder is very strong, and composed of a peculiar fibrous substance from which isinglass is obtained, but it is lined internally with a thin and delicate mem- brane. The shape of the swimming-bladder varies considerably in different tribes. In the Perch it is a simple cylinder closed at both extremities : sometimes it gives off branched appendages ; some- times, as in the Cyprinid&i it is divided into two portions, one anterior and the other posterior, by a deep central constriction ; but, whatever its shape, its office is the same, namely, to alter the specific gravity of the fish, and thus to cause it to rise or sink in the medium it inhabits. By simply compressing this bladder by approximating the walls of the abdomen, or occasionally by means of a muscular apparatus provided for the purpose, upon a principle with which every one is familiar, the fish sinks in proportion to the degree of pressure to which the contained air is subjected ; and, * The different opinions on the nature or homology of the opercular bones may be reduced to two principles : first, that they are modifications of parts of the ordinary skeleton ; secondly, that they are superadded bones peculiar to fishes : the latter view is that taken by Cuvier. According to the former, which is the more philosophical mode of considering them, three opinions have been offered ; the first by Spix and Geoffrey, that they are gigantic representatives of the ossicles of the ear, otherwise absent in the skeleton of fishes, this view has been adopted by Professor Grant ; secondly, that they are dismemberments of the lower jaw, which by the detachment of the opercular bones from the ramus is rendered more simple in its composition than in reptiles, a view proposed by M. de Blainville and temporarily adopted by Bojanus and Oken, but refuted by the complicated structure of the lower jaw in certain sauroid fishes, as the Lepi- dosteus, which likewise possesses the opercular bones; thirdly, that they are parts of the dermal skeleton, in short, scales modified in subserviency to the breathing func- tion ; an opinion first proposed by Professor Owen, in his Lectures on Comparative Anatomy at St. Bartholomew's Hospital in 1835, and which is the view here adopted. 510 PISCES FISHES. as the compressed air is again permitted to expand, the creature becoming more buoyant rises towards the surface. In the Perch, and many other fishes, this organ is entirely closed, so that there is no escape for the contained air; and in such it has been found that if they are suddenly brought up by means of a line from any great depth, the gas being no longer compressed by the weight of the column of water above, and having no exit, bursts the swimming-bladder, and sometimes distends the abdomen to such an extent, that it pushes the stomach and oesophagus into the fish's mouth. In other cases, however, a provision is made apparently with a view of obviating such an accident, and a kind of safety-valve pro- vided, through which the air may be permitted to escape : thus, in the Carps a tube communicates between the interior of the air- bladder and the oesophagus, and in the Herring a similar commu- nication is met with between this organ and the stomach. The gas which fills the air-bladder has been found in many cases to be nearly pure nitrogen, but in fishes that live at a great depth Messrs. Configliacchi * and Biot ascertained that oxygen was substituted, whence it has been presumed that this apparatus was in some way or other an auxiliary in respiration ; and some authors have even gone so far as to see in the swimming-bladder the representative of the lungs of aerial Vertebrata. But, however this may be, the gas enclosed is indubitably a product of secretion, being derived either from the lining membrane of the viscus, or from a glandular structure which may frequently be distinctly pointed out in its interior. Cuvier justly observes, that, whatever opinions may be enter- tained relative to the use of the air-bladder, it is difficult to explain how so considerable an organ has been refused to so many fishes, not only to those which ordinarily remain quiet at the bot- tom of the water, as Skates and Flat-fishes, but to many others that apparently yield to none either in the rapidity or facility of their movements, such as the Mackerel, for instance ; yet even while the common Mackerel (Scomber scomber) has no air-blad- der, a very nearly allied species (Scomber pneumatophorus) is provided with one, and of this many other instances might be adduced. (544.) From the circumstances under which fishes seize and swallow their prey, it must be evident that they are incapable of * Suir analisi dell' aria contenuta nella vesica natatoria del Pesci. Pavia, 1809. 4to. PISCES FISHES. 511 enjoying any very refined sense of taste. Those species which are carnivorous are of necessity compelled to catch with their mouths, and retain a firm hold of the active and slippery food they are destined to devour : to divide or masticate their aliment would be impracticable ; and, even were they permitted so to do, the water which perpetually washes over the interior of their mouths would obviously preclude the possibility of appreciating savours. In the construction of the mouth of a fish we therefore find, generally speaking, that every part has been made subservient to prehen- sion : teeth, sometimes in the form of delicate spines, or else pre- senting the appearance of sharp recurved hooks, have been fixed in every possible situation where they could be made available as prehensile organs ; not only are the jaws densely studded with these penetrating points, but they are occasionally placed on every bone which surrounds the oral cavity, or supports the entrance of the pharynx. The intermaxillary, the maxillary, and the palatine bones, the vomer, the branchial arches, the pharyngeal bones, and even the tongue itself, may all support a dental apparatus, either of the same description or composed of teeth of different shapes ; generally, however, some of these bones are unarmed, and occa- sionally teeth of any kind are altogether wanting. But if such is the most usual arrangement of the dental appa- ratus in fishes, we must be prepared to find, in a class so extensive as that we are now investigating, various modifications both in the form and arrangement of the teeth, adapting them to the diverse habits and necessities of individual species ; and a few of these we must not omit to notice in this place. The Myxine, or Hag-fish, one of the lowest of the entire class, possesses no osseous framework whereunto teeth could be attached ; and yet, from the parasitical life which this creature leads, it has need of dental organs of considerable efficiency. The Myxine, feeble and helpless as the casual observer might suppose it, is in reality one of the most formidable assailants with which the larger fishes have to contend, since neither strength nor activity avail aught in defending them against a foe apparently so despicable : fixing its mouth firmly to the skin of its comparatively gigantic victim, the Myxine bores its way into its flesh by means of a dental apparatus of a very extraordinary description. A single fang-like tooth is fixed to the median line of the palate, and the tongue is armed on each side with two horny plates deeply serrated : thus provided, the Myxine, when it attacks its prey, plunges its palatine 512 PISCES FISHES. hook into its flesh ; and, thus securing a firm hold, the lingual saws, aided by the suctorial action of the mouth, tear their way to its very vitals.* In the Lamprey the whole interior of the mouth is studded with horny teeth, not merely fixed to the palate and tongue, but to the cartilaginous representative of the inferior maxilla, and to the inner surface of the lips. In the Carp tribe (Cyprinidte) the jaws are destitute of teeth, but in the throat there is a singular apparatus serving for the masti- cation of their food. The basilar bone at the base of the skull sup- ports a broad three-sided dental plate, which might be compared to an anvil ; while the two inferior pharyngeal bones are each armed with four or five large teeth, so disposed, that, by working upon the piece first-mentioned, they bruise and triturate the aliment before it is permitted to pass into the digestive cavity. In Skates (Raidte) the internal surface both of the upper and lower jaws are so covered with teeth, that they have the appearance of a tesselated pavement : these teeth are sometimes flat and smooth, so as to be merely useful in crushing prey ; but in many species they are prolonged into sharp hooks adapted to prehension. In the Sharks a beautiful provision is met with. Several rows of teeth placed one behind the other are found laid flat, and con- cealed behind the jaw. One row only, composed of triangular cut- ting teeth, stands erect and ready for use ; but when these fall off, blunted and unfit for service, the next row rises to take their place ; and thus a succession of efficient weapons are given to these terrific monsters of the ocean. We will not enlarge further upon this portion of our subject ; enough has been said for our present purpose, and the reader will find elsewhere abundant information. -f- The teeth of osseous fishes are generally firmly anchylosed to the bones ^that support them, although in a few instances they are found fixed in sockets, as in the rostral teeth of the Saw-fish (Pristis), and in the mouth of Sphyrfena, Acanthurus, Dicty- odus, &c4 But there are other modes of attachment only met with among fishes, some of which are not a little curious ; and X * Professor Owen. " ODONTOGRAPHY, or a Treatise on the Comparative Anatomy of the Teeth, their physiological relations, mode of developement, and microscopic structure," &c. 4to. Bailliere, 1840. f Vide Yarrell's British Fishes. 8vo. 2 vols. J Owen. Odontography, p. 6. PISCES FISHES. 513 Professor Owen, in his truly splendid work above referred to, thus describes the most important. " In the Cod-fish, Wolf-fish, and some other species, in propor- tion as the ossification of the tooth advances towards its base and along the connecting ligamentous substance, the subjacent portion of the jaw-bone receives a stimulus, and developes a process cor- responding in size and form with the solidified base of the tooth. In this case the inequalities of the opposed surfaces of the tooth and maxillary dental process fit into each other, and for some time they are firmly attached together by a thin layer of ligamentous substance ; but in general anchylosis takes place to a greater or less extent before the tooth is shed. The small anterior teeth of the Angler (Lophtus) are thus attached to the jaw, but the large pos- terior ones remain always moveably connected by highly elastic, glistening ligaments, which pass from the inner side of the base of the tooth to the jaw-bone. These ligaments do not permit the tooth to be bent outwards beyond the vertical position, when the hollow base of the tooth rests upon a circular ridge growing from the alveolar margin of the jaw ; but the ligaments yield to pressure upon the tooth in the contrary direction, and its point may thus be directed towards the back of the mouth ; the instant, however, that the pressure is remitted, the tooth flies back, as by the action of a spring, into its usual erect position ; the deglutition of the prey of this voracious fish is thus facilitated, and its escape pre- vented. ' " The broad and generally bifurcate osseous base of the teeth of Sharks is attached by ligaments to the ossified or semi-ossified crust of the cartilaginous jaws. The teeth of the Salarias and certain Mugiloids are simply attached to the gum. The small and closely crowded teeth of the Rays are also connected by ligaments to the subjacent maxillary membrane. The broad tesse- lated teeth of the Eagle-Rays have their attached surface longi- tudinally grooved to afford them better holdfast, and the sides of the contiguous teeth are articulated together by true serrated or finely undulating sutures; which mode of fixation of the dental apparatus is unique in the animal kingdom. " If the engineer would study the model of a dome of unusual strength, and so supported as to relieve from its pressure the floor of a vaulted chamber beneath, let him make a longitudinal section of one of the pharyngeal teeth of a Wrasse (Labrus). The base of this tooth is slightly contracted, and is implanted in a shallow 514 PISCES FISHES. circular cavity, the rounded margin of which is adapted to a circular groove in the contracted part of the base ; the margin of the tooth which immediately transmits the pressure to the bone is strength- ened by an inwardly projecting convex ridge. The masonry of this internal buttress, and of the dome itself, is composed of hollow columns, every one of which is placed so as to transmit in the due direction the superincumbent pressure. " In another case, in which long and powerful piercing and lacerating teeth were evidently destined, from the strength of the jaws, to master the death-struggles of a resisting prey, we find the broad base of the tooth divided into a number of long and slender processes, which are implanted like piles in the coarse osseous substance of the jaw ; they diverge as they descend, and their extremities bend and subdivide like the roots of a tree, and are ultimately lost in the bony tissue. This mode of implantation, which I have detected in a large extinct Sauroid fish (Rhizodus), is, perhaps, the most complicated which has yet been observed in the animal kingdom." For a full account of the growth and developement of the teeth of fishes, we must refer the reader to the same source from which we have extracted the preceding paragraphs ; nevertheless, the following is a brief abstract of Professor Owen's views upon this subject. In all fishes the first step in the formation of a tooth is the production of a simple papilla from the surface either of the soft external integument, as in the formation of the rostral teeth of the Saw-fish (Pristis), or of the mucous membrane of the mouth, as in the rest of the class. In these primitive papillae there can be very early distinguished a cavity containing fluid, and a dense membrane (membrana propria) surrounding the cavity, and itself covered by the thin buccal mucous membrane, which gradually becomes more and more attenuated as the papilla increases in size. The pulp- substance, or contents of the membrana propria, remains for some period in a fluid or semi-fluid condition ; granules are ultimately developed in it, which at first float loosely, or in small aggregated groups, in the sanguineo-serous contents of the pulp. These gra- nules soon attach themselves to the inner surface of the membrana propria^ if they be not originally developed from that surface. The whole of the contents of the growing pulp becomes soon after condensed by the numerous additional granules, which are rapidly developed in it after it has become permeated by the capillary PISCES FISHES. 515 vessels and nerves. The particles become arranged into linear series or fibres ; an appearance which is first apparent at the super- ficies of the pulp, to which the fibres are vertical. At this period ossification commences in the dense and smooth membrana propria of the pulp, and is thence continued centripetally in the course of the above-mentioned lines towards the base of the pulp. Lastly, around the capillaries of the pulp the granules become condensed into concentric layers, which then form the walls of minute tubes, visible on a microscopic examination of the substance of the tooth. In some genera, as Balistes and CAryaopry*, an enamel-pulp is developed from the inner surface of the capsule which surrounds the bone-pulp, and by this organ the surface of the teeth of such fishes is coated with enamel in a manner to be described more at large hereafter. In most osseous fishes, in addition to the lips, which even when fleshy, being destitute of proper muscles, would be unable to retain food in the mouth, 'there is generally behind the front teeth in each jaw a valve formed by a fold of the lining membrane of the mouth, and directed backwards so as efficiently to prevent the alimerjt, and more especially the water swallowed for the purpose of respiration, to escape again from the oral orifice.* (545.) Fishes have no salivary glands, as saliva to them would be entirely useless : their esophagus (Jig. 227, g ; Jig- 236, d) is capacious ; and, from the circumstance of their having neither neck nor thorax, extremely short, so that the food when seized is con- veyed at once into the stomach. (546.) The stomach itself is generally a wide cul-de-sac (fig- 227, A), the shape and proportionate size of which varies of course in different species. Its walls are most frequently thin, and the lining membrane gathered into large longitudinal folds (Jig. 286, e), so as to admit of considerable distension ; but occa- sionally, as for example in the Mullets, its muscular walls are so thick that it might almost deserve the name of gizzard, and in such fishes its power of crushing the food is no doubt consi- derable. (547.) The intestinal canal in the osseous fishes is a simple tube (Jig. 227, ) folded in sundry gyrations proportioned to its length ; but in the cartilaginous families, such as the Sharks, the Rays, and the Sturgeons, it presents internally a very remarkable arrange- ment, evidently intended to increase the extent of surface over * Cuv. et Valenciennes, op. cit. p. 367. 516 PISCES FISHES. winch the digested aliment may be spread, for the purpose of absorbing its nutritive portions. In these tribes a spiral valve (Jig' 236, A) winds in close turns from the pyloric to the anal extremity of the capacious intestine ; so that, although externally the intestine appears short in proportion to the size of the animal, its mucous lining is exceedingly extensive. Fis- 227. (548.) In addition to the biliary secretion which we have met with in the lower animals, another system of chylopoietic glands for the first time makes its appearance in the class before us, from which a fluid termed the pancreatic is poured into the intestine. In the osseous fishes this viscus presents the simplest condition of a gland, consisting of simple caeca (fig. 227, n, n) ; sometimes, as in the Perch, only three in number ; at others, as for instance in the Salmonida, extremely numerous. From these appendages a glairy fluid, resembling saliva in composition, is abundantly secreted, and becomes mixed with the bile immediately upon its entrance into the intestine. In the cartilaginous fishes, such as Sharks and Rays, the pan- creas exhibits a more perfect developement, and already presents the appearance of a conglomerate gland (fig. 236, /), from which the pancreatic fluid is conveyed into the intestine through a com- mon duct. PISCES FISHES. 517 (549.) The liver of fishes is proportionately very large, and generally contains abundance of oil. The bile derived from it is received into a gall-bladder (Jig. 227, c), from which a duct of variable length in different species conveys it into the intestine, in the immediate vicinity of the pylorus. (550.) It is in these animals that we for the first time find the biliary secretion separated from venous blood; and consequently they are provided with a new arrangement of the blood-vessels of the abdomen, which they possess in common with the other Vertebrata, forming what is termed by anatomists the system of the Vena Porta. The veins derived from the stomach, the intestines, and the spleen, which last viscus now makes its appearance, instead of conveying their contents to the heart, plunge into the substance of the liver, and there again subdivide into capillary tubes ; thus fur- nishing to the liver abundance of venous blood from which the hepatic secretion is elaborated. (551.) The Spleen, now for the first time met with in the animal creation, is a highly vascular organ, generally enclosed in the mesentery between two folds of the intestine (Jig. 227, m ; Jig. 236, x), and evidently, in position, presenting no precise relations with the stomach. It receives a large supply of arterial blood, which becomes converted into venous as it circulates through this organ, and in that state is transmitted to the liver through the portal system of veins. (552.) Another important addition to the animal economy, pe- culiar to the Vertebrate division of animals, is the lymphatic or ab- sorbent system of vessels, which in fishes are abundantly distributed through the body, and ramify like a rich net- work over the walls of the intestines. These pour the materials absorbed from the body, and the products of digestion, into the principal venous trunks, to be mixed up with the circulating blood.* (558.) The circulation of the blood in fishes is carried on by the assistance of a heart composed of two cavities only, which re- ceives the vitiated blood after it has circulated through the system, and propels it through the branchise, where it is exposed to the influence of the oxygen contained in the surrounding medium. After being thus purified, the blood is collected from the respiratory organs by the radicles of the branchial veins ; and these latter ves- * For a detailed account of the lymphatic system of fishes the reader is referred to the following authors Monro, Anat. and Physiol. of Fishes, fol. ; Hewson, Phil. Trans. 1769 ; Fohman, Hist. Generate des Lymphatiques des Verteb. ; Heidelberg and Leipzig, fol. 1827. 518 PISCES FISHES. sels, by their union, form the aorta. There is, therefore, no sys- temic heart in fishes, the aorta itself serving to propel the slow- moving blood in its course through the arterial system. Fig. 228. (554.) The heart (Jig. 227, o) is enclosed in a pericardium, and situated beneath the pharyngeal bones and branchial apparatus ; the cavity in which it is lodged being separated from the peri- toneum by a kind of tendinous diaphragm, and also by a capacious sinus, in which the venous blood derived from all parts of the body is collected preparatory to its admission into the heart. The auricle of the heart (Jig. 228, B, b) is contained within the pericardium : it varies greatly in form in different fishes, but its capacity is generally considerably greater than that of the ven- tricle ; and its walls are thin, but, nevertheless, present distinct fleshy columns. The blood derived from the great sinus before mentioned enters the posterior part of the auricle of the heart by a large orifice, which is guarded by two membranous valves so disposed as to pre- vent the reflux of the blood during the contraction of the auricular chamber. The ventricle is strong and fleshy, and at its communi- cation with the auricle there is a strong mitral valve. The com- mencement of the branchial artery (Jig. 228, A, d), is so muscular and capacious, that it might almost be considered as forming a second ventricular chamber : this portion, which has been distin- guished by the name of the bulb (bulbus arteriosus), is separated from the ventricle by strong valves ; and in the cartilaginous fishes, as, for instance, in the Shark (Jig. 228, B, e), there are several rows of semilunar valves so disposed as most efficiently to prevent PISCES FISHES. 519 the blood from being driven back again into the ventricle. In the heart of Lophius (Jig. 228, A), the conformation of the cavities is very peculiar. The auricle (b) is large and pyriform, and the ven- tricle (c) of a globular shape ; but the most singular feature in its structure is the valve between the ventricle and the bulb (d). This is a soft fleshy protuberance (e), perforated in the centre, which projects into the cavity of the bulb, and allows the blood to pass freely in one direction ; but the sides of the canal collapse, and close the orifice, if the blood is forced back from the bulb towards the ventricle. Issuing from the pericardium, the branchial artery runs beneath the centre of the branchial apparatus, dividing into as many trunks as there are branchial arches, to each of which a vessel is given off. To each branchial arch are attached a great number of vascular lamellae placed parallel to each other, like the teeth of a comb. The branchial artery, which runs in a groove situated upon the con- vexity of the corresponding arch, sends off a twig to every one of these laminae ; and this vessel, after twice bifurcating, divides into an infinite number of little ramuscules, which run across both sur? faces of the branchial fringe, and terminate by becoming converted into capillary veins. The radicles of the branchial veins all open into a venous canal which runs along the internal margin of each lamella, and these last terminate in the great vein of the corresponding branchial arch, which runs in the same groove as the artery, but is more deeply situated, and, moreover, runs in the opposite direction ; that is to say, that the branchial artery derived from the heart, and coming from the ventral aspect of the body, diminishes in size as it mounts towards the back, and gives off twigs to the branchial fringe, whereas the branchial vein, on the contrary, receiving blood from the lamellae of the branchia, increases in diameter as it approaches the dorsal region. On leaving the gills, the branchial veins assume the appearance and perform the function of arteries. The anterior, even before escaping from the branchial arch, gives off ramifications to different parts of the head, and the heart and parts adjacent likewise receive their supply of arterial blood from a branchial vein. The veins derived from all the branchial arches ultimately unite and form the aorta, which evidently corresponds to the aorta of Mammalia, although it has neither auricle nor ventricle at its com- mencement. 520 PISCES FISHES. The aorta, while in the abdomen, runs beneath the spine, and gives arteries to the viscera in the usual manner ; but at the com- mencement of the tail it becomes enclosed in the inferior vertebral arches, by which it is defended to its termination. (555.) There is yet another set of organs, which, as we ascend from inferior to higher forms of animal life, we encounter for the first time in the class before us ; an apparatus for elaborating the urinary secretion, which is peculiar to the Vertebrate classes. The kidneys in fishes are very voluminous : they are situated on each side of the mesial line, immediately beneath the bodies of the vertebrae ; and extend along the whole length of the abdomen, not unfrequently reaching to the base of the skull, where their anterior portion (Jig. 27, e) lies above the branchial apparatus. The ure- ters (Jig. 27, f) generally terminate in a kind of bladder-like di- latation, the orifice of which is found behind that of the vulva (s). Examined minutely, the substance of the kidney is found to be entirely composed of microscopic tubules, which terminate in the ureters : these uriniferous tubes are variously contorted, but of equable diameter throughout ; and they end towards the periphery of the kidney by blind extremities. (556.) The skin of these aquatic animals is perpetually lubri- cated by an abundant mucous secretion furnished by muciparous follicles, or secreted in long tubular organs placed beneath the skin. In the Skate the vessels last mentioned are remarkably large, and their distribution very extensive. Fig. 229. PISCES FISHES. 521 (557.) The brain of an adult fish occupies but a small portion of the cranial cavity ; the space between the pia mater , which invests the brain, and the dura mater, which lines the skull, being occu- pied by a loose cellular tissue filled with fluid : there is consequently no serous or arachnoid cavity, such as exists in man. It has been remarked, that the interval between the cranium and the brain is considerably less in young than in mature fishes ; a fact which sufficiently proves that in them the brain does not grow in the same proportion as the rest of the body ; and, indeed, the size of the brain is nearly equal in individuals of the same species, even although the body of one be twice as large as that of the other.* In these, the lowest forms of Vertebrata, the brain consists of several masses placed one behind the other, either in pairs or singly; these masses in fact may be regarded as so many distinct ganglia, the complexity and perfection of which we must expect to become gra- dually increased as we proceed upwards towards mammiferous quadrupeds. The anterior pair of ganglia (figs. 229 and 234, c ; Jig. 232, a) invariably give origin to the olfactory nerves, and consequently may be justly looked upon as presiding over the sense of smell. These ganglia are, in fact, the representatives of those masses which in man are erroneously called the " olfactory nerves ;" for even in the human subject, although their real nature is obscured by the enor- mous developement of other parts of the encephalon, the so-called nerves are not nerves at all, but really lobes of the brain from which the true nerves emanate. Fig. 230. (558.) The olfactory nerves of fishes, derived from the lobes al- luded to, vary greatly in composi- tion and proportionate size : some- times they are quite capillary ; sometimes thick, though still sim- ple ; occasionally they are double or triple, and in some cases are composed of numerous fibres bound up in fasciculi. (559.) The organs of smell to which these nerves are destined are of very simple structure : Two excavations are found near the an- terior part of the snout, lined with * Cuv. et Val. op. cit. 522 PISCES FISHES. a delicate pituitary membrane, which is variously folded, in order to increase the extent of the sentient surface (Jig- 230) ; and it may be presumed, that from the number of plicse, which varies amazingly, some estimate may be formed of the relative perfection of the sense of smell in different genera. Into each olfactory chamber the water is freely admitted by two distinct orifices, while behind the pituitary membrane the olfactory nerve swells out into a ganglion (Jig. 232, 1), from which nervous fibrils radiate, to be distributed over the plicated lining of the nose (). (560.) The second pair of ganglia met with in the brain of a fish (Jig. 232, b) give origin to the optic nerves (2), and may therefore very properly be regarded as representing the tubercula quadrigemina of the mammiferous brain. The nerves of vision derived therefrom have no commissure, and present in many species a peculiar structure which is not a little remarkable ; each nerve being composed of a broad band of nervous substance, folded up like a fan, and enclosed in a dense membrane, so that when un- folded it presents the appearance delineated at fig. 231, A. (561.) The eye itself differs in many points of structure from that of terrestrial Vertebrata, its organization being of course ad- apted to bring the rays of light to a focus upon the retina in the denser element in which the fish resides ; the power of the crystal- line lens is therefore increased to the utmost extent, and the antero-posterior diameter of the eye-ball necessarily contracted in the same ratio, in order that the retina may be placed exactly in the extremely short focus of the powerful lens. The eyes of all the Vertebrata are constructed upon principles essentially similar, and present the same tunics and lenses as are met with in the human eye, and, generally speaking, arranged in the same manner as in man. It is not our intention, therefore, in the following pages minutely to describe the anatomy of the eye in every class which will come under our notice ; but taking the human eye, with the construction of which we presume our readers to be intimately acquainted, as a standard of comparison, point out those modifications of the general type of structure common to this divi- sion of animated nature. The first thing which strikes the attention of the anatomist, when examining the eye of a fish, is the size of the crystalline lens, and its spherical form. This shape, and the extreme density of texture which the lens exhibits, are, indeed, perfectly indispens- able. The aqueous humour, being nearly of the same density PISCES FISHES. 523 as the external element, would have no power in deflecting the rays of light towards a focus, and consequently the aqueous fluid in fishes is barely sufficient in quantity to allow the free sus- pension of the iris : the vitreous humour, from the same reason, would be scarcely more efficient than the aqueous in changing the course of rays entering the eye, and hence the necessity for that extraordinary magnifying power conferred upon the lens. B Fig. 231. C But the focus of the crystalline will be short in proportion as its power is increased ; every arrangement has therefore been made to approximate the retina to the posterior surface of the lens : the eye-ball is flattened, by diminishing the relative quantity of the vitreous humour ; and a section of the eye (Jig. 231 , B, c) shows that its shape is very far from that of a perfect sphere. This flattened form could not, however, have been maintained in fishes, had not special provision been made for the purpose in the con- struction of the sclerotic ; the outer tunic of the eye, therefore, generally contains two cartilaginous plates imbedded in its tissue, which are sufficiently firm in their texture to prevent any alteration in the shape of the eye-ball ; and in some of the large fishes the sclerotic is actually converted into a cup of bone presenting orifices at the opposed extremities, one for the insertion of the trans- parent cornea, the other for the admission of the optic nerve. The vitreous humour and crystalline lens in many fishes are kept in situ by a ligament placed for the purpose. This is a deli- cate falciform membrane derived from the retina (Jig- 331, B, c), which plunges into the vitreous humour, and, being continued along 524 PISCES FISHES. the internal concavity of tlie eye, is fixed to the capsule of the lens. In some fishes, as the Salmon, this ligament is of a dark colour ; and in the Conger, there are two such bands, by which the crystalline is suspended as by its opposite poles. Another peculiarity in the structure of the visual apparatus of osseous fishes is the existence of a vascular organ placed at the back of the eye-ball, and interposed between the choroid tunic and a brilliant metallic-coloured membrane which invests the choroid externally. This organ, generally called the " choroid gland " by the older anatomists (Jig* 231, A,g, g), is of a crescentic form, and always of a deep red colour. It is principally made up of blood- vessels, which run parallel to each other ; and from it issue other vessels, frequently very tortuous, and always much ramified, which form a vascular net-work in the choroid. The nature of this organ it is not very easy to determine. Some have believed it muscular ; but the strise perceptible in it are vascular, and not fibrous : others have thought it to be glandular, but it has no excretory duct. Most probably it is an erectile tissue analogous to that of the corpus cavernosum, and has some influence in accommodating the form of the eye to distances, or to the density of the surround- ing medium.* The pupil of the eye in the animals we are describing is very large, so as to take in as much light as possible ; but generally motionless. In some genera the shape of the aperture is curious : thus in the Rays a broad palmate veil hangs in front of the pupillary aperture ; and in one case, the Anableps, there are two pupils to each eye. (562.) The eyes of osseous fishes are lodged in the bony orbits of the face, imbedded in a soft glairy cellulosity ; but in many of the cartilaginous tribes, such as the Sharks and Rays, each eye-ball is moveably articulated to the extremity of a cartilaginous pedicle fixed to the bottom of the orbital cavity (Jigs. 232, t, and 231, c). (563.) Six muscles serve to turn the eye in different directions : namely, four recti, arising, as in man, from the margin of the optic foramen ; and two oblique muscles, derived from the anterior part of the orbit, and inserted transversely into the globe. These muscles are well represented in Jig.%3l 9 wherein the reader will observe that the superior oblique (g) does not pass through a pulley, as is the case in the human subject. (564.) It is extremely remarkable, that even in fishes the muscles * Cuv. et Val. op. cit. p. 338. PISCES FISHES. 525 of the eye have special nerves appropriated to them, and those pre- cisely the same as in the highest Mammalia. The third pair of Fig. 232. nerves animates them all, except the external rectus and the superior oblique ; and also sends off filaments to be distributed to the cho- roid, although no ophthalmic ganglion has yet been discovered. The fourth pair is exclusively appropriated to the superior oblique ; and the external rectus, or abductor muscle, invariably receives its supply from the sixth pair. (565.) To animals whose eyes are constantly washed by the water in which they live any lachrymal apparatus would obviously be superfluous ; and consequently, in the class before us, neither lachrymal gland, nor lachrymal puncta, nor even eyelids properly so called, are ever met with. (566.) Behind the optic lobes of a fish's brain the ganglia from which the other cerebral nerves emanate become confused into one mass, so that they are no longer distinguishable from each other. The nerves themselves, however, are easily recognised, and, with the exception of the ninth pair (the lingual or hi/poglossal nerves), which are not met with in fishes, both in their distribution and 526 PISCES FJSHES. number precisely accord with those with which the human ana- tomist is familiar. We have already traced the third, fourth, and sixth pairs to the muscles of the eye. The fifth issues through the great ala of the sphenoid, and divides, as in man, into an oph- thalmic branch (fig. 229, a), which runs through the orbit to be distributed to the parts about the nose ; a superior maxillary branch (|3), that supplies the parts about the upper jaw ; and an inferior maxillary branch (^), destined to the lower jaw : the ge- neral distribution of the nerve, as far as regards the face, is in fact exactly similar to that of the same nerve in man ; but in fishes it is found to give off other branches not met with in the human subject, one of which (p) is destined to the operculum. Another () takes a very remarkable course : it mounts up to the top of the skull, joins a large branch of the eighth pair (0), and, issuing from the cranium through a hole in the parietal and interparietal bones, passes along the whole length of the back on each side of the dorsal fin, receiving twigs from all the intercostal nerves, and sup- plying the muscles of the fin and the fin-rays themselves. This branch is superficial until it reaches the little muscles that move the fin. It has, sometimes, other branches equally superfi- cial, which descend to the anterior parts of the muscles of the trunk above the pectoral fins ; and others, which run as far as the anal fin, where they form a longitudinal nerve similar to that of the back. (567.) The seventh pair of cerebral nerves (fig* 229, s, *) in fishes, as in all other Vertebrata, is devoted to the organ of hearing, and brings to the sensorium the impressions of sound. (568.) The sense of hearing in these creatures must necessarily be very imperfect ; they have neither an external ear nor a tym- panic cavity, and consequently are entirely destitute of a membrana tympani, and of the ossicles of hearing : they have neither Eusta- chian tube nor fenestra ovalis ; the labyrinth alone, and that more simple in its composition than the labyrinth of the human ear, is all that the anatomist meets with in this first appearance of an auditory apparatus among the Vertebrate classes. The accompanying figure (fig. 233) represents the ear of a very large fish, the Lophius piscatorius ; and the student will have little difficulty in at once recognising all the parts of which it consists. The soft parts of this simple ear are not enclosed in bony canals, as in the human subject ; but the membranous labyrinth is lodged in a wide cavity on each side of the cranium : so that little dissection is necessary to expose the entire organ, which is surrounded on all PISCES FISHES. 527 sides with the same kind of oily or mucilaginous fluid, which filte up the wide interspace that exists between the brain and the dura mater lining the inner surface of the skull. Fig. 233. As in all other Vertebrata, there are three semicircular canals, disposed nearly as in the human ear, and each dilated in like man- ner into an ampulla which receives the filaments of the acoustic nerve. Two of the semicircular canals coalesce before they open into the vestibule, so that there are only five orifices whereby the three semicircular canals communicate with the vestibular cavity. The membranous vestibule (supported in the figure by two pins), is of variable shape, and its walls are very delicate. Its cavity, as well as the interior of the semicircular canals, is filled with a trans- parent glairy fluid ; and it moreover encloses certain hard bodies (otolitkt*), generally three in number, suspended by delicate fila- ments in its interior. The otolithes of osseous fishes are of a stony hardness, resembling shells, and their structure is nothing at all like that of bone. Their shape varies in different species, but, nevertheless, is so con- stantly the same in fishes of the same kind, that the forms of these pieces might be employed as an important zoological character. In the cartilaginous fishes the otolithes are quite soft, resembling starch : in both classes they are composed principally of chalk, and effervesce strongly when dissolved in acids. The auditory nerve gives a filament to each of the semicircular canals, which penetrates into the ampulla of the canal to which it is destined, and there spreads out ; but the larger portion of the nerve 528 PISCES FISHES. iS distributed over the vestibular sacculus, where it forms a beauti- ful net-work. There is.no Cochlea, although some writers imagine that they can distinguish a rudiment of this part of the ear in a slight pro- jection from the walls of the vestibule. (569.) The ears of fishes are, therefore, much less perfect than those of other Vertebrata : * deprived of tympanum, of ossicles, and of Eustachian tube, they can scarcely receive the impressions produced by the vibrations of the ambient element, except by those vibrations being communicated through the cranium ; and, more- over, the membranous labyrinth not being enclosed in bone, the skull can only transmit these movements in a very feeble and im- perfect manner. The absence of a cochlea would go far to prove that the ear of fishes cannot appreciate the differences of tones. All that it offers to the physiologist is a membranous apparatus en- dowed with great sensibility, in which the nervous filaments distri- buted in the ampullae of the semicircular canals must necessarily partake of all the movements of the fluid in which they are plunged, and where those appropriated to the vestibule must be still more strongly agitated by the shocks that these movements give to the otolithes contained in its cavities. It is probable, therefore, that fishes hear ; that noise produces in them a powerful sensation ; but that they cannot distinguish or appreciate differences of tone, as the higher animals are enabled to do. (570.) The nerves composing the eighth pair, preside over the same functions in all the Vertebrata. The glosso-pharyngeal sends twigs to the first branchial arch, the fauces, and the tongue. The nervus vagus (Jig. 222, t) supplies the three posterior branchiae, and the lower part of the pharynx; it is then continued along the (oesophagus to the stomach, where it terminates : it thus presides over the same functions in all the Vertebrate classes ; and it is not a little interesting to see it even in fishes distributed to the organs of respiration, notwithstanding the peculiarity of their structure and position. In these creatures, however, it likewise furnishes nerves to other parts of the body, and sends a long branch, which generally runs in the substance of the lateral muscles of the trunk, communicating with the spinal nerves, and giving off fila- ments to the skin ; an arrangement the physiology of which is not as yet understood. The next pair of cerebral nerves in the ani- * Cuv. et Val. op. cit. p. 347. PISCES FISHES. 529 mals under consideration would seem to represent the spinal recur- rent of the human subject ; it supplies the swimming-bladder and the muscles of the shoulder. (571.) All the above nerves posterior to the optic arise from a chain of ganglia constituting the medulla oblongata ; but above these are situated other important masses entering into the composition of the encephalon, from which no nerves take their origin, viz. the cerebral hemispheres and the cerebellum. (572.) The cerebral hemispheres in all the Vertebrata are un- doubtedly the seat of the mental powers; and, as this portion of the brain becomes developed and perfected, brutality and stupidity give place to sagacity and intelligence. In the higher quadrupeds, and more especially in man, the pro- portionate size of the hemispheres of the brain is so enormous that they overlap and conceal all the parts we have been describing ; but, as we descend to lower forms, their relative dimensions become gradually smaller and their structure less complicated, until in fishes, the least intelligent of all the creatures belonging to this great division of the animal kingdom, they are found in such a rudimentary condition that they are frequently far inferior in size even to the olfactory or optic ganglia (Jig. 232, c). The lobes representing the hemispheres in fishes (Jig- 284, b) are quite smooth externally, and within are hollowed into a large ventricle, in the floor of which is seen the upper surface of the optic ganglia (fig. 234, B, d). They present none of that complica- tion of parts met with in the brains of higher orders : their inner surface is lined with transverse fibres (A), and a simple com- missure passes pi^ 234. across the an- terior part of the ventricle, bringing the two sides into kA communication with each other; behind the com- missure a pas- sage leads to the third ventricle, ABC the infundibulum, and the pituitary gland. (573,) The cerebellum (Jig. 234, a) is at (/nee recognisable from 530 PISCES FISHES. its position and singleness. In the Perch its form is that of a blunted cone, with the summit directed slightly backward, but the shape and relative dimensions of this part of the brain are extremely variable. It consists, in fishes, only of the central portion (pj'o- cessus vermiformis), so that there are neither lateral lobes nor pons Varolii : its surface is composed of cineritious substance, and in its centre is a ramified medullary axis containing a ventricle that com- municates with the fourth. One very remarkable feature in the structure of the encephalon of fishes is the existence of supplementary lobes (Jig. 234, g) placed behind the cerebellum, which sometimes are united by a commissure : occasionally, as in the Trigla, there are as many as five pairs of such supplementary masses ; but probably, instead of regarding these as belonging to the brain, it would be more proper to consider them as being merely the first ganglia composing the spinal cord enormously developed in proportion to the importance of the nerves which they give off to the pectoral fins. (574.) The spinal nerves of fishes arise by double roots from the sides of the medulla spinalis, which generally extends from one end of the canal formed by the superior vertebral arches, to the other. The posterior roots are dilated into ganglia soon after their origin, but the ganglia are extremely minute. The spinal cord of the Moon-fish (Orthagoriscus Mold) is, however, an exception to the usual conformation : in this remarkable fish the spinal ganglia are all collected into a stunted mass placed imme- diately behind the brain ; and from this all the spinal nerves are given off, in the same manner as those forming the cauda equina in the human subject. (575.) The Sympathetic system in the creatures we are now examining is of very small size, when compared with that met with in the higher Vertebrata ; nevertheless, it occupies the usual position, and communicates as in man with the commencements of the spinal nerves. (576.) There are few subjects more calculated to arrest the attention of the physiologist than the progressive developement of the generative system in the Vertebrate classes ; and it is not a little interesting to watch the gradual appearance of additional organs, both in the male and female, as we advance upwards in the series of animated beings from the cold-blooded and apathetic fishes. In its simplest condition, the whole generative apparatus, even of a vertebrate animal, is in both sexes merely a capacious PISCES FISHES. 531 gland provided with an excretory duct, wherein, in the female, ova are secreted, and in the male a fecundating fluid is elaborated from the blood. The eggs of the female when mature are expelled from the nidus in which they were formed, and cast out into the surround- ing water. The male, urged apparently rather by the necessity of getting rid of a troublesome burden than by any other feeling, ejects the seminal secretion in the same manner; and the fecun- dating fluid, becoming diffused through the waves, vivifies the eggs with which it is casually brought into contact. Such is the whole process of reproduction in the osseous fishes. (577.) In the females of such fishes, the ovary, or roe as it is generally called, consists of a wide membranous bag, ordinarily divided into two lobes, but sometimes, as in the Perch, single (fig. 227, q). This extensive organ, when distended with ova, fills a large proportion of the abdominal cavity, and its lining membrane is folded into broad festoons, wherein the ova are formed, and lodged until sufficiently mature for expulsion. When ripe, the eggs escape into the cavity of the ovary, and are expelled in countless thousands into the surrounding element through the orifice of the ovarian sac (Jig. 227, r), which is situated imme- diately behind the anus (&), and in front of the urinary canal (Y). (578.) Generally, as has been already stated, the ova of fishes are fecundated after their expulsion ; but there are a few instances, as for example the Viviparous Blenny (Zoarcus viviparus) of our own shores, in which the young are hatched in the ovary, and grow to a considerable size before they are born : in such cases impreg- nation must take place internally, and the males in these species have, in fact, a nipple-like prolongation of the orifice of the duct, through which the semen escapes, probably for the purpose of introducing the seminal fluid into the interior of the ovary of the females. Nevertheless, even in these the ovaria present the same structure as in ordinary fishes ; the only difference being that their eggs are retained until the embryo is far advanced in its develope- ment, instead of being prematurely extruded. (579.) The testicle in the males of osseous fishes, generally named " the milt," equals in bulk the ovary of the other sex, and the quantity of the secretion furnished by it must be exceedingly great. The entire organ is composed of slender and very delicate convoluted caeca, in which the semen is elaborated. These tubes towards the circumference of the testis all terminate in blind extremities, but by their opposite ends they communicate with the 532 PISCES FISHES. general excretory duct ; so that, by blowing air into the latter, the entire organ becomes amazingly distended. In some cases the seminiferous tubules run parallel to each other, and become fur- cate as they approach the exterior of the testis : in others, after dividing and subdividing to some extent, as they diverge from the common duct, they become converted into innumerable anastomos- ing ramifications ; so that the whole substance of the testis appears to be made up of reticulate tubes, which during the spawning sea- son, when they are filled with the creamy fluid that they secrete, are visible even with the naked eye.* (580.) It will be observed by the anatomical reader, that while in the OSSEOUS FISHES the ova escape into the interior of the ovary, and are expelled through an excretory orifice resembling the duct of an ordinary gland, in the CARTILAGINOUS FISHES and in all other VERTEBRATA the germs burst from the exterior of the ovarium, where they are generally seized by Fallopian tubes, and either conveyed out of the body as eggs, or, being hatched internally, the offspring are nourished in receptacles provided for the purpose, until they arrive at a considerably advanced state of developement. But it is only by degrees that these more perfect ovigerous organs make their appearance, and we would particularly solicit the attention of the student to the different gradations of structure met with in this part of the animal economy. In the Eel and the Lamprey we have the first appearance of an ovary, such as is common to the higher Vertebrata. It consists of a very extensive vascular membrane covered by the peritoneum, and attached in broad folds beneath the spine, extending nearly from one end of the abdomen to the other (fig- 235). This viscus is not hollow, neither has it any excretory duct, so that naturalists were long at a loss to explain how the ova of these creatures were expelled. The extensive membrane above alluded to, as is now sufficiently well determined, produces in its substance the germs of the future progeny ; and these, as they become mature, break loose from the nidus wherein they were generated into the interior of the peri- toneal cavity of the Eel, and float loosely in the abdomen : there is no Fallopian tube as yet developed ; but two simple orifices, placed on each side of the anal opening, serve to give exit to the countless eggs, which thus escape into the surrounding water. The male organs of the Lamprey and Eel, together with the * Miiller, de Glandularum structura penitiori. Lipsiae, fol. 1830. PISCES FISHES. 538 ovaria of the female, and the kidneys and ureters, were accurately described by Hunter, in the Catalogue of his Collection, and their form and structure are illustrated by the pre- parations and drawings still preserved in the College of Surgeons ;* but in such fishes the testis of the male so exactly resembles the female ovary, that it was even imagined by Sir E. Home that no males existed, or that the females were themselves herma- phrodite : according to Rathke,*f* how- ever, the testes of the male are com- posed of solid granules precisely like the female ova ; and the secretion de- rived from them is in like manner al- lowed to escape into the abdomen, from which it is expelled through si- milar openings in the peritoneum. (581.) In the Sharks and Rays we meet with a very important addi- tion to the female sexual apparatus, namely, an oviduct, by which the germ is seized on its escape from the ova- rium, and furnished . with additional coverings necessary in such fishes for the security of the fetus. In these genera the folds of the ovarian membrane become less exten- sively spread out; and, from the size of the yolks of the eggs formed therein, the organ assumes a racemose appearance. The ovaries now form two large bunches placed on each side of the spine; and the ova when mature would necessarily escape into the abdominal cavity, as those of the Lamprey and Eel do, were they not seized by the patulous orifices of the two long and membranous oviducts whereby they are conveyed out of the body. (582.) There is, moreover, in the CHONDROPTERYGIOUS FISHES a necessity for defending the young during the earlier stages of their growth, by means which it would have been quite foreign to the purposes of Nature to have adopted in the other * See Physiol. Catalogue, vol. iv. pp. 48. 129, pi. 59 and 60. t Neueste Schriften der Naturforschenden Gesellschaft zu Danzig. Halle, 1824. 534 PISCES FISHES. division of this extensive class. The earth is peopled only at its surface, and the vegetable banquet there spread is abundantly suffi- cient for the support of terrestrial beings. The ocean, however, being densely populated at every assignable depth, could never have supplied vegetable food to anything like the extent required to satisfy her progeny ; hence, therefore, the necessity for that astonishing fertility so remarkable in the osseous fishes nine mil- lions of ova have been calculated to be spawned at a birth by a single cod-fish : such spawn, being naked and unprotected, is eagerly devoured by thousands of hungry mouths, or the feeble young soon fall a prey to countless voracious persecutors. If, how- ever, it was obviously requisite that the progeny of osseous fishes should be thus multitudinous, in order to provide a sufficiency of needful food, it is equally clear that it would have been incom- patible with the design of the Creator that the ravenous Sharks should be endowed with equal fecundity : their eggs are conse- quently few in number ; and, in proportion to their scarcity, jealous precaution must be taken to insure the safety of the included young, in order to prevent the complete extinction of the race. The means employed for this Fig. 236. end are simple and beautiful. About the middle of the ovi- duct of the female there is a thick glandular mass, destined to secrete a horny shell in which the yolk and white of the egg be- come encased. The egg when complete has somewhat the shape of a pillow-case, with the four corners lengthened out into long tendril-like cords (fig. 236), whereby the egg is entangled amongst the sea-weed at the bottom of the ocean. A brittle egg-shell would soon be destroyed by the beating of the waves, hence the necessity for the corneous nature of the envelope ; and yet how is the feeble embryo to escape from such a tough and leather-like cradle ? This likewise has been provided for: the egg remains permanently open at one extremity, or, to carry out our humble simile, one end of the pillow-case is left unsewn ; the slightest pressure from within, PISCES FISHES. 535 therefore, separates the valvular lips of the opening, and no sooner has the little Shark thus extricated itself from its confinement than the two sides close again so accurately that the fissure is not at all perceptible.* Fig. 237. * According to Cuvier, in those Sharks which are viviparous, that is, whose young are hatched in the oviduct prior to their expulsion, this egg-shell is never formed, and the investments of the fetus remain permanently membranous. Loc. cit. p. 397. 536 PISCES FISHES. (583.) The sexual organs of the male Chondropterygii ate very remarkable, and their real character is not properly understood. The testicle (fig. 237, n) is large, and occupies the same posi- tion as the ovary of the female ; but the singularity of this testis consists in its being made up of two portions, one of which has an excretory duct, while the other, although equally bulky, has none. The former portion, when minutely examined, is composed of an immense assemblage of flexuous secerning vessels, that pour their secretion into a long and tortuous vas deferens (o), which, after run- ning in a zig-zag course nearly the whole length of the abdomen, dilates into a capacious reservoir of semen (/?), and ultimately ter- minates with its fellow of the opposite side in a conical fleshy organ (&), which may be presumed to answer the purpose of an intromittent apparatus. The second portion of the testis appears to consist of globular bodies having no excretory duct whatever ; and it is not impossible that this is an organ analogous to the testis of the Lamprey, and that its secretion escapes into the abdominal cavity, to be ex- pelled through two orifices (s, s) situated on each side of the anus, whereby a free communication exists between the interior of the peritoneal sac and the external surface of the body. (584.) In these highly organized genera impregnation takes place internally, and the male is furnished with two strong prehen- sile organs called claspers (/), by means of which he seizes and securely holds the female during copulation. 