is ^/jyy ! 'i I SI: *&$///$$; "wl Pllil Wwwfi$ imm %$m^ jfjfisg "i m sfiw/&Pji ; vW , GENERAL OUTLINE OF THE ORGANIZATION OP THE ANIMAL KINGDOM. GENERAL OUTLINE OF THE ORGANIZATION THE ANIMAL KINGDOM, MANUAL OF COMPARATIVE ANATOMY. BY THOMAS RYMER JONES, F.R.S., PROFESSOE OF COMPAEATIVE ANATOMY IN KING'S COLLEGE, LONDON ; LATE FULLEEIAN PROFESSOR OF PHYSIOLOGY TO THE ROYAL INSTITUTION OF GREAT BRITAIN, ETC. ETC. ILLUSTRATED BY FOUR HUNDRED AND TWENTY-THREE ENGRAVINGS. LONDON: JOHN VAN VOORST, PATERNOSTER ROW. M.DCCC.LXI. LONDON: PRINTED BY TAYLOE AND FRANCIS, RED LION COURT, FLEET STREET. TO RICHARD OWEN, ESQ., F.R.S., ETC. ETC. ETC., THE FOLLOWING PAGES ABE INSCBIBED BY HIS SINCEEE FBIEND, THE AUTHOR. M367752 PREFACE TO THE SECOND EDITION. THE object of the writer of the present work has been twofold : first, to lay before the Naturalist a complete view of the organi- zation and physiological relations of every class of living beings ; and secondly, to offer to the Anatomical Student a succinct account of the structure and development of the vital organs through all the modifications they present in the long series of the animal creation. Such were the intentions of the Author, as announced at the commencement of his undertaking ; and the reception the first edition received at the hands of the public has been such as to afford gratifying proof that his efforts to facilitate the progress of the cultivators of a science the importance of which is becoming every day more conspicuous have not been unsuccessful. Since the publication of the preceding edition, however, great and important advances have been made in our knowledge : many and earnest have been the labourers in this enticing field, and proportionately encouraging have been the results. The inde- fatigable industry of Professor Owen, conspicuous in every de- partment of our science, has, by his invaluable analysis of the vertebrate skeleton, not only re-modelled the nomenclature of the osteologist, but placed in the hands of the Geological Student a light wherewith to guide his steps amid the darkness of departed worlds. The improvements in our microscopes, and the zeal of our microscopists, have much advanced our knowledge of the In- fusorial organisms. The researches of Van Beneden and Siebold relative to the embryogeny of parasitic worms open before us a new field of research; while the observations of Steenstrup, Vlll PREFACE. Daly ell, and Agassiz, on the " alternation of generations" among the Hydriform Polyps and Acalephse, promise results of the utmost interest to the Naturalist. The discoveries of Milne-Edwards have importantly increased our information concerning the organization of the Mollusca as well as of the Alcyonoid Polyps ; and those of Muller, revealing the metamorphoses of the Echinodermata, add new lustre to a name already so distinguished in science. To particularize our own countrymen and fellow-labourers whose names give value to the following pages would be an in- vidious task ; suffice it to say that the Author has endeavoured, to the best of his ability, to keep pace with their diligence and onward progress, so as adequately to record and acknowledge their contributions to the general stock of scientific lore. To Mr. Van Voorst, the liberal Publisher of the present volume, the Author cannot but offer his best thanks ; the nu- merous and costly illustrations that adorn the work speak for themselves, while his endeavours to publish it at a price placing it within the reach of every student will, it is hoped, be exten- sively appreciated. PREFACE TO THE THIRD EDITION. THE short interval which has elapsed since the preceding preface was written affords additional evidence of the increasing useful- ness of this work. Encouraged by such success, the. Author, in revising the present edition, has supplied sundry omissions, and added such new observations as the onward progress of ana- tomical science seemed to require. Important alterations in the arrangement of the Animal Series have likewise been introduced, among which may be pointed out the complete separation of the PROTOZOA from the CILIATED INFUSORIA, the introduction and redistribution of the class HELMINTHOZOA, the transference of the classes ROTIFERA and CIRRHOPODA into close proximity with the CRUSTACEA, to which they are related in many particulars of their economy, and the establishment of the POLYZOA as legiti- mate members of the MOLLUSCOUS division of Creation. As a general rule, however, the Author has been careful to avoid un- necessary changes in zoological classification, from a conviction that they are rather calculated to embarrass than to facilitate the progress of the student of Comparative Anatomy. GENERAL INDEX. CHAPTER I. ON CLASSIFICATION. Page Sec. System of Aristotle 1 . 2 Linneeus 1,3 Hunter -. -.' v ; 9 . '4 Cuvier 3 '..;'.' 5 Arrangement founded on the condition of the nervous system ... 4 . 10 CHAPTER II. PEOTOZOA. RIIIZOPODA 5 . 13 Structure of Gromia oviformis 5. 13 Locomotion of Rhizopods 7 . 14 Their mode of nutrition and reproduction 7.15 Some of them enclosed in shells 7 . 16 FORAMINIFERA 7 . 17 Their organization 8 . 18 Pseudopodia 8 . 19 Structure and growth of their shells 8 . 20 Shell of Orbitolite 9 . 21 Professor Max Schulze's observations relative to the reproduction of the Foraminifera 10 . 23 Their multitudes and geological importance 11 . 25 POLYCYSTINA 12 . 27 ACTINOPHRYS 13 . 28 Mode of taking and digesting food 14 . 29 Contractile vesicle . , 15 . 33 Reproduction by self-division 16 . 35 NOCTILUC.E 16 . 36 Their mode of reproduction 17 . 38 Phosphorescence of the sea 17 . 40 AMCEB.E 18 . 41 Structure of Amoeba princeps 19 . 42 Development from ovules 20 . 43 SIEGES . 20 . 44 General structure of Sponges 20 . 45 Siliceous and calcareous spicula 21 . 47 Currents visible in the living Sponge 21 . 49 Proteiform structure of the living substance of Sponges 23 . 52 Mr. Carter's observations on the nature of Sponge-cells 23 . 53 Seed-like bodies of the fresh-water Sponge 25 . 58 Sexual generation of Sponges 26 . 60 Structure and evolution of spermatozoa 27 . n. Reproduction of Sponges from ciliated gemmules 27 . 61 THALASSICOLLA 29 . 62 xii GENERAL INDEX. CHAPTER III. INFUSORIA. Page Sec. INFUSORIA Restriction of the term _ . , . 31 . 63 Boundary between the Animal and Vegetable Kingdoms .... 32 . n. Locomotive apparatus of the Infusoria 34 . 66 Cilia 34 . 67 Ciliary action 34 . 68 Mechanism of the stem of Vorticella 34 . 69 Trichocysts 35 . 70 Arrangement of the digestive apparatus, according to Ehrenberg . 86 . 73 Objections to Ehrenberg's views 36 . 74 Organization of the Infusoria, according to M. Dujardin .... 38 . 79 Observations of M. Meyen upon this subject 39 . 80 Acineta, observations of Dr. Lachmann 40 . 83 Internal structures observable in the Infusoria 41 . 84 " Bound corpuscles" 41 . 84 " Gastric juice" 41 . 85 Contractile space 41 . 87 Nucleus . . 44 . 90 Horny integument 44 , 91 Reproduction of Infusoria 44 . 92 by external gemmation 44 . 93 spontaneous fissure 44 . 94 as exhibited in Convattaria 45 . 97 Wonderful productiveness of the fissiparous mode of generation . 46 . 99 Reproduction by encystment 47 . 101 Stein's views relative to the metamorphoses of Vorticella .... 47 . 101 Encapsulation of Kerona pustulata 48 . 103 Ploesconia Charon . 49 . 104 Propagation by embryos or internal germs 50 . 105 Metamorphoses of Trichoda Lynceus . 53 . 110 CHAPTER IV. ANTHOZOA. ANTHOZOA External resemblance to plants 54. Ill General description 54 . 112 Fungice and Meandrln 54 . 113 Structure of Fungia agariciformis 54. 114 Nature of its gelatinous investment . . . . 55 . 115 Reproduction of Fungice 55 . 116 Calcareous polypary deposited in the areolse of the living tissue . . 55 . 118 CORTICAL COMPOUND ANTHOZOA 56 . 119 Then* various forms 56 . 120 ALCYONID^E 56 . 121 Organization of Cydonium Miilkri 57 . 122 Inquiry concerning the compound nature of the Alcyonidaj ... 57 . 123 Milne-Edwards' s researches relative to the structure of Alcyonidium ekgans 58 . 125 Anatomy of the individual polyps 58 . 126 Organization of the common polypary 60 . 128 Gemmiparous mode of reproduction 62 . 133 Reproduction by gemmules . .-..,.' 63 . 135 Alcyonium stellatum, . -, * 64 . 141 Vascular canals of the polypary 64 . 141 Mode of reproduction . . . .. .. .-. --v-'-'j" -.'.- .... 65 . 142 MADHBPORID/E ?--. - : . . . . 67 . 144 Formation of coral reefs .':.,. . 67 . 145 GENERAL INDEX. xiii Page Sec. CORALLID/E 68 . 147 Corallium rubrum . . . . 68 . 148 Isis hippuris . ,", 68 . 149 Gorgonia verrucosa 69 . 151 PENNATULID^: . .' ". ".""T'Y ; / / f ' / % ' / _ >; . . a . . 70 . 153 TUBIPORIM: i.'., *. y^ 70 . 154 Tubipora musica . . ..."......,.. 71 . 155 Anatomy of the polyp of Tubipora . . . ^--vc-'..'..* ; J- .'^ . . 71 . 155 Growth of the polypary of Tubipora . . . / . . w, ,v ~i . . 72 . 158 Reproduction and development of Tubipora . .v.- ',"'-,;. . 73 . 162 ACTINIFORM POLYPS J- f ^:.- . *;^ * -.-" - r- 73 . 163 Anatomy of Actinia . ...'..'../. .Xa:*. v .- , .... 74 . 164 Reproduction of Actinia . . . . . . . . _> i /.;*. . 76 . 169 Development of the embryo i ..;*.;,-. . 77 . 172 Filiform tubes 78 . 173 Power of reproducing mutilated parts 79 . 174 Mechanism of the oral tentaoula 79 . 175 Filiferous capsules 79 . 176 CHAPTER V. HYDKOZOA. H YDROZOA General account of the Hydrce or Fresh-water Polyps . 80 . 177 Their mode of catching prey .'..''. .... 80 . 179 Filiferous capsules of the Hydra 81 . 180 Digestion of food -.;-n-,v, . v . 81 . 181 Coloured globules 81 . 182 Eeproduction of Hydra v iridis . . . "" .' '" ' /'-"V 1 ' '-'. ..... 82 . 183 by external gemmation . . . . . . . . . . . 82 . 184 mechanical division . '; f -' i rL } '^ */"' i>\' ''.''. . 82 . 186 ova -.-/'..-.', '.'-; .V . 82 . 187 Spermatic capsules . .'-"i ' i 83 . 188 Ovigerous capsules 84 . 189 Structure of ova 84 . 190 Hermaphroditism of Hydra viridis . . . ... . ... . 84 . 191 CORYNB v.K- $*>* ^ '.. . , '. . 84 . 193 TUBULARIDJE 85 . 194 Structure of Tubularia indivisa . . . .'"-^ ' ; . l;; v ' '"; ' ;'- f .'" . . 85 . 194 Propagation of Coryne and Tubularia 86 . 195 by continuous gemmation 86 . 197 free gemma? 88 . 201 simple ova 91 . 212 SERTULARID.E 92 . 215 Structure of Sertularia 92 . 217 Circulation observable in stem 93 . 218 Growth of polypary 93 . 220 Structure of polyps 94 . 221 Structure of tentacula 94 . 222 Proboscidiform mouth 94 . 223 Digestive cavity 95 . 224 Reproduction by cuttings 95 . 226 the formation of new branches 95 . 227 Planula . 97 . 230 of Sertularian zoophytes, according to the observa- tions of Lov6n . .97 . 231 Development of the embryo of Sertularian zoophytes 98 . 235 CAMPANULARID^: 99 . 243 Structure of polypary 99 . 244 Embryogeny of the Campanularian zoophytes 99 . 247 xiv GENERAL INDEX. CHAPTER VI. HYDKOZO A (continued ) ACALEPILE. Page Sec. ACALEPHJS General description of the 104 . 267 PULMONIGRADA 105 . 270 Tissues entering into their composition . . . 106 . 271 Physiological interest attaching to their organization 106 . 272 Digestive system of Rhizostoma .' . a -; . . . 106 . 273 Cassiopea Borbonica 107 . 275 Cyanea aurita 108 . 277 Marginal excrementitious orifices . . . 110 . 283 Movement of fluid in the alimentary tubes 1 10 . 284 Muscular structure observable in some species . Ill . 285 Circulation of globules enclosed in vascular canals Ill . 286 Nervous system of the Medusae . . . Ill . 287 Oculiform specks 112 . 288 Auditory vesicles 112 . 289 Reproduction by gemination 113. 292 Alternate generations in the Medusiform Acalephae 113 . 294 Biseruality of the Acalephas 114 . 295 Female generative organs of Cyanea aurita 114. 296 Ovigerous pouches 114 . 297 Structure of ova in the ovaria 115 . 298 Three forms of ova contained in the ovigerous pouches . . . . 115 . 299 The ciliated embryos on leaving the ovigerous pouches assume the form of Hydra 115 . 300 Reproduction of the hydriform embryos by stolons, gemma, and bulblets 115 . 301 Segmentation of the Hydra tuba into Medusiform young . . . . 115 . 302 Development of the young Medusa 116. 304 Arrangement of the alimentary and generative apparatus in Cu- vieria carisochroma and Mguorea violacea 117 . 306 Generative apparatus of Mquorea 118. 307 CILIOGRADA 119 . 308 Structure of the locomotive organs in the Beroeform Acalephse . 119 . 309 Organization of the cilia 119 . 310 Diffused condition of the vital principle 120 . 312 Alimentary system of Beroe 120 . 313 Vascular apparatus ...-, . -. . . . . . 120 . 314 Circulation 121 . 315 Anal tubes 121 . 316 Emunctories of Beroe Forskahlii 121 . 316 Oculiform speck 121 . 318 Oculiform organ and nervous system of Lesueuria . . . . . . 122 . 319 Arrangement of generative system 122 . 321 Anatomy of Cestum Veneris 123 . 323 Secretions of the Acalephse 124 . 324 Their power of stinging 125 . 325 Phosphorescent light 125 . 325 PHYSOGRADA 125 . 326 PhysaUa 125 . 326 ClRRIGRADA 126 . 327 Porpita . 126 . 328 Sail of Velella and Rataria 127 . 328 DIPHTEA 127 . 329 Their structure and general history ...". .'/;.;....... . 127 . 330 GENEKAL INDEX. CHAPTER VII. HELMINTHOZOA. Page Sec. HELMINTHOZOA General history of the 128 . 335 ENTOZOA TV . . 129 . 336 Ccenurus cerebralis 129 . 339 Cysticercus 130 . 340 CESTOIDEA . V , 130 . 341 Tanice or tape-worms *,'.. 130 . 341 Anatomy of Tcenia solium 131 . 342 Longitudinal canals 132 . 343 Structure of mature segment or proglottis 132 . 344 Mode of impregnation 135 . 346 Development of Tamioid Entozoa 135 . 347 Structure of the ovum 135 . 348 "Scolex" 135 . 349 Cysticercus a Scolex of Ttsnia 135 . 350 " Strobile" or body of the Entozoon . . , 136 . 350 Developed by gemmation from the Scolex or " head " .... 136 . 351 Separation of the " proglottis " or adult Tamia 136 . 352 Development of proglottis 136 . 352 Segments of the Strobile enjoy a community of life 137 . 354 Growth of " proglottis" after its detachment from the " Strobile " 137 . 355 Enormous fecundity of the Tsenioid Helminthozoa 137 . 356 Envelope of the ova 137 . 357 Oviposition of " proglottis " 138 . 358 M. Miescher's observations relative to the metamorphoses of the Helminthozoa 138 . 359 M. Van Beneden's account of the metamorphoses of Tetrarhynchus 138 . 360 CYSTOIDEA Transformation of Cystiform Entozoa into Tasniae . . 140 . 367 Echinococcus veterinorum 140 . 368 TREMATODA 141 . 369 Distoma (Fasciola, Linn.) hepaticum 141 . 370 General history of Distoma 141 . 370 Alimentary apparatus 141 . 370 Nervous system 142 . 371 Generative system of Distoma 142 . 372 They are hermaphrodite 142 . 373 Structure of female generative organs 142 . 374 male generative organs 142 . 375 Development and metamorphoses of the Trematode Helminthozoa 143 . 376 Observations of Nitzsch, Bojanus, Steenstrup, Siebold, &c., relative to the " alternate generation " of the Distomata 143 . 378 Steenstrup's account of the metamorphoses of Cercaria echinata . 144 . 379 Monostomum mutabile 147 . 388 HELMINTIIOZOA ACANTHOCEPHALA 148 . 390 Anatomy of Echinorhyndius gigas 148 . 391 Structure of retractile proboscis 148 . 391 alimentary canal 149 . 392 Lemnisci 149 . 393 The Echinorhynchi are dioecious 149 . 394 Structure of female generative organs 149 . 394 Floating ovaria described by Siebold as existing in some Acantho- cephalous Helminthozoa 149 . 395 Male organs of Echinorhynchus 150 . 396 TURBELLARLE 150 . 397 PLANARI.E 150 . 398 Nervous system of PlanaricB 151. 400 Their power of surviving mechanical division . . .'-.- . . . 151 . 403 Structure of their digestive system 152 . 404 Visceral cavity 152 . 405 xvi GENERAL INDEX. Page Sec. Vascular system . . . 153 406 Hermaphroditism of Planarice 153 407 Structure of generative apparatus in Planar i a tremellaris . . . 153 408 NEMERTEAN HELMINTHOZOA 153 409 Ciliated integument 154 410 Alimentary apparatus 154 411 Circulatory system .,..,... 155 413 Reproductive organs 156 417 CCELELMINTHA Ascaris lumbricoides, general history of 157 419 Muscular parietes of the cavitary Helminthozoa 157 420 Structure and arrangement of the muscular system 157 421 Nervous system of Ascaris 158 422 Digestive system . 158 423 Caecal appendages to alimentary canal met with in some species . 159 424 Nutritive appendages v.v 159 425 In the Ccelelmintha the sexes are separate 159 426 Structure of female sexual apparatus ........... 159 426 Formation of the ova in the pvaria 160 427 Sexual organs of male Ascaris 160 428 Trichina spiralis 161 429 CHAPTER VIII. ECHINODERMATA. ECHINODERMATA General view of the class 161 . 430 CRINOID^E 162 . 433 Structure of Encrinus 162 . 433 Organization of fossil Encrinites 163 . 434 ASTERIDJS '.' ".""*! ; . .". ' :Vw .'.'.- '... . 163 . 436 Comatula 1 I I . 1 ."'".... 163 . 436 Gorgonocephalus 164 . 436 Ophiura , 165 . 437 Asterias . . * 166 . 438 Locomotive apparatus of Star-fish 166 . 439 ECHINIDJS V. , ... 167 . 440 Scutetta 167 . 440 Echinus ' .- . . 168 . 441 HOLOTHURIDJS '. 169 . 442 Holothuria . . . .,!?.?.. t .;...,. ,.' 169 . 442 FISTULARIDJS 169 . 443 Fistularia .>.,. ;' . . . . . . . 170 . 443 Anatomical structure of Asterias . . * ..:.-. .. ....,,,* ,j, ', ... . . 170 . 445 External coriaceous integument 170 . 446 Dermic secretions 171 . 447 Tegumentary spines 171 . 448 Pedicellariae 171 . 449 Calcareous framework of skeleton 172 . 450 Dorsal reticulations 172 . 451 " Vertebral " plates 172 . 452 Composition and arrangement of " vertebras " 172 . 453 Ambulacra! apertures 172 . 453 Ambulacra! grooves 172 . 453 Ambulacra! suckers 172 . 454 Mechanism of the ambulacra! suckers 172 . 454 Peritoneal lining of visceral cavity ....... . .' .' . 175 . 457 Alimentary system of Asterias 175 . 458 Caecal appendages to the stomach . . . 175 . 459 Functions of cnecal appendages 175 . 460 Food of Star-fishes 177 . 465 GENERAL INDEX. xvii Page Sec. Their power of destroying shell-fish . 177 . 465 Absorption of nutriment from digestive cavities 178 . 467 Intestinal veins 178 . 468 Vascular system of Aster ias . . . ."".'. ^ 178 . 469 Respiratory function in Asierias 180 . 474 Madreporic plate and " sand -canal " -. ">\ ~-\ ** : --V '.. '' v . . . 181 . 476 Ciliary apparatus of Star-fish . . -. -. . . . 181 . 477 Reproductive system (female) ] 82 . 479 Male generative organs 182 . 480 Ova 183 . 481 Anatomical structure of ovum '. 183 . 481 Development of embryo 183 . 481 Growth of young Star-fish, according to the observations of Sars . 183 . 482 Agassiz 184 . 484 Miiller's researches relative to the metamorphoses of Ophiurus . . 185 . 487 Pluteus paradoxus 186 . 488 Phenomena accompanying its development into a Star-fish . . . 187 . 491 Development and growth of rays 187 . 492 Nervous system of Asterias 1 88 496 Oculiform specks 189 . 497 General sensibility of the Echinodermata 189 . 497 ECHINI 190 . 498 Shell of Echinus 190 . 500 Necessity for its elaborate construction 191 . 505 Tubular suckers 192 . 506 Locomotive spines 193 . 507 Mode of their articulation with the shell 193 . 507 Growth of the spines of Echinus 194 . 509 Internal structure of spines 194 . 510 Mouth of Echinus 195 . 511 " Lantern of Aristotle " 196 . 511 Mechanism and muscular apparatus of the " jaws " 197 . 512 Digestive system of Echinus 198 . 518 Circulatory system 199 . 520 Respiratory function how performed f . . 199 . 522 Pinnated tentacula 199 . 523 Nervous system 200 . 524 Reproductive organs 200 . 525 Development of embryo 200 . 526 Miiller's researches Metamorphoses of Echini 200 . 526 HOLOTHUEIDJS 202 . 531 Structure of muscular integument * . 202 . 531 Locomotive suckers 202 . 532 Digestive system of Holothuria 204 . 533 Respiratory apparatus 204 . 536 Circulatory system 206 . 537 Generative organs 208 . 539 Metamorphoses of embryo 208 . 540 Nervous system 209 . 541 Muscular irritability 209 . 541 FlSTULARID^E 209 . 542 Sipunculus 210 . 543 Cuticle 210 . 545 Muscular system 210 . 546 Digestive apparatus 210 . 547 Vascular system 211 . 550 Course of the circulation 212 . 551 Sacculi 213 . 552 Nervous system 213 . 553 b xviii GENERAL INDEX. CHAPTER IX. HOMOGANGLIATA. Page Sec. HOMOGANGLIATA General type of structure 213 . 555 Review of the classes composing the Homogangliate division of the animal kingdom ,.-.... . 214 . 557 Arrangement of the nervous system ..,,,* 215 . 562 CHAPTER X. ANNELIDA. ANNELIDA General characters of the class 216 ABEANCHIATA 217 Anatomy of the Leech (Hirudo medicinalis) . . . . + , :. , . 217 Structure of muscular integument 217 Locomotive apparatus 218 Structure of the mouth 218 Mechanism of dental saws . . . . , 218 Internal digestive apparatus 219 Structure of alimentary canal 219 Respiratory and circulatory systems of the Leech 220 Nervous system of the Leech 224 Splanchnic gangl ionic nerves of Leech 225 Senses of the Leech 225 Organs of vision 225 Hermaphrodite condition of the generative system 225 Arrangement of male apparatus 225 Structure of female organs of generation as heretofore described . 226 Generative system of the Leech, as deciphered by Dr. Williams . 227 ABRANCHIA TERRICOLA 228 Anatomy of the Earthworm (Lumbricus terrestris) 228 Locomotive apparatus 229 Alimentary system 230 Arrangement of the circulation 231 Generative system of the Earthworm 233 , according to Duges 234 Dr. Williams 235 Evolution of ova 238 Development of embryo 238 Anatomy of Nais filiformis 240 Circulatory system of Nais 240 Generative system of Nais, after Duges 241 - Dr. Williams 242 Reproduction by spontaneous division 244 DORSIBRANCHIATA 245 Arrangement of respiratory apparatus 245 Cirri 246 Seta 246 Structure of the mouth in the Dorsibranchiata 246 Goniada 246 _ .- Amphinome 247 PhyUodvee 248 Laodicea 248 Alimentary canal of the Dorsibranchiate Annelida . . * -., -,.- . 248 Mechanism of alimentation 248 Circulatory system of the Dorsibranchiata . 249 Amphinome 249 in Eunice mnguinea 250 Multiplication by gemmation 253 Respiratory apparatus of Arenicola 255 GENERAL INDEX. xix Circulation in Arenicola Reproductive organs of the Dorsibranchiata Multiplication by spontaneous division Aphrodite aculeata ...... . -. -. TUBICOLA Anatomy of Terebella Branchial apparatus Cephalic tentacula ........... Circulatory system of Terebella nebulosa Generative apparatus of Terebella Embryology of the Annelidans Development and metamorphoses of Terebella Growth of Annelida by the successive formation of new segments Nervous system of Annelidans Structure of the eye of Torrea vitrea " Auditory capsule " of Arenicola Luminosity of marine Annelidans 259 261 262 264 266 268 268 269 271 274 275 277 277 278 279 279 Sec. 668 670 673 674 680 681 682 683 686 695 705 706 711 712 713 714 715 CHAPTER XI. MYRIAPODA. MYUIAPODA General remarks on the 280 . 716 JULJD.E 281 . 723 Anatomy of Julus terrestris 281 . 723 Male generative organs of Julus 282 . 727 Female generative organs of Julus 283 . 728 Ova, of Julus 283 . 729 Development and growth of Julus terrestris Observations of Mr. Newport 284 . 732 SCOLOPENDRID.E 287 . 739 Anatomy of Scolopendra 287 . 740 Mouth, structure of 288 . 742 Alimentary canal 288 . 743 Structure of the heart in the Scolopendridce 288 . 744 Arterial system 289 . 746 Generative organs of male Scolopendra 289 . 747 female 290 . 748 Phosphorescent and electrical phenomena exhibited by some species 290 . 749 CHAPTER XII. INSECTA. INSECTA Definition of the class 290 . 750 Metamorphosis of insects 290 . 752 Larva 291 752 Pupa and Imago 291 753 Insecta Ametabola 291 754 Distribution of insects into orders 291 755 HEMIPTERA 292 757 OKTIIOPTERA 292 758 DICTYOPTERA 293 759 NEUROPTERA 293 761 DIPTERA 294 763 LEPIDOPTERA 294 764 HYMENOPTERA 295 766 COLEOPTERA , 295 767 External anatomy of perfect insect . . . ;" * v 225 769 Structure of the legs of insects 298 778 [ GENERAL INDEX. Page Sec. Arrangement of the wings of insects 304 . 793 Texture of the wings of insects ft 305 . 797 Organization of the external skeleton 306 . 801 Anatomy of the wings of insects ..,,,,.. . . . 306 . 802 Appendages to external skeleton .... 307 . 804 Spines 307 . 805 Hairs 307 . 806 Scales of Lepidoptera 307 . 807 Muscular system of insects 308 . 808 Muscles of the leg of Melolontha vulgaris 309 . 815 Food of insects 310 . 817 Structure of the mouth of insects 311 . 818 Digestive system of insects 316 . 831 Structure of alimentary canal 317 . 832 Crop or "sucking-stomach" 317 . 833 Gizzard 318 . 834 Stomach proper 318 . 835 Small intestine 318 . 836 Colon 318 . 836 Salivary vessels 319 . 838 Hepatic vessels 319 . 840 Anal vessels 319 . 841 Absorption of nutriment 320 . 842 Respiratory system of insects 320 . 843 Structure of tracheae 322 . 844 Mechanism of respiration 324 . 848 Circulatory system of insects 324 . 849 Structure of the dorsal vessel or heart 325 . 850 Diffused character of the circulation 326 . 853 Structure of secerning vessels ....... 326 . 854 Nervous system of insects 327 . 855 Brain or encephalic ganglion, and nerves derived therefrom . . 327 . 856 Ventral chain of ganglia . 327 . 857 Motor and sensitive tracts 328 . 858 Nervus vagus or recurrent nerve 329 . 859 Sympathetic or splanchnic system 330 . 860 On the senses of insects 330 . 861 Sense of touch 330 . 862 taste 331 . 863 smell 331 . 864 Hearing of insects 331 . 865 Eyes of insects, of two kinds 332 . 867 Structure of simple eyes 332 . 870 compound eyes 332 . 871 Inquiry concerning vision with compound eyes 334 . 874 Sexes of the insect races distinct 335 . 875 Structure and arrangement of male organs in Melolontha vulgaris 335 . 876 Varieties observable in other insects 336 . 879 Female generative system of Melolontha 336 . 880 Formation of the eggs in the ovaria 337 . 883 Appendages to oviduct 338 . 884 Gluten-secretors 338 . 885 Spermatheca 338 . 886 Physiological use of spermatheca 338 . 887 Generative apparatus of Meloe variegatus 339 . 889 Structure of ovipositor 339 . 890 of Sires 339 . 891 Tenthredo 339 . 891 Number of eggs laid by insects 340 . 892 Fecundity of Aphides 341 . 894 Impregnation of several successive generations by a single coitus . 341 . 894 This mode of reproduction referable to the nursing system of Steenstrup ..... .. .. . . . .. . -.<.;'* X. . 341 . 895 GENERAL INDEX. xxi Page Sec. This mode of reproduction compared with that of Zoophytes and Plants 342 896 Fertility of the Hive-Bee 345 904 Termite Ants. . . V ." . . * 345 905 Variously shaped eggs of insects V- V ''. . . . 347 906 Phenomena of insect-metamorphosis ' . 347 907 Metamorphoses of Dragon-fly 349 912 Gnat 350 915 Anatomy of Caterpillar 352 916 Digestive apparatus 352 916 Respiratory system 354 917 Refe, epiploon, or fat-mass 354 919 Silk-spinning apparatus 354 920 Moulting of Caterpillar 356 922 Change from larva to pupa 357 923 Conversion of pupa into imago or perfect insect 359 925 Phenomena attending the healing of wounds in insects .... 359 926 Internal changes accompanying metamorphosis 359 927 Changes in the arrangement of the alimentary canal 359 927 Absorption of fat-mass 360 928 Disappearance of silk-secreting apparatus 360 929 Development of generative system 360 930 Progressive concentration of nervous system 360 931 Perfection of the organs of the senses * 362 931 Sounds emitted by insects 362 932 Phosphorescence of Elater and Lampyrls 362 933 CHAPTER XIII. ARACHNID A. ARACHNIDA Separation of Arachnidans from Insects 363 . 935 ARACHNIDA TRACHEARIA 364 - . 936 Their external anatomy 364 . 937 Structure of locomotive apparatus 364 . 938 Organization of mouth 365 . 939 Remarkable structure of digestive system 365 . 940 Respiratory organs 366 . 942 Singular respiratory apparatus of Trombidium 366 . 944 Breathing-organs of Hydrachna and the aquatic Acaridans . . . 367 . 946 Nervous system 367 . 947 Eyes 367 . 948 Generative apparatus 367 . 949 ARACHNIDA PULMONARIA 368 . 951 PEDIPALPI 368 . 952 ARANEID.E 368 . 953 Apparatus of the mouth in Scorpions 369 . 955 Spiders 370 . 956 Alimentary canal 370 . 958 Anal vessels of the Spider 371 . 960 Respiratory apparatus 372 . 961 Circulatory system in Spiders 373 . 964 Scorpions 374 . 965 Heart or dorsal vessel of Scorpion 375 . 966 Arterial system 376 . 969 " Portal system" of blood-vessels 376 . 970 Structure of " pulmonibranchiae" 377 . 971 Venous sinuses 377 . 972 Course of the circulation in the Scorpionidse 378 . 973 Nervous system of the Arachnida 379 . 974 Ocelli or eyes of Arachriidans 380 . 975 Generative apparatus 380 . 976 di GENERAL INDEX. Page Sec. Generative apparatus of male ^.. . 380 . 976 -of female ........ 380 . 977 Spinning-organs of the ARANEIDJB 381 . 978 Uses to which the thread of the spider is converted 382 . 979 Cavern of Mason Spider (Mygate) 382 . 979 Tent of Clotho Durandii . 383 . 980 CHAPTER XIV. CRUSTACEA. CRUSTACEA Tegumentary system of the 386 . 982 Progressive coalescence of the elements of the exoskeleton and of the centres of the nervous system 386 . 983 External organization of Talitrus 387 . 988 the Lobster (Astacus marinus) .... 387 . 990 Crab 387 . 991 King-Crab (Limulus) 387 . 992 Parallelism between these gradations and the progress of Insect- metamorphosis 388 . 993 Structure of articulated appendages 388 . 994 in the Lobster 389 . 994 Hermit-Crab ... 390 . 999 Crab 392 . 1001 Casting of the shell in Crustaceans 392 . 1002 Structure of the articulations 394 . 1005 Arrangement of muscular system 394 . 1005 Structure of the alimentary canal 394 . 1006 Hepatic apparatus 395 . 1010 Organs of respiration and circulation 395 . 101 1 Respiratory organs of the Lobster 395 . 1012 Crab 395 . 1013 Branchial chambers 397 . 1014 Mechanism of the respiratory apparatus 397 . 1015 Circulatory apparatus in the lower Crustacea 398 . 1018 Decapoda 398 . 1019 Course of the circulation 399 . 1022 Lacunar system in the Crustacea 400 . 1023 Nervous system 401 . 1024 of Talitrus 402 . 1025 Oniscus Asellus 402 . 1026 Crab 402 . 1027 Centralization of the nervous system during embryonic develop- ment 403 . 1028 Conclusions from the above facts 403 . 1029 Distribution of the nerves 403 . 1030 Nervus vagus 403 . 1031 Sensitive and motor tracts of the nervous system 404 . 1033 Susceptibility of pain in the Articulata 405 . 1035 Structure of tegumentary crust 406 . 1038 Power possessed by the Crustacea of voluntarily breaking off their limbs 407 . 1039 Capability of reproducing them .... 408 . 1040 Physiology of the process of reproducing cast-off limbs .... 408 . 1041 Senses of the Crustacea 408 . 1042 Structure of the organs of vision 409 . 1043 Simple eyes 409 . 1044 Agglomerated eyes 40^ Compound eyes ":.:*. v', ^ ; ~ . . . 409 . 1046 Auditory apparatus 409 . 1047 Generative system ~ * - 411 . 1053 Male organs of Astacus fluviatilis, 411 . 1054 GENERAL INDEX. Page Sec. Male organs of the BRACHYURA 412 1057 Female organs of Astacus fluviatilis 413 1060 Spermatheca 413 1061 Ova carried about by the female 413 1062 Metamorphoses of the Crab 413 1063 BRANCIIIOPODA 415 1067 Daphnia 415 1067 Squitta 416 1068 Reproductive system of the ENTOMOSTRACA 416 1069 " Womb-legs " of Apus 417 1070 Dorsal cavity of Daphnia 417 1071 Impregnation of several generations by a single coitus .... 417 1072 Ephippial eggs 418 1074 Development of embryo 419 1077 Cyclops 419 1078 Artemia 421 1079 EPISSOA 421 1080 General history of the Epizoa 421 1081 Lerneans 422 1082 Achtheres percarum 423 1085 Rudimentary articulated appendages 423 1088 Alimentary system of Achtheres 424 1091 Muscular system . . 425 1094 Nervous system 425 1095 Generative system (female) 425 1096 Internal ovaria 426 1097 Egg-sacs 426 1097 Metamorphoses of embryo 426 1098 Lamproglena pulchella 426 1100 Muscular system , 427 1101 Alimentary apparatus 427 1102 Senses 427 1103 Reproductive organs 427 1104 Nicothoe Astaci 427 1105 Alimentary system 428 1106 Phlebenterism 428 1107 Diminutive size of male 429 1108 Gemmiparous reproduction 429 1108 Female generative apparatus 429 1109 Metamorphoses of embryo 429 1110 PYCNOGONID.E 430 1111 Remarkable disposition of alimentary canal 430 1112 Nervous system 431 1116 CHAPTER XV. ROTIFERA. ROTIPERA General characters of the class 432 . 1117 Stephanoceros Eichhornii 432 . 1119 Tegumentary cell of Brachionus 433 . 1120 Caudal appendage 433 . 1121 Arrangement of cilia 433 . 1122 Nature of ciliary action 434 . 1123 Cause of apparent rotatory movement 434 . 1124 Observations of Dr. Arthur Farre 434 . 1124 M. Dujardin's explanation 435 . 1126 Uses of ciliary apparatus 436 . 1127 Mechanism whereby the " wheels " are protruded and retracted . 436 . 1128 Arrangement of digestive system 437 . 1130 Alimentary canal of Stephanoceros 437 . 1131 Brachionus . 437 . 1132 civ GENERAL INDEX. Page Sec. Structure of gastric dental apparatus 437 . 1133 Professor Williamson's account of the digestive organs of Meli- certaringens . . 438 1134 Nervous system of Eotifera 440 Views of Ehrenberg 441 Williamson . 441 Leydig 442 Huxley 442 Tentacular apparatus ., 442 Circulation in> the Rotifera 443 Respiratory system ; . 443 " Water-vascular system " 445 Organs of generation 445 Generative organs of Melicerta 445 Embryogenic development 446 Formation of tube of Melicerta . . f \ 446 Generative system of Asplanchnia 447 Female produces three kinds of ova 447 Remarkable organization of male Asplanchnia 447 Difference of form in the two sexes . . . . 448 1439 1140 1143 1144 1145 1146 1147 1148 1152 1154 1155 1155 1157 1158 1158 1159 1162 CHAPTER XVI. CIRRHOPODA. CIRRHOPODA Characters of the class 449 . 1164 External anatomy of Pen telasmis vitrea 449 . 1164 Nervous system of Pen telasmis 451 . 1166 Muscular system of Pentelasmis 451 . 1167 Digestive apparatus 452 . 1169 Circulatory system 452 . 1170 Senses of the CIRRHOPODA 453 . 1171 Visual organs 453 . 1171 " Olfactory apparatus " 453 . 1172 " Acoustic sacculus " 454 . 1173 Generative system 454 . 1174 , views of John Hunter 455 . 1 1 74 Cuvier 455 . 1175 , researches of Mr. Darwin . 456 . 1177 " Complements! male " 456 . 1177 Structure of the " complemental male " of Ibla Cummingii . . 456 . 1178 Hermaphroditism of Ilia quadrivalvis 457 . 1180 Larvae of Ibla quadrivalvis 457 . 1181 Development of ova 458 . 1182 Balani 459 . 1184 Metamorposes of the Balani 459 . 1185 Lepades 460 . 1186 Conversion of the locomotive larva into the fixed and pedunculate Cirrhopod 460 . 1187 Structure of the antennae 460 . 1188 Cement-gland 461 . 1189 Structure of the newly attached larva 461 . 1190 CHAPTER XVII. HETEROGANGLIATA (OWEN) ; MOLLUSCA (CUVIER). General review of the Molluscous division of the Animal Kingdom 462 . 1191 GENERAL TNDEX. CHAPTER XVIII. POLYZOA. BRYOZOA(EHRENB.); CILIOBRACHIATE POLYPI (FARRE). POLYZOA General survey of the class Anatomy of Bowerbankia densa 464 Structure of tentacula 464 Action of the tentacular cilia 464 Power of seizing food 464 Alimentary apparatus of the Polyzoa 465 Process of digestion 465 Structure of the cell of Bowerbankia 466 Mechanism of setse around its orifice 466 Muscular system of Bowerbankia 467 FLUSTR.E and ESCHAR/E 468 Arrangement of the polyp-cells 468 Their structure 468 Operculum 469 Structure of tegumentary sac 469 Changes of form at different ages 469 Indications of vital action 469 Anatomical structure of Flustra and Eschara 470 Arrangement of their muscular system 470 digestive apparatus . . 470 Mechanism of opercular lid 470 Nervous system of Flustrce .... 471 Organs of sense 471 Propagation of the Polyzoa 471 by gemmation 471 Gemmiparous reproduction of Pedicellina 471 Laguncula 471 Reproduction by true ova 472 Arrangement of male apparatus 472 Spermatozoa 473 Structure of ova 473 Impregnation and escape of mature ova 473 Ova of Pedicellina 473 Division of vitellus 473 Development of embryo 474 Growth of mature embryo 474 Reproduction by ciliated gemmules 474 Propagation of Halodactylus diaphanus 474 FLUVIATILE POLYZOA 475 Their general history 475 Structure of external envelope 475 Anatomy of Cristatella mucedo 475 Arrangement of the tentacula in different genera ....... 476 Structure of their alimentary canal 476 Circulatory system replaced by ciliary action 477 Respiratory function 477 Reproduction of the Fresh- water Polyzoa 477 Sexes frequently conjoined, sometimes separate 478 Ova of two kinds 478 Two modes of embryogenic development 478 xxvi GENERAL INDEX. CHAPTER XIX. TUNICATA. Page TUNICATA General survey of the class 479 Anatomy of Phallusia nigra . . ....', , : . . 479 External envelope 479 Mantle 480 Respiratory sac 480 Heart ; : '. . = 481 Pericardium 481 Nature of the circulation 481 Mode of procuring food by means of the respiratory cilia . . . 482 Alimentary apparatus 483 Reproductive system '. .; ./. '. .-., , , . 484 Hermaphroditism of Amaroucium Argus . . . . *.V. f . . 484 Structure of testes 484 Structure of ovarium 484 Composition of the ova 484 Development of embryo 484 Structure of larva 484 The compound Ascidians at first solitary 485 Growth of the compound Botrytti 485 Generative system of Cynthia ampulla 486 Ascidia grossularia 487 Nervous system 487 SALP^E Anatomy of 487 Alternate generations of isolated and concatenated Salpa . . . 488 Observations of Chamisso, Krohn, Steenstrup, Milne-Edwards, &c. 488 PYROSOMA 490 POLYCLINUM 490 CHAPTER XX. CONCHIFERA. CONCHIPERA General survey of the class 491 OSTRACEA Anatomy of Pecten Jacob&a 491 Mantle 491 Branchiae 492 Arrangement of the viscera 492 Branchial cilia mode of procuring food 493 Alimentary system of the Oyster 493 Respiratory apparatus 494 Structure of heart and arrangement of the vascular system . . . 495 Locomotive apparatus structure of the " foot " 496 Uses of the "foot" 496 in the Solenida 496 Pholades 497 Cardiaceee . ' . . . 497 Byssus '". ... ,...'. . . 497 Formation of the byssus in the Mussel 497 Varieties in the structure of the byssus . . . '..'..' f -. . . 498 Mechanism for opening and closing the shell . .''/".. -. . . 498 Hinge .?*?': . . 498 Adductor muscle .'..,'.,'.-. . . . 498 Growth and formation of shells .'...- .' . 499 Structure of the glandular margin of the mantle 499 Superficial enlargement of the shells 499 Lines of growth 499 Formation of ridges, spines, &c 500 Deposition of coloured pigment 500 GENEEAL INDEX. Page " Epidermis " 500 Increase of the thickness of the shell 500 Nacre or mother-of-pearl 501 Formation of pearls adherent to shell 501 detached globular pearls 501 Progressive displacement of adductor muscle 501 SlPHONIFEROUS CONCHIFERA 502 Uses of the siphons 502 Family of " Borers," Pholas, Teredo, &c 504 Nervous system 504 in the T>imyaria 501 Monomyaria 504 " Ocular points " around the mantle of Pecten 504 " Auditory sacculus " in Cyclas cornea 505 Generative system of the Conchifera 505 Ovaria 505 Organ of Bo j anus .... 506 Oviposition 507 Transfer of ova to the branchial laminse . ... 508 CHAPTER XXI. BRACHIOPODA (Cuv.) ; PALLIOBRANCHIATA (OWEN). BRACIIIOPODA General characters of the class 509 Shells 509 Muscular system 510 Mantle 510 Mouth 510 Mechanism of the arms 511 in Terebratula psittacea 511 Terebratula Chilensis 511 Use of the arms in procuring food 512 Alimentary system in the Brachiopoda 512 Respiratory structure of mantle 513 Circulatory system 513 Nervous system 516 Organs of sense 516 Reproductive organs 516 CHAPTER XXII. GASTEROPODA. GASTEROPODA General characters of the class 518 Formation of shells 519 Anatomy of the Snail 523 Muscular envelope 523 "Foot" . . 523 Tentacula 523 Dental apparatus of the mouth 524 Tongue 524 Alimentary canal 524 Salivary glands 524 Liver 524 Respiratory cavity 525 Circulation of the blood 525 Slime-glands of the integument 525 " Secerning organ of the viscosity " 525 " Sacculus calcareus " 526 Hermaphroditism of the Snail 527 tviii GENERAL INDEX. Page Sec. Mode of copulation 527 . 1390 Internal generative viscera 527 . 1391 " Sac of the dart ".. . .........!.... 527 . 1391 Male generative system .......... ..f "J - . ; \ 528 . 1392 Testicle ..*;-.... 528 . 1393 Vas deferens ....:. 528 . 1393 Intromittent apparatus 528 . 1393 Female generative system 528 . 1394 Ovary 528 . 1395 Oviduct and "uterus" 528 . 1395 Spermatheca 528 . 1396 Multifid vesicles 529 . 1397 Nervous system 529 . 1393 Senses 529 . 1399 Structure of the cephalic tentacula 529 . 1400 Peculiar arrangement of tentacular nerves 530 . 1401 Distribution of the GASTEROPODA into orders 530 . 1402 PULMOBRANCHIATA 531 . 1403 Branchiae of marine Gasteropoda 531 . 1404 NUDIBRANCHIATA 532 . 1406 INPEROBRANCHIATA 532 ." 1407 TECTIBRANCHIATA 532 . 1408 PECTINIBRANCHIATA 532 . 1409 TUBULIBRANCHIATA 533 . 1411 SCUTIBRANCHIATA , 533 . 1412 CYCLOBRANCHIATA 533 . 1413 HETEROPODA 534 . 1414 Circulating system of the GASTEROPODA 534 . 1415 of Buccinum 534 . 1415 Tritonia 535 . 1416 Doris 535 . 1416 Aplysia 535 . 1417 Structure of the venous system of Aplysia 535 . 1417 No proper absorbent system exists in the Mollusca 536 . 1418 Structure of the heart in Aplysia 536 . 1419 Lacunar system of the Gasteropoda 536 . 1420 Milne-Edwards' s discoveries relative to the circulation in the Mollusca 537 . 1421 Prof. Huxley's observations relative to the circulation in Firola . 537 . 1422 Nature of the circulation in Haliotis 538 . 1423 Patella 540 . 1428 Structure of the heart in Chiton, Haliotis, Fissuretta, &c. . . . 541 . 1430 Respiratory and circulatory organs of Pterotrachea 541 . 1431 " Water-vascular system " of the Gasteropoda 541 . 1432 Digestive system 541 . 1433 Varieties in the structure of the mouth 542 . 1434 Mouth of Pleurobranchus 542 . 1435 Tritonia Hombergii 542 . 1436 PECTINIBRANCHIATA 542 . 1439 Proboscis of Buccinum 543 . 1440 Alimentary apparatus of Buccinum 544 . 1446 Salivary glands 544 . 1447 Alimentary apparatus of HETEROPODA 544 . 1449 Structure of gizzard in Scyllaa 545 . 1450 Aplysia 545 . 1450 Hepatic system of the Gasteropoda 545 . 1452 Usual arrangement of the biliary ducts 545 . 1453 Remarkable exceptions as to the arrangement of the biliary canals 545 . 1454 Termination of the bile-ducts in Scyllcea 545 . 1454 Onchidium 545 . 1454 Doris . i> ;..._.. .. . 546 . 1455 Excrementitious secretions of Aplysia . . . 546 . 1456 Heterogangliate condition of the nervous system in the Gasteropoda 546 . 1457 GENERAL INDEX. xxix Page Sec. Nervous system of Buccinnm 546 . 1458 Pterotrachea 546 . 1459 Snail 546 . 1460 Structure of tentacles in the marine GASTEROPODA 547 . 1461 " Auditory capsules " 548 . 1462 of Tergipes Edwardsii 548 . 1463 Generative system of the GASTEROPODA ; .i ~. . 549 . 1464 Structure of the generative apparatus in the CYCLOBRANCHIATA, SCUTIBRANCHIATA, and TuBULIBRANCHIATA 549 . 1465 The Pectinibranchiata all dioecious 549 . 1466 Structure of male generative organs 549 . 1467 female generative organs 550 . 1468 Hermaphroditism of the HETEROPODA 550 . 1469 Female apparatus 550 . 1469 Male apparatus 550 . 1470 Hermaphroditism of the TECTIBRANCHIATA, INFEROBRANCHIATA, NUDIBRANCHIATA, and PULMONATED GASTEROPODS .... 550 . 1471 Male apparatus of Patella 550 . 1472 Female apparatus of Patella 550 . 1473 Structure of ova and development of embryo of the NUDIBRAN- CHIATA 551 1474 CHAPTER XXIII. PTEEOPODA. PTEROPODA General character of the class 552 1478 Anatomy of Clio borealis 553 1480 External structure of Clio 553 1481 . Cephalic appendages and extraordinary prehensile apparatus . . 554 1482 Hoods 554 1483 Tentacula 554 1484 Mouth .... 555 1485 Structure of jaws 555 1486 Muscular cylinders wherein the jaws are lodged 555 1487 Action of these remarkable dental organs 556 1488 Structure of the tongue 556 1489 Arrangement of the alimentary system 556 1490 Respiratory apparatus of Clio 556 1491 Heart and systemic vessels 556 1493 Nervous system of Clio 556 1494 Senses 557 1495 Structure of eyes 557 1495 Generative apparatus 557 1496 Singularly formed penis 558 1497 Hyalaa 558 1501 Pneumodermon 559 1502 CHAPTER XXIV. CEPHALOPODA. CEPHALOPODA General characters of the class . . . -v ... ... . 559 1503 Octopus vulgaris 559 1505 Structure of "cephalic arms" 559 1506 " Arms " of the Calamary (Loligo) 560 1508 Organization of tentacular suckers 561 1509 inthePoulpe . . ,.,,.,; ,, . 561 1510 Onychoteuthis .... 561 1511 Argonaut or Paper Nautilus v ;.;,!.. 563 1512 Nautilus Pompilius 565 1517 ;x GENERAL INDEX. Page Sec. Structure of its camerated shell 566 . 1517 Supposed object of camerated structure 566 . 1518 External organization of Nautilus 566 . 1519 Structure of the muscular integument of the Cephalopoda . . . 567 . 1520 Lateral fins of Loligopsis and Onyckoteuthis 567 . 1521 First appearance of internal organized skeleton . , ; . -.- . . 567 . 1522 Disposition of cartilaginous skeleton in the CEPHALOPODA . . . 568 . 1523 Cranial cartilage 568 . 1524 Rudimentary endoskeleton 568 . 1524 Exoskeleton 568 . 1525 Gladius of Loligo 568 . 1526 Dorsal plate of the Cuttle-fish (os Sepia) 569 . 1527 Its microscopic structure and uses 569 . 1528 Mode of growth of the " Cuttle-bone " 569 . 1529 Shell of the Argonaut 569 . 1530 Observations of M. Sander Rang on the natural history of the Argonaut 570 . 1531 Experiments of Madame Power 572 . 1534 Formation of the shell of Nautilus 572 . 1535 Mouth of the Cephalopoda 573 . 1536 Oral apparatus of the Cuttle-fish 573 . 1537 Nautilus 574 . 1538 Tongue of the CEPHALOPODA 574 . 1539 Salivary glands 575 . 1540 Alimentary canal 576 . 1541 Crop 576 . 1541 Gizzard 577 . 1542 Rudimentary representative of the pancreas in Nautilus . . . 577 . 1543 the Dibranchiate Cephalopods 577 . 1544 Intestine 577 . 1545 Liver 577 . 1546 Secretion of bile 578 . 1547 Ink-bag 578 . 1549 Respiratory system 578 . 1550 Branchial apparatus 578 . 1551 Mechanism of respiration 578 . 1552 " Great venous cavity," or " pericardium " 580 . 1553 Mechanism of siphon of the shell in Nautilus 580 . 1554 Venous system of the Cephalopoda , 580 . 1555 Spongy appendages to vena; cava3 580 . 1555 Supposed functions of spongoid appendages 581 . 1556 Cribriform parietes of vena cava in Nautilus 582 . 1557 General arrangement of the circulatory system 582 . 1558 Branchial hearts . 582 . 1558 Appendages to branchial hearts 583 . 1559 Branchial hearts do not exist in Nautilus Pompilius 583 . 1560 Systemic circulation , . . . 583 . 1561 Venous sinuses or " lacunae " of Octopus 583 . 1562 Do not exist in Sepia and Loligo 583 . 1562 Nervous system of the Cephalopoda 584 . 1564 Superior development of encephalic ganglia 584 . 1565 Structure of encephalon . . . . 584 . 1566 Nervous system of Nautilus Pompilius 585 . 1567 Supra-cesophageal mass 585 . 1568 Inferior cesophageal collars 585 . 1569 Nerves derived from the anterior collar in Nautilus 585 . 1569 in the DIBRANCHIATA . . 586 . 1570 Represent the fifth pair in vertebrate animals 586 . 1571 Posterior suboesophageal ring comparable to the medulla oblon- gata of Quadrupeds 586 . 1572 Nerves derived from it in Nautilus 586 . 1572 Senses of the Cephalopoda 586 . 1573 GENERAL INDEX. Sense of touch . 586 Structure of the cephalic apparatus in Nautilus Pompilius . . . 587 Lateral processes 587 Antenniform tentacula - * 587 Labial processes and their cirri .- . . . . . . 587 Nerves of the prehensile arms of the DIBRANCHIATA . . ... 589 Sense of taste . . . ; i .".<. 589 Olfactory apparatus 589 in Nautilus 590 Eyes of the CEPHALOPODA 590 Nature of the eye of Nautilus 590 Structure of the pedunculated eyeball of Nautilus 591 Structure of the eye in the DIBKANCHIATA 592 Orbital cavity 592 Anatomy of the eye of Sepia officinalis 592 Muscles of the eyeball 596 Progressive development of the organ of hearing 596 Auditory apparatus of the DIBRANCHIATE CEPIIALOPODS .... 596 Structure of the ear in Sepia officinalis 596 Generative system of the Cephalopoda 598 Ovaria of the female Cuttle-fish 598 Structure of oviduct 598 Ova of the Cephalopoda 599 Generative apparatus of male Cuttle-fish 600 Pouch of Needham 601 Male sexual organs of the Argonaut 601 Hectocotylus Argonauts 601 Penis 602 Embryonic condition of the young 603 Chromatophores 604 Fossil Cephalopoda 604 Ammonites 604 Hamites, Lituites, Orthoceratites, &c 605 Spirula 605 Bekmnites . 605 CHAPTER XXV. VEETEBEATA. VERTEBRATA General view of this division of the animal kingdom . 606 Internal osseous skeleton 606 Progressive ossification of the bony framework 606 Observations on the skeleton in general 607 Composition of typical skeleton 607 Vertebral column 608 Cranial vertebrae 608 Framework of the face 608 Lateral arches 608 Hyoid apparatus 608 Eibs 609 Limbs 609 Elements of the skeleton 610 Composition of a vertebra 610 the thorax 611 Skeleton made up of vertebral segments 612 Elements constituting a complete or typical vertebra 612 Vertebras of osseous fishes 614 air-breathing Vertebrata 614 " Diverging appendages " 614 convertible into limbs 614 txii GENERAL INDEX. Page Simplification of the vertebral column as it approaches the caudal region 615 Composition of the cranial portion of the spinal column . . . 615 Cranium divisible into four vertebrae 615 Composition of occipital vertebra . . . . . u . , . . . 616 parietal vertebra . . ., r . ' . * i ' . "-,. -*:i ;-. 616 frontal vertebra 616 nasal vertebra 616 Composition of typical vertebrate skeleton 616 General view of the nervous system of the Vertebrate . . . . 618 Brain 618 Spinal medulla 618 Sensitive and motor columns of the spinal cord 618 Origin of spinal nerves 618 Their compound structure .... *W:-*' 618 Sympathetic system - v 619 Senses of the Vertebrate . . . *>?.'. 1-4^ - v v- .:..-.-v *v ;- 619 Additions to alimentary system 619 Pancreas .v\. v i; *.,.. 619 l ic portal system 619 . 620 nphatics and lacteal vessels 620 Circulatory and respiratory systems 620 Tegumentery system 620 Instincts and affections 621 CHAPTER XXVI. PISCES FISHES. FISHES General survey of the class ; 621 Transitional condition of the skeleton in Myxine and the Lamprey 622 Amphioxus 622 Rudimentary condition of osseous system 623 Entire absence of cranial cavity 623 Structure of the mouth of Amphioxus 623 Arrangement of branchial chamber 624 Digestive system of Amphioxus 624 Reproductive organs 625 Circulatory system of Amphioxus 625 " Arterial heart " 625 " Bulbs " of the branchial arteries 625 " Aortic arch " performing the functions of a systemic heart . . 626 " Heart of the vena porta " 626 " Heart of the vena cava " ' : ', . ... 626 Mechanism of the circulation in Amphioxus 626 Apparent absence of brain 626 Gradations in the structure of the vertebral column of fishes . . 627 Structure of the bodies of the vertebrae in fishes 628 Vertebral processes 628 Structure of caudal vertebra 628 Ribs 628 Structure of the azygos fins of fishes 630 Interspinous bones 630 Fin-rays 630 Articulation between fin-rays and interspinous bones . . . . . 631 Composition of the skull of osseous fishes 631 Cranium . .. v . 631 composed of twenty-six bones 632 Principal frontal 632 Anterior frontal . . 632 Posterior frontal ". *"Y '...*". . 632 GENERAL INDEX. Page Ethmoid .. 632 Basilar '...-.: 632 Body of the sphenoid 632 Parietal bones 632 Interparietal 632 External occipitals 632 Lateral occipitals 632 Alar bones 632 Mastoid bones 632 Petrous bones 632 Anterior sphenoid 632 Orbital alee of sphenoid 632 Vomer 632 Bones composing the upper jaw 632 Intermaxillary bones 633 Maxillary bones 633 Bones of the face 633 Nasal bones 633 Suborbital 633 Supra-temporal 633 Pterygo-palatine and temporal system of bones 633 Palatine 633 Transverse bone 633 Internal pterygoid 633 Temporal bones . 634 Tympanic bones 634 Jugal bones 634 Articulation of the gill-covers 634 Opercular bones 634 Prceoperculutn 634 Operculum 634 Suboperculum 634 Interoperculum 634 Lower jaw 634 Dental 634 Articular 634 Angular 634 Opercular 634 Os hyoides and branchiostegous rays 635 Styloid bones 635 Branchiostegous membrane 635 Branchial apparatus 636 Structure of branchial arches 636 Pharyngeal bones 636 General disposition of the skeleton in osseous fishes 636 Muscular system 637 PLEURONECTID.E or FLAT-FISHES 638 Osteology of the Flounder (Pleuronectes flesus) . ...... 638 Skeleton of the CHONDROPTERYGII 640 TEGUMENTARY or EXO-SKELETON of fishes 642 Structure and growth of scales 642 Cuticular spines 643 Association of the exo- and endo-skeleton 643 Convertibility of the exo-skeleton into true bone 643 Nature of dorsal fins and interspinous bones 644 opercular bones 644 Swimming-bladder of fishes 645 Structure of the mouth in fishes 646 Modifications in the arrangement of their dental system . . . 646 Dental apparatus of the Myxine . . . . . - - - 647 Lamprey 647 CyprinidtB 647 Eaidce 647 d GENERAL INDEX. Page Sec. Dental apparatus of Sharks 647 . 1770 Attachment of the teeth 648 . 1772 Growth and development of the teeth . . . .... . . . 649 . 1774 Postlabial valve 650 . 1776 Arrangement of alimentary canal 650 . 1777 Pancreatic appendages in osseous fishes 651 . 1780 Structure of the pancreas in Sharks and Rays 651 . 1781 Liver and biliary ducts 651 . 1782 Portal system of veins 651 . 1783 Spleen 651 . 1784 Lymphatic and lacteal system 652 . 1785 Circulation of the blood in fishes 652 . 1786 Structure of the heart 653 . 1787 Branchial circulation . . 653 . 1790 Systemic circulation 654 . 1793 Kidneys 654 . 1797 Their minute structure 654 . 1798 Mucous secretions of the skin 654 . 1799 Lodgment of the brain in fishes 654 . 1800 Composition of the encephalon 655 . 1801 Olfactory lobes 655 . 1802 nerves 656 . 1803 Organs of smell 656 . 1804 Optic lobes and nerves of vision 656 . 1805 Anatomy of the eye in fishes 656 . 1806 "Choroid gland" 658 . 1811 Pedunculated eyes of Sharks and Bays 658 . 1813 Muscles of the eyeball * . . 658 . 1814 Nerves of the ocular muscles 659 . 1815 Absence of lacrymal apparatus 659 . 1816 Distribution of fifth pair of nerves 660 . 1817 Auditory nerves 660 . 1819 Anatomy of the organ of hearing in fishes 660 . 1820 Glosso-pharyngeal nerve 662 . 1831 Nervus vagus 662 . 1831 Spinal recurrent 662 . 1831 Medulla oblongata 662 . 1832 Cerebral hemispheres 662 . 1833 Cerebellum 663 . 1836 Postcerebellic lobes 663 . 1837 Spinal nerves 664 . 1838 Sympathetic system 664 . 1839 Generative system of the osseous fishes 664 . 1840 Structure of the ovary or female roe 664 . 1841 Viviparous fishes ' V V- ! V >.^ . . 665 . 1842 Testicle or milt of the male in osseous fishes 665 . 1843 Progressive development of oviduct . . . . r . ' . -. . . . 665 . 1844 Female generative organs of the Eel and Lamprey . . . . . 666 . 1846 Male organs of the Eel and Lamprey 666 . 1848 Female generative system in Sharks and Kays 666 . 1849 Egg of the Shark i-V^v. . 667 . 1851 Sexual organs of male CHONDROPTERYGII 669 . 1853 CHAPTER XXVII. EEPTILIA. REPTILIA General characters of the class 670 . 1857 AMPHIBIA or BATRACHIAN REPTILES . . . . v .' :.' .... 670 . 1859 Lepidosiren annectans J p'.'*v~v ~v . 670 . 1860 Siren lacertina . . v -v . . 671 . 1863 Proteus anguinus .-? . 671 . 1864 GENERAL INDEX. xxxv Page Sec. Menobranchus ' 672 . 1865 Axolotl 672 . 1865 Caducibranchiate Amphibia 672 . 1866 History of the Water-Newt (Triton cristatus) 672 . 1867 OPHIDIAN KEPTILES 674 . 1874 SAURIAN KEPTILES 675 . 1875 Bimanes and Seps 675 . 1876 CHELONIAN KEPTILES 675 . 1877 Osteology of the Crocodile 675 . 1879 Frog 679 . 1899 Boa constrictor 680 . 1904 Tortoise . . . 682 . 1908 Muscular system of the Reptilia 684 . 1912 Metamorphosis of muscular system in the Tadpole 684 . 1913 Mechanism employed for the prehension of food 685 . 1914 Structure of the tongue of the Frog and Toad 685 . 1915 Tongue of the Chameleon 685 . 1916 Jaws of the Chelonian Reptiles 686 . 1917 Structure of the mouth of non-venomous Serpents 686 . 1918 Poison-teeth of venomous Serpents 687 . 1920 Supplementary poison -fangs 688 . 1921 Rattle of the Rattle-snake (Crotalus) 688 . 1922 Mouth of the Crocodile 688 . 1923 Structure of the teeth of the Crocodile 690 . 1926 Alimentary canal of Keptiles 690 . 1927 Stomach of the Crocodile 690 . 1928 Intestinal canal of the Keptilia 691 . 1930 Salivary apparatus 691 . 1932 in OPHIDIA 691 . 1933 Liver 691 . 1934 Pancreas 691 . 1934 Spleen 691 . 1935 Lymphatic and lacteal systems 692 . 1936 Lymphatic hearts 692 . 1937 Respiratory system of Reptiles 692 . 1939 Structure of the lungs 694 . 1944 Trachea 695 . 1946 Mechanism of respiration 695 . 1948 Structure of the heart in Reptiles 696 . 1951 Course of the blood in Mcnopoma 698 . 1956 Circulation in the PERENNIBRANCHIATE REPTILIA 699 . 1958 Course of the blood in Proteus 699 . 1958 Changes that occur in the distribution of the vascular system during the metamorphosis of the CADUCIBRANCHIATA .... 700 . 1961 Changes in the arrangement of the hyoid apparatus 701 . 1966 Circulation and respiratory system of Lepidosiren 703 . 1972 Nervous system of the Reptilia 703 . 1976 Anatomy of the brain of the Tortoise 704 . 1976 Distribution of the cerebral nerves of Keptiles 704 . 1978 Olfactory nerves 705 . 1979 Sense of smell 705 . 1980 Optic nerves 705 . 1983 Structure of the eye 705 . 1984 Muscles of the eyeball 706 . 1986 Eyelids and lacrymal apparatus 706 . 1987 Nerves of the third, fourth, and sixth pairs 707 . 1992 Fifth pair of nerves 707 . 1993 Facial nerve 707 . 1994 Auditory nerve 707 . 1995 Anatomy of the organ of hearing 707 . 1996 Glosso-pharyngeal and pneumogastric nerves 709 . 2004 Spinal system of nerves 709 . 2006 Sympathetic system 709 . 2007 txvi GENEEAL INDEX. Page Sec. Sense of touch -. 710 . 2008 Tegumentary skeleton 710 . 2009 Kidneys and urinary excretion 711 . 2010 Alantoicsac 711 . 2012 Generative system in the Eeptilia 711 . 2013 Male sexual organs of the Frog 712 . 2014 Sexual apparatus of the female Frog 712 . 2017 Male organs of the Newt 713 . 2018 Female generative system of the Newt, Proteus, and Siren . . . 714 . 2019 Male organs of generation in the Ophidian, Chelonian, and Sau- rian Eeptiles 714 . 2020 Structure of in tromittent apparatus in the Chelonians . . . . 714 . 2021 Saurians 715 . 2024 Female generative system in the higher Eeptilia 715 . 2025 Formation of the egg and development of the embryo .... 715 . 2026 CHAPTER XXVIII. AVES BIEDS. Osteology of birds .... ... 717 2030 2043 2045 2046 2047 2047 2048 2048 2049 2051 2053 2057 2058 2059 2060 2061 2062 2064 2065 2067 2068 2070 2072 2075 2077 2078 2080 2081 2082 2084 2085 2087 2088 2089 2090 2091 2091 2092 2093 2094 2095 ... 721 Alimentary apparatus .... 722 Beak ... 722 Sense of taste ... 722 Structure of the tongue and hvoid apparatus 722 . ... 723 ... 723 . . . 723 . ... 724 . ... 724 . ... 725 Pancreas ... 725 ... 726 . ... 726 . ... 726 ... 728 . ... 728 ... 728 Upper larynx . . . . 729 ... 729 P-. -. - * 730 ... 730 ... 731 Olfactory apparatus ... ... 732 . ... 732 Osseous zone of the sclerotic .... ... 734 ... 734 ... 735 . . . 735 ... 736 Glandulo, HaTdwi ... 737 ... 737 . ... 737 737 ... 737 Structure of the kidney ... 737 . ... 738 ... 739 f*K . . 739 TntrnTnittent annaratus . . 739 GENEKAL INDEX. Page Male intromittent organ of Geese, Ducks, &c. 740 Female generative system 740 Atrophy of ovarium on one side of the body 740 Structure of fertile ovary 740 Oviduct . :-i .... 741 Formation of the egg 741 Organization of the calyces or ovarian ovisacs 741 Structure of the ovulum 741 Phenomena attending conception 742 Addition of the " albumen " and " chalazae " 742 Formation of the " membrana putaminis " and egg-shell . . . 742 Anatomy of the egg prior to incubation 743 Development of embryo in the egg 743 Formation of vascular system 744 Branchial arches 746 Branchial fissures 746 Changes in the branchial apparatus 746 Establishment of pulmonary circulation 748 Formation of " ductus ar teriosi " 748 Separation of pulmonic from systemic circulation 748 Omphalo-mesenteric system of vessels 749 Ductus vitello-intestinalis 750 Corpora Wolfiana 750 Allantoic system 751 Urachus 751 Comparative view of the ova of Vertebrata 751 CHAPTER XXIX. MAMMALIA. MAMMALIA General characters of the class 753 Osteology of the mammiferous skeleton 754 MONOTREMATA 760 Skeleton of Ornithor hy nchus paradoxus 760 MARSUFIALIA 761 Skeleton of Hypsiprymnus 762 PLACENTAL MAMMALIA 762 CETACEA 763 Skeleton of Balcena mysticetus 763 PACHYDERMATA 764 Skeleton of Hippopotamus 765 SOLIDUNGULA Or SOLIPEDS 765 Skeleton of the Horse 766 RUMINANTIA 766 Skeleton of the Stag 767 EDENTATA 768 Skeleton of the Armadillo 768 Sloth 769 RODENTIA 769 Skeletons of EODENTIA 770 Jerboa (Dipus} 770 CARNIVOROUS MAMMALIA ... * 771 AMPHIBIA 772 Skeleton of the Seal 772 Walrus ( Trichechus rosmarus) 772 DlGITIGRADA 773 Skeleton of the Weasel 773 "Mechanism of the claws of the Felidce 773 The Dog 773 PLANTIGRADE CARNIVORA 774 INSECTIVORA 774 txviii GENERAL INDEX. Page Skeleton of the Mole 774 CHEIROPTERA 774 Skeleton of the Bat 774 QUADRUMANA ,..... 775 Skeleton of the Orang-Outang 776 Muscular system of the Mammalia 778 Diaphragm -..-. ." 778 Panniculus carnosus 778 Muscles of the CETACEA 779 anterior extremity 779 posterior extremity 780 Articulations of the skeleton 780 Dental system of the Mammalia 781 Mouth of the Whalebone Whale 781 Dental apparatus of the Porpoise 783 1 Narwal 783 Tusks of the Elephant , 784 Dentes scalprarii of the RODENTIA 784 Teeth of CARNIVORA 785 Molar teeth of HERBIVORA ;<..; $::, . . 786 Grinders of the Elephant 786 Succession of the teeth 787 in the CARNIVORA 787 in the Elephant 787 Movements of the lower jaw 788 Muscular apparatus of the lower jaw 789 Structure of the tongue in Mammalia 789 of the Ant-eater and Echidna .... 789 Sense of taste 790 Armature of the tongue in the Felidcs and Porcupine .... 790 Salivary apparatus 790 Parotid glands 790 Submaxillary and sublingual glands 790 Buccal, molar, and labial glands 790 Salivary apparatus in the Seals (Phocidce} 790 Ant-eater and Echidna 790 deficient in the CETACEA . . 791 Hyoid apparatus 791 Fauces and epiglottis 792 Pharynx 793 (Esophagus 793 Stomach 793 Complex stomachs 793 Compound stomachs of RUMINANTIA 794 1 CETACEA 795 Small intestines 796 Caecum and appendix vermiformis 796 Liver 796 Pancreas 797 Spleen 797 Portal system of veins 797 Peritoneum 797 Lacteal system 797 Lymphatic system ' 798 Respiratory apparatus 798 Circulatory apparatus 798 in the CETACEA 799 Seal-tribe . ........ ..... . 800 Eete mirabile 801 Brachial and femoral plexus of the Sloth 801 Tegumentary system of Mammalia 801 Structure of the skin 801 Nature and growth of hair 802 GENERAL INDEX. Page Epidermic appendages, horns, claws, &c 802 Horns of the Deer-tribe . . ........ . . . . . . . 803 Blubber of the CETACEA ....... 803 Cutaneous glands 804 Castor-glands of the Beaver 804 Musk -glands of Moschus tnoschiferw 804 Temporal glands of the Elephant . . . . ',' V^." , :. . . 805 Anal glands of the Skunk and Polecat 805 Nervous system of Mammalia 805 Olfactory lobes of the brain 806 nerves 806 Structure of the organ of smell 807 Nasal apparatus of the CETACEA 807 " Blowing-organs " of the Porpoise 808 Optic lobes of the brain " tubercula quadrigemina " 809 Optic nerves 810 Anatomy of the eye 810 Muscles of the eyeball 811 Nictitating membrane 811 Lacrymal apparatus 812 Medulla oblongata 812 Distribution of the third, fourth, and sixth pairs of nerves . . . 812 Fifth pair or trigeminal nerves 812 Facial, glosso-pharyngeal, pneumogastric and lingual nerves . . 812 Auditory nerves 812 Structure of the organ of hearing 812 Tympanic chain of ossicles 813 Structure of external ear 814 Organ of hearing in the GET ACE A 814 Cerebral hemispheres 814 Corpus callosum 815 Cerebellum 815 Pons Varolii 816 Spinal cord 816 Sense of touch 816 Sympathetic system of nerves 816 Genito-urinary apparatus 816 Characteristic peculiarities of the male 817 female 817 Arrangement of the renal system in the Mammalia 817 Minute structure of the kidney 817 Lobulated kidney 818 Urinary bladder and ureters 818 Gradations in the organization of the genito-urinary apparatus . 818 Genito -urinary organs of Ornithorhynchus paradoxus and Echidna 819 In the male 819 female 820 Dissection of foetal Ornithorhynchus 822 Genito-urinary system of the MARSUPIALIA 823 Anatomy of the male organs of the Kangaroo 824 female organs of the Kangaroo 825 Organization of the Marsupial ovum 826 Peculiarities in the anatomy of the Marsupial foetus 828 Genito-urinary organs of the PLACENTALIA 830 Organization of male generative system 830 Testes, their structure 831 Situation of testes 831 " Succenturiate glands " 832 " Vesicula seminales" 832 "Prostate" 832 " Cowper's glands " 833 Canal of the urethra 833 Structure of penis 834 GENERAL INDEX. Page Sec. Female generative system in the Placental Mammalia .... 835 . 2478 Progressive development of uterus 835 . 2479 Oviducts, or " Fallopian tubes" 836 . 2481 Ovaria 838 . 2484 Development of the ovum 838 . 2485 Placenta 839 . 2487 Combination of allantoic and placental circulations 839 . 2488 Changes that occur in the foetal circulation 840 . 2491 Mammary glands 841 . 2494 PHYSIOLOGICAL INDEX. NERVOUS SYSTEM. Anatomy of the nervous system of Page Sec. Medusiform Acalephae Ill . 287 Lesueuria 122 . 319 Distoma 142 . 371 Planariaj 151 . 400 Ascaris lumbricoides 158 . 422 Asterias 188 . 496 Echinus 200 . 524 Holothuria 209 . 541 Sipunculus 213 . 553 HOMOGANGLIATA 215 . 562 Leech 224 . 584 ANNELIDA 277 . 712 MYRIAPODA 287 . 741 INSECTA 327 . 855 Changes that take place in the condition of the nervous system during the metamorphoses of Insects 360 . 931 ARACHNIDA 367 . 947 CRUSTACEA ; 386 . 983 KOTIPERA 440 . 1439 CIRRHOPODA 451 . 1166 HETEROGANGLIATA 462 . 1191 POLYZOA 471 . 1225 TUNICATA 487 . 1279 CONCHIFERA 504 . 1326 BRACHIOPODA 516 . 1356 GASTEROPODA 529 . 1398 PTEROPODA 556 . 1494 CEPHALOPODA 584 . 1564 Nautilus pompilius 585 . 1567 VERTEBRATA 618 . 1653 FISHES 655 . 1801 EEPTILIA 703 . 1976 BIRDS 730 . 2072 MAMMALIA 805 . 2356 ORGANS OF THE SENSES. SENSE OP TOUCH in Actinia 74 . 165 in Holothuria 209 . 541 in the Leech 225 . 586 in Terebella 268 . 683 in Myriapoda 281 . 719 in Insects 330 . 862 in Crustacea 408 . 1042 Tentacula of Gasteropoda 523 . 1379 xlii PHYSIOLOGICAL INDEX. SENSE OP TOUCH (continued] Page Sec. in Cephalopoda 586 . 1574 in Fishes . -. - 659 . 1817 in Reptiles 710 . 2008 in Birds 731 . 2075 in Mammals 816 . 2404 SENSE or SMELL in Insects <**i . 804 in Crustacea 409 . 1047 in Nautilus pompilius 590 . 1581 in Fishes 656 . 1804 in Reptiles 705 . 1980 inBirds 732 . 2077 in Mammals 807 . 2362 SENSE OP VISION. " Pedunculated eyes " of Acalepha; . . 112 . 288 Oculiform organ of Lesueuria 122 . 319 _ Asterias 189 . 497 Eyes of Leech 225 .' 587 Structure of the eye of Torrea vitrea 278 . 713 Simple oceUi of Insects 332 . 870 Compound eyes of Insects 332 . 871 Eyes of Arachnida 380 . 975 Crustacea 409 . 1043 _ simple 409 . 1044 agglomerated 409 . 1045 _ compound 409 . 1046 Cirrhopoda 453 . 1171 the Scallop (Pecteri) 504 . 1329 Snail 529 . 1400 other Gasteropoda 547 . 1461 Pteropoda (Clio) 557 . 1495 Nautilus pompilius 590 . 1583 Cuttle-fish (Sepia) 592 . 1587 Fishes . 656 . 1806 Reptiles 705 . 1984 Birds 732 . 2078 Mammalia 810 . 2374 SENSE OF HEARING. ' Auditory capsules " of Hydrozoa 112 . 289 Medusae 112 . 289 _ Annelidans (Arenicola) 279 . 714 Hearing of Insects 331 . 865 __ Crustacea 409 . 1047 " Acoustic sacculus " of Cirrhopoda 454 . 1173 . Conchifera 505 . 1330 Gasteropoda 548 . 1462 Ear of the Cuttle-fish 596 . 1598 Organ of hearing in Fishes 660 . 1820 Reptiles 707 . 1996 Birds 737 . 2090 --Mammalia 812 LOCOMOTIVE SYSTEM. Locomotive apparatus of the Infusoria 34 . 