537 CHAPTER XXVIII. REPTILIA. (585.) THE globe that we inhabit is usually said to be made up of land and water, and, perhaps, for the purposes of the geogra- pher, such a division of the surface of our planet is all that is requisite. A slight investigation of this subject, however, is sufficient to convince the naturalist, that a very considerable pro- portion of the world around us can scarcely be strictly referred to either one or the other of the geographical sections referred to ; that there are extensive marshes, for instance, equally ill-adapted to be the habitation of aquatic animals, or of creatures organized for a purely terrestrial existence ; that some localities may be alternately deluged with water and parched with drought ; that the margins of our lakes, the banks of our rivers, and the shallow ponds and stream- lets of warm climates, could only be adequately populated by beings of an amphibious character, alike capable of living in an aquatic or in an aerial medium, and combining in their structure the condi- tions necessary for enabling them to reside in either element. Aquatic animals, strictly so called, breathe by means of gills ; for a vertebrate animal to respire air, it must be provided with lungs : but if a creature is destined to live both in air and water, it must obviously have both gills and lungs coexistent, either of which may be employed in conformity with the changing necessities and altered circumstances. We cannot, therefore, be surprised to find that in the lowest Reptiles this is literally the arrangement adopted ; that they respire like fishes by means of branchiae while in the water, whereas on emerging into the air they have lungs ready for use. (586.) The AMPHIBIA (Batrachia Cuv.) are to the anatomist amongst the most interesting animals in the whole range of zoo- logy, as we trust will be made sufficiently evident when we come to investigate their internal economy ; but it is to their outward forms and habits that we must first introduce the reader, leaving the details of their organization to be discussed in the sequel. From whatever form or race of animals the zoologist advances 538 REPTILIA. towards the next succeeding it in the great scale of Nature, he will find himself insensibly led on by such gentle gradations that the transition from any one class to another is almost imperceptible. Nihil per saltum is one of the most obvious laws in Creation ; and of this, perhaps, we could not select a more striking illustration than is afforded by the Lepidosiren (Jig. 238). Two distinct species of this most remarkable animal have been met with : one, the Lepidosiren paradoxa, discovered by Dr. Nat- terer in the river Amazon ; the other, Lepidosiren annectans, was found by T. C. B. Weir, Esq. and is a native of the African con- tinent, inhabiting the river Gambia. An individual of the species last mentioned has been minutely anatomized by Professor Owen,* and both in its outward form and internal organization is so pre- cisely intermediate between a Reptile and a Fish, that, while Dr. Natterer regards it as an Amphibian, Professor Owen considers that, notwithstanding that it possesses lungs, the icthyic characters predominate, and it ought rather to be ranked among the Fishes. The body of the Lepidosiren annectans (Jig. 238) is about a foot long, and covered with scales, resembling those of the cycloid fishes : the tail gradually tapers to a point, but is fringed above and below with a membranous fin, supported by numerous soft, elastic, trans- parent rays, articulated to the superior and inferior spines of the caudal vertebrae ; the gills are covered by opercula, not being exposed, as in the proper Amphibia ; and, moreover, it has four rudimentary fins, or legs, as the reader may choose to call them. These rudimental extremities are round, filiform, and gradually attenuated to an undivided point ; being supported internally by a single-jointed soft or cartilaginous ray. The nostrils of the Lepi- dosiren, however, are merely two blind sacs as in fishes, and do not communicate with the mouth or fauces ; a character which Professor Owen regards as the only decided evidence that the animal ought in preference to be ranked among the class Pisces. * Transactions of the Linnean Society for 1840. REPTIL1A. 539 (587.) The Siren lacertina, a creature which inhabits the marshes of Carolina, is another amphibious animal, scarcely further removed from the fishes than the last. The Siren attains the length of two or three feet ; it has a body very nearly resembling that of an eel ; but instead of pectoral fins it has two rudimentary feet, each provided with four fingers, its hind feet, the representatives of the ventral fins, being entirely wanting ; it is, moreover, furnished with gills placed on each side of the neck, while internally it possesses two capacious membranous lungs adapted to aerial respi- ration. (588.) In the Proteus anguinus, an animal only met with in the subterranean waters of Carniola, the body, of which a figure is given in a subsequent page (Jig. 254), is equally anguilliform ; but the legs are now four in number, although still very imperfectly developed. Its gills are fringes of blood vessels placed externally upon the sides of the neck, and its thin, and delicate lungs (, z) extend nearly the whole length of the abdomen. The Amphibia above-mentioned, as well as the Menobranchus and the Axolotle, both animals of very similar construction, pre- serve their branchiae through the whole period of their lives, and are for this reason denominated Amphibia perennibranchiata : but there are other genera which, although in the early part of their exist- ence they are equally provided with both gills and lungs, ultimately become sufficiently perfect in their organization to enable them to enjoy a more or less complete terrestrial existence ; and, conse- quently, their branchiae become obliterated as the lungs grow more efficient, until at length no vestiges of the former remain percep- tible. These are called A. caducibranchiata. (589.) The most remarkable examples of the CADUCI BRAN- CHIATE AMPHIBIA are the Frogs, the Toads, and the Newts, so common in our own country ; and the metamorphosis of these creatures from the tadpole, or fish-condition under which they leave the egg, to their perfect air-breathing and four-footed state, is a matter of common observation. We select the Newt (Triton cristatus) as an example of the changes which these amphibians undergo as they advance towards maturity. Immediately before leaving the egg, the tadpole of the Salaman- der, or Water-Newt (Jig. 239, A), presents both the outward form and internal structure of a fish. The flattened and vertical tail, fringed with abroad dorsal and anal fin, the shape of the body, and the gills appended to the sides of the neck, are all apparent ; so 540 REPTILIA. that, were the creature to preserve this form throughout its life, the naturalist would scarcely hesitate in classing it with fishes properly so called. When first hatched (fig. 239, B),* it presents the same fish-like Fig. 239. body, and rows itself through the water by the lateral movements of the caudal fin. The only ap- pearance of legs as yet visible consists in two minute tuber- j cles, which C seem to be sprouting out from the skin immediately behind the branchial tufts, and which are, in fact, the first buddings of ante- rior extremities. Nevertheless, to compensate to a certain extent for this total want of those prehensile limbs which afterwards become developed, two supernumerary organs are provisionally furnished, in the shape of two minute claspers, seen in the figure, situated on each side of the mouth ; by means of these the little being holds on to the subaquatic leaves, and thus prevents itself from being washed away by the slightest current. Twelve days after issuing from the egg, the two fore-legs, which at first resembled two little nipples, have become much elongated, and are divided at their extremity into two or three rudiments of fingers (fig. 239, c). The eyes, which were before scarcely visi- ble, and covered by a membrane, distinctly appear. The branchiae, at first simple, are divided into fringes, wherein red blood now cir- culates ; the mouth has grown very large, and the whole body is so transparent as to reveal the position of the viscera within. Its activity is likewise much increased ; it swims with rapidity, and darts upon minute aquatic insects, which it seizes and devours. About the twenty-second day (fig. 240, u) the Tadpole, for the first time, begins to emit air from its mouth ; showing that the lungs have begun to be developed. The branchiae are still large. The fingers upon the fore-legs are completely formed ; * Vide Rusconi, Amours des Salamandres Aquatiques, et developpement du Tetard de ces Salamandres depuisl'ceuf jusqu'a 1'animal parfait. 4to. Milan. 1821. REPTILIA. 541 the hind-legs begin to sprout beneath the skin ; and the creature presents in a transitory condition the same external form as that which the Siren tacertina permanently exhibits. Fig. 240. By the thirty-sixth day the young Salamander (Jig. 240, E) has arrived at the developement of the Proteus anguinus ; its hind- legs are nearly completed, its lungs have become half as long as the trunk x>f its body, and its branchise more complicated in structure. At about the forty-second day the tadpole begins to assume the form of an adult Triton (Jig- 240, F) : the whole body becomes shorter, the fringes of the branchise are rapidly obliterated, so that in five days they are reduced to simple prominences covered by the skin of the head ; and the gill-openings at the sides of the neck, which, as in fishes, allowed the water to escape from the mouth, and were in like manner covered with an operculum formed by a fold of the integument, are gradually closed : the membranous fin of the tail contracts, the skin becomes thicker and more deeply coloured, and the creature ultimately assumes the form and habits of the perfect Newt (Jig. 241), no longer possessing branchiae at all, but breathing air, and in every particular completely converted into a Reptile. But, however curious the phenomena attending the developement of the tadpoles of the amphibious Reptiles may be to the observer 542 REPTILIA. who merely watches the changes perceptible from day to day in their external form, they acquire a tenfold interest to the physio- Fig. 241 . logist who traces the progressive evolution of their internal viscera ; more especially when he finds that in these creatures he has an opportunity afforded him of contemplating, displayed before his eyes, as it were, upon an enlarged scale, those phases of develope- ment through which the embryo of every air-breathing vertebrate animal must pass while concealed within the egg. The division, therefore, of Reptiles into such as undergo a metamorphosis, and such as do not, is by no means philosophical, although convenient to the zoologist : all Reptiles undergo a metamorphosis, though not to the same extent. In the PERENNIBRANCHIATA the change from the aquatic to the air-breathing animal is never fully com- pleted ; in the CADUCIBRANCHIATA the change is accomplished after the embryo has escaped from the ovum ; and in the REP- TILIA proper, as well as in BIRDS and MAMMALS, which are gene- rally said to undergo no metamorphosis, the changes referred to are accomplished in ovo during the earliest periods of the formation of the fetus. (590.) The second order of Reptiles (OPHIDIA) includes the Serpent tribes, animals entirely deprived of external locomotive ex- tremities, and nevertheless endowed with attributes at once formid- able and surprising. Absolutely without limbs or any apparent means of progression, the scale-clad Serpent makes its way in either element with equal facility ; and walks or leaps, or climbs or swims, at will. Destitute of any prehensile members, it seizes and devours the strongest and most active prey : it binds its victim in a living rope ; or, with a single scratch inflicted by its venomed fangs, speedily destroys the stoutest assailant. (591.)The transition from the OPHIDIA to the Lizards (SAURIA), composing the third order of Reptiles, is very gradually accom- plished by several intermediate forms, in which the first buddings REPTILIA. 543 of legs make their appearance ; and these locomotive organs, be- coming more and more completely developed in other genera, at Fig. 242. length conduct us from the flexible and apodous Serpents to the strong and four-footed Reptiles which are the types of the Saurian division (fig. 243). The progressive developement of the locomo- tive extremities is not a little curious : even among some of the Ser- pents properly so called, as, for example, in the Anguis fragilis of our own country, the rudiments of these limbs may be detected beneath the skin ; more especially those of the hinder extremity, wherein a little pelvis and femur mav be distinctly recognised, while a minute sternum, clavicle, and scapula indicate the first appearance of the thoracic legs. In Bimanes, the lowest of the Saurian genera, two little feet, each provided with four toes, are appended to the framework of the shoulder; and in Seps, which equally possesses the body of a 544 REPTILIA. serpent, all four extremities first make their appearance externally. As the legs become increased in their relative size and importance, Fig. 243. the trunk is proportionately shortened and its flexibility diminished (Jig. 243), until at length we are conducted almost by impercep- tible gradations to the strong and voracious Crocodiles, the most perfect of the Reptile families. (592.) The fourth order of Reptiles (CHELONIA) comprises a series of animals of most anomalous conformation, in which the greater part of the skeleton is brought quite to the exterior of the body, and the limbs are absolutely enclosed within the cavity form- ed by the ribs. Such are the Tortoises and the Turtles (Jig. 244) ; but, as we shall describe the anatomy of these animals more at length hereafter, we need only in this place point out to the reader their outward form and general appearance. (593.) Commencing our researches concerning the internal organ- ization of this extensive class by examining the osteology of the Reptilia, we shall, as we have hitherto done, select one skeleton for special examination ; and afterwards, taking that as a standard of comparison, observe the most conspicuous modifications of struc- ture met with in the different divisions of this important group. REPTILIA. 545 Fig. 244. (594.) The skeleton we choose for particular description is that of the Crocodile, one of the most interesting that can possibly be offered to the contemplation of the comparative anatomist ; inas- much as it exhibits, developed to a medium extent, a greater num- ber of the elements which we have supposed to enter into the composition of a perfect or typical skeleton than any other with which we are acquainted : we, therefore, beg the attention of the student while we investigate this important piece of osteology. (595.) A glance at the skeleton of the Crocodile (jg. 245) at once shows us that in consequence of the addition of a thorax, and the connection which now necessarily exists between the pelvis and the spine, the vertebral column becomes- divisible into dis- tinct regions : viz. the cervical, containing seven vertebrae ; the dorsal, formed by those vertebrae which support the thoracic ribs ; and the lumbar vertebrae intervening between these and the sa- crum. The number of bones entering into the composition of the sacrum, that is, which are connected with the ossa ilii of the pelvis, are in this case two in number; while, behind these, six and thirty vertebrae enter into the composition of the tail. In the cervical, dorsal, lumbar, and sacral regions, no inferior spinous processes exist ; but in the caudal portion of the vertebral column these elements are found greatly developed, as in fishes, and obviously with the same intention, namely, to increase as much as possible the vertical extent of the tail, and thus convert this 546 REPTILIA. part of the body, which is here of extraordinary length and great flexibility, into a powerful instrument of propulsion. (596.) The transverse processes of the cervical vertebrae are re- markably large, and so extended that they materially interfere with the lateral movements of the neck ; an arrangement evidently designed to afford a sufficient extent of insertion for the powerful muscles of the cervical region. (597.) The thorax is com- Fi s- 245. posed of a Sternum and two sets of ribs ; one set being ar- ticulated with the transverse processes of the dorsal verte- brae, and hence called dorsal ribs; while the others, being fixed to the sides of the ster- num, are named sternal ribs : the contiguous extremities of the dorsal and sternal ribs are, moreover, united by inter- vening cartilages, which, as they are generally more or less perfectly ossified in the adult Crocodile, might almost be regarded as additional ele- ments of the thorax. The posterior dorsal ribs are far less perfectly deve- loped than those situated more anteriorly; and it is not a little interesting to observe how gradually, even in the same skeleton, the transition is effected from the simple condition already noticed in the ribs of fishes, in which each rib is merely appended to the extremity of the transverse process of a ver- tebra, to ribs perfectly adapt- ed to enter into the compo- sition of a true thoracic REPTJLIA. 547 cavity, and united by a double articulation both with the trans- verse processes and the bodies of the vertebra. The head of the last rib of the Crocodile is, in fact, simple, and merely articulated with the apex of the transverse process of the corresponding verte- brae ; the next is slightly bifid at its origin, but both the divisions are still connected with the transverse process : as we advance still further forwards, the division of the origin of the rib becomes more and more decided, until at length, at about the fifth rib, we have two distinct heads, one firmly articulated with the body of the vertebra, the other with the transverse process ; presenting an arrangement pre- cisely similar to that met with in the structure of the thorax of a bird. (598.) The sternal apparatus is not less interesting to the osteo- logist. The anterior extremity of the sternum is osseous, and considerably prolonged forwards, to be articulated with the clavi- cles, and thus afford a support to the anterior extremity. Behind this it becomes cartilaginous, and affords attachment to the sternal ribs, which enter into the composition of the thorax : it does not, however, terminate at the posterior margin of the thoracic cavity, but is continued along the mesial line of the abdomen quite to the pubis, and gives off eight abdominal sternal ribs, to which no dorsal correspondents are met with. These abdominal ribs serve to support the muscles of the abdomen, and here present their maximum of developement : rudiments of them are, however, still met with in the higher animals, and even in the human subject we find, in the transverse tendinous bands which intersect the substance of the rectus muscle of the abdomen, the last remains of these appendages to the sternal portion of the skeleton. (599.) In the anterior extremity of the Crocodile we have most of the parts enumerated as entering into the composition of a per- fect or typical skeleton ; the shoulder, however, is composed of only two pieces, the Scapula and the Clavicle, the last of which articu- lates with the sternum : the bones of the arm, fore-arm, and hand, are completely developed. (600.) The posterior extremities are fully formed, the pelvis being connected by means of the ossa ilii to the transverse pro- cesses of two vertebrae, which therefore, as we have seen, constitute the Sacrum. (601 s ) In examining the bones which enter into the composi- tion of the head of the Crocodile, or indeed of most Reptiles, the anatomist finds his studies much facilitated by the circumstance that the sutures separating the individual bones never become 548 REPTILIA. obliterated, so that the elements of this portion of their skeleton remain permanently detached and separate ; and for this reason we shall take the present opportunity of going a little into detail concerning the composition of the skull of the Crocodile, as it is well calculated to illustrate the real structure of the cranium in the Vertebrata generally. The bones of the face are easily recognised ; the Intermaxillary (fig. 246, 17), the Maxillary (18), and the Nasal (20), the Zy- gomatic (b) and the Lacrymal (c), all occupy their usual relative positions. The roof of the mouth is formed, as in Man, anteriorly by a process of the upper jaw (Jig- 246, A, 18), and posteriorly by the palate-bone (22). J Fig. 246. A The Frontal consists of five pieces ; viz. the Principal Frontal (1), which probably in the fetus consisted of two lateral halves, the Anterior Frontal (2, 2), and the Posterior Frontal (4, 4). The Parietal (7) is, as is generally the case in Reptiles, repre- sented by a single bone. The Occipital consists of four pieces, which remain permanently detached; namely, the Basilar (5), the two Lateral Occipital (10), and the Superior Occipital placed above the foramen magnum. REPTILIA. 549 The Sphenoid, which in Man is regarded as a single bone, is here represented by several distinct parts. The body is divided into two portions {fig. 246, A, 6), called respectively the Anterior and the Posterior Sphenoids. The great or Temporal Ala (11) are also separate bones, as also are the Internal Pterygoids (25). A bone (24), which is not met with either in Mammalia or Birds, passes from the Internal Pterygoid to the point of junction between the Zygomatic, the Maxillary, and the Posterior Fron- tal : this has been named by Cuvier the Transverse bone. The Ethmoid and the Vomer (16) are but very imperfectly ossified, so that the septum between the nostrils is in the skeleton extremely incomplete, and the sense of smell of course propor- tionately obtuse. But the most interesting of the cranial bones is the Tem- poral, which, although considered as one bone by the human osteo- logist, is in Reptiles evidently composed of at least four distinct and separate parts. These are, 1st, the Petrous bone (fig. 246, A, e), which partially encloses the organ of hearing ; 2dly, the Tym- panic bone (a), which supports the membrana tympani ; 3dly, the Mastoid bone (12), which is the homologue of the Mastoid process of Man ; and 4thly, the Temporal bone, properly so called (23), which represents the squamous portion of the human Temporal bone. (602.) Each lateral division of the inferior maxilla of Reptiles is separable into at least five and generally six pieces, which are united together by suture; these are named the dental (84), which support the teeth, the angular (36), the opercular (37), the arti- cular (35), and two small pieces seen upon the inner surface of the jaw. Having thus described at some length the composition of the skeleton in the Crocodile, which we have chosen for minute ana- lysis, as being the type of the Saurian Reptiles, we shall now pro- ceed to examine the osteology of the other orders, so as to appre- ciate more correctly the peculiarities of structure that they indivi- dually exhibit. (603.) In the AMPHIBIA, as for example in the Frog, one of the most striking circumstances connected with their history is the extraordinary change which takes place in the condition of every part of the framework of the body during the evolution of the tad- pole, and its metamorphosis into the perfect frog. The skeleton of a Tadpole is, in every particular, that of a fish : 550 REPTILIA. its texture is soft and cartilaginous, the caudal portion of the spine prolonged and flexible ; neither are there any external limbs con- nected with the vertebral column, so as to trammel the lateral movements of the tail; and yet in the mature frog (jig. 247) let the reader observe the amazing difference. The head, it is true, still preserves somewhat of the character of that of the fish, especially in the disproportionate developement of the face, when compared with the size of the cranial cavity ; but all the bones of the spine have become consolidated into ten vertebrae, firmly con- nected together by strong articulations, while the flexible tail of the tadpole has become converted into a strong and immoveable os coccygis, composed of a single piece. No ribs whatever are met with in the Frog ; and, even in those Amphibia which are Fig. 247. possessed of these elements of the ske- leton, they are mere rudiments appended to the extremities of the transverse pro- cesses of the verte- brae. The sternum, however, is largely developed, and gives extensive attachment to the muscles of the abdomen. The an- terior extremities are supported by a semicartilaginous zone, in which the three elements of the shoulder the scapula, the clavicle, and the coracoid bone, are distinctly recognisable ; and the bones of the arm, fore-arm, and hand, are very perfectly formed. The pelvis is large, and firmly ossified in correspondence with the strength and magnitude of the hinder extremity ; the ossa ilii being articulated to the ends of the transverse processes of the last vertebra, which from this circumstance may be called the sacrum. The tibia and fibula are consolidated into one bone ; while two of the bones of the tarsus, the astragalus and the os calcis, are so excessively elongated, that they might almost be taken for a second tibia and fibula, did not their position indicate their real nature. One circumstance is remarkable in the construction of the BEPTILIA. 551 shoulder-joint of these reptiles, which are found to have a strong ligament passing between the head of the humerus and the scapula, exactly in the same manner as the ligamentum teres of the human hip-joint. The use of such a deviation from the ordinary structure of the articulation is obvious ; the frog, as it alights from those long and vigorous leaps which form its ordinary mode of progres- sion, receives the whole shock of its fall upon its fore-legs, and thus this ligament becomes needful as an additional security to the ar- ticulation in question. (604.) The skeleton of an Ophidian Reptile presents a strange contrast to that of the Batrachian last described. Taking the Boa Constrictor as an example of this order, we find the spine of this enormous serpent composed of three hundred and four distinct vertebrae, of which two hundred and fifty-two support ribs : flexi- bility is, therefore, abundantly provided for in the construction of these lithe and elegant beings, inasmuch as the division of their spinal column into so many pieces allows the utmost pliancy in any required direction. Flexibility, however, is not the only condition requisite in this case ; strength and precision of movement are equally indispensable, and the question is, how are these apparently opposite qualities to be so combined and associated as not in the slightest degree to interfere with each other. The mechanism con- spicuous in the construction of the spine of a serpent is in this re- spect truly admirable. The anterior extremity of the body of every vertebra is rounded into a smooth and polished ball (Jig' 248, c), which exactly fits into a hemispherical cup excavated in the sub- stance of the vertebra next succeeding : a perfect ball-and-socket joint is thus formed between every vertebra and that which precedes or follows it; and thus the spine is rendered capable of the utmost lati- tude of movement, and offers, at the same time, a firm purchase to the muscles acting upon the vertebral column. To provide, how- ever, against undue extent of motion in certain directions, we now meet with other processes derived from the vertebral arches : in addi- tion to those given Fig. 248. merely as levers for the attachment of muscles,secondary apophyses, called oblique or articu- lating processes, become develop- 552 REPTILIA. ed ; and, contiguous vertebrse being likewise mo veably connected together by means of these appendages, unnecessary flexure is not allowed, and all danger of dislocation prevented. (605.) Serpents, being entirely deprived of external limbs, have neither shoulder nor pelvis ; their ribs alone affording them the means of progression. These extend on each side in an uninter- rupted series from the first vertebra behind the head to the origin of the tail, so that the division of the spine into regions is here out of the question. Every rib (fig. 248, a) is attached at its origin by a kind of ball-and-socket joint to the extremity of the correspond- ing transverse process of a vertebra (6), and is therefore freely move- able. There is no sternum here, neither are there sternal ribs ; but the dorsal ribs, wielded as they are by innumerable and powerful muscles connected with them, literally perform the office of inter- nal legs, and materially assist the creature in progression. (606.) Having already enumerated the bones which enter into the composition of the cranium of a Saurian Reptile, it would be superfluous again to mention in detail those met with in the skull of a serpent, more especially as they will be easily recognised by a glance at the annexed figure, in which the corresponding bones are all indicated by the same references : one peculiarity only re- quires special notice, namely, the extreme mobility of the principal bones of the face, and more particularly of the pieces composing the lower jaw, by which provision these reptiles are enabled to swallow entire animals of astonishingly large dimensions when compared with the size of their mouths. In order to allow of this, the bones composing the superior maxilla (17, 18) are only loosely joined together by ligamentous REPTILTA. 553 bands, and even the arches of the palate are moveable. The two halves of the lower jaw (34, 34) are connected together at the symphysis by a ligament so loose and elastic that separation to a great extent is easily allowed ; and, moreover, those two elements of the temporal, the Mastoid (12), and the Tympanic (a), which form the bond of connection between the inferior maxilla and the cranium, are here lengthened out into long pedicles, so that by their mobility the entrance to the throat can be dilated in a sur- prising manner, and prey of apparently very disproportionate bulk thus introduced into the stomach. (607.) The most extraordinary skeleton met with among Rep- tiles, and, indeed, among the Vertebrata generally, is that of the Chelonia ; in which the ribs and sternum are both placed quite at the exterior of the body, so as to form a broad dorsal shield called the Carapax, and an equally strong ventral plate named the Plastrum, between which the limbs and the head can be more or less completely retracted. Yet, notwithstanding this apparent total inversion of the osseous system in the creatures before us, it is interesting to observe by what slight modifications in the arrangement of the elements of the skeleton such prodigious changes are accomplished. This is well exemplified in the construction of the Carapax of the common Tor- toise (EmysEuropaus). In this well-known animal (Jig. 250) the vertebrae of the neck, and of the tail, present nothing particu- larly remarkable in their structure ; but, being connected together in the ordinary manner, the neck and caudal region of the spine present their usual flexibility. The dorsal vertebrae, however, are strangely distorted ; the elements of the upper arch being dispropor- tionately developed, while the bodies remain almost in a rudimen- tary condition. The superior spinous processes of these vertebras are flattened, and converted into broad osseous plates, which form a longitudinal series along the centre of the back, and are connected together by sutures resembling those of the human cranium. The ribs are changed into broad flat bones, firmly united by suture to each other, and also to the lateral margins of the spinous processes of the vertebrae, so that they all form, as it were, a single broad plate : the heads of the ribs are very feebly developed, and the intervals 'between them and the bodies of the vertebrae filled up with ligament. The margin of the shield thus formed by the dorsal ribs is further enlarged by a third set of flat bones, apparently representing the sternal ribs of the Crocodile, fixed by suture 554 REPTILIA. around the whole circumference of the Carapax, which they assist in completing. p . g ^ (608.) The Plastrum, or Sternum, is made up of nine pieces, which have been proved by M. Geoffroy St. Hilaire to be the ele- ments of this portion of the skeleton in the most complete state of developement in which they are met with. Of these nine elements, eight are disposed in pairs ; but the ninth, which is always placed between the four pieces composing the two anterior pairs, is single, and occupies the mesial line: in birds we shall afterwards find this element of the sternum performing a very important office. (609.) The bones of the shoulder, and of the hip, in the Tor- toise (fig- ^50), are absolutely placed within the thorax, and articu- lated to the sides of the vertebral column. The precise homology REPTILIA. 555 of the scapular apparatus has not been as yet decidedly pointed out ; there are, however, three branches, probably representing the Sca- pula, the Clavicle, and the Coracoid bone ; but, in the construction of the pelvis, the Ilium, the Ischium, and the Pubis are identified with facility. (610.) The muscular movements of Reptiles are ordinarily slow and languid, a circumstance which no doubt depends upon the impurity of their blood consequent on the imperfect manner in which the circulating fluid is exposed to the influences of respir- ation. The muscles of these animals are, however, peculiarly tena- cious of life, and preserve their irritability and power of contraction for an astonishing length of time after they have even been separated from the body. The muscles of a Turtle will continue to live for days after the creature has been decapitated ; and the heart will still contract, when irritated, even many hours after its removal. But, perhaps, the most interesting phenomenon connected with the muscular system of the Reptilia, is the progressive develope- ment of entirely different sets of muscles as the metamorphosis goes on by which they are converted from their earliest fish- condi- tion to their mature and perfect state. This series of changes, which doubtless takes place in all the higher Vertebrata, is well exemplified in the tadpole of the Frog or Toad, and the different phases of developement are in such creatures easily investigated. At first the tadpole presents the muscular structure of a fish, both in the muscles of the expanded and vertical tail, and in those of the branchial apparatus. As growth proceeds, the broad muscles of the abdomen become developed, and ultimately those of the limbs are superadded as those members successively make their ap- pearance ; the muscles of the shoulder and pelvic region being first recognisable, and subsequently those of the legs and feet. In the mean time, as the abdominal muscles, and those of the extremities, become gradually perfected, those peculiar to the fish-state are rapidly removed : the broad tail becomes atrophied and absorbed, diminishing in length nearly at the rate of a line a day ; the flaky lateral muscles of the caudal region disappear altogether ; and, moreover, the entire muscular apparatus of the branchial and hyoid systems is altered as the character of the respiratory organs becomes changed, in a manner to be explained hereafter, from the aquatic to the aerial condition. (611.) As Reptiles, for the most part, must from necessity swallow their prey entire, organs of taste would be scarcely more 556 REPTILIA. useful to them than to the fishes described in the last chapter ; and we are, therefore, not at all surprised to find the tongue in almost every family appropriated to a totally different use, and not unfrequently converted into an apparatus of prehension, whereby the food is seized and conveyed into the mouth. In the Batracoid Amphibia, for instance, we have a remarkable example of this provision. The Frog and the Toad, notwith- standing their slow and clumsy movements, are destined to feed upon insects, and consequently must be provided with some instru- ment by which such active prey may be caught. The organ pro- vided for this purpose is the tongue, which, by a slight modifica- tion in its structure, becomes changed into a prehensile forceps, admirably adapted to such an office. The tongue of the Frog, instead of presenting the usual arrangement, is found to be fixed to the symphysis of the lower jaw, and folded back upon itself, so that its point, which is free and bifid, is lodged in the throat. Thus provided, the Frog is enabled to seize its victim with the greatest ease. No sooner does a fly approach sufficiently near than this living forceps is rapidly everted ; and the insect, being seized by its furcate extremity, is as speedily brought between the jaws of its destroyer. The teeth of the Batrachia very much resemble those of the generality of fishes ; being simple points soldered to the sur- face of the jaws, but not implanted in sockets, sufficient to give a secure hold of their food, but quite unadapted to mastication. (612.) The Cameleon is another curious example of a reptile obliged to employ its tongue in securing insect prey. The Came- leon is arboreal in its habits : its feet, cleft, as it were, into two por- tions, firmly grasp the boughs upon which it climbs ; while its well- known power of changing the colour of its skin, so as to imitate that of the branches around it, efficiently conceals it from obser- vation. The tongue of this creature, when extended, is as long as its whole body, and is terminated by a club-shaped extremity, smeared over with a viscid secretion : when an insect comes within a distance of five or six inches from the Cameleon, the end of this tongue is first slowly protruded to the distance of about an inch, and then, with the rapidity of lightning, launched out with uner- ring aim ; the fly, glued to its extremity, is with equal velocity conveyed into the mouth. (613.) The jaws of the Chelonian Reptiles are not armed with teeth, but cased in horny coverings so as to resemble the beak of a bird, with which they crop the vegetable aliment upon which they generally subsist. REPTILIA. 557 Serpents, as regards their means of destroying prey, may be di- vided into two great groups ; the first including those which are not venomous, the second embracing such as are armed with poison-teeth. (614.) In the non-venomous serpents, as for example in the Boa constrictor, the upper jaws and the palate-bones are all lined with sharp teeth, so that there are four rows of dental organs, two placed along the margins of the maxilla, and two projecting from the roof of the mouth : all these teeth are simple, very sharp, and point backwards. Each division of the lower jaw is likewise armed with a single row, which are also directed towards the back of the mouth. It must be evident, from a mere inspection of these teeth, that they can be of little use in holding, much less in destroying, such strong and large animals as the Boa devours ; and upon a little consideration we shall find that they are intended for a very diffe- rent office. These serpents kill their victims by coiling their lengthy bodies around the chest, and then by strong muscular contraction they compress the thorax of their prey so firmly, that, its move- ments being completely prevented, respiration is put a stop to, and the animal so seized speedily perishes from suffocation. But, hav- ing succeeded in extinguishing life, the most difficult task still re- mains to be accomplished : how is the serpent, utterly destitute as it is of all external limbs, to force down its throat the carcase of a creature many times thicker than its own body ? The mode adopted is as follows : Once more winding itself around the slain animal, it commences at the head, which by main force it thrusts into its mouth ; the elastic ligament at the symphysis of its lower jaw gives way, and the branches of the inferior maxilla be- come widely separated, so that the mouth is stretched enormously as the food is thus forced into it. Deglutition is here a very lengthy and laborious process ; and, was there not some special con- trivance to guard against such an accident, no sooner were the efforts of the snake relaxed in the slightest degree, than the mus- cles of the throat and jaws, being in a state of extreme tension, would force out of the mouth what had already been partially swallowed. To provide against this, the teeth are in this case con- verted into a sort of valve : pointing backwards as they all do, they permit the bulky food to pass into the fauces, but at the same time, their sharp points being directed towards the throat, efficiently pre- vent it from being pushed back again in the opposite direction.* * In the collection of Professor Bell there is a small snake, which having by mis- hap attempted to swallow a mouse of too large size, and being quite unable, in conse- 558 REPTILIA. (615.) In the venomous serpents those teeth, which are fixed to the margin of the superior maxillary bone of the innoxious genera, are generally deficient ; and instead of them there is found an appa- ratus of poison-fangs, constituting perhaps the most terrible weapons of attack met with in the animal creation. The poison-teeth (fig. 251, a) are two in number, one fixed to each superior maxillary bone : when not in use, they are laid flat upon the roof of the mouth, and Fig. 251. covered by a kind of sheath formed by the mucous membrane of the palate ; but when the animal is irritated, or about to strike its prey, they are plucked up from their concealment by muscles in- serted into the upper maxillary bone, and stand out like two long lancets attached to the upper jaw. Each fang is traversed by a canal ; not, as it is generally described, excavated in the substance of the tooth, but formed by bending as it were the tooth upon itself, so as to enclose a narrow channel through which the poison flows. The canal so formed opens towards the base of the tooth by a large triangular orifice, but at the opposite extremity it terminates near the point of the fang by a narrow longitudinal fissure. The gland wherein the poison is elaborated occupies the greater part of the temporal fossa, and is enclosed in a white and tendinous capsule (Jig. 251, ); the substance of the organ is spongy, and composed of cells communicating with its excretory duct (c), by which the venom is conveyed to the opening at the base of the fang.* The poison-gland is covered by a strong process of the tem- quence of the mechanism referred to, to disgorge it, was found dead, and the skin and muscles of its neck absolutely rent from excessive stretching. * M6moire sur les caracteres tir6s de 1'Anatomie pour distinguer les Serpens venimeux des Serpens non-venimeux ; par M. Duvernoy, D. M. Annales des Sc. Nat. torn. xxvi. KEPTILIA. 559 poral muscle (d) 9 which is attached to a thin aponeurotic line (e). The greater portion of the fibres of this muscle take their origin from the capsule of the secreting apparatus, which they partially envelope ; and then winding round all the posterior part of the gland, and passing behind the commissure of the lips, the lower part of the muscle is firmly implanted into the lower jaw very far anterior to the angle of the mouth. The process of the temporal muscle which thus surrounds the gland is very thick and strong, so that it is easy to imagine with what force the poison will by this mechanism be injected into the wounds inflicted by the fangs, seeing that the same muscles which close the jaw at the same time com- press the bag of venom with proportionate energy. Behind the large poison-fang in use, the capsule that encloses it generally contains the germs of several others, ready to supply its place should the former be broken off; and, on the event of such an accident, one of these supplementary teeth soon becomes consoli- dated with the superior maxilla, and adapted in all respects to take upon itself the terrible office of its predecessor. (616.) Dreadful as are the means of offence thus conferred upon the poisonous serpents, it is impossible to avoid noticing in this place that admirable provision of Nature, which, in one genus at least, serves to give timely warning of the vicinity of such dan- gerous assailants. We need merely mention the rattle of the Rattle-snakes (Crotalus) ; an organ, the intention of which is so obvious, that the most obtuse cannot contemplate it without at once appreciating the beauty of the contrivance. This singular rattle is formed of numerous horny lings, that are in fact merely modifications of the general scaly covering of the reptile, so loosely articulated together, that the slightest movement of their formida- ble possessor is betrayed by the startling noise produced by the collision of the different pieces composing the organ ; even when at rest, the creature announces by rapid vibrations of the tail the place of its concealment, apparently to caution the inadvertent in- truder against too near an approach. (617.) In the grand police of Nature, the scavengers are by no means the least important agents. In hot climates especially, where putrefaction advances with so much rapidity, were there not efficient and active officers continually employed in speedily re- moving all dead carcases and carrion, the air would be perpetually contaminated with pestilential effluvia, and entire regions rendered 560 REPTILIA. uninhabitable by the accumulation of putrefying flesh. Perhaps, however, no localities could be pointed out more obnoxious to such a frightful cause of pestilence than the banks of tropical rivers ; those gigantic streams, which, pouring their waters from realm to realm, daily roll down towards the sea the bloated remains of thousands of creatures which taint the atmosphere by their decom- position. Such are precisely the situations inhabited, by Crocodiles and Alligators, the largest of the Saurian Reptiles now in existence, animals in every way designed by Nature to feed upon putrefying materials : their tongue (Jig- 252, d) scarcely projects from the lining membrane of the mouth, and its surface (e) is studded with large glands ; the whole interior of the mouth is in fact, from its construction, little adapted to gustation. The Crocodile nevertheless likewise kills living prey, which, from the structure of its teeth, it is obliged to effect by dragging Fig. 252. its victim into the water and there drowning it. This mode of pro- ceeding, however, simple as it might appear, involves many difficul- ties : as the reptile has no other instruments of prehension besides its mouth, and is obliged to hold its struggling prey submersed by the strength of its formidable jaws, it is manifest that, without some special contrivance, the water rushing into the throat of the Cro- codile would prevent it from breathing quite as effectually as the REPTILIA. 561 animal it endeavours to drown ; it might therefore become a question which of the two would survive immersion longest. The mechanism employed under these circumstances to give the Cro- codile the advantage over its prey is very complete : A broad cartilaginous plate (fig. 25, f) stands vertically from the os hyoides, and projects upwards into the back part of the mouth ; a similar valve (g) hangs down from the back of the palate, so that the two together form a kind of flood-gate, which, when the mouth is widely opened, effect a complete partition between the cavity of the mouth and the fauces, where the aperture of the larynx (h) is situated. The nostrils, moreover, are placed quite at the extremity of the snout, and the nasal passages leading from them are prolonged through the whole length of the upper jaw until they communicate with the fauces behind the velum of the palate (g). Such being the arrangement, it is immediately ob- vious, that, when the communication between the mouth and the fauces is cut off by means of the two valves (gjO, the Crocodile, by merely keeping the tip of its snout above the water, breathes with the utmost facility, and it is thus enabled to keep its prey submerged for any length of time that may be requisite to extin- guish life. (618.) The teeth of the Crocodile, and of the higher Saurians are not merely consolidated with the bones of the skull to which they are appended, but are implanted in sockets formed in the bones composing the upper and lower jaws. Each tooth is a simple hollow cone, and encloses a vascular pulp, from the surface of which the bony matter of the tooth was formed. When a tooth becomes old and worn, a second is secreted by the same pulp within, the cavity of the first, and the original one is shed, so that a succession of teeth thus make their appearance. (619.) The alimentary canal of Reptiles offers little that requires special description. The oesophagus (Jig. 257,jf, f) is generally extremely capacious, and the stomach of very variable shape and capacity. The latter viscus is for the most part pyriform, tapering gradually towards the pylorus ; such is the case in the CHELONIA and in the BATRACOJD AMPHIBIA : in SERPENTS it resembles a long bowel, and is capable of extraordinary dilatation ; and in the PERENNIBRANCHIATE AMPHIBIA, as in the Proteus /?g.254,z) and the Menopoma (Jig. 257, g), it looks like a mere dilatation of the intestine. The stomach of the Crocodile is remarkable as affording ano- 562 REPT1LIA. tlier among the innumerable instances that might be adduced of that gradual transition everywhere observable as we pass from one class of animals to Fig. 253. that which next suc- ceeds it in the series of creation. The Crocodile is the con- necting link be- tween REPTILES and BIRDS, and in almost every part of its body it presents a type of structure almost intermediate between the two. The stomach of this creature (Jig. 253) might in fact be almost mistaken for the gizzard of a rapacious bird. The oesophagus (c) terminates in a globular receptacle, the walls of which are very muscular, and the muscular fibres (a) radiate from a central tendon (b) precisely in the same manner as those of a bird. The pyloric orifice is closely approximated to the termination of the oesophagus, and the commencement of the duodenum dilated into a round cavity (d) ; an arrangement which, as we shall see in the next chapter, exactly resembles that met with in the feathered tribes. In the neighbourhood of the pylorus, the walls of the stomach in all the REPTILIA become perceptibly thickened : the intestine is generally short and usually divided into two portions, representing the small intestines and the colon, the division between the two being marked by a prominent valve analogous in function and posi- tion to the ileo-colic valve in the human subject ; and sometimes, moreover, as, for instance, in the Iguana, there is a distinct ceecum developed at the commencement of the large intestine. The auxiliary secretions subservient to digestion in the class before us, are the Salivary, the Hepatic, and the Pancreatic. (620.) The Salivary glands are of very peculiar construction.* * Cuvier, Le9ons d'Anatomie Compare, torn. iii. p. 223. REPTILIA. 563 In theCHELONiAN,the SAURIAN, and tlieBATiiACHiAN orders, tlie substance of the tongue seems to be principally made up of a thick glandular mass, formed by a multitude of little tubes united at their bases, but, becoming separate towards the surface of the tongue, they give the whole organ a papillose or velvety appear- ance. This glandular apparatus rests immediately on the muscles of the tongue, and upon its sides a multitude of pores are visible through which the salivary secretion exudes. (621.) In the OPHIDIAN REPTILES, from the manner in which they swallow their prey, the bulk of the tongue is necessarily re- duced to the utmost extent ; the whole organ seems converted into a slender bifid instrument of touch, and is covered with a delicate membrane. Instead of the salivary apparatus described in the last paragragh, two glandular organs (Jig. 251, s, ), placed immediately beneath the skin of the gums, surround the margins both of the upper and lower jaws ; and from these an abundant salivary secre- tion is poured into the mouth, through orifices situated externally to the bases of the teeth. (622.) The Liver of Reptiles (Jig. 254, h) requires no parti- cular description : its secretion, as well as that of the pancreas (Jig. 254, o), is poured into the intestine in the usual manner at a little distance from the pylorus. (623.) The Spleen, and system of the Vena Porta^ are dis- posed in the same manner as in other Vertebrata. The spleen (Jig- 254, /) is generally more or less closely connected with the stomach ; and the large vein derived from it, being joined by those proceeding from the other viscera of the abdomen, forms the trunk of the portal vein (wi), which soon divides again into numerous branches that ramify in the substance of the liver. (624.) The Lymphatic and Lacteal systems are very im- portant parts of the economy of these creatures ; and, from the large size of the absorbent vessels, their disposition is more easily traced in the class before us than in any other. The principal trunks surround the aorta and other large blood-vessels, and com- municate very extensively with the veins in different parts of the body. From the imperfect condition of the valves in their inte- rior, the lacteals of many tribes may be readily injected from trunk to branch ; and, when thus filled with mercury, they are found to spread out between the coats of the intestines like a dense network of silver. 2 o 2 564 REPTILTA. (625.) But the most remarkable circumstance connected with the absorbents of this class of animals is the discovery, made by Professor M tiller of Berlin,* of a system of lymphatic hearts destined to propel the products of absorption from the chief lym- phatic trunks into the veins. In the Frog four of these pulsating cavities are easily displayed by simply raising the skin covering the regions of the body where they are situated. The posterior pair of hearts are appendages to the lymphatic trunks which convey the absorbed fluids derived from the hinder extremities into the ischiadic veins : they are situated on each side midway between the extremity of the long bone which represents the os coccygis and the hip-joint, and are placed immediately beneath the integument. They each consist of a single cellular cavity, and pulsate regularly ; but their pulsations are quite independent of those of the heart, neither are the contractions of the two lymph-hearts synchronous with each other. Another pair of these contractile cavities is situated beneath the posterior margin of the scapula close to the transverse process of the third vertebra : this pair forces the contents of the lymphatics of the anterior portions of the body into the jugular veins. (626.) Fishes respire water by means of gills. Reptiles, breathing a lighter medium, are provided with lungs, membra- nous bags into which the external element is freely admitted, and again expelled in a vitiated condition, its oxygen having been em- ployed in renovating the blood which circulates in an exquisite network of delicate vessels, that ramify in rich profusion over the walls of the pulmonary chamber. This important difference between Fishes and Reptiles as relates to their mod^ of respiration would seem, at first sight, to draw such a distinct line of demarcation between these two great classes of Vertebrata that it would be impossible for the most superficial zoologist to confound one with the other, or to be for a single moment at a loss in attempting to assign to any creature belonging to either of these divisions of the animal world its pro- per position ; indeed, to mistake an air-breathing Reptile for a Fish properly so called, would appear to be an error which the most ignorant naturalist could hardly be in danger of committing. We have, however, again and again had opportunities of observ- ing how nearly animals of neighbouring classes approximate each * V r ide Berlin Annals for 1832; and also Panizza, sopra il sistema linfatico dei Ret- tili. Fol. Pav. 1833. HKPTILIA. 565 other, not only in their outward form, but in their anatomical con- struction ; and, in considering this portion of our subject, we shall have another most striking illustration of Fig. 254. this great law in zoology. The perfect and typical Reptile, as the Lizard, the Tortoise, and the Serpent, breathes air and air only, and is there- fore only provided with lungs adapted to this kind of respiration : but the Peren- nibranchiate Amphibia, possessing both lungs and gills, participate to a greater or less degree in the characters of Fishes, so that in some, as, for example, in the Lepidosireu (Jig. 238), so near is the approximation, that it becomes almost impracticable for the most accomplished anatomist precisely to determine whether the animal ought rather to be called a Reptile or a Fish ; and lastly, in the Batrachian Amphibia, as we have al- ready seen, we have the same animal gradually changed from a Fish into a complete and perfect Reptile. In considering the apparatus provided for circulation and respiration in the ani- mals comprised in the class before us, we shall therefore first describe the organiza- tion of these viscera in Reptiles furnished with lungs only ; secondly, of those hav- ing permanent gills as well as lungs ; and thirdly, the metamorphoses that take place in the construction of the breath- ing organs during the developement of the lungs, and the obliteration of the branchiae in those forms in which the branchiae are not persistent. (627.) The lungs of Reptiles are two capacious membranous sacs occupying a considerable portion of the visceral cavity which, as there is no diaphragm as yet developed, cannot properly be di- 566 REPTILIA. vided into thorax and abdomen, as it is in Mammalia. From the internal surface of the walls of each lung membranous septa project inwards, so as partially to divide the interior of the organ into numerous polygonal cells, which are themselves subdivided into smaller compartments in a similar manner. This structure is well seen in the lung of the Tortoise (Jig. 255). The pulmonary Fig. 255. cells are most nume- rous and complete towards the anterior extremity of the lung, and it is here that the pulmonary vessels principally ramify : towards the hinder part of the viscus the cells be- come larger, and the breathing sur- face proportionately less extensive, until in some cases, as in Serpents, the cells being quite oblite- rated, the lung ter- minates posteriorly in a simple mem- branous bladder. The air is brought into the lungs through a long tra- chea composed, as in other Vertebrata, of a series of cartilaginous rings ; but there is this peculiarity in the construction of the Reptile lung, the trachea never divides into bronchial ramifications, but terminates abruptly by one or more orifices, which open at once into the general pulmonary cavity. It must be evident, from the whole construction of a lung of this description, that, owing to the comparatively limited surface that it presents internally, it is far less adapted efficiently to expose the circulating fluid to the influence of the atmosphere than the REPTILIA. 567 more complex apparatus of Birds and Mammalia: the respiration of Reptiles is consequently proportionately imperfect ; and hence that coldness of their blood, and feebleness of muscular movement, which are so characteristic of the entire class. The air required for purifying the blood is, of course, conti- nually changed ; being alternately takeu into the lungs, and again expelled in a deteriorated condition, by a mechanism which will be found to vary in different Reptiles in accordance with the peculia- rities of their organization. No Reptile possesses a diaphragm, and, being destitute of this important muscle, the movements whereby inspiration and expiration are accomplished are, in such genera as are furnished with moveable ribs, entirely dependent upon the mobility of the framework of the chest : the dilatations and con- tractions of the thorax consequent upon the alternate elevation and depression of the ribs being sufficient to ensure the inhalation and expulsion of air, such is the case in the Serpent and the Lizard. In the AMPHIBIA, however, there are not even ribs developed, or, if they exist at all, they are such mere rudiments as to be quite useless as instruments of respiration ; and on the other hand, in the CHELONIAN REPTILES, the large and expanded bones of the thorax are so consolidated together, and so immoveably fixed to the broad and osseous sternum, that respiration in the ordinary manner F would be altogether impracticable. Under these circumstances, as a compensation for the want of mobility in the chest, the os hyoides and the muscles of the throat are converted into a kind of bellows, by which the air is forced mechanically into the lungs, and they are thus distended at pleasure. Any one who watches a Frog or a Tortoise with a little attention will at once understand the mechanism by which this is effected. The mouth is kept closely shut ; and the nostrils, which open immediately into its cavity, are each provided with a muscular valve so disposed as freely to permit the entrance of air into the mouth, but also effectually preventing its return by the same chan- nel. By this arrangement the descent of the hyoid apparatus fills the mouth with air ; and the subsequent contraction of the broad muscles of the throat, the nostrils and the pharynx being of course both closed, forces the air into the opening of the larynx, and distends the lungs, from which it ig again expelled by the pressure of the abdominal muscles. The structure of the heart and the course of the circulation in Reptiles afford interesting subjects for investigation. The heart 568 REPTILIA. consists of three cavities, namely, a strong and muscular ventricle (Jig- 256, a), and two membranous and very capacious auricles, both of which communicate by valvular openings with the ventri- cular cavity. The right auricle (b) receives the venous blood from all parts of the body through the venae cavse (/, o,j?), the termina- tions of which are guarded by strong valves ; the left auricle (c) is appropriated exclusively to the lungs, from which it receives arte- rial blood through the pulmonary veins (w, m). It is obvious, therefore, that the ventricle receives two kinds of blood from the two auricles, venous blood from the systemic auricle, and arterial blood from the pulmonic auricle ; and as the interior of the ventri- cular cavity is crossed by innumerable column earner, giving it almost a spungoid appearance, the vitiated and purified blood derived from these two sources are more or less completely mixed together, and blood only partially arterialized is distributed to the system. Fig. 256. Two sets of vessels take their origin from the single ventricle, viz. the pulmonary and aortic. The pulmonary artery soon di- vides into two trunks (/,/), one destined to each lung ; so that a KEPTILIA. 569 part of the impure blood expelled from the ventricle is at once driven to the organs of respiration to be further oxygenized. The aorta) immediately after its origin, likewise separates into two trunks (d, e), the right and the left ; which, winding backwards, ultimately join to form one great vessel (/), from which the arteries of the viscera (i, A:), and those destined to the posterior parts of the body, are given off. From the commencement of the right aortic trunk a very large vessel is furnished, "which bifurcates to form two arteria innominate (g*, g), from which the carotid and subclavian arteries take their origin. (628.) Although the above description refers more immediately to the construction of the heart of the Tortoise, in all essential par- ticulars it is equally applicable to all Reptiles of the Saurian, Che- Ionian, and Ophidian orders ; and when we thus see that, in addition to the comparatively imperfect condition of their lungs, the blood which circulates through the body is in these creatures a mixed and semi- venous fluid, we need not be surprised at the contrast which they offer when compared with the hot-blooded and vigorous ani- mals to be described in the subsequent chapters of this work. Cuvier committed a serious error in describing the Batrachian Reptiles as having a heart composed but of two cavities : our illus- trious countryman John Hunter had already ascertained that, in Frogs, Toads, and Salamanders, the heart possessed a pulmonary as well as a systemic auricle ; and his observations have since been abundantly confirmed by Dr. Davy, Dr. Martin St. Ange, and Professor Owen. The pulmonic auricle in these creatures is in- deed comparatively of small size ; but it exists as a perfectly distinct chamber, and receives the blood from the lungs preparatory to its admission into the common ventricle. With regard to the use of the additional auricle in the Reptilia, Professor Owen has well remarked,* that from the impediments which frequently occur to a free and regular circulation of blood in these cold-blooded and slow-breathing creatures, the venous side of the heart is subject to great distension ; hence the large size of the auri- cles, and of the sinus which receives the systemic veins, and also the perfect developement of the valves intervening between the venae cavse and the auricle, of which the Eustachian valve of the Mammi- ferous heart still presents a rudiment. Had the pulmonary veins terminated along with the systemic in the same cavity, their orifices would have been subjected to the pressure of the accumulated con- tents of that cavity, and there would have been a disproportionate * Transactions of the Zoological Society of London, vol. i. p. 217. 570 REPTILIA. Fig. 257. obstacle to the passage of the aerated blood into the ventricle. This is obviated by providing the pulmonary veins with a distinct receptacle, which is equally ready with the right auricle to render its contents into the ventricle during the diastole of that cavity. (629.) Passing from the consideration of the more perfect Reptile circulation as it exists in those genera which in their adult condition possess lungs only, to those which may pro- perly be called Amphibious, and are provided with both lungs and gills throughout the whole period of their lives, we must still pause to notice one or two in- termediate forms, which, not- withstanding that they lose their branchiae at an early stage of their growth, are evi- dently closely related to the Perennibranchiata, as may be gathered from the arrange- ment which their blood-ves- sels permanently exhibit ; such is the Menopoma, or Great South American Sala- mander, an animal met with in the rivers and lakes of the South American continent. In the annexed figure, taken from the Catalogue of the Hunterian Collection, the principal vessels of this crea- ture are delineated as seen from the dorsal aspect. The lower jaw (a) has been re- moved from the head, so that in the drawing are exposed the cut edge of the masseter muscle (6), the tongue (c), and the opening of the larynx, into which a bristle (d) has been introduced, one end of which is seen passing into the cavity of the right lung : the bag of the pharynx (/,/) has been left entire, and upon this the REPTILIA. 571 main vascular trunks are supported. From the heart, situated upon the opposite side of the oesophagus, is given off a large vessel representing the bulbus arteriosus of fishes, which terminates by dividing into four branchial arteries ; but, as in the adult Meno- poma there are no branchiae, these vessels (o, o, o) wind round each side of the neck, and again unite into two trunks (r, r) which by their union form the aorta (t, t). It will easily be perceived that this arrangement is precisely that met with in fishes ; only that, as there are here no gills intervening between the terminations of the branchial arteries and the commencements of the branchial veins, these vessels are immediately continuous with each other. Moreover, from the lowest branchial arch (o) a pulmonary artery is given off, which ramifies over the surface of the as yet rudimentary lung (e), and thus gives rise to a distinct pulmonary circulation. Having carefully considered the disposition of the vessels in the Menopoma above described, the reader will be able to appreciate the arrangement of the vascular system in those Amphibia which, being provided both with gills and lungs through the whole of their lives, literally combine the blood-vessels of a fish with those of an air-breathing reptile. In the PERENNIBRANCHIATA, as, for example, in the Proteus, instead of the bulbus arteriosus being immediately continuous with the aorta, as it is in the Menopoma, through the interposition of the vessels o, o, o, (Jig. 257,) the blood derived from the heart is obliged to pass more or less completely through gills appended to the sides of the neck before it arrives in the vessels (r, r), which may be said to represent the branchial veins of fishes. The branchiae are either vascular tufts or pectiniform organs, (fig. 258, b, b,) essentially analogous in structure to those of a fish. The blood, however, which is propelled from the heart is not here entirely venous, but consists of a mixed fluid, partially derived from the systemic and p- lg , 253. partially from the pul- monary auricle, the two having of course been mingled together in the common ventricle of the tripartite heart. The con- traction of the heart forces the blood into the bulbus arteriosus, from which it is in great part driven into the branchiae : arrived there, it passes along the great 572 KEPTIL1A. branchial artery (Jig.Q58, ), is made to circulate over the branchial fringes (&), and, being again collected into the branchial vein (c), in a purified condition, it is poured into those large trunks, the re- presentatives of the vessels r, r, (Jig. 257,) which form the aorta. But, besides the branchial circulation, these creatures likewise possess lungs (Jig. 254, z 9 t), and a pulmonary circulation of greater or less importance in different genera. Nevertheless, the pulmonary artery is merely a small twig given off from the aortic system of vessels, through which semi-arterial ized blood passes to the lungs, to be returned in a still purer condition to the left auricle of the heart. (630.) If the student has fully comprehended the permanent condition of the blood-vessels as it exists in the perfect Reptile and in the Perennibranchiate Amphibian, he will have little difficulty in understanding the changes which occur in the dis- tribution of the vascular system during the metamorphosis of the CADUCIBRANCHIATA. In the Salamander, when the lungs begin to be developed and are co-existent with the branchial apparatus, the arrangement of the circulating system is precisely similar to that described as being permanent in the Perennibranchiata ; as may be seen by a reference to the appended diagram, which would equally illustrate the dis- tribution of the blood-vessels in both cases. In this early stage of the tadpole's life, the contraction of the heart and bulbus arteriosus drives the greater part of the blood through the branchial veins (Jig. 259, a, a, a) to the gills, from which it is returned in a purified con- dition by the branchial veins (f, y,y, ), which by their union at length form the aorta, as in fishes. At this period the pulmonary artery (b), which is very small in correspondence with the as yet rudimentary condition of the lungs, is merely a branch derived from the aortic system, and reinforced by a vessel (c) given off from the bulbus arte- riosus. The greater propor- tion of the blood, therefore, Fig. 259. REPTILIA. 573 evidently goes to the branchiae, and a very small part to the lungs. The reader must, however, here remark, that there are small anastomosing vessels (e, e, e), uniting the branchial arteries with the trunks of the branchial veins, and that these are situated just at the roots of the gills, since these vessels become of the utmost importance during the subsequent stages of the metamor- phosis. The branchise gradually become diminished in size, and a smaller quantity of blood passes through them, and as this goes on the vessels (a, a, a ; f,f->f) shrink in the same proportion. Meanwhile the lungs are progressively more and more developed, and the pulmo- nary artery (b) expands in an equal ratio. As the blood forces its way with more difficulty through the branchise, the anastomos- ing vessels (e, e, e) dilate, and a freer supply of blood is poured into the pulmonary system ; until at last, when the lungs are fully formed, and the branchial arteries (a, , a) and veins (/,/,/) quite obliterated, all the blood necessarily passes immediately through the anastomotic trunks (e, e, e), which of course then represent the vessels (0,0, o) of the Menopoma (Jig- 257), and the mode of respiration is thus completely converted from that of a Fish into that of a true Reptile. (631.) But, during the progress of these changes in the dispo- sition of the vascular system, others not less wonderful take place in the form and uses of the entire hyoid apparatus, and, in those muscles of the throat which are connected with the function of respiration. The hyoid apparatus of the tadpole is, in fact, a very compli- cated structure,* and, like that of the fish, supports the branchise, and facilitates the entrance and expulsion of the water ; moreover, by opening or closing at pleasure the communication which exists through the branchial apertures between the mouth and the ex- terior of the body, it thus allows air to be taken into the lungs at pleasure. The os hyoides of the tadpole, at an early period of its deve- lopem en t, supports four branchial arches (Jig. 260, A, 1, 2, 3, 4), which bound three branchial fissures, through which, as in a fish, the water escapes from the mouth. The branchial arches 2 and 3 * Recherches anatomiques et physiologiques sur les organes transitoires et la meta- morphose des Batraciens ; par J.G. Martin St. Ange. Annales des Sciences Naturelles, vol. xxiv. 574 REPTILIA. are studded on eaeli side with cartilaginous points ; and the arches 1 and 4 have similar points on one side only, so that when the arches are approximated, as they can be by an elaborate temporary set of muscles provided for the purpose, the cartilaginous teeth lock into each other so accurately, that the branchial fissures are completely and firmly closed ; a provision which is evidently indispensable, in order to allow the tadpole to fill its lungs with air. The above is the condition of the branchial portion of the hyoid apparatus before the metamorphosis of the tadpole has made much progress ; and from this time a series of changes begin of a most curious and interesting description. When the me- rig. 260. tamorphosis has commenced, the os hyoides and branchial arches assume the ap- pearance repre- sented at Jig. 260, B. The pieces 8 and 9 are no longer both cartilagi- nous, the latter having become entirely ossified. The branchial arch 1 is likewise converted into bone ; and its upper surface, being considerably enlarged, is now connected with both the pieces marked 10 and 11. The three cartilaginous pieces 5, 6, 7, in Jig. 260, A, are consolidated into one, while the branchial arches 2, 3, 4, become much reduced in size, the branchiae approach each other, and the cartilaginous points with which they are provided adhere together, so that from hour to hour, so to speak, the mass (2, 3, 4) composed of the three united branchial arches becomes insensibly obliterated, and in a very few days is entirely absorbed. While this absorption is going on, the branchial arch 1 assumes greater consistency, its inferior extremity becomes directed outwards, and it loses the little cartilaginous teeth previously appended to it ; the os hyoides thus assumes the simple form represented in Jig. 260, c. Lastly, the cartilage 6 disappears, and the complex branchial apparatus of the tadpole be- REPTILIA. 575 Fir.261. comes converted into the permanent and comparatively simple os hyoides of the Salamander, depicted in Jig. 260, D. The branchial arches 2, 3, 4, Dr. St. Ange remarks, are ab- sorbed in proportion as the circulation becomes modified, their atrophy depending upon the change which takes place in the course of the blood, owing to the dilatation of the anastomotic ves- sels (Jig. 259, e, e, e), and the enlargement of the pulmonary arte- ries (b). It is, therefore, owing to a kind of revulsion produced by the afflux of the blood towards the pulmonary organ, instead of towards the branchiae, that the atrophy of the branchial capillaries, and subsequently of the whole branchial apparatus, is produced. (632.) We must, in the last place, before leaving the considera- tion of the circulating system of the REP- TILIA, describe that of the Lepidosiren, a crea- ture so exactly interme- diate between the two classes, that it is really difficult to determine whether it ought most properly to be called a fish provided with lungs, or a reptile with the circulatory organs of a fish. The heart resembles that of a fish, and con- sists of a single auricle (fg. 261, a), a ventri- cle (6), and bulbus arteriosus (c). The vena cava (e), bringing the vitiated blood from the system, terminates at once in the auricle, which is represented in the figure as laid open ; but the pulmonary vein (/), whereby the aerated blood is brought from the lungs (w, m), passes along as far as the auriculo-ventricular opening, where it empties its contents into the 576 KEPT ILIA. ventricle by a distinct orifice, protected by a cartilaginous valvular tubercle. It is, therefore, only necessary in this case to dilate the pulmo- nary vein previous to its termination, to make a heart with two auricles ; but, as Professor Owen observes, the same advantage is secured to the Lepidosiren in a different manner, for, while it still retains the diccelous type of the heart of the fish, the continuation of the pulmonary vein prevents the admixture of the respired with the venous blood until both have arrived in the ventricle. The aorta, or rather the bulbus arteriosus (g), in this interesting creature, fulfils at once the office of a systemic, a branchial, and a pulmonary artery. It gives off on each side six vessels, which correspond to the six cartilaginous branchial arches : of these arches four, namely, the 1st, 4th, 5th, and 6th, support gills, so that the arteries belonging to them (1, 4, 5, 6) are, as in fishes, distributed over the branchial fringes, and are thus true or func- tional branchial vessels. But the 2nd and 3d arches have no gills appended to them, so that the arteries (2, 3) belonging to these arches do not divide, but are continued round to the dorsal region, where they unite to form an aorta, as in Menopoma (Jig. 257) ; moreover, before their union to form the systemic trunk, they give off the pulmonary arteries (/, m) by which the pulmonary circula- tion is supplied. Thus each contraction of the ventricle of the heart drives the mixed blood derived from the verise cavse and pulmonary veins, first, to the gills ; secondly, to the aorta, through the vascular trunks (2, 3) ; and, thirdly, to the lungs through the pulmonary artery (/, m) ; so that from this arrangement, whether the creature be placed in water or in air, respiration is carried on efficaciously either by the pulmonary or branchial apparatus vica- riously. (633.) The principal difference observable between the brain of Reptiles and of Fishes, is the increased proportionate size of the cerebral hemispheres (Jig. 261, 5), but they are still ex- tremely small when compared with the bulk of the body. The appended figure, which represents the brain of the Tortoise in three different aspects, may easily be compared with that of the fish already given. The olfactory lobes (c) might now be mis- taken -for prolongations of the anterior extremity of the hemi- spheres ; they contain distinct ventricles, and of course give origin to the olfactory nerves (o, o). The hemispheres (b) are much more developed than in the last class ; their surface is always KEPTILIA. 577 smooth and without convolutions; and they are hollowed out into capacious ventricular chambers, in which are contained the corpus striatum and choroid plexus (j#g. 262, c), and the two sides are moreover brought into communication by an anterior and posterior commissure. The optic lobes (e) are as yet uncovered by the extension of the hemisphere backwards ; and each, when laid open, is found to enclose a ventricle {Jig. 262, c). The cerebellum (a) is still small, and consists but of the median portion : behind it is a sup- plementary lobe (g), extending over the fourth ventricle, as in Fishes. The student will easily recognise the pituitary body (f) ; but neither this, nor the origins of the nerves, present any pecu- liarity worthy of more particular description. Fig. 262. Taking the cerebral nerves in the order in which they arise, we will now proceed briefly to trace their general distribution ; and this we shall find to correspond most exactly in all essential points throughout the different classes of Vertebrata. The olfactory nerves leave the olfactory lobes of the brain as single round cords ; and are not, as in the Mammalia, divided into 2 p 578 REPTILIA. numerous filaments : there is, consequently, no cribriform plate to the ethmoid bone ; but the nerve of each side (Jig. 264, e) is received into a simple canal, partly osseous and partly car- tilaginous, through which it is conducted to the cavity of the nose. The nasal apparatus of Reptiles differs from that of Fishes in one important particular. Breathing air as these creatures do, the sense of smell now becomes connected with the respiratory function; and, a communication being established between the nasal cavities and the larynx, the air which passes through this channel into the lungs must necessarily come in contact with the sentient surface formed by those portions of the lining mem- brane of the nose to which the nerves of smell are distributed; and, in proportion as the extent of that surface becomes developed, the power of appreciating the presence of odorous particles in the atmosphere will necessarily be increased. The physiologist is thus enabled to estimate with great exactness the relative per- fection of the sense of smell in different classes, or even in dif- ferent families of the air-breathing Vertebrata, simply by observing the complication and extent of surface presented by the lining membrane of the olfactory organ. Taking this as our guide, we must suppose that in all Rep- tiles the sense in question is extremely obtuse, since in these creatures there are neither turbinated bones nor ethmoidal plates as yet distinguishable ; a few folds of the membrane lining the nose, even in those species which are most highly gifted in this particular, being the only provision for extending the olfac- tory surface ; and in many cases, as for example in the Am- phibia, the nose seems merely a simple canal leading into the mouth. On reaching the nasal cavity, the olfactory nerve spreads out into delicate filaments (Jig. 264, d), which are distributed to the Schneiderean membrane covering the septum and upper part of the nose. (634.) The optic nerves of Reptiles (Jig. 262, n), soon after their origin, become confounded together by a commissure, in the same way as in the human subject; and, again separating, they are continued through the optic foramina to the eyes. The eye-ball itself presents few peculiarities in its structure. In the Tortoise, and many Lizards, the sclerotic contains a circle of bony plates imbedded in its substance, and surrounding its REPTILIA. 579 anterior margin : these are obviously the rudiments of that os- seous zone which in the class of Birds, as we shall find, performs a very important office. The ciliary processes of the choroid are generally very feebly developed. The pupil is frequently round, but it is sometimes of a rhomboidal figure, as for example in the Gecko ; and in the Crocodile and some serpents the pupil- lary aperture is a vertical fissure like that of a Cat. The optic nerve enters the eye in the same way as in qua- drupeds, and, having passed the choroid, it terminates in a round papilla, from the margin of which the retina spreads out : as to the rest, the eye of a Reptile differs so little in any essential cir- cumstance from that of Man as to render any more elaborate description superfluous. The eye-ball is moved by six muscles, disposed as in Fishes ; the four recti arising from the margin of the optic foramen, while the two obliqui are derived from its anterior margin. In Fishes, from the circumstances under which they live, there is no occasion for the presence of any lacrymal apparatus, or for eyelids adapted to defend and moisten the surface of the cornea ; but in the class before us, especially in the more elevated tribes, these appendages to the eye make their appearance, and gradually assume a complexity of structure even greater than that which they present in the human subject. In Serpents, and in some of those Lizards which are most nearly allied to the Ophidians, there are still no eyelids ; and con- sequently in such genera there can be neither any lacrymal appa- ratus, nor a conjunctiva, properly so called : the skin of the head merely passes like a delicate film over the transparent cornea, offering no fold worthy of the name of an eyelid. In ordinary Lizards* the skin forms a kind of veil stretched over the orbit, and pierced by a horizontal fissure, which is closed by a sphincter muscle. The lower eyelid is the most moveable, and encloses a small cartilaginous plate; and there is besides ge- nerally a fold of the conjunctiva at the inner canthus of the eye, which is the first appearance of a third eyelid or membrana nictitans. In the Chelonian Reptiles, and in the Crocodiles, the upper and lower eyelids are sufficiently perfect accurately to close the eye ; but there are no eyelashes as yet present. Moreover, these * Cuv. Le9<>ns d'Anat. Comp. vol. ii. p. 433. 580 REPTILIA. animals possess an additional eyelid or nictitating membrane, si- milar to that of Birds, which can be drawn at pleasure over the front of the eye, so as entirely to conceal it. This is effected by a special muscle provided for the purpose, which arises from the posterior part of the globe of the eye, and, after winding round the optic nerve, passes beneath the eye-ball, to be inserted into the free margin of the membrana nictitans. In Frogs and Toads the upper and lower eyelids are nearly motionless ; but the third is largely developed, and moved in the same way as that of the Crocodile. In the higher Reptilia a distinct lacrymal gland and puncta lacrymalia are met with, occupying the same positions as those of the human subject. (635.) The third, fourth, and sixth pairs of the cerebral nerves, have the same distribution in all the Vertebrata; and represent respectively the oculo-muscular, the pathetici, and the abducentes of man. (636.) The nerves belonging to the fifth pair likewise corre- spond both in their distribution and office with the trifacial nerves of mammiferous Vertebrata. (637.) The facial nerve, or portio dura of the seventh pair, is small in proportion to the limited developement of the soft parts of the face ; but it is constantly present. (638.) The auditory nerve of course is destined to the ear, and its distribution is almost the same as in Fishes ; nevertheless, in the general construction of the organ of hearing, Reptiles present very important and interesting advances towards a higher form of the acoustic apparatus, which we must proceed to notice. The ear of Fishes, being only adapted to hear sounds conveyed through a watery medium, was found to consist only of the mem- branous labyrinth, enclosed in the cavity of the skull, and without any communication with the exterior of the body. Reptiles, on the contrary, living in air, must be enabled to appreciate the sono- rous vibrations of the atmosphere, and are consequently provided with an auditory apparatus, capable of responding to pulsations of sound of far greater delicacy than those transmitted through the denser element. The first great improvement therefore which the anatomist notices in the composition of the ear of a Reptile, is the addition of a tympanic cavity, and of a tense and delicate membranous drum, the vibrations of which are communicated to the labyrinth REPTILIA, 581 or internal ear through the intervention of an ossicle that represents the stapes of Mammalia. The drum of the ear is situated immediately beneath the skin, the parts composing the external ear of Fig. 263. quadrupeds being as yet entirely deficient. The membrana tym- pani, that now for the first time makes its appearance in the series of animals, is tensely stretched across the tympanic aperture; being covered exter- nally by the integu- ment of the head. In the Turtle (Jig. 263) the tympanic mem- brane is represented by a cartilaginous plate (a). The ossicle, or columnella as it is here called, is single and trumpet-shaped : it passes quite across the tympanic cavity (b), its external extremity being inserted into the drum ; while at its opposite end it expands into a disc (c), which closes an aperture (foramen ovale) that com- municates with the membranous vestibule of the internal ear. It is obvious therefore that every tremor impressed upon the mem- Irana tympani will be conveyed by the columnella to the fora- men ovale, and thus communicated to the fluid contained in the labyrinth, upon which, as in Fishes, the auditory nerve is dis- tributed. The cavity of the tympanum communicates with the interior of the mouth by a wide opening, that represents the Eustachian tube ; a circumstance evidently intended to prevent air or fluid from being pent up in the tympanic chamber, and thus interfering with the free vibration of the drum. In Serpents, on account of the peculiar disposition of the pieces of the temporal bone before described (J 606), there is no tympanic cavity, and the columnella (Jig. 249, v) is absolutely im- bedded in the flesh ; the arrangement, however, in other respects is the same as in the generality of Reptiles. The lower tribes of Amphibia, as we might be led to expect 582 REPTJLIA. from their close approximation to Fishes, have neither tympanum nor columnella ; and thus, like Fishes, can only hear in an aquatic medium. (639.) The membranous labyrinth of Reptiles (jig> 264, #, &, c) corresponds in its general conformation with that of Fishes, pre- senting the same semicircular canals, ampullse,and vestibular cavity ; Fig. 264. and moreover, the sacculus contains cretaceous concretions, or oto- lithes of a similar character. But in this class the membranous canals become enclosed in a bony sheath, moulded as it were upon their outer surface ; which is another very important step towards perfecting the auditory apparatus. (640.) Neither must we omit to mention, that in the highest of the Reptilia, as for example in the Crocodile, the first rudiment of a cochlea makes its appearance, although as yet in a form of extreme simplicity. This portion of the organ of hearing, which, from the elaborate structure that it presents in the higher Verte- brata, must be regarded as being importantly connected with cor- rect audition, is seen in this, the earliest stage of its developement, to be a simple conical appendage to the sac of the vestibule ; and, on opening it, it is found to be divided by a central cartilaginous septum into two compartments, which are however continuous with each other at the apex of the cone. One of these com- partments or canals opens at one extremity into the vestibule, while the other communicates with the tympanic cavity by a very small aperture closed with a thin membrane. Thus, there- fore, although the entire organ resembles a simple canal bent REPTILIA. 583 upon itself, the representatives of the scala vestibuli, of the scala tympani, and of iliefenestra rotunda of the human ear can be dis- tinctly identified. (641.) The glosso-pharyngeal and pneumogastric nerves in Rep- tiles supply the same or- Fig. 265. gans to which they are dis- tributed in the human sub- ject ; the former being destined to the base of the tongue and the muscles of the pharynx; while the lat- ter, assuming a plexiform arrangement, are appro- priated to the lungs and heart, as well as to the oesophagus and the sto- mach. (642.) The hypoglossal pair of the cerebral nerves, which was not met with in Fishes, now becomes dis- tinctly apparent ; and, as in the higher Vertebrata, may be traced in the mus- cles of the tongue. (643.) The spinal system of nerves offers no peculia- rity worthy of special de- scription. In the annexed figure, taken from Boja- nus, the nerves derived from the medulla spinalis are seen to issue in the usual manner from the intervertebral foramina ; and they evidently essen- tially correspond with the grand type of structure common to the vertebrate classes. In the apodous Reptilia, as for example in the Serpents, to attempt to divide them into the usual regions is clearly absurd ; but in quadrupedal forms, as 584 REPTILIA. for instance in the Tortoise, the cervical nerves, the brachial plexus, from which are derived the nerves of the anterior extre- mity, the intercostal nerves, and those forming the lumbar and sacral plexuses, are at once distinguishable ; and the correspond- ence between their distribution in the reptile and in the human subject must forcibly strike the student who makes the com- parison. (644.) Neither does the sympathetic system of the Reptilla offer any important aberration from that arrangement with which the human anatomist is familiar. The ganglia are smaller in their pro- portionate size; those of the neck and face are, indeed, scarcely per- ceptible : but the thoracic ganglia are found in their usual positions, communicating on the one hand with the spinal nerves, and on the other giving off filaments which form plexuses around the arterial trunks, and ramify extensively to be distributed to the viscera of organic life. (645.) The sense of touch in all the members of the class under consideration must, from the nature of their integument, be extremely imperfect : many of them, as for example the Serpent tribes, are, in fact, absolutely deprived of any limbs which can be regarded as tactile organs ; and, even in those forms which are provided with efficient locomotive extremities, they are but ill adapted to exercise the functions of an apparatus of touch. The cuticular investments of the body are formed of dense and unyielding materials, consisting, in the higher Reptiles, of broad horny plates, or of imbricated scales. In the Amphibia, indeed, the skin is smooth, and the epidermis only forms a delicate cor- neous film ; yet even in these the cuticle is thrown off at certain seasons of the year, as the old coat becomes too small for the in- creasing size of the animal: a phenomenon which in the Lizard and Serpent tribes is still more remarkably witnessed ; for these animals strip themselves of their old scales as the hand would be drawn out of a glove, and cast away in one piece the entire epidermic inte-. gument, even to the film which covers the transparent cornea of the eye. (646.) The urinary excretion in Reptiles becomes of very consi- derable importance, and the structure of the kidneys and excretory ducts proportionately elaborate. The kidneys (fig- 267, o, p) are generally situated very far back, even within the cavity of the pelvis where a sacrum exists, as in the Chelonian and Saurian orders ; and in these tribes they are very partially covered by the peritoneum REPTILIA. 585 being firmly imbedded in the sacral region. But in the Serpents, in consequence of the elongated form of the body, and the complete flexibility of every portion of the spine, the kidneys are peculiar both in their position and general structure. Instead of being placed upon the same level as in other Vertebrata, the right kidney of an Ophidian is situated much more anteriorly than the left ; a circumstance which much facilitates the packing of the abdominal viscera, and contributes greatly to ensure the free movements of the vertebral column at this place. For the same reason, the kidneys of a serpent are divided into numerous lobes, placed in a longitudinal series upon the outer side of the commencement of the ureter, and loosely connected to each other and to the spine by cellular tissue and a fold of the peritoneum. As relates to the minute structure of the kidneys in the Rep- tilia, these viscera are invariably composed of convoluted tubes, which pour their secretion into the commencement of the cor- responding ureter. The ureters of course vary in length accord- ing to the position of the renal organs ; they ultimately terminate in the cloaca (Jig. 267, u) ; a cavity or general outlet through which, in the female, the ova, the fseces, and the urine are dis- charged, and which in the male gives passage to the contents of the rectum, the secretion of the kidneys, and the semen. (647.) In connection with the urinary apparatus of Reptiles, it will be convenient to mention a bladder that exists in Chelonian and Amphibious Reptiles, and is also found in some Saurian tribes, to which the name " urinary bladder"" has been erroneously applied. This bladder, in the Tortoise (Jig. 267, A) and Proteus (Jig. 254, q) is of considerable size, and in the Frog forms a very capa- cious receptacle, having its upper part divided into two cornua. It is generally filled with a clear limpid fluid, which in the case of the Frog is forcibly ejected if the animal be alarmed : but that this fluid is not urine is obvious from the fact already stated, that the ureters open into the cloaca (Jig. 267, u), and not into the bag referred to ; the latter, in fact, is the unobliterated remains of the ALLANTOIS of the embryo, concerning which further particulars will be given in the next chapter, and the fluid contained in it is most probably the product of cutaneous absorption.* (648.) In tracing the developement of the generative apparatus * Vide Cyclopaedia of Anatomy and Physic, art. AMPHIBIA, by Professor Bell, p. 104. 586 EEPTILIA. through the different orders of Reptiles, the student will not fail to observe many beautiful illustrations of progressive improvement. The finny tribes, incapable of social intercourse, were content with the simple extrusion of their eggs into the sea, leaving them to be impregnated by the casual approach of a male of the same species : but even in the Amphibious Reptiles some steps are gained in associating the sexes with each other ; and although the eggs are still impregnated out of the body of the mother, in the Frog this is accomplished in exitu, and not subsequent to their expulsion. Frogs, during the breeding season, are found to pair, and the male having selected his mate mounts upon her back, clinging to her with unwearying pertinacity during the whole period of oviposition, and vivifying her eggs by the aspersion of the seminal secretion as they are successively expelled in long gelatinous chains. During this protracted embrace the male Frog is assisted in retaining his hold by the developement of a peculiar papillose structure upon the first toes of the fore-feet, which disappears at the end of the time appropriated to reproduction. Of course no intromittent apparatus is as yet required, and we may naturally expect to find the male organs still exhibiting great simplicity of construction. (649.) The testes and their excretory ducts are, in fact, the only parts as yet met with ; but the anatomy of these parts, although most accurately investigated by Swammerdam upwards of a century ago, is still very generally misunderstood. The testicles are situated in the loins, surrounded by several tongue-like masses of fat, pre- senting a peculiar granulated appearance. Each testis is invested by a delicate capsule, and, on removing this very carefully, the entire viscus is seen to be made up of short caeca ; the blind extremities of which alone appearing at the periphery of the organ caused Cuvier to describe it as being " an agglomeration of little whitish grains interwoven with blood-vessels." The semen elaborated by these caeca is taken up by several small excretory ducts that pierce the kidney, in the immediate vicinity of which the testis lies, and open into the ureter, that here forms the com- mon excretory duct, whereby the urine as well as the seminal fluid is discharged, both escaping into the cloaca at a little distance from the orifice of the allantoid bladder, to be ultimately ejected through the vent. (650.) Neither is the generative system of the female Frog less EEPTILIA. 587 worthy of notice. The ovaria resemble in their essential structure those of the Lamprey ( 580), only they are much less extensive ; consisting of a few festoons of the highly vascular membrane wherein the ova are secreted, fixed at the pelvic extremity of the abdominal cavity. On each side of the body is a long and very tortuous oviduct, which when unravelled is found to be many times the length of the animal. The fimbriated commencement of this oviduct is firmly bound down by folds of peritoneum in the imme- diate vicinity of the pericardium, and, of course, as remote as possible from the ovary ; it therefore becomes a question of no inconsiderable interest to determine the manner in which the ova are conveyed from the ovarian nidus to the orifice of the oviduct : it is obvious that they must first break loose into the abdominal cavity, as we found them to do in the Lamprey and the Eel, and that at length, having made their way into the neighbourhood of the pericardium, they are seized by the patulous extremity of the Fallopian tube, and thus conveyed out of the body. As the ova make their transit through the oviduct, they become imbedded in a tenacious albuminous secretion, "and are at length lodged in a dilated portion of the tube, to which the name of uterus has been very im- properly given, preparatory to their expulsion through the cloaca. After the eggs have been discharged into the surrounding water, the albuminous mass in which they are imbedded swells considerably; and, when the young tadpoles are hatched, this material no doubt serves to nourish them during the earlier period of their existence. (651.) In the Newt (Triton) impregnation takes place internally, although the male is still without any rudiment of an intromittent apparatus, so that we are compelled to believe that in the case of these Amphibia the simple ejection of the male fluid into the water in the vicinity of the female is sufficient to ensure its admission to the ova while still in the oviduct. An improvement is likewise visible in the construction of the internal viscera subservient to generation ; and a vas deferens, quite distinct from the ureter, makes its appearance. In the male Salamander (Triton cristatus) the testis -during the breeding season consists of two pyriform masses, from which the seminal ducts (Jig* 266, c, c) are derived. These soon unite to form a single convoluted tube (d), through which the semen is conveyed into the cloaca. The kidneys (w), and their excretory ducts (i 9 i), are here placed considerably further back ; but the ureters terminate in the cloaca at the same point (m) as the vasa deferentia. Two other large glands (o, o) are appa- 588 REPTILIA. rently connected with the generative functions, and their excretory ducts likewise open into the cloacal outlet. (652.) In the female Triton, as Fi s> 266. also in the Proteus and Siren, the ovaria and oviducts offer precisely the same arrangement as that met with in the Frog already de- scribed.* (653.) In the Ophidian, Chelo- nian, and Saurian orders, the testes of the male sex are situated in the loins ; and, in fact, they occupy the same position throughout the oviparous Vertebrata : they offer no peculiarity of structure ; only differing from those of the Frog in the increased length of the now contorted seminal cseca of which they are essentially composed. From each testis a long and flexu- ous vas deferens conducts the se- men into the cloaca. Here, how- ever, in these more elevated forms of theReptilia,we have another im- portant addition to the male sexual apparatus ; instruments beinggiven to facilitate the impregnation of the female during that union of the sexes which now becomes es- sential to fecundity. The earliest appearance of the copulatory organ is seen in Serpents and in the Lizard tribes ; and in such rep- tiles it will be observed, that the penis is rather a provision for securing the juxta-position of the sexual apertures of the male and female than an instrument of intromission. The two lateral halves of the penis, or corpora cavernosa, as we shall have to call them hereafter when they become conjoined in the mesial line, are as yet quite separate, and placed at each side of the cloacal fissure, from which they protrude when in a state of erection ; so that there ap- pear to be two distinct organs of excitement, or, more properly speaking, of prehension ; for each division, being of course im- * Vide Rusconi. Observations Anatomiques sur la Sirene mise en parallele avec Je Protee et le tetard de la Salamandre Aquatique. A Pavie, 1837. RErTTMA. 589 perforate, is covered with sharp spines, and is obviously rather adapted to take firm hold of the cloaca of the female than to form a channel for the introduction of the seminal fluid. (654.) In the Chelonian Reptiles the penis is much more perfectly developed, and really constitutes a very efficient intromittent in- strument. The two corpora cavernosa, after commencing separately, approach each other, and become united along the mesial line so as to form a single organ of considerable size, terminated at its ex- tremity by a glans-like dilatation. There is, however, no corpus spongiosum, or urethral canal, properly so called : the latter is re- presented by a deep groove, which runs along the upper surface of the penis from the cloaca to the extremity of the organ ; and it is along this groove that the spermatic fluid is conveyed during coitus. On making a section of this strange apparatus, two canals are dis- covered, running one on each side of the central furrow, along the whole length of the organ as far as the glans, where they terminate, without at all communicating with the exterior ; but, on tracing them in the opposite direction, they are found to be derived from the peritoneal cavity, into which they open by distinct orifices.* Two retractor muscles, derived from the pelvis, and extending along the under surface of the penis quite to its extremity, fold the whole organ back into the cloaca, where it lies concealed when not in use. In the Crocodiles and higher Saurians the penis in its structure resembles that of the Tortoise ; and, instead of an urethra, there is merely a deep groove traversing the upper surface of the organ, along which the semen trickles out of the cloaca. (655.) Throughout all the Reptile families the organization of the female generative system is so extremely similar, that one example will be abundantly sufficient for our purpose ; the same description in fact being equally applicable to the Saurian, the Chelonian, and the Ophidian orders. The ovaries occupy their ordinary position in the lumbar region of the abdomen, where they are attached on each side of the vertebral column by a broad fold of peritoneum : their structure is in all essential pojnts precisely similar to those of the Amphibia ; but, owing to the increased proportionate size of the individual ova formed by their vascular membrane, they resemble a string of beads, or assume somewhat of a racemose appearance. The oviducts are long and flexuous ; they commence by a wide orifice {Jig. 267, b M), by which the germs are taken up from the * Cuv. Anat. Comp. torn. v. p. 115. 590 REPTILIA. ruptured ovisacs of the ovaria in the same way as those of Mam- malia are seized by the fimbriated extremities of the Fallopian tubes. The first portion of the Fig. 267. oviduct is thin and intestini- form; but lower down, where the investments of the egg are form- ed, its walls become thicker, and assume a glandular character (n> o, p) : they finally open into the cloaca; and the mode of their termination in the Tortoise is ex- hibited in the accompanying fi- gure, where (M, m, e M) indicate the terminal portion of the right oviduct laid open ; the left (a M, b M) being shown through its entire length. (656.) The formation of the egg and the developementof the embryo is similar in all the ovi- parous Vertebrata ; it will there- fore be more convenient, and prevent unnecessary repetition, if we defer the consideration of this important subject to the next chapter ; the reader bearing in mind that in all essential par- ticulars the details which will be given there, when we come to consider the growth of the bird in ovo, are equally applicable to the Chelonian, Ophidian, and Saurian Reptiles. 591 CHAPTER XXIX. AVES BIRDS. (657.) THE class of Vertebrate animals which now offers itself to our notice contrasts remarkably with the cold-blooded and apathetic inhabitants of the water ; and even with the slow-moving Reptile, that languidly crawls upon the surface of the ground, or drags on an amphibious existence in the marsh or on the shore. The Bird, ordained to soar into the regions of the air, and not only to sustain itself in that thin medium but to skim from place to place with astonishing rapidity, needs a strength of muscle and activity of limb even greater than that conferred upon the mammiferous qua- druped. Senses of the utmost acuteness are now requisite, com- bined with instinct and intelligence of a high order ; and accord- ingly, both as regards their faculties and enjoyments, the feathered tribes far surpass the other oviparous Vertebrata. Next to that improvement in the condition of the nervous system, which we have all along been able to trace advancing part passu with the increase of sagacity and the expansion of the bodily faculties, the most remarkable circumstance observ- able in the economy of Birds is the elevated temperature of their bodies and the heat of their circulating fluids. In the Reptile an impure and semi-oxigenized blood was slowly propelled through the system from the undivided ventricle of their trilocular heart ; and we found their energies, their instincts, and their affections proportionately feeble and obtuse : but now, not only does the heart become divided into four cavities, one ventricle being appro- priated to transmit venous blood to the lungs, while the other drives a pure and highly arterialized fluid in copious gushes to the remotest regions of the body; but, as though even this was not suf- ficient tomeet the necessities of the case, the whole interior of the bird is permeated by the atmospheric air, which penetrates even into the bones ; and the respiratory function being thus rendered as complete as possible, all parts of the muscular system are abun- dantly supplied with blood arterialized to the utmost, and every fibre, quivering with life intense, is ready to exert that vigorous 592 AVES BIRDS. activity which brings down the falcon upon his quarry like a thun- derbolt from the clouds, or sustains the migratory bird through long and perilous journey ings. But increase of muscular energy is by no means the only conse- quence resulting from more perfect respiration, and a consequently increased temperature of the blood : the clothing of the body must now be changed for a warmer covering than scales or horny plates; feathers are therefore at once provided as the lightest, warmest blan- ket that could be given : maternal care, which to the cold-blooded Ovipara would have been a useless boon, can now be beneficially ex- ercised ; the eggs, no longer left to chance, are cherished by the vital heat of the parent ; and the callow brood, during the first period of their lives, are dependent for support upon the watchful attentions of the beings from whom they derived their existence. (658.) The skeleton of a vertebrate animal formed for flight must obviously be constructed upon mechanical principles widely different from any that have yet come under our notice. The utmost lightness is indispensable ; but still, in a frame-work which has to sustain the action of muscles so vigorous, strength and firmness are equally es- sential : it is in combining these two opposite qualities that the human mechanician displays the highest efforts of ingenuity, and by the scientific disposition of his materials exhibits the extent of his resources and the accuracy of his knowledge ; but let the best informed and most ingenious mechanic carefully and rigidly investi- gate the skeleton of a bird, and we doubt not that in it he will find all his art surpassed, and derive not a little instruction from the survey. In the spinal column of a bird we find three principal regions, each of which will merit distinct notice. The anterior or cervical region is exceedingly variable in its pro- portionate length, and forms the only flexible portion of the spine : it performs, indeed, the office of an arm, at the extremity of which the beak, the chief instrument of prehension, is situated. The number of vertebrae entering into the composition of this part of the spinal column is very variable : in the Swan there are as many as twenty-three ; in the Crane, nineteen ; while in the little Sparrow nine only are met with : their bodies are joined together by articu- lating facets inclosed in synovial capsules, and not by the interposi- tion of intervertebral substance ; an interarticular cartilage, however, is generally met with, by which the movements of the chain are facilitated. The spinous and transverse processes are short ; while AVES BIRDS. 593 the oblique processes, united by articulating surfaces, limit the mo- bility of the neck. Although this portion of the spine is very properly designated the " cervical region," we are not on that account to imagine that the vertebrae composing it are unprovided with ribs : on the con- trary, rudimentary costal appendages are generally found connected with their transverse processes, which, in the young bird, are obvi- ously separate elements, although they afterwards become united by anchylosis. (659.) But if flexibility is thus abundantly provided for in the cervical portion of the vertebral column, it is quite evident that in the thoracic portion of the skeleton, which has to support the framework of the wings, and sustain the efforts of the muscles connected with flight, firmness and rigidity become essential requisites ; and accord- ingly everything has been done to prevent those movements which in the neck were so advantageously permitted. The bodies and spinous processes of the contiguous vertebrae are therefore here firmly consolidated together by anchylosis ; and, moreover, splints of bone, derived from the transverse processes, overlap each other, and still further add to the stability and strength of the back. The ribs appended to the dorsal vertebrae may be called the true ribs; these enter into the composition of the thorax, and mate- rially assist in strengthening that region. Each rib, as in the Croco- dile, presents a dorsal and a sternal portion connected together by a joint : the former are attached to the vertebrae by a double articula- tion, their spinal extremity being furcate; while the latter are articu- lated to the sides of the sternum. A thorax is thus formed, possessing sufficient mobility to perform the movements connected with respiration, but still affording a strong basis to support mus- cular action ; and, in order to give the greatest possible strength, from the posterior margin of each dorsal rib a broad flat process is prolonged backwards and upwards to overlap the rib next behind, so as in this manner to bind the whole together into one strong frame- work. The sternum itself is developed in proportion to the enormous size of the three pectoral muscles which constitute the great agents in flight : it is principally composed of the central azygos element before noticed in the Tortoise, which is here remarkably dilated, and in birds of flight prolonged inferiorly into a deep keel-like pro- cess, so as to increase materially the extent of surface from which the muscles of the breast take their origin ; but in the cursorial 594 AVES BIRDS. Fig. 268. genera, such as the Ostrich, the Emeu, &c. where the wings are not available for flying, the keel is entirely wanting, and the sternum forms merely a kind of osseous shield, covering comparatively a very small portion of the breast. (660.) Whoever considers the position of the hip-joint in the fea- thered tribes, and reflects how far it is necessarily removed behind the centre of gravity when the bird walks, carrying its body in a horizon- tal position, will at once perceive that the pelvic portion of the spine, having to sustain the whole weight of the trunk under the most unfavourable circum- stances, and at the same time to give origin to the strong and massive muscles wielding the thigh, must be consoli- dated and strengthened in every possible man- ner; and that even the slight degree of move- ment permitted in the dorsal region would here be inadmissible. The lumbar and the sacral vertebrae, and the entire pelvis, are therefore at an early period solidly united together by anchylosis into one bone, and the num- ber of the vertebrae composing this part of the skeleton is only distinguishable from the situation of the intervertebral foramina through which the spinal nerves are given off. In very young birds the pelvis is evidently formed by the three elements that usually enter into its composition ; and the ilium, the ischium, and ihepubes, as well as the ischiadic notch and obturator foramen, will all be at once recognised by the anatomist, occupying their usual relative positions ; although he will not fail to notice one re- markable circumstance, namely, that except in one instance, the Ostrich, the ossa pubis do not meet in front, so that there is no pubic arch or symphysis. (661.) The anterior extremity of a bird, although an instrument of flight, is found, when stripped of those feathers and long quills that form the extensive surface presented by this member during AVES BIRDS. 595 life, still closely to adhere to die general type in accordance with which this part of the skeleton is invariably constructed. The framework of the shoulder exhibits the scapula (fig. 269, 6), the clavicle (of), and the coracoid element (c) ; notwithstanding that these bones, form- Fig. 269. ing, as they do, the basis of a limb so vigorous, and wielded by such powerful muscles, are necessarily mo- dified in their form and general ar- ran gement, so as to constitute strong buttresses adapted to keep the shoul- der-joint firm and steady during flight. The scapula (b) is a long and slender bone placed upon the ribs, and lying parallel to the spine along the dorsal region of the thorax, imbedded in the mus- cles to which it gives attachment, while at its fixed extremity it assists in forming the cavity of the shoulder-joint. The coracoid bone (c) is the great support of the shoulder ; for, while at one ex- tremity it sustains the wing, at the opposite it is firmly and securely united to the sternum by a broad articulation. But the most pecu- liar element of this apparatus is the/wrcM/wra, or forked bone (d), composed of the conjoined clavicles; which, being anchylosed to- gether in the mesial line, and also strongly connected with the shoulder-joint, materially add to the stability of the whole. In the wing itself the humerus (f) is at once recognised, as also the ulna (g) and the radius (h) ; but in some birds, as in the Penguin, the student might be at a loss to identify one or two small bones (/?), forming a kind of patella to the elbow-joint ; these appear to be the representatives of the olecranon process de- tached from the ulna. The carpus (i) consists of only two small bones. The metacarpus is formed of two pieces (, /), anchylosed together at their two extremities ; and these, with two, or in some cases three, rudimental fingers complete the wing. The largest finger consists of two, or sometimes three, phalanges (m, o) : a second (n) offers but a single joint ; and the third, which is a a 2 596 AVES BIRDS. mere rudiment when present, is an appendage to tlie radial side of the carpus. In the pelvic extremity (Jig. 268) the femur is a short and strong bone : to this succeeds the tibia, upon the outer side of which is fixed a rudiinental^fttf/a. The tarsus can scarcely be said to exist, being at a very early age confused with the metatarsus ; the whole forming a single tarso-metatarsal bone, which, in the Wading Birds especially, is of very great length : at its distal extremity are three articular surfaces that support the three anterior toes, while a fourth toe, the hallux, directed backwards, is attached to it posteriorly by the intervention of a small accessory piece ; and in Gallinaceous Birds an osseous spur, consolidated with the posterior face of the tarso-metatarsal bone, is generally considered as a fifth toe. The number of toes varies in different tribes of birds. Thus, in the Ostrich there are only two ; in many genera there are three ; in by far the greater number, four ; and in the Gallinacea, five. But whatever the number of toes may be, the number of phalanges pe- culiar to each is remarkably constant : thus, the outermost toe always consists of five phalanges; the fourth toe invariably of four; the third as constantly of three ; the second, when it exists, has only two ; and, lastly, in the spur or innermost toe there is but a single piece. (662.) So rapidly is the progress of ossification accomplished in the skeleton of a bird, that it is only in very young animals the indi- Fig. 270. vidual bones or elements composing the cranium can be identified, as the sutures speedily become obliterated : when, however, they AVES BIRDS. 