66 Acalephae 119 . 309 Locomotive organs of Asterias 166 . 439 _ Echinus 192 . 506 Sipunculus * ."., . 210 . 546 PHYSIOLOGICAL INDEX. Locomotive organs of the Leech 218 the Earthworm 229 Dorsibranchiate Annelida 246 Julus 281 Scolopendra 287 Legs of Insects 298 Wings of Insects 304 Muscular system of Insects 308 Spinning-organs of Arachnida . 381 Locomotive apparatus of Crustacea 387 Cirrhopoda 451 Mantle of the Ascidia 480 " Foot " of Conchifera 496 Byssus of Mussel and Pinna 497 Apparatus for opening and closing shells of Conchifera .... 498 Arms of Brachiopoda 511 Muscular system of Gasteropoda 523 Locomotive organs of Pteropoda 553 Tentacula and suckers of Cuttle-fish 560 " Sails " (so called) of Argonaut 563 Arms and float of Nautilus 565 Fins and muscular system of Fishes 637 Muscular system of Eeptiles 684 Locomotion of Birds 721 Muscular system of Mammals 778 SKELETONS OF INVEETEBEATA. Solid framework of Sponges 21 Po'yparies of Fungiae and Meandrinse 54 cortical Anthozoa 56 Alcyonidse 60 Madreporidae 67 Corattium and Iris hippuris 68 Gorgonia 69 Pennatulidse 70 Tubiporidaa 70 Tubularia 85 Sertularidae 92 Internal plates of Porpita and Velella 126 Skeletons of Echinodermata : Crinoidae 162 Asteridee 172 Echinidse 190 Myriapoda 280 Insecta 295 Arachnida 363 Crustacea 386 Cirrhopoda 449 Conchifera 491 Brachiopoda 509 Gasteropoda 519 Cephalopoda 568 First appearance of internal osseous skeleton 568 SKELETONS OF VEETEBEATA. a. Cuticular or Exo-skeleton. Dermo-skeleton of Fishes 642 . 1748 Hair and other epidermic structures 802 . 2347 Horns of Deer . . 803 . 2349 xliv PHYSIOLOGICAL INDEX. b. Osseous or Endo-skeleton. Page Sec. General view of the skeleton of Vertebrate 606 . 1615 Osteology of Fishes 627 . 1685 Reptiles . . . ... .;,,.; 675 . 1879 Birds 717 . 2030 Mammalia , .-. .. . . 754 . 2147 ALIMENTARY SYSTEM. Rhizopoda 7 . 15 Foraminifera 8 . 18 Actinophrys Sol 14 . 29 Noctiluca 16 . 36 Amoeba princeps 19 . 42 Sponges < i^v . 20 . 45 Infusoria 36 . 73 Fungi ,1 , .... 55 . 115 Alcyonidae - v -. ... 56 . 121 Cydonium Mulleri 57 . 122 Alcyonidium elegans 58 . 125 Alcyonium stellatum 64 . 141 Tubipora musica 71 . 155 Actinias 74 . 164 Hydrae 81 . 181 Tubularia 85 . 194 Campanularia 99 . 243 Acalephse : Ehizostoma 106 . 273 Cassiopea Borbonica 107 . 275 Cyanea aurita 108 . 277 Cuvieria carisochroma 117 . 306 JEquorea violacea 117 . 306 Beroe 120 . 313 Cesium Ven&ris 123 . 323 Helminthozoa : Ccenurus cerebralis 129 . 339 Cystic&rcus 130 . 340 Tainia 131 . 342 Distoma hepaticum 141 . 370 Echinorhynchus gigas 149 . 392 Planariae 152 . 404 N&mertes 154 . 411 Ascaris lumbricoides 158 . 423 Echinodermata : Asterias 175 . 458 Echinus 198 . 518 Holothuria 204 . 533 Sipunculus 210 . 547 Annelida : Leech 219 . 575 Earthworm 230 . 598 Dorsibranchiate Annelida 248 . 649 Terebella 266 . 681 Myriapoda : Julus terrestris 281 . 723 Scolopendra 288 . 743 Insecta ,. '. ~ . . 316 . 831 Arachnida : Trachearia 365 . 940 Pulmonaria '..... 370 . 958 Crustacea -. 1 -. . 394 . 1006 Nicothoe Astaci 428 . 1106 Pycnogonidae 430 . 1112 PHYSIOLOGICAL INDEX. xlv Epizoa : Page Sec. Achtheres percarum . . . 424 . 1091 Lamproglena . . ....-,. .i . . . . 426 . 1100 Eotifera : Stephanoceros 437 . 1131 Brachionus 437 . 1132 Melicerta ringens 438 . 1134 Cirrhopoda : Pentelasmis 452 . 1169 Polyzoa : BowerbanJcia densa 465 . 1200 Flustrse and Escharse 470 . 1222 Freshwater Bryozoa 476 . 1249 Tunicata : Phattusia nigra 483 . 1267 Salpge 487 . 1280 Conchifera : Oyster 493 . 1293 Siphoniferous Conchifera 502 . 1322 Brachiopoda 512 . 1346 Gasteropoda : Snail 524 . 1382 Structure of the mouth in the Gasteropoda 542 . 1435 Digestive canal of Buccinum 544 . 1446 Heteropoda 544 . 1449 Scyttaa 545 . 1450 Aptysia 545 . 1450 Pteropoda : Clio borealis 556 . 1490 Cephalopoda 576 . 1541 Vertebrata : Fishes :-- Amphioxus 624 . 1672 Dental apparatus of Fishes 646 . 1765 Alimentary canal 650 . 1777 Eeptiles : Prehension of food 685 . 1914 Alimentary canal 690 . 1927 Birds 722 . 2045 Mammalia : Dental apparatus 781 . 2250 Alimentary canal 793 . 2302 EESPIEATOEY AND CIECULATOEY SYSTEMS. Asterias 180 . 474 Echinus 199 . 522 Holothuria 204 . 536 Sipunculus 211 . 550 Leech 220 . 578 Earthworm 231 . 600 Dorsibranchiata 249 . 652 Amphinome 249 . 652 Eunice sanguined 250 . 654 Arenicola 255 . 665 Aphroditaceffi 262 . 674 Myriapoda 288 . 744 Insecta , 320 . 843 Arachnida : Trachearia 366 . 942 Pulmonaria , . 372 . 961 xlvi PHYSIOLOGICAL INDEX. Crustacea : Page Sec. Decapoda .-.-...-.... v^ . . 395 . 1012 Lacunar system in the Crustacea . .,-"..-*.- . -, > 400 . 1023 Entomostraca 415 . 1067 SguiUa 416 . 1068 Pycnogonidse .,-.. .., ... . 430 . 1111 Phlebenterism 430 . 1112 Cirrhopoda 452 . 1170 Tunicata 480 . 1262 Conchifera 494 . 1294 Brachiopoda 513 . 1348 Snail 525 . 1386 Gasteropoda 534 . 1415 Cephalopoda 578 . 1&50 Fishes 652 . 1786 Eeptiles 692 . 1939 Birds 726 . 2061 Mammalia , . 798 . 2328 EEPEODUCTIVE SYSTEM. Sponges 25 Reproduction of Infusoria 44 Fungice , 55 Alcyonidse 57 Alcyonidium * 62 Tubipora 73 Actiniee 76 Hydra viridis 82 Coryne and Tubularia Sertularian Zoophytes 95 " Medusiparous generation " 99 Alternate generation in the Medusiform Acalephse 113 Generative organs of Cyanea aurita 114 Mguorea 118 Multiplication of the naked-eyed Medusse by gemmation . . . . 115 Generative organs of Lesueuria 122 system of the Cestoid Entozoa 132 Trematode Entozoa 142 "Alternate generation" of the Distoma 143 Reproductive system of Acanthocephalous Entozoa 149 Planariae 153 the Coalelmintha 159 Polyzoa 471 Generative system of Asterias 182 Echinus 200 Holothuria 208 Sipunculus 213 Leech 225 Earthworm 233 Nais 241 . Dorsibranchiata 259 Myriapoda 282 Insects 335 Arachnida 380 Crustacea 41 1 Cirrhopoda 454 Tunicata 486 Conchifera 505 Brachiopoda 516 Snail 527 Gasteropoda .-..., 549 58 92 116 122 133 162 169 183 195 226 247 294 295 307 301 321 344 372 376 394 407 426 1227 479 525 539 552 588 608 632 670 726 875 976 1053 1174 1277 1331 1358 1390 1464 PHYSIOLOGICAL INDEX. xlvii Page Generative system of Pteropoda 557 Cephalopoda 598 Fishes 664 Reptiles 711 Birds 739 Mammalia 818 1496 1600 1840 2013 2094 2416 LYMPHATIC SYSTEM. In Fishes 652 . 1785 In Reptiles 692 . 1936 InBi'ds 726 . 2060 In Mammalia 9o SEW URINARY SYSTEM. In Fishes . 654 . 1797 In Reptiles 711 . 2010 In Birds 737 . 2091 In Mammalia 817 . 2411 EMBRYOGENESIS. Development of Tubipora 73 162 Actinia 77 172 Embryogeny of Tubularidse 86 195 I Sertularidse 97 231 Cyanea aurita 116 304 the Cestoid Entozoa 135 350 Trematode Entozoa 143 376 Epizoa 426 1098 Rotifera 446 1155 Marine Polyzoa 474 1240 Fluviatile Polyzoa 478 1258 Astoria* 183 481 Ophiurus 185 487 Echinus 200 526 Earthworm 238 625 Terebella 275 706 Julus terrestris 284 732 Phenomena of Insect-metamorphosis 347 907 Embryogeny of the Crustacea 413 1063 Cirrhopoda 459 1185 Tunicata 484 1272 Cephalopoda 603 1610 Fishes 665 1842 Reptiles 715 2026 Birds 743 2113 Monotremata 822 2429 Marsupialia 828 2449 Placentalia . . 838 2485 A GENERAL OUTLINE OF THE ANIMAL KINGDOM. CHAPTER I. ON CLASSIFICATION. (1.) FROM the earliest periods to the present time, the great desi- deratum in Zoology has been the establishment of some fundamental 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 attempted 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 admitted but two great sections, in one or other of which all known beings were included, the highest comprehending creatures possessed of blood (i. e. red blood), corre- sponding to the YERTEBRATA of modern authors ; the lowest embracing animals which in his view were exsangueous, or provided with a colour- less fluid instead of blood, and corresponding to the INVERTEBRATA of more recent zoologists f. (3.) Linnaeus, like Aristotle, selected the circulatory system as the * Historia Animalium. t IIpos $e TOVTOIS TO. f^ev evctifjia rvy^avei ovra, olov dv9p} Kai TUV OaXarriiov (T7/7rta Kai KcipajSos Kai TrdvO' oaa TrXeiovs Trodas e%ei rerrapwv. Hepi Zwa 'laroptov, Ke0. A. B 2 ON CLASSIFICATION. 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 cold red 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 invertebrate animals, to which latter he gives the general name of YEEMES, Worms. We shall not in this place comment upon the want of anatomical knowledge conspicuous in the above definitions, or the insufficient data afforded by them for the purposes of Zoology. The apparatus of cir- culation, being a system of secondary importance in the animal economy, was soon found to be too variable in its arrangement 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 obtained 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 importance of each, and sketch- ing out with a master hand the outlines of that arrangement since adopted as the most natural and satisfactory f. The result of the labours of this illustrious man cannot but be of deep 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 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 Linnccus, * Sy sterna Naturce. Vindobonse, 1767. 13th 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. parti. 1835. SYSTEMS OF HUNTER AND CUYIER. 3 founded upon the same basis. He arranges in this manner all animals in five groups. I. Creatures whose hearts are divided into four cavities Mammalia and Birds. II. Those having a heart consisting of three cavities Reptiles and Amphibia*. III. Animals possessing a heart with two cavities Fishes and most Mollusca. IY. Animals whose heart consists of a single cavity Articulated Animals. Y. 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 that is satis- factory; but the researches of this profound physiologist upon the employment of the nervous system for the purpose of zoological distri- bution did much to inaugurate a more natural method of classification, afterwards carried out with important results. (5.) The appearance of the " Animal Kingdom distributed in ac- cordance with its organization," of Cuvier, formed a new and important era in Zoology. In this we find all creatures arranged in four great divisions, YEETEBEATA, MOLLUSCA, AETICULATA, and RADIATA. 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 exclusively drawn from their internal organization, the arrangement of the nervous system being essentially the primary character of distinction, and have been found to be strictly natural ; whilst the last division, characterized by the appellation of EADIATA, in the formation of which the structure of the nervous system has been allowed to give place in importance to other characters of secondary weight, obviously embraces creatures of very dissimilar and incongruous formation. (6.) The YEETEBEATA are distinguished by the possession of an in- ternal 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 dis- tributed 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. (7.) The MOLLTJSCA have a nervous system constructed upon a very different type, and do not possess any vertebral column or articulated * 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.). B2 4 ON CLASSIFICATION. 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. (8.) The class of ARTICULATED ANIMALS is likewise well characterized 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. (9.) The fourth division of Cuvier, namely that of ZOOPHYTES or RADIATED ABTIMALS, is confessedly made up of the most heterogeneous materials, comprising creatures 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. (10.) 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 establishment 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 more perfectly organized of the group ; these, therefore, have been classed by themselves, and designated by Professor 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. Macleay into a group by themselves, to which he has given the denomination of AcRiTAf. (11.) There can be no doubt that the nervous matter must be re- garded 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 immediate relation with the de- velopment 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 non-existence of certain senses, the capability of locomotion, and the means of procuring food must be in strict correspondence with the powers centered in the nervous masses of the body, or in that arrangement of nervous particles which represents or replaces them. , a thread ; vevpov, a nerve. t a priv. ; Kplvw, to discern. EHIZOPODA. 5 (12.) 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 predicate 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 our- selves 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. It is, however, by no means our intention in the present work to enter the arena of discussion relative to the juxtaposition or precedence in the scale of animal existence which ought to be assigned to any particular group as denned by modern zoologists. The classification employed in the following pages is simply adopted as being the most convenient for our present object ; we shall therefore arrange our studies in accordance with the following sequence. CHAPTER II. PKOTOZOA*. (13.) EHIZOPODA f. On carefully examining the contents of a marine aquarium, or a glass vessel casually filled with sea- water, the micro- scopic observer will not unfrequently perceive, adherent to the sides, numerous beings which, from their minute size and transparency, have until a very recent period entirely escaped notice, although the part they are destined to play in the economy of this world is by no means unimportant. The body of one of these remarkable organisms (fig. 1, 1) consists of a minute spherical vesicle, something resembling a globular flask provided with a short narrow neck, filled with a fawn-coloured glutinous substance containing numerous minute granules, and appa- rently unprovided with any external appendages. On placing one of these creatures, however, in a glass of sea-water (its native element), it is found in the course of a few hours to have attached itself to the sides of the ves- sel by means of numerous long ramified filaments of hyaline transparency, which soon begin to reveal their office to be that of a locomotive appa- * Trpoiros, first; wov, animal. t Vide D'Orbigny, Diet. Univers. d'Hist. Nat. 1845, v., and Foraminiferes Fossiles, 1846 : Ehrenberg, Berlin Trans. 1838 and 1839, or Weaver's abstract, Ann. Nat. Hist. 1841, vii. pp. 296, 374: Dujardin, Ann. Sc. Nat. 1835, iv. & v. : Clark, Ann. Nat. Hist. 1849, iii. 380; 1850, v. 161 : Williamson, Trans. Micr. Soc. ii. and Micr. Journal, i. : Carpenter, Trans. Geol. Soc. 1849, and Phil. Trans. 1856 : Carter, Ann. Nat. Hist. 1852, x. PROTOZOA. ratus, by whose aid the animal can transport itself from place to place, but with such extreme slowness that its movements are hardly per- ceptible. The locomotive filaments thus displayed are perceptible by the naked eye, their length being, when fully extended, four or five times 1. Gromia oviformis, with the rhiziform tentacles displayed. 2. Filaments fused together into a kind of network. the transverse diameter of the body ; still they exhibit in their interior no appearance of organization, but resemble so many threads of molten glass. When protruded, each of these filaments, at first simple and of equable diameter through its entire length, soon begins to elongate itself in a very mysterious manner, moving in different directions, as though seeking some basis of support. As the elongation of the filament con- tinues, apparently owing to a constant influx of new material into its substance, it is seen to give off here and there secondary branches, which, in turn dividing dichotomously, give to the whole structure the root-like appearance represented in the figure. The retraction of these singular organs is accomplished by a sort of inversion of the above pro- cess, each filament shrinking as it were into itself until it totally disap- pears. The most remarkable circumstance, however, observable in the economy of these creatures is, that the protruded filaments are able to coalesce and, as it were, to become fused together, forming a gela- tinous network that spreads out in all directions (fig. 1, 2). FORAMINIFERA. (14.) When a Rhizopod, having all its filaments thus extended, wishes to advance in any given direction, those threads which are directed in front become elongated, and those placed behind, on the contrary, are drawn forward, while the intermediate move so as to accommodate themselves to each change of position, thus evidently exhibiting a consentaneity of action. (15.) Internally these creatures present no traces of any special nutri- tive apparatus ; neither are there any organs appropriated to reproduction, their multiplication being apparently accomplished either by gemmation or by simple division, as any portion of the mass separated from the rest seems capable of living and of forming a new centre of organization. (16.) The delicate body of Gromia, above described, is unprovided with anything like a shell; but there are many races presenting an organization in every way analogous (such as the Hiliolce, the Cristel- larice, the Vorticialce, and others), that possess the power of secreting shells of very exquisite texture, many of which form extremely beautiful objects when examined under the microscope. (17.) The FOKAMINIFERA constitute a very curious and remarkable group, important from the immense num- bers in which they occur in a fossil state, and interesting from the peculiarities of structure whereby they are distinguished. The shells of these singular organisms (fig. 2) are divided into distinct com- partments*, so as almost exactly to re- semble in their form the camerated shells of the Nautili, Ammonites, and other highly-organized mollusca. Examined, however, in a living state, they are found to belong to animals of a very different type, as remarkable for the simplicity of their organization as for their elegance and delicacy. The shell, as represented in the figure, consists of numerous chambers divided from each other by calcareous septa, and perfo- rated by innumerable minute orifices, or foramina, from which circumstance is derived the characteristic name. In- ternally these chambers are entirely filled with a homogeneous, transparent, glairy Fig. 2. 1. Nonionina, exhibiting pseudcpodia protruded through the foramina in substance, which, being Soft and diffluent, 2 . The Rafter" th'eloWon of the like the arms of Gromia described above, she11 in weak acid - * Whence the group has also received the name of POLYTHALAMIA, i. e. many- chambered. 8 PEOTOZOA. can be protruded through the numerous apertures in the periphery of the shell in the shape of long contractile filaments (pseudopodia). (18.) On removing the delicate calcareous shell by the assistance of a weak acid, the body of the animal denuded of its covering (fig. 2, 2) is found to be entirely soft : that portion which is lodged in the first compartment of the shell is colourless and of a crystalline transparency ; but in each of the succeeding segments there may be detected a granu- lar mass of a brownish colour, and not unfrequently the minute silicious shells of Naviculce, Barillarice, and other forms of Infusorial organisms, the remains of which may be traced nearly as far as the umbilicus of the spiral. (19.) The pseudopodia of the Foraminifera probably entangle and lay hold of the minute bodies which serve as food, consisting of Diato- maceee, Desmidiese, the smaller forms of Conferva^ &c., and draw these by their contraction into the substance of the animal, within which they may be seen through the transparent shell. It is not by any means constantly that their indigestible residua are cast forth again, for they sometimes accumulate in such numbers as even to choke up a con- siderable part of the cavity. The living gelatinous substance is occa- sionally seen to extend itself around the exterior of the shell ; and pseudopodia may then be put forth from this extension as well as from the ordinary outlets. (20.) The Eoraminifera are evidently composite fabrics evolved by a process of continuous gemmation, each gemma remaining in connexion with the body by which it was put forth ; and according to the plan on which this gemmation takes place will be the configuration of the shell. Thus, if a bud should be put forth from one of these creatures in the direction of the axis of its body, and a second shell should be formed around this bud in continuity with the first, and this process should be successionally repeated, a straight rod-like shell will be pro- duced, having many chambers communicating with each other by the openings that originally constituted their mouths, the mouth of the last- formed chamber being the only aperture through which the gelatinous body, thus composed of a number of segments connected by pedicles or stolons of the same material, can receive a supply of food. The succes- sive segments may be all of the same size, or nearly so, in which case the entire rod will approach the cylindrical form, or resemble a line of beads ; but it often happens that each segment is somewhat larger than the preceding, so that the composite shell has a conical form, the apex of the cone being the original segment, and its base the one last pro- duced. If each of the successively formed segments instead of being developed exactly in the axis of its predecessor should be directed a little to one side, it is obvious that a curved instead of a straight rod will be the result ; and this curve may be increased until it become a spiral. The character of this spiral will depend in a great degree upon SHELLS OF FORAMINIFEKA. 9 the enlargement or non-enlargement of the successively formed cham- bers ; for sometimes it opens out very rapidly, every whorl being con- siderably broader than that which it surrounds, in consequence of the great excess of the size of each segment over that of its predecessor (fig. 2, 1) ; but more commonly there is little difference between the successive segments after the spiral has made two or three turns. In many genera the new segments are added in concentric rings, each surrounding its predecessors, so as to form flattened disks varying in size from that of a pin's head to that of a sixpence. When such disks are subjected to microscopic examination, they are seen to be composed of concentric circles of cells (fig. 3) which communicate with each Fig. 3. Structure of the calcareous disk of an Orbitolite : a, the central cell; 5, circumambient cell; c, c, concentric zones; d, d, annular passages of the outermost zone. The same parts are shown by a vertical section passing in a radial direction at e, e ; and a,tf,f, following the course of one of the zones. other by means of lateral passages, and which in the living state are each of them filled by the sarcode whereof the living portion of the animal consists. In this case there can be no reasonable doubt that the radial extensions of the outermost zone issue forth as pseudopodia from the marginal pores, and that they search for and draw in alimentary materials in the same manner as do those of other Foraminifera. (21.) Where the growth of the disk takes place with normal regu- larity, it is probable that a complete circular zone is added at once. When the sarcode body has increased beyond the capacity of its enveloping disk, it may be presumed that its pseudopodial extensions proceeding from the marginal pores coalesce, so as to form a complete annulus of sarcode round the margin of the outermost zone ; and probably it is by a deposit of calcareous matter in the surface portion of this annulus that the new zone of shelly substance is formed, which constitutes the walls of the 10 PKOTOZOA. cells and passages occupied by the soft sarcode body. Thus we find this simple type of organization giving origin to fabrics of by no means mi- croscopic dimensions, in which, however, there is no other differentia- tion of parts than that concerned in the formation of the shell, every segment and every stolon (with the exception of the two forming the nucleus or centre) being, so far as can be ascertained, a precise repeti- tion of every other, and the segments of the nucleus differing from the rest in nothing but their form. The equality of the endowments of each segment is shown by the fact (of which accident has frequently furnished proof), that a small portion of a disk entirely separated from the re- mainder will not only continue to live, but will so increase as to form a new disk, the loss of the nucleus not appearing to be of the slightest consequence from the time that active life is established in the outer zones. (22.) In Faujasina and the Nummulites, apparently the most highly organized of the Foraminifera, the shell is of more complicated struc- ture, being permeated by a system of radiating interseptal canals com- municating with the exterior. (23.) For the following observations relative to the reproduction of the Foraminifera we are indebted to Professor Max Schulze *. Remarking that an individual of the genus Triloculina (D'Orbigny) had become stationary for several days, and enveloped, as is not usual, in a thin layer of brownish slime, Professor Max Schulze f paid par- ticular attention to it. At the end of a few days after it had become quiescent, minute spherical, sharply denned granules were detached from the brownish slimy envelope, and in the course of a few hours the animal was surrounded with about forty of these corpuscles, which gradually became more and more separated from it. Microscopic exa- mination proved that these were young Foraminifera. When viewed by transmitted light, they presented a pale-yellowish-brown calcareous shell, consisting of a central globular portion partly surrounded by a closely- applied tubular part, and having no septum in the interior. In a short time the young animals protruded their contractile processes from the anterior opening of the shell and crawled about upon the glass. The parts of the body contained within the shell could be examined with great accuracy under the highest magnifying powers, and were seen to consist of a transparent, very finely granular, colourless material, of which the protruded filaments were an immediate continuation. From the circumstances under which the young Foraminifers made their appearance, they must necessarily quit their parent in a tolerably per- fect condition. (24.) When the calcareous shell of the parent animal was carefully * Muller's Archiv, 185G, p. 163. t Professor Max Schulze, Ueber den Organismus der Polythalamien (Foramini- feren). Leipzig, 1854. GEOLOGICAL IMPORTANCE OF THE FORAMINIFERA. 11 broken up, it was found to contain only trifling remains of a finely granular organic substance, which, after careful and continued observa- tion, exhibited no trace of motion such as is often, under other circum- stances, presented in separated particles of the animal substance, nor could he perceive any vestige of a young one in process of development. The almost complete absence of any organic contents in the shell of an individual which from eight to fourteen days previously was creeping about, renders it probable that the whole (or, at any rate, part) of its body had been transformed into young ones. (25.) It is astounding to reflect upon the multitudes of these micro- scopic shells which crowd almost every sea-beach. In some cases at least one-half of the bulk of the sand seems to consist of these elegant organisms. Plancus (Ariminensis, De Conchis minus notis) counted 6000 in a single ounce of sand from the shores of the Adriatic ; and D'Orbigny estimated that an ounce of sand procured from the Antilles contained not fewer than 3,800,000 ! The numbers therefore contained in a square yard are beyond all human calculation ; and yet, what is that when compared with the extent of sea- coast in all parts of the globe ? Probably therefore no race of animals is more numerically im- portant than that we are now considering. Their remains constitute a great proportion of the so-called sand-banks which often so materially interfere with navigation by obstructing the entrance to bays and straits, or, as is the case with the port of Alexandria, blocking up harbours. They enter largely into the formation of coral islands, and not unfrequently compose extensive geological deposits. One solitary species of the genus Fusulina has, in Kussia, given rise to enormous beds of calcareous shells. The cretaceous formations both of France and England contain them in immense quantities. The tertiary strata abound with numerous species; and the very stones of which the largest of the Pyramids of Egypt is built are principally composed of shells (Nummulites) belonging to this important group. The tertiary basins of the Gironde, of Austria and of Italy, and more especially the " calcaires grossiers " of the vast Parisian basin, are in some parts so filled with them, that 58,000 have been counted in a cubic inch, or about 3,000,000,000 in a cubic yard figures that may well spare us further calculation. In fact, it might be stated without exaggeration, that the city of Paris, as well as many of the towns and villages in the surrounding departments, are almost entirely built of stones that seem to be mere agglomerations of these microscopic shells. (26.) The substance of the shell in the Foraminifera varies to a certain extent in accordance with its mode of growth. When the calcareous investment is made up of segments involving each other, it is of a dense texture, resembling porcelain. When the segments alternate without a spire, or when the spiral revolution is oblique, the shell is porous, and perforated, more especially in the last-formed com- 12 PKOTOZOA. partments, by numerous apertures through which the pseudopodia are protruded. When the segments are all placed in a straight line, or when they are rolled in a spiral upon the same plane, or when they are alternate and the shell is inequilateral, their texture is very generally transparent like glass *. (27.) Nearly related to the Foraminifera, or at least apparently be- longing to the same general type of structure, are the Polycystina, an extensive group of very interesting microscopic bodies possessing great beauty and variety of form and structure. These are minute silicious shells, which appear, from the recent observations of Professor MUller 't, to contain in the living state an olive-brown sarcode extending itself into pseudopodial prolongations that pass through the large apertures by which the shells are perforated. The sarcode does not seem always to fill the shell, but only its upper part or vault, and to be very regularly divided into four lobes. It is a peculiar feature in these Polycystina that their shells are often prolonged into spines or other projections, which Fig. 4. are sometimes arranged in such a manner as to give them a very singular aspect (fig. 4). It seems probable that these creatures are at the present time as widely diffused as the Foraminifera, although, from their extreme minuteness, they have not been so often recog- nized. They were first disco- vered by Professor Ehrenberg at Cuxhaven, on the North Sea; they were afterwards found by him in collections made in the Antarctic seas; and have been recently de- scribed by Professor Bailey as presenting themselves (with Foraminifera and Diatomaceaa) in the de- 1. Podocystis Schomburgkii. 2. Bhopalocanium ornatum. * The following table indicates the proportions of genera and species of Forami- niferous shells which have been met with in various geological epochs : Carboniferous series 1 genus, 1 species; Jurassic series 4 genera, 20 Cretaceous series 30 250 Tertiary series 55 460 Now existing 68 900 so that 1631 species of these minute organisms have already been distinguished by classical naturalists. t Vide Miiller, liber die Thalassicollen, Polycystineen und Acanthometren des Mittelmeeres, in Monatsbericht der Konigl. Akademie der Wissenschaften zu Berlin, 13 Nov. 1856. ACTINOPHRYS. 13 posits brought up by the sounding-lead from the bottom of the Atlantic Ocean at depths of from 1000 to 2000 fathoms. They appear to have been much more abundant, however, during the later geological periods, inasmuch as in a single deposit in Barbadoes, Professor Ehrenberg detected no fewer than 282 forms which he considers to be specifically distinct. (28.) ACTINOPHEYS. Among the most interesting contributions to our knowledge of these simple organisms are those of the distinguished German micrologist Kolliker, whose researches relative to the organiza- tion of Actinophrys /Sol* are calculated to clear up many doubtful points connected with the physiological history of numerous allied genera of kindred structure. The Actinophrys (fig. 5, l) is a minute animalcule Actinophrys Sol. 1. a, the cortex; b, nucleus of the animalcule; c, homogeneous basal sub- stance ; d, vacuoles; e, tentacular filaments. 2. The same, less magnified, at the moment of feed- ing: a-e, as above; f, an infusorium which has just entered the substance of the body, while the surrounding filaments enclose it on all sides. 3. Another specimen : a-e, as in fig. 1 ; f, a Vau- cheria-spore wholly imbedded in the cortical substance, the opening through which it entered entirely closed, although its situation is indicated by a slight depression ; g, another spore already entering the nuclear substance ; h, an infusorium lying in a special cavity ; i , a spore in the nuclear substance ; k, half-digested morsels ; I, a swallowed Lynceus ; m, excrementitious matter beginning its exit from the cortical substance. The other figures represent the sarcode highly magnified. nearly spherical in its shape, having the surface of its body covered with closely-set delicate filaments, the length of which frequently exceeds the diameter of the creature. It does not present a trace of mouth, stomach, intestine, or anus, but consists entirely of a perfectly homo- geneous substance of soft and delicate consistence. Examined under a very high power, the whole animalcule appears to be made up of a most regular and delicate tissue of round or polygonal cells, although on closer inspection such is found not to be really the true structure. When the animal is torn or crushed, it becomes evident that it is entirely composed of a semifluid material (sarcode) enclosing vacuoles; for it will be found that the supposed cells may at pleasure, under pressure, * Siebold and Kolliker's Zeitschr. vol. i. p. 198. 14 PEOTOZOA. be made either to coalesce into larger or be divided into smaller cavities presenting in all respects the character of the normal ones. The filamentary appendages to the periphery of this animalcule are essentially tentacular organs, composed of the same substance as the rest of the body, from which they differ only in never having vacuoles in their interior ; and if granules are to be detected in their structure, these are very few in number. (29.) The mode in which the Actinophrys is nourished is a subject of the highest interest. Although, as has been stated, the creature has neither mouth nor stomach, yet it lives upon solid nutriment and re- jects such parts as are indigestible. The Actinophrys, indeed, feeds upon Infusoria of all kinds, on the lower Algse, such as the Diatomaceae, and even on minute Crustacea, as the young of Lynceus, Cyclops, &c., which it accomplishes in the following manner : When, in its progress through the water, it comes in contact with fitting food, the object, whether of animal or vegetable nature, as soon as touched by one of the tentacular filaments, usually becomes adherent thereunto. The fila- ment, with prey thus attached, then slowly shortens itself, dragging the object seized towards its devourer, all the surrounding filaments bending their points together, so that the captive becomes at last en- closed on every side (fig. 5, 2, /). (30.) That the tentacles, however, possess some other power than that of mere prehension appears evident, because nearly every creature of moderate and even immoderate size which strikes against them is at once for a time rendered immoveable. When a Rotifer, in crossing the field with velocity, strikes against any object, the rotatory organ is often seen at once to suspend its operation, more particularly should its cilia strike the cilia of another animalcule; and frequently no notice whatever is taken of the shock ; not so, however, with the victim of the Actinophrys Sol, on the instant of contact with whose tentacles it appears to be paralysed. In some cases the prisoner is held for some seconds on the exact spot where it struck, and then, without any visible means, becomes attracted towards the body of the Actinophrys, gliding slowly down the tentacle until it is jammed between its base and a neighbouring one. In other instances, instead of the prisoner being arrested on or near the extremity of the tentacle on which it strikes, it is shot down to the base with extreme rapidity, to occupy the same position as in the former case. Sometimes it would seem as if the appetite of the Actinophrys were sated, or that the captive was not approved of, for after remain- ing stunned for a few seconds, ciliary action is feebly recommenced, not sufficient to produce motion, but as if a return to vitality had been effected ; shortly it is seen to glide off the tentacle (as if that organ possessed the power both of appropriation and rejection), and fre- quently, with but little sign of recovered life, it floats out of the field. (31 .) But should the Actinophrys be hungry, the spot upon which STRUCTURE OF ACTINOPHKYS. 15 the captured animalcule is lying slowly retracts, and forms at first a shallow depression, in which the prey, apparently adherent to the sur- face and following it in its retraction, is finally lodged. The depres- sion, by the continued retraction of the substance, becomes deeper ; the imprisoned animalcule, which up to this time had projected from the surface of the Actinophrys, entirely disappears within it, and at the same time the tentacula, which had remained with their extremities applied to each other, again erect themselves and stretch out as before the capture. Finally, the depression assumes a flask-like shape by the drawing-in of its margin, the edges of which coalesce, and thus a cavity closed on all sides is formed wherein the prey is lodged. In this situation it remains a longer or shorter time, gradually, however, approaching the central portion of the body. In the mean time the periphery of the Actinophrys regains in all respects its pristine condi- tion. The engulfed morsel is gradually digested and dissolved, as is readily seen by its change of appearance from time to time. If entirely soluble, as, for instance, an Infusorium, the space in which it is con- tained contracts as the dissolution of its contents goes on, and finally disappears altogether : should there, however, be an indigestible residue, a passage for its exit is formed, and it is expelled by renewed contrac- tions of the homogeneous substance, and in the same direction, or nearly so, as that which the morsel followed in its introduction. The passage and the opening through which the expulsion was effected dis- appear again without leaving a trace. (32.) The number as well as the size of the morsels taken at one time, in the manner above described, by an Actinophrys, is very various. Sometimes there may be two, four, or six swallowed simultaneously ; occasionally more than ten or twelve. (33.) A remarkable contractile vesicle is always visible in these ani- malcules, which Mr. Weston* regards as a valvular orifice. It is best distinguished when about the edge of the seeming dislc, and is never still night nor day, being slowly but without cessation protruded, occu- pying from ten to seventy or eighty seconds in its development, and then, like the bursting of a vesicle, rapidly and totally subsiding : for an instant it totally disappears, but only to be as gradually and as certainly reproduced. Should that side of the creature where the valve is placed be turned from the observer, the effects of the contraction are distinctly seen, although the valve itself is not ; for at the instant of its bursting and closure, some half a dozen or more of the tentacles situated on or about it, which have been gradually thrust from their normal position by the act of its protrusion, now approach each other with a jerk-like motion caused by the sudden bringing together of their bases. (34.) The valve seems to be formed of a double layer of the external hyaloid membrane, the edges of which appear to adhere to each other * Quarterly Journal of Microscopical Science, vol. ir. p. 116. PEOTOZOA. tenaciously, notwithstanding the growing distension from within, until the force becomes so great that the lips, as they may be called, sud- denly separate, apparently to give vent to some gaseous product. (35.) With regard to the reproduction of the species, Mr. Weston assures us that self-division is one mode. First may be noticed a deep depression above and below, not far from the centre of the body ; this, as it increases, throws the tentacles across each other, as a necessary consequence of the depressions in the surface and the position into which the outer membrane (in which the tentacles are inserted) is drawn. As the division proceeds, the two animals steadily, but rather quickly, increase the distance between them, until there is only a long membranous neck, apparently composed first of four, then three, then two irregular lines of cells, which ultimately diminish into a single cord composed of three simple cells, elongated like the links of a chain, and becoming more attenuated till the division is complete. All this latter part of the process is rather rapidly performed ; that is, from the first formation of the rows of cells to the time of the final separation occupies only about a quarter of an hour. (36.) The NOCTILUC^: may perhaps be classed with the Rhizopods. The general shape of the Noctiluca* (fig. 6, l) is that of a minute Fig. 6. melon deeply indented at one extre- mity, at which point is attached a sort of proboscidiform appendage or tail: externally its body seems to consist of two membranes of extreme delicacy, which are apparently filled with a clear fluid. At the bottom of the indenta- tion above-mentioned, close to the insertion of the appendix, there is always found a little mass of mud, or other detritus, which it is very diffi- cult to wash away ; but when this is accomplished, it becomes perceptible that this foreign matter is adherent to a semitransparent granular sub- stance, which here protrudes through a little aperture generally called the mouth, and which is continuous with a quantity of the same material situ- ated in the interior of the little globe. No digestive apparatus is visible; but numerous vacuoles of variable size (fig. 6, 2) are discovered in the gra- nular substance within, together with a central nucleus. No rhizopodic * M. de Quatrefages, Observations sur les Noctiluques, Ann. des Sc. Nat. 1850. 1. Noctiluca, magnified, and viewed as a transparent object. 2. A portion of its in- ternal tissue magnified 150 diameters, show- ing vacucles and rhizopodic expansions. NOCTILUCA. 17 expansions are in these organisms protruded externally; but in the interior the microscope reveals a delicate network of irregular filaments that ramify in every direction, and exactly resemble in their character the anastomosing threads of Gromia, represented in a preceding figure. (37.) In the vacuoles it is easy to perceive particles of green matter or other foreign substances, which seem to afford nourishment to the animal; so that these cavities doubtless perform the functions of tempo- rary stomachs, although they are constantly changing their shape and situation in a most remarkable manner. (38.) No reproductive apparatus is apparent in these little beings ; yet sometimes individuals are to be seen with double bodies, and, from the observations of Colonel Baddeley as recorded by Mr. Brightwell*, there seems to be little doubt that the Noctiluca multiplies by spon- taneous fissure. Colonel Baddeley 's researches lead him to infer that this process "begins by the gradual formation of a second nucleus, which after its commencement rapidly arrives at the size and appear- ance of the other. A second globular substance also (termed by some previous writers on the subject the mouth) is formed, in addition and near to the nucleus ; and a constriction, small at first, but gradually increasing, takes place, until the perfect Noctiluca3 are developed, united at last by a thin band which is speedily ruptured, the whole process of division not occupying more than twelve hours." The observations of Dr. Buschf, and more particularly those of Mr. GosseJ, clearly de- monstrate that the Noctiluca3 increase also by germs or gemmaa. (39.) It will surprise some of our readers to find that the N"octiluca3, small as they are, feed upon Diatomaceae, and that in these microphagists we have the means of supplying our cabinets with specimens of some of the rarer forms. Colonel Baddeley observes that he finds that, when newly captured, each Noctiluca has several Diatoms in its interior, lying in the various chambers or pouches distributed through the body of the animal. These Diatoms all disappear in a few days, leaving nothing visible but the vacuoles or alimentary sacs filled with granular particles. A very careful examination shows an orifice near the tail or peduncle, the opening of which may be detected by carefully pressing it ; and from this is protruded, by continuous gentle pressure, a very thin hyaline sac, filling gradually with fluid and small granular particles, till it attains about one-third of the size of the animal, when it bursts and disappears. (40.) The name Noctiluca is indicative of the extraordinary faculty that these little creatures possess of emitting a brilliant phospho- rescent light. When a vase filled with sea- water containing them is placed in a dark chamber, the slightest agitation is sufficient to excite this * Quarterly Journal of Microscopical Science, vol. v. p. 186. t Microscopical Journal, vol. iii. p. 203. | Eambles on the Devonshire Coast, p. 257. C 18 PEOTOZOA. phenomenon, and the smallest undulations upon the surface are indicated by luminous circles. On examining one of the animalcules attentively with the microscope, it is further observable that the light given out is not universally diffused through the substance of its body, but is con- fined to minute luminous points scattered here and there, which make their appearance in rapid succession and as suddenly vanish ; so that evidently there is no special organ to which the luminous appearance can be referred, as in the case of the glow-worm and other phospho- rescent creatures. In size these stars of ocean are almost microscopic, the largest of them not much exceeding the dimensions of a pin's head : but the amazing numbers in which they crowd the billows amply makes up for their minuteness ; at certain seasons, indeed, it may be literally said that every drop of every wave contains one or more individuals belonging to the brilliant host. On taking up at random a flask of sea- water, and allowing the little creatures to accumulate, as they always do when at rest, at the top, it will be seen that their bodies will form a stratum equalling in thickness from one-seventh to one-third part of the entire contents of the vessel. After such demonstration as this, it is easy to comprehend how the entire sea, rendered luminous by the pre- sence of Noctilucae, seems to burn with phosphorescent fire. When the surface is tranquil in some well- sheltered bay, these living gems form a kind of cream of liquid light ; or if a wave disperses their myriads and at the same time calls forth by agitation all their brightness, it is easy to imagine how a flame is thus evoked that spreads for miles, giving at a distance the appearance of a uniform sheet of light, but, when closely examined, resolvable, like the nebulae in the firmament, into constituent stars. (41.) AMOZB^:. Yery nearly al- lied to theBhizopods in their organi- zation are certain minute gelatinous beings found in our fresh waters, which have long been puzzles to the microscopist, and a fruitful theme of discussion among naturalists (fig. 7). Fig. 7. These creatures appear under a good glass as minute patches of transpa- rent jelly, having, under ordinary circumstances, a diameter of from s-j-jj-th to -g-J-g-th of an inch, but re- markable for perpetually changing their form at one time shrinking into the appearance of a little globe, then expanding into a flattened ra- diating disc, and again shooting out processes of their substance in various directions, so as to assume all sorts of shapes with the greatest Amoeba, showing the vacuoles in its sarcodic substance, a, 6, c, d, some of the various shapes which it assumes. AMOEBA. 19 facility, deserving well the names of Proteus and Amoeba bestowed upon them by zoologists. (42.) When a drop of water containing these creatures is placed be- neath the microscope, the observer at first discovers nothing but a few semitransparent or cloudy-looking motionless globules, from which flows, as from a drop of oil, a kind of semifluid stream, which, fixing itself upon the object-glass, seems to draw the entire mass slowly after it. In this way numerous expansions make their appearance from different parts of the body, which after spreading to a little distance again shrink and become completely blended with the central portion. The young Amceba3 are perfectly diaphanous, and with difficulty perceptible except under favourable circumstances ; but as they become older they lose this transparency in consequence of the accumulation of foreign particles in their interior, which seem to have been introduced from without by the simple pressure of the semifluid body of the animalcule as by the con- tractions and expansions of its various portions it crawls or rather flows over them. Fig. 8. Amoeba princeps (Ehr.), magnified 300 diameters. The figures 1, 2, 3 exhibit the same animal and its protean changes of form. There are, however, other corpuscles or granules, besides those above indicated, found in the interior of these creatures. Some, extremely minute and irregular in their shape, appear to differ only in density from the surrounding glutinous substance, and these are considered by Dujardin to be rather products of secretion than ova. They move about, appearing to flow in accordance with the variable expansions of the creature which contains them. But besides these, in large specimens of Amoebae, other granules are met with (fig. 8, 1, 2, 3), which on account of the uniformity of their appearance might with more plausibility be regarded as reproductive germs ; but their nature is very doubtful. The Amcebse are capable of multiplication by spontaneous fissure, or by de- taching a lobe from their bodies, which will continue to live upon its c2 20 PKOTOZOA. own account just as well as when forming a part of the original animal- cule. On cutting one of these creatures in two, or tearing it to pieces, there is no escape of fluid perceptible ; but each portion contracts itself, and commences a separate individuality. (43.) In one species of Amoeba (A. verrucosa, Ehr.) Mr. Carter* has witnessed ovular development, the Amoeba perishing as the ovules are perfected, and ending in becoming a mere ovisac. When first formed, the ovules, which are spherical, consist of a hyaline capsule enclosing a sphere of glairy, refractive fluid; but as they begin to increase, this glairy matter becomes transformed into a granuliferous mucus which is spread over the inner surface of the capsule ; and finally the granules present motion whether of themselves or by the aid of the mucus in which they are imbedded is uncertain. The history of their further development has not yet been made out; but Mr. Carter thinks that the next stage of their growth consists in the whole ovule becoming poly- morphic. (44.) SPONGES. However dissimilar apparently, both in form and structure, from the simple organisms described above, it is in their im- mediate vicinity that we must place the extensive group of SPONGES, which has until recently held a very dubious position upon the confines of the animal and vegetable kingdoms. (45.) The common sponge of com- merce is, as every one knows, made up of horny, elastic fibres of great deli- cacy, united with each other in every possible direction, so as to form in- numerable canals which traverse its substance (fig. 9, c). To this struc- ture the sponge owes its useful pro- perties, the resiliency of the fibres composing it making them, after compression, return to their former state, leaving the interstitial canals open, to suck up surrounding fluids by capillary attraction. (46.) The dried sponge is, how- ever, only the skeleton of the fabric. 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 that constitutes the living part of the sponge, secreting, as it extends itself, the horny fibres which are imbedded in it. Fig. 9. Spicula and horny skeleton of various Sponges. * Ann. & Mag. Nat. Hist. 2nd ser. vol. xx. p. 37. SPONGES. 21 (47.) Many species, although exhibiting the same porous structure, have none of the elasticity of the officinal sponge, a circumstance to be attributed to the difference observable in the composition of their skeleton or ramified framework. In such, the living investment forms within its substance not only tenacious bands of animal matter, but great quantities of crystallized spicula, sometimes of a calcareous, at others of a silicious nature, 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 blowpipe or by the caustic acids or alkalies, the spicula remain, and may readily be examined under a microscope : they are then seen to have determinate forms, generally in relation 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. (48.) Crystallized spicula of this description form a feature in the structure of the sponge which is common to that of many vegetables, resembling the formations called rapTiides by botanical writers. Some of the principal forms they exhibit* are depicted in fig. 9, a, b, d, e, /, g, which likewise will give the reader a general idea of the appear- ance 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, and g likewise represent detached spicula of different shapes highly magnified. The most 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 ex- amined with ordinary powers. (49.) 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 f. The entire surface is seen to be perforated by innumerable pores and apertures, some ex- ceedingly minute, opening on every part of its periphery; others of larger dimensions, placed at intervals, and generally 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. The annexed diagram, fig. 10, A, will give the reader an idea of the most usual direction of the streams. The entering fluid rushes in at the countless pores distributed over the gene- ral surface 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, such as neces- * Savigny (Jules Cesar), Zoologie d'Egypte : gr. fol. Paris, 1809. t Dr. Grant, in the New Edinburgh Philosophical Journal, 1827. 22 PROTOZOA. Fig. 10. sarily abound in the water of the ocean, are thus introduced into the sponge on all sides, and are probably employed as nutriment, whilst the superfluous or effete matter is continually cast out with the issuing streams as they rush through the fecal orifices. The growth of the sponge is thus provided for ; the living gelatinous por- tion continually accumulates and, as it spreads in every di- rection, secretes and deposits, in the form peculiar to its spe- cies, the fibrous material and earthy spicula constituting the skeleton. (50.) It is by no means easy to explain the cause of the per- petual flow of water through the substance of the sponge in currents so powerful and so constant. In the various spe- cies of Grantia, however, Mr. Bowerbank and Dr. Dobie have succeeded in detecting the pre- sence of cilia. These sponges \ A, a common Sponge : the arrows indicate the di- rection of the entrant and issuing currents. B, a ciliated gemmule magnified. have a very simple structure, each being a sort of bag, whose walls are so thin that no system of canals is required, the water absorbed by the outer surface passing directly towards the inner, and being expelled from the mouth of the bag. The cilia may be plainly seen with a -J-th inch objective on the cells of the gelatinous substance scraped from the interior of the bag, or they may be observed in situ by making very thin sections of the substance of the sponge. Mr. Bowerbank*, however, has satisfactorily proved that some sponges possess a power of opening and closing the oscula at pleasure. He found that in a specimen of Spongilla fluviatilis about half an inch in dia- meter, which had attached itself to a watch-glass, there was at the summit of a large oval inflation a single osculum, which opened or closed according to the necessities of the animal, and from which, when in full action, a constant stream of water was poured forth. The inha- lation of the water by the porous system presented some remarkable peculiarities : when in a state of repose, the dermal membrane appeared to be completely imperforate ; but when about to commence vigorous inhalant action, a slight perforation appeared here and there over its surface, the orifices gradually increased in size until the full diameter of the pores was attained, and their margins then became thickened and * Quarterly Journal of Microscopical Science, vol. vi. p. 78. STEUCTURE OF SPONGE. 23 rounded. On a little indigo being diffused in the water, it was seen to be absorbed with avidity ; and the inhalant action continued for a con- siderable period, the interior of the sponge becoming strongly coloured with indigo. After a time the rapid inhalant process ceased, either abruptly or gradually, a very languid action only remained, and nearly the whole of the pores were closed. When this operation was about to take place, the rounded margin of the orifice lost its form and became thin and sharp, while the circumference gradually melted inwards until the ori- fice entirely closed, and not a vestige of the organ previously existing remained : the operation of closing occupied rather less than a minute. When once closed, these orifices do not appear to be reopened, but fresh pores are produced. The colouring matter absorbed during the period of active inhalation was apparent in the sponge from twelve to eighteen hours ; and during this period the stream from the osculum was ex- tremely languid. The structure and habits of the freshwater Sponges are entirely in accordance with those of marine species. (51.) Prom 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. (52.) In Cliona celata, one of the freshwater sponges, M. Dujardin* discovered, mixed up amongst the pin-like spicula that constitute their skeleton, irregularly shaped globules, composed of a contractile gluti- nous substance, which, when examined under the microscope, were seen continually to change their shape, presenting a constantly varying outline, exactly similar to what is witnessed in the protean animalcule, Amceba diffluens, above described ; and to this contractile substance, whereof the living substance of the sponge seems principally to consist, he proposed to give provisionally the name ofHalisarca (sponge-flesh f). Subsequent observations have shown that these proteiform bodies are not only thus changeable in their shape, but are able to exercise a distinct power of locomotion by agitating long flagelliform filaments that are appended to their bodies (fig. 11, l) ; in fact, the whole of the living portion of the sponge seems to be made up of agglomerations of these amorphous bodies, spread over the spicula or skeleton of the sponge, all individually capable of changing their form by emitting processes in different directions, so as to increase their means of contact with the surrounding fluid, from which they evidently derive materials for assimilation. (53.) These sponge-cells, as they are called by Mr. Carter , are about * Ann. des Sc. Nat. torn. x. 1838. t Lit. " sea-flesh." } On the Freshwater Sponges of Bombay. Ann. Nat. Hist. 1849. 24 PROTOZOA. the 10 1 0u th part of an inch in diameter. If one of them be selected for observation, it will be found to be composed of its proper cell-wall, a number of granules fixed to its upper and inner surface, and towards its centre generally one or more hyaline vesicles. Fig. 11. 1. Kemarkable forms assumed by Proteans developed from the matter of the seed-like bodies of Spongilla, magnified. 2. General form of large spiculum. 3. Spiniferous spiculum. (After Mr. H. J. Carter). (54.) The granules are round or ovoid, translucent, and of an emerald- or yellowish-green colour, varying in diameter below the la> 1 oou th part of an inch, which is the average linear measurement of the largest. In some cells they are so minute and colourless as to appear only under the form of a nebular mass, while in others they are of the largest kind, and few in number. (55.) The hyaline vesicles, on the other hand, are transparent, colour- less, and globular, and, although variable in point of size like the green granules, are seldom recognized before they much exceed the latter in diameter. They generally possess the remarkable property of slowly dilating and suddenly contracting themselves, and present, in their interior, molecules of extreme minuteness in rapid commotion. (56.) The sponge-cell when in situ is constantly changing its form, both partially and wholly ; its granules also are ever varying their posi- tion, in unison with, or independently of, the movements of the cell ; and its pellucid vesicle or vesicles may be seen dilating or contracting them- selves, or remaining passively distended, exhibiting in their interior the molecules above mentioned in rapid commotion. When first separated from the common mass, an isolated cell for a short time assumes a glo- bular form, and afterwards, in addition to its becoming polymorphic, evinces a power of locomotion ; it emits expansions of its cell-wall in the form of obtuse or globular projections or digital and tentacular prolongations. If in progression it meets with another cell, both com- bine ; and if more are in the immediate neighbourhood, they all unite together into one globular mass. Should a spiculum chance to be placed in the path of a cell thus in motion, it will ascend it and traverse it REPRODUCTION OF SPONGES. Fig. 12. from end to end, subsequently quitting it ; or else, assuming its globular form, it will embrace some part of the spiculum and remain stationarily attached to it. The changes in shape and position of the sponge-cell are for the most part effected so imperceptibly that they may be likened to those which take place in a cloud. Its granules, however, are more active ; but there appears to be no motion in any part of the cell (ex- cepting among the molecules within the hyaline vesicle) which in any way approaches to that characteristic of the presence of cilia. (57.) The intercellular substance that forms the bond of union be- tween the sponge-cells is of a mucilaginous appearance. When observed in the delicate pellicle which, with its imbedded cells, it forms over the surface and throughout the canals of the sponge, it is transparent ; but when a portion of this pellicle is cut off from its attachments, it collapses and becomes semi-opake. In this state the detached portion immediately evinces a tendency to assume a spheroidal form; but whether the intercellular substance participates in this act or remains passive while the contraction is wholly performed by the habit of the cells im- bedded in it to approximate themselves, is not evident. (58.) The freshwater sponges are reproduced from seed-like bodies found in the substance of the oldest or first- formed portions of the sponge, never in its periphery. They are round or ovoid according to the species, and each pre- sents a single infundibular depression on its surface which communicates with the interior. At the earliest pe- riod of development at which it is re- cognizable, it is composed of a number of cells united together by an intercel- lular substance similar to that de- scribed above. In this state, appa- rently without any capsule, and about half the size of the fully developed seed-like body, it seems to lie free in a cavity formed by a condensation of the common structure of the sponge immediately surrounding it. The cells of which it is now composed ap- pear to differ from those of the fully-developed sponge-cell only in being smaller, in the colourless state of their contained granules, and in the absence of hyaline vesicles. The seed-like body gradually passes from the state just mentioned into a more circumscribed form, then becomes surrounded by a soft, white, compressible capsule, which finally thickens, turns yellow, and developes upon its exterior a firm crust of silicious spicula, presenting in some species a hexagonally tessellated appearance (fig. 12, c). The spicula are arranged perpendicularly to the surface of Magnified section of a seed-like body of Spongilla Meyeni, showing, f, spicular crust; g, coriaceous capsule; k, internal cells ; i, infundibular opening, c, portion of coriaceous membrane, magnified, to show the hexagonal divisions with trans- parent centres ; d, small spiculum, mag- nified; e, one of its toothed disks with central aperture, magnified. (After Mr. H. J. Carter.) 26 PROTOZOA. the capsule, and the interval between them is filled up with a white, sili- cious, amorphous matter which keeps them in position. Each spiculum extends a little beyond this matter, and supports on its free end a toothed disk, similar to a corresponding one on its fixed end, which rests on the capsule, so that the external surface of the seed-like body is studded with little stellate plates (fig. 12, d, e). In other species, where there appears to be no such regular arrangement of these spicula, a number of smooth spiniferous points is presented. (59.) If a seed-like body which has arrived at maturity be placed in water, a white substance will after a few days be observed to have issued from its interior, through the infundibular depression on its sur- face (fig. 12, i), and to have glued it to the glass : if this be examined with the microscope, its circumference will be found to consist of a semitransparent material, the edge of which is notched or extended into digital or tentacular prolongations, precisely similar to those of the protean cell, which in progression or in polymorphism throws out parts of its substance in the same way. In the semitransparent substance may be observed hyaline vesicles of different sizes, contracting and dilating, as well as green granules, so grouped together as almost to enable the practised eye to distinguish in situ the passing forms of the cells to which they belong. Subsequently to the development of this fleshy substance comes that of the horny skeleton and its spicula (fig. 11, 2), which are at first membranous, and at an early period of their development pliable; they afterwards become firm and brittle. They are hollow, and the form of their cavity corresponds with their own shape ; sometimes, moreover, they contain a green matter like the endochrome of the cells of Confervse*. (60.) In the genus Teihya, Mr. Huxley has described a true sexual generation to exist, a portion of the spongy mass being found to consist of a granular substance in which ova and stellate crystalline bodies are imbedded. " The ova are of various sizes ; they have a very distinct vitellary membrane, which contains an opake, coarsely granular yelk. A clear circular space, about l ^ () O th of an inch in diameter, marking the position of the germinal vesicle, is seen in each ovum, and within this a vesicular germinal spot fi J^ O th of an inch in diameter is some- times visible. The stellate bodies are about y^nnfth f an i ncn ^ n ^ a ~ meter. The granular uniting substance is composed entirely of small circular cells about . 