597 are examined under very favourable circumstances, as for example in the skull of a young Ostrich, it is by no means difficult to distin- guish them, and by comparing them with those of other Vertebrata, to observe the modifications they have undergone both in form and position. In the annexed figure the principal pieces, both of the cranium and face, have been indicated by the same figures as were used to point out the correspondent bones in the skulls of the Cro- codile (Jig. 246) and the Serpent (Jig. 249), so that it would be needless again to enumerate them in this place. (663.) The muscular system of the feathered tribes, as far as acti- vity and energy of motion is concerned, contrasts strikingly with that of the Vertebrata we have as yet considered ; for, with the exception of Insects, no animals in creation are comparable to Birds, either in the vigour or velocity of their movements. This perfection of muscular power, which is obviously essential to enable the bird to sustain itself in the air, and there perform the varied evolutions connected with flight, is no doubt mainly con- nected with the highly arterialized condition of the blood, and the completeness of the respiratory apparatus. Neither is it uninterest- ing to observe, that while in the Insect respiration was effected by the admission of air to every part of the system by means of tra- cheal tubes, in Birds likewise the air freely penetrates to the interior of the body, and, as we shall afterwards find, is there most exten- sively diffused. (664.) In the construction of the alimentary system there are many interesting peculiarities to invite our notice. Their mouth constitutes the apparatus whereby the prehension of food is ac- complished ; it is in no instance provided with teeth, or adapted to masticate food, but forms a beak encased in a dense, horny sheath, which, from the varieties of form that it assumes in different genera, becomes adapted to very various purposes. In the Rapacious tribes, for instance, the bill is a strong and for- midable hook, calculated to tear in pieces the animals devoured. In Granivorous Birds it is a simple forceps for picking up the seeds of vegetables. In the Snipe and the Curlew it forms a probe, whereby insects are extracted from the soft and marshy ground. In the Parrot it is partially an assistant in climbing, as well as an organ for seizing food ; and, not to mention innumerable other modifica- tions, in the Flamingo and Duck tribes it constitutes a shovel, by the aid of which alimentary matters are obtained. (665.) The sense of taste, even in these highly gifted animals, is 598 AVES BIRDS. as yet but very imperfectly developed ; and their tongue, instead of being soft and flexible, as in the Mammalia, is supported by one or two bony pieces, derived from the os hyoides (Jig. 271), and covered with a horny sheath, obviously ill adapted to gus- tation, but simply assisting in the deglutition of food. We Fig. 271. must not, therefore, be at all surprised if even in birds the tongue is convertible into various instruments assisting in the apprehension or preparation of nourishment : thus, in the Parrot it is a thumb opposable to the upper mandible, and eminently serviceable in holding and turning nuts or morsels of fruit: in the honey-eating tribes the tongue is armed at its extremity with a tuft of horny filaments, resembling a camel-hair pencil, which, be- ing plunged into the bell of a flower, sucks up the nectar from the bottom ; and in the Woodpecker it is absolutely converted into a harpoon, whereby the insect is speared in its lurking-place, and dragged into the mouth. (666.) In most birds, in consequence of the very small size of the cavity of the stomach, or gizzard as it is generally called, some other receptacle for the aliment becomes indispensable ; and accordingly various provisions have been made for lodging food in sufficient quantities in situations where it may be retained until the gizzard is ready to receive it. In birds that catch insects on the wing, this is most conveniently effected by dilating the fauces and upper part of the throat into a capacious chamber, wherein the insects as they are seized accumulate : this is remarkably the case in the Swifts. In the Pelican a very peculiar plan is adopted ; the beak is amazingly prolonged, and beneath the lower jaw is suspended a wide pouch, formed by the skin of the throat, wherein large quantities of fish may be contained and carried about. In other fishing birds the whole esophagus is extraordinarily capacious, and will hold a considerable supply ; but the most usual arrangement in birds requiring such a AVES BIHDS. 599 Fig. 272. reservoir, is the existence of a crop, or dilatation of some part of the gullet into a wide bag (tngluvict), wherein grain or other sub- stances hastily picked up may be stored preparatory to digestion. After expanding into the crop in those birds that possess this cavity, the oesophagus again contracts to its former dimensions (Jig. 21%, a) ; but just before terminating in the gizzard it again dilates to form a second but smaller cavity (6), called the proven- triculuS) or bulbus glandulosus^ in which the food undergoes further preparation. The walls of the proventiculus are thickly studded with large glandular follicles, variously disposed ; from whence a copious secretion of " gastric juice? as it is called, is poured out and mixed with the aliment. Having, therefore, undergone mace- ration in the juices of the crop, and become subsequently saturated with the gastric fluid, that constitutes so important an agent in digestion, alimentary substances are at length received into the giz- zard (c), where further preparation is necessary. (667.) The gizzard in such birds as feed upon vegetable substances is an organ possessing immense strength ; and constitutes, in fact, a crushing mill, wherein nutritive materials are bruised and triturated : its cavity is very small, and lined with a dense, coriaceous cuticu- lar stratum ; and its substance is almost entirely made up of two dense and enormously powerful masses of muscle, the fibres of which radiate from two central tendons (fig. 272, c), situated upon the opposite sides of the viscus. The action of these late- ral muscles will obviously grind and crush with great force what- ever is placed in the central ca- vity ; a process that is materially expedited by the presence of hard and angular pebbles, swallowed for the purpose, by the assistance of which the contained food is speedily comminuted. Another and much feebler set of muscles (d) bounds the cavity of the gizzard in the intervals between the great lateral masses, which, receiving the food from the proventriculus, perpetually feed 600 AVES BIRDS. this living mill, and retain the material to be ground within the in- fluence of the crushers until it is properly prepared, when other fibres, acting the part of a pylorus, allow it pass on into the duode- num (e). (668.) The intestinal canal of Birds is, as in other classes, very variable in its relative length as compared with that of the body: its calibre is pretty equal throughout, and the division into large and small intestines can scarcely be said to exist. Commencing from the pylorus, the duodenum (Jig- 273, rf, A) is always found to make a long and very characteristic loop, embracing the lobes of the pancreas (e, e), and then, after sundry convolutions, the intestine is con- tinued to its termination in the cloaca. The division between the large and small intestines is indicated by the presence of one, or more generally two, csecal appendages, which communicate with the cavity of the gut at no great distance from its cloacal extremity. (669.) In Birds, the auxiliary secretions subservient to the digestive process are the salivary, the gastric, the hepatic, and the pancreatic. The salivary apparatus varies much in structure and disposition in different tribes. In its simplest form it consists of distinct secerning follicles, placed immediately beneath the mucous membrane of the mouth, into which the secretion is poured by numerous orifices. In the Gallinaceous Birds the glands assume a conglomerate cha- racter. In the Turkey there are two pairs :* the first pair forms a cone, having its apex directed towards the extremity of the beak ; and the two glands of the opposite sides touch each other along the mesial line through almost their entire length, filling up anteriorly the angle of the lower jaw. These glands are situated immediately beneath the skin, but in front they touch the mucous membrane of the rnouth ; and their secretion is poured into the buccal cavity by several orifices. The second pair of glands is smaller, of an elongated form, and is placed above the posterior third of the former, this is immediately in contact with the mucous lining of the mouth. In the Woodpeckers the glands that secrete the fluid whereby the tongue is lubricated are of very considerable size. They pass further back than the angle of the lower jaw, extending even to beneath the occiput ; and their secretion, which is viscid and tenacious, enters the mouth by a single orifice situated under the point of the tongue. * Cuvier, Leyons d'Anat. Comp. torn. iii. p. 221. AVES BIRDS. 601 In the generality of birds, however, there is only one pair of salivary glands; and these, in many cases, seem to be united into a single mass, separated posteriorly into two lobes, and situated beneath the palatine membrane, behind the angle of the rami of the lower jaw. From these glands a thick, white, and viscid fluid is poured into the mouth through numerous orifices, principally disposed along the mesial line, which separates the two glands. (670.) We have already spoken of the gastric glands which densely stud the coats of the proventriculus, and furnish the "gastric juice ;" and therefore pass on to notice the other subsidiary chylo- poietic viscera, namely, the liver, the pancreas, and the spleen. The liver is a viscus of considerable magnitude, consisting of two principal lobes, and firmly suspended in situ by broad liga- ments and membranous processes. The vena portse, supplying that venous blood from which the bile is elaborated, is formed by vessels derived from numerous sources, receiving not only the veins of the stomach, spleen, and intestines, as in Mammalia, but likewise Fig. 273. the renal and sacral veins ; another proof, if any were wanting, that no arrange- ment by which the decarbon- ization of the blood can be facilitated has been omitted in the organ- ization of the class before us. Thehepatic ar- teries and the hepatic veins present no- thing remark- able in their disposition, but the course of the bile from the liver into the intestine merits our 602 AVES BIRDS. notice. Two sets of ducts are provided for this purpose : the first (Jig. 73, i) carries the bile directly from the liver into the gall-bladder (g), from which another duct conveys the bilious fluid into the duodenum ; but the second set of bile-vessels conducts the secretion of the liver at once into the intestine by a wide canal (o), that has no communication whatever with the gall- bladder, there is, therefore, no arrangement like that of the " ductus communis choledochus" of Mammals : if the bile is want- ed immediately, it passes at once into the intestine through the duct o ; but, if digestion is not going on, it is conveyed into the gall-bladder through the duct i, to be there retained until needed. The pancreas (Jig. 218, e, e) is a conglomerate gland of con- siderable size, situated in the elongated loop formed by the duo- denum : it generally consists of two portions more or less inti- mately connected, and from each portion an excretory duct (n) is given off; these two ducts terminate separately in the intestine, in the immediate vicinity of the openings of the biliary canals. In some birds even three pancreatic ducts are met with, as is the case in the common fowl ; but under such circumstances the third duct, instead of opening into the intestine at the same point as the other two, issues from the opposite extremity of the pancreas, and enters the middle of the duodenum at the place where the gut turns upon itself. The spleen (Jig. 273, f) is of very small size in all birds ; it is situated near the anterior extremity of the pancreas, and is loosely connected to the side of the proventriculus (b). The distribu- tion of its vessels, and its general structure, is the same as in Mammalia. The lymphatic system is well developed, and the course of the lymphatic vessels has been investigated with great care by various anatomists. The vessels themselves are thin, and have but few valves ; they principally accompany the larger blood-vessels from all parts of the body to the aorta, around which they form a plexus, and ultimately join to give rise to two principal trunks or thoracic ducts: these terminate severally in the right and left jugular veins, and into these vessels the greater proportion of the lymph and chyle absorbed is of course poured, to be mixed with the circulating blood. (671.) Before describing the circulatory apparatus of birds, it will be advisable in the next place to consider the nature and disposition of their organs of respiration ; which, from what has been already AVES BIRDS. 603 stated concerning the heat and purity of the blood in these crea- tures, we are prepared to find presenting the highest possible con- dition of developement. Birds, in fact, breathe not only with their lungs, but the vital element penetrates every part of the interior of their bodies, bathing the surfaces of their viscera and entering the very cavities of their bones ; so that the blood is most extensively subjected to its influence. The lungs, in fact, are no longer closed bags as those of Reptiles are, but rather resemble spongy masses of extreme vascularity, firmly bound down in contact with the dorsal aspect of the thorax ; their poste- rior surface being fixed to the ribs on each side of the vertebral column, and entering deeply into the intercostal spaces. Such lungs are obviously incapable of alternate dilatation and contrac- tion, so that inspiration and expiration must be provided for by a mechanism specially adapted to the emergency. From an exa- 604 AVES BIRDS. mination of Jig. 274, the arrangement adopted will easily be under- stood : the bronchi derived from the bifurcated inferior extremity of the trachea plunge into the anterior face of the lungs (c, c), and by innumerable canals distribute air throughout their spungoid substance ; but the main trunks of the bronchial tubes, passing right through the pulmonary organs, open by wide mouths, repre- sented in the figure, into the cavity of the thorax, into which the air likewise freely penetrates. The whole thoracico-abdominal cavity is moreover divided by septa of serous membrane into numerous intercommunicating cells, all of which are freely per- meated by the atmospheric fluid, which in most instances is admitted into the very bones themselves, and even penetrates to the interspaces between the muscles of the neck and limbs ; thus, in some birds of powerful flight, gaining free access to almost every part of the system. The mechanism by which the air is drawn into, and then expelled from, this extended series of respiratory cells, is sufficiently simple ; the whole being accomplished by the movements of the expanded sternum, assisted slightly by the abdominal muscles. The descent of the sternum from the vertebral column necessarily enlarges the capacity of the chest, and, acting like a great bellows, sucks in air through the trachea, which not only fills all the spongy substance of the lungs, but penetrates to all parts whereunto air is admitted; while the ascent of the sternum, and consequent contraction of the thoracico-abdominal space, alternately effects its expulsion. The results obtained by this unusual arrangement are of great importance in the economy of the feathered races. In the first place, the perfect oxygenization of the blood is abundantly secured. Secondly, from the high temperature of the blood, the air drawn in becomes greatly rarified, and thus materially diminishes the specific gravity of the bird. Thirdly, from the inflation of the whole body, the muscles, more especially those of flight, act with better lever- age and firmer purchase ; so that their efforts are materially favoured. And, lastly, it is owing to the capacity of the air-cells that the Sing- ing Birds are enabled to prolong their notes to that extent which renders them pre-eminent among the vocalists of creation. (67&.) In connection, therefore, with the respiratory system of the feathered races, it will be advisable, in the next place, to con- sider the construction of the air passages, whereby the atmospheric fluid passes into and out of the body; and more especially of the organs of voice connected with them. AVES BIRDS. 605 A. The trachea is of very great proportionate length in correspond- ence with the elongated neck ; commencing at the root of the tongue, and extending into the thoracic cavity, where it divides into two bronchial tubes, one appropriated to each lung (Jig. 274, /, /). The trachea of birds is composed of cartilaginous rings, which are very generally ossified ; each ring, with the excep- tion of two or three immediately beneath the upper larynx, forming a complete circle (Jig. 275, A) surrounding the tracheal tube : these rings are enclosed between the soft membranes of the trachea, and thus keep the air-passages constantly permeable to the atmosphere. In many birds, especially among the web- footed tribes, the trachea suddenly dilates into wide chambers, or cavities of different forms and dimensions ; a circum- Fig. 275. stance the object of which has not as yet been satisfactorily explained: and, what is still more inexplica- ble, in some genera, and those too with the longest necks, as for ex- ample the Wild Swan, and many of the Wading Birds, the lower part of the trachea is lengthened out and variously contorted before it terminates in the chest. This long trachea is provided with mus- cles whereby the rings may be approximated, and thus the length of the tube is considerably modified : these muscles (fig. 274, A, B, h) arise from the sternum, and sometimes also from the furcula, and are continued along the sides of the windpipe throughout its whole length. The upper larynx, or rima glottidis, is in birds but of secondary importance in the production of vocal sounds ; it is a simple fissure bounded by two osseous pieces (Jig. 275, A, B,/) corresponding with the aryienoid cartilages of Mammalia : these, however, in the Bird are not connected with chorda vacates ; but simply, as they are se- parated or approximated, open or close the fissure of the glottis. When, therefore, we compare the framework of this organ with the cartilaginous pieces found in the larynx of Mammalia, considerable difference is perceptible, insomuch that it is not easy positively to recognise the analogous portions, more especially as in the Bird the cartilages are more or less completely ossified. If the broad an- terior plate (Jig. 275, b) be considered as the thyroid cartilage, we 606 AVES BIRDS. must suppose the cricoid to be represented by three distinct ossicles, two of which (c, c) are lateral, while the third or central portion (e) supports the arytenoid bones (/,/), which are moveably arti- culated with its anterior margin. The arytenoid bones themselves are of an elongated form, and each presents a long process (g, g) for the insertion of the muscles that act upon them. These arytenoid bones are moved by two pairs of muscles ; the superficial pair (thyro-arytenoidei, Jig. 276, B) serving to pull asunder, while the more deeply seated (constrictores glottidis^Jig. 276, A) bring together the lips of the glottis. (673.) It is the lower larynx, situated at the opposite extremity of the trachea, at the point where that tube gives off the bronchi, that the real vocal apparatus of birds is situated ; and in the more perfect Singing Birds a very important set of muscles is appropriated to perform those delicate movements that regulate the condition of the air-passages at this part, and thus give rise to all the varieties of tone of which the voice is ca- pable- In the Insessorial Birds, by far the most accomplished song- sters, five pairs of muscles are connected with the inferior larynx ; and so disposed as to influence both the diameter and length of the bronchial tubes (Jig. 274, A, B, n, o, z 9 s, h). In the Parrots, three pairs only are met with ;* some of the Natatores have two ; other natatorial birds, as well as the Rasores and Grallatores, only one ; and in a few, as the King of the Vultures and the Condor, the vocal muscles are quite deficient. (674.) Not only is the respiration of these highly gifted Vertebrata thus abundantly provided for, but, as an immediate consequence of the necessity for supplying the system with pure and highly oxygenized blood, the heart, hitherto but imperfectly divided, becomes now separated into two distinct sets of cavities, each composed of an au- ricle and of a strong ventricular chamber. The right side of the heart receives the vitiated blood from all parts of the system, * Vide Yarrell on the Organs of Voice in Birds. Linn. Trans, vol. xvi. AVES BIRDS. 607 which is poured into the corresponding auricle by three large veins, viz. one inferior and two superior vena? cavce. The contraction of this auricle drives the blood into the right ventricle ; theauriculo- ventricular opening being guarded by a broad fleshy valve, formed by the muscular substance of the heart itself; and hence the venous blood is forced through all the ramifications of the pulmonary arteries. The aerated blood is then returned from the lungs by two veins, which pour it into the left auricle ; and the left ventricle, now en- tirely appropriated to the systemic circulation, diffuses it through the body ; thus all mixture of the venous and arterial fluids being prevented, the system is supplied by the left side of the heart with pure and highly vitalized blood. (675.) In the nervous system of Birds there is a very perceptible improvement when compared with that of Reptiles, more especially in the increased proportional developement of the cerebral hemi- spheres : still, however, there are no convolutions seen upon the surface of the cerebrum ; neither are those extensive communica- tions between the lateral halves as yet developed, which in the higher Mammalia assume such size and importance : the corpus callosum and fornix are both wanting, a simple commissure being still sufficient. Neither has the cerebellum in these animals as- sumed its complete developement, presenting only the central por- tion ; so that the pons Varolii^ or the great commissure, which in Man unites the lateral cerebellic lobes, is of course deficient. The olfactory and optic lobes are even here recognisable as distinct ele- ments of the cerebral mass, and the origins of the nerves strictly conform to the arrangement already described in the brain of Rep- tiles. The rest of the cerebro-spinal axis presents no peculiarity worthy of special notice ; and the general distribution of the cere- bral and spinal nerves is so similar in all the Vertebrata, that it would be useless again to describe them in this place. The sympathetic system in Birds is well developed, and its ar- rangement differs in no essential particular from what is seen in the human body ; the situation of the cervical ganglia is, however, peculiar, inasmuch as they are lodged in the bony canal formed by the transverse processes of the vertebrae of the neck for the recep- tion of the vertebral artery, and are thus securely protected in spite of the unusual length and slenderness which the neck not unfre- quently exhibits. But if in the general arrangement of the nervous system of the feathered races there is little to arrest our notice, we shall find 608 AVES BIRDS. in the construction of the organs of their senses many circumstances of considerable interest to the physiological reader ; and, conse- quently, these will require a more extended description. (676.) The sense of touch must obviously be extremely imperfect in these animals : their body, enveloped in feathers, can be little sensible to impressions produced by the contact of external objects ; and their limbs, covered as they are with plumes, or cased in horny scales, are but little adapted to exercise the sense in question. The beak alone offers itself as calculated to be a tactile instrument ; but even this, enclosed as it is in the generality of birds by a dense corneous case, must be very inefficient in investigating the outward surfaces of substances : nevertheless, in some tribes the beak is un- doubtedly extremely sensible, and is used to search for food in marshy soils, or to find it in the mud at the bottom of shallow waters ; this is the case, for instance, in many of the long-billed Wading Birds, and also in the flat-billed aquatic families, such as the Goose and Swan ; in these, in fact, the covering of the beak is comparatively soft, and the nerves that supply it, derived from the fifth pair, are of very considerable size. (677-) Taste is evidently one of the last indulgences granted, as we advance from the lower to the more highly gifted races of the animal creation ; and even in birds it is only necessary to inspect the structure of the tongue in order to be convinced that they can derive but small enjoyment from this source. The skin of the tongue in these creatures is totally devoid of gustatory papillae, and frequently, indeed, enveloped in a horny sheath; so that, if the sense of taste exists at all, it must be, to the last degree, limited and obtuse. (678.) In return, however, for the imperfection of the above senses, the olfactory apparatus in this class of animals begins to assume far greater importance than in the cold-blooded Vertebrata ; Fig. 277. ." : -- ;; AVES BIRDS. (j()9 and the nasal cavity indicates, by its extent, that it is now well adapted to investigate the odorous properties of the air taken in for respiration. The septum narium completely divides the nose into two lateral chambers of considerable extent, which individually communicate with the pharynx (fig- 277, c) ; and, upon the outer wall of each compartment, three convoluted laminae, covered with a most delicate Schneiderian membrane, re- present the turbinated bones of Mammalia, and increase the olfactory surface. Of these, the middle turbinated bone (jig* 277, a) is the largest ; but the superior appears to be the most important, as it is upon this that the olfactory nerve is principally distributed, inso- much that Scarpa considered that the comparative powers of smell possessed by different birds might be estimated by the develope- ment of this portion of the olfactory organ. The olfactory nerves (Jig. 277, i), as in Reptiles, still enter the nose without dividing, so that there is no cribriform plate to the ethmoid bone. The nostrils are simple apertures, perforating some part of the horny beak covering the upper mandible, and are never provided with moveable cartilages or muscles, as those of Mammalia will be found to be. (679.) The eye of a Bird is an optical instrument of such ad- mirable construction, that, did not the nature of this work compel us to adopt the strictest brevity in our descriptions, it might well tempt us to indulge in lengthened details relative to the adaptation and uses of its various parts. If we contrast the Bird with the Reptile, or more especially with the Fish, and consider the totally different circumstances under which these animals exercise the sense of vision, we might well expect extraordinary modifications in the structure of their organs of sight. The Fish, immersed in a dense medium, can see but to a very limited distance around it ; and the sphericity of the crystalline lens, with the consequent contracted antero-posterior diameter of the eye-ball, at once testifies how small is the sphere of vision commanded by the finny tribes. The Bird, on the contrary, dwelling in the thin air, and not unfrequently soar- ing into regions where that air is still further rarified, must sur- vey an horizon even more extensive than that enjoyed by the terrestrial Mammal ; while, from the rapid movements of the feathered races, it becomes absolutely requisite that the focus of the eye shall continually vary between the extremes of long and short sighted vision. The birds of prey, as they fan the air at an altitude which places them almost beyond the reach of human sight, or sail 610 AVES BIRDS. in broad gyrations through the sky, are scanning from that height the surface of the ground, and looking out for mice or other little animals on which to feed : but when the prey is seen, and the bird, shooting down with the rapidity of a thunderbolt, stoops upon the quarry, it must obviously be indispensable that it should see with equal clearness and distinctness when close to its victim, as it did when far remote; and to enable it to do this special provisions have been made in the structure of the eye-ball. A glance at figure 279, exhibiting a section of the eye of an Owl, will show the anatomist that in its general composition the organ is similar to that of Man. The sclerotic and the choroid tunics present the same arrangement, the transparent hu- mours of the eye occupy the same relative positions, and the iris and ciliary folds exist as in the human subject. Descending from generalities, however, he will find many points in the organization of a bmTs eye eminently deserving separate examination, and it is to these we would specially invite his notice. First, the shape of the eye-ball is peculiar : it is not spherical, as in man, nor flattened anteriorly, as in fishes and aquatic reptiles ; but, on the contrary, the cornea is rendered extremely prominent, and the antero-pos- terior axis of the eye considerably lengthened. This is remark- ably exemplified in the Owl ; in which bird, as Dr. Macartney* pointed out, such is the disproportion between the anterior and posterior spheres of the eye, that the axis of the anterior portion is twice as great as that of the other. The obvious consequence of this figure of the globe of the eye is to allow room for a greater proportion of aqueous fluid, and for the removal of the crystalline lens from the seat of sensation, and thus produce a greater con- vergence of the rays of light, by which the animal is enabled to discern the objects placed near it, and to see with a weaker light ; and hence Owls, which require this sort of vision so much, possess the structure fitted to effect it in so remarkable a degree. (680.) But it is evident, that, in order to retain this conical shape of the eye-ball, some further mechanical arrangements are ne- cessary, which in the spherical form of the human eye are not requi- site. In Fishes, where the eye-ball is constructed upon entirely opposite principles, being compressed anteriorly, cartilaginous sup- ports are found imbedded in the sclerotic tunic, which, in some cases, is absolutely ossified into a bony cup. In many Reptiles the same end is obtained by placing a circle of bony plates around * Rees's Cyclopaedia, art. Birds. AVES BIRDS. 611 the cornea ; and this latter plan is again adopted in Birds, to main- tain their eyes in a shape precisely the converse of the former. In the Owls these ossicles are F*x 278. most largely developed; in such birds they form a broad zone (Jig. 278), extending from the margin of the cor- nea, embracing the anterior conical portion of the eye, and imbedded between two fi- brous layers of the sclerotic. The figure which is thus given to the eye, from the in- creased space obtained, is evi- dently calculated to allow the humours, forming the refracting media whereby the rays of light are brought to a focus upon the retina, to become materially changed in shape ; and both the convexity of the cornea, and the position of the lens, may thus be altered so as to adjust them in cor- respondence with the distance at which an object is viewed. The cornea is rendered more convex, and the shape of the aqueous humour consequently adapted to examine objects close at hand, by the simple action of the muscles that move the eye-ball ; for these, seeing that the edges of the pieces composing the bony circle overlap each other so as to be slightly moveable, as they compress the globe of the eye, cause the protrusion of the aqueous humour, and the cornea becomes prominent ; or, if the bird surveys things that are remote, the cornea recedes, and becomes flattened, an effect caused by the recession of the aqueous humour, and, as some authors assert,* by muscular fibres disposed around the circumference of the cornea, and attached to its inner layer, which draw back the cornea in a manner analogous to the action of the muscles of the diaphragm upon its tendinous centre. But the most beautiful piece of mechanism, if we may be par- doned the expression, met with in the eye of a bird, is destined to regulate the focal distance between the crystalline lens and the sentient surface of the retina, in order to insure the clearest pos- sible delineation either of near or distant objects. The provision for this purpose is peculiar to the class under our notice ; and con- sists of a vascular organ, called the marsupium, or pecten, which is * Vide Cyclop, of Anat. and Phys. p. 304. 2 R 2 612 AVES BIRDS. lodged in the posterior part of the vitreous humour (Jig- 279 a). This organ is composed of folds of a membrane resembling the choroid coat of the eye, and, Fig. 279. being in like manner covered with pigment, might easily be mistaken for a process derived from that tunic ; with which, in fact, it has no connec- tion, being attached to the optic nerve just at the point where it ex- pands into the retina. Its substance seems to be made up of erectile tis- sue, and it is most copiously supplied with blood derived from an arterial plexus formed by the arteria centralis retinae ;* so that there is little doubt that, being like the iris endowed with an involuntary power of dilatation and contraction, as it enlarges from the injection of blood, it distends the chamber of the vitreous humour, and pushes forward the lens ; while, as it again collapses, the crystalline is allowed to approach nearer to the retina, and thus the focus of the eye is adjusted upon the same principle as that of a telescope. Four recti and two obliqui muscles preside over the movements of the eye-ball ; but, as in the Reptilia, the superior oblique arises from the anterior part of the orbit, as well as the obliquus inferior, and its tendon is not reflected over a trochlea. (681.) Birds have three eye-lids : an upper and a lower, resem- bling those of mammalia ; and a third, which, when unemployed, is concealed in the inner canthus of the eye, but can be drawn down vertically by muscles specially appropriated to its motions, so as to sweep over the entire cornea, which it then covers like a curtain. The upper and the lower eye-lids differ but little in their struc- ture from those of Man ; nevertheless, a few trivial circumstances are worthy of the notice of the student. In the first place, there are seldom any eye-lashes attached to the palpebral margins ; and, secondly, the lower eye-lid is the most moveable of the two, and not only contains a distinct tarsal cartilage, but is provided with a special depressor muscle, which arises from the bottom of the orbit like the levator palpebra superioris of the human subject : the ele- vator of the upper eye-lid, and orbicularis palpebrarum, are like- wise well developed. * Vide Barkow, in Meckel's Archiven, Band xii". AVES BIRDS. 618 The third eye-lid, or nictitating membrane, is represented in fig. 280, A, e ; the upper and the lower eye-lids having been divided through the middle, and turned back to display it: it is necessarily, to a certain extent, transparent, for birds sometimes look through it ; as for instance, when the eagle looks at the sun :* it is, therefore, of a membranous texture ; and a Fig. 280. most admirable and peculiar muscular apparatus is given, by which its movements are effected. This is placed at the back of the eye-ball, and may easily be displayed by turning aside the recti and obliqui muscles, as in Jig. 280, B. Two muscles are then perceived arising from the globe of the eye, taking their origin from the outside of the sclerotic coat : one of these (c), named the quad- ratus membrane nictitantis, arising from near the upper aspect of the eye, descends towards the optic nerve ; but instead of being inserted into anything, as muscles usually are, it terminates in a most remarkable manner, ending in a tendi- nous sheath or pully, through which the tendon of the next muscle passes as it winds around the optic nerve. The second muscle (d) 9 called the pyramidalis memb. nictitantis, arises from the inner aspect of the eye-ball ; and its fibres are collected into a long, slender tendon, which, as it turns round the optic nerve, passes through the tendinous sheath formed by the quadratus, as a rope through a pully, and then is continued in a cellular sheath formed by the sclerotic, underneath the eye, to the lower angle of the third eye-lid, into which it is inserted. The reader will at once perceive how beautifully these two muscles, acting simultaneously, cause the nictitating membrane to sweep over the cornea, which returns again into the inner canthus of the eye by its own elasticity. (682.) Being thus provided with moveable eye-lids, a lacrymal * Cuv. Lemons, d'Anat. Comp. torn. ii. p. 431. 614 AVES BIRDS. apparatus is, of course, indispensable ; and, accordingly, birds are supplied with two distinct glands, one being appropriated to the secretion of tears, while the other furnishes a lubricating fluid, appa- rently destined to facilitate the movements of the membrana nic- titans. The lacrymal gland is situated, as in Man, at the outer angle of the eye, and its duct pours the lacrymal secretion upon the eye- ball near the external canthus. The lacrymal canal, whereby the tears, after moistening the cornea, are discharged into the nose, commences by two orifices (j^g*. 280, A, c) situated just behind the internal commissure of the eye-lids ; and is continued, into the nasal cavity, where it terminates in front of the representative of the middle turbinated bone. The second gland, the glandula Harden, seems to supply the place of the Meibomian glands of the human eye-lids : it forms a considerable glandular mass, situated behind the conjunctiva at the nasal angle of the eye-lids ; and through its excretory duct, which opens behind the nictitating membrane, the lubricating secre- tion that it furnishes is poured out. (683.) Besides the secreting organs above described, a third very large gland is found, generally lodged in a depression beneath the vault of the orbit, although in some genera it is situated external to that cavity : the secretion of this gland is, however, poured into the nose by one or more ducts, and thus serves copi- ously to moisten the Schneiderian membrane. (684.) The auditory apparatus of a Bird is almost precisely si- milar in its structure to that of one of the more perfect Reptiles, such as the Crocodile. There is still no external ear, or osseous canal worthy of being called an external meatus, yet in a few rare instances, such as the Bustard, the feathers around the ear are so disposed as to collect faint impressions of sound ; and in the Owls, besides possessing a broad opercular flap, that forms a kind of external ear, there are sinuosities, external to the membrana tym- pani, which resemble, not very distantly, those found in the ear of Man. Entering into the composition of the organ of hearing in the class before us, we have the membrana tympani (Jig. 281, a), and tympanic cavity, from which a wide Eustachian tube (d) leads to the posterior nares. The labyrinth presents the vestibule (c), the semi- circular canals (b), and the rudimentary cochlea (e); all of which so exactly correspond in structure with what has already been de- AVES BIRDS. 615 scribed when speaking of the ear of Reptiles ( 639, 640), as to render repetition needless. A single trumpet-shaped bone, the representative of the stapes, fig. 281. communicates immediately between the mem bran a tym- pani and iliefenestra ovalis ; but two or three minute car- tilaginous appendages, con- nected with the membranous drum of the ear, are regard- ed as being the rudiments of the malleus, incus, and os orbiculare met with in the next class. (685.) The kidneys in the Bird (Jig. 282, e, e, e) are very large : they are lodged in deep depressions, situated on each side of the spine in the lumbar and pelvic regions ; their posterior aspects being moulded into all the cavities formed by the bones in that situation. In their essential structure each kidney is made up of innumerable microscopic flexuous tubes ; which, joining again and again into larger and still larger trunks, ultimately terminate in the ureter, without the interposition of any infundibular cavity analogous to the pelvis of the human kidney. From the manner in which the kidneys are imbedded, the ureters are necessarily derived from their anterior aspect. After receiving all the terminations of the urinary tubules, they pass behind the rectum to the cloaca, into which they discharge the urinary secretion. The cloaca, therefore, receives the terminations of the rectum, of the ureters, and also, as we shall immediately see, of the sexual pas- sages : no urinary bladder is as yet developed, nevertheless vestiges of its appearance begin to become visible. The cloaca is, in fact, in some birds divided into two compartments, distinct both in their appearance and in their office ; they are, moreover, separated by a constriction, more or less well defined in different species. It is into one of these compartments that the rectum opens, while the other (Jig. 282, TO, m) contains the orifices of the ureters and genera- tive canals ; the latter is, therefore, generally distinguished by the name of urethro-sexual portion of the cloaca, and is in truth a remnant of the allantois, and a rudiment of a bladder for the ac- cumulation of the urine. (686.) An unctuous secretion, peculiar to the class under 616 AVES BIRDS. Fi*. 282. consideration, has been provided for the purpose of oiling the feathers ; and in water birds the fluid alluded to becomes of very great importance to their welfare, as it causes their plumy covering to repel moisture so efficiently that it is never wet. The gland given for this purpose is called the " uropygium" and is situated upon the back of the os coccygis ; from this source the bird dis- tributes the oily material thus afforded to all parts of its plumage. (687.) The male generative organs in Birds are fully as simple in their structure as those of the Reptilia. The testes are two oval bodies (/g. 282, g), in- variably situated in the lumbar re- gion, lying upon the anterior por- tion of the kid- ney. In their intimate structure they consist of contorted and ex- tremely slender tubes, wherein the semen is elabo- rated, contained in a strong cap- sule. The sperm- secreting tubules of each testis terminate in a slightly flexuous vas defer ens (A, i), that opens into the cloaca by a simple orifice (m, m). In most birds it can scarcely be said that a penis exists at all, two simple rudimentary vascular papillae at the termination of the vasa deferentia constituting the entire intromittent apparatus ; so that copulation between the male AVES BIRDS. 617 and female must, in the generality of species, be effected by a sim- ple juxta-position of the sexual orifices : nevertheless, in the web- footed tribes, which copulate in the water, and in the Ostrich, the penis of the male is much more perfectly organized, as will be seen by the following description extracted from Cuvier.* The structure of the penis is far from being the same in all birds provided with such an organ : it offers, in fact, two types extremely different from each other; whereof the Ostrich and Drake may be taken as examples. The penis of the Ostrich is of a size pro- portioned to that of the bird. Its form is conical ; and a deep, narrow groove runs along its upper surface from the base to the point. The vasa deferentia open into the cloaca opposite to the commencement of the groove; so that the semen flows directly into this furrow. This penis consists, first, of two solid conical bodies, entirely composed of fibrous substance, supported at their base within the sphincter of the cloaca upon its inferior wall. The fibrous cones are placed side by side, but not confounded together ; and the right is smaller than the left, no doubt to allow this organ, which never becomes soft as that of quadrupeds, to be more easily folded back into the cloaca. Secondly, of a fibro- vascular body, which constitutes the bulk of the inferior aspect of the penis, and is con- tinued to its extremity. Thirdly, of a cellular portion, capable of erection, placed beneath the skin lining the urethral groove. This last is doubtless the first appearance of the corpus spongiosum, which in Mammifers completely encloses the canal of the urethra; while the two others represent the corpus cavernosum. The whole apparatus, when not in use, is drawn into the cloaca by two pairs of retractor muscles. (688.) In Geese, Ducks, and many wading birds, such as the Stork, the structure of the male intromittent organ is totally dif- ferent. When in a state of repose, it is lodged in a pouch under the extremity of the rectum, and curved, so as to describe three parts of a circle. When the penis is opened in this condition, it is found to be made up of two portions, each composing half of its substance. The parietes of one half are thick, elastic, and slightly glandular. The other presents internally a great number of trans- verse grooves and folds. This latter portion during erection unrols itself outwards like a glove ; and, at the same time, the half first mentioned introducing itself into the hollow cylinder formed by the * Lejons d'Anat. Comp. torn. v. p. 108. 618 AVES BIRDS. Fig. 283. second, fills it up, and constitutes the firmest part of the organ. Most of the grooves and folds visible during non-erection become much less apparent when the penis is protruded ; and their direc- tion being oblique, they prevent it from stretching out in a straight line, but oblige it to assume a cork-screw appearance. A deep groove runs along the whole length of this singular organ ; and it is into the commencement of this groove that the vasa deferentia pour the seminal secretion. (689.) The females of species whose males possess a large penis, are provided with a rudimentary clitoris of similar con- struction. (690.) The female ge- nerative system in the fea- thered tribes offers a remark- able exception to what we have as yet seen in the ver- tebrate Ovipara. Instead of being symmetrically develop- ed upon the two sides of the body, the right oviduct, and most frequently the corre- sponding ovarium, remain permanently atrophied ; and, although they do exist in a rudimentary condition, they never arrive at such dimen- sions as to allow them to assist in the reproductive process. (691.) The fertile ova- rium presents in all essen- tial circumstances the same organization as those of the Reptilia ; and is in the same Avay attached by folds of peritonaeum in the vicinity of the spine (fig. 283, /). The contained ova are found in all stages of maturity ; and, being con- nected together by narrow pedicles, the viscus assumes a distinctly racemose appearance. The oviduct (d, e) commences by a wide funnel-shaped aperture, and soon assumes the appearance of a convoluted intestine. Its AVES BIRDS. 619 lining membrane varies in texture in different parts : near the in- fundibular orifice it is thin and smooth ; further down it becomes thicker and corrugated ; and at last, near the termination of the canal, where the egg is completed by the calcification of its out- ward covering (g), it presents a villose texture. The oviduct ultimately opens into the corresponding side of the urethro-sexual compartment of the cloaca. (692.) We must, in the next place, proceed to describe, with as much brevity as is consistent with the importance of the subject, first, the nidus, or ovisac, in which the rudiment of the future being is produced; secondly, the structure of the germ (ovulum) when it escapes from the ovary ; thirdly, the additions made to the ovulum as it passes through the oviduct ; and, lastly, the pheno- mena that take place during the developement of the embryo by incubation. (693.) If the ovarium of a bird be examined whilst in functional activity, such of the pedunculated ovisacs (calyces, Jig. 283, f) as have within them ovula ripe for exclusion, will be found to consist of two membranes.* Of these, the exterior is very vascular, and is surrounded with a pale zon& (stigma) , occupying the centre of the calyx. The lining membrane of the ovisac, on the contrary, is thin and pellucid, but studded with minute corpuscles, which are probably glandular, or perhaps little plexuses of vessels. Within this ovisac the basis of the future egg (ovulum) is formed. (694.) The ovulum produced in the ovisac, when mature, is made up of the following parts. The bulk of it consists of an orange-coloured oleaginous material, enclosed in a most delicate and pellucid membrane (membrana vitelli ) ; this is the yolk of the future egg. Upon the surface of the yolk there is visible a slightly elevated opaque spot (cicatricula), wherein is lodged the repro- ductive germ : this last, which is apparently the most important part of the ovulum, is a minute pellucid globule ; and has been named after its discoverer the " vesicle of Purkinje" or the germinal vesicle. (695.) The phenomena attending conception are therefore sim- ply these : The membranes of the ovisac are gradually thinned by absorption ; and, being embraced and squeezed by the infundibular commencement of the oviduct, the transparent zone or stigma gives way, allowing the ovulum, covered only by its membrana vitelli, to escape into the oviductus. The rent ovisac is soon removed by * Vide Purkinje, Symbolae ad ovi Avium historiam ante incubationera. 4lo. Lipsise,1830. 620 AVES BIRDS. absorption ; and the ovulum, with its cicatricula, is left to be clothed with other investments : but the germinal vesicle is now no longer to be seen ; its delicate covering having been, as Ptirkinje supposes, ruptured by the violence to which it has been subjected. (696.) It is during the passage of the ovulum through the canal of the oviduct that it becomes enclosed in the other parts entering into the composition of the egg : these are, the albumen, the chalazas, the membrana putamim's, and the calcareous shell. The albumen, or glairy fluid forming the white of the egg, is secreted by the mucous membrane that lines the commencement of the oviduct ; and being laid on, layer upon layer, gradually coats the membrana vitelli. Some of the albumen meanwhile becomes inspissated so as to form an almost invisible membrane, the chalaza, which being twisted by the revolutions of the yolk, as it is pushed forward in the oviduct, is gathered into two delicate and spiral cords (Jig- 285, c, c), whereby the yolk is retained in situ after the egg is completed. The ovulum, now covered with a thick coating of albumen, and furnished with the chalaza, at length approaches the terminal extremity of the oviduct, where a more tenacious material is poured out : it is here that the whole becomes encased in a dense mem- brane resembling very thin parchment, called " membrana puta- minis ;" and ultimately, on arriving in the last dilated portion of the canal (Jig. 283, g), the lining membrane of which secretes cre- taceous matter, the shell is formed by the gradual accumulation of extremely minute, polygonal, calcareous particles, so disposed upon the surface of the egg that imperceptible interstices are left between them for the purpose of transpiration. Thus, as the oviduct is traced from its infundibular commence- ment, the different portions of it are seen successively to discharge the following functions : the orifice of the infundibulum receives the ovulum from the ovisac ; the succeeding portion, extending nearly three fourths of its entire length, secretes the albumen and the chalazas ; it in the next tract furnishes the membrana putami- nis ; and in the last place, the shell ; after which, the complete egg is expelled through the cloaca. (697.) The anatomy of the egg prior to the commencement of incubation is therefore sufficiently simple. Immediately beneath the shell is the membrana putaminis ; which, however, we must here remark consists of two layers ; and at the larger end of the egg these layers separate, leaving a space (Jig. 284, a, 6), called AVKS BIRDS. 621 the vesicula aeris , so formed is filled with air containing an unusual propor- tion of oxygen, des- tined to serve for the respiration of the future embryo. En- closed in the mem- brana putaminis the student next finds the albumen and chalazas (Jig. 285, c) ; and lastly, the we may further notice, that the chamber Fig. 284. the mem- yolk, enclosed in its proper membrane (Jig. 284, c), brana vitelli. (698.) We must, however, dwell a little more at length upon the composition of the yolk. The cicatricula (Jig. 284, g) is made up of a thin membrane, which originally enclosed the vesicle of Purkinje (/); but this latter, although introduced into the diagram for the purpose of illustration, is in reality, as we have already seen, no longer visible ; and we must now change the word cicatricula for that of blastoderm, which may be presumed to consist of the original cicatricula and the ruptured vesicle of Purkinje : it is from this blastoderm, or germinal membrane, as it is sometimes called, that the future being is developed. Immediately over the blastoderm the membrana vitelli is slightly thickened (Jig. 284, F^.285. h) ; and beneath it is a canal (e), which leads to a chamber (d) placed in the centre of the yolk; this cavity is filled with a whitish granular substance. (699.) Such is the composition of the complete egg of a Fowl ; and with the exception of trifling circumstances, hereafter to be noticed, of that of vertebrate animals in general. The develppement of the embryo is accomplished in the following manner. 622 AVES BIRDS. Fig. 286. No sooner has incubation* commenced, than the blastoderm be- comes distinctly separate from the yolk and the membrana vitelli; and, as it begins to spread, assumes the form of a central pellucid spot, surrounded by a broad dark ring (Jig. 285, g, h) : it at the same time becomes thickened and prominent, and is soon separable into three layers; of these, the exterior (Jig. 286, c) is a serous layer ; the internal, or that next the yolk (A), a mucous layer ; and between the two is situated a vascular layer, B, in which vessels soon become apparent. These three layers are of the utmost im- portance ; as from the first mentioned, all the serous structures, from the second all the mucous structures, and from the third the entire vascular system of the embryo originate. (700.) Towards the close of the first day of incubation the blastoderm has already begun to change its appear- ance, and two white filaments are apparent in the middle of the cen- tral pellucid cir- cle. Supposing a longitudinal sec- tion of it at this period, the mem- brana vitelli will be found to have become more pro- minent where it passes over the germinal space (Jig. 286, 1, p). The outer layer of the blastoderm (c) has become thickened at e into the first rudiment of the dorsal portion of the embryo ; but the mucous layer A, and the vascular layer B, have as yet undergone little alteration. At the commencement of the second day (Jig. 286, 2), the an- terior portion of the embryo is dilated, and bent down so as to inflect the three membranes of the blastoderm at this point. At the conclusion of the second day this inflection is carried still further; and from the vascular layer, a single pulsating cavity (Jig. 286, 3, A), the punctum saliens, the first appearance of a * Dr. Karl Ernst v. Baer iiber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. 4to. 1837. AVES BIRDS. heart lias become developed : so that considerable advance is already made towards that disposition of the fetus and its membra- nous investments represented in the next figure, to which we now beg the reader's attention. (701.) The serous membrane (Jig. 287, c) has at the third day become reflected to a considerable distance over the back of . the fetus ; at one extremity investing the head with a serous covering, while at the opposite it in like manner covers the tail : it is this reflection of the serous layer which forms the amnion^ as will be observed infig. 288, where the amniotic sac, C, is completed. The mucous layer, A, is now seen to line the as yet open space which is to form the abdominal ca- F ^ 287 ' vity ; and by its inflections gives birth to the rudi- ments of the ab- dominal viscera. From the vascu- lar layer, B, has been developed the heart, now composed of two chambers (a, Z>), and the branchial arteries (c), which join to form the aorta (w), exactly as in the Menopoma (fig. 257). The allantois (p), the uses of which will be described hereafter, likewise begins to make its appearance.* (702.) At the fifth day (fig. 288) the lineaments of the viscera become tolerably distinct. The sac of the amnios, c, is completed; the liver ', and the lungs e, begin to show themselves ; and the bag of the allantois (/?) is largely developed : still, however, the heart (a, b) is that of a fish, and the aorta (ra) formed by the union of the branchial arches (c) ; so we have yet to trace how, as the lungs increase in size, the circulatory apparatus becomes changed, and the branchial organs obliterated. On the third day of incubation there exist four vascular arches (fig. 287, c) on each side, having a common origin from the bulb (6), which obviously represents the bulbus arteriosus of Fishes and Reptiles (vide figs. 259, 261) ; these encircle the neck, and join on arriving in the dorsal region to form the aorta, which com- mences by two roots, each made up of the union of the four bran- * Des Branchies et des Vaisseaux branchiaux dans les Embryons des animaux ver- tebres, par Prof. Ch. Ernst v. Baer. Annales des Sciences Nat. torn. xv. 624 AVES BIRDS. chial vessels of the corresponding side. The vascular arches are developed one after the other, the most anterior being visible even on the second day: shortly, a second appears behind the first, the former in the mean time becoming considerably larger ; and at length the third and the fourth are formed, the fourth being still very small at the commencement of the third day. At this period three fissures are perceptible between the bran- chial arches, and in front of the first pair is the first appearance of the oral orifice ; which, however, is not, properly speaking, the aperture of the mouth, since at this epoch the jaws and buccal cavity are not as yet formed ; but, physiologically considered, it rather represents the pharynx. At the close of the third day this branchial apparatus is already slightly changed ; the branchial fissures are wider, and the fourth vascular arch is become nearly as large as the others. On the fourth day the first vascular arch is almost imperceptible, and that for two reasons : in the first place, it becomes covered up with cel- lular tissue ; and, secondly, it is so much diminished in size towards the second half of the fourth day, that it merely gives passage to a most slender stream of nearly colourless blood. By the close of the fourth day it is no longer recognisable ; but, before its disappearance, it is seen to have given off from its most convex point a vessel, which becomes the carotid artery, so that, when the arch itself is atrophied, that portion of it which was connected with the bulb of the aorta becomes the trunk of the carotid. The second arch then becomes diminished in size, insomuch that the third and fourth receive the greater part of the blood ; while in the meanwhile a fifth arch makes its appearance behind the fourth, so that in this way there are still four permeable arches. While these changes are going on in the vascular canals, the first branchial fissure gradually closes ; and, to make up for this,' a new one is formed between the arch which originally was the fourth, and that last developed. At the commencement of the fifth day there are consequently again four vascular arches and three branchial fissures on each side ; but not the same as those of the third day, since one arch and one fissure have disappeared, and have been replaced by similar parts. During the fifth day the vascular arch, which at first was the second, is obliterated, and the two succeeding ones become increased in size ; but at the end of the fifth day all the branchial fissures are effaced, being filled up with cellular tissue, and no trace of them is AVES BIRDS. 