3 ./ O th of an inch in diameter, and of spermatozoa in every stage of development from those cells. The cell throws out a long filament which becomes the tail of the spermatozoon, and, becoming * Besides the seed-like bodies above described, other reproductive bodies are met with in Spongitta : 1. some which, from their resemblance to the motile spores or zoospores of many plants, have also been termed swarming-spores (Schwarm- sporen); and 2. others which, from their resemblance to the spermatic filaments elsewhere met with, are denominated zoosperms. SPERMATOZOA. longer and more pointed, itself forms the head. It is remarkable that the ova are in no way separated from the spermatozoa, but lie imbedded in the spermatic mass like eggs packed in sand*." (61.) The multiplication of marine sponges, however, is effected in * The Spermatozoa, until recently considered as animalcules, generally present themselves under the form of long slender filaments or corpuscles, the shape of which varies to a remarkable extent, and nevertheless is so constant in individuals belonging Fig. 13. to the same species that it is frequently possible to identify by their form the particular creature to which each mo- dification is peculiar. Generally speak- ing, among the higher animals the Spermatozoa are found to consist of an extremely attenuated linear body, either filiform throughout or swollen and enlarged at one end, so as to pre- sent something like the appearance of a microscopic tadpole (fig. 13, 1). They are exceedingly minute, seldom exceed- ing a line in length, but much more generally of far smaller dimensions, so that the highest powers of the micro- scope are requisite for their examina- tion. These microscopic atoms may be regarded not merely as abounding in the seminal secretion of all animals, but in fact as constituting that im- portant agent, the presence of a fluid or liquor seminis appearing, when re- garded in a physiological point of view, merely the vehicle in which the active Spermatozoa are suspended. Until very recently these minute bodies were regarded as individual animated creatures ; and many authors have fancied that several forms of them at least presented a somewhat complicated organization, such as an intestine, gastric sacculi, and even gene- rative organs 1 . More recent researches have, however, satisfactorily proved that they are in all cases composed of a uniform homogeneous substance of a yellowish colour, in which no traces of complexity of structure are discernible. Their move- ments, however, are in most cases exceedingly vivacious ; and were it not for the now well-ascertained fact that many other constituent elementary tissues, both animal and vegetable, exhibit equal activity even long after their separation from the organisms to which they belong, we might still be tempted to assign to them a much higher position in the scale of vitality than that to which they are really entitled. The motions of the Spermatozoids are, however, evidently only com- parable to the automatic movements of cilia, and the relationship which they bear This figure represents the several stages of evo- lution of the Spermatozoa in the common creeper (Certkia familiaris), magnified about a thousand diameters. I, an adult Spermatozoon, taken from the orifice of the vas deferens; a, b, c, seminal granules, which are probably nothing more than altered epithelial cells ; d, e, f, cysts or vesicles en- closing one or more round granular globules ; g, a similar cyst containing, besides the two globules, a finely-granular mass, in which the Spermatozoa may be seen to form ; h, the cyst, still containing finely-granular matter, has assumed an oval form, and the bundle of spermatic animalcules, increased in size, lies bent up within it ; i, a cyst still more developed ; the involucrum, pear-shaped, covers the bundle of animalcules where their spiral extremities lie; k, a cyst arrived at maturity, still covered by the involucrum. (After Wagner.) 1 Vide Leuwenhoeck, vol. iv. pp. 268, 284 : Ehrenberg, Infusionsthierchen, p. 465 : Valentin, Nov. Act. Acad. Leopold, vol. xix. p. 239. 28 PROTOZOA. 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 channels in its interior are found to have their walls studded with yellowish gelatinous granules, developed in the parenchymatous tissue ; these granules arc the germs or gemmules from which a future race will spring ; they seem to be formed indifferently in all parts of the mass, sprouting, as it were, from the albuminous crust that coats the skeleton, without the appear- ance of any organs specially appropriated to their development. As they increase in size, they are found to project more and more into the canals ramifying through the sponge, and to be provided with an appa- ratus of locomotion of a description such as we shall frequently have occasion to mention. The gemmule assumes an ovoid form (fig. 10, B), and a large portion of its surface becomes covered with innumerable vibrating hairs, or cilia, as they are denominated ; these are of incon- ceivable minuteness, yet individually capable of exercising rapid move- ments, whereby they produce currents in the surrounding fluid. As soon therefore as a gemmule is sufficiently mature, it becomes detached from the nidus where it was formed, and being whirled along by the issuing streams, is expelled through the fecal orifices of the parent, and escapes into the water around. Instead, however, of falling to the bottom, as so apparently helpless a particle of jelly might be expected to do, the ceaseless vibration of the cilia upon its surface propels it rapidly along, until, being removed to a considerable distance from its to ciliated epithelium-cells is rendered abundantly manifest by the revelations of the microscope to modern observers 1 . From these researches it would appear that the origin of the Spermatozoa is invariably to be traced to nucleated cells, in the interior of which they are individually developed. These developing-cells, or vesicles, as they ave termed, are found at certain seasons crowding the seminiferous tubes of the testes in immense numbers. Taken from the body after death they are seen to be perfectly transparent and filled with a fluid which on coagulating becomes somewhat granular. Most of these developing-cells (fig. 13, a, b, c} are found freely floating in the minute seminal canals, but frequently they are enclosed in another cell-like envelope, either singly (d) or in numbers of three, four, six, or seven in each; the existence, however, of a more considerable number (e, /) in one common cyst is unusual. Whether single or more numerous, however, it is in the developing- cells that the Spermatozoa are formed by a kind of endogenous growth, at first appearing like dim shadows lying amongst the contained granules, but gradually assuming a sharper outline as the body and, subsequently, the tail are perfected. The entire Spermatozoon at length becomes visible coiled up in the interior of the cell, which, when the development is completed, bursts and discharges its contents. * Professor Grant. i Vide Von Siebold, in Miiller's Archiv, 1836 and 1837: E.Wagner, Fragment e zur Physiologic der Zeugung ; Beitriige zur Geschichte der Zeugung und Entwicke- lung, in den Abhandlung. der Konigl. Bayerisch. Akad., Munich, 1837 : Kolliker, Beitrage zur Kenntniss der Geschlectsverhaltnisse und Samenfliissigkeit wirbellosen Thiere, Berlin, 1841 ; Die Bildung der Samenfaden in Bliischen, Nuremberg, 1840. THALASSICOLLA. 29 original, it attaches itself to a proper object, and, losing the now useless locomotive cilia, it becomes fixed and motionless, and 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 reproduction of animals, the object of which is evident, as the result is admirable. The parent sponge, de- prived of all power of movement, would obviously be incapable of dispersing to a distance the numerous progeny that it furnishes ; they must inevitably have accumulated in the immediate vicinity of their place of birth, without the possibility of their distribution to other lo- calities. 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 pro- geny 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. (62.) Very widely distributed through the ocean, whether in tropical or extra-tropical climates, peculiar gelatinous bodies may be found float- ing upon the surface of the water ; indeed they are occasionally among the most constant of all the various products of the towing-net. The THALASSICOLLA* (for so these simple organisms are designated by Professor Huxley f) is found in transparent, colourless, gelatinous masses of very various form elliptically elongated, hourglass-shaped, contracted in several places, or spherical, varying in size from an inch in length down- wards, showing no evidence of contractility nor any power of locomo- tion, but floating passively on the surface of the water. Of such bodies there appear to be two very distinct kinds. In one, the mass consists of a thick gelatinous crust containing a large cavity. The crust is struc- tureless ; but towards its inner surface minute spherical, spheroidal, or oval bodies are imbedded (fig. 14, 2), each of which appears to be a cell with a thin but dense membrane, and containing a clear fatty-looking nucleus surrounded by granules, the whole substance, in fact, resembling an animal Palmella. Very commonly the central part of each mass, instead of containing a single large cavity, consists of an aggregation of clear, closely appressed spaces resembling vacuoles (fig. 14, 3) ; and fre- quently each cell is surrounded by a zone of peculiar crystals, somewhat like the stellate spicula of a sponge, consisting of short cylinders, from each end of which three or four conical spines radiate, each of these again bearing small lateral processes (fig. 14, 4 & 5). Frequently the * QaXaaea, the sea; jcoXXa, glue. t Vide Ann. and Mag. of Nat. Hist. ser. 2. vol. riii. p. 433. 30 PEOTOZOA. connecting substance in which the cells are imbedded appears to be quite structureless ; but in some specimens delicate, branching, minutely granular fibrils may be seen radiating from each cell into the connecting substance (fig. 14, 4). Fig. 14. Structure of ThalassicoUa (after Professor Huxley). In the second form of ThalassicoUa , the creature consists of a sphe- rical mass of jelly, as large as the middle-sized specimens of the last variety, with an irregular blackish central mass. Enveloping this, and forming a zone about half the diameter of the sphere, are seen nume- rous clear spaces (vacuoles), and among these are scattered numerous yellow cells and a multitude of very dark granules. Delicate, flattened, branching fibrils radiate from the innermost layer, passing between the vacuoles ; and in one specimen Professor Huxley observed these fibrils thickly beset with minute dark molecules which were in active motion, as if circulating along the fibrils, but without any definite direction. INFUSOEIA. 31 CHAPTER III. INFUSOEIA*. (63.) IF we examine a drop of water taken from any pond or ditch in which vegetable or animal substances have been permitted to undergo incipient decay, with a microscope even of very limited power, we must soon perceive that it swarms with innumerable organisms, which are evidently endowed with life and exhibit considerable activity. Prom the circumstance of their extreme minuteness, these microscopic beings were designated by their first discoverers " Animalcules" to which appella- tion, from the fact of their generally making their appearance in vege- table infusions, the term " Infusorial " was very generally superadded by their earlier investigators. Progressive improvements in the struc- ture of the microscope, however, soon made it apparent that the so- called Infusorial or Microscopic Animalcules embraced a vast variety of different forms of living beings possessed of little in common except their invisibility to ordinary observation : the larvae and even the adult states of innumerable Insects, Crustaceans, Worms, and Zoophytes were all comprehended under a term so general ; and even microscopic Algae, Desmidieae, and Diatomaceae, now universally acknowledged to be mem- bers of the vegetable domain of Nature, were included in this chaotic assemblage organisms widely dissimilar from each other both in their shape and structure. It would be foreign to our present purpose to analyse the succes- sive steps whereby something like order has at length been established in a scene of such apparently inextricable confusion, and how pari passu with the improvement of the microscope has been the rapid advancement of knowledge in connexion with these until so late a period unknown existences ; suffice it to say that, in accordance with * Vide Miiller, 1786 : Ehrenberg, Infusionsthierchen, 1837 : Dujardin, Hist. Nat. des Zoophytes : Pineau, Ann. Sc. Nat. 3 e se>. tomes iii. v. ix. : Stein, Wiegm. Archiv, 1849; id. Sieb. and Zol. Z. iii.; id. Die Infusionsthierchen, Leipzig, 1854: Peltier, PInstitut, 1836 : Focke, Isis, 1836, and Physiolog. Studien : Kutorga, Naturgesch. d. Infusionsthierchen : Meyen, Muller's Archiv, 1839 : Pritchard, Infus. Anim. : E. Jones, Ann.Nat. Hist. 1839 : Werneck, Ber. d. Berl. Akad.1841 : Erdl, Mull. Archiv, 1841 : Griffith, Ann. Nat. Hist. 1843, xii. : Siebold, Lehrbuch Vergl. Anat. : Cohn, Sieb. and K61. Z. iii. 260 : Kolliker, Sieb. and Kol. Z. i. 198 : Claparede, Wiegm. Arch. Dec. 1854, translated in Ann. Nat. Hist. 2 ser. xv. 211 : Schneider, ibid. p. 191, translated ibid. xiv. p. 322 : Carter, Notes on the Infusoria of Bombay, Ann. Nat. Hist, for 1856 and 1857 : A memoir by Dr. N. Lieberkuhn in Miiller's Archiv for 1856, translated in Ann. Nat. Hist, for Oct. 1856. 32 INFUSORIA. the more refined characteristics now adopted in zoological classifi- cation, Crustaceans and Insects, as well as the larvae of Annelidans, Zoophytes, and Echinoderms, have been successively withdrawn from the group and located in their appropriate stations, while innumerable zoospores and embryonic plants, together with the DesmidiaceaB and the Diatomacese generally, are by common consent conceded to the botanical series of Creation*. Still, as it would appear, the zoologist is reluctant to dissever forms of life which habit has accustomed the micro- scopical observer to associate with each other ; and even M. Dujardin, one of the latest and most unprejudiced writers upon the history of these * It is by no means an easy task to indicate the boundary -line which separates the animal from the vegetable kingdom. The most important difference, that the vegetable cell^membrane contains no azote, while the animal cell-membrane does, cannot be applied in doubtful cases, the tenuity of the membrane not allowing of the investigation. That animals possess the power of locomotion, but plants not, is incorrect as applied generally, and is still less applicable here, because many uni- cellular Algse exhibit motion, frequently very energetic motion (when swarming), whilst the ova of multicellular Alga are quiescent. The unicellular Algae differ from the Infusoria in this, that their membrane and its appendages are not motile, and that consequently they have a rigid form, whilst the latter in some instances change their figure, and in others are furnished with motile cilia. The presence of starch is, further, not invariably decisive as to the vegetable nature of a cell. The ova of multicellular animals, the figure of which is rigid and unchangeable, may also be recognized as not belonging to the unicellular Algse from their want of colour- ing matter, which is present in the latter. We can scarcely expect Chemistry to decide what is animal and what plant. The non-nitrogenous cellulose, which at first sight appears to be an exclusive attribute of the vegetable, is also found pretty generally in the animal kingdom, as we learn from the researches of G. Schmidt on Cynthia mammillaris, and those of Kolliker and Lowig on a great number of the most various of the lower animals. Just as little does chlorophyll appear to be exclusively characteristic of the vegetable world, since the green granules and vesicles which occur imbedded in the parenchyma of Hydra viridis, of various Turbellaria (Hypostomum mride and Tryphoplana viridata, Schm.), and of Infusoria (Stentor polymorphus, Bursaria vernalis, Loxodes bursaria, when moderately expanded (fig. 34), is a fleshy cylinder, generally found attached by one extremity to a rock, or some other sub- marine support, whilst the opposite end is surmount- ed by numerous tentacula arranged in several rows around the oral aperture (fig. 35). When these ten- tacula are expanded, they give the animal the appear- ance of a flower, the decep- tion being rendered more striking by the beautiful colours they not unfre- quently assume ; and hence, in all countries, these or- ganisms have been looked upon by the vulgar as sea- flowers, and distinguished by names indicative of the fan- cied resemblance. Their ani- mal nature is, however, soon rendered evident by a little attention to their habits. When expanded at the bot- tom of the shallow pools of salt water left by the re- treating tide, they are seen to manifest a degree of sen- sibility, and power of spon- taneous movement, such as we should little anticipate 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 them shrink from the touch ; and if rudely assailed, they completely contract their bodies, so as to take the appearance of a hard coriaceous mass, scarcely distinguishable from the substance to which they are attached. (165.) 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 any passing animal that chance may place at their disposal, and when the oppor- tunity arrives are not a little remarkable for their voracity and for their GENERAL STRUCTURE OF ACTINIA. 75 capability 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 overpowered 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 disk, expands to an extraordinary size ; and the creature is soon engulfed in the digestive bag of the Actinia, where the solution of all its soft parts is rapidly effected, the hard undigestible remnants being subsequently cast out at the same orifice. (166.) The Actiniae possess the power of changing 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 disk that supports them, or detaching themselves entirely, and swelling themselves with water ; they become nearly of the same specific gravity as the element they inhabit, and the least agitation is sufficient to drive them elsewhere. When they wish to fix themselves, they expel the water from their distended body, and, sinking to the bottom, attach themselves again by the disk at their base, which forms a powerful sucker. (167.) From the above sketch of the outward form and general habits of these polyps, the reader will be prepared to examine their in- ternal economy and the more minute details of their structure. On examining 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 aperture of the mouth, is found to coat the surface of the stomach itself : this epidermic secretion forms, in fact, a deciduous tunic, that the creature can throw off at intervals. On removing this, the substance of the animal is found to be made up of fasciculi of muscular fibres, some running perpen- dicularly upwards towards the tentacula, while others, which cross the former at right angles, pass transversely round the body ; the meshes formed by this interlacement are occupied by a multitude of granules, apparently of a glandular nature, giving the integument a tuberculated aspect : these granules are not seen upon the sucking-disk at the base. The tentacula are hollow tubes, composed of fibres of the same descrip- tion. 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, somewhat modified in texture. (168.) On making a section of the animal, as represented in fig. 36, the arrangement of these parts is distinctly seen, a being the muscular integument ; 6, the tentacula, formed by the same fibrous membrane ; and c, the stomach. 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, I, into numerous compart- 76 ANTHOZOA. ments, which, however, communicate freely with each other and like- wise with the interior of . 3g the tentacula, as seen at e. Every tentacle is per- ^ forated at its extremity by a minute aperture, b, whereby the sea-water is freely admitted into these compartments, so as to bathe the interior of the body; and when, from alarm, the animal con- tracts itself, the water so admitted is forcibly ex- pelled in fine jets through the holes by which it entered. There can be no doubt that the surround- ing 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 keep- ing Actiniae in glass-vessels for the purpose of watching their proceed- ings must have noticed that, as the fluid in which they are confined becomes less respirable, from the deficiency of air, the quantity imbibed is enormous, stretching the animal until it rather resembles an inflated bladder than its original shape. (169.) It is in the compartments thus (at the will of the creature) distended with water that we find the organs of reproduction, which here assume a development far exceeding what we have noticed in other zoophytes. On raising a portion of the membrane forming the stomach, as at /, we see lodged in each partition an immense number of granular corpuscles attached to a delicate transparent membrane, and arranged in large clusters, g. The ovigerous membrane that secretes these corpuscles is represented unravelled at h ; it is through its whole extent bathed with water admitted into the compartment wherein it is lodged a circumstance which provides for the respiration of the em- bryos during their development. (170.) We learn from the researches of MM. Kb'lliker and Erdl that the Actiniae are bisexual, and that the male and female organs are allotted to different individuals, the testes of the male and the ovaria of the female being so similar in their structure and appearance that the difference between them is only appreciable with the microscope. Section of Actinia. BEPEODUCTION OF CERIANTHUS MEMBKANACETJS. 77 In both sexes the reproductive apparatus consists of riband-like convo- lutions attached by delicate membranous folds to the free margins of the septa, and filled with multitudes of the granular-looking bodies above mentioned, which in the females are the ova, in the males the spermatic capsules. In neither sex is there any excretory duct ; so that the eggs, when mature, must escape immediately into the general cavity of the body by bursting through the delicate membranous envelope in which they are enclosed. The whole exterior of the organ is densely ciliated. When examined under the microscope, the granules extracted from their investment are found, in the male, to contain immense numbers of caudate and very active spermatozooids, while those of the female are real ova, of spherical shape, and furnished with a distinct yelk and germinal vesicle. (171.) It is during the months of August and September that the generative system of our native species is in full activity and develop- ment. The ovum, it would seem, when arrived at maturity, breaks loose from the ovarian nidus ; and as the number of males in a given locality is pretty nearly the same as that of the females, and they are always more or less found in company notwithstanding their sedentary habits, the eggs of the female would seem to be impregnated by the seminal fluid of the other sex, diffused through the surrounding water. (172.) In Cerianihus membranaceus, however, a complete hermaphro- ditism exists, all the convolutions of the reproductive ribands being equally supplied with both the male and female elements of generation, in the shape of minute capsules, some of which enclose an ovule, whilst the others are filled with spermatozoa. These two kinds of capsules are scattered promiscuously, without any regular order, but always so that the ovigerous and spermatogenous organs are closely in contact. The ovules on quitting their ovarian nidus fall into the general peri- visceral cavity, where they may be found of a spherical or oblong shape, and each presenting a distinct Purkinjean vesicle. The act of fecun- dation in this case appears to take place in the ovarian laminae, and probably by the rupture of the delicate walls which circumscribe the contents of the male and female capsules. The eggs found in the ovaria are round and of a yellow colour, resembling minute grains of sand, and densely ciliated. There exists a considerable opening in the base of the stomach, whereby a free communication is established between the interseptal spaces and the general abdominal reservoir, through which the young Actinia3 are expelled in a very advanced state of development into the stomach, and from thence pass through the mouth into the surrounding water. The smallest germs are semi-opake spherical bodies, while the more advanced present every gradation of form from the simple sphere up to the complete tentaculate polyp. The largest are about the size of peas. On section they present appearances similar 78 ANTHOZOA. Fig. 37. to those exhibited in the annexed diagrams (fig. 37), intended to illustrate the manner in which the morphological changes are brought about and the several special or- gans of the Actinia unfolded. The figures may be thus explained : 1. Outline of the mature embryo- nic corpuscle after the disappear- ance of the cilia with which, at an earlier stage, it is furnished. 2, 3, 4. Primary involution of the integumentary membrane. 5, 6. Re-induplication of the external membrane and formation of a stomachal cavity. In the two latter figures may likewise be seen the commencement of the tentacula (a) and ovarian septa (6), which are all formed by the same process of involution*. (173.) The perigastric spaces of the Actiniae enclose, in addition to the reproductive apparatus, abundant convolutions of very remarkable fili- form organs of great length and tenuity (fig. 38), concerning the nature of which much diversity of opinion still exists. These convoluted threads have, in fact, no communication whatever with the generative organs ; they seem to consist of long, semi-capillary csecal tubes, attached by a short mesentery to the lower margin of the perigastric septa ; and from Fig. 38. Convoluted filiform organs of Actinia. each of them a cord, slightly larger than the filament from which it is * " Observations on the Anatomy of Actinia," by T. Spencer Cobbokl, M.D., Annals of Nat. Hist., Feb. 1853. FILIFEROUS CAPSULES. 79 derived, may be traced upwards along the corresponding septum as far as a little above the inferior termination of the alimentary cavity, and then running along the external wall of the stomach as far as the py- loric opening. The structure of the terminal portion as well as that of the convoluted filament is tubular, and their office apparently important in the economy of the animal : M. de Blainville regards them, with some probability, as representing the biliary system. (174.) 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 tentacles in search of food, which, on being swallowed, sometimes passed through its mutilated body, but was occasionally retained and digested. In about two months, tentacles grew from the cut extremity of the other portion, and this soon afterwards began to seize prey. By similar sections he even succeeded in making an animal with a mouth at each end. (175.) After the account .above given of the general structure of the Actinia, the mechanism whereby 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 their interior. We have seen already that the cavity of each tubular arm communicates freely with the space intervening 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 placed 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 in- jected, they shrink and collapse. This circumstance, so easily seen in the Actiniae, will likewise enable us to account for similar phenomena observable in other polyps, the internal economy of which is by no means so conspicuous. (176.) On cutting off a portion of one of the arms of an Actinia and subjecting it to pressure, it is seen to have, imbedded in the substance of its gelatinous parietes, an immense number of minute organs, now universally known by the name of filiferous capsules. These remark- able structures, which are found to exist very extensively throughout the entire group of polypoid organisms, consist of minute sacculi, wherein may be perceived a slender and highly elastic filament coiled up spirally, but which, on compression, suddenly shoots forth from one extremity of the capsule to a length that is perfectly surprising. It is upon the presence of these filiferous capsules that the adhesive power of the tentacula is supposed to depend ; and from the rapidity wherewith prey, when seized, is destroyed by their grasp, it is probable that a poisonous fluid is emitted along with the thread, to the virulence of 80 HYDEOZOA. which its paralysing effects are to be attributed. The presence of these remarkable organs is, however, by no means restricted to the tentacula ; on the contrary, they are dispersed over various parts of the body, and exist even in the folds of the ovarian membrane. In the latter situa- tion, indeed, they are frequently extremely numerous, and compara- tively of large size. CHAPTER V. HYDEOZOA. (177.) THE HYD:ELS;, or freshwater polyps, are common in the ponds and clear waters of our own country ; they are generally found creeping upon confervse or submerged twigs, and may readily be procured in summer for the purpose of investigating the remarkable circumstances connected with their history. (178.) The body of one of Kg 3a these simple animals consists of a delicate gelatinous tube, con- 6 tracted at one extremity, which is terminated by a minute suck- er, and furnished at the oppo- site end with a variable number of delicate contractile filaments, placed around the opening that represents the mouth. (179.) When the Hydra is watching for prey, it remains expanded (fig. 39, 1, 2, 5), its tentacula widely spread and perfectly motionless, waiting patiently till some of the count- less beings that populate the stagnant waters it frequents are brought by accident in con- tact with it : no sooner does an animal touch one of the fila- ments than its course is arrested as if by magic ; it appears instantly fixed to the almost invisible thread, 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 gra- dually dragged towards the orifice of the mouth, that opens to receive it, and slowly forced into the interior of the stomach. HYDRA VIEIDIS. 8t (180.) To the earlier observers of the habits of the Hydra nothing could be more mysterious than this faculty, possessed by the creature, of seizing and retaining active prey, in spite of all its efforts at resist- ance, but which is now satisfactorily explained as depending upon the presence of a prehensile apparatus, allied in its nature to the filiferous capsules of the Actinice, described in the last chapter. These wonderful organs are not only thickly dispersed over the whole surface of the tentacles, but are likewise met with, though less numerously distributed, over the general surface of the body. They appear, under high powers of the microscope, to be composed of minute oval vesicles, from each of which can be protruded a long delicate filament, having its free ex- tremity slightly swollen, and apparently of a soft viscid texture, the whole being not inaptly compared by Agassiz to a lasso. The neck of each vesicle is furnished with three recurved booklets, which, when the skin of the animal is irritated, or when the arms are prepared to seize prey, remain erect and prominent. The modus operandi oi^ these structures is as simple as the result is efficacious : the " lasso-threads" with their viscid extremities, speedily involve the victim seized, in their tenacious folds, and closely bind it against the booklets wherewith the surface of the tentacula is thickly studded : these, probably, in their turn constitute prehensile organs, and moreover form an apparatus of poison-fangs of a very deadly character ; for it is observable that an animal once seized by the Hydra, even should it escape from its clutches, almost immediately perishes. (181.) Arrived in the stomach of the polyp, the animal that has been swallowed is still distinctly visible through the transparent body of the Hydra, which seems like a delicate film spread over it (fig. 39, 4) ; gradually the outline of the included victim becomes indistinct, and the film that covers it turbid; the process of digestion has begun: the soft parts are soon dissolved and reduced to a fluid mass, and the shell or hard integument is expelled through the same aperture by which it entered the stomach. (182.) No traces of vessels of any kind have as yet been detected in the granular parenchyma of which the creature is 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 Planarice, the granules of the body acquire a similar hue, but the fluid wherein they float remains quite transparent ; 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 it has devoured ; sometimes the granules thus tinted are forced into the tentacula, from whence they are driven again by a sort of reflux into the body, producing a kind of cir- culation, or rather mixing up, of the granular matter, and distributing it 82 HYDEOZOA. to all parts. If, after having digested coloured prey, the polyp is made to fast for some time, the vesicles gradually lose their deepened hue and become comparatively transparent. The granules, therefore, would seem to be specially connected with the absorption and distribution of nutriment. (183.) When mature and well supplied with food, minute gemmules or buds are 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 surface. These gemmaB appear at first like delicate gelatinous tubercles 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 tentacles characteristic of their species. (184.) This mode of propagation, termed " gemmation," differs from the development of the Hydra ab ovo, inasmuch as the germ -cell, whicH sets on foot the process, is derivative and included in the body of the adult, instead of being primary and included in a free ovum. (185.) Sometimes six or seven gemmae have been observed to sprout at once from the same Hydra ; and although the whole process is con- cluded 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. (186.) 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 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 speedily developes the wanting parts of its structure ; if longitudinally divided, both portions soon become complete animals ; if even it be cut into several parts, every one of them will in time assume the form and functions of the original. (187.) It has recently been discovered that the Hydrce, at certain seasons of the year, are reproduced from real ova*, at which period various observers have proved them to be possessed of a male ap- paratus of a most remarkable character. This strange organism makes its appearance under the form of two, three, or four minute conical tubercles, which become developed from the sides of the body, at a short distance below the tentacula ; and in these, under the microscope, * " On a species of Hydra found in the Northumberland Lakes," by A. Hancock, Esq., Ann. and Mag. of Nat. Hist, for 1850. PROPAGATION OF HYDRA. 83 a glandular-looking body and innumerable active particles are seen to be contained. (188.) The conical eminences, which constitute the spermatic cap- sules, appear to derive their origin from the greater degree of develop- ment of one or more of the superficial cells in the vicinity of the base of the arms. These capsules sometimes occur in considerable numbers, as from eight to sixteen, on the brown polyp ; but in the green species Fig. 40. Oviparous reproduction of Hydra viridis. 1. Body of Hydra magnified : a, the ovum contained in the ovigerous capsule sprouting from the side of the polyp ; 6, 6, spermatic capsules. 2. Mature ovum of Hydra crushed, its contents escaping. 3. Spermatic capsule broken by pressure, showing the contained spermatozoa. only two or three are generally seen, placed on opposite sides of the body, and invariably situated somewhere in the vicinity of the oral extremity (fig. 40, 1, 6 6). The interior of the capsules has a slightly ribbed or striated appearance ; and at the summit a small aperture is sometimes perceptible, through which, when the development is com- plete, the spermatic filaments are observed to issue. On breaking up the capsule, under the microscope, large numbers of these filaments are seen united in bundles by their minute globular heads, the filamentous part being free, and vibrating with great rapidity in the manner which is known to be characteristic of these bodies in all animals (fig. 40, 3) . These| spermatic capsules were observed by Dr. Allen Thompson* on many individuals in which no ova existed. The ova are developed in the lower portion of the body, which, at the time when the male apparatus makes its appearance, becomes considerably enlarged, presenting an opake swelling, in the interior of which an ovum makes its appearance * Edinburgh New Philosophical Journal, 1846. G2 84 HYDEOZOA. (fig. 40, l, a) ; when mature, this ovum becomes detached from the parent animal and fixes itself to some foreign body. (189.) The ovigerous capsule is, when fully developed, of such a size as to be seen with the naked eye. It is attached to the side of the polyp, nearer to the foot than the spermatic capsules, and is distin- guished from the rest by its spherical form and yellowish-brown colour. In the Hydra viridis only one of these ova appears to be developed on the body of the polyp at the same time ; but a number, varying from five to seven, have been occasionally observed upon the Hydra fusca. (190.) The ovum appears, at first, as a small granular mass in the thickness of the wall of the animal. As the spherical yelk-mass en- larges, it projects from the side, seeming to carry along with it the outer or clearer layer of the animal's body ; then the cells of this layer grow thinner, and recede from the outer covering or capsule enveloping the egg-like mass, which at the same time becomes much thicker, and is left attached to the animal only by a narrow portion or pedicle. As the development proceeds, a similar atrophy of the cells of the pedicle is followed at last by the separation of the spherical mass, which is thus detached from the parent polyp. (191.) From various observations it would seem that while some of the individuals of the Hydras are hermaphrodite, others produce the organs of one sex only ; but generally both kinds are developed from the same Hydra. (192.) The fertilization of the ova cannot take place until after the rupture of the spermatic cyst and that of the ovisac also ; so that the parent has no more participation in it than has the Fucus in the ana- logous fertilization of its germ-cells after their discharge. Although the production of a new Hydra from such an egg has not yet been wit- nessed, there seems no reason to doubt the fact. It would seem that this alternation of reproduction between the gemmiparous and the sexual is greatly influenced by temperature, the eggs being produced at the approach of winter and serving to regenerate the species in the spring, while the budding process naturally takes place only during the warmer seasons of the year, but may be made to continue through the whole by a sufficiently high temperature. (193.) Nearly related to the Hydrce is the remarkable group of Gela- tinous Polyps, as they are named by Cuvier in his classification of the Animal Kingdom, constituting the genus CORYNE (Corine of Gaertner). One of these little animals, seen with the naked eye, observes Mr.Gosse*, to whom we are indebted for the following particulars of their history, looks like a very slender branching plant. It is altogether about as thick as fine sewing-cotton, creeping along a frond of sea- weed, or other substance upon which it grows, like an irregularly-winding thread. This creeping root sends off frequent rootlets, which, crossing each other, * Rambles on the Devonshire Coast. 1853. TUBULAEIDyE. 85 appear to anastomose, making a sort of network from which free stalks shoot up here and there, sometimes to the length of three inches or more, sending forth the polyp -branchlets irregularly on all sides. The creeping fibre, the stalk, and the branchlets are seen under the micro- scope to be tubular ; and the two latter are marked throughout their course with close-set rings or false joints, apparently produced by the annular infolding of a small portion of the integument. The tube is of a yellowish-brown colour, sufficiently translucent to reveal a core or central axis of flesh running along its centre and sending oif branches into the polyp -branchlets, from the open lips of which the flesh emerges in the form of a thickened oblong head, somewhat club-shaped, whence the name Coryne (from Kopvvr], a club). The tube or sheath becomes membranous, or rather gelatinous, at its margin, the ultimate three or four rings being evidently soft and scarcely consistent, of undefined out- line, and larger than the rest. The club-shaped head of the polyp is studded with short tentacles, of curious and beautiful structure. These vary much in number in each polyp ; but the full complement appears to be from twenty-five to thirty, arranged somewhat in a whorled manner in four or five whorls, which, however, especially the lower ones, are often irregular and scarcely distinct. The tentacles spring from the axis, with a graceful curve ; they are rather thick and short when con- tracted, but slender when elongated, and nearly equal in diameter, except at the termination, where each is furnished with a globose head studded all over with tubercles, each of which is tipped with a minute bristle. The neck or body of the tentacle is perfectly transparent, and appears to be a tube with thin walls, but containing a colourless thickish axis freely permeating its centre, marked with delicate parallel rings. The tentacles are endowed with the power of free motion, and they fre- quently throw themselves to and fro with considerable energy. The whole polyp likewise can be tossed together from side to side at pleasure. (194.) TUBTJLAKID^E. In the Tubular Hydrozoa the structure of the tentacula is widely different from what has been described in Tubijpora mmica and other anthozoic polyps. 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 considerable distance beneath it (fig. 41, l) ; and, nearly on a level with the inferior circle, a second aperture (fig. 41, l, a) is observable, communicating with that portion of the body which is lodged within the tube, and re- sembling 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 between the two rows of tenta- cula, which becomes gradually dilated into a globular form (fig. 41, 2,3). * Lister, " On the Structure and Functions of Tubular and Cellular Polypi," Phil. Trans. 1834. HYDEOZOA. Fig. 41. 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 repetition 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, b, which encloses the polyp is perfectly diaphanous, allowing its contents to be readily investigated under the microscope. When thus ex- amined, a continual circula- tion of particles is visible, moving in even, steady cur- rents in the direction of the arrows (fig. 41,1) 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 are glo- bular, but they have generally no regular form. (195.) The mode of propagation in Coryne, Tubularia, and other genera of Hydriform polyps has occupied the attention of many diligent inquirers, the results of whose labours are extremely interesting and important ; it is, however, to the researches of Professor Van Beneden that science is principally indebted for information upon this subject. According to the observations of that indefatigable naturalist*, the re- production of these zoophytes is effected in no fewer than three different ways : 1. By continuous gemmation. 2. By the production of free gemmae. 3. By simple ova. (196.) Observation has moreover shown that in every species pro- pagation is effected by more than one of these modes of reproduction, and sometimes by three or four ; and it must be remarked that in none of them is the cooperation of a male apparatus requisite, neither have any male organs or spermatozoa been as yet detected. (197.) Development by continuous gemmation is the simplest possible, * Nouveaux M6moires de PAcademie de Bruxelles, 1843-1844. Tubularia indn DEVELOPMENT OF TTJBULARIA COKONATA. 87 and is effected by mere growth from the original polyp at certain de- terminate points of its substance, which points are similarly situated with respect to each other in all the individuals belonging to the same species. At these points appear gemmee exactly similar, both in texture and mode of growth, to the body from which they spring ; and the buds thus produced give birth to others in a precisely similar manner. (198.) In like manner when a stem is cut off transversely, a bud is developed from the cut extremity, which by its growth prolongs the original trunk. When this kind of gemma has attained to a sufficient size, there arises from its extremity a little crown of tubercles ; sub- sequently a second becomes manifest, at some distance from the first ; and as the growth of these tubercles continues, each of them becomes at length developed into a tentacle. The tentacle, therefore, grows from the body exactly in the same way as the bud from the stem, the only difference being that the former is solid, and the latter tubular. (199.) The growth of the horny sheath of the polypary (fig. 42,6) Tubularia coronata. b, the polypary, or horny sheath ; c, living substance of the animal ; d, boundary between the individual and the common stock ; g, tentacular arms ; &, tentacular zone ; h, mouth ; n, reproduction by continuous gemmation ; o, ovigerous capsules. (After Van exactly keeps pace with the development of the soft substance, and even advances beyond it. 88 HYDEOZOA. (200.) Below the tentacula the body of the polyp appears constricted, marking the boundary between it and the stem ; and soon the flower- like head, becoming too large to be contained in its sheath, issues forth, and, expanding its tentacula, displays itself perfectly unfolded (fig. 42, g, h). The ovigerous pedicles, o, hereafter to be described, are developed subsequently. (201.) Second mode of propagation, by free gemmce. The free gemmae are produced upon distinct pedicles, which in the genus Tubu- laria are developed within the lower circle of tentacula. They resemble numerous appendages disposed in a circle, and forming a crown around the body of the polyp (fig. 42, o). These pedicles grow in the same manner as the buds and the tentacula described above ; that is to say, a hollow tubercle first makes its appearance, which seems to be merely an extension of the external covering of the polyp. Each tubercle slowly expands, and soon divides into one or more branches, which are all hollow, and the same fluid that circulates in the general substance of the polyp may be observed to pass into their interior (fig. 44, A'). (202.) At the free extremity of each of the pedicles thus formed, a distinct cell is soon perceptible (fig. 43, A, B, c, ), situated immediately beneath the surface, which cell is the rudiment of a new individual. No nucleus has been remarked in its interior. This primitive cell may be regarded as the analogue of the vitelline sac, or, perhaps, as the vesicle of Purkinje or of Wagner ; most probably, however, it is the vitelline vesicle, from the circumstance that it becomes organized inter- nally, in which case the reproductive process assumes the third or the fourth form, subsequently to be noticed ; or else it serves for the point of departure, or, it might almost be said, the mould for the formation of a free gemma, which becomes organized around it at the expense of the pedicle itself. It is, in effect, a part of the reproductive appendage that will subsequently become detached ; but at this period of its development it is impossible to determine after which of the four modes of reproduc- tion the embryo will be formed. The vesicle (fig. 43, A, B, c, a) now increases rapidly in size, and beneath it another membrane (fig. 43, A, B, c, 6) is soon perceptible, which by its inner surface is in contact with the circulating fluid. This membrane is the origin of ttje new indi- vidual ; or, in other words, it is a blastoderm, formed by the internal skin, and not by the vitellus. Soon there is seen, projecting from its centre, a little cone (fig. 43, B, b) which, compressing the vesicle, a, forms a depression upon its inferior surface, so that the vesicle begins to assume the appearance of a serous membrane, yielding to the pressure of the organs over which it spreads, and which it ultimately covers much in the same way as the pleura covers the lungs. The tubercle, b, will afterwards form the walls of the digestive cavity of the new animal, and may be seen to have a circulating fluid, derived from the body of the polyp, moving in its substance. Around the base of the cone, 6, may DEVELOPMENT OF TUBULARIA CORONATA. 89 now be seen four other tubercles (fig. 43, c, c), which become developed like the preceding; but instead of compressing the vesicle, a, they surround it, and ultimately completely enclose it. They are united together by a thin membrane, so as to present the appearance of a trans- parent vase, having four longitudinal prominent bands, the free edge slightly enlarged and rounded, a pedicle in the middle like the stem of the vase, and the transparent vesicle lining its interior throughout (fig. 43, E). (203.) The different phases of the mode of development above de- scribed, however, will be best understood by a reference to the series of figures which we have appended, carefully copied from Professor Yan Beneden's elaborate illustrations. (204.) The young Tubularia has now assumed the appearance of a Beroe, and in this condition has doubtless been often mistaken for an individual belonging to the class Acalephae, to be described in the next chapter ; lively contractions of its body are frequently witnessed, although it still remains attached to its pedicle. (205.) At the extremity of each of the four longitudinal vessels a little tubercle is next developed (fig. 43, F, e), which, as it becomes elon- gated, is converted'into a tentacle ; or sometimes, as in Eudendrium, by its bifurcation, two tentacula are formed from each tubercle. (206.) At this period of its development the young Tubularia spon- taneously detaches itself from the parent stem, presenting, at the moment of its separation, the appearance of a balloon, or rather, of a melon (fig. 43, G). Its contractions become more and more lively; and it is by the aid of these movements that its separation is effected. The two poles of its globular body may be seen to approach each other, and to separate alternately with a movement of systole and diastole, similar to what is observable in many Acalephse. No traces of cilia are observable either externally or in the interior of its body. In this condition it presents an external covering, which is, so to speak, merely a derivation from the integument of the parent-polyp ; this covering presents some- what more consistence than the internal parts, and is open in front. (207.) A second membrane lines the preceding throughout its whole extent ; like the former it is quite transparent, and at the anterior open- ing is prolonged internally to a little distance, forming a sort of funnel. These walls enclose four vessels (fig. 43, H, c), which extend from the base of the embryo, and open in front into the hollow zone from which the tentacula take their origin. These longitudinal vessels, therefore, communicate with each other by a transverse canal, and at their origin open into the central or digestive cavity. Prom this disposition it re- sults that the contents of the stomach can pass as far as the extre- mities of these four vessels, and by means of the transverse canal can be transferred from one to the other. Professor Van Beneden observed a fluid containing globules moving in this direction in their interior. 90 HYDEOZOA. The communication by means of transverse canals is another arrange- ment exactly similar to what exists in the adult Beroeform Medusae. Fig. 43. Development of Tubularia by free gemmse. (208.) The outer membrane presents eight longitudinal canals (fig. 43, G, H, b), which are found to be filled with cellules, but in which no movement has been observed. It is to the presence of these longitu- dinal bands that the embryo in this stage of its development owes its resemblance to certain fruits, more particularly to a melon. (209.) From the anterior part proceed four appendages (fig. 43, G, d), which were still undeveloped at the period of the detachment of the young polyp, but now insensibly unfold themselves. These are the tentacula. In the centre there projects a rounded opake body, gene- rally of a red or yellowish tinge, which is the stomach. This viscus communicates, as has been stated above, with the four longitudinal vessels, and is the only opake part of the embryo. It opens in front by an orifice that constitutes the mouth ; the whole organ is emi- nently contractile, turning in all directions like the body of a Hydra, sometimes elongating itself like a worm, and at others shrinking so as to be almost imperceptible. (210.) If the embryos examined in this condition be vigorous, their movements are very varied, and the forms that they assume extremely singular. The regular contractions above noticed are the most simple actions ; the two poles separate and approach each other alternately, whence results the progression of the little creature. But this con- traction may be -carried to a still higher degree : the rounded stomach in the middle of the embryo not only moves itself about in every direc- tion, but it seems to make efforts in the middle of its transparent DEVELOPMENT OF TUBULARIA CORONATA. 91 envelope, like a worm in search of a passage by which to get out ; and at length it pushes its free extremity through the opening in front of it, and elongates its body still more, until the two poles of the balloon getting approximated, the whole embryo becomes somewhat disk- shaped, or the four vessels that communicate with the stomach (if vessels they really are), by moderately contracting, form as many depressions di- viding the disk into four lobes (fig. 43, H, i), or by a more forcible con- traction give it the appearance of a Greek cross ; and all these changes of form may take place in a few seconds. (211.) Observations are wanting relative to the manner in which the free acaleph gives origin to the fixed polyp ; for although Professor Van Beneden observed the latter at a very early period after they had become attached, he was unable to witness any further changes that they undergo, and therefore gives a hypothetical outline of the forms through which he supposes them to pass, preparatory to their final establishment as young Tubulariae. (212.) Third mode of propagation,, by simple ova. This mode of repro- duction approximates the nearest to what occurs in the higher animals. Cells are observed in process of gradual organization in the middle of a vesicle, in the same manner as the vitelline cells, which are converted into an embryo. In this case the vitelline cells become aggregated and modified, so as to give rise to a new individual, which is isolated from the commencement of its existence. The point of departure for the formation of the embryo is the same as in the preceding mode of development, and the reproductive vesicle has at first precisely the same structure ; but instead of preserving its transparency, this vesicle soon exhibits numerous cells, which render it more and more opake, and give it the appearance of a vitellus. In this case, moreover, there is a great difference in the relations which the red pedicle (fig. 44, A, B), bears to the embryo. In the preceding mode of development this pedicle constitutes an integrant part of the newly-formed being, form- ing, in fact, its stomach ; but in the oviparous mode, there is no organic connexion between the one and the other, the vitellus being formed between the pedicle and the integument of the offset, and on pressing the latter between two plates of glass, these structures readily separate without any laceration. (213.) As the vitellus (fig. 44, B, a) increases in size, it becomes im- pacted between the integument and the pedicle, and its augmentation of bulk still increasing, the upper part of the pedicle becomes covered with it as with a hood, and at last almost entirely enveloped by it (fig. 44, c, D, E). At this period the margins of the vitellus become indented on that side nearest the pedicle, and the tubercles between the inden- tations soon show themselves to be the rudiments of tentacula. The tentacula become more and more elongated, the embryo separates itself slightly from the pedicle, and a protuberance (fig. 44, p, G, 6) is then per- 92 HYDROZOA. ceived in the centre of the tentacular zone, which becomes the proper body of the polyp, or rather, forms the walls of its stomachal cavity. (214.) The walls of the bud which has hitherto contained the embryo now become ruptured, and it gains its liberty (fig. 44, H). In this condition it almost exactly resembles a young Hydra in its con- tracted state ; and, in fact, both its body and its tentacula seem to have Fig. 44. Development of Tubularia from ova. the same anatomical structure as those of that simply organized polyp. Having attained to this condition, its development proceeds rapidly, and it soon begins to assume the specific form of the Tubularia from which it sprung (fig. 44, i). (215.) SERTULABID^E. In the Sertularian Hydrozoa, the fleshy sub- stance of the animal is enclosed in a ramose horny sheath, which it traverses like the pith of a tree, following all the ramifications of the branched stem of the polypary. (216.) 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 satisfac- torily, it is necessary to be provided with several glass troughs, of dif- ferent depths, in which the living animals immersed in their native element may be placed : in this situation, if the water be carefully re- newed at short intervals, they will live for some time. (217.) Besides the cells which contain the polyps, others, specially destined to the development of the ova, exist at certain periods of the year ; they are larger than the preceding, and of a very different shape ; but of these we shall have occasion to speak more fully hereafter. SEETULAEID^S. 93 (218.) The general stem of the polypary is entirely filled with a fleshy substance exactly resembling in its nature the tissue composing the body of the polyp, whereby all the individuals belonging to the common stock are brought into communication with each other. In- ternally it seems to be hollow, and to contain a fluid, in which nume- rous globules may be observed in active motion. It is from this central fleshy substance that the buds or lateral offsets derive their origin. (219.) On examining attentively the stem of a living Sertularia, the globules are all seen to follow each other in distinct currents, and sometimes may be observed to move in opposite directions in the same branch : on arriving at the bifurcation of a stem, some seem to stop, whilst others continue their course to the right or left. If the branch of a living polypary, while in a state of activity, be divided and slightly compressed, the globules that escape from the cut extremity still con- tinue their movements for some considerable time, somewhat after the manner of zoosperms ; and as this kind of motion, when observed ex- ternally to the polypary, resembles very closely that which they exhibit in its interior, it is apparently not dependent upon any pressure from the walls of the general fleshy substance, but seems to be inherent in the globules themselves. The general movement of the fluid contained in a branch, however, more especially as relates to its direction, depends upon the pressure exercised by the polyps ; so that if several individuals on one side of a branch contract simultaneously, they sometimes even force the contained liquid through the mouths of those upon the oppo- site side. (220.) It has been generally stated that the living pith exudes from its surface the horny matter which, by its concretion, forms the tube or external skeleton investing the whole ; the accuracy of such a sup- position, nevertheless, may well be questioned. We have already seen, in the Tubipora muswa, that the calcareous tube investing that polyp was produced by the interstitial deposits of earthy matter in the mem- brane that originally constituted its outer case. In the tribe of zoo- phytes 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. 45), the mode of its growth will be rendered intelligible. The soft part, or living axis of the polypary, is seen to be contained in two distinct layers : the inner one, a, being continuous with the digestive sac of the polyp, and immediately embracing the granular matter, seems to be the special seat of the nutritive process ; the outer or tegumentary layer, 6, 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 development the entire skeleton. HYDROZO^. Fig. 45. As it expands, it gives origin to the cells and branches characteristic of the species ; and, from being at first quite soft and flexible, it gra- dually acquires hardness and solidity by the depo- sition of corneous matter in its substance. (221.) The cells thus formed are inhabited by polyps analogous to those that provide nourish- ment for the cortical fa- milies, though differing in the number and ap- pearance of the tenta- cula, which are here studded with minute tu- bercles, but never pro- vided with cilia. Few objects are more admi- rable than these polyps, when watched with a good microscope. Pro- truding themselves be- yond the mouths of their cells, they inflect their bodies in all direc- tions in quest of prey, waiting till some passing object impinges upon their tentacula, when it is at once seized and conveyed into the stomach with a rapidity and dexterity almost beyond belief. (222.) The tentacula in the Sertularian Hydrozoa are all arranged in a single row, and form a sort of funnel-like appendage to the oral orifice of the body. They are susceptible of considerable elongation and con- traction, like those of the Hydra, but in a less degree. Their number is constant throughout the different periods of growth in each species, but varies in different genera. Internally they are not hollow as is the ease in the Anthozoa, but under the microscope they have the appear- ance of being divided into compartments by delicate transparent dia- phragms, giving them an appearance like that of some Confervse ; they are throughout of equal thickness, and no movement of fluids is percep- tible in their interior. (223.) In the centre of the tentacular circle may be observed a fleshy protuberance of variable shape, which might be compared to a proboscidiform elongation of the mouth ; sometimes this appendage is elongated into the form of a tube, sometimes it shrinks into a globular Diagram representing section of a Sertularian zoophyte, a, inner or nutritive layer; 6, outer or tegument?, ry layer; c, oral tentacles of the polyp ; d, e, gemmules ; f, polypiform external capsule ; g, polypiferous cell; h, reproductive cell. GROWTH OF POLYPIDOM. 95 mass, or occasionally it may be seen so completely contracted as simply to form a broad lip-like ring around the oral opening. (224.) The stomach, as in the Hydra, is a simple cavity excavated in the interior of the body, without any proper parietes, which inferiorly communicates immediately with the fleshy substance contained in the common polypary ; so that the contents of the stomachal sac may not unfrequently be seen to pass into the living pith, and in like manner the globules there circulating to return into the stomach. (225.) The multiplication of these beautiful zoophytes appears to take place in three different modes : 1st, by cuttings, as in plants ; 2ndly, by offshoots, or the formation of new branches bearing polyps ; 3rdly, by Planulce capable of locomotion. (226.) 