625 left. The remainder of the metamorphosis seems to depend principally upon changes that occur in the bulbus arteriosus (b) 9 which is by degrees converted into the bulb of the aorta. This part of the arterial system, from being a single cavity, about the fifth day divides into two canals, which become gradually more and more separated, and bent upon themselves. The separation of the bulbus arteriosus into two vessels is, in the opinion of Pro- fessor Baer, owing to the circumstance that the ventricles gradu- ally become separated by a septum, which, as it becomes more complete, causes two distinct currents of blood to be propelled from the heart. The current coming from the right ventricle arrives sooner than the other at the vascular arches, and rushes through the two posterior and through the middle arch of the left side ; while the gush of blood from the left ventricle fills the two anterior arches, and the middle arch of the right side ; a circumstance depending upon the course impressed upon the currents derived from the two ventricles. Each current becomes more and more distinct ; and at last each is provided with a proper channel, forming the trunks of the future pulmonary artery and of the future aorta. It will be seen, that as yet the real aorta does not exist ; for at Fig. 288. this period of the metamorphosis all the blood passes through the vascular arches that remain into the dorsal vessel ( fig. 288, m), which is formed in the same manner as the aorta of Fishes by the union of the branchial vessels. While the branchial fissures penetrated into the pharyngeal cavity, the branchial vessels were contained in the corresponding branchial arches ; but, as soon as these fissures disappear, the vas- cular trunks abandon the neighbourhood of the pharynx, and begin to assume the character that they afterwards present. The most posterior arch of the left side gradually disappears, 2 s 626 AVES BIRDS. and on the seventh day of incubation is no longer recognisable ; whilst in the mean time the current of blood from the right ven- tricle is directed in such a manner as to pass in front of this arch, and enters the posterior arch of the right side, and the last but one on the left. As, moreover, the two arches, that were formerly the most anterior, have become obliterated, while the third and fourth, on the contrary, are increased in size, the blood, passing backwards through these arches into the roots of the aorta, enters also the carotid artery, which now resembles a prolongation of the com- mencement of the aorta towards the head. Thus, one part of the primitive root of the aorta becomes the trunk of the carotid artery. There exist, consequently? on the eighth day three vascular arches on the right side, and only two on the left ; and these five arches are derived from the heart, as are also two small vascular trunks now quite distinct, which have been formed from the bulb. The anterior arch of both sides and the middle arch of the right side proceed from the left ventricle ; the posterior arches issue from the right : but all of them as yet unite to form the two roots of the aorta, which are still of pretty equal size, and each root gives off a carotid artery. At the point where the anterior arches join the roots of the aorta, they are now seen to give off newly- formed trunks, which go to the anterior extremity of their respec- tive sides ; and as these limbs and the head increase in size, and require more blood, the anterior arch propels a greater proportion of blood in that direction, and insensibly less and less into the aorta. The consequence is, that the anterior arch becomes more and more decidedly the brachio-cephalic trunk ; and in short, on the thirteenth day, it only communicates with the dorsal aorta by a small vessel, and ultimately becomes quite detached, forming the arteria innominata of the corresponding side. Meanwhile the posterior arches on both sides send out branches destined to the contiguous lungs. On the eighth day these vessels are still very small, and difficult to find ; but they soon grow larger, and, during the last half of the period of incubation, they show themselves as the immediate continuations of the arches from which they are derived ; their junctions with the aorta becoming more and more imperfect, and constituting the two ductus arteriosi. These canals are of very unequal size ; that of the right side is much shorter than that of the left, which is now the only remnant of the original root of the aorta on that side, and considerably AVKS BIRDS. G27 narrower than the root of the aorta on the right side. On the right side, in fact, the middle arch now becomes of great import- ance, and really constitutes the commencement of the descending aorta, receiving the other communications as subordinate parts. The bird having escaped from the egg, and having breathed for some time, all the blood from the right ventricle passes into the lungs, the ductus arteriosi become totally imperforate, and two dis- tinct circulations are thus established; one proceeding from the right side of the heart through the lungs into the left side of the heart, the other from the left side of the heart through the system into the left side of the heart. We see, therefore, that of the five pairs of vascular branchial arches which at first by their union formed the aorta as in Fishes, those of the first pair on both sides and of the fifth on the left side speedily disappear. The third on each side become the brachio- cephalic trunks, the fourth of the right side becomes the descending aorta, while the fifth of the right side and the fourth of the left side are converted into the pulmonary arteries. The very short trunk common to the two pulmonary arteries, as well as the equally short trunk of the aorta, properly so called, are produced by the transformation of the single cavity of the original " bulbus arte- riosus" into two distinct canals, and thus this wonderful meta- morphosis is accomplished. (703.) About the hundred and twentieth hour from the com- mencement of incubation, the vascular layer of the blastoderm has spread extensively over the yolk (Jig. 289, b ) ; and, as the vessels formed by it become perfected, they are found to converge to the Fig. 289. 628 AVES BIRDS. fig. 290. navel of the embryo, and to constitute a distinct system of arteries and veins (omphalo-mesenteric), communicating with the aorta and with the heart of the fetus, and forming a vascular circle surrounding the yolk. The omphalo-mesenteric arteries, (Jig. 291, b, c,) which thus ramify over the vitelline sac, are derived from the mesenteric arteries ; and the blood distributed through them is returned, by the omphalo-mesenteric veins, to the superior vena cava of the young chick. (704.) As soon as the intestinal system of the embryo bird is distinctly formed, the membrane enclosing the yolk (vitellicle) is seen to communicate with the intestine by a wide duct (ductus vitello-intestinalis), whereby the nutritive substance of the yolk enters the alimentary canal to serve as food, and the mucous membrane lin- ing the vitellicle becomes thrown into close wavy folds, so as to present a very ex- tensive surface. Gradually, as growth advances, the yolk di- minishes in size ; and at length, before the young bird is hatch- ed, the remains of it are entirely withdrawn into the abdominal cavity, (Jigs. 292, 293,) where its absorption is completed : but even in the adult bird a little caecal appendage, or diverticulum, still indicates the place formerly occupied by the ductus vitello-intestinalis. (705.) While the above phenomena are in progress, another important system of vessels provided for the respiration of the bird in ovo are developed, and obliterated before the egg is hatched. At about the period represented in Jig. 288, the sides of the abdominal cavity, which is still open anteriorly, are occupied by transitory secreting organs, named corpora Woifiana ; these, apparently, are the rudiments of the genito-urinary system : and, to receive their secretion, a bladder is developed, called the allan- toid sac, a viscus which is moreover destined to play an impor- tant part in the economy of the embryo, and soon becomes AVES BIRDS. 629 its principal respiratory organ. The allantois first makes its ap- pearance as a delicate bag (Jig. 288, p), derived from the anterior surface of the rectum, but Fiv 29 i it expands rapidly, and soon occupies a very con- siderable portion of the interior of the egg (Jig. 289, c), until at last "it lines nearly the whole ex- tent of the mcmbrana putaminis, and, becoming thus extensively exposed to the influence of the air that penetrates the egg- shell, it ultimately takes upon itself the respiratory function. When fully de- veloped (Jig. 290), it is covered with a rich net work of arteries and veins (a, b) spread upon its surface. The arteries (fig. 291, a) are derived from the com- mon iliac trunks of the em- bryo, and of course repre- sent the umbilical arteries of the human fetus ; the vein enters the umbilicus, and, passing through the fissure of the liver, pours the blood, which it returns from the allantois in an arterialized condition, into the inferior cava, as does the umbilical vein of Mammalia. About the nineteenth day of incubation, the air-vessel at the large extremity of the egg (Jig. 290, c) is ruptured, and the lungs begin to assume their function, by breathing the air that this vesicle contains. The circulation through the allantois then gra- dually diminishes, and it is slowly obliterated, until merely a liga- mentous remnant, called the urachus, is left. In Reptiles, how- ever, as we have already seen, a portion of the allantoid bag re- mains even in the adult creature (fig. 254, q); and in Birds that compartment of the cloaca in which the genital and urinary passages terminate are vestiges of the same organ. 630 AVES KIHDS. Fig. 292, in 293. (706.) Although the above description gives the reader a ge- neral view of the process of oviparous generation in its most perfect and consequently most com- plex form, the reader, in applying it to the deve- lopement of the ovum in the inferior OVIPARA, must bear in mind the following important dif- ferences : 1st. That in the air-breathing REPTI- LIA the white of the egg is almost, if not entirely, wanting; but the other phenomena are similar to those witnessed in the Bird. 2dly. That FISHES not only is there no white formed, but for ob- vious reasons the allantoid apparatus is not developed. The egg in these lower tribes con- tains only the yolk and the cicatricu- la ; it swells from absorbing the sur- rounding water, and the fetus is developed upon the surface of the yolk ; the latter, which, as in Birds, communicates with the intestine, be- ing slowly received into the abdomi- nal cavity. AVES BIRDS. 631 (707.) The subsequent changes that occur in the circulatory system of a Bird, namely, the obliteration of the foramen ovale, and of the ductus arteriosi, whereby the pulmonary and systemic circulations become quite distinct, are similar to those which take place in the Mammiferous fetus, and will be described in the next chapter. 632 CHAPTER XXX. MAMMALIA. THE highest boon conferred upon the lower animals, " Hea- ven's last best gift," is parental affection. The cold-blooded Ovipara, unable in any manner to assist in the maturation of their offspring, were necessarily compelled to leave their eggs to be hatched by the agency of external circumstances ; and their progeny, even from the moment of their birth, were abandoned to chance and to their own resources for a supply of nourish- ment. In Birds, the duties and the pleasures inseparable from the necessity of incubating their ova, and of providing nutriment for their callow brood, are indeed manifested to an extent unparalleled in the preceding orders of Vertebrata ; but it is to the Mammalia alone, the most sagacious and intelligent of all the inhabitants of this world, that the Creator has permitted the full enjoyment of paternal and maternal love, has thrown the offspring absolutely helpless and dependent upon a mother's care and solicitude, and thus confers upon the parent the joys and comforts that a mother only knows, the dearest, purest, sweetest, bestowed upon the animal creation. (708.) The grand circumstance whereby the entire class of beings generally designated under the name of QUADRUPEDS may be distinguished from all other members of the animal kingdom is, that the females of every species are furnished with mammary glands, secerning organs appointed to supply a secretion called milk, whereby the young are nourished from the moment of their birth, until they have reached a sufficient age to enable them to live upon such animal or vegetable substances as are adapted to their maturer condition. The possession of these lactiferous glands would indeed be in itself a sufficiently decisive characteristic of the whole group ; and if to this we add that their visceral cavity is separated into a thorax and abdomen by a mus- cular diaphragm^ and that they breathe by means of lungs pre- cisely similar to our own, we need not in this place dwell upon any more minute definition of the Mammiferous Vertebrata. MAMMALIA. 633 (709.) The MAMMALIA, as we might be prepared to antici- pate from their importance, are extensively distributed. The generality of them are terrestrial in their habits, either browsing the herbage from the ground, or, if of carnivorous propensities, leading a life of rapine, by carrying on a blood-thirsty warfare against animals inferior to themselves in strength or ferocity. Many inhabit the trees ; some burrow beneath the surface of the soil ; a few can raise themselves into the air and flit about in search of insect prey; the Otter and the Seal persecute the fishes even in their own element ; and the gigantic Whales, wal- lowing upon the surface of the sea, " tempest the ocean" in their fury. (710.) With habits so diverse, we may well expect correspond- ing diversity in their forms, or in the structure of their limbs ; and, in fact, did we not compress our description of these particulars into reasonable limits, we might easily test the perseverance of the most patient reader in following us through the mass of details connected with this part of our subject. We shall, therefore, commencing as we have hitherto done, with the osteology of the class, first describe, in general terms, the characters of a Mam- miferous skeleton ; and then, as we arrange the Mammalia under the various orders into which they have been distributed, speak of the most important aberrations from the given type. (711.) The vertebral column of all Mammals, with the remark- able exception of the Cetacea, is divisible into the same regions as in the human skeleton, viz. the cervical, dorsal, lumbar, sacral, and coccygeal or caudal portions. The cervical vertebrae are invariably seven in number. The Sloth (Bradypus tridactylus) was, until recently, regarded as forming a solitary exception, it having been supposed to possess nine cervical vertebrae ; the researches of Professor Bell, however, show, that even this animal conforms to the general law. The dis- tinguished naturalist referred to* has demonstrated, " that the pos- terior two of these vertebrae have attached to them the rudiments of two pairs of ribs, in the form of small elongated bones articulated to their transverse processes ; they must, therefore, be considered as truly dorsal vertebrae, modified into a cervical form and function suited to the peculiar wants of the animal/ 1 Professor Bell fur- ther observes, that " the object of the increased number of ver- * Cyclop, of Anat. and Phys. art. EULNTATA. 634 MAMMALIA. tebrse in the neck of the Sloth is evidently to allow of a more ex- tensive rotation of the head ; for, as each of the bones turns to a small extent upon the succeeding one, it is clear that the degree of rotation of the extreme point will be in proportion to the number of pieces in the whole series. When the habits of this extra- ordinary animal are considered, hanging as it does from the under surface of boughs, with the back downwards, it is obvious that the only means by which it could look towards the ground must be by rotation of the neck ; and as it was necessary, to effect this without diminishing the firmness of the cervical portion of the vertebral column, to add certain moveable points to the number possessed by the rest of the class, the additional motion was acquired by mo- difying the two superior dorsal vertebrae, and giving them the office of cervical, rather than by infringing on a rule, which is thus preserved entire, without a single known exception." (712.) The occipital bone articulates with the atlas by two lateral condyles, instead of by a single central articulating surface ; a circumstance which depends upon the greatly increased develope- ment of the encephalon, and the consequent expansion of the cranium. (713.) The number of dorsal vertebrae depends upon that of the ribs : thus, in the Bat tribe there are only eleven ; while in some of the Pachydermata, as, for example, in the Elephant and Tapir, as many as twenty dorsal vertebrae may be counted. The lumbar and sacral vertebrae will likewise be more or less numerous in dif- ferent genera ; and in the number of pieces composing the coccyx, or tail, there is every variety, from four to five and forty. (714.) The thorax is enclosed by ribs, that in structure, and in their mode of connection with the dorsal vertebrae, resemble those of Man. At its dorsal extremity each rib is articulated by its head to the bodies of the vertebrae, and to the intervertebral substance ; while its tubercle, or the representative of the second head of the rib of a Bird, is moveably connected with the corre- sponding vertebral transverse process. There are no sternal ribs ; but these are represented by cartilaginous pieces, whereby towards the anterior part of the thorax each rib is attached to the side of the sternum ; posteriorly, however, this connection does not exist. The anterior ribs are therefore called true ribs, and the posterior, false or floating ribs, precisely as in the human skeleton. (715.) The sternum is composed of several narrow pieces, placed in a line behind each other along the middle of the breast. MAMMALIA. 635 These pieces are generally consolidated : by their lateral margins they give attachment anteriorly to the clavicles, if these bones be present ; and, behind these, to the costal cartilages of the true ribs. From the whole arrangement of the thorax, it is evident that the ribs are capable of extensive movements of elevation and de- pression, whereby the capacity of the whole thoracic cavity may be increased or diminished ; movements which, aided by those of the diaphragm, draw in and expel the air used for respiration. (716.) The anterior extremity is appended to a broad scapula, generally unconnected with the rest of the skeleton except by muscular attachments. In quadrupeds that use this extremity as an instrument of prehension or of flight, a clavicle is interposed between the scapula and the sternum ; but most frequently this ele- ment of the shoulder is deficient, and even the coracoid bone, if a vestige of it remains at all, is reduced to a mere appendage to the scapula, known to the human anatomist as the coracoid process. The rest of the limb presents the arm, the fore-arm, the carpus, metacarpus, and phalanges ; but these are so altered in appearance in different orders, that no general description will suffice, and we must therefore defer this part of our inquiry ^frr tho present. (717.) In the posterior extremity there is equal dissimilarity in the construction of the distal portions of the limb ; but the pelvis, although much modified in form, consists of the same pieces as in the human subject, and in like manner has the pubic arch and fora- mina fully completed. (718.) The cranium and face are made up of numerous bones, easily recognizable, as they correspond in their general arrangement with those composing this part of the skeleton in the lower Verte- brata. Their developement in the facial region is large in propor- tion to the strength of the muscles moving the lower jaw; and they are so disposed as to form buttresses to resist the powerful pressure of the teeth, as well as to enclose cavities wherein are lodged the organs connected with the senses of smell and of vision. One example will answer our present purpose, and we have selected the skull of the Pig as one calculated to show a medium develope- ment of the whole series. In the face we find on each side two bones entering into the composition of the upper jaw, into which teeth are implanted ; these are the superior maxillary (fig- 94, 18), and the intermaxil- lary (17). These bones, moreover, bound extensively the cavity of the nose ; and, together with the palatine process of the palate 636 MAMMALIA. bone (Jig. 295, 22), constitute the bony palate, or roof of the mouth. The nasal bones (20, 20) complete the upper part of the face; and, being in contact along the mesial line, arch over the nasal chamber. Fig. 294. The orbit is boundeci anteriorly by the lacrymal bone (c), and ihejugal or malar bone (6). Its posterior boundary is generally wanting, as the external angular processes of the jugal and frontal bones do not meet. The orbital cavity is principally formed by processes derived from the os frontis, the sphenoid, the lacrymal, and the malar bone ; the ethmoid and the palatine rarely entering into its com- position. The os ethmoides, the vomer, and the turbinated bones will be described minutely when we speak of the olfactory apparatus, which they contribute to form. The inferior maxilla in Mammals is characterized by two cir- cumstances, which distinguish it from that of other Vertebrata. It consists, in the first place, of only two lateral pieces, exactly similar to each other, joined together at the chin by a symphysis in many orders, but in others even this symphysis is obliterated at an early age, and in the adult the two lateral halves would seem to form but one piece. Another character peculiar to the lower jaw of a Mammal is, that it is moveably articulated with the temporal bone by means of a convex and undivided condyle. These marks, identifying the MAMMALIA. 637 Mammiferous lower jaw, ought to be well remembered by the geologist. We shall hereafter have occasion to describe the teeth that arm the jaws of the different tribes of quadrupeds ; and therefore now Fig. 295. proceed to examine their cranial cavity, and the bones that enter into its formation. The frontal bones (Jigs. 294, 295, 1, 1) are generally two in number ; and even when, as in Man, they seem to form but one bone, the two lateral halves are produced from separate points of ossification, and only coalesce as age advances: sometimes, indeed, even in the adult, they remain permanently separated by suture. The parietal bones (7, 7) occupy their usual position ; and, although generally double, as in the human skeleton, they are not unfrequently consolidated together, even at an early age, so as to represent but a single bone. The occipital bone consists primarily of the same pieces as in the Reptile ; but in the Mammifer these are at an early period conso- lidated into one mass, situated at the back of the cranium. Its basilar portion (5) articulates with the atlas by two condyles ; while the lateral wings (10) and the superior arch (8) surround the foramen magnum, and protect the cerebellic regions of the encephalon. The sphenoid (6), although composed of fewer separate pieces than in the Reptilia, and even regarded by the human anatomist as a single bone, is still distinctly divisible, especially in young animals, into two very important portions, one anterior, and the other posterior ; each, as we shall soon see, forming the body of a distinct cranial vertebra. The posterior half (6) consists of the 638 MAMMALIA. body, including the posterior clinoid processes, and of the greater alse and pterygoid processes (Jig. 295, 25). The anterior half is formed by the anterior clinoid processes and alse minores (Jig- 295, 11). These two halves may therefore be called, respec- tively, the anterior and posterior sphenoids. Lastly, we have the temporal bone, exhibiting but one piece, although made up of all the parts which in the Reptile were so obviously distinct elements. The petrous portion wedged into the base of the cranium, still encloses the internal car. The tympanic element (Jig* 294, a) supports the membrana tympani. The mastoid process (Jig. 295, 12) is the homologue of the mas- toid bone of the Crocodile ; and, lastly, the squamous element with which the lower jaw is articulated (Jig. 294, 23) in the Reptilia, was visibly a distinct bone. Even to these may be added the zygomatic process, which Professor Owen regards as an inde- pendent elemental part. (719.) Reviewing, therefore, all that has been said relative to the composition of the skull in the different classes of Vertebrata, the following deductions may be arrived at.* 1. That, as we advance from lower to higher forms, the propor- tionate size of the cranium relative to that of the face becomes greater. 2. That the number of bones met with upon the inferior and lateral aspects of the head gradually diminishes : for in Mammalia the pterygoid and tympanic bones which even in Birds are sepa- rate pieces, become very generally confounded with the sphenoid and the temporal ; and also the petrous and squamous portions of the temporal become blended together. 3. The number of bones normally entering into the composition of the cranium of adult Mammalia varies considerably. When most numerous, there are twenty-eight, eleven in the cranium, and seventeen in the face. In this case the cranial bones are the follow- ing, one occipital, one sphenoid, the two squamous portions of the temporal, the two tympano-petrous portions of the temporal, the two parietal, the two frontal, and the ethmoid. The bones of the face are, two superior maxillary, two inter- maxillary, two nasal, two lacrymal, the vomer, two inferior turbi- nated bones, two palate bones, two jugal bones, and, lastly, the two halves of the lower jaw. It is true that some slight exceptions occur : thus, for example, * Meckel, Trait6 G6nerale d'Anatomie Comparee, torn. iii. seconde partie, p. 195. MAMMALIA. 639 in the Cetacea the pterygoid bones remain detached; in the Ro- dentia the occipital is divided into a superior and inferior portion ; but, in the latter, the two frontal and the two parietal become con- solidated into one bone. In Man the bones of the cranium become much less numerous, inasmuch as all the elements of the occipital, of the temporal, of the frontal, the intermaxillary, and the maxillary, composing the upper jaw, and the two halves of the lower jaw, respectively coalesce ; and the skull consists of but one-and-twenty bones, seven in the cra- nium, and fourteen in the face. Even this number is not the smallest ; for in some Monkeys the nasal bones unite and become consolidated into one piece. (720.) Having thus enumerated the different osseous pieces forming the crania of all classes of vertebrate animals, we must next consider them in another point of view, namely, as being continua- tions of the spinal chain of bones, or real vertebrae modified in form and proportions in conformity with the increased volume of the nervous masses they are destined to enclose. We must, however, premise that it is by no means our intention to adopt unreservedly the theoretical opinions of those Continental writers who find verte- bral elements in the bones of the face, and even in the nasal carti- lages ; still, without overstraining the facts, it is easy to demonstrate very satisfactorily, that the cranial pieces that immediately enclose the cerebral masses are strictly vertebrae, and present the same essential structure as those of the spinal region. That this is the case in the skull of a Reptile, no one, indeed, who examines the subject, can hesitate to admit ; but even in the Mammiferous cranium, where, from the enormous proportionate size of the encephalon, the cranium is most distorted, it is not diffi- cult to perceive the relationship. The cranial vertebrae are three in number, the occipital, the parietal, and the frontal : these are exhibited in the subjoined diagram, after Carus, representing those of the Sheep. The occipital vertebra (Jig. 296, A) has for its body thebasilar portion ; the arches bound the foramen magnum laterally ; and above, the spinous process, flattened out and expanded in proportion to the size of those lobes of the brain and cerebellum which it de- fends, forms the posterior portion of the skull. The body of the second or parietal vertebra (B) is the body of the sphenoid; that is, more properly speaking, the poste- rior sphenoid bone, whose large alse, curving upwards, meet the 640 MAMMALIA. parietal, and thus an arcli is formed of sufficient span to cover the middle lobes of the cerebrum. The anterior, or frontal vertebra, Fig. 296. has for its body the anterior sphenoid (al<E minores) ; its arch being com- pleted by the cavity of the os frontis, which encloses anteriorly the cribriform plate of the ethmoid bone. From this analysis of the compo- sition of the cranium, it is apparent that the temporal bones, although in Man they assist so materially in com- pleting the cranial cavity, are only intercalated between the real vertebral elements ; as indeed might almost have been anticipated, seeing how different- ly the pieces belonging to this bone are arranged in different classes of Vertebrata. (721.) Such is the general organ- ization of the Mammiferous skeleton. Let us now proceed to consider the osteology of the different orders into which the Mammalia have been dis- tributed, and observe in what respects they individually differ from each other. The transition from Birds to Qua- drupeds, remotely separated as they might appear to be, is effected by gentle gradations of struc- ture ; and the MONOTREMATA, notwithstanding their quadru- pedal form and hairy covering, are so nearly allied to the feathered Ovipara in many points of their organization, that they evidently form a connecting link between these two great classes of animals. It is true that they have mammary glands, and must therefore be supposed to give suck to their offspring ; but it is not even yet satisfactorily determined whether they lay eggs, or produce living young. The structure of their generative apparatus would seem, in fact, to be rather allied to the Oviparous than the Mammiferous type; and, as in Birds, the rectum, the urinary passages, and the sexual organs, all discharge themselves into a common cloacal MAMMALIA. 641 chamber, so that there is still but a single vent, a circumstance from which the name of the order is derived. Even their skeleton, in many points, presents a very close affinity to that of a Bird, as will be evident on examining the osseous sys- tem of the Ornithorhynchus paradoxus (Jig. 297). Fig. 297. The mouth of this quadruped indeed resembles that of a Duck, whence the name of " Duck-bill," whereby it is usually distin- guished. It has, moreover, a distinct furcular bone in addition to what would seem to be the ordinary clavicles ; but in reality these are the coracoid bones still largely developed. Moreover, the an- terior or sternal ribs are ossified, and a spur is attached to the hind foot of the male, not remotely resembling that of a Cock : this last appendage is perforated by a duct, and has a gland connected with it, situated on the inner side of the thigh, by which a poisonous secretion was formerly supposed to be elaborated. (722.) The MARSUPIALIA, it will be afterwards explained, as regards the conformation of their generative system, are organized in accordance with a type intermediate between that common to Birds and that which characterizes Mammalia properly so called. The Marsupial quadrupeds bring forth their young alive, but in such an imperfect condition, that at the period of their birth scarcely the vestiges of their limbs have become apparent ; and in this state they are conveyed into a pouch formed by the skin of the female's abdomen, where they fix themselves by their mouths to the nipples of their mother, and, sucking milk, derive from this source the materials for their growth. These animals are peculiar to the Australian and American continents; nay, in Australia, so anomalous in all its productions, with one or two exceptions, and those perhaps brought there by accidental importation, all the quadrupeds are constructed after the Marsupial type. The great characteristic whereby to distinguish the skeleton of a Marsupial 642 MAMMALIA. Mammifer, is the existence of two peculiar bones attached to the anterior margin of the pubis, which in the living animal are im- bedded in the muscular walls of the abdomen, and thus support the pouch of the female. The marsupial bones, however, exist in the male likewise ; and even in the MONOTREMATA, that are evidently nearly allied to the proper MARSUPIALS, although no pouch is met with even in the female sex, the bones alluded to are found connected with the pubis. This great section of the Vertebrate creation, which, perhaps, ought rather to be regarded as a class by itself, is composed of numerous families, of diverse forms and very opposite habits. The Opossums (Didelphis) of the American continent live in trees, and devour birds, insects, or even fruits : in these, the thumb of the hind foot is opposable to the other fingers, and adapted for grasp- ing the boughs, whence they are called Pedimanes; their tail is like- wise prehensile. Others are terrestrial in their habits, wanting the prehensile thumb. Fig. 298. The Kangaroo Rat, or Potoroo (Hypsiprymnus), of whose ske- leton we have given a drawing (fg. 298), is remarkable for the dis- proportionate size of its hind legs: these, moreover, have no thumb, and the two innermost toes are joined together as far as the nails; so that there appear to be but three toes, the inner one being fur- nished with two claws. Such legs are well adapted to make strong MAMMALIA. 643 and vigorous leaps over a level plain ; and in the Kangaroos (Macropus) the extraordinary developement of the posterior extre- mities is even yet more wonderful. In other respects, the skeletons of the Marsupialia conform to the general description already given. (723.) All other Mammiferous Vertebrata produce their young alive, and not until they have attained a considerably advanced state of developement during their intra-uterine existence. The con- nection between the maternal and fetal systems in these orders is maintained during the latter periods of gestation by the develope- ment of a peculiar viscus, called the placenta: nevertheless, after birth, the young animals are still dependent upon the mother for support, and live upon the milk supplied by her mammary organs. (724.) The lowest order of PLACENTAL MAMMALIA com- prises those forms which, although they breathe air by means of lungs, and have hot blood like ourselves, are appointed to inhabit the waters of the ocean, wherein they pass their lives, and even bring forth and suckle their young. In order to live under such circumstances as these, the CETACEA must necessarily, in many points of their structure, be organized after the model of fishes ; and we cannot be surprised, if in their outward form, and even in the disposition of their limbs, they strikingly resemble the finny tribes. Their head is large, frequently indeed of enormous propor- tions : there is no neck apparent externally ; the head and trunk, as in fishes, appearing continuous. The anterior extremities are con- verted into broad fins, or paddles ; whilst the pelvic extremities are absolutely wanting : posteriorly, the body tapers off towards the tail, and terminates in a broad, horizontal fin, which latter, however, is not supported by bony rays, as in the fish, but is entirely of a cartilaginous and fleshy structure. Frequently there is even a ver- tical dorsal fin ; but this, too, is entirely soft and cartilaginous, so that in the skeleton no vestiges of it are apparent.* In the Whalebone- Whale (Bal&na mysticetus) the peculiari- ties of the Cetaceous skeleton are well exhibited. In this gigantic animal (fig. 299), which sometimes measures upwards of a hun- dred feet from the snout to the tail, the head forms nearly a fourth part of the entire length of its stupendous carcass ; so enormously developed are the bones of the face that form the upper and the lower jaws. The cranial cavity, wherein the brain is lodged, does not of course participate in this excessive dilatation, but * It is interesting to see these fins still formed by the skin (exoskeleton,) where the osseous system could not enter into their composition without deviating altogether from the Mammiferous type. 2x2 644 MAMMALIA. corresponds to the size of the brain lodged within it. It, however, presents one point of physiological in- Fig. 299. terest, serving to prove still more de- monstratively, that the temporal bone is merely an adjunct to, and not essentially a constituent part of, the cranium ; for here the petrous portion of the temporal bone, wherein is lodged the organ of hearing, is entirely detached from the skull, to which it is only fastened by a ligamentous connection. This remarkable arrangement is no doubt intended to pre- vent the stunning noises that would else be conveyed from every side to the ear, by cutting off all immediate communica- tion between the auditory apparatus and the osseous framework of the head. The cervical vertebrae, in conformity with the shortness of the neck, are exceed- ingly thin ; and some of them are not unfrequently anchylosed into one piece. The thorax is composed in the ordinary manner ; but the posterior ribs are only fixed to the transverse processes of the corresponding vertebras. Behind the tho- rax the whole spine is flexible, its move- ments being untrammeled by any pelvic framework, or posterior extremity ; so that, as in fishes, the broadly expanded tail is the great agent in locomotion ; and, from the horizontal position of this mighty oar, it is better adapted to enable the animal to plunge headlong into the depth, and to rise again to the surface, with all expedi- tion, than if it had been placed vertically, as it is in fishes. The only vestiges of a pelvis met with in the Whale are, the rudimentary ossa pubis represented in the figure. These are imbedded in the abdominal muscles, and serve to support the external organs of MAMMALIA. 645 generation : tlie caudal vertebrse are, however, distinguishable by the inferior spinous processes, developed from their under surfaces. As to the construction of the anterior extremity, the shoulder is composed of the scapula alone. The arm and fore-arm are much stunted, and are not moveable at the elbow ; therefore the muscles for pronating and supinating the arm do not exist, but are represented by aponeurotic expansions spread over the surfaces of the bones. The bones of the carpus are flattened, and more or less consolidated together. The fingers, likewise, are flat; and the whole limb so covered with tendinous bands, and enveloped in skin, as to form merely a fin, whereby the creature guides its course through the water. (725.) In the Herbivorous Cetacea, as the Manatus and Du- gong, the head is smaller in proportion to the sides of the body, and the hands better developed, so as to be useful in creeping on land, or in carrying their young. These genera inhabit the mouths of tropical rivers. (726.) The relationship between the Cetacea and the next order that offers itself to our notice is too evident not to be immediately appreciated. The thick and naked skin, the gigantic body, the massive bones, the bulky head, and even the variable and irregular teeth that arm the ponderous jaws, are all again conspi- cuous in the PACHYDERMATA; and the river and the marsh, the localities frequented by the latter, as obviously indicate the inter- mediate position which these animals occupy between the aquatic and the terrestrial Mammalia. Ffc. 300. 646 MAMMALIA. Fig. 301. The skeleton of the Hippopotamus (Jig. 300) offers a good ex- ample of the general disposition of the osseous system in the Pachy- dermata. The spinous processes of the last cervical and anterior dorsal vertebrae are necessarily of prodigious strength, giving origin as they do to the muscles that support the weighty skull : the ribs are numerous, broad, and flat ; they extend nearly along the entire length of the trunk, and thus assist in sustaining the bulky viscera of the abdomen. The pelvis is massive in proportion to the weight of the body ; and both the thoracic and pelvic extremities short, thick, and strong, forming, as it were, pillars upon which the trunk is raised. The most important differences ob- servable between the different genera of Pachydermatous Mammalia are found in the structure of their feet, and in the num- ber and disposition of their toes. In the Elephant there are five to each foot ; but in the living state they are so encased in the callous skin which forms a sort of hoof to the foot of this monstrous animal, that they are scarcely perceptible exter- nally. In the Hippopotamus above de- lineated there are four, and also in the Hog tribes; but in the latter the two middle toes are disproportionately large. The Rhinoceros has only three toes to each foot ; and other varieties in this respect might easily be pointed out. (727.) In the SOLIDUNGULA, or So- LIPEDS, regarded by Cuvier as a family belonging to the order last mentioned, we have a tribe of animals quite peculiar as relates to the construction of their loco- motive extremities. In the Horse, for example, a creature obviously formed to be an assistant to the human race, so completely has every other consideration been sacrificed, in order to ensure the utmost possible strength and solidity in the structure of the foot, that all the toes appear exter- MAMMALIA. 647 nally to have been solidified into one bony mass; which, being encased in a single dense and horny hoof, is not only strong enough to support the weight of the quadruped, and to sustain the shock produced by its most active and vigorous leaps, but becomes abundantly efficient to carry additional burdens, or to draw heavy loads in the service of mankind. In the anterior extremity of a Soliped (Jig. 301) the shoulder consists only of the scapula, there being no clavicle to connect it with the sternum. The humerus is short and very strong: the radius and ulna are partially consolidated together, so that all movements of pronation and supination are impossible. The car- pus is composed of seven short bones disposed in two rows. The metacarpus is a single bone (the cannon bone), which, from its length and size, is commonly called the " fore-leg" of the horse ; the carpo-metacarpal articulation being looked upon as the " knee." Lastly, the foot consists of three great phalanges ; whereof the proximal is named the " pastern" the second the "coronary" and the distal phalanx the " coffin bone" In the macerated skeleton, however, the vestiges of two other toes are visible ; but they are merely rudiments resembling osseous splints attached to each side of the metacarpus or cannon bone. In the posterior limbs of the Horse the same peculiarities are observable, both in the construction of the leg and foot. (728.) The RUMINANTIA constitute another order of qua- drupeds of very great importance to mankind, distinguished by their remarkable habit of chewing the cud ; that is, of bringing up the food again from the stomach into the mouth, for the pur- pose of undergoing a second process of mastication. They all have well-developed incisor teeth in the lower jaw, but none in the upper. The patient and thirst-enduring Camel, the stately Giraffe, the Ox, the Sheep, the Goat, the nimble Antelope, and the fleet and elegant Stag, are all examples of this extensive order ; but it is the skeleton of the last-mentioned alone that we shall select for de- lineation Og. 302). The most remarkable feature observable in the Ruminant order of quadrupeds is, that, with the exception of the Camel tribe and the Musk-deer, the males, and sometimes the females, are provided with two horns attached to the os frontis, appendages not met with in any other Vertebrata. In some, as the Giraffe, these horns consist merely of a bony protuberance developed from each frontal bone, which is coated with a hairy skin derived from the 648 MAMMALIA. common integument of the head. In others, as in the Ox, Goat, Antelope, Sec. the Fig. 302- bony nucleus of the horn is covered over with a sheath of cor- neous matter, giving it a hard and smooth surface. Both the above kinds of horns are persistent; but in the Deer tribe the de- fences of the head, which are large and branched, are deci- duous, being formed every year from a vascular skin that covers them exter- nally during the pe- riod of their growth, but shrivels up and dries when they are completed. These horns fall off after a certain time, to be renewed again the following season ; the mode of their formation will, however, be examined in another place. (729.) In consequence of the weight of the horns in such spe- cies as possess weapons of this description, the head is necessarily extremely heavy ; and in genera where the horns are wanting or feebly developed, as in the Camel or the Giraffe, such is the length of the neck, that, even with a disproportionately small head attach- ed to the extremity of so long a lever, incessant and violent mus- cular exertion would be needed to sustain or to raise it from the ground. This difficulty is obviated by a very simple and elegant contrivance : a broad band of ligament, composed of the same elastic tissue as that composing the ligamenta sub/lava of the human spine, is extended from the tips of the elongated spinous processes of the back, and sometimes even as far backwards as the MAMMALIA. 649 lumbar and sacral regions. This ligament, strengthened by addi- tions derived from most of the vertebral processes over which it passes, runs forward to be fixed anteriorly to the crest of the occipital bone, and to the most anterior of the cervical vertebrae. The whole weight of the cranium and neck being therefore fully counterbalanced by the elasticity of this suspensory ligament, the muscles of the neck act with every possible advantage ; and all the movements of the head are effected with the utmost grace and facility. The RUMINANT i A are generally distinguished as having " cloven feet ;" and, in fact, both the hind and fore feet present a very cha- racteristic formation. The bones of the fore-arms, as well as the tibia and fibula, are more or less completely consolidated, espe- cially towards their distal extremities. The carpal and tarsal bones resemble those of the Horse, and are similarly situated. The metacarpal and metatarsal or cannon bones are respectively composed of two lateral halves united along the mesian line ; and to each of these halves is attached a toe composed of three pha- langes, the last phalanx of each being encased in a strong hoof. In some genera two rudimentary lateral toes are also distinctly recognisable, but these are too small to be used in locomotion. fig. 303. (730.) The EDENTATA, forming the next order of quadrupeds, are so called from the deficiency of teeth observable in the fore part of their mouth. In the most perfect tribes, as, for example, in the Armadillo (Jig. 308), the skeleton is well developed in all its parts, and presents nothing to attract our special notice, except, perhaps, the large proportionate size of the distal joints and claws that arm the toes ; but in the Sloths (Bradypus) so unusual is 650 MAMMALIA. the conformation of the limbs, that it had at one time become quite the fashion for naturalists to bestow a passing expression of sympathy in alluding to these, so called, miserable and imperfect members of the animal creation. " The Sloths," says Cuvier,* u derive their name from their exces- sive slowness, the result of a structure truly heteroclite, where Na- ture seems to have wished to amuse herself by producing something imperfect and grotesque. These animals have their fingers joined together by the skin, and only indicated externally by enormous compressed and hooked claws, which are bent when in repose to- wards the palms of the hands or the soles of the feet. The hind feet are articulated obliquely with the leg, and only rest upon their external edge ; the phalanges of the fingers are articulated by tight hinge-joints, and the proximal ones become consolidated at a certain age with the bones of the metacarpus or metatarsus, even these last become anchylosed with each other for want of use. To this inconvenience in the organization of the extremities may be added one equally great, consequent upon their proportions. The arms and the fore-arms are much longer than the thighs and the legs^ so that when these creatures walk they are obliged to drag themselves upon their elbows ; their pelvis too is so wide, so much directed sideways, that they cannot approximate their knees. Their deportment is the natural consequence of such dispropor- tionate structure. They remain upon trees, and never quit one till they have stripped it of its leaves, so difficult is it for them to get to another ; nay, it is even asserted that they let them- selves fall from their branch to avoid the trouble of crawling down." Well may humanity pause before it ventures to accuse Nature of having " wished to amuse herself by producing something im- perfect and grotesque ;" and we should not have inflicted upon ourselves the task of quoting so painful a passage, did it not emanate from such a source, and had not ample opportunities of observation shown that the very structure so accurately described by Cuvier is better than any other adapted to the arboreal life for which the Sloth is destined. It is not upon the ground, but in the tree, that this animal must be criticized; and there, as we learn, among its native branches, hanging securely by means of its hooked toes and peculiarly organized hind legs, it feeds in situations which otherwise would be left unoccupied ; or, using its * Regne Animal, vol. i. p. 223, etseq. MAMMALIA. 651 Fig. 304. long arms, it swings from bough to bough with a facility little to be expected from its appearance. (731.) The herbage that covers the plain, or the foliage of the trees, are not, however, the only vegetable materials that have been made available for the support of Mammiferous quadrupeds. The RODENTIA are furnished with teeth adapted to gnaw even the wood and the bark, or to crack nuts and other hard fruits, from which they derive nourishment. This order of Mammals is, therefore, distinguished by the possession of two incisor teeth in each jaw, so constructed as to erode hard substances, and which moreover by a peculiar mechanism, to be described in another place, are always kept sharp and tren- chant: such are the incisor teeth of the Beaver or of the Hare (fig. 304). The skeletons of the RODENTIA are slight and feeble, adapted to the bird-like activity of their habits. Their fingers and toes are well developed, and the bones of the leg and fore-arm free throughout their whole length, although the movements of pro- nation and supination are as yet very limited. In many genera, more especially in such as climb trees like the Squirrels, the clavicles are very perfectly formed, so that the fore legs can be employed to a certain extent as hands, for conveying food to the mouth. Very generally, the hind legs of the RODENTIA are consider- ably longer than their anterior extremities ; hence such genera run by bounds or leaps, and their course is very rapid. In the Jerboa (Dipus) (Jig. 305) this disproportionate size of the hind legs is excessive, insomuch that the creature moves by leaps, like a Kangaroo ; and, the metatarsal bones of the three middle toes being consolidated into one bone, the whole limb resembles more that of a bird than of a quadruped. 652 MAMMALIA. Fig. 305. (732.) Araong all the countless races of the ani- mal kingdom, Man alone is permitted, in a state of na- ture, to arrive at old age; that is to say, at such an age as to allow fee- bleness and decre- pitude to usurp the place of strength and activity. Man only is capable of such a privilege, because he alone possesses that fore- sight which ena- bles him to prepare in youth against the decline of his faculties, and is endowed with sym- pathies and af- fections directing the young and the vigorous to maintain the aged and the infirm. Among the lower animals, sickness and decay are not permitted to exist. Activity and health alone are conspicuous throughout the broad creation : disease and decline are banished from the world. Does any creature lack but for a brief period its accustomed powers of escape, the destroyer is at hand instantly to remove it from its appointed sphere of action. Butchers are placed on all sides ready to perform their office ; and nothing is permitted to live but what possesses its faculties and its strength unimpaired and unen- feebled. The great character that distinguishes the Carnivorous quadru- peds is, the high degree of intelligence and activity for which they MAMMALIA. 653 are so remarkable. The perfection of their limbs, and the acute- ness of their senses, at once indicate their superiority over the Herb- ivorous races ; and their jaws, armed with powerful fangs, usually distinguished by the name of canine teeth, show at a glance the na- ture of their appointed food, and their murderous propensities. The distribution of these tyrants of the animal creation we shall find to be coextensive with that of the victims they are appointed to destroy. (733.) The aquatic tribes of the Carnivora (Amphibia, Cuv.) are obviously constructed for swimming. Their bodies, covered over with short, close, and polished hair, taper off towards each extremity, resembling in form those of the CETACEANS. The cervical, thoracic, and lumbar regions of the spine are light and flexible ; and the pelvis contracted and placed as far back as possi- ble. Both the anterior and posterior extremities, although com- Fig. 306. pletely formed, are short ; and in the living animal are only free externally as far as the carpal and tarsal joints. The feet, more- over, are broadly webbed, and thus become converted into most efficient paddles, by the aid of which these creatures swim with astonishing ease and elegance, the hinder pair performing at once the functions of oars and rudder. Upon land, however, their movements are, as might be supposed, extremely clumsy : it is true that they not unfrequently scramble on to the beach, there to bask in the sun, or to suckle their little ones ; but, if danger threatens, they immediately take to the water, and fall easy victims if their retreat towards the sea be intercepted. Such being the helplessness of the Seals when they quit the water for the shore, it is not surprising that, in some of the larger 654 MAMMALIA. and more unwieldy forms, assistant locomotive organs have been given, derived from unlocked for sources. Thus, in the Walrus (Trichecus rosmarus), which apparently obtains nourishment from the fuci of the shore, as well as by destroying living prey, even the canine teeth of the upper jaw are converted into instru- ments of progression, and serve as crutches to drag the animal along. In these creatures the upper jaw is extremely dilated and massive, and the canine teeth implanted in it not unfrequently project downwards to a distance of from one to two feet from the mouth. The strength of the tusks so formed is proportionate to the bulk of this gigantic Seal, and by their aid the Walrus is en- abled to climb on to the rock in order to repose after its labours in the ocean. (734.) The Terrestrial Carnivora, that live upon flesh, are na- turally divisible into two great sections. Of these, the most cruel and blood-thirsty, called from this circumstance " Digitigrada," walk only upon their toes, and bound along with an elasticity and swiftness that are abundantly provided for in the construction of every part of their osseous system. In this section are classed the extensive tribes of Weasels (Jig. 307), and of Civets, the Hyenas, and the race of Cats, the most formidable and ravenous of quadrupeds. In the Feline Carnivora, indeed, to which belong the Lion and the Tiger, so justly celebrated for their strength and ferocity, a peculiar and beautiful provision is visible in the construction of the foot, whereby the claws that arm the last phalanges of the toes are kept constantly sharp, their points never being allowed to become worn by touching the ground ; hence they are in these crea- tures terrific instruments of attack. The mechanism provided for effecting this is as follows : three elastic ligaments, derived from MAMMALIA. 655 the penultimate joint of the toe, are inserted into the last phalanx in such a manner that, by their elasticity, under ordinary circum- stances, they keep the claw laid back upon the upper aspect of the foot ; so that, the soft cushions beneath the toes being the only parts brought in contact with the ground, these creatures always walk with a stealthy and noiseless tread. But when the Tiger springs upon his prey, the tendons of the flexor muscle of the toes, implanted into the opposite surface of the phalanx, overcoming the elasticity of the retractile ligaments, pluck forward the curved claws, and, burying them deeply into the flesh of the victim, the strongest animals struggle vainly to shake off a gripe so tenacious. But, among the Digitigrade Carnivora, none are of so much importance as the Dog ; an animal specially provided for the use of mankind, to be his companion in the field, and his assistant at the chase. Nor has Nature, in the case of the Dog, merely given to man a servant endowed with sagacity and zeal : man has need of help in various ways, and under very different circum- stances. In bodily strength he is unable to cope with ferocious enemies that surround him on all sides ; his senses are imperfect, when compared with those of some of the lower animals ; in speed he is outstripped by the very creatures appointed to be his food how then are all these deficiencies to be compensated ? The Dog has been placed at man^s disposal : its instincts, its size, its form, its senses, and its corporeal attributes, are all subjugated to his control; and thus whatever aid he may require, is to be obtained by the cultivation of its faculties. (735.) The PLANTIGRADE CARNIVORA, as their name indi- cates, in walking apply the entire sole of the foot to the ground, as far back as the end of the os calcis : such are the Bear (Ursus), the Glutton (GWo), the Badger (J/e/es), and others of similar organ- ization. These tribes are less exclusively carnivorous in their habits than the preceding, and their nails are not retractile, so that their points are blunted by dragging upon the ground. (736.) The INSECTIVORA form another section of these de- structive quadrupeds, distinguished by their molar teeth being studded with sharp points, and thus calculated to devour insect prey : the Hedgehog (Erinaceus), the Shrew (Sorex), and the Mole (Talpd), are well-known examples of this division, and their habits are known to all. We need scarcely mention the peculiar circumstances under which the Mole passes its subter- ranean existence, or the extraordinary conformation of its anterior 656 MAMMALIA. extremities, whereby they are converted into most efficient instru- ments for digging beneath the soil. The extended scapula, the strong and well-developed clavicle, the square and massive hume- rus, and, moreover, the broad and rake-like hand, all proclaim the office of this strange limb ; while the long and carinated ster- num indicates with equal plainness the size and power of those muscles by which the apparatus is wielded.