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 con- tinue to grow and perform the functions of the entire animal. (227.) The second mode of increase, namely, by the formation of new branches and polyps, seems more like the growth of a plant than the development 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 ex- amining any growing branch, it will be found to be soft and open at the extremity, and the soft tegumentary membrane (above described as forming the tube by its conversion into hard substance) is seen to pro- trude through the terminal orifice ; the skeleton is therefore not 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 ramifica- tions peculiar to its species. (228.) Having thus lengthened the stem to a certain distance, the next step is the formation of a cell and a new polyp, which is accomplished in the following manner*. The newly-formed branch has at first pre- cisely the appearance and structure of the rest of the stalk of the zoo- phyte (fig. 46, 1), being filled with granular matter, and exhibiting in its interior the circulation of globules (already described) moving to- wards 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 dilated. After a few hours the branch is visibly longer, its extremity more swollen, and the living pith is seen to have partially separated itself from the sides of the tube, the bounda- ries 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 * Lister, Phil. Trans, loc. cit. 9C> HYDEOZOA. of the bell-shaped polyp (3), but no tentacula are yet distinguishable ; a rudimentary septum becomes visible, stretching across the bottom of the cell, through the centre of which the granular matter, now collected into a mass occupying only a portion of the stem, is seen to pass. The polyp and cell gradually grow more denned (4, 5, 6), and the tentacula become distinguishable ; the cell, moreover, is seen to be continued in- wards by a membranous infundibular prolongation of its margin (V), that immediately reminds us of the funnel-shaped membrane of Tubi- pora ( 157), and its office is no doubt similar. As the development proceeds, the tentacles become more perfect (8,9), and the polyp at length rises from its cell to exercise the functions to which it is destined. Fig. 46. Diagram illustrating the mode of growth of a Sertularian polypidom. (229.) The main feature that distinguishes the Sertularian Zoophytes from the Hydrce seems to consist in the fact that, whereas in the latter each newly-formed offset becomes detached from the parent stock and enjoys a separate existence, in the former each new sprout remains permanently adherent, the successive generations being united into a ramified stem, which is common to the entire group. The Hydra, having no polypary or outer covering, when it dies, entirely perishes : but in the Sertularida3, every sprout, when it dies, leaves its horny in- tegument attached to the general community ; and thus, in time, there results an elaborately-branched stem, the complexity of which increases with the age of the colony. PKOPAGATION OF SEKTTJLABIAN ZOOPHYTES. 97 (230.) The third mode of multiplication, or that by Planulce, seems to be specially adapted to the diffusion of the species ; and as it presents many points of peculiar interest, we shall dwell upon it at some length. At certain periods of the year, besides the ordinary cells adapted to con- tain 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 form (fig. 48, A) ; 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 membrane (fig. 45, 5), 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 operculum. The cell being thus constructed by the expansion and subsequent hardening of the tegu- mentary membrane, it remains to explain the origin of the reproductive germs which soon become developed in its interior. (231.) According to the observations of Loven on the reproductive process in Sertularia, the first appearance of the reproductive germ is a slight elevation (derived from the central mass contained in the ovarian vesicle), in the centre of which an active circulation of nutritious globules seems to be concentrated. This protuberance gradually enlarges and assumes a spherical form, the part whereby it is attached to the central mass becomes constricted, and at the same time its cavity becomes enlarged, and divided into several compartments. (232.) Upon the outer aspect of the newly-formed germ a little spherical body may be detected, composed of coloured granular sub- stance, in which a circular transparent spot speedily becomes per- ceptible. (233.) A delicate translucent capsule envelopes the parts described above, which, after a time, exhibits at its upper and outer surface a circle of minute elevations. This capsule Loven regards as the body of a female polyp, of which the little elevations are the rudimentary tentacula ; and its contents manifestly constitute an ovum, enclosing a Purkinjean vesicle. Several of these ova are formed in the ovarian vesicle, presenting different degrees of development, the upper ones being the most advanced in growth. In proportion as each ovum in- creases in size, the original sacculus, which is merely a prolongation of the central living substance of the polypary, and which at first formed the larger part of the germ, becomes proportionally smaller, owing to the rapidly-increasing dimensions of the ovum, and soon the vesicle of Purkinje is no longer discoverable. Meanwhile the canal whereby its cavity communicates with the central mass becomes elongated; so that its union with the common substance of the polypary is not de- stroyed even when the "female polyp " has burst through the external 98 HYDBOZOA. membrane and the thin operculum of the ovarian capsule in which it was formed. (234.) When the " female polyp" has thus escaped from the ovarian vesicle of the common polypary, it has the appearance of a globular transparent capsule attached by a short pedicle to the operculum through which it has made its way, the orifice whereby it escaped having closed around its stem. The tentacles are twelve in number ; and from the circle surrounding their base, four canals may be traced, de- scending in the substance of the globular body to terminate in the pedicle or sacculus that occupies the lower part of the (Medusiform) capsule. (235.) On the rupture of the external membrane of the ovum en- closed in the " female polyp," the young animal escapes, under a form not at all resembling that of the parent animal. (236.) It presents at this period the appearance of a little worm, of an elliptical shape, slightly flattened. Its entire surface is thickly covered with vibratile cilia, by the agency of which it moves about even while still imprisoned in the body of its mother, from which it subse- quently makes its escape through the oral orifice. Generally each " female" gives birth to two embryos, occasionally to three. (237.) No sooner has the young larva got free than it begins to swim about, by means of its cilia, with a uniform gliding motion : sometimes it turns round incessantly upon its axis, either horizontally or in a vertical direction, at the same time varying its shape from that of an egg to that of a pear. It is of a whitish colour, but still suffi- ciently transparent under the microscope to show that it contains a cavity filled with a coloured fluid, and composed of two membranes, whereof one, the outer, is transparent as glass, the internal slightly opake. (238.) Repeated observations render it improbable that in this con- dition the little embryo is nourished by means of a mouth. (239.) After swimming about for some time in the above condition, the young creature fixes itself to some foreign body, such as a fucus, or other marine production ; and then its form begins to be entirely changed, and it is converted into a flat, circular disk, around which the cilia, now quiescent, form a circular transparent fringe. In the centre of its internal cavity an opake round spot makes its appearance, the size of which is about a fifth part of that of the whole body, composed of a mass of granules placed concentrically, and occupying the situation whence the stem of the nascent polypary is to be developed. At this point the external membrane becomes slightly thickened, and, as it were, furrowed with vessels proceeding from the internal cavity. From the opake central spot arises a hemispherical protuberance ; and at the same time the central cavity loses its semicircular form and becomes divided into four or five irregular lobes, which subsequently become the horizontal supports of the fixed polypary. (240.) Already the whole expansion is covered with a horny layer ; CAMPANULAEID^l. 99 but this only becomes distinctly recognizable at a more advanced stage of growth. (241.) The trunk continues to rise vertically upwards, and ulti- mately produces at its summit a solitary cell, in which a " male " (nutritive) polyp is gradually developed ; and then, as growth ad- vances, secondary ramifications are developed, after the pattern peculiar to the species. (242.) Another important fact in connexion with the history of the Hydrozoa is, that in the compound species there exist male branches as well as female branches upon the same polypary, the latter pro- ducing ovigerous vesicles, whilst in the former the ova are replaced by seminal capsules; these almost precisely resemble the "female cap- sules " (Beroeform, c/emmce) figured by Loven, and are, in like manner, surmounted by a circle of tentacula. (243.) CAMPANTTLARIDJE. The polypary or common integument of these zoophytes is composed of a semigelatinous horny substance (fig. 47). The older stems assume a dark-brown colour and a consistence resem- bling that of horn. The young branches, on the contrary, and more particularly the polyp-cells, are thin and perfectly diaphanous. The polypary always exhibits a principal trunk, from which the different branches proceed, every one of the latter being terminated by -a bell- shaped cell. (244.) In the earlier stages of growth the polypary consists of a pri- mary trunk, from which alternating pedicles are given off at regular distances. These pedicles soon become transformed into branches, on which new pedicles in turn make their appearance, as they did on the original stem, exhibiting a dichotomous or trichotomous arrangement. (245.) At the base of each branch transverse rings are formed (fig. 47, g}, which are persistent during the life of the polypary. All the branches, as well as the common trunk, increase in their dimensions in accordance with the age of the zoophyte ; and, as in vegetables, there is a relation preserved between the thickness of the trunk and the number and extent of the branches. (246.) Each polyp-bearing cell at the extremities of the branches presents externally a bell-shaped cup, having at its bottom a horny dia- phragm, perforated in the centre. It is through this perforation that the body of the polyp is brought into communication with the common fleshy substance of the polypary, and through its intermedium, with the other polyps. (247.) From the recent observations of Van Beneden* relative to the embryogeny of the Campanularian polyps, it would appear that they frequently undergo, during their development, a series of changes not less wonderful than those exhibited by the other Hydriform races whose history has been carefully traced. * Nouveaux Mem. de 1'Acad. de Bruxelles, vol. xvii. (1843). H2 100 HYDKOZOA. Fig. 47. (248.) The ovum developed in the ovarian vesicle is, at its first ap- pearance, of a spherical shape, and imbedded in the substance where- with the deciduous capsule is filled (fig. 47, e,e'). It is sur- rounded by a mem- brane analogous to the calyx of the ovary in buds, which, being torn, discovers the de- nuded ovum, wherein the vesicles of Pur- kinje and of "Wagner are easily detected; but these very speed- ily disappear, without any other change in the interior of the egg being discernible. (249.) The next step in. the process of development seems to be the conversion of the external vitelline cells into a layer, situated immediately beneath the vitelline membrane, which may be regarded as the representative of the blastoderm. (250.) The blasto- derm now becomes thickened around the Campanularia gelaUtwsa. a a, tegumentary skeleton, or horny j. 11 f polypary; 6, 6', buds in progress of development into polyps ; VltellUS, terming a SOrt c c c , terminal polyp-cells empty; d,d',d",d'", polyps in dif- Of elevated ring (fig. ferenfc 8ta es of S rowth J ovarian cell containing an embryo . ready to escape; e', another ovarian cell containing several em- 48, B), and the pOSl- bryos in various states of development ; /, living substance filling tirm nf fhp rliffprrmf the interior of the horny polypary ; ^, annular constrictions of ent the horny skeleton. (After Van Beneden.) organs hereafter to be developed become recognizable. (251.) Certain cells now begin to make their appearance in the interior of the blastoderm, the arrangement of which is particularly remarkable (fig. 48, c & D, 6) ; these cells arrange themselves in groups of five around the circumference of the blastoderm, and have the ap- pearance of so many crystals : the form of each group is quadrilateral ; EMBBYOLOGY OF CAMPANULARIA GELATINOSA. 101 but subsequently, at each angle there is developed another cell, con- necting the two groups together, and making the whole number to amount to twenty-four. These twenty-four cells will afterwards be- come the tentacles of the polyp. Fig. 48. Embryology of Campanularia gelatinosa. A, an ovarian vesicle from which an embryo is in the act of escaping ; others, in a less advanced state, are seen in the interior. B, a detached embryo in a very early stage, showing the vitellus and blastoderm. C, another embryo, more highly magnified : 6, cells formed around the vitellus. D, the same, more advanced : a, cavity enclosing the remains of the vitellus ' 6, elongated cells subsequently developed into tentacula ; c, other cells, eight in number, which are the rudiments of the organs of the senses. E, the same in a more advanced condition. F, an embryo at the moment of its escape from the ovarian vesicle, magnified : a, fleshy pedicle ; 6, mouth ; d, muscular fasciculus ; e, nervous ganglion ; ff t organs of sense; g, tentacles. G-, the same, as it swims in the water, presenting all the characters of a Medusa. (252.) Cells of another order now make their appearance (fig. 48, D, c), grouped together in pairs, behind the preceding, with similar regularity. These will become organs of sense. (253.) It is difficult to avoid making the comparison between the above appearances and those of crystallization ; the cells, in fact, arrange themselves precisely like crystals, with perfect symmetry, and always in accordance with the number four or its multiple. (254.) The embryo, at this period of its development, presents the shape of a thick lens-like disk ; and shortly, from the centre of its in- 102 HYDROZOA. ferior surface, there is developed a tubercle, destined hereafter to become the body of the polyp ; it is by this part that it will ulti- mately become attached. (255.) The four cells formed between the individual groups above mentioned are, in this stage of the growth of the little being, so com- pressed that they seem to be quite lost ; soon, however, they expand so as to press upon their neighbours, and then the disk appears to be surrounded with a regular series of cellules, twenty-four in number, which, as they become developed, shoot out externally, and soon present themselves under the appearance of so many tubercles (fig. 48, E). (256.) The eight interior cells (c c) take another direction, but their form remains unchanged; and they exhibit, up to the termination of this embryo condition of the animal, a nucleus in the centre of each, which might be regarded either as a crystalline lens or an otolith, according as these organs are judged to be eyes or auditory capsules ; for such are the designations applied to them by modern zootornists, as will be ex- plained in the next chapter. (257.) The marginal tubercles situated around the disk (fig. 48, E) now become sensibly elongated, and the whole embryo presents the appear- ance of a minute star-fish, the elongate tubercles representing the rays. (258.) The nuclei in the interior of the marginal (tentacular) tubercles next become elongated, together with their containing cells, rendering the rays hollow in the centre ; and soon new cells are dis- coverable in their interior, the number of which is limited, and probably the same, in all the rays. The appearance of these secondary cells causes a rapid increase in the length of the tentacula, and their remains give rise to numerous septa, producing an appearance somewhat ana- logous to that of the transverse striae of muscular fibre. (259.) The embryo animal, be it observed, is as yet still contained in the ovary of the polyp; but it is already capable of distinct and con- tinual movements, perceptible through the walls of the ovarian vesicle. (260.) A tubercle (fig. 48, F, 6) is developed from the centre of the under surface of the disk, which represents the central pedicle met with under various forms among the Medusae, and which may be called the proboscidiform appendage. This organ can contract or extend itself in all directions, constantly changing its form, and resembling in no slight degree the body of a Hydra. At an early period an opening is perceptible at the extremity of this appendage, which evidently repre- sents a mouth, being in direct communication with the vitelline cavity. (261.) The vitelline or digestive cavity, now that there is a mouth, increases in size in proportion to the growth of the embryo, but still preserves its sac-like shape. It is partially filled up with irregular granules, which become perceptible at a very early period at first colourless, but gradually becoming of a yellowish tinge. Towards the close of this period of development the granules seem to be heaped EMBEYOLOGY OF CAMPANULARIA GELATINOSA. 103 tog-ether into a mass, from which all the nutritive part appears to have been extracted. This constitutes the meconium. (262.) In some instances, the nutritious fluid that circulates in the interior of the parent polypary may be seen to penetrate as far as the interior of the vitelline cavity of the embryo, which thus seems to derive its nourishment at the expense of the general community ; and when it is taken into consideration that the ova are formed in the common fleshy substance lining the walls of the ovarian vesicle, and that the nutritious fluid is diifused throughout its entire mass, it is easy to understand how, after the (external membrane surrounding the em- bryo is ruptured, it is enabled to penetrate, by means of the mouth, into the interior of its stomachal cavity. (263.) Mention has been made, in the above description ( 256), of cells which give origin to organs of sensation, and which make their appearance at a very early period. These present the same appearance as the eyes and the ears of the lower mollusca and other inferior ani- mals, and moreover present a similar organization, being composed of two spherical vesicles enclosed one within the other. That the young polyp possesses these organs of relation with the external world is undeniable, although no traces of them remain when the animal has acquired its full development ; but what is still more surprising, accord- ing to the researches of Van Beneden, coexistent with these instruments of sense, there are perceptible a muscular system and an apparatus of nerves and nervous ganglia which, like the preceding, are only of a tem- porary character. While the young polyp is still enclosed in its cell, two bands, apparently composed of muscular fibre (fig. 48, P, d), make their appearance ; these run from one margin of the disk to the opposite edge, crossing each other at right angles, in the centre, so as to present a cruciform arrangement. These bands are quite isolated, and their muscular fibres distinct and transparent. By their action the margins of the disk are approximated, enabling these little animals to imitate the movements so characteristic of the Medusae. (264.) Situated upon the course of the bands above described, close to the edge of the vitelline sac, are little rounded bodies (fig. 48, p, e e), presenting an irregular and slightly tuberculated surface, considered by Van Beneden to be nervous ganglia. These little bodies are four in number. No filaments of intercommunication or nervous cords have as yet been detected even proceeding to the organs of sensation, but the ganglia seem to be adherent to the muscular bands apparently by the intermedium of nerves. (265.) It may appear a little rash, says the eminent observer to whom science is so much indebted for these researches, to speak of muscles, nerves, and organs of sensation in the embryo of a polyp, which at a later period presents no traces of the existence of such ap- paratus ; nevertheless the polyp, during its free state, must necessarily 104 HYDEOZOA. require such instruments of sense, to enable it to select a situation adapted to the reception of the new colony to which it gives birth : when once it has made choice of a fit locality, such organs become as useless as they were formerly needful, seeing that all the functions of life are restricted to those of alimentation and reproduction. (266.) The young Campanulariae, arrived at this stage of develop- ment, abandon the ovarian vesicle of the parent polypary and swim freely about in the surrounding medium, exactly resembling so many young Medusae (fig. 48, G). CHAPTER VI. HYDEOZOA (continued). ACALEPHJE (CllV.). (267.) THE ocean, in every climate, swarms with infinite multitudes of animals which, from their minuteness and transparency, are almost as imperceptible to the casual observer as the Infusoria themselves ; their existence, indeed, is only indicated by the phosphorescence of some spe- cies, which being rendered evident on the slightest agitation, illuminates the entire surface of the sea. All, however, are not equally minute, some growing to a large size ; and their forms are familiar to the inhabitants of every beach, upon which, when cast up by the waves, they lie like masses of jelly, melting, as it were, in the sun, incapable of motion, and exhibiting few traces of organization, or indications of that elaborate structure which more careful examination discovers them to possess. Their uncouth appearance has obtained for them various appellations by which they are familiarly known, as Sea-jelly, Sea-blubber, or Jelly- 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 cir- cumstance. They were known to the older naturalists by the title of Urticce marince ; 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 (a/caXr/0?7, a nettle). (268.) 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 com- position 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 an Acaleph, 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 trans- parent cellular matter almost as delicate as a cobweb, which apparently EHIZOSTOMA CUVIEKI. 105 formed all the solid framework 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 col- lected 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 Acalephse, the sea- water col- lected and deposited in the delicate cells of an almost imperceptible film becomes, in some inscrutable manner, instrumental to the exercise of the extraordinary functions with which these creatures are endowed. (269.) The ACALEPBJE 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 his- tory, the reader will be made acquainted with the principal modifications of outward form exhibited by various races of these interesting beings. (270.) PTJLHONIGRADA. The most ordinary examples of the Acalephae found in our climate, when examined in their native element, are seen to be composed of a large ^ 49 mushroom-shaped gelati- nous disk, from the in- ferior surface of which various processes are pen- dent, some serving as tentacula, others for the prehension of food. In Rhizostoma (fig. 49) the central pedicle resembles in structure and function the root of a plant, being destined to absorb nou- rishment from the water in which the creature lives. The body of one of these Medusae is specifi- cally 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 surface, and in some measure to row it from place to place, is the umbrella-shaped expansion or disk, which is seen continually to perform movements of contraction and dila- tation, repeated, at regular intervals, about fifteen times in a minute, hav- ing 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 disk, the Medusa can strike the water with sufficient force to ensure its progression in a certain direction when swimming in smooth water; but of course such efforts are utterly ineffi- Khizostoma Cuvieri. 106 ITYDEOZOA. cient in stemming the course of the waves, at the mercy of which these animals float. The tentacular appendages, situated around the margin of the disk 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 disk, when cut into, seems perfectly homoge- neous in its texture, nor is any fibrous appearance easily recognizable, to which its movements could be attributed ; nevertheless in the larger species its inferior surface appears corrugated, as it were, into minute radiating plicee, which seem to contract more energetically than the other portions, and resemble a rudimentary development of muscular fibre. (271.) In the Acalepha?, indeed, the substance of the body is gene- rally entirely soft and gelatinous, emulating, in the delicacy of its tex- ture and perfect translucency, the structure of the vitreous humour of the eye, its entire organization apparently consisting of a transparent aqueous fluid contained in innumerable polyhedral hyaline cells. In some species, however, certain parts of the animal are of semicartilagi- nous tissue ; and in a few instances cartilaginous or calcareous lamellsa are found imbedded in their substance, which may be compared to a rudimentary polypary or internal skeleton. (272.) Interesting as these creatures 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 circulating fluid to atmospherical influence ; these vital operations 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. (273.) In the Pulmonigrade AcalepTice we have the best illustration of this arrangement : in these, the stomach or digestive cavity is excavated in the centre of the disk, and is supplied with food by a mechanism that differs in different species. In Rhizostoma (fig. 49), 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 * pi%a, a root ; trro/ia, n mouth. CASSIOPEA BOKBONICA. 107 hang from the centre of the disk. Each of these appendages is found to contain ramifying canals, opening at one extremity by numerous minute apertures upon the external surface, whilst at the opposite they are collected into four large trunks communicating with the stomach ; as the Ehizostoma, 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 microscopic 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. (274.) But it is not upon this humble prey that some of the MedusaB feed ; many are enabled, in spite of their apparent helplessness, to seize and devour animals that 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. Incredible as this may seem when we reflect upon the structure of these feeble beings, obser- vation 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 many species speedily paralyses and kills the animals which fall in their way. The mouth of these Acalephas is a simple aperture leading into the gastric cavity, and sometimes sur- rounded with tentacula, that probably assist in introducing the food into the stomach. (2 75 . ) In Cassiopea Borbonica, the principal Fig< **? agents in procuring nou- rishment are numerous retractile suckers (fig. 50, a), terminating in small violet - coloured disks, which are dispersed over the fleshy appendages to the under surface of the body ; the stem of each of these suckers is tubular, and conveys into the sto- mach nutritive materials absorbed from animal substances to which they are attached during the process of imbibing food. (276.) The above examples will suffice to give the reader an idea of Cassiopea Borbonica. 108 HYDEOZOA. Fig. 51. Cassiopea Borbonica, 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 that 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 (fig. 51, 6) is a large cavity placed in the centre of the inferior surface of the disk, and is ap- parently divided into four com- partments by a delicate cruciform membrane arising from its inner walls. Into this receptacle all the materials collected by the ab- sorbing suckers are conveyed through eight large canals, and by the process of digestion be- come reduced to a yellowish semi- fluid pulpy matter constituting the pabulum destined to nourish the whole body. Erom the central stomach sixteen large vessels arise (fig. 51, c), which radiate towards the circumference of the disk, dividing and subdividing into numerous small branches that 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 (fig. 51, e) which runs round 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 inves- tigate 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, conveying 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 loco- motive disk, as obviously perform the part of respiratory organs, inas- much as the fluids permeating 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 disk itself. (277.) The umbrella-like disk of Cyanea aurita, whose anatomy has been most carefully studied by Ehrenberg, is composed of a highly organized gelatinous substance invested by three membranous integu- CYANEA AUEITA. 