* (737.) The CHEIROPTERA, or family of BATS, present a striking contrast to the Mole both in form and habits : neither would it be easy to conceive that a skeleton, consisting almost of precisely the same elements, could be converted to uses so diame- trically opposite. Fig. 308. In these Mammalia the anterior extremities are converted into wings, enabling them to emulate the very birds in their powers of flight, and in the velocity of their movements, when upon the wing pursuing insect prey. In creatures destined to such a life, the whole skeleton must of course be lightened, and the bones attenuated to the utmost. The skull, the spine, the thorax, the pelvis, and the hind extremities, all testify by the delicacy of their structure that no unnecessary weight is here permitted. It is, however, in the construction of the anterior limbs that the Cheiroptera present the most remarkable peculiarities. The scapulae are broad and expanded, covering a considerable portion of the back of the thorax, thus giving a firm basis to the wing. The clavicles are large and perfectly formed, in order to resist the powerful action of the pectoral muscles used in depressing * For an admirable history of the habits of the Mole, the reader is referred to Bell's British Quadrupeds, page 85. MAMMALIA. 657 the wings during flight ; and, in order to give those muscles a sufficient extent of origin, the sternum, although exhibiting the general characters of that of a quadruped, is deeply carinated along the mesial line. The hurnerus is of moderate length, but the fore-arm prolonged and slender; it consists, in fact, of but one bone, so that all movements of pronation and supination are necessarily impracticable. The carpal bones present their usual structure and arrangement at the base of the hand ; but those of the metacarpus, excepting that of the thumb, are so extraordinarily lengthened, that they themselves form a consider- able portion of the framework of the wing, which is completed by the phalanges of the fingers appended to their extremities. All these wire-like fingers are connected together by a broad duplicature of skin, derived from the sides of the body, which is continued along the whole length of the hind legs, and even fills up the interspace between these last and the tail ; this membrane forms an expansion sufficiently extensive to become converted into an organ of flight. The fingers composing this strange hand are obviously incapable of closing towards the palm, as ours do when grasping an object : their only movements are such as fold up the wing against the side of the body, by laying the fingers close along the side of the fore-arm, as in closing a fan. The thumb alone is left free; and this being short, and armed with a strong nail, is employed in enabling the creature to cling to some elevated object in those gloomy lurking-places wherein it hides during the day. (738.) The QUADRUMANA, next to mankind the most ele- vated members of the animal creation, are, as is evident from every point of their organization, the destined inhabitants of the trees ; neither will it appear astonishing, when we consider the extensive provision that has been made for the support of animal life amid the dense and pathless forests of tropical climates, that animals so intelligent, and capable of enjoyment, should have been widely disseminated through extensive regions of our globe. The great distinction characteristic of the Quadrumana is found in the organization of their feet, all of which are converted into prehensile instruments, whereby they can seize the boughs of the trees wherein they reside, and thus securely swing themselves from branch to branch, or even leap from one tree to another, with won- derful activity and precision. Their hands are constructed upon the same principle as those of Man ; their thumbs, although less 658 MAMMALIA. perfectly formed than our own, being opposable to the other fingers, and thus secure a firm and steady grasp. The bones of the fore- arm are free, and accurately articulated with each other ; the pro- nation and supination of the hand are, therefore, now accomplished with, facility. In the construction of the feet the same provisions have been made to enable them to take a firm grasp : the toes, like the fingers of the hand, are long and flexible, and the repre- sentative of the great toe is converted into a very perfect thumb, easily opposable to the rest ; the foot, or posterior hand, therefore, equals, or even surpasses in its powers of prehension, the hand which terminates the anterior limb. For many of the American monkeys a fifth hand has been provided, formed by their long and muscular tail, which, from its extreme flexibility, can be forcibly twisted around any foreign object, and holds it with a tenacious grasp. Thus abundantly furnished with prehensile instruments, the Quadrumana are obviously most excellent and accomplished climbers ; springing fearlessly through the forest by strong and vigorous leaps, or chasing their prey even to the topmost branches of the trees wherein they live. (739.) But, however grotesquely some of the more anthropoid Quadrumana resemble the human race, the approximation, even in their outward form, is at best exceedingly remote. The lower tribes, such as the Lemurs of Madagascar, walk on all fours like cats, and are still remarkable for their long and fox-like muzzle. The brutal and ferocious Baboons are scarcely more human in their appearance ; and even in the most elevated species, called by the vulgar ' wild men of the woods," the interval that separates them from humanity is wide indeed ! Taking the skeleton of the Orang-Outang (Simia Satyrus) as one of the most perfect examples met with in the class under con- sideration, it is at once evident that such an animal is by no means adapted to walk in an erect position, although well fitted to main- tain a semi-upright attitude, such as is best calculated for climb- ing. The skull, whose very outline indicates brutal ferocity, is armed with canine teeth, scarcely less formidable than those of the Tiger ; and the massive jaws of this creature are moved by muscles almost equally powerful. It is true that the protu- berance of the face is considerably diminished, and the facial angle thus materially enlarged ; but to make up for the feeble- ness of the upper jaw, consequent upon this reduced size of the bones composing it, additional strength is needed to re- MAMMALIA. 659 Fig. 309. sist the strong pressure of the enormous temporal muscles. This is given by adding strong buttresses to the outer angle of the orbit formed by the union of the frontal and the jugal bones, and thus the whole out- line of the face becomes more humanized. Another advance towards the condition of the human skull is apparent in the position of the foramen magnum, and of the condyles of the occipital bone, which are now considerably ad- vanced forwards beneath the base of the cranium, thus allow- ing the head to be articulated to the atlas at a very considerable angle with a line drawn through the axis of the spine ; a condi- tion evidently favourable to the erect posture. The thorax is well formed and capacious, giving great free- dom of respiration ; but the spi- 1 nal column is short and clumsy, neither does it present those graceful sigmoid curves that con- vert the human spine into a per- fect spring, upon the top of which the head is carried. The arms are of inordinate length and extremely powerful ; the joints perfect, and the clavicle well formed. But in the con- struction of the pelvic extremities the differences between this and the human skeleton become strikingly apparent. The pelvis is long, and the ossa ilii narrow ; the thighs and legs so short, that, when the creature stands erect, the tips of the fingers almost touch the ground. The protuberance of the os calcis is very slight ; and thus the posterior hands, although well adapted for taking hold of any object, are but ill calculated to sustain the weight of the body in an upright posture. Upon the ground, indeed, the living 660 MAMMALIA. animal puts the spectator in mind of a human being crippled in the lower extremities; but, in its native trees, these members, like those of the Sloth, are admirably suited to the circumstances under which the Orang is ordained to live. (740.) Having thus introduced the reader to the different orders of Mammalia, as well as to the principal differences observable in the arrangement of their osseous system, we must briefly glance at some few points connected with their myology, selecting those that seem most worthy of being specially pointed out to the notice of the anatomical student. To enumerate all the varieties that occur in the disposition of the muscular system in vertebrate animals, would, of course, be incompatible with the extent of this work ; and perhaps, even were it practicable, the details would scarcely possess much interest to the beginner in comparative anatomy. Considered generally, in- deed, the muscular system of quadrupeds conforms very accurately in its arrangement to that of the human subject ; and for the most part the same names are applicable to the individual muscles, al- lowance being made for such modifications in the manner of their origins and insertions as are rendered necessary by the disposition of the skeleton, or in order to accommodate them to the perform- ance of special functions. To enumerate, therefore, the muscles of the jaws, of the neck, of the spine, of the chest, of the abdomen, or even of the extremities, in such genera as have the members last mentioned completely developed, would only be to repeat cir- cumstances with which the human anatomist is already familiar : nevertheless, there are some points of practical importance con- nected with this part of our subject that must not be altogether passed over in silence. (741.) The diaphragm is a muscle only met with in the class before us, and in all Mammalia it forms the great agent in respiration ; dividing the thoracic from the abdominal cavity by a broad musculo-tendinous septum, and presenting a disposition in all essential particulars similar to that of Man. (742.) Another muscle of considerable anatomical interest is the cutaneous muscle provided for the movements of the inte- gument. In many tribes, more especially those which, like the Hedgehog, the Echidne, and the Porcupine, have the skin covered with spines, this muscle is extremely developed, invest- ing the greater part of the body with a thick layer of muscular fibres, called not improperly the panniculus carnosus. In Man, MAMMALIA. 661 too, this muscle exists, but under a very different aspect ; being only found in certain regions of the body, where it forms nume- rous cutaneous muscles adapted to different offices. In the neck, where it is principally developed, it is called the platysma myoides : in the facial region it is likewise of great importance ; the occipito-frontalis, the corrugator supcrcilii^ and other mus- cles connected with the expression of the countenance, being indu- bitably but portions of the fleshy pannicle. In the palm of the hand it is slightly visible, forming the palmaris brevis ; and even the little muscles connected with the external ear may be referred to the same series. (743.) In Whales no pelvis or posterior extremities exist ; it is needless, therefore, to remark, that the whole of the muscular system appropriated to those parts in higher animals must be totally wanting : but, in return, the muscles connected with the caudal portion of the spine are amazingly powerful, so as to render the horizontally expanded tail an instrument of propulsion, ade- quate to the necessities of these unwieldy animals. A large tri- angular muscle is found in the CETACEA, apparently replacing the quadratus lumborum, the psoas, and the iliacus, which arises from the lower surface of the last rib, from the last dorsal vertebra, and also from those of the loins and sacrum : from this powerful assem- blage of muscular fasciculi tendons are given off, to be inserted into the lower surface of the bones that support the tail, converting this organ into a mighty oar, adapted by its position to bring the creature with all speed to the top of the ocean in search of air. It is, as might be supposed, in the muscles of the limbs that the most important differences exist. In the anterior extremities, for example, the presence or absence of a clavicle will materially affect the disposition of the muscles of the shoulder, as will also the ex- istence of a coracoid process to the scapula ; nevertheless in their general arrangement they conform to those of Man. The rhom- boid muscles, which to creatures walking on all fours must be im- portant agents, are generally found in quadrupeds to take their origin as far forward as the head ; the serrati magni likewise, whereby in the prone position the weight of the body is as it were suspended from the scapula, must be immensely strong. The muscles acting upon the arm are similar in all the Mam- malia; but in the fore-arm, as might be expected from the very variable condition of this part of the skeleton, the disposition of the muscular system varies too, and even the existence of many 662 MAMMALIA. muscles could not be expected : thus as the movements of pro- nation and supination are, from the immovable condition of the bones of the fore-arm, impracticable in the CETACEANS, the RU- MINANTS, the SOLIPEDS, and others, the pronators and supina- tors are denied; or, if their representatives exist, they become simply assistants in flexion and extension. The flexors and exten- sors of the wrist are pretty constant, but the muscles devoted to the hand and fingers will vary in almost every order. The pal- maris longus, although generally present where the hand is flexi- ble, is wanting where its action upon the palmar fascia would be useless, as, for example, in the ungulate tribes. In quadrupeds there are two extensor tendons appropriated to each of the fingers that correspond to the four outer fingers of the human hand ; whilst in Man the index and little fingers only have auxiliary extensors. The abductor and extensor muscles of the thumb are not so per- fectly developed in any animals as they are in the human hand. The short extensor is, in fact, wanting even in Monkeys ; and in the lower orders of quadrupeds even the extensor longus and abductor are blended together, or totally wanting. The deep and superficial flexors of the fingers are very generally met with, the number of tendons furnished by each corresponding of course to that of the fingers themselves ; but in the Solipeds the two muscles are almost blended together. Even in the Rumi- nants, although these muscles remain separate, their tendons be- come confounded together, and divide again, to be inserted into the phalanges to which they are appropriated. In these Ungu- lata too, as we need scarcely say, the lumbricales and mterossei are quite deficient; and the short muscles of the thumb are com- pletely developed only in Man and in the Quadrumana. It is in the human species only that the lower extremities are organized so as to maintain the body in the erect position, and, in consequence, the glutsei muscles in the human body are enormously developed when compared with those of the lower animals ; but the other muscles derived from the pelvis and thigh present but slight differences throughout the whole class under consideration. In the leg and foot likewise it is not difficult to identify the muscles that correspond to those found in the human subject, but, as in the an- terior extremity, modified in their disposition and mode of insertion in accordance with the construction of the skeleton. The articulations whereby the different pieces composing the MAMMALIA. 663 Mammifcrous skeleton are connected to each other are constructed upon the same principles as in the human body, insomuch that to describe them even in general terms would be useless. The bones of the cranium and face, as in Man, are joined toge- ther by harmony or by suture. The articulations of the lower jaw are double, each presenting an interarticular cartilage ; except in the Cetacea, where, instead of such a structure, a very thick matted ligamentous substance, having its interstices filled with oil, passes directly from the condyles of the jaw to the temporal bones. The joints of the spine, thorax, and pelvis are all constructed upon the same principles as the corresponding articulations in the human subject ; and the same may, with slight exceptions, be said of those of the extremities. The chief differences will be found in the connection between the radius and ulna, the movements of rotation becoming gradually less manifest as we descend from Man : the tibia and fibula, too, ultimately become completely anchy- losed to each other. The hip-joint contains an internal liga- mentum teres ; but in a few instances, e. g. the Ornithorhyncus, the Echidne, the Sloths, the Elephant, the Seals, and the Orang Outang, this round ligament is deficient. The arrangement of the other articulations will be at once apparent, on reference to the figures of the different skeletons already given. (744.) Turning to the digestive system of Mammiferous ani- mals, their teeth first in- Fi s- 310. vite our at- tention. We have already, when describ- ing the os- seous frame- work of these elevated be- ings, exposed their general arrangement in the jaws of the diffe- rent orders ; but it still remains for us 664 MAMMALIA. to explain the varieties of their structure and the mode of their formation. The most remarkable form of teeth, one indeed that is unique, is met with in the Whalebone Whale (Balana mysticetus). The teeth in this Cetacean are -not, indeed, instruments of mastication ; but form a very curious apparatus, adapted to strain the waves of sea as through a sieve, and thus obtain from the ocean a suffi- ciency of food for the sustenance of its monstrous body. The whalebone (as it is improperly called) is attached to the gums of the upper jaw, being arranged in thin flat plates of some breadth, and varying in length according to the size of the whale.* These plates are placed in several rows, similar to teeth in other animals; they stand parallel to each other, having one edge di- rected towards the circumference of the mouth. The outer row is composed of the longest plates, and these are in proportion to the varying distances between the two jaws, some being fourteen or fifteen feet long, and twelve or fifteen inches broad, but towards the anterior and posterior part of the mouth they are very short. Tnferiorly each plate of whalebone is terminated by a broad fringe of horny fibres resembling hair ; and, seeing that in some whales there are above three hundred plates composing the outer row on each side of the mouth, the reader may form some idea of the extent of this enormous strainer, whereby the little Clio Borealis, and other small Mollusca, that swarm so abundantly in the Northern ocean, are caught by shoals preparatory to their being swallowed. For what is known concerning the growth of whalebone, we are indebted to John Hunter ; and, as it would be difficult to curtail his clear and concise description of the process, it is here given in his own words. -f- " The formation of whalebone is extremely curious, being in one respect similar to that of hair, horns, spurs, &c. ; but it has besides another mode of growth and decay, equally singular." " These plates form upon a thin vascular substance, not imme- diately adhering to the jaw-bone, but having a more dense sub- stance between, which is also vascular. This substance, which may be called the nidus of the whalebone, sends out thin, broad processes answering to each plate, on which the plate is formed, as the cock^s spur or the bulPs horn on the bony core, or a tooth on * J. Hunter, on the Structure and (Economy of Whales. Philos. Trans. 1787. t Vide supra. MAMMALIA. 665 its pulp ; so that each plate is necessarily hollow at its growing end, the first part of the growth taking place on the inside of this hollow." " Besides this mode of growth, which is common to all such substances, it receives additional layers on the outside, formed from the above-mentioned vascular substance, extended along the surface of the jaw. This part also forms upon it a semi-horny substance between each plate, which is very white, rises with the whalebone, and becomes even with the outer edge of the jaw. This intermediate substance fills up the spaces between the plates as high as the jaw ; acts as abutments to the whalebone; or is similar to the alveolar processes of the teeth, keeping them firm in their places. 1 ' "As both the whalebone and intermediate substance are con- stantly growing, and as we must suppose a determined length ne- cessary, a regular mode of decay must be established, not depend- ing entirely on chance, or the use it is put to. In its growth, three parts appear to be formed : one from the rising cone, which is the centre ; a second on the outside ; and a third, being the in- termediate substance. These appear to have three stages of dura- tion ; for that which forms on the cone, I believe, makes the hair, and that on the outside makes principally the plate of whalebone : this, when got a certain length, breaks off, leaving the hair project- ing, becoming at the termination very brittle : and the third, or intermediate* substance, by the time it rises as high as the edge of the skin of the jaw, decays and softens away like the old cuticle of the sole of the foot when steeped in water." (745.) Other kinds of teeth, met with among Mammals, are com- posed of calcareous earths deposited in a nidus of animal matter, and consequently resemble bones in the hardness of their texture. In F 'g- 31 * their simplest form these teeth consist of but one kind of mate- rial, called ivory; and in such cases there' is no distinction into classes as in the human sub- ject, every tooth being conical, and formed upon a simple pulp. Such are the teeth of the Por- * Mr. FTunter means, by " intermediate," interposed between the contiguous plates, not between the " hair" and the laminated whalebone. 666 MAMMALIA. poises (Delphinidte), and of tlie Caclielot Whales (Physeler). The example selected to illustrate their structure and mode of growth is a preparation of a portion of the jaw of the Bottle-nose Whale (Delphinus Tursio) contained in the Hunterian collection.* From this it is seen (Jig. 311) that each tooth of the Cetaceans in question is a hollow cone of ivory (a, >, c, d), which, on being split longitudinally, is found to contain a vascular pulp, exactly filling up its internal cavity. Tt is upon the surface of this pulp that the ivory matter is produced and deposited, stratum inter stratum, within the tooth, thus gradually adding to its substance as growth proceeds. In animals possessing a dental apparatus of this descrip- tion, Mr. Hunter observed that the teeth are not at first developed in the jaw, but appear to form in the gum upon the edge of the maxillary bones ; and that they either sink into the jaw as they lengthen, or, as is more probably the case, the alveoli rise to en- close their roots as growth advances. It would moreover appear that these creatures do not shed their teeth ; but that, as the jaw enlarges, new teeth are constantly produced from behind, while those towards the symphysis fall off, and their sockets become absorbed : thus the size of the teeth is made to keep pace with the increasing dimensions of the jaw.-)- The exact number of teeth met with in any species of these Whales will evidently be un- certain. In the male Narwal (Monodon) there are no teeth implanted along the margins of the jaws ; but from the intermaxillary bone of the left side of the face there projects a single tusk of great strength, which sometimes attains the length of eight or ten feet. This formidable weapon is fully developed only upon one side of the body ; nevertheless, the corresponding tooth exists in a rudimentary condition, enclosed in the opposite intermaxillary bone. In the Elephant, a creature which so obviously forms a connect- ing link between the gigantic Cetacea and terrestrial quadrupeds, tusks, more ponderous even than that of the Narwal, project from both intermaxillary bones : but these, as well as the tusks of other PACHYDERM AT A, grow upon a simple pulp, such as that which forms the teeth of the Bottle-nose Whale; are formed of ivory, without any enamel ; and their growth is only limited by the abrasion to which they are subject. * Preps. No. 327 and 328. t The Animal CEconomy, by John Hunter, with notes by Richard Owen, Esq. F.R.S. p. 353. London, 1837. MAMMALIA. 667 In by far the greater number of quadrupeds the teeth pre- sent a more complex structure, and consist of two distinct sub- stances of very different texture : the one analogous to the ivory of the simple teeth described in the last paragraph ; the other called enamel, of crystalline texture, and such extreme density as to withstand being worn away by acting upon the hardest materials used as food. Teeth of this description may be advantageously divided into two principal groups : first, those whose growth is continuous during the entire lifetime of the animal ; and, second, those which are completed at an early period, and then cease to grow. The first division includes the incisor teeth of the Rodentia, or denies scalprarii, as they have been termed. Such teeth are, in fact, chisels of most admirable construction, destined to gnaw the hardest kinds of food, and yet never to all appearance wearing away or becoming blunted by use. The annexed figure (312) represents a section of the incisor tooth, and of the left ramus of the lower jaw of a Porcupine Fig. 312. (Hystrix cristatd), and from this example the structure of such teeth will be readily understood. The bulk of the tooth consists of solid ivory (a), which in its texture and mode of growth resembles that of a simple tusk, being continually growing from behind by the addition of new matter produced from the vas- cular pulp (c), so that, were such a tooth not worn away constantly at the point, it would curl up over the face like the tusk of the Babiroussa ; and if by accident the opposing tooth in the upper jaw should be broken off, this circumstance in fact really takes place. But, besides the ivory-forming pulp (c), there is a vascular membrane (e) which exists only upon the anterior surface of the GC8 MAMMALIA. socket, its limits on each side being distinctly marked by a defined line. This membrane secretes enamel, and coats the convex surface of the tooth with a thin layer (b) of that dense substance. From this beautiful arrangement it results, that while the ante- rior end of the tooth is perpetually worn away by attrition against hard substances, the ivory is abraded more rapidly than the enamel that coats it in front ; thus, therefore, the tooth constantly preserves its chisel-like shape, and presents the sharp cutting edge formed by the layer of enamel. (746.) The second kind of teeth, composed of bone and enamel, are limited in their growth ; and the entire crown or projecting portion is invested with enamel covering its surface. The teeth of all the CARNIVORA, of the QUADRUMAXA, and also of MAN, are of this description. From marked differences in their form in different regions of the mouth, such teeth are conveniently divisible into different groups, called respectively incisores, lani- ares or canine teeth, pseudo-molar es or false grinders, and molar es or grinding teeth. Whatever may be the shape of teeth of this class, their mode of growth is similar to that observed in those of our own species. We have chosen, in order to illustrate this, the growing perma- Fi^.313. nent teeth of a young Lion, wherein the different organs employed in their formation are easily distinguishable. The ivory that forms the bulk of the tooth (Jig. 318, b) is formed by the surface of an internal pulp (a) ; and as it slowly accumulates, encroaching upon the central cavity, and penetrating more deeply into the socket, the fang is gradually formed, and the central pulp shrinks until, in the fully formed tooth, it becomes reduced to a thin membrane, richly supplied with vessels and nerves, which lines the small cen- tral cavitv that remains. MAMMALIA. 669 Before the progressively advancing tooth issues from the nidus wherein it is produced, the enamel is deposited upon the surface of the ivory by the lining membrane of the capsule (c), and becomes arranged in crystalline fibres placed perpendicularly to the surface of the ivory, until the whole crown of the tooth is adequately coated with this important additional substance. Meanwhile the growth of the tooth still proceeds by the lengthening of its root, until at last the crown issues from the jaw, and the enamel-secreting membrane (c) becomes obliterated. (747.) The most complex condition of the dental organs is that found in the molar teeth of herbivorous quadrupeds, which, being destined to act the part of mill-stones in grinding down and comminuting vegetable substances, must necessarily, like the mill-stones of human contrivance, have a grinding surface, presenting prominent edges and deep sulci, not liable to become worn even by the continual abrasion to which they are subjected. In order to obtain this end, the ivory and enamel indigitate, as it 'were, in the substance of the tooth ; and are, moreover, imbedded in a third material, not met with in the simpler forms, called the cementum or crusta petrosa. In consequence of this arrangement, seeing that the plates of ivory, of enamel, and of cement, are all of different degrees of hardness, the softer substances are most easily worn away; and thus these compound teeth always offer an efficient grinding surface. By inspecting the accompanying figure (fig. 314), representing a section of the tooth of an Elephant, the Fig.3\4. disposition referred to will be better un- derstood : the layers of enamel are seen to alternate with plates of ivory, while all the interstices are filled up by the cir- cum fused cementum. During the growth of a compound tooth of this descrip- tion, the enamel- secreting membranes derived from the capsule of the tooth, of 670 MAMMALIA. course, intercligitate with the ivory-forming pulps that arise from the bottom of the sockets, and thus the hard materials formed by them take the same arrangement. After these structures have been completed, one or other of the sets of pulps, most probably the enamel pulps, changing their action, fill up all the intervening spaces with the crusta petrosa. (748.) As during the growth of a quadruped the size of the jaws is continually increasing, a necessity exists for changing the teeth once or oftener during the life of the animal, in order to adapt these organs to the altered conditions required : hence the necessity for shedding the teeth of young animals, and replacing them with others of larger dimensions or more numerous than the first set. This is effected in two different ways, each of which demands our separate notice. In most quadrupeds, as, for example, in the Carmvora, the Quadrumana, and the greater number of herbivorous genera, the succession of the teeth is provided for precisely in the same way as in our own persons, namely, by the formation of a new tooth below each of the deciduous ones (fig- 313, d, d) ; so that, when the latter falls out in consequence of the absorption of its fangs, the former is ready to take its place. The germ of the second tooth is at first found imbedded in the jaw-bone, in the immediate vicinity of the roots of the one which it is destined to replace ; and, as its growth advances, the old and used tooth is gradually removed to make way for the new comer. The steps of this pro- cess are exactly similar to those by which the milk-teeth of a child are changed, and the details connected with it are familiar to every anatomist. But in the Elephant, and some other genera of PACHYDER- MATA, the succession of the teeth is effected in a different man- ner ; the place of the first formed being supplied by others that advance from behind as the former become used. Animals exhi- biting this mcde of dentition have the grinding surfaces of their molar teeth placed obliquely ;* so that, if they were to issue altogether from the gum, the anterior portion would be much more prominent than the posterior, notwithstanding that the opposed teeth act upon each other in a horizontal plane. The consequence of this arrangement is, that the anterior portion of these teeth is ground down to the roots, and worn away sooner than the poste- * Cuv. Lemons d'Anat. Comp. torn. iii. p. 122. MAMMALIA. 671 rior portion. Moreover, the posterior part of the tooth is consi- derably wider than the anterior ; so that, as the succeeding tooth advances from behind, there is always sufficient room to receive it ; and in this way, by the time that the first tooth is quite destroyed and falls out, a new one from behind has already taken its office. There is, therefore, no absorption of the roots of these teeth, but they are ground down from the crown to the stump. The new tooth that thus advances from behind is always of larger dimensions than that to which it succeeds ; because the animal itself has grown in the interval, and the jaws have become proportionally developed. The Elephant in this way may have a succession of seven or eight teeth on each side in both jaws, or from twenty-eight to thirty-two in all ; and nevertheless, seeing that the anterior ones successively fall out, there are never more than two visible at once above the gums on each side, or eight in all : generally, indeed, there is only one visible at a time. Every successive tooth is composed of more laminae than that which immediately preceded it, and a longer time is required to perfect its growth. Nearly the same account of this process was found in the Manuscripts of John Hunter,* who lucidly accounts for such an aberration from the ordinary course of proceeding. " These crea- tures," says that distinguished observer of Nature, " do not shed their teeth as other animals do that have more than one ; for those that have more than one tooth can afford to be for some time without some of their teeth : therefore the young tooth comes up in many nearly in the same place with its predecessor, and some exactly underneath ; so that the shedding tooth falls sometimes before the succeeding tooth can supply its uses. But this would not have answered in the Elephant ; for if the succeeding tooth had formed in the same situation with respect to the first, the animal would have been for some time entirely deprived of a tooth on one side, or, at least, if it had one on the same side in the oppo- site jaw, that one could have been of no use ; and if this process took place in both sides of the same jaw, and in either jaw, the animal would have been entirely deprived of any use of the two remaining." (749.) The teeth" of Mammalia being thus adapted to so many various offices, and serving under different circumstances to hold, * Descriptive and Illustrated Catalogue of the Physiol. Series of Comp. Anat. in the Mus. Roy. Coll. Surg. Lond. Part i. p. 100. 672 MAMMALIA. to bruise, to cut, to tear, or to grind alimentary substances, we must naturally expect the movements of which the lower jaw is capable, to be in correspondence with the nature of the dental apparatus. In MAN, as the student well knows, in consequence of the laxity of the ligaments that connect the inferior maxilla with the temporal bone, and the thickness of the articular cartilage that is interposed between the convex surface of the condyle and the shallow glenoid cavity, every kind of motion is permitted in con- formity with the omnivorous habits of the human race; and the temporo-maxillary articulation is no longer a mere hinge, but the teeth can be made to act upon each other by rubbing their grinding surfaces in all needful directions. In the Herbivorous quadrupeds these triturating motions are likewise extensive. In the RODENTIA the movements of the lower jaw are principally backwards and forwards, thus giving free play to their chisel-like teeth whilst employed in eroding hard substances ; and in the CARNTVORA, where there is no necessity for any grinding motion, the condyle is so locked into a deep and transverse glenoid cavity, that the movements of a hinge only are permitted. (750.) But, whatever the degree of motion conferred upon the lower jaw, the muscles that act upon it are exactly comparable to those of the human subject. The masseter is strengthened in proportion to the hardness of the substances used for food ; the temporal covers a greater or less extent of the cranium, as the jaws are stronger or more feeble; and even the pterygoid muscles differ only in relative size and form from those of Man. The digastric muscle, however, which is an important agent in depressing the lower maxilla, does not preserve the same ar- rangement in the lower quadrupeds that it presents in the human species. In Monkeys indeed it still exhibits two fleshy bellies, and a central tendon that traverses the stylo-hyoideus ; but in general it is a single fleshy muscle, arising from the neighbourhood of the mastoid process, and inserted near the angle of the jaw. (751.) The tongue in nearly all the Mammifera is composed of the same muscles as in Man ; and their disposition is so similar, as to render any detailed enumeration of them quite unnecessary. The only exceptions worthy of notice are found in the Ant-eaters (Myrmecophaga), and in the Echidna, animals possessing tongues of remarkable length and slenderness, by means of which they secure their insect prey. MAMMALIA. 673 In botli these animals the tongue suddenly becomes much con- tracted at the place where it begins to be free from the surround- ing parts. It then appears to be made up of two very long and slender muscular cones, laid one upon the back of the other, their apices being at the end of the tongue.* Each of these cones consists of two muscles: one external, composed of a multitude of distinct fasciculi investing the internal muscle in a circular manner, and forming around it numerous little rings resembling the annelli of an earth-worm. The internal muscle, on the contrary, is of great length ; it arises from the middle and upper part of the sternum, runs forward along the neck, passes between two layers of the mylo-glossus, and afterwards becomes surrounded by the annular muscle. It is composed of distinct fasciculi, rolled upon themselves in an elongated spiral ; the external fibres terminate at the first rings, those beneath attain the rings that succeed, and so on until the innermost fibres reach quite to the extremity of the tongue. It is easy to perceive that, by its action, this muscle will shorten the tongue until it lies in a very small compass, or bend it in any direction ; whilst the annular muscle will lengthen it, exactly in the same way as the body of a leech is extended or contracted. In the Ant-eater the annular muscle does not appear so dis- tinctly double as it does in the Echidna; but it forms by itself almost all the substance of the tongue, which is thus capable of being elongated to a wonderful extent. (752.) Regarding the tongue with reference to the sense of taste, the Mammalia may be looked upon as the only animals capable of receiving much enjoyment from this source, since in them alone the lingual mucous lining seems to be perfectly adapted to gustation. Even among these highly endowed crea- tures, it is only in Man, and those Herbivorous orders that pre- pare their food in the mouth by a prolonged mastication, that the sense in question exhibits much delicacy of perception ; for the Carnivorous quadrupeds, seeing that they tear to pieces and swallow their food in large morsels, can scarcely be supposed to pay much attention to its sapid qualities. In the Cat tribe (Felida), indeed, all the middle portion of the surface of tongue is covered over with sharp, recurved, and horny spines, adapted as it were to file off remnants of soft flesh from the * Cuv. Le9ons d'Anat. Comp. torn. iii. p. 264. 674 MAMMALIA. bones of their victims ; and the gustatory papillae are elsewhere of small dimensions. The tongue of the Porcupine, likewise, is armed on each side near its extremity with broad, horny, and sharp scales ; but, with these exceptions, the mucous covering of the tongue, the various kinds of papillae upon different parts of its sur- face, and, moreover, the distribution of the nerves supplied to it, differ in no important circumstance from what is observed in the human organ of taste. (753.) Importantly connected with the perfection of the sense of taste, and materially assisting in the mastication of food, is the salivary apparatus, which, throughout all the Mammalia, is made up of the glands, that offer the same general arrangement as in Man. The parotids vary principally in their proportionate size, and their ducts always perforate the lining membrane of the mouth in the vicinity of the molar teeth. The submaxillary and the sublingual glands are also very ge- nerally present ; and, as in the human subject, the saliva that they furnish enters the mouth beneath the under surface of the tongue. The mucous lining of the lips and cheeks is likewise studded with muciparous follicles, called from their situation buccal, molar, or labial glands ; these likewise serve to lubricate the oral cavity. In the Seals (Phocida) there are no parotids, neither are these glands found in the Echidna hystrix, or in the Ant-eater (Myrme- cophaga) ; but in the last-named genus their place is supplied by two other secreting organs, of which Cuvier gives the following de- scription.* One is in contact inferiorly with the upper edge of the masseter muscle, and fills up a great part of the space that repre- sents the temporal, zygomatic, and orbital fossae, where it partially embraces the globe of the eye : the excretory duct derived from this gland opens into the mouth, behind the superior maxillary bone. The other, which is probably destined to furnish the viscid secretion that coats the worm-like tongue of this animal, is oval and flat, lying in front of the tendon of the masseter behind the angle of the lips, and then running along the edge of the lower lip as far as its middle. Its canal opens externally in a groove at the commissure of the lips, and a white, thick, and tenacious fluid may be pressed out, from the cells of which the gland seems to be made up. In a few species, in addition to the salivary glands met with in * Lefons d'Anat. Comp. torn. iii. p. 215. MAMMALIA. 075 Man,* there is a group, apparently a continuation of the molar, which mounts up along the superior maxillary bone, beneath the zygoma, even to behind the globe of the eye. The excretory ducts derived from this group pierce the mucous membrane near the posterior margin of the superior alveolar ridge; such an arrange- ment is met with in the Ox, the Sheep, and the Horse. In the AMPHIBIOUS MAMMALIA the salivary system is very feebly developed ; and in the GET ACE A, as might be expected from their habits, no salivary glands whatever are to be detected. (754.) Before considering the mechanism of deglutition in the Mammalia, we must, in the next place, briefly describe their hyoid apparatus ; more especially as this remarkable system of bones, which in the lower Vertebrata was so importantly connected with the respiratory function, is now reduced to an extremely simple condition, and, although it is still intimately connected with the larynx, is more particularly remarkable, as forming a centre of attachment for almost all the muscles of the throat. Perhaps there is no part of the bony framework of the body that exemplifies more strongly than the os hyoides the impossi- bility of attaining correct physiological views relative to the compo- sition of the skeleton by the mere examination of the human sub- ject. Let the student, for instance, compare for a moment the os hyoides of Man with that of the Fish, or of the Amphibious Rep- tile, and endeavour, in the simple segment of a circle presented by the one, to find the analogues of the body and complicated arches of the others; then, doubtless, he will find that, without some inter- mediate gradations of form, it is not easy to trace the slightest rela- tionship between them. The human os hyoides consists of a central portion and two cor- nua; but these are generally so completely consolidated as to form but one bone, which is connected by the interposition of a broad ligament with the upper margin of the thyroid cartilage ; moreover, two smaller appendages, called the lesser cornua, are articulated with the upper surface of the hyoid bone, close to the point of junction between the cornua majora and the body; from whence ligaments, called the stylo-hyoid, pass upwards and backwards to the styloid processes of the temporal bone. All the apparatus of hyoid arches passing between the body of the bone and the base of the cranium, which were so largely * Le9ons d'Anat. Comp. torn. iii. p. 210. 2x2 676 MAMMALIA. developed in the lower Vertebrata, have therefore totally disap- peared ; and the question to be solved is, how we may identify the remaining portions with any of the elements of the more complex structures that have come under our notice. (755.) Difficult as this would be to the student who had con- fined his attention to the human body, on referring to the os hyoides of a quadruped, one of the Carnivora for instance, the analogies become at once perceptible. The body (jig- 315, a) is evidently the representative of the J . Fig. 315. central portion of the hyoid apparatus in Fishes (Jig. 221, 42), in Reptiles (fig. 260, 5), and in Birds (fig. 271). The lingual elements found even in birds are quite obliterated ; but two arches still remain. The posterior of these (fig. 315, <f), which represent the larger cornua of the human os hyoides, do not reach the cranium, but, as in Man, are attached by muscle and ligament to the thyroid cartilage ; while the anterior cornua, so small in Man, are in quadrupeds by far the largest, each consisting of two pieces, of which the second are articulated with the extremities of the styloid bones (c, c), and these last are in turn joined to the tem- poral bones by means of articulating surfaces. In Man the styloid bones (c) become anchylosed with the temporal, giving rise to the i( styloid processes ;" and the intermediate pieces of the anterior cornua (b) have their places supplied by ligaments (the stylo- hyoid) : in this way, therefore, the hyoid apparatus attains the form that it exhibits in the human skeleton. (756.) The muscles connected with the os hyoides in quadru- peds correspond with those met with in the human body ; and their action in effecting the deglutition of food is well known to the anatomical reader. (757.) The passage of the fauces in the Mammalia presents an organization peculiar to the class, and exhibits structures adapted to prevent alimentary materials from entering the air-passages during the operation of swallowing. The most remarkable of these is the epiglottis, forming a valvular fibre-cartilaginous lid, that accurately closes the opening of the larynx during the transit of MAMMALIA. 677 food into the throat. The communication between the posterior nares and the faucial cavity is likewise protected by a musculo- membranous valve, called the velum pendulum palati ; but as, with the exception of the CETACEA hereafter to be noticed, the arrangement of these parts exactly resembles what is seen in the human subject, it would be superfluous to describe them more minutely in this place. (758.) The bag of the pharynx in all the Mammalia is similar in its structure to that of Man ; and its muscles, namely, the stylo-pharyngeus, and the three constrictors, although stronger than in our own species, offer no differences worthy of more parti- cular notice. (759.) The oesophagus, leading from the termination of the pharynx into the stomach, is a long muscular tube, that traverses the chest in front of the bodies of the dorsal vertebrae, and, having pierced the diaphragm, reaches the abdominal cavity. Its lining membrane is loose and much plicated, so as to allow of consider- able dilatation ; but externally its walls are very muscular, the surrounding muscles being arranged in two distinct layers. In Man the outer stratum of muscular fibres is disposed longitudi- nally, while the inner layer consists of circular fibres ; but in most other Mammalia both these layers assume a spiral course, and cross each other obliquely as they embrace the cesophageal tube. (760.) The stomach itself presents such endless diversity of form, that merely to enumerate all the details that have been amassed relative to this part of our subject would fill many vo- lumes, without perhaps at all advancing our real knowledge con- cerning the process of digestion ; we must, therefore, content our- selves with a very general view of the organization of this important viscus, and regard the Mammalia as possessing either simple, complex, or compound stomachs, each of which will deserve a distinct notice. (761.) In the simple form of stomach the organ consists of a single cavity, as is the case in the human species, let the shape of the viscus be elongated, pyriform, or globular; for in this respect there is every possible variety ; but whatever its form, or the relative positions of the cardiac and pyloric orifices, its struc- ture corresponds with that of Man in all essential particulars. This kind of stomach exists in by far the greater number of Mammals. (762.) In the complex stomach the viscus is made up of several compartments communicating with each other, but without pre- 678 MAMMALIA. sen ting any difference of organization, such as in the present state of physiological knowledge would lead us to suppose them to pos- sess different functions: neither are we at all able to find any con- nection between such an arrangement and the nature of the sub- stances used as food. The Kangaroo (Macropus major), the Kangaroo Rat (Hypsiprymnus), the Porcupine (Hystrix), and the Hi/rax, are amongst the most striking examples. (763.) The compound stomach is that possessed by the Ru- MINANTIA, or animals that chew the cud ; and consists of four distinct cavities, differing very materially both in their size and in the arrangement of their lining membranes. The first and by far the largest cavity (Jig. 316, d) is called the paunch (rwwiew), and is of very great size, occupying a considerable portion of the abdominal cavity, and forming the great receptacle into which the crude vegetable aliment is received when first swallowed : this chamber is lined with shaggy villi. The second cavity (reti- culum) (c) is much smaller, and its walls are covered with numerous polygonal cells, from whence it derives the name it bears. The third chamber (e), called the psalterium^ has its lining membrane disposed so as to form deep lamellse, arranged longitudinally in alternating large and small layers, and thus presenting a most extensive surface. The fourth stomach (abomasus) (f) also exhibits very numerous folds of mucous membrane : it is of a pyriform shape, and by its smaller end terminates at the pylorus (g). The three first stomachs are lined internally with a thin cuticular investment ; but the last, apparently the representative of the single stomach of those quadrupeds that have but one stomachal cavity, is coated with a soft membrane that furnishes abundantly the ordi- MAMMALIA. 679 nary gastric secretions, and appears to be more especially the di- gestive stomach. The passage of the food through these different chambers will be easily understood on referring to the preceding figure, in which the course of the aliment before and after rumination is indicated by the direction of the probes a, b. The oesophagus, it will be observed, communicates on the one hand with the paunch d, and on the other with the cavities c, e,jf; and, moreover, by means of a muscular fold formed by the walls of the second cavity, a pas- sage may be formed leading directly into the third stomach (e) without communicating with the second (c). The process of rumination would, therefore, seem to be effected in the following manner. The herbage when first swallowed in an unmasticated condition passes into the capacious paunch (rf), where it accu- mulates, and undergoes, no doubt, a kind of preliminary mace- ration. When the RUMINANT has done grazing, and is at lei- sure, the food is again regurgitated into the mouth, to undergo more careful and complete mastication : for this purpose, a part of it is admitted into the reticulum (c), and there formed into a smooth and lubricated bolus ; which, being expelled into the oeso- phagus, is immediately seized by the spiral muscles surrounding that canal, and forced forwards into the mouth. After undergoing a thorough triturition, the aliment is once more swallowed, and it then enters into the third stomach e, passing along the muscular fold that leads from the oesophagus into that compartment. Here it is spread out over the extensive surface formed by the laminated walls of the psalterium, and is prepared for admission into the last cavity f, which, as has been said, is the true digestive stomach. (764.) While the young Ruminant continues to be nourished by its mother's milk, the three first cavities are undeveloped and com- paratively very small ; so that the milk passes on immediately into the fourth stomach, to be at once appropriated as aliment. (765.) In the Camel, the Dromedary, and the Llama, the walls of the reticulum and of a portion of the paunch are ex- cavated into deep cells or reservoirs bounded by muscular fasci- culi, wherein water may be retained in considerable abundance, un- mixed with the contents of the stomach ; it is in consequence of this arrangement that these animals are able to subsist for many days without needing a fresh supply of water even during long journeys in a tropical climate. 680 MAMMALIA. (766.) In the CETACEA the stomach consists of several bags that communicate with each other. These bags vary from five to seven in number ; but in the present state of our knowledge concerning the physiology of digestion it is difficult to divine what is the purpose of such an arrangement, more especially as rumina- tion is here out of the question. The first stomach of the Whale is, however, no longer merely a reservoir,* as the food under- goes a considerable change in it. The flesh of its prey is entirely separated from the bones, which proves that the secretion of this cavity has a solvent power. This was found to be the case in the Bottle-nose Porpoise and in the large Bottle-nose Whale ; in both of which several handfuls of bones were contained in the first cavity, without the smallest remains of the fish to which they had belonged. In others the earth had been dissolved, so that only the soft parts remained ; and, indeed, it is only partially digested materials that can be conveyed into the second and third cavities, the orifices being too small to permit bones to pass. (767.) The rest of the alimentary canal in most quadrupeds, like that of Man, is divisible into the small and the large in- testines ; the division between the two being marked by one or even two appendages, called respectively the ctecum and the appendix vermiformis. The small intestines require no particular description, as in all minor circumstances, such as their proportionate length and diame- ter, or in the number and arrangement of the valvula conniventes, they do not differ from the human. The large intestines, how- ever, offer very great variations of structure, and will therefore merit our more attentive consideration ; we shall accordingly lay before the reader the following resume of the principal facts connected with this subject, as given by the indefatigable Cuvier.'J' (768.) In Man, the Orangs (Simia), and the Wombat (Phas- colomys), both caecum and vermiform appendage are met with. (769.) In the other QUADRUMANA, the DIGITIGRADE CAR- NIVORA, the MARSUPIALIA, the RODENTIA, the PACHYDER- MATA, the RUMINANTIA, the SOLIPEDS, and the AMPHIBIOUS MAMMALS, there is a csecum without any vermiform appendage. (770.) Neither ceecum nor appendix vermiformis are found in the EDENTATA, the PLANTIGRADE CARNIVORA, nor in the CE- TACEA. * Sir E. Home, Lectures on Comp. Anat. vol. i. p. 225. t Lejons d' Anatomic Comparee, torn. iii. p. 465. MAMMALIA. 681 Numerous exceptions, of course, occur to the above summary ; but it would be useless to notice them in a survey so general as the present. Even where no caecum exists, the separation between the large and small intestines is generally indicated by a valve (ileo-cotic) formed by the lining membrane of the bowel : this, for example, is the case in the Sloths and Armadillos. (771.) In all the Mammalia that possess a caecum, this organ appears to be a prolongation of the colon beyond the point at which the small intestine enters its cavity. The caecum thus formed varies materially, both as relates to its size, shape, and structure : in animals that live upon vegetables, and even in some that are omnivorous, it is generally very large, gathered into sac- culi, and often distinctly glandular ; but in such as live upon flesh it is always small, and its cavity smooth, resembling a small intes- tine. (772.) The assistant chylopoietic viscera, namely, the liver, the pancreas, and the spleen, are constructed upon the same prin- ciples as in the human subject, and, except in a few minor circum- stances, offer little to arrest our particular notice. (773.) The liver occupies the same position as in Man, being principally situated in the right hypochondrium, where it is se- curely suspended by broad folds of peritonaeum connecting it to the abdominal surface of the diaphragm and to the circumjacent parts. It is most frequently, especially in the more active carni- vorous families, divided by deep fissures into several lobes ; a dis- position whereby the free movement of this part of the body is evidently facilitated. The gall-bladder, when present, which is not invariably the case, receives the bile indirectly through a cystic duct derived from the hepatic, so that the biliary fluid, poured into the duodenum through a ductus communis choledochus, is de- rived either immediately from the liver, or is regurgitated from the gall-bladder as occasion requires. The pancreas resembles the human in every particular, and its secretion enters the duodenum at the same point as that of the liver. The spleen is always attached to the stomach by a duplicature of the peritonaeal lining of the abdomen, and is organized in the same manner as that of Man, except in the CETACEA, where this viscus is divided into several small portions quite distinct from each other. 682 MAMMALIA. (774.) The system of the vena port<p is made up of the ve- nous trunks derived from the spleen, the stomach, the pancreas, and the intestinal canal : these all unite to form one large cen- tral trunk, which after entering the liver again divides and sub- divides minutely in that viscus, and furnishes the venous blood, from which the bile is principally if not entirely elaborated. (775.) The peritonaeum, or the serous membrane lining the abdominal cavity, forms in the Mammalia a shut sac, and by its numerous inflexions invests all the chylopoietic viscera, forming broad mesenteric folds to support the intestines ; it thus encloses between its laminae the entire system of mesenteric vessels, and also the lacteals derived from the alimentary canal : as to the rest, its structure and disposition, even to the formation of the omental sacs, differ in no important respect from what is found in the hu- man body. (776.) The chyle, the result of the digestive process, is taken up from the mucous lining of the intestinal canal by innumerable microscopic orifices that form the commencement of the lacteal system, which in the Mammalia seems to assume its most perfect developement. This important system of absorbent vessels con- sists of slender canals enclosed between the two layers of the me- sentery, to the root of which they converge from all the tract of the intestine. The valves formed by the lining membrane of these tubes are in Mammals so numerous and perfect that it is no longer possible to inject them from trunk to branch. Before ter- minating in the thoracic duct, these vessels permeate numerous 4t mesenteric glands," as they are called, by means whereof they appear to communicate freely with the venous system ; but the bulk of the matter absorbed enters a kind of reservoir called the " receptaculum chyli" whence, by means of the thoracic duct, the chyle is conveyed to be mixed up with the mass of the circulating fluid, and is ultimately poured into the vena innominata at the junction of the jugular and subclavian veins of the left side of the body. (777.) The lymphatic system of Mammals, as far as it has been studied, conforms in its arrangement to that of Man. (778.) Neither will it be at all necessary to describe at any length the construction of the respiratory and circulatory organs in the class now under consideration ; seeing that the structure of the lungs, the mechanism of respiration, the arrangement of the pulmonary vessels, the cavities of the heart, and the general dis- MAMMALIA. 683 position of the arteries and veins of the systemic circulation differ in no material circumstance from what is met with in our own per- sons. The lungs, occupying the two sides of the chest, are each con- tained in a distinct chamber, formed by the ribs and diaphragm, without in any part adhering to its walls. Each lung is enclosed in a serous cavity formed by the pleura, which, after lining the ribs, the intercostal muscles, and the thoracic surface of the dia- phragm, is reflected on to the lung itself at the point occupied by the roots of the pulmonic vessels, and invests the entire surface of the viscus ; it moreover passes deeply into those fissures that separate the lung into several distinct lobes. In the interspace between the two pleurae, called the medias- tina, is lodged the heart, contained in a fibro-serous envelope (the pericardium) ; and behind this the oesophagus, accompanied by the principal trunks of the vascular system, passes through the thorax into the abdomen. (779.) Each lung is a closed bag, composed of innumerable cells that communicate with the terminations of the bronchial tubes, and collectively present an immense surface, over which the blood contained in the capillaries of the pulmonary vessels is made to circulate. The inspiration and expiration of air are effected by the alter- nate movements of the diaphragm and of the walls of the thora- cic cavity, whereby the atmospheric fluid is drawn into and ex- pelled from the pulmonary cellules, and is thus constantly renewed as it becomes deteriorated by the abstraction of the oxygen con- sumed during the process of converting the venous into arterial blood. The purified blood, after passing through the pulmonary capil- laries, is collected in an arterialized condition by the pulmonary veins, and conveyed to the systemic side of the heart, which offers the same arrangement throughout the entire class, consisting of an auricular chamber (Jig. 317, c), and of a very muscular ventricle, a, the auricula-ventricular opening being guarded by mitral valves and columns earner similar to those found in the human heart. From the left ventricle the blood is driven into the aorta, e, the commencement of which is guarded by three semilunar valves, and thus it passes through the entire system. When again collected from the periphery of the body, the now vitiated fluid is returned to the heart by the venous system, and 684 MAMMALIA. Fig. 317. poured through the vena cavte into the right or pulmonic auricle ; and hence it passes into the right ventricle (Jig. 317, b), to be again returned through the pulmonary artery to the lungs, thus completing the circulation. (780.) But although the general arrangement of the circulatory and respiratory organs in all Mammals thus in every respect re- sembles that which exists in the human body, there are of ne- cessity variations in the distribution of certain parts of the san- guiferous system, adapted to the peculiarities of organization pre- sented by the different orders and even families of this great class, which must not be wholly passed over in silence. (781.) In the GET ACE A, for instance, many interesting circum- stances are observable in the arrangement of the vascular system. In the herbivorous ge- nera, as for example in the Dugong, the two sides of the heart are separated to a consider- able extent by a deep fissure (Jig. 317, cr, &), so that the pulmonary and systemic hearts are much more evidently dis- tinct viscera than they ap- pear to be in the quadru- pedal forms ; neverthe- less, in the Whalebone and Spermaceti Whales the heart assumes the usual appearance, and is only remarkable for its amazing size ; this, indeed, may well have attracted the notice of Hunter,* while investigating such gigantic beings. " In our examination of par- ticular parts," says that eminent anatomist, " the size of which is generally regulated by that of the whole animal, if we have only been accustomed to see them in those which are small or middle- sized, we behold them with astonishment in animals so far exceed- ing the common bulk as the Whale. Thus the heart and aorta of the Spermaceti Whale appeared prodigious, being too large to be contained in a wide tub, the aorta measuring a foot in diameter. * The Animal (Economy, by J. Hunter, with Notes by Professor Owen, p. 366. a MAMMALIA. 685 When we consider these as applied to the circulation, and figure to ourselves that probably ten or fifteen gallons of blood are thrown out at one stroke, and moved with an immense velocity through a tube of a foot in diameter, the whole idea fills the mind with wonder." (782.) In the arrangement of the blood-vessels of the CETA- CEA many interesting peculiarities are met with.* The general structure of the arteries, indeed, resembles that of other Mam- mals, and where parts are nearly similar their distribution is likewise similar. But these animals have a greater proportion of blood than any others known, and there are many arteries appa- rently intended as reservoirs, wherein a large quantity of arterial blood may accumulate, apparently for important purposes, where vascularity could not be the only object. Thus the intercostal arteries divide into a vast number of branches, which run in a ser- pentine course between the pleura and the ribs, and penetrate the intercostal muscles, everywhere lining the walls of the thorax. These plexiform vessels, moreover, pass in between the ribs near their articulation, and anastomose extensively with each other. The medulla spinalis is likewise surrounded with a net-work of arteries in the same manner, more especially as it comes out from the brain, where a thick substance is formed by their ramifications and convolutions, and these vessels most probably anastomose with those of the thorax. The precise function assigned to this ex- tensive plexus of arteries has not been as yet satisfactorily deter- mined, although it is doubtless a receptacle wherein arterial blood is stored up during the long-continued submersion to which these animals are so frequently subjected. (783.) As the GET ACE A have no pelvic extremities, the aorta, instead of bifurcating into iliac arteries, is entirely appropriated to supply the enormous tail beneath which it is continued, enclosed in a canal formed by the roots of the inferior spinous processes of the caudal vertebrae, that are here again developed as in fishes. (784.) The venous system in the Cetacean order is equally remarkable for the plexuses formed by it in different parts of the body ; of these the most important communicates with the abdo- minal cava, and is of immense extent. The veins of these crea- tures, moreover, are almost entirely deprived of valves, so that every possible arrangement has been made to delay the course of * Hunter, ut supra, p. 365. 686 MAMMALIA. the circulating blood during the temporary suspension of respira- tion that occurs whenever the animal plunges beneath the surface of the water. (785.) In other aquatic Mammals that dive, and are thus sub- jected to prolonged immersion, large dilatations are found con- nected with the principal trunks of the venous system in the neighbourhood of the heart, in order to prevent a dangerous dis- tension of these veins while the circulation is impeded and re- spiration put a stop to. This is particularly remarkable in the Seal tribe ; and in these Carnivora we are assured by good authorities that it is not uncommon to find the foramen ovale of the heart, and the ductus arttriosus, which in the fetus allows blood to pass from the pulmonary artery directly to the aorta, still open even in the adult animal ; but this arrangement, as we are well satisfied, is by no means to be regarded as the normal structure of the heart in a Seal. (786.) In many of the long-necked herbivorous quadrupeds a peculiar provision has been made in the disposition of the internal carotid arteries, apparently intended to equalize the force of the blood supplied to the brain in different positions of the head: for this purpose the arteries referred to, just as they enter the skull, divide into several branches, which again unite so as to assume a kind of plexiform arrangement, forming what is called the rete mirabile of old authors. The effect of this subdivision of the main trunk into so many smaller channels will evidently be to moderate the rapidity with which the blood would otherwise enter the cranium, and thus preserve the brain from those sudden in- fluxions to which it would otherwise be constantly liable. (787.) We must likewise notice a structure, in some respects similar to the above, that exists in the arteries both of the an- terior and posterior extremities of the Sloths (Bradypus). In these slow-moving animals, the axillary and iliac arteries, just be- fore entering the limbs to which they are respectively destined, suddenly divide into numerous small channels, which again unite into one trunk before the arteries of the member are given off. No doubt such an arrangement will very materially retard the course of the blood as it flows through these multiplied canals, and perhaps is materially connected with the long-enduring strength of muscle that enables these creatures to cling without fatigue to the branches whereby they suspend themselves. Innumerable other minor differences in the course and distribu- MAMMALIA. 687 tion of the blood-vessels might of course be pointed out, a few of which may require notice elsewhere ; but, generally speaking, the arrangement of the vascular system in all quadrupeds is so similar, that the anatomical student who may push his researches thus far will never be at a loss in identifying the different vessels, and com- paring them with those found in the human body. (788.) Although the respiration of Mammalia is inferior, as regards the extent to which their blood is exposed to the influ- ence of the atmosphere, to the perfection of this process in Birds, nevertheless, such is the elevated temperature of the body in these hot-blooded animals, that a warm covering of some non-conduct- ing material is here absolutely requisite to retain the vital warmth, and defend them against the thermometrical changes of the ele- ment they inhabit. Their skin is generally, therefore, clothed with a warm covering of hair ; a cuticular structure, the nature and growth of which it behoves us now to examine. We must first, however, notice the organization of the skin itself, and then the nature of the various structures employed to defend it will be readily understood. The skin of all Mammals, like that of the human body, con- sists of the cutis, or vascular true skin ; of the epidermis, or cuticle ; and of a thin layer of pigment interposed between the two, which is a diversely coloured secretion, deposited like the cuticle upon the surface of the cutis. The hairs that cover the quadruped, whatever be their form or thickness, are cylin- ders of horny or cuticular substance, that grow upon so many minute vascular pulps, from the surface of which the corneous mate- rial is perpetually secreted. Some kinds of hair are per- manent, and, if constantly cut, will continue to grow during the whole life of the 318. 688 MAMMALIA. animal ; such is the hair of Man, and that which forms the mane and tail of the Horse : but generally the hair is shed at stated periods, to be replaced by a fresh growth. For the most part, these structures are so minute, that the apparatus employed in forming them escapes observation ; but in very large hairs, such as those that compose the whiskers of the Seal, or of the Lion, it is not difficult to display the organs by which they are secreted. The appended figure, taken from one of the drawings in the Hunterian collection, represents a section of the lip of a young Lion, and in it all the parts connected with the growth of the larger hairs are beautifully displayed. A bulb or sacculus, formed by an inward reflection of the cutis (Jig- 318, B, e), and lined by a similar inflection of the cuticle (/), contains in its fundus a vascular pulp (g, g, $), well supplied with large vessels and nerves (h). It is from the surface of the pulps (g), exhibited upon a mag- nified scale at A, that the horny stem of the hair is gradually se- creted, and its length of course increases in proportion to the accumulation of corneous matter continually added to the root. (789.) Various are the appearances, and widely different the uses, to which epidermic appendages, in every way analogous to hair, both as relates to their composition and mode of growth, may be converted : the wool of the Sheep, the fur of the Rabbit, the spines of the Hedgehog, the quills of the Porcupine, the scaly covering of the Manis, and even the armour that defends the back of the Armadillo, are all of them but modifications of the same struc- tures, adapted to altered conditions under which the creatures live. Even the horn upon the snout of the Rhinoceros is but an ag- glomeration of hairy filaments, formed upon a broad and com- pound pulp. The nails and claws that arm the fingers and toes, the corneous sheath that invests the horns of the Ox and Ante- lope, nay, the hoofs of herbivorous quadrupeds, are all epidermic secretions from the vascular cutis ; or, in other words, are hairs altered in their form and extent, according to the exigencies of the case. (790.) Widely different, however, are the so-called horns of the Deer tribe, which in reality consist of bone, and, being deci- duous, have to be reproduced from year to year by a most pecu- liar and interesting process. No sooner does the return of genial weather again call forth the dormant reproductive energies of the system, than the budding antlers begin to sprout from the forehead of the Stag, and rapidly expand in their dimensions from day to MAMMALIA. 689 day. On making a longitudinal section of the young horn, it is found to be continuous with the os frontis, having its outer sur- face covered with a vascular periosteal membrane derived from the pericranium, which in turn is protected by a fine velvety skin. Moreover, when a growing antler is injected minutely, and its earthy matter removed by means of an acid, vessels derived from the periosteum are found to traverse it in all directions, proving its identity with real bone. As growth goes on, the external carotid arteries, thus called upon rapidly to furnish a prodigious supply of materials, dilate in a remarkable 'manner, and soon the palm and the antlers of the horn have acquired their full dimensions. No sooner is this accomplished, than a prominent ring or burr is formed around the base ; which, projecting outwards, compresses and soon obliterates the vessels that have hitherto supplied the growing defences. The circulation being thus put a stop to, the soft teguments and periosteum peel off in strips ; and the bone, denuded of its covering, becomes a formidable weapon. At the close of the breeding season the removal of the horns is speedily effected : the connection between their bases and the os frontis is gradually weakened by interstitial absorption, until at length a slight effort is sufficient to detach the branching honours of the Stag, and they fall off, leaving a broad cicatrix ; this soon skins over, and the succeeding year calls forth a repetition of the process.* (791.) The CETACEA form a very remarkable group among the hot-blooded Mammifers, as relates to the external covering of their bodies. No covering of hair or wool would have been effi- cient in retaining the vital heat under the circumstances in which these creatures live ; and, even if such clothing could have been made available, it would have seriously impeded their progress through the water. Another kind of blanket has therefore been adopted : the cuticle is left perfectly smooth and polished, with- out any vestige of hair upon its surface ; but, beneath the skin, fat has been accumulated in prodigious quantities, and, enveloped in this non-conducting material, the Whales are fully prepared to inhabit an aquatic medium, and to maintain their temperature even in the Polar Seas. * In a physiological point of view this rapid production of osseous matter is truly wonderful. The horns of the Wapiti Deer, thus annually reproduced, will weigh up- wards of thirty pounds ; and in the fossil Irish Elk the weight of these deciduous defences must have been greater than that of the entire skeleton. 2 Y 690 MAMMALIA. (792.) The skin of all quadrupeds contains innumerable se- cerning follicles, whereby lubricating fluids are continually furnish- ed for the purpose of maintaining the surface in a moist or supple condition ; but not unfrequently these glandular follicles are ag- gregated together in considerable numbers, so as to form secreting pouches. In many species of Stags and Antelopes, for example, large pouches of this description are found below the margin of the orbit, that furnish a secretion vulgarly regarded as the Stag's " tears." In most instances some of the cutaneous glands secrete a highly odorous material, especially in the vicinity of the parts of generation ; and their secretion being most abundant during the rutting season, it is not without reason that these organs are looked upon as destined to attract the sexes, and perhaps to stimu- late the sexual passions. The preputial glands, so called because they furnish an odoriferous fluid that lubricates the prepuce and glans of the penis in the male, and of the clitoris in the female, are of this kind.* For the most part, these are simple sebaceous folli- cles contained in the thickness of the prepuce ; but occasionally they are replaced by true conglomerate glands, formed of lobes and lobules, and having but a single excretory duct, that opens upon Fig. 319. the sides of the glans penis or clitoridis be- neath the pre- puce. Many of the Roden- tia are furnish- ed with glands of this descrip- tion, and they are situated on each side of the penis, im- mediately be- neath the skin that covers the pubic region. (793.) It is with the pre- putial glands that we must notice the still more elaborately de- * Cuv. Lemons d'Anat. Comp. torn. v. p. 252 et seq. MAMMALIA. 691 veloped secreting organs of the Beaver, that furnish the drug called " castor" These organs, represented in the annexed figure (Jig. 319), consist of large glandular pouches, g, A, that discharge their contents in the vicinity of the anal and preputial apertures ; but of what the importance of the material thus abundantly se- creted may be in the economy of the animals so provided, it is not easy to conjecture. (794.) The secreting apparatus of the Musk Deer, (Moschus moschiferus,) which produces musk, is of analogous conforma- tion. This is an oval pouch situated beneath the skin of the lower part of the belly : its walls are thin and apparently membranous, but the membrane that lines them is rugose and plicated. The orifice leading to this pouch is small, and opens in front of the prepuce. (795.) Lastly, in connection with these odoriferous glands we may mention the " temporal glands" of the Elephant, from the duct of which, situated on each side midway between the eye and the ear, there flows a viscid and fetid liquid ; and likewise the u anal glands" met with in most CARNIVORA. The ducts of the glands last mentioned open near the margin of the anus ; and in some genera, as the Skunk and the Polecat, the stench produced by the fluid poured from these sources is so into- lerable as to become a most efficient defence against a foreign enemy. (796.) We now come to consider the nervous system of the MAMMALIA, and are of course prepared to anticipate that in pro- portion as they surpass all other animals in intelligence, so will the encephalic masses assume a complexity and perfection of structure such as we have not hitherto witnessed in the whole series of the animal creation . Their senses likewise may be presumed to have attained the utmost delicacy of organization in correspond- ence with the exalted attributes conferred upon this important class, and consequently to exhibit appendages and accessory parts, adapting them most accurately to repeat to the sensorium impres- sions derived from without. (797.) Abstruse as the study of the brain has been rendered by the chaotic assemblage of names applied by the earlier anatomists in their bewilderment to every definable portion of its substance, we have little doubt that, when the grand laws that have hitherto guided us in investigating the nervous system of the lower animals are had recourse to, the student will soon perceive how little diffi- culty there is in comparing even the brain of Man with the ence- 2 Y 2 692 MAMMALIA. phalon of the humbler Vertebrata examined in preceding pages, and thus tracing the progressive advances from simple to more complex organization. (798.) The great lessons deducible from all that we have as yet seen relative to the essential organization of the nervous system are obvious enough. First, that all nerves, whether connected with sensation or the movements of the body, emanate from or are in communication with nervous masses called ganglia, which are in fact so many brains presiding over the functions attributa- ble to the individual nerves. Secondly, that in the lower animals where these ganglia exist, they are comparatively small, and more or less completely detached from each other ; but that in the Ver- tebrata such is the increased developement of the central masses of the nervous system, that they coalesce, as it were, into one great organ called the cerebro-spinal axis ; and thus that the encephalon and medulla spinalis are both made up of symmetrical pairs of ganglia appointed to different functions, but so intimately blended together that they are no longer distinguishable, except from the pairs of nerves with which they are connected. (799.) Taking the above for axioms, and they are incontro- vertible, let us proceed to analyze the cerebro-spinal axis of the Mammalia, and to compare it in simple terms with that of Birds, Reptiles, and Fishes already examined. (800.) Commencing at the anterior extremity of the series, the first encephalic masses that present themselves are the " olfactory nerves^ as the human anatomist has been pleased to call them, although in every one of the details connected with their anato- mical structure and relations they confessedly differ from every nerve in the body. They are, in truth, not nerves at all, but brains, the ganglia or brains of smell, from which the olfactory nerves properly so called invariably emanate. In Fishes ( 557) they were found to equal, or even to surpass in size, the hemi- spheres themselves. In Reptiles and Birds they became gradually concealed by the developement of the hemispherical masses ; and in the Mammalia such is their diminutive appearance when com- pared with the cerebrum, that they are scarcely recognized as ele- ments of the encephalon at all. In all the oviparous Vertebrata the nerves of smell were two simple cords, one derived from each of the olfactory ganglia, from which they proceeded through osseous canals to the nose. But in the Mammifers these nerves are extremely numerous in proportion to MAMMALIA. 693 the extent of tlie surface to be supplied, and escape from the skull through the cranial plate of the ethmoid bone, which, from the number of apertures that it offers for their passage into the nose, richly merits the name of " cribriform," more especially in the carnivorous quadrupeds possessed of the most acute smell. (801.) The interior of the nasal cavity is divided by a median septum into chambers, in each of which a very large surface is pro- duced by the complicated convolutions of the thin nasal plates of the ethmoid (Jig .230, F ig. 320. a), and of the in- ferior turbinated bone (6), over which the air is made to pass in its progress to the lungs before it arrives at the pos- terior nares (c). The whole of this complication of bony lamellae is covered with a de- licate and highly lubricated mucous membrane, wherein the olfactory nerves termi- nate ; and from the figure given, representing the left nasal cavity of a Lion, some idea may be formed of the acuteness of the sense in question conferred upon the predaceous Garni vora. (802.) With this perfection of the olfactory sense a corre- sponding mobility of the outer nostrils is permitted to the Mam- miferous races. In the Reptiles and Birds the external apertures leading to the nose were merely immoveable perforations in the horny or scaly covering of the upper mandible ; but now the nostrils become surrounded with moveable cartilages, and appropriate mus- cles, adapted to dilate or contract the passages leading to the nose, or even to perform more important and unexpected duties, as, for example, in the proboscis of the Elephant. (803.) The CETACEA, as regards the conformation of their nostrils, and indeed of the whole of their nasal apparatus, form a remarkable exception to the above description. Inhabiting the water as these creatures do, they are compelled to breathe atmo- spheric air. Are they then to smell through the intervention of 694 MAMMALIA. an aquatic or aerial medium ? To smell in water would require the nose of a fish, which could not be granted without infringing upon the laws that regulate the progression of animal organiza- tion. To smell in air would be useless to the Whale ; and, more- over, its nasal passages are required for another function, with which the exercise of smell would apparently be incompatible. Thus circumstanced, we find the whole nasal apparatus com- pletely metamorphosed, and so disposed as to answer two impor- tant purposes : viz. first, to allow the Cetacean to breathe air whilst its mouth is immersed in water ; and, secondly, to provide an outlet whereby the water that is necessarily taken into the mouth may escape without being swallowed. The arrangement adopted to attain both these ends is very beautiful. The nostrils, instead of occupying their usual position, are situated quite upon the top of the head (Jig. 321, a), so that, as soon as the vertex reaches the surface, air is freely obtained. But another difficulty remains to be overcome : how is the Cetacean to breathe air while its mouth is full of water ? To allow this, the upper extremity of the larynx is prolonged, so as to form a thick cartilaginous plug (c). When the creature breathes, this elongated larynx is introduced into the posterior nares, as represented in the figure ; and, being firmly embraced by a sphincter muscle whilst in that situation, the air is admitted into Fig. 321. the trachea through the passages a, 6, without ever entering the oral cavity. MAMMALIA. 695 It only remains to be seen how the Cetacean gets rid of the water taken into the mouth, without being obliged to swallow it ; and the same figure, representing a vertical section of the head of a Porpoise, will enable us to understand the mechanism whereby this is accomplished. The two canals forming the posterior nares (b) are defended superiorly by a fleshy valve,* which is closed by means of a very strong muscle placed above the intermaxillary bones. To open this valve the force must be applied from below ; and, when the valve is shut, all communication is cut off between the posterior nares and the capacious cavities placed above them. These cavities are two large membranous pouches lined with a black skin, which, when they are empty, as represented in the figure, falls into deep folds ; but, when full, the walls are distended so as to form capacious oval receptacles. Externally these cham- bers are enveloped by a very strong expansion of muscular fibres, by which they can be violently compressed. Let us now suppose that the Cetacean has taken into its mouth a quantity of water that it wishes to expel : it moves its tongue and its jaws as though it would swallow ; but, at the same time closing its pharynx, the water is forced upwards through the pos- terior nares (b), till it opens the interposed valve, and distends the pouches placed above. Once in these reservoirs, the water may remain there until the creature chooses to expel it, or in other words " to blow." In order to do this, the valve between the pouches and the posterior nares being firmly closed, the sacs are forcibly compressed by the muscles that embrace them, and the water is then spouted up through the " blow-holes," or nostrils, to a height corresponding to the violence of the pressure. (804.) It must be evident that it would be impossible that a nose, through which salt water is thus continually and violently forced,, could be lined with a Schneiderian membrane of sufficient delicacy to be capable of receiving odorous impressions. In the CETACEANS therefore the nerves of smell, and even the olfactory lobes of the brain, are totally deficient. (805.) The second pair of ganglia entering into the composi- tion of the encephalon, and giving origin to nerves, are the optic lobes ; from which are derived the nerves of vision. In the Fish and in the Reptile these were at once recognizable as primary ele- ments of the brain ; but in the Mammifer, owing to the excessive developement of the surrounding parts, they are quite overlapped * Cuvier, Le9ons d'Anat. Comp. torn. ii. p. 673. 696 MAMMALIA. and concealed by the hemispheres. Nevertheless the " tubercnla quadrigemina (Jig. 322, rf, d) occupy the same relative position as in the Tortoise, (vide Jig. 262, B, c, e,) and in like manner still give origin to the nerves appropriated to the instruments of sight, of which they are the proper ganglia. (806.) The two optic nerves before passing to their final des- tination partially decussate each other, as in the human subject, they then proceed forward into the orbit, and penetrating the globe of the eye expand into the retinae. (807.) Minutely to describe the Fi^.322. construction of the eye-ball in the Mammalia would be quite super- fluous, seeing that in every essen- tial particular it exactly corresponds with that of Man. The disposi- tion of the sclerotic and choroid coats, the structure of the cornea, the arrangement of the humours and of the retina, the organization of the iris 9 in short, the whole economy of the eye is the same throughout the entire class. Ne- vertheless, there are a few points of secondary importance deserving our attention, whereby the organ is adapted to peculiarities of cir- cumstance in which different tribes are placed. In the Cetacea 9 and also in the amphibious Garni vora that catch their prey in the water, the shape of the lens is nearly sphe- rical as in Fishes ; and the antero-posterior diameter of the eye is in consequence considerably diminished by the extraordinary thickness of the sclerotic at the posterior aspect of the eye-ball, an arrangement approaching very nearly to that already described (560). (808.) Instead of the dark brown paint which lines the choroid of the human eye, in many Mammals the Ruyschian tunic secretes a pigmentum of various brilliant hues, that shines with metallic splendour. This membrane, called the " tapetum" partially lines the bottom of the eye-ball, but its use has not as yet been satis- factorily pointed out. (809.) The shape of the pupil likewise varies in different MAMMALIA. 697 quadrupeds : for the most part, indeed, the pupillary aperture is round, as it is in Man ; but in Ruminants, and many other Her- bivora, it is transversely oblong. In the Cats (Felida), that hunt in the gloom, and consequently require every ray of light that can be made available, the pupil is a long vertical fissure ; but this only obtains among the smaller genera, for in those Feline Car- nivora that surpass the Ocelot in size, such as the Leopard, the Lion, and the Tiger, the pupil again assumes a round form. (810.) The eyes of Mammalia are lodged in bony orbits, as in the oviparous Vertebrata, and in like manner are supported in their movements by a quantity of semifluid fat, with which the orbital cavities are filled up. In Man, as in Birds, Reptiles, and Fishes, six muscles are appropriated to the movements of each eye-ball, viz. four recti and two obliqui. The four recti mus- cles have the Fig.3-23. same disposi- tion in Mam- malia as in Birds ; that is, they arise from the margin of the optic fora- men, and run forward to be inserted oppo- site to each other upon the superior, infe- rior, and late- ral surfaces of the sclerotic coat. The inferior oblique likewise offers a similar arrangement in all the Vertebrata, arising from the margin of the internal wall of the orbit, and running outwards to be attached to the external surface of the globe of the eye. But the superior oblique, in the class before us, takes a very peculiar course. Aris- ing like the rest, it passes forward to the upper and inner margin of the orbit, where its tendon is reflected over a little cartilaginous pulley (fig. 323, c), and turns back again to be inserted into the external and posterior aspect of the eye-ball. (811.) In addition to the six muscles appointed for the movements of the eye in MAN and the QUADUUMANA, other 698 MAMMALIA. Mammalia have a seventh, called the choanoid or funnel-shaped muscle. This likewise arises from the borders of the optic fora- men, and, gradually expanding, forms a hollow cone interposed be- tween the recti muscles and the optic nerve ; the base of the cone being attached to the sclerotic, behind the insertion of the recti. Frequently, indeed, this choanoid, or suspensory muscle, is di- vided into four portions, in which case the animals so provided would seem to have eight recti muscles. (812.) The eye-lids of Mammalia resemble the human in every respect, excepting that in the lower orders a remnant of the nicti- tating membrane is still met with ; but it is of small dimensions, and unprovided with muscles. (813.) The lacrymal apparatus exists in all quadrupeds, and the lacrymal gland occupies the same situation as in Man; the tears being poured on to the conjunctiva near the external canthus of the eye-lids. The lacrymal ducts, likewise, whereby the tears are conveyed into the nose, so nearly resemble the human as to re- quire no particular description. The carunculte, lacrymales are also met with at the inner canthus of the eye-lids. In some quadrupeds, indeed, an additional gland exists, called the glandula Harderi : this is situated behind the internal angle of the eye, and secretes a lubricating fluid, that is discharged beneath the rudiment of the third or nictitating eye-lid. (814.) In Whales, as might be expected from their aquatic Jhabits, no vestige of a lacrymal apparatus is to be seen. (815.) Behind the optic lobes of the encephalon the nervous centres, from whence the other cerebral nerves take their origin, are so intimately blended together, that the anatomist is no longer able to distinguish them from each other. They form, in fact, the " medulla oblongata" and are the commencement of that long series of sentient and of motor ganglia that forms the spinal cord. All the nerves derived from the medulla oblongata, and from the spinal cord, are throughout the Mammiferous class exactly comparable to those met with in our own species, and therefore will require but brief notice. (816.) The third, fourth, and sixth pairs are destined to the muscles of the eye, and their distribution is the same as in Man. (817.) The fifth pair, or trigeminal nerves, consist of both motor and sentient fasciculi, both of which are distributed to the different parts of the face exactly as in the human subject ; allow- MAMMALIA. 6'99 ance of course being made for the varying form of the jaws, and for the proportionate size of the different organs connected with mastication. (818.) The seventh, or facial nerve, as also the glosso-pharyn- geal, the pneumogastric, and the lingual, have the same origin and general distribution throughout the whole class. (819.) The eighth pair of nerves are here, as in all the Verte- brata, devoted to the sense of hearing, which in the Mammifera attains its highest developement and perfection. The sensitive por- tion of the auditory apparatus, or the internal ear, is now enclosed in the petrous portion of the temporal bone, and imbedded in osseous substance of such stony hardness, that, except in very young sub- jects, it is by no means easy to display its different parts. As in Fishes and Reptiles, it consists of several membranous chambers or canals, filled with a limpid fluid, over which the filaments of the auditory nerve spread out. The whole apparatus, indeed, except in its proportionate size, very accurately resembles the auditory organ of the lower Vertebrata : the semicircular canals exhibit nearly the same arrangement, and in like manner commu- nicate with the vestibule by five orifices. The vestibule itself is small, and no longer contains any chalky concretions : it commu- nicates on the one hand with the cavity of the tympanum, by means of the foramen ovale ; and on the other sends off a canal (scala) to form the cochlea, an organ which in the Mammifer assumes its full developement and perfection. In the Reptilia and Birds, as the reader will remember, the cochlea was a simple canal bent upon itself (Jig> 281, e), one end of which (scala vestibuli) opened into the vestibule, while the other (scala tympani) terminated at the tympanic cavity, from which it was separated by the membrane of the fenestra rotunda ; but in the Mammalia the two scalse of the cochlea are consi- derably elongated, and wind in a spiral direction around a central axis (modiolus), so as very accurately to resemble the whorls in the shell of a snail, whence the name of the organ is derived.* It is in the increased complexity of the cochlea, therefore, that the chief character of the labyrinth of the Mammal consists ; but in the tympanic cavity the differences between the Mammi- * In Man, and by far the greater number of Mammals, the scalae of the cochlea make two turns and a-half around the modiolus; but in a few Rodent quadrupeds, as, for example, in the Guinea-pig, the Cavy, and the Porcupine, there are as many as three turns and a-half. 700 MAMMALIA. ferous car and that of the Bird are still more striking and de- cided. The cavity of the tympanum in the class before us is very extensive, and not unfrequently its extent is considerably enlarged by the addition of capacious mastoid cells. By means of the Eustachian tube it communicates freely with the throat. Upon its inner wall it offers the fenestra ovalis and the fenestra rotunda, closed by their respective membranes ; and externally is the mem- brana tympani, the vibrations of which are to be conveyed to the labyrinth. In Reptiles and Birds the communication between the drum of the ear and the membrane of the fenestra ovalis was effected by the interposition of a single ossicle, called the " columnella ;" but in Mammals a chain of four ossicles, named respectively the mal- leus, the incus, the os orbiculare, and the stapes, intervenes be- tween the labyrinth and the membrana tympani : these ossicles, both in their disposition and connections, are precisely similar to those of Man, and, moreover, are acted upon by little muscles in every respect comparable to those of the human subject. However remote the structure of the tympanic chain of ossicles in the Mammal may appear to be from that of the simple co- lumnella of the Bird, it is interesting to see how gradually the transition is effected from one class to another even in this par- ticular of their economy ; for in the Ornithorynchus, the Echidna, and the Kangaroo, so bird-like is the form of the stapes, that it might easily be mistaken for the ossicle" of one of the feathered tribes,* and every intermediate shape is met with as we advance from this point towards the stirrup-shaped bone of the most per- fect quadrupeds. It is in the class under consideration, that for the first time an external ear properly so called makes its appearance, for the fea- thered appendages of the Owl or of the Bustard ( 684) are scarce- ly entitled to such an appellation. In the Mammifera, however, with a very few exceptions, such as the CETACEA, Moles, and the Seal tribe, a moveable cartilaginous concha is appended to the exterior of the head, adapted by its form and mobility to collect the pulses of sound and convey them inwards towards the drum of the ear. The basis of this external auricle is composed of fibro- cartilage covered with a delicate skin, and its cavity is moulded * Vide Sir Anthony Carlisle, " on the Physiology of the Stapes" Phil. Trans, for 1805. MAMMALIA. 701 into various sinuosities, so disposed, no doubt, as to concentrate sonorous impressions. In Man, as the anatomist is aware, nume- rous small muscles act upon the auricular cartilages ; but in quad- rupeds possessed of moveable ears the number and size of these muscles are prodigiously increased, and the ears are thus directed with facility in any required direction. (820.) More minutely to describe the structure of the auditory apparatus in the Mammiferous class would be foreign to our pre- sent purpose : nevertheless, we must not omit to notice one most remarkable provision whereby the Whales, strangely circumstanced as those creatures are, are permitted to hear either through the me- dium of the air they breathe, or of the water in which they pass their lives. The reader will at once appreciate the difficulties of the case : the ear of a fish, without any external communication, although best adapted to receive the stunning concussions convey- ed through the denser element, could never appreciate the more delicate vibrations of the air, and the ordinary Mammiferous ear would be perpetually deafened by the thundering of the water. How is the Whale to hear what is going on in either the sea or the atmosphere ? The plan adopted is simple and efficacious: The external meatus of the ear is reduced to the smallest possible diameter, the canal being barely wide enough to admit a small probe ; this is the hydrophonic apparatus, and is all that is exposed for the recep- tion of aquatic sounds. The Eustachian tube, on the contrary, is very large, and opens into the blow-hole through which the Whale respires atmospheric air : if, therefore, the Cetacean comes to the top of the water to breathe, it is the Eustachian tube that conveys aerial sounds to the ear, and thus it hears sufficiently under both conditions. (821.) So far, as the student will have perceived, the different portions of the encephalon to which we have adverted correspond most exactly to similar parts met with even in the brain of a reptile : where then are we to look for those grand differences whereby the Mammiferous brain is peculiarly characterized ? The peculiarities of the brain of a Mammal are entirely due, first, to the increased proportional developement of the cerebral hemi- spheres ; and, secondly, to the existence of lateral cerebellic lobes, in connection with both of which additional structures become re- quisite. In those Marsupial tribes that form the connecting links 702 MAMMALIA. Fie. 324. between the Oviparous and Placenta! Vertebrata, the brain still exhibits a conformation nearly allied to that of the Bird, and the great commissures required in the more perfect encephalon are even yet deficient ; but in the simplest brain of a Placental Mam- mifer the characteristic differences are at once apparent. In the Rabbit, for example, (Jig. 322), the cerebral hemispheres (b) are found very materially to have increased in their propor- tionate dimensions ; and although, even as yet, convolutions upon the surface of the cerebrum are scarcely indicated, additional means of intercommunication between the hemispheric masses become indispensable. The corpus callosum there- fore, or great transverse com- missure of the hemispheres, (fig. 322, c,) is now super- added to those previously in existence; while other medul- lary layers, called by various ridiculous names, bring into unison remote portions of the cerebral lobes. In proportion as intelli- gence advances, the surface of the cerebral hemispheres becoming more extensive is thrown into numerous con- volutions separated by deep sulci ; until at length in the Carnivora, as, for instance, in the Lion, (Jig. 324,) the cerebrum (e, e) attains such enormous di- mensions that the other elements of the encephalon are, as it were, hidden among its folds. (823.) But, in addition to this increased complexity of the cere- brum, the cerebellum likewise has assumed a proportionate im- portance. In the Oviparous races this important element of the brain consisted only of the mesian portion, so that no cerebellic commissure was requisite ; but in the Mammal it exhibits in addi- tion two large lateral lobes (Jig. 324, c, c), and co-existent with these the pons Varolii (Jig. 324, d) makes its appearance, em- bracing the medulla oblongata and uniting the opposite sides of the cerebellum. MAMMALIA. 703 (824.) The structure of the spinal cord and the origins of the spinal nerves throughout all the Mammalia are precisely similar, and exactly correspond with what occurs in the human body ; nei- ther does the anatomical distribution of the individual nerves de- rived from this source require any special notice, since, generally speaking, it differs in no important particular from the arrangement with which every anatomist is familiar. (825.) The sense of touch in Mammalia is diffused over the whole surface of the body ; its perfection in different parts being of course influenced by the nature of the integument, and the number of sentient nerves appropriated to any given region. All the nerves derived from the sensitive tract of the spinal medulla, and the three divisions of the fifth pair of encephalic nerves, are equally susceptible of tactile impressions ; so that, in a class so extensively distributed as that before us, we need not be sur- prised to find a special apparatus of touch developed in very different and remote parts adapted to particular exigencies. Thus the whiskers of the Seals and of nocturnal Carnivora, the lips of the Horse, the trunk of the Elephant, the hands of Man, the hind feet of the Quadrumana, and even the extremity of the tail where that organ is prehensile, are all in turn made available as tactile instruments, and exercise the sense in question with the utmost delicacy. In the Bats, where the sense of vision becomes inadequate to guide them through the dark recesses where they lurk, that of touch assumes its utmost developement, and every part of the body that could by possibility be furnished with it has been abundantly provided for in this respect. Not only is the broad expanse of the wing acutely sensible, but the very ears have been converted into delicate feelers ; nay, from the tip of the nose in some species, membranes of equal sensibility have been largely developed, so that the Bats, as was ascertained by Spallanzani, even when deprived of sight and hearing, will fly fearlessly along, and avoid every obstacle with wonderful precision, guided apparently by the sense of touch alone. (826.) The sympathetic system of the Mammifera differs in no important particular from the human, the arrangement of the gan- glia and the distribution of the plexuses being in all respects the same. (827.) In the conformation of the genito-urinary apparatus in Mammalia the physiologist will find many circumstances of ex- treme interest. 704 MAMMALIA. (828.) Even in Birds, as the reader will remember, the secre- tions of the testes and of the kidneys were both poured into the common cavity of the cloaca, and discharged through the anal orifice. No bladder was provided for the reception of the urine ; and a simple, grooved but imperforate penis, even where that organ was most fully developed, was sufficient for the purposes of impregnation. (829.) Widely different, however, is the arrangement of the male genito-urinary system in the class we are now considering. The cloacal cavity is no longer met with, the terminations of the rectum and of the sexual ducts being now remotely separated ; the penis is traversed by a complete urethral canal, through which the seminal fluid is forcibly ejaculated ; and, moreover, subsidiary glands, not met with in any of the preceding classes, add their se- cretions to that of the testes, and thus facilitate the intromission of the fecundating fluid. A urinary bladder is now superadded to the renal apparatus, wherein the urine is permitted to accumu- late in considerable quantities, prior to its expulsion through the urethra, the excretory duct common to both the urinary and generative organs. (830.) Not less remarkable are the corresponding changes ob- servable in the disposition of the female reproductive organs. The Mammifers are appointed to bring forth living young ; a uterine receptacle is, therefore, necessarily provided for the reception of the foetus, and mammary glands are given to support the tender offspring during the earlier portion of its existence : but the history of these organs cannot be laid before the reader at a glance, and we must therefore patiently trace out their developement step by step, and gradually ascend from the Oviparous type up to the most complete forms of the genito-urinary system. (831.) Commencing with the urinary apparatus, the first parts that offer themselves to our notice are the kidneys, the ureters, and the bladder ; in describing which the same remarks will be found applicable to both sexes. The kidneys in all the Mammiferous orders occupy a similar position, being situated in the loins on each side of the aorta, from whence they receive a copious supply of arterial blood by the renal arteries, which, after having supplied the urinary secretion, is re- turned to the circulation by the emulgent veins that empty them- selves into the inferior cava. As relates to their intimate structure, the kidneys of all qua- MAMMALIA. 705 drupeds are essentially similar to those of our own species, each of these organs being composed of uriniferous tubules of extreme te- nuity that terminate in central papillae from which the urine flows. These tubules, as they advance into the medullary substance of the kidney, bifurcate again and again, until they arrive at the cortical or external portion, where they spread out on all sides, and, be- coming exceedingly flexuous, are inextricably intervolved among each other, so that the entire cortex is composed of their gyra- tions. At last all the uriniferous vessels terminate in blind ex- tremities, and according to M tiller* have no immediate communi- cation with the vascular system. In form the kidneys of Mammals more or less resemble the human ; but there is one important circumstance, observable in many tribes, which is well calculated to show that these organs, even when they appear most simple, are in reality formed by the coalescence of several distinct glands. In the human fetus the kidneys pre- sent a lobulated appearance ; that is to say, they are evidently composed of numerous divisions, each having the same structure : but in the adult the lines of demarcation between these lobes be- come entirely obliterated. In many genera, however, this division into lobes remains permanent during the whole lifetime of the crea- ture ; such, for example, is remarkably the case in amphibious CAR- NIVORA, as the Otter and the Seal tribes, and still more strikingly in the CETACEANS, where the kidneys are not inaptly comparable to large bunches of grapes. But whatever the form of the organ, or the number of lobules entering into its composition, the urine secreted by each kidney is received into a common excretory duct called the ureter, and is thus conveyed into the bladder prepared for its reception. The urinary bladder exists in all the Mammalia, and receives the ureters by valvular orifices in precisely the same manner as in the human subject. In the male its excretory duct, the urethra, is common to the urinary and generative systems, and terminates at the extremity of the penis ; but in the female the urethral canal is of much simpler structure, opening by a distinct orifice into the vulva.-)- (832.) We have preferred laying before the reader the above gene- De Gland. Structura, p. 102. t The Lemurs and the Mole form remarkable exceptions, for in these creatures the female urethra traverses the clitoris precisely as in the other sex. 2 z 706 MAMMALIA. ral view of the urinary system of Mammalia, to noticing in detail those varieties that occur in the disposition of the bladder and ure- thra of some of the lower tribes, in conformity with the different types of organization presented by their sexual organs ; these, how- ever, must not be lost sight of in following out the developement of the reproductive apparatus, from the oviparous races to the most perfect and highly gifted members of the animal creation. It is to this important subject that we must now invite the attention of the reader. (833.) The oviparous Vertebrata lay eggs, and their young are perfected without further nourishment derived from the maternal system than is contained within the egg itself. In our own species, and throughout all the races of Mammalia found on the European continent, the females produce their young alive and fully formed, capable of independent existence, but, nevertheless, nourished for a considerable period by milk derived from the breast of the mother. The distinction, therefore, between an oviparous and a viviparous creature would appear to be sufficiently broad, and the physiological relations between them as remote as possible. The student, however, who has followed us thus far through the long series of living beings that have successively presented themselves to our notice, must naturally expect that between ani- mals so dissimilar in their economy as the Bird and the Mammal, intermediate types of organization must occur, and that the trans- ition from one to the other is here, as elsewhere, gradually ac- complished. In this respect his expectations will be by no means disap- pointed. The Ornithorynchus paradoxus and the Echidna, ani- mals met with only in the continent of New Holland, are most obviously connecting links between these two grand classes ; and it is, therefore, with the history of these strange animals that we must commence our examination of the Mammiferous generative system. The Ornithorynchus paradoxus well deserves the specific epi- thet applied to it by zoologists. It has, indeed, the form of a quadruped, and its body is covered with hair, and not with feathers; but its mouth is the beak of a duck, and upon its hind feet, which are broadly webbed, the male carries a spur not unlike that of a barn-door fowl. Having the beak of a bird, how is the creature to suck ? Nevertheless the females have mammary glands well developed, but destitute of prominent nipples, so that the mode in MAMMALIA. 707 Fig. 325. which the young animal obtains the milk provided for it is even yet a puzzling question. Does the Ornithorynchus lay eggs, or produce living young ones ? This is a query that has not been satisfacto- rily answered ; and its generative apparatus is so nearly related to that of an oviparous animal, that even anatomy throws but little light upon the subject. Both in the male and female there is, in fact, but one vent, that leads to a cloacal chamber resembling that of a bird, and the entire organization of the sexual organs is rather that of an egg-laying than of a vivi- parous creature, as will be evident from the following details respecting them. The penis of the male Ornithorynchus is perforated by a urethral canal, through which the semen passes, but not the urine; its extremity, moreover, is terminated by two tubercles, giving it almost a bifid ap- pearance. This penis when in a relaxed state is lodged in a little pouch in the floor of the cloaca, from which it projects when erected. The cloacal cavity, as in birds, gives pas- sage to the feces and to the urine. The tes- tes (a) and the vasa deferentia (b) resemble those of an oviparous animal ; but, on the other hand, there is a complete urinary bladder (c), and moreover a pair of auxi- liary (Cowpers) glands (d, d), organs never met with except in the Mammiferous class. (834.) The anatomy of the female or- gans is not less singular. The ovaria (Jig. 327, a, a) are large and racemose, like those of a bird ; while the two oviducts or uteri (Jig. 326, a, a), as the reader may choose to call them, open into the cloaca by two distinct orifices (c, c), situated on each side of the urethra, derived from the bladder (b). It is to Professor Owen that science is indebted for all that is known relative to the anatomy of the female Ornithorynchus when in a gravid state, and his researches upon this subject appear to establish the following interesting particulars. First, 708 MAMMALIA. Fig. 326. a that the ovaria, notwithstanding their racemose appearance, ex- hibit all the essential characters of the Mammiferous type of struc- ture ; and corpora lutea were formed where the reproductive germs had escaped from them. Secondly, that the eggs contained in the uterine cavities (Jig. 327, c, e) had no con- nection whatever with the walls of the uterus. Thirdly, that each ovum exhibited the usual parts of an egg, viz. the cor- tical membrane, the al- bumen, and the yolk ; and that upon the latter a membrana vitelli and the blastoderm or germinative membrane were plainly perceptible. Fourthly, that the uterine walls assume an increased thick- ness when in an impreg- nated state, but that not the slightest trace of a decidual or adventitious membrane is apparent in the cavity of the womb. From all these circumstances, the distinguished author of the paper referred to* was led to adopt the subjoined train of rea- soning as to the probability of the Ornithorynchus being a vi- viparous Mammal. The form, the structure, and the detached condition of the ova, observes Professor Owen, may still be re- garded as compatible with, and perhaps favourable to, the opinion that they are excluded as such, and that the embryo is developed out of the parent's body. But the following objections present themselves to this conclusion : the only part of the efferent tube of the generative apparatus which can be compared in structure or relative position with the shell-secreting uterus of the Fowl, is the dilated terminal cavity in which, in all the specimens examined, the ova were situated ; and upon the oviparous theory it must be * On the Ova of the Ornithorynchus paradoxus, by Richard Owen, Esq. Phil. Trans. Part II. for 1834, page 563. MAMMALIA. 709 supposed either that the parietes of this cavity, after having secre- ted the requisite quantity of soft material, suddenly assume a new function, and complete the ovum by providing it with the calca- reous covering necessary to enable it to sustain the superincum- bent weight of the mother during incubation ; or that this is effected by a rapid deposition from the cuticular surface of the external passages ; or lastly, according to a more recent but still more improbable supposition, by a calcareous secretion of the ab- dominal glands poured out upon the ovum after its exclusion. Fig. 327.* But granting that the egg is provided in any of these ways with the necessary external covering, yet, from the evidence afforded by the specimens examined, the ovum is deficient in those parts of its organization which appear to be essential to successful incubation, viz. a voluminous yolk to support the germinal membrane, and the mechanism for bringing the cicatricula into contiguity with the body of the parent. Add to this, that such a mode of developement of the fetus requires that all the necessary nutritive material be accu- mulated in the ovum prior to its exclusion. Now the bony pelvis of the Bird is expressly modified to allow of the escape of an egg, both large from the quantity of its contents, and unyielding from its necessary defensive covering ; but, whatever affinities of structure * Owing to an error on the part of the draughtsman, who has neglected to reverse the drawing, the left uterus in the above figure is represented on the right side, and vice versa. 7JO MAMMALIA. may exist in other parts of the Ornithorynchus, it is most import- ant to the question of its generation to bear in mind that it mani- fests no resemblance to the Bird in the disposition of its pubic bones. From the above considerations it is therefore probable. that the young Ornithorynchi are produced alive ; yet still the reader will perceive by the closeness of the reasoning brought to bear upon the subject, how nearly the oviparous and mammiferous modes of generation are approximated by the interposition of these con- necting forms of Vertebrata. (835.) But if from these arguments, derived from the anato- mical construction of the female parts, it is allowable to conjecture that the Ornithorynchus is ovo-viviparous, using that term in a strictly philosophical sense, the difficulties of the case are by no means removed ; and granting that the contents of the ovum are barely sufficient to nourish the embryo during the very earliest stages of its developement, we have yet to learn how the fetus is matured after the exhaustion of this supply. There is no reason whatever to suppose that a placenta exists at any period of uterine gestation ; neither is there a marsupial pouch in which the pre- maturely born young can be carried about and supplied with milk ; so that whether the young Monotreme be developed in the uterus, or out of the uterus, we are equally at a loss to understand how its nutrition is provided for. In this state of uncertainty, the anatomy of the young Ornithorynchus, examined at as early a period as pos- sible, becomes a subject of extreme interest ; and fortunately Professor Owen has been enabled to add obser- vations upon this subject to his other valuable researches relative to the generation of these creatures.* The annexed figure (Jig- 328) is a por- trait of one of the specimens dissected, and from every appearance it could not have been more than a few days old, that is, supposing it to have been born at an advanced period of its de- velopement. It was as yet blind, and the situation of the eyes was * Owen, on the Young of the Ornithorynchus paradoxus. Trans. Zool. Society, vol. i. Fig. 328. MAMMALIA. 711 only indicated by the convergence of a few wrinkles to one point ; but, when these were put upon the stretch, the integument was found entire, and completely shrouding or covering the eye-ball anteriorly : its skeleton was, moreover, quite in a cartilaginous condition, and it was obviously in every respect helpless, and still dependent upon its mother for sustenance. The stomach was found filled with milk, a sufficient proof that at that period, at least, it was nourished by the lacteal secretion ; but, with regard to its previous fetal condition, the difficulties that have been above alluded to remained in their full force. No trace of an umbilical cicatrix was visible upon the ventral surface of the body, even when examined with a lens, a sure proof that no pla- centa had existed. The ileum was carefully examined, but there was no appearance of the pedicle of the vitelline vesicle ; neverthe- less, the other vestiges of fetal organization were more obvious than in the ordinary marsupial or ovo-viviparous Mammalia. The umbilical vein was seen extending from a linear cicatrix of the peritoneum, opposite the middle of the abdomen, along the ante- rior margin of the suspensory ligament to the liver. It was re- duced to a mere filamentary tube filled with coagulum. From the same cicatrix the remains of the umbilical arteries extended down- wards, and near the urinary bladder were contained within a dupli- cature of peritoneum, having between them a small flat oval vesicle, the remains of an allantois, which was attached by a contracted pedicle to the fundus of the bladder ; but still as both the em- bryo of a Bird and that of the ovo-viviparous Reptile have an allantois and umbilical vessels developed, no certain inference can be drawn from the above appearances as to the oviparous or vivi- parous nature of the generation of the Ornithorynchus. (836.) Such is the present state of our knowledge relative to the first type of Mammiferous generation, viz. that met with among the MONOTREMATA. In the second, or MARSUPIAL TYPE, the phenomena, although equally strange, are better under- stood, and to these we must now beg the attention of the student. The MARSUPIALIA, from the variety of their forms and exten- sive distribution, constitute a most important section of Mammi- ferous quadrupeds, distinguished by the peculiarities that occur in the organization of their generative apparatus, and by the singular mode in which they produce and suckle their young. Animals of this kind are only met with in the American and Australian regions of our globe ; and so widely do they differ, as far as their MAMMALIA. reproduction is concerned, from all the Mammiferous inhabitants of the Old World, that they might even be regarded as forming quite a distinct and separate group in the animal creation, serv- ing to accomplish another step in that grand transition by which the physiologist is conducted from the oviparous to the placental Vertebrata. The Marsupialia are, strictly speakingj ovo-viviparous, that is to say, the uterine ovum never forms any vascular connection with the maternal system, but after a very brief intra-uterine gestation the embryo is expelled in a very rudimentary and imperfect condition, even its extremities being as yet but partially developed ; and in this helpless state the fetus is conveyed from the uterus into a pouch or marsupium,) formed by the integument of the abdomen, there to be nourished by milk sucked from the mammary glands, until it arrives at such a state of maturity as enables it to assume an independent existence. We may naturally expect, therefore, that, with habits so remark- able, the structure of the generative apparatus, both in the male and female Marsupial, will offer important peculiarities, and these accordingly first present themselves for description. (837.) We select the Kangaroo as an example of the entire group ; beginning, as we have hitherto done, with the organization of the male organs of generation. The first circumstance that strikes the attention of the anatomist in a male Marsupial is the extraordinary position of the testes, which, instead of being situated behind the penis, as in most pla- cental Mammals, are placed in front of that organ in a kind of scrotum that occupies the same place as the pouch of the female, and is in like manner supported by two marsupial bones derived from the pubes, around which the cremaster muscle winds in such a manner as to enable it powerfully to compress the testicles during the congress of the sexes. The vasa deferentia derived from the testes open into the commencement of the urethra, which now, for the first time, forms a complete canal leading from the bladder to the extremity of the penis. The auxiliary glands that pour additional secretions into the urethra are of great size, and more numerous than those met with in the human subject. In the first place, the commencement of the urethral tube is em- braced by a bulky and conical prostate^ to which succeed three pairs of large secreting organs (Cowper's glands), each enveloped in a musculo-membranous sheath, apparently intended to compress MAMMALIA. 713 their substance, and thus efficiently discharge their secretion into the canal of the urethra, there to be mixed up with the seminal fluid. But, perhaps, the most decided peculiarities that characterize the males of Marsupial quadrupeds are met with in the construc- tion of the penis itself. The two roots or crura of the corpora cavernosa are not, as in the higher Mammals, attached to the branches of the ischium by ligamentous bands, but each swells into a large bulb enclosed F . g29 in a powerful mus- cular envelope. The bulbous portion of the urethra is like- wise double, and em- braced by powerful muscles. In the Kangaroo, more- over, the spongy erectile tissue that encloses the urethra passes with that ca- nal through the cen- tre of the body of the penis, formed by the corpora caverno- sa, so that a glans can scarcely be said to exist ; but in other Marsupials, as, for example, in the Opossums (Didel- phis), the extremity of the intromittent organ is bifid, thus forming another ap- proximation to the oviparous type. (838). In the female Kangaroo, and other Marsupials, there are still two distinct uteri, opening into the vagina by distinct orifices ; and even the vagina itself is double, exhibiting a very peculiar and interesting arrangement, represented in the preceding figure 714 MAMMALIA. (jig. 329). The ovaria (a, a) are now reduced to compara- tively small dimensions when compared with those of the Ovipara; a circumstance that depends upon the reduced size of the ovarian ovules, which no longer present the bulky yolks peculiar to ovipa- rous generation, the necessity for the existence of such a large store of food being now superseded by the provision of another kind of nourishment derived from the mammary glands. The Fallopian tubes commence by wide fimbriated apertures, and each leads into a separate uterine canal (6), in which the first part of gestation is accomplished. The two uteri open by two orifices (e, f) into the two vaginae (g, g), which remain quite distinct from each other from their commencement to their termination in the urethro- sexual canal (A), a kind of cloaca into which both the vaginae and the urethra empty themselves. (839.) Such being the arrangement of the generative apparatus of the female Kangaroo, we are prepared, in the next place, to con- sider the structure of the Marsupial ovum, and to trace its progress from the ovary, where it is first formed, into the marsupial pouch, where the developement of the fetus is ultimately completed. The ovary of a Marsupial animal, as has been already observed, resembles that of ordinary Mammalia, and presents the same dense structure. But the ovarian ovules, although characterized by the paucity of yolk as compared with the oviparous classes, yet have a larger proportion than exists in the placental Mammalia. When impregnation is effected in the Marsupial animal, the Graqfian vesicle or ovisac is ruptured, and the little ovulum escapes into the Fallopian tube, whereby it passes into the uterine cavity ; from whence of course it must absorb the materials destined to support the future embryo, in the same manner as the egg is furnished in the oviduct with the albumen that invests the yolk. The deve- lopement of the embryo from the blastoderm or germinal membrane is, no doubt, accomplished in the same manner in all Mammalia as it is in Birds, up to a certain stage of maturity ; but at that stage of growth, when, in the case of the Bird, the yolk is required to contribute to the nourishment of the newly-formed being, in the Mammifera where no adequate supply of yolk exists other means must be resorted to ; and accordingly the Marsupial embryo is born prematurely, in order to supply it with milk, and in the or- dinary Mammal a placenta is developed, forming a means of vascular communication between the mother and the fetus. (840.) The important investigations of Professor Owen upon MAMMALIA. 715 this subject* cannot be too highly appreciated. In the gravid uterus of a Kangaroo, examined by this indefatigable labourer in the cause of science, a fetus was met with that had apparently arrived very nearly at the term of its intra-uterine existence ; and the following is a summary of its anatomy at this period. The ovum (Jig- 330, c) was lodged in one of the uterine cavi- ties, and the fetus was about an inch and four lines in length. The walls of the gravid uterus were obviously dilated, and its parietes varied in thickness from one to two lines, being in the unimpreg- nated state about half a line ; but this increase was not in the mus- cular coat, but in the lining membrane, which was thrown into irre- gular folds and wrinkles. There was, however, not the slightest trace of any vascular connection between the uterus and the ovum, neither placenta nor villi, nor any determination of vessels to a given point on either of the opposed surfaces of the chorion or uterus : on the contrary, the external membrane of the ovum (cho- rion) exhibited not the slightest trace of vascularity, even under the microscope, and seemed in every respect to resemble the mem- brana putaminis that lines the egg-shell. (841.) The body of the fetus itself was immediately enclosed in a transparent membrane Fig. 330. (/;), the amnios. (842.) Between the chorion (a) and the am- nios (b) was an extensive vascular membrane (c, d, d, c, e) ; its figure seemed to have been that of a cone, of which the apex was at the umbilicus of the fetus. Three vessels could be distinguished diverging from the umbilical cord, and ramifying over it. Two of these trunks con- tained coagulated blood ; while the third was smaller, empty, and evidently the arterial trunk. No trace of any other membrane could be seen extending * On the Generation of Marsupial animals, with a description of the impregnated uterus of the Kangaroo, by Richard Owen, Esq. Phil. Trans. 1834. 716 MAMMALIA. from the fetus besides the three above mentioned, the chorion (a), the amnios (i), and the interposed vascular membrane, the nature of which becomes the next subject of inquiry. (843.) On tracing the three vessels above alluded to, as ramify- ing over the vascular membrane, through the umbilicus into the abdomen, the two larger ones, filled with coagulated blood, were found to unite, and after being joined by the mesenteric vein pene- trated the liver : these, consequently, were the representatives of the omphalo-mesenteric or vitelline vein of the embryo bird ( 703). The third vessel passed between the convolutions of the small intestine along the mesentery to the abdominal aorta, corre- sponding to an omphalo-mesenteric or vitelline artery. The mem- brane, therefore, upon which they ramified answers to the vascular layer of the germinal membrane which spreads over the yolk in the Oviparous animals, or to the vitelline vesicle of the embryo of ordinary Mammalia. A filamentary pedicle connected this membrane to the intestine near the termination of the ileum, thus completing the resem- blance between this apparatus and the vitelline system of Birds. But here we must caution the student not to be misled on one im- portant point : the contents of the vitelline sac in the Marsupials, although doubtless intended to afford nourishment to the embryo animal, and thus representing the yolk of the bird's egg, differs from it in one very essential circumstance. The yolk of the Ovi- parous ovum is ready formed in the ovary and exists prior to con- ception ; but in the Mammal, where the ovarian yolk is met with in extremely small quantities, the contents of the vitellicle must obviously be derived from some other source, most probably from absorption from the uterine cavity. (844). In the Marsupial ovum the vascular membrane of the vitellicle is doubtless sufficient for the respiration of the little creature up to the time of its birth, and, accordingly, the allan- toic system ( 705) is but very partially developed. In the ovum delineated in the last figure, there was, as yet, no perceptible trace either of an allantois or of a urinary bladder ; but, as has been proved by another dissection, during the latter week of uterine ges- tation, the urinary bladder is prolonged beyond the umbilicus so as to form a small allantois destined to receive the renal secretion, which becomes more abundant as the little fetus increases in size and completeness.* * See Proceedings of the Zool. Society for August, 1837. MAMMALIA. 717 In the mammary fetus of a Kangaroo a fortnight old, Profes- sor Owen detected both an urachus and umbilical arteries, but these only extended from the bladder and iliac vessels as far as the umbilicus ; neither could any umbilical vein be found penetrating the liver. It is in the placental Mammals that we shall find these vessels assuming their full importance, and developing them- selves into a new system, whereby the communication between the mother and her offspring is still more effectually provided for. (845.) When we consider the very early period at which the young Kangaroo is born, namely, at about the thirty-ninth day after conception, it is only reasonable to suppose that the organs most immediately connected with the vital actions are precociously ma- tured ; and accordingly, even in the embryo above delineated (fig. 330), the intestines, the liver, the kidneys, and the testes were all conspicuous, and the diaphragm, the heart, and the lungs were in such an advanced condition as to show that they would soon be capable of prematurely taking upon themselves the exercise of the circulatory and respiratory functions. (846.) This rapid developement of the viscera connected with circulation and respiration, is in truth essentially requisite ; for no sooner has the embryo arrived at the size represented in the next figure (Jig. 331, A), and while the limbs are still in a most rudi- mentary condition, the embryo is transferred from the uterus into the marsupial pouch, where it is found attached by its mouth to one of the nipples, from whence the materials of its support are to be obtained, until it has acquired sufficient strength and size to leave the strange portable nest in which its fetal growth is accomplished, and procure food adapted to a maturer condition. (847.) A very beautiful provision is met with in the construc- tion of the respiratory passages of the young Marsupial, intended to obviate the possibility of suffocation consequent upon the admis- sion of milk into the trachea, a circumstance that without some peculiar arrangement might easily happen ; but of this we must quote the original description, extracted from the paper already re- ferred to.* '" The new-born Kangaroo," observes Professor Owen, " possesses greater powers of action than the same sized embryo of a Sheep, and approximates more nearly in this respect to the new- born young of the Rat; yet it is evidently inferior to the latter. For although it is enabled by the muscular power of its lips to grasp and adhere firmly to the nipple, it seems to be unable to draw sus- * Page 348. 718 MAMMALIA. /A tenance therefrom by its own unaided efforts. The mother, as Professor Geoffroy* and Mr. Morgan -f* have shown, is therefore provided with a peculiar adaptation of a muscle (analogous to the cremaster) to the mammary gland, for the evident purpose of in- jecting the milk from the nipple into the mouth of the adherent fetus. Now it can scarcely be supposed that the fetal efforts of suction should always be coincident with the maternal act of injec- tion ; and, if at any time this should not be the case, a fatal acci- dent might happen from the milk being forcibly injected into the larynx. Professor Geoffroy first described the modification by which this purpose is effected, and Mr. Hunter appears to have foreseen the necessity for such a structure, for he has dissected two small fetuses of the Kangaroo for the especial purpose of showing the relation of the larynx to the posterior nares. J The epiglottis and arytenoid cartilages are elongated and approximated, so that the rima glottidis is thus situated at the apex of a cone- shaped larynx (Jig. 331, B, a), which projects, as in the CE- TACEA, into the posterior nares, Fig. 331. where it is closely embraced by the muscles of the soft palate. The air- passage is thus completely separated from the fauces, and the injected milk passes in a divided stream, on either side of the larynx, into the oesophagus." " Thus aided and protected by modifications of structure, both in the system of the mother and in its own, designed with especial refer- ence to each other's peculiar condi- tion, and affording therefore the most irrefragable evidence of creative foresight, the feeble off- spring continues to increase from sustenance, exclusively derived from the mother, for a period of about eight months. The young Kangaroo may then be seen frequently to protrude its head from the mouth of the pouch, and to crop the grass at the same time * Memoires du Muse~e, torn. xxv. p. 48. .f Trans. Lin. Society, vol. xvi. p. 61. t " See Nos. 3731, 3734, 3735, in the Physiological series of the Hunterian Mu- seum, in which there are evidences that Mr. Hunter had anticipated most of the anatomical discoveries which have subsequently been made upon the embryo of the Kangaroo." MAMMALIA. 719 that the mother is browsing. Having thus acquired additional strength, it quits the pouch, and hops at first with a feeble and vacillating gait ; but continues to return to the pouch for occa- sional shelter and supplies of food, till it has attained the weight of ten pounds. After this it will occasionally insert its head for the purpose of sucking, notwithstanding another fetus may have been deposited in the pouch ; for the latter, as we have seen, attaches itself to a different nipple from the one which had pre- viously been in use." Thus therefore are we conducted by the Ovo-vivipara, as the MAIISUPIALIA are properly called, to the most perfect or pla- cental type of the generative system. (848.) Commencing our account of the reproductive organs of VIVIPAROUS MAMMALIA, by examining those of the male sex, we have another striking example of the insufficiency of the no- menclature employed by the anatomist who confines his studies to the human body, when it becomes necessary to describe cor- responding organs even in animals organized after the same type. True it is, that there is the same general arrangement of the generative apparatus ; and it is convenient, as far as possible, to apply the same names to structures that apparently represent each other : but a very superficial examination of the facts will serve to show that great differences exist between them ; and, accordingly, we are not surprised to find the utmost perplexity and confusion in the descriptions of these parts, arising from the indiscriminate application of the terms employed in human anatomy to totally dissimilar structures. It is not, however, our business here to criticise the labours of authors upon this subject; we must content ourselves with select- ing an example of one of the more complex forms under which the male genitals present themselves, and leave the reader to contrast the various organs with those met with in the human subject. The annexed figure (Jig. 332, A) represents the generative viscera of the male Hedgehog. The rectum (a) and the neck of the bladder (h) remain in situ ; but the rest of the latter viscus has been removed, and the first portion of the urethra (e) slit open, in order to show the relations of the surrounding parts. The testes (&, b) present the same structure in all the class, and consist essentially of an immense assemblage of extremely delicate tubuli semi?iiferi, enclosed in a dense albugineous tunic from 720 MAMMALIA. which septa pass internally, whereby the seminiferous tubes are divided into several fasciculi : after piercing the proper fibrous tunic of the testes, the sperm-secreting tubes are collected into an extremely tortuous duct, that by its F'g- 332. convolutions forms the epididymis, as in Man, and is then continued, under the name of vas defe- rens, to the com- mencement of the urethra, into which the two ducts open (B, 6, b). In the Horse, and many Ruminants, the vas deferens presents a remarkable struc- ture : before its ter- mination it suddenly swells to a consider- able diameter, de- pending upon the increased thickness of the walls of the canal, which at the same time become cellular, and secrete a gelatinous fluid that escapes into the cavity of the duct. (849.) In their situation the testes of placental Mammals are found to offer very striking differences. In the Cetacea, the Elephant, and the Seal tribes, they remain permanently in the abdomen, bound down by a process of the peritoneum. In Man, and most quadrupeds, on the contrary, they pass out of the abdo- minal cavity through the inguinal rings, and are suspended in a scrotal pouch formed by the skin, and a cremaster muscle, and lined by a serous prolongation of the peritoneal sac. The sper- matic cords, therefore, formed by the vessels and excretory canal of the testes will take a different course, in conformity with the variable position of these organs, and, where a scrotum exists, must enter the abdomen through an inguinal canal. Still, from their horizontal posture, quadrupeds are but little liable to hernise, MAMMALIA. 721 even where the inguinal passages are much more open than in the human subject. (850.) The quantity of the seminal fluid furnished by the testes is very small, as must be evident from the extreme narrow- ness of the duct through which it passes into the urethra. Ne- vertheless, as the impregnation of the female now requires the forcible injection of this fluid, it is absolutely requisite to increase the bulk of the vivifying secretion, in order to enable the muscles that embrace the urethral "tube efficiently to expel it. For this purpose additional glands are given, whereby different fluids are poured into the urethral cavity, apparently for the sole purpose of diluting the spermatic liquor, and thus forming a vehicle for its expulsion. These succenturiate glands, as they are named, are not found in any oviparous animal ; but in the Mammal such is their size and importance, that there may be just reason for supposing them to exercise a more important office than that usually assigned to them by physiologists ; and this supposition seems to obtain additional weight when we consider the great diversity of structure that they exhibit in different quadrupeds. (851.) The vesicultE seminales are the first of these accessory secreting organs that require our notice. In Man the seminal vesicles, as they are erroneously termed, resemble two membranous reservoirs, situated beneath the neck of the bladder, and were once supposed to be receptacles for containing the semen. When opened, however, they are found to be composed of the windings of a very sinuous secreting surface ; and, as their excretory ducts open into the urethra in common with the vasa deferentia, they obviously add the fluid that they elaborate to the secretion of the testes. But, notwithstanding their apparent importance in the human species, these organs do not exist at all in by far the greater num- ber of CARNIVOUA ; neither are they found in the RUMINANTS, nor in the cetaceous Mammals. In other quadrupeds, on the contrary, they are found ; and their proportionate size is extremely remarkable. This is spe- cially the case in the Rodent tribes, and among the INSECTIVORA. In the Hedgehog, for example, their bulk is enormous. In this creature they form two large masses (Jig. 33, A, c, c), each com- posed of four or five bundles of long and tortuous secerning ves- sels folded upon themselves in all directions, and pouring the 3 A 722 MAMMALIA. product of their secretion into the urethra by two ducts (Jig. 332, B, c, c), quite distinct from the vasa defer entia. (852.) The prostates are the next succenturiate glands, super- added to the essential generative organs of the placental Mam- mals ; and so diverse is their structure in different tribes, that it is not always easy to recognise them under the varied forms that they assume. In Man the prostate is a solid glandular mass, that embraces the commencement of the urethra, into which it discharges its secretion by numerous small ducts ; and this is the most common arrangement throughout the Mammiferous orders. In RUMINANTS, SOLIPEDS, and in the Elephant, there are two or even four prostates of a very different kind ; each gland having a central cavity, into which smaller cavities open by wide orifices. In these creatures, therefore, the prostatic secretion accu- mulates in the interior of the gland, from whence it is conveyed into the urethra by appropriate excretory canals. In most of the RODE NT i A, in the Mole and in the Hedgehog, the structure of the prostate is so peculiar, that many distinguished comparative anatomists refuse to apply the same name to organs that obviously represent the gland we are describing, preferring with Cuvier to call them " accessory vesicles.' 1 '' In the Hedgehog, the prostate is replaced by two large masses (Jig. 332, A, d, d), each composed of parallel, flexuous, and branch- ed tubes, all of which unite into ducts common to the whole group, whereby the fluid elaborated is conveyed into the urethra through minute orifices (Jig. 332, B, e, e). (853.) A third set of auxiliary secreting bodies, very generally met with, are called by the name of" Cowper's glands." These in our own species are very small, not exceeding the size of a pea ; but in many quadrupeds they are much more largely developed. In the Hedgehog (Jig. 332, A,/) they are obviously composed of convoluted tubes, and their ducts open by distinct apertures (B, g, g) into the floor of the urethra. (854.) The canal of the urethra, through which the urine as well as the generative secretions are expelled from the body of the male Mammal, is a complete tube, and no longer a mere furrow, as we have seen it to be in all the Ovipara possessed of an intromittent ap- paratus. It extends from the neck of the bladder to the extremity of the penis; but in this course, owing to its relations with the sur- rounding parts, it will be necessary to consider it as divisible into two MAMMALIA. 723 or three distinct portions, each of which offers peculiarities worthy of remark. The first part of the urethral tube is not unfrequently, as in the human subject, more or less completely surrounded by the prostate gland, and in such cases merits the name of " pro- static portion;" but where, as in the Hedgehog, the prostates do not enclose the commencement of the canal, this division of the urethra does not exist. The second is the " muscular portion," extending from the prostate to the root of the penis, and it is into this part that all the generative secretions are poured from their respective ducts (Jig. 33%, B, b, c, e, g, h). Externally, this division of the urethra is enclosed by strong muscles (Jig. 332, A, i, i), which by their convulsive contractions forcibly ejaculate the different fluids concerned in impregnation, and thus secure an efficient in- tromission of the seminal liquor into the female organs. The third portion of the urethra is enclosed in the body of the penis, and surrounded by the erectile tissue, of which that organ essentially consists ; but in all quadrupeds this part of the canal is not so decidedly continuous with the muscular portion as it appears to be in Man and the generality of Mammalia. In many RUMINANTS, and in some of the Hog tribe, the muscular division of the canal opens into the upper part of the third or vascular division, in such a manner that a cul-de-sac occupies the com- mencement of the vascular bulb of the urethra, as it is called by anatomists, into which the secretion of Cowper's glands is poured, without having been previously mixed with the seminal or prostatic fluids. In some RODENTS, as, for example, in the Squirrel and the Marmot, the arrangement is still more curious ; for the cul- de-sac of the bulb of the urethra in these creatures, which receives the secretion of Cowper's glands, is lengthened out into a long tube that runs for some distance beneath the proper urethra, and only joins that canal near the extremity of the penis. (855.) The body of the penis in the Mammalia, as in all other Vertebrata possessed of such an organ, is composed of vascular erectile tissue ; but now, besides the corpora cavernosa, which in Reptiles and Birds formed the entire organ, another portion is superadded, destined to enclose the canal of the urethra in a thick erectile sheath, and, moreover, to form the glans or most sensitive part of the intromittent apparatus. The corpora cavernosa are now securely fixed to the bones of the pelvis by two roots or crura ; and even in the CETACEA,where 3 A 2 MAMMALIA. no pelvis is met \vith, the ossa ischii exist, apparently only for the purpose of giving firm support to the origin of the parts in ques- tion. The size of the corpora cavernosa in Man, and many other animals, is of itself sufficient to give the needful rigidity to the parts during sexual excitement ; but in some tribes an additional provision is required to ensure adequate firmness. Thus in Monkeys, Bats., the CARNIVORA, the RODENTIA, and the Baltenidte among CETA- CEANS, a bone is embedded in the substance of the male organ, of which it forms a considerable part. Where this bone exists, the corpora cavernosa are proportionately small, and the fibrous walls of the penis are confounded with its periosteal covering. The corpus spongiosum, likewise composed of erectile tissue, is quite distinct from the cavernous bodies, and, as we have said before, is only found in the Mammifera. It commences by a bulb- ous origin that embraces the urethra, and it accompanies that canal quite to the extremity of the penis, where it dilates into the glans. The size and shape of the male organ varies of course in every genus of quadrupeds, as does the form and texture of the glans. To describe these would lead us into details of too little import- ance to be noticed in a survey so general as that we are now taking ; nevertheless, we cannot entirely omit to notice the strange and unaccountable structure met with in some of the Rodent tribes, whereby the penis is rendered a most formidable-looking apparatus, the object of which it is not easy to conjecture ; although as an instrument of excitement no one will be disposed to deny its efficiency. Thus, in the Guinea-pig tribe (Cavia, Ilig.) the penis is strengthened by a flat bone that reaches forward as far as the extremity of the gland beneath which is the termination of the urethra ; but behind and below the orifice of this canal is the opening of a pouch, wherein are lodged two long horny spikes. When the member is erect, the pouch alluded to becomes everted, and the spikes (Jig. 333, d) are protruded externally to a consi- derable length. Both the evected pouch (b) and the entire sur- face of the glans are, moreover, covered densely with sharp spines or hooklets ; and, as though even all this were not sufficient to produce the needful irritation, still further back there are, in some species, two sharp and strong horny saws (c, c) appended to the sides of the organ. From this terrible armature of the male MAMMALIA. -") Cavys, it would be only natural to expect some corresponding peculiarity in the female parts ; but, however inexplicable it may appear, the female vagina offers no uncommon structure. (856.) We have in the Hfft 333> last place to examine the generative system of the female placental Mamma- lia, and thus to trace the developement of this im- portant system to its most j complete and highest form. In theMARSupiALiA,as the reader will remember, there were still two dis- tinct uteri, that were ob- viously the representatives of the oviducts of the oviparous classes. In the human female, on the contrary, the uterus is a single central viscus, into which the germs derived from the ovaria are introduced through the two " Fallopian tubes" as the oviducts are now designated ; but we shall soon see that the viviparous Mammals offer in the anatomical structure of the generative system of the female so many inter- mediate gradations of form, that we are almost insensibly con- ducted even from the divided uteri of the Ornithorynchus up to the most elevated and concentrated condition that the uterine apparatus ultimately attains in our own species. In the female Rabbit, for example, we have a placental Mam- mal that in every part of the organization of its reproductive organs testifies its near affinity to the Marsupial type. The .ovaria (Jig' 334, A:, /), although widely different as regards the size of the contained ovules from those of oviparous animals, still retain faint traces of a botryoidal or racemose appearance. The oviducts (w, o,) or the Fallopian tubes as we must now call them, are reduced in their diameter to very small dimen- sions, and testify by their tenuity how minute must be the ovule to which they give passage. To these succeed the uteri (e, f) 9 still entirely distinct from each other throughout their whole ex- tent, and even opening into the vagina (g) by separate orifices, into which the probes i, A, have been introduced. As far as its anato- my is concerned, such a uterine apparatus might belong to a mar- 726 MAMMALIA. supial Maramifer ; and even in the rest of the sexual parts obvious relations may be traced between the rodent we are describing and the ovo-viviparous quadrupeds. It is true that there are no longer two vaginae terminating in a single cloacal cavity, but let the reader observe how nearly the va- gina of the Rabbit (a, b) approximates the condition of a cloacal chamber. Anteriorly it receives the contents of the bladder (d, m) ; while the rectum (s) terminates by an anal orifice (r), so closely conjoined with the aperture of the vulva, that the anatomist is almost in doubt whether the external opening might not be de- scribed as common both to the vagina and intestine. Advancing from this lowest form of a placental uterine system, it is found that the two uteri before their termination become united so as to form a central portion common to both, called the body of the uterus, through the intervention of which they communicate with the Fig. 334. vagina by a single passage named the os tinea ; still, however, the cornua uteri, especially in those tribes that are most remark- able for their fecundity, become during gestation far more capaci- ous than the mesial portion of which they appear to be prolonga- tions. It is, in fact, in the cornua that the numerous progenv of such animals are lodged during the whole time of their retention in the uterus ; and consequently such an arrangement is absolutely requisite, as must be evident from simply inspecting the gravid uterus of a Sow (fig. 335), where the cornua uteri (c, c,) are of remarkable dimensions. MAMMALIA. 727 As we ascend from the more prolific inferior races to the Qua- drumana and the Human species, the proportionate size of the body of the uterus becomes materially increased, and that of the cornua diminishes in the same ratio, until in the Monkeys and in Woman the latter become quite "lost, and the now pyriform central part appears to compose the entire' viscus, into the cavity of which the Fallopian tubes seem immediately to discharge themselves. Thus gradually, therefore, does the oviparous sexual apparatus assume the viviparous type ; and then, passing through numerous intermediate forms, ultimately attains its most concentrated condition in the uterus of the human female. (857.) In every other part of the generative system we shall likewise find the characters of the type at length completely estab- lished. The ovaria (Jig. 335, a) entirely lose all traces of their original racemose condition, for now the quantity of granular matter enclosed along with the germ in each Graafian vesicle, the last rein- Fig. 335. nant of the yolk, has become almost inappreciable, and the little ovarian ovules are enclosed in a dense parenchymatous substance 728 MAMMALIA. enveloped by a smooth albugineous tunic. The Fallopian tubes (b) correspond, in the smallness of their diameter, with the minute- ness of the globules they are destined to convey from the ovaries into the uterine receptacle ; and lastly, the excretory canal of the bladder (cf) becomes quite separated from the vagina (e), and the anal and generative apertures are found completely distinct from each other. (858.) After the above brief sketch of the anatomy of the or- gans of generation in the higher Mammalia, it now remains for us to trace the developement of the germ from the moment of im- pregnation to the birth of the fetus, and observe in what particu- lars placental generation differs from the oviparous and ovo-vivipa- rous types already described. In the viviparous or placental Mam- mifer, the effect of impregnation is the bursting of one or more of the Graqfian vesicles, and the escape of the contained germs from the ovisacs wherein they were formed. In the Ovipara, owing to the delicacy of the ovisacs, the vascular membranes composing them when once ruptured are speedily removed by absorption ; but in the Mammal this is not the case, and a cicatrix remains perma- nently visible upon the surface of the ovary, indicating where the rupture has occurred : such cicatrices are known by the name of corpora lutea. (859.) On the rupture of the ovarian ovisac, the vesicle of Pur- kinje, or the essential germ, accompanied only by a most minute quantity of granular fluid, or yolk, is taken up by the fimbriated extremity of the Fallopian tube, and conveyed into the interior of the uterus, where its developement commences. Observations are wanting to teach us precisely what are the first appearances of the embryo ; but there is not the least doubt that the materials for its earliest growth are absorbed in the cavity of the womb, and that its formation from a blastoderm, or germinal membrane, is ex- actly comparable to what occurs in the egg of the Bird, already minutely described in the last chapter ( 699 et seq.), and that in every particular, as relates to the growth and functions of the vitelline or omphalo-mesenteric as well as of the amniotic systems, the phenomena are the same as in the marsupial Mammal up to the period when the young Marsupian is prematurely born, to be afterwards nourished in the pouch of its mother from materials de- rived from the breast. But precisely at that point of developement where the Marsu- pial embryo is expelled from the uterus of its parent, namely, when MA MM ALT A. 729 the functions both of the vitellicle and of the allantoic apparatus become no longer efficient either for nutrition or respiration, a third system of organs is developed in the placental Mammifer, whereby a vascular intercommunication is established between the fetus and the uterine vessels of the mother, forming what has been named by human embryologists the Placenta. In the ovum of a Sheep, at that period of the growth of the fetus which nearly corresponds with the end of utero-gestation in the prematurely born Kangaroo, all the three systems alluded to are coexistent and easily distinguishable, as will be seen in the accom- panying figure (j&\ 336). The fetus (0), enclosed in its amnio- tic membrane (7>), has its limbs as yet but very imperfectly formed, exhibiting pretty nearly the condition of a nascent Marsupial (vide Jig. 331) ; but here it will be seen that the umbilical systems exhi- bit very striking differences in the two races. The vitellicle (f), Fig. 336. with its pedicle (e), are of very small dimensions ; the allantoid sac (g), on the contrary, is of considerable bulk, and, having ceased to act as a respiratory organ, becomes adapted to receive the urinary secretion through the canal of the urachus. The most important feature, however, is the" rapid extension of the umbilical vessels (rf), which in BIRDS and MARSUPIALS were distributed only to the allantois ; but in the placental Mammals these vessels rapidly spread over the chorion (A), and, coming in contact with the vascular sur- face of the womb, they soon form a new bond of communication be- tween the mother and the fetus, constituting the placenta ; and thus the offspring is nourished, until, its intra-uterine growth being ac- complished, it is born in an advanced condition of developement, and becomes the object of maternal care during that period in which it is dependent upon the breast of its mother for support. 730 MAMMALIA. (860.) The appearance of the placenta varies much in different tribes : thus, in the Sheep and other RUMINANTS it consists of numerous detached masses of villi (z, z), that indigitate with corresponding processes derived from the maternal womb ; in the Mare it covers the whole surface of the chorion ; but in the greater numbers of Mammals, and in the Human female, it forms a single vascular cake, whence is derived the name appropriated by anatomists to this important viscus. (861.) After the developement of the placental system, it is obvious that the arteries derived from the common iliac trunks of the fetus, which at first were distributed only to the allantois, as in the case of the Bird ( 705), on the developement of the pla- centa become transferred to the latter viscus, and form the umbili- cal arteries of the navel-string. The vein likewise, notwithstand- ing its prodigiously increased extent of origin after the placenta has been formed, takes the same course on entering the umbilicus of the fetus as it did when it was derived only from the allantois ; so that, although the placenta completely usurps the place of the allantois, both the allantoic and placental circulations are carried on through the same umbilical arteries and veins. (862.) In order to complete our history of fetal developement up to the full establishment of the permanent double circulation that characterises all the hot-blooded Vertebrata after birth, it only remains for us to notice the changes that occur in the vessels of the fetus, whereby, on the cessation of the functions of the pla- centa, the pulmonary circulation is at length brought into action. Up to the period of birth the arrangement of the fetal circula- tion remains essentially that of a Reptile, inasmuch as both the venous blood derived from the system and the arterialized blood that comes from the placenta, are mixed together in the as yet im- perfectly separated chambers of the heart. Under these circum- stances the arrangement of the vascular system is as follows : Pure blood, supplied from the placenta is brought into the body by the umbilical vein, which passes partly into the portal system of the liver, but principally through the ductus venosus into the inferior cava, and thence into the heart. From the construction of the heart during this portion of fetal existence it is obvious, that, in that viscus, all the blood derived from the placenta, from the ve- nous system of the fetus, and also from the as yet inactive lungs, is mingled together prior to its distribution through the arterial sys- tem. The two auricles communicate freely with each other through MAMMALIA. 731 the foramen ovale ; and by means of the ductus arteriosus the greater portion of the blood driven from the right ventricle during the systole of that cavity passes into the aorta, a very small pro- portion only finding its way into the pulmonary arteries. Such a heart therefore supplies a mixed fluid to the fetal system ; of which a portion, having passed through the arterial trunks, finds its way back to the placenta through the two umbilical arteries, there to recommence the same circle. (863.) Immediately after birth, however, the whole arrangement is altered, and the adult condition fully established. The lungs assume their functions, and the pulmonary arteries attain their full proportions ; while the placenta at once ceases from its office, and all the umbilical vessels become obliterated. The ductus venosus is no longer permeable, so that the portal system and that of the vena; cava are quite separated : the foramen ovale closes, thus completely separating the right from the left auricle : the ductus arteriosus is reduced to a mere ligament ; all the blood, therefore, driven from the right side of the heart must now pass into the ex- panded lungs, and be returned through the pulmonary veins to the left side of the heart. Thus the pulmonary and systemic circula- tions being rendered totally distinct, arterialized blood alone enters the arterial system, to be distributed through the body ; and, the umbilical arteries disappearing, the highest form of the circula- tory apparatus is fully established. (864.) After birth the mammary glands supply the first nutri- ment to the still helpless offspring. These vary in number and position in different species of placental Mammifers, their number being of course greatest in the most prolific races. Where the arms or anterior limbs can be used for supporting or clasping the feeble young, as in the QUADRUMANA, the BATS, and the females of our own species, it is upon the breast that these nutrient founts are placed ; but in less gifted tribes the mammte are situated beneath the abdomen or in the inguinal region. Their structure, however, is similar throughout the entire class ; each gland consisting of in- numerable minute secreting cells, grouped together in lobules and in lobes. Delicate excretory ducts, derived from all these ultimate cells, unite together again and again until they form capacious ducts, or rather reservoirs for milk. In the Human female the lac- tiferous canals terminate by numerous orifices upon the extremity of the nipple ; but, where the nipples are of large size, they gene- rally contain a wide cavity wherein the milk accumulates in consi- 10% MAMMALIA. derable quantities, to be discharged through one or two orifices only. Such are the modes by which Supreme Beneficence has provided for the infant progeny of Mammiferous beings, and con- ferred the endearments of maternity where He has bestowed intel- ligence to appreciate affection. But even this is not all : from the superabundance of the store provided there may be yet to spare ; and Man is privileged to bid his lowing herds yield him their milk for food, and Ihus obtains no slight addition to the bounteous table spread for his enjoyment. THE END. LONDON: PRINTED BY SAMUEL BEXTLEY, Bangor House, Shoe Lane. WORKS ON NATURAL HISTORY, &c. MOSTLY ILLUSTRATED, PUBLISHED, AND IN PREPARATION, BY JOHN VAN VOOKST, 1, PATERNOSTER ROW, BOOKSELLER TO THE ZOOLOGICAL SOCIETY OF LONDON. A HISTORY OF BRITISH QUADRUPEDS, INCLUDING THE CETACEA. BY THOMAS BELL, F.R.S., F.L.S., V.P.Z.S., &c. Professor of Zoology in King's College, London. The letter-press of this volume contains an account of the habits, utility in food, manufactures, agriculture, or domestic economy, and the noxious qualities of such as are in any way injurious to man, of each species, of this the highest class in the animal kingdom ; and an attempt has been made to define the characters of many of the species with more accuracy than had been done by previous authors. The illus- trations comprise a figure of every species, and of many varieties, with numerous pic- torial tail-pieces and anatomical diagrams, illustrative of the text, amounting, in all, to two hundred. Price of the work, in demy 8vo. 28s. A few copies are also printed in royal 8vo. price 21. 16s. and a very limited number in impeiial 8vo. price 4/. 4s. A HISTORY OF BRITISH BIRDS. BY WILLIAM YARRELL, F L.S., V.P.Z.S., &c. The History contains descriptions of species, their synonymes, generic and specific characters, geographical range, habits, food, nidification, sometimes with nests, eggs, and other interesting particulars. The illustrations include one representation of each species, and frequently of both male and female : the distinctive difference be- tween the young and adult bird is sometimes given in a third figure, and, occasionally, the variation from summer to winter plumage is shown. Other illustrations, comprising modes of capture, anatomical distinctions, or the most interesting features of internal or external structure, are introduced the more fully to illustrate the descriptions. In the 1st vol., price 28s., 105 species are figured and described. The second vol., price 35s., contains 109 species. The third vol. now in course of publication will complete the work. Published in parts, each alternate month, price 2s. 6d. A limited number is also printed on royal 8vo. price 5s. each part, and fifty only on imperial 8vo. The latter size will not be delivered until the work is complete. A HISTORY OF BRITISH REPTILES. BY THOMAS BELL, F.R.S., F.L.S., V.P.Z.S., &c. Professor of Zoology in King's College, London. The Reptiles of this country, although few in number, are not devoid of consider- able interest ; their habits are popularly much misunderstood, and several innocent and useful species are shunned and destroyed, from a mistaken notion that they are directly or indirectly noxious to man. The elucidation of their habits, the distinctive descrip- tion of the species, their geographical distribution, and the history of the transformation of all the amphibious forms, are amongst the subjects discussed. In addition to a figure of each species, and of some of the most important varieties, the illustrations comprise many of structure, development, and transformation. In one vol. demy 8vo. with more than 40 illustrations, price. 8s. 6d. Royal 8vo. 17s. Imperial 8vo. 25s. 6rf. A HISTORY OF BRITISH FISHES. BY WILLIAM YARRELL, F.L.S., V.P.Z.S., &c. &c. This work, which contains a complete history of the Ichthyology of Great Bntain, including many species never before noticed, is illustrated with figures of the Fishes, mostly taken from the objects themselves, and numerous vignettes, drawn and en- graved by the most eminent artists. In two vols. demy 8vo., illustrated by nearly 400 beautiful wood-cuts, price 2/. 8.s. [loyal 8vo. 4/. 16s. Imperial 8vo. price 11. 4s. A SUPPLEMENT TO THE FISHES, Containing about thirty additional species new to Britain, which the Author has de- rived from various sources since the publication of the work, some of which are also new to Ichthyology. Price 7s. 6d. demy 8vo. 15s. royal 8vo. 22s. 6d. imp. 8vo. BY THE SAME AUTHOR, A Paper on the Growth of the Salmon in Fresh Water ; with six illustrations of the fish of the natural size, exhibiting its character and exact appearance at various stages during the first two years. 12s. sewed. A HISTORY OF BRITISH STARFISHES, SEA URCHINS, AND OTHER ANIMALS OF THE CLASS ECHINODERMATA. Containing an Account and Figure of every Species met with on the British Coasts. By EDWARD FORBES, M.W.S., For. Sec. B.S. &c. One vol. 8vo., with above 120 Illustrations, price 15s., or Royal 8vo. 30s. A HISTORY OF BRITISH CRUSTACEA. By Professor BELL. In preparation. A HISTORY OF BRITISH FOREST TREES. BY PRIDEAUX JOHN SELBY, F.R.S.E., F.L.S. &c. A wood-engraving of each species, and such Vignette or other Illustrations as may be necessary to elucidate any subject under review, will be embodied with the description. About eight Parts, in 8vo., at 2s. 6d. each, or royal 8vo., 5s. each, will complete the volume. A HISTORY OF BRITISH FERNS. BY EDWARD NEWMAN, F.L.S. This work contains a Figure of each British species and variety, and shows the distri- bution of the veins, the mode of fructification and manner of growing. The letter-press gives the geographical range, every known locality, a list of synonymes, and a full description of each species and variety. 8vo., with Eighty-seven Illustrations, price 10s. A MANUAL OF BRITISH (ALG^) SEA-WEED. BY THE HON. W. H. HARVEY. With references to the figures in other works. In the press. BRITISH OOLOGY. Being illustrations of the Eggs of British Birds, drawn and coloured from Nature, with descriptions, by WILLIAM C. HEWITSON. 2 vols. royal 8vo. 61. 16s. 6d. A NOMENCLATURE OF 1 BRITISH BIRDS. Being a systematic catalogue of all the species hitherto discovered in Great Britain and Ireland, intended for labelling collections of British Birds and their Eggs. By HENRY DOUBLEDAY. 3rd edition, Is. 6d. sewed. A FLORA OF SHROPSHIRE. BY W. A. LEIGHTON, B.A., F.R.S.E., c. Comprising the flowering plants indigenous to the county, arranged on the Linnaean system. 8vo. 24s. A FLORA OF THE NEIGHBOURHOOD OF REIGATE, SURREY, CONTAINING THE FLOWERING PLANTS AND FERNS. BY GEORGE LUXFORD, A.L.S., F.R.S.E. 12mo, with a map of the district, 5s. A HISTORY OF THE FOSSIL FRUITS AND SEEDS OF THE LONDON CLAY. BY JAMES SCpTT BOWERBANK, F.G.S. &c. The First Part, in royal 8vo. price 16s., contains the description, and 423 figures en- graved on 17 copper plates by Mr. JAMES DE CARL SOWERBY. The Work will be completed in about Five Parts. NOTES ON NETS; OR, THE QUINCUNX PRACTICALLY CONSIDERED, BY THE HON. AND REV. CHARLES BATHURST, LL.D. 12mo., price 4s. THE HONEY BEE; ITS NATURAL HISTORY, PHYSIOLOGY, AND MANAGEMENT. BY EDWARD BEVAN, M.D. A new edition, considerably extended and carefully revised by the Author, one volume, 12mo., with many illustrations, 10s. 6d. THE NATURAL HISTORY OF THE SPERM WHALE, AND A SKETCH OF A SOUTH SEA WHALING VOYAGE. BY THOMAS BEALE, LATE SURGEON TO THE " KENT" AND " S.lRAH AND ELIZABETH*' SOUTH SEAMEN. PostSvo., 12s. AN ANGLER'S RAMBLES. BY EDWARD JESSE, F.L.S. AUTHOR OF " GLEANINGS IN NATURAL HISTORY." Contents: Thames Fishing. Trolling in Staffordshire. Perch Fishing-club. Two Days' Fly fishing on the Test, Luckford Fishing-club. Grayling Fishing. A Visit to Oxford. The Country Clergyman. Post 8vo., price 10s. 6d. CATTERMOLE'S ILLUSTRATED EDITION OF DR. AIKIN^S CALENDAR OF NATURE; OR, NATURAL HISTORY OF EACH MONTH OF THE YEAR. With additions, by a Fellow of the Linnaean and Zoological Societies, and eighteen designs by Cattermole. Small 8vo., 4s. 6d. In ordering this volume " Cattermole's edition" should be particularly expressed. BEAUTIES OF THE COUNTRY; OR, DESCRIPTIONS OF RURAL CUSTOMS, OBJECTS, SCENERY, AND THE SEASONS. BY THOMAS MILLER, AUTHOR OF "A DAY IN THE WOODS," " ROYSTON GOWER," " RURAL SKETCHES," &C. Post 8vo., with twenty-six Illustrations, 12s. 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Price, in royal 8vo., 2s. 6d. plain, 5s. coloured. A few copies will be printed on large paper. THE CANADIAN NATURALIST. BY PHILIP HENRY GOSSE, COB. MEM. OF THE NAT. HIST. SOC. OF MONTREAL, AND OF THE LIT. AND HIST. SOC. OF QUEBEC. This volume is the result of a residence of several years in Lower Canada, and con- tains brief and popular notices of subjects in the different departments of natural his- tory, presenting to the reader a picture of the face of nature in that interesting country. Forty-four illustrations of the most remarkable animal and vegetable productions are embodied in the text. Post 8vo. 12s. DOCUMENTS CONNECTED WITH THE HISTORY OF LUDLOW AND THE LORDS MARCHERS. EDITED BY THE HON. R. H. CLIVE. Imperial 8vo. 31s. 6<l. LITTLE FABLES FOR LITTLE FOLKS. Selected for their moral tendency, and re-written in Familiar Words, of One and Two Syllables. Designed to amuse and instruct. Illustrated. 18mo. \s.6d. ELEMENTS OF PRACTICAL KNOWLEDGE; OR, THE YOUNG INQUIRER ANSWERED. 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Elegantly printed, in 1 vol. post 8vo., 9s. A Polyglot Edition of this volume, with inter-paged translations in the Greek, Latin, German, Italian, and French languages. 12s. THE BARD, BY GRAY. With Illustraiions from Drawings, by the Hon. MRS. JOHN TALBOT. Uniform with the Elegy of Gray, to which it forms an appropriate companion volume. 7s. ILLUSTRATIONS OF THE SEVEN AGES OF SHAKSPEARE. Drawn on the Wood expressly for this Volume by W. MULREADY, R.A. ; C. R. LESLIE, R.A. ; J. CONSTABLE, R.A.; SIR DAVID WILKIK, R.A. ; W. COLLINS, R.A. ; A. E. CHALON, R.A.; A. COOPLR, R.A. ; Sin A. W. CALLCOTT, R.A.; EDWIN LANDSEER, R.A.; W. HILTON, R.A. 4to., 15.*. RETURN 14 DAY USE ESKFROMWl OWED This book is due on the last date stamped below, or on the date to which renewed. Renewed books are subject to immediate recall. r2 MM 2 1 ^ 5Z DEH in 1963 2SMo LD 9 1 50m 12 Y 1 General Library (C4796slO) 476 UniVefS Be' r k f eS lifOrnia