109 ments, the structure of which is by no means so simple as has been generally imagined. The exterior of these tegumentary membranes, covering the convex surface of the disk, consists of a dense tissue made up of hexagonal cells containing a soft whitish substance mixed up with little granules, and presents upon its outer surface innumerable little suckers or agglomerations of granular bodies, which are visible to the naked eye. (278.) The concave or ventral surface of the disk is furnished with a double investment, consisting of an outer and inner layer, the external of which resembles in its structure the dorsal membrane described above, and constitutes a sort of epidermic covering. The inner layer, which in its intimate texture likewise consists of hexagonal cells, encloses nothing but a number of isolated granules, clear and translu- cent as water. The interspace between this inner layer and the dorsal integument is considerably greater than that which separates it from the ventral surface ; both these spaces, however, are filled up with a clear gelatinous mass, wherein are distinguishable numerous isolated granular bodies, of a rounded shape and of unequal size, that seem to be all connected with each other by fibres or extremely delicate vessels, and not supported by expansions of cellular membrane. The rest of the gelatinous mass is too transparent to allow any organization to be detected ; this, however, is in small proportion, and encloses the large vessels belonging to the nutritive apparatus, immediately to be described. (279.) The opening of the mouth is situated in the centre of the lower surface of the disk, between the four arms suspended from that portion of the body. The mouth itself consists of a short quadrangular tube, from the angles of which the arms are dependent. Each arm is composed of a thick central cartilage, whereunto are attached two membranous laminae, variously plaited and puckered throughout their entire length, and moreover at certain seasons gathered into little pouches or pockets, to be hereafter mentioned in connexion with the generative apparatus. (280.) Superiorly the oral aperture terminates in four short tubes arising from its four angles; and these, diverging, mount upwards, supported by a cartilaginous prolongation derived from the central supports of the arms. These four tubes evidently represent the oeso- phagus and lead into four ample stomachs of a subglobular shape, which are smooth internally and lined by a special membrane, wherein may be seen numerous little granular bodies, but no vessels. (281.) From the above stomachal cavities proceed several large canals that diverge towards the circumference of the disk, and consti- tute a part of the digestive apparatus. One of these vessels arises immediately from the dilated portion of each cesophageal tube ; and these, dividing and subdividing dichotomously, ramify towards the margin of the disk. From each of the four stomachs three other 110 HYDROZOA. large canals take their origin, and run in the same direction ; of these, the two lateral ones are simple and unbranched, but that in the centre ramified dichotomously. These sixteen large vascular trunks, together with all their numerous ramifications, sometimes anastomotically united, ultimately terminate in a wide circular vessel that surrounds the margin of the disk. The nutrient canals are situated beneath the inner membrane, described above, whereby they are partially enclosed and supported. (282.) Before closing our description of the alimentary system of the Pulmonigrade Acalephae, we must mention some accessory organs, of recent discovery, which are in connexion with it. Eschscholtz* de- scribes a series of elongated granular bodies, placed in little depressions around the margin of the disk, which seem to be of a glandular nature, and apparently communicate by means of minute tubes with the nu- trient 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. (283.) Prior to the publication of Ehrenberg's important researches relative to the anatomy of the Cyanea aurita't, it was generally believed that in the Pulmonigrade Medusae the alimentary canals were unprovided with any excrementitious orifices ; these, however, were discovered by the illustrious Prussian observer, occupying the situations indicated by eight dark-brown-coloured spots situated at equal dis- tances around the margin of the disk, and which had previously been suspected to be the analogues of a biliary organ. By keeping the living Medusae for some time in sea-water deeply coloured with indigo, and thus causing all the ramifications of the alimentary apparatus to become filled with the coloured fluid, while the rest of the body re- mained transparent and colourless, it appeared that, opposite each of the above-mentioned spots, the circular marginal canal into which the nutritive tubes, radiating from the stomach, empty themselves becomes dilated into a sort of cloacal cavity, in which the debris of digested materials, such as the shells of minute Conchifera, Rotifera, Bacil- laria, &c., were easily distinguishable ; from each of these cloacal dila- tations, canals can readily be traced communicating with the exterior ; and on irritating the living animal, it is easy to witness the discharge of excrementitious matter through the eight marginal orifices of the disk. (284.) A distinct movement is frequently perceptible in the interior * System der Acalephen. Berlin, 1822. Annalesdes Sciences Naturelles,vol.xxviii. p. 251. f Abhandl. der Konigl. Akad. der Wissenschaften zu Berlin, 1835. SENSES OF THE MEDUSAE. Ill of the ramifying alimentary tubes, which has been mistaken for a circulation, but which is merely the effect of ciliary action, or of peri- staltic movement in the walls of the intestine. (285.) Up to the period when Ehrenberg made the important re- searches we are laying before the reader, relative to the anatomy of these creatures, it was impossible to account for the capability of loco- motion which the Pulmonigrade Acalephse evidently possess, but which his researches serve to render perfectly intelligible. The canals formed by the ramifications of the alimentary apparatus he observed to be all bordered by two delicate lines of a pale red colour, which, under the microscope, are evidently of a muscular character; by the contractions of these, therefore, the most important movements of the animal are accomplished. Besides the above, however, other muscles are discernible. In Cyanea the disk is surrounded with a fringe of tentacula, each of which exhibits at its base a muscular structure ; consequently the possession of muscular fibre is evidently established as a part of the economy of these animals. (286.) It is very probable that the older writers, who speak of a circulation of blood in the Medusae, only alluded to the movements ob- servable in the contents of the intestinal ramifications; it appears, however, from Ehrenberg's observations, that in the MedusaB there exist distinct globules, which are of a uniform round shape, enclosed in distinct vessels, wherein a kind of circulation is carried on : these glo- bules he describes as being colourless, spherical, simple, and varying from yirg-tk ^ lTFo^ n ^ a ^ ne *- diameter. (287.) Although the Medusae have always been admitted to possess considerable sensibility, no traces of a nervous system had been de- tected in their soft and delicate tissues until Ehrenberg pointed out a structure apparently of a nervous character. On carefully examining the eight brown-coloured spots which are disposed at equal distances around the margin of the disk (fig. 48, F, /), he found them to present a very elaborate and remarkable organization. Each of these coloured spots is seen, when accurately observed, to be composed of a little button-like appendage, of an oval or cylindrical shape, attached to the extremity of a slender pedicle, which in turn takes its origin from a kind of vesicle, wherein may be perceived, by means of the microscope, a glandular-looking substance. On the dorsal aspect of each of the pedunculated brown -coloured appendages is situated a distinctly marked round spot of a bright red colour, supposed by Ehrenberg to be an ocular organ, while he considers the glandular-looking substance above mentioned to constitute a nervous ganglion. In addition to this ar- rangement, he considers that there exist, running all along the margin of the disk, in each of the interspaces between the marginal tentacles, a series of ganglia of a similar character, giving off nerves to the ten- tacula, whilst other ganglia are to be detected in the tentacular append- 112 HYDEOZOA. ages situated in the vicinity of the oesophagus, as well as in the oviferous cavities. In short, he states the general distribution of the nervous matter in the Medusiform Acalephse to be as follows : Four groups of nervous ganglia are situated around the oesophagus in the oviferous cavities close to the ovaria, which are in communication with as many groups of tentacula. Upon the outer border of the disk, close to the base of the marginal tentacles, is another series of nervous nodules, interrupted at regular intervals by the eight brown- coloured corpuscles. Lastly, there exists a series of isolated ganglionic masses, eight in num- ber, situated at the bases of the supposed ocular organs, to which they give off nervous filaments. (288.) The so-called ocular organs, named by Ehrenberg unhesitatingly ft pedunculated eyes," present a very remarkable structure. Each " pe- dunculated eye " is directed towards the dorsal aspect of the disk, and has, situated beneath its lower surface, a minute sacculus of a yellowish colour, but variable in its shape, wherein is contained a number of solid crystals, clear as water, and which the action of acids proves to be com- posed of carbonate of lime. (289.) Not only eyes, however, but ears also are conceded by modern naturalists to these favoured occupants of the ocean. (290.) At the base of the marginal tentacula, or cirri, says Pro- fessor Forbes*, there are present, in a great many of these animals, coloured spots or bulbs ; and in some species these points are so strongly coloured, that, from this circumstance and their magnitude, they indi- cate the course of the animal when in motion, appearing like a circle of gems in the water. When these bulbs are examined with the mi- croscope, they are found to contain a small cavity, quite distinct from any coloured matter that may be present ; the former is regarded by modern naturalists as an otolithic vesicle) the latter as an ocellus, or eye-spot. (291.) The otolithic vesicle, which, from analogy and its peculiar structure, is considered as an organ of hearing, is a small spherical sac, developed in the midst of the granular substance of the bulb, and con- taining more or fewer minute vibrating bodies. Will has described the otolithic vessel and its contents, as they are found in Geryonia, as follows : " The auditory vesicles are seated in the course of the mar- ginal circular vessel, in very uncertain number ; usually, however, one at each side of the larger marginal cirri. They are round, measuring T \jth of a line in diameter, and consist of a tolerably thick membrane ; they contain from one to nine, and even more, round globules. If there is only one, it is situated exactly in the centre of the vesicle ; but if there are several, they are found lying together, either in two groups, or joined to each other. I have never observed them move. Muriatic acid dissolves them, and causes the vesicle to burst." The existence of similarly- * Monograph on the British Naked-eyed Medusae. PROPAGATION OF MEDUSA. 113 Lizzia octopunctata. constructed organs has been recognized in many other species by various observers. (292.) It was discovered by Sars*, that Fig. 52. certain forms of naked- eyed Medusae multiply their species by means of gemmation, the buds being produced either from the walls of the peduncle or stomachal proboscis, or from the surface of the ovaries. In both cases the new individuals were not different from, but similar to, their parents; and in one in- stance, provision seemed to be already made in the newly-formed offshoots for continuing to propagate by the same mode other indivi- duals similar to themselves. Prom a certain part of the body roundish knobs grow forth, which gradually assume the shape of a bell, by opening themselves at the free end, and soon present the form of young Acalephs, being merely attached to the mother by means of a short peduncle, derived from the back of the disk. These develope in themselves all essential organs whilst still attached to the mother, like the buds of a plant, until at length, after a certain time, they separate from the parent and swim about as inde- pendent individuals. (293.) Professor Forbes, in his admirable monograph upon the British Naked-eyed Medusae, not only confirms the above important observa- tion of the Norwegian naturalist, but describes four different modes of gemmiparous reproduction as occurring in that group of the Acalephae. 1st, gemmation from the ovaries, as noticed by Sars in Thaumantias multicirrata ; 2nd, a mode of gemmation from the pedunculated sto- mach, which he calls subsymmetrical, because in this case four gemmae are symmetrically arranged round the peduncle, one of which is con- stantly in a more advanced condition of development than the other three ; 3rd, gemmation irregularly from the walls of a tubular pro- boscis in which there is no order of development with respect to position, individuals springing indifferently from various parts of the peduncle (fig. 52) ; and a fourth mode, which is very remarkable, in a new British species named Sarsia prolifera, in which the buds are produced at the bases or tubercles of the four marginal tentacles, and hang from them in bunches like grapes. The degree of development is not equal in all four bunches, and in each case buds are seen in very various stages of advancement, from embryo wart-like sproutings to miniature Medusae, simulating, in their essential characters, the parent animal. (294.) We have already seen, at the close of the last chapter, that * Fauna Norvegica. 1 114 HYDEOZOA. the progeny of the Hydriform polyps, during one phasis of their develop- ment, were strictly medusoid in their form and organization ; and in like manner it is now incontrovertibly established that the Acalephce are, at a certain stage of their growth, to all intents and purposes Hydriform polyps, as will be immediately evident. (295.) The Acalephs are now universally admitted to be bisexual ; and the generative apparatus in both sexes is invariably found to be more or less intimately in relation with the alimentary canal : that is to say, as in the case of the polyps, the reproductive organs are append- ages derived from the internal or nutritive system of the body. In both the males and females of the great majority of genera, the testes of the former and the ovaria of the latter are similarly disposed, and present externally precisely the same structure, consisting of duplica- tures of a delicate membrane, between which, in the case of the female, ova are developed in great numbers, generally of a rich orange or purple colour, so as to be conspicuously visible. In the male Acaleph, instead of ova, the generative membrane secretes a vivifying fluid, rich in sperma- tozoa, and consequently easily recognizable under the microscope. (296.) In Cyanea aurita the generative apparatus of the female con- sists of four membranous ovaria, easily recognizable on account of their bright colour, which is usually violet, or deep yellow. Their form is generally semicircular (fig. 53), and they are lodged in as many distinct cavities, situated in the immediate vicinity of the central stomachs. Each of these cavities com- municates freely with the external element by means of a large round or oval orifice, furnished internally with tentacula having suckers at their extremities (fig. 53, d.) The four semicircular ovaries are each com- posed of a simple con- torted tube (fig. 53, a, b) : when full of eggs, its colour is a beautiful vio- let ; but when empty, or when the ova are only partially developed, a yellowish brown. (297.) The ova are not retained in the ovaria during the whole time 1. Ovarium of Cyanea aurita. 2. Ciliated embryo after its escape. EMBRYOLOGY OF CYANEA AURITA. 115 of their development, neither do they remain in the ovigerous cavity, but escape from the orifice of the latter into the surrounding water, from whence they are again taken up by the tentacula and by the two laminae of the arms, and become lodged in little pouches formed by the laminated margin, in which they undergo further metamorphosis and development. These ovigerous pouches are only met with at certain seasons, disappearing when their functions are accomplished. (298.) The eggs are of a rounded form and covered with a smooth, thin, membranous envelope whilst they remain in the ovary ; internally they are filled with a finely- granular mass of a violet hue. (299.) The ova contained in the arm-sacculi are destitute of any shell, and present themselves under three distinct forms, which are very remarkable. Some resemble blackberries, and are of a pale violet hue ; others have the shape of minute thick disks, likewise violet, resembling little Medusae deprived of arms and without any nutrient canals ; lastly, others are met with (and these latter are the most numerous) which have a cylindrical shape, truncated at both ends, and of a brownish- yellow colour. The two last-mentioned forms are densely covered with cilia, and swim about with facility ; the largest among them measure about -Jth of a line in diameter. (300.) Subsequently the ciliated embryos, escaping from their con- finement, detach themselves from the cradles wherein they have been nursed up to this period, and swim freely about in the surrounding water until ripe for a further change in their economy; they then settle down upon some foreign object, such as a piece of sea- weed, to which they attach themselves by one extremity (fig. 54, 3), assuming the appearance of a contracted Hydra, but, as yet, unprovided either with mouth or tentacula; gradually, however, an oral aperture and stomachal cavity, surrounded by tentacular organs, become apparent ; and as these progressively increase in number (fig. 54, 4, 5, 6, 11), the little creature assumes completely the polyp form, and, what is still more wonderful, acquires in this early and, as it might be called, larva- condition of its existence the power of multiplying itself under the same shape, apparently ad infinitum. (301.) This kind of reproduction is effected by the development of stolons, gemmce, and lulblets from any portion of the surface of the polypoid animal, which in turn give origin to similar offsets (fig. 54, 12, 13, 14), precisely resembling, when mature, the original polypoid body. (302.) The next phasis in the development of these Acalephs is one of the most remarkable circumstances connected with their history, and, were it not for the accumulated testimony of numerous observers, might appear almost incredible. The polyp, much in the condition represented at fig. 54, 11, is immovably fixed by its basis to the surface of a Eucus, or some similar support ; in length it is about -1-th, and in diameter J^-th of an inch ; its surface is smooth, and its texture alto- i2 116 HYDKOZOA. gether gelatinous ; its tentacles are movable in all directions and ex- ceedingly irritable, and its whole structure and appearance, in short, that of a gelatinous polyp or Hydra. But a great change is now in pre- paration : the body of the Hydriform polyp gradually increases in size ; and transverse folds begin to make their appearance at equal distances, one below the other, partitioning off its body into numerous rings or segments (fig. 54, 15). Fig. 54. Development of fyanea capillata (after Sara, ' Annalee des Sciences Naturelles ' for 1841, plates 15 B, 16, & 17, pp. 19, 50). 1. Young Acalephs newly hatched (natural size). 2. One magnified, showing infusorial condition of development. 3, 4, 5, 6. The same animal now become attached by a pedicle, and gradually assuming the polypoid form. 7. A still more advanced condition, showing the mouth surrounded by numerous retracted tentacula : the mouth is dilated, exhibit- ing four longitudinal eminences, situated in the stomachal cavity. 8. The same individual cut open longitudinally, and spread out so as to show the longitudinal eminences in the interior : the transverse lines are caused by the contraction of the body. 9, 10. Two polypoid Acalephs, with stolons developed from the upper part of the body : in fig. 10 the stolon has become attached to the supporting surface. 11. Fully-developed polyp. 12. Another individual giving off a stolon, from which proceeds a second that in like manner gives off a third offset. 13. Stolons growing off from the base of the polypoid Medusa, which, creeping along the surface of the substance to which it is attached, give origin to new polyps, a, b. 14. Three young gemmae sprouting from the body of a polypoid Acaleph. 15. A polypoid larva magnified (the natural size is shown at 15 a), having its body divided by numerous transverse wrinkles. (303.) In the course of a short time the segments thus formed be- come surrounded with marginal rays dichotomously divided at their extremities. These rays or arms are free, having their apices directed upwards, and disposed with such regularity that the once polypoid body seems to be furnished with eight longitudinal ribs (fig. 55, 16). (304.) "We now arrive at the fourth period of the process, when the different segments into which the original polyp has become divided separate from each other, so as to form so many distinct disks (Planulce, Dalyell), each of which on its separation becomes a complete animal. This separation commences at the upper extremity of the series of newly-formed beings, and is repeated, segment after segment, towards DEVELOPMENT OF CYANEA CAPILLATA. 117 the base, each segment as it becomes detached presenting the form, characters, and attributes of a free Acaleph, and in this condition as- suming an independent existence, under the appearance represented in fig. 55, 17, a. Fig. 55. ir* Transformation from the polypoid form to the third, or Acaleph, condition (after Sars). 16. The polypoid larva (16 a, natural size) in a more advanced state, now divided into segments piled upon each other, each of which is a young Medusa, having its disk surrounded with radiating processes bifurcated at their extremities. These segments becoming detached one by one from the summit of the pile successively, assume the medusiform condition. 17. Another example, in progress of division, in which only four segments remain undetached, and of these the three uppermost are at the point of separation. 17 a. A segment of the preceding detached, now become a free Acaleph : it is represented as seen from below, and already exhibits in its centre the square oral orifice, round which are perceptible rudimentary tentacula, together with the radiating nutritive canals, &c. 18. The same, in a still more advanced stage, exhibiting the rudiments of marginal tentacula. 19. The same, arrived at its perfect form, furnished with its four buccal arms, now completely di- vided and pendent, and fully provided with the marginal tentacles of the adult. (305.) The now free Acaleph, the disk of which is not as yet much more than i-th of an inch in diameter, exhibits, when magnified, the characteristic organization of a true Medusa, the oral orifice (fig. 56, a), the positions of the ovaria 6, the radiating nutritive canals, c, the circular marginal vessels, d, the oculiform points, e, the anal aper- tures, /, and the rudimentary tentacula surrounding the disk, g g, being all easily recognizable. The Medusa being thus far complete, its further advance is rapid, the rays become gradually shorter in propor- tion to the disk, the marginal tentacles are more and more developed, and at length the young Acaleph, complete in all its parts (fig. 55, 19), will in time, by the production of multitudinous ova, give birth to another generation, destined, during their development, to exhibit a parallel series of changes. (306.) In some of the Medusa3 which are destitute of a central pe- dicle, such as Cuvieria carisochroma (fig. 57), the arrangement, both of the alimentary and generative apparatus, is considerably modified. In ^Equorea violacea, examined by Milne-Edwards*, the gastric cavity, which is very large, occupies nearly a third of the whole diameter of * Ann. des Sci. Nat. for 1841. 118 HYDEOZOA. Fig. 56. the disk ; the oral aperture, instead of being pedunculated, is simply surrounded with a mem- branous border, hanging loosely down when in a state of repose, and so short as to be quite in- adequate to close the opening of the mouth. Superiorly this membra- nous border is attached to a ring, slightly more callous in its structure than the rest of the body, immediately above which is a circle of tolerably wide orifices placed very close to each other, all of which lead into radi- ating canals that diverge towards the circumfe- rence of the disk. These canals, seventy-four in number, becoming narrower as they recede from the stomachal cavity, run in straight lines to terminate in a circular vessel that surrounds the disk near its margin, from which little canals are given off, apparently ana- logous to the emunctory vessels above described as existing in Medusa aurita. (307.) The generative system in JEquorea is in relation with this arrangement of the nutritive canals. Arising from the under surface of the disk there are numerous membranous lamellse disposed in a radiating man- ner around the gastric cavity. These Iamella3 seem to be suspended from beneath each radiating canal; con- sequently they are seventy-four in number; and being much folded upon themselves, each has the appearance of being formed of a double mem- brane. In their interior they exhibit Young Medusa (after Sars). a, the mouth, surrounded with the as yet undeveloped buccal arms ; 6666, ovaria, or testes ; c, radiating nutritive canals ; d, marginal circle of nu- trient vessels ; e, oculiform organs ; fff, anal apertures situ- ated on the margin of the disk. Fig. 57. 1, 2. Cuvieria carisochroma. numerous striae of a violet hue, which the microscope shows to con- CYDIPPE PILEUS. 119 stitute the generative apparatus. In some individuals these folded membranes enclose the ova, and in others contain a fluid filled with spermatozoa ; so that they evidently represent the ovaria and testes of these Acalephs. (308.) CILIOGEADA. In the Ciliograde Acalephce (CTEJ^OPHOEA), the organs of motion consist of narrow bands of vibratile cilia, variously disposed upon the surface of the animal. (309.) In the globular forms of the Beroes (fig. 58), these cilia are Fig. 58. 1. Cydippe pileus : a, tentacula unfolded. 2. Supposed nervous system. 3, 4. Isolated cilia. arranged in eight longitudinal rows, and appear to be attached to sub- jacent 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 afterwards close over and completely conceal them ; their mo- tion is extremely rapid, and sometimes only recognizable by the currents they produce, or by the iridescent hues that play along the arches. (310.) The arrangement of the locomotive apparatus appended to the eight longitudinal costal bands is extremely beautiful. The series of vibratile fringes is attached to a row of minute transverse ridges, disposed almost like the steps of a ladder, and, moreover, in their essen- tial structure they differ very materially from vibratile cilia of the ordinary character. In shape they are not filiform, but resemble membranous laminae deeply fringed around their free margin, having the shape of so many little semi-oval palletts. The movements of these flabelliform appendages are very rapid, and are seldom interrupted while the animal is in vigorous health ; the slightest contact, however, is suffi- cient to stop them. The different laminae, moreover, belonging to the same row are quite independent of each other ; neither does interference with one produce the slightest effect upon the action of the rest. The animal, nevertheless, seems to possess the power of arresting or con- trolling their motions at pleasure. It is likewise remarkable that the vibratory movement is kept up for a very long time in fragments sepa- rated from the rest of the body, without at all changing its character ; 120 HYDEOZOA. but it may be observed that, in portions thus detached, the sensibility appears to be destroyed before the contractile power, inasmuch as, after a certain time, the vibration is kept up unintermittingly in spite of such contact as would previously have caused a suspension of vibratory action. (311.) The cilia, which are placed on the longitudinal ridges, are linear-lanceolate in form, flat, and not hollow. They are not webbed together, and have no communication with the vessels that run beneath the ciliary ridges. Each row of cilia is mounted on a transverse base,' of a more solid texture and less transparent than the rest of the body . The substance of this base consists of globules irregularly imbedded in a homogeneous substance*. When one of the cilia of a Cydippe is cut off, it has of itself no power of motion ; but if the smallest portion of the substance of its base remains attached, it moves with great vivacity. Hence it is concluded that the ciliary motion is effected by undulatory movements of this peculiar tissue. (312.) In the Beroeform Acalephae, it would seem that the vital principle was equally diffused throughout every part of their fragile substance, which the slightest violence is sufficient to break up into pieces ; indeed it is not uncommon to find the surface of the sea covered with fragments of their bodies, on which the locomotive cilia may still be seen in rapid action, producing, by their decomposition of the light, a splendid iridescent appearance. (313.) The capacious cavity that occupies almost the entire length of the body of the Beroe, and communicates freely with the exterior through the inferior orifice, is perfectly smooth internally, and consti- tutes a kind of wide pharyngeal sac, at the bottom of which is situated a transverse aperture guarded by two thickened lips, the texture of which is firmer than that of the rest of the body. These lips only come in contact with each other near the centre of their free margins, and consequently leave on each side a gaping orifice. The cavity that they thus partially close is very small, and evidently corresponds with the cen- tral stomach of the discophorous Medusae, and in like manner constitutes a central reservoir, from whence the vascular system is derived. (314.) The digestive receptacle is filled with a fluid that is continually in movement, and which may be seen to pass into two lateral tubes that soon each divide into four branches and, arriving at the surface of the body, terminate in eight longitudinal canals that convey the con- tained fluid to the cilia, which latter organs, as they are in constant vibration, appear to perform the functions of a respiratory apparatus. From the lateral parietes of each of the eight longitudinal costal canals there arise an infinite number of small vessels or transverse sinuses ; these, after intercommunicating with each other, are lost in the sur- rounding parenchyma*. Arrived at the margin of the wide opening * Milne-Edwards, Ann. des Sei. Nat. torn, xvi., 1841. BEEOE FOESKAHLII. 121 situated at the inferior extremity of the body, the eight longitudinal trunks terminate in a transverse annular canal that communicates in its turn with two vertical trunks much more deeply seated than the preceding vessels : these lateral vessels, mounting upwards, terminate in the stomachal cavity. (315.) The vascular apparatus above described is filled with a fluid in constant circulation, in which may be perceived innumerable round colourless globules. The course of the current is directed from the inferior vascular ring through the eight superficial canals situated beneath the ciliated ribs towards the summit of the body, whence it subsequently descends in a contrary direction through the two deep- seated trunks above described into the annular vessel, thus completing the circulatory round. The movement of the circulating fluid is tole- rably rapid ; nevertheless no traces appear of any central organ of im- pulsion, neither do the vessels exhibit the slightest contractility ; in some of -the larger trunks, however, the presence of cilia is distinctly per- ceptible, by the agency of which the circulatory current is produced. (316.) From the researches of Milne -Edwards, it appears that the vascular system of the Beroeform Acalephs communicates with the exterior by means of emunctory canals analogous to the anal tubes situated on the margin of the disk in Medusa aurita, described above. In Beroe ForsJcahlii, Milne-Edwards was enabled to assure himself of the existence of two such outlets, situated not at the inferior margin of the body, as in other Acalephs, but at its upper extremity. When this portion of the animal is fully extended, it frequently occurs that a little ampulla suddenly makes its appearance on one side or the other of the terminal fossa, which, quickly increasing in size, exhibits in its interior movements of rapid rotation ; then, suddenly opening at its summit, it discharges its contents and immediately disappears, leaving no traces of its excretory functions except a minute pore, which is easily distin- guishable. These excretory ampulla communicate with the gastric cavity that forms the central reservoir of the vascular apparatus, and are evidently emunctories through which feculent matters are expelled. (317.) The body of the Beroes has generally been described as having the form of a bag open at both ends, a mistake which is explicable from the circumstance that, when the animal is not completely unfolded, its superior extremity is retracted and puckered up in such a manner as to give the appearance of a wide orifice placed opposite to that which occupies the inferior extremity : this appearance, however, is deceptive ; for if one of these Acalephs is carefully examined while swimming freely in its native element, it becomes evident that the supposed upper orifice is only a deep cavity the bottom of which is furnished with a delicate contractile arborescent fringe, in the centre of which is situated a little pyriform papilla, regarded as constituting an ocular apparatus. (318.) This oculiform speck, which is situated immediately in the axis 122 HYDKOZOA. of the body, presents, at its base, a globular spot of a red colour and granular appearance, in which are contained numerous minute crystal- line corpuscles. The whole apparatus is immediately connected with a minute rounded mass, apparently of a ganglionic character, from which, in some genera, filaments are distinctly seen to issue. (319.) In Lesueuria, for example, on carefully examining the bottom of the wide excavation that exists at the anterior extremity of the egg- shaped animal, four mammillated processes are apparent, each occupy- ing the median line of one of the four principal lobes ; and in the midst of these is seen an oculiform tubercle, situated precisely in the axis of the body, which is remarkable for its bright red colour. It is of a spherical shape, and presents a granular surface similar to that of the brilliant red spots distributed around the margin of the disk in the Medusae, which Ehrenberg designates the eyes. Immediately beneath the oculiform spot is situated a subpyriform body, which is apparently of a ganglionic nature ; its substance is more opake than that of the neighbouring tissues, and from it proceed a great number of filaments, apparently of a nervous character. These form four fasciculi, which run obliquely downwards towards the inferior and external margin of the principal lobes of the body : some very delicate filaments appear to terminate near the base of the accessory lobes; but the greater number are continued as far as the row of filiform appendages situated near the margins of the principal lobes, many of them apparently giving off ramifications in their course. Moreover, besides the above, a small lon- gitudinal filament may be traced along the middle of each of the ciliated zones, probably of a nervous character, and which give origin to a mul- titude of little filaments that are distributed in a very regular manner, in fasciculi, beneath each of the little transverse ridges whereupon the vibratile fringes are attached, as well as to the mid-spaces intervening between them : it would even seem that there is a little ganglion at the origin of each of these ciliary branches; but whether this be the case or not is doubtful. At the upper extremity of the body, the vertical or ciliary filaments are prolonged beyond the ciliated ridges, and becoming united in pairs, run towards the central ganglion situated beneath the oculiform spot, with which, in all probability, they communicate. (320.) From the above description it will be evident that the nervous system of Lesueuria differs widely in its arrangement from that supposed by Dr. Grant to exist in Cydippe*, resembling more the arrangement of the nerves in the Tunicated Mollusca, with which the Beroida3 pre- sent many natural affinities. (321.) The arrangement of the generative system in the Beroeform AcalephaB is very imperfectly understood ; or perhaps we ought rather to say that nothing is satisfactorily known concerning this part of their * Dr. Grant's figure of the nervous system as he supposed it to be arranged in Cydippe pilctts, is given in a preceding page, fig. 57, 2. CESTTJM VENEEIS. 123 economy. M. Delle Chiaje* states that, upon the inner surface of each of the eight ciliated ribs, there is discoverable a longitudinal oviduct, to both sides of which are appended bunches of ovules, an observation the accuracy of which is doubted by Milne-Edwards, who finds, indeed, on each side of the ciliated bands a multitude of little racemose bunches, of a rose colour, having the appearance of ovaria, but to whom it seemed that these bunches were contained in the substance of the walls of the body, and were simply dilatations of the lining membranes of the sub- ciliary vascular canals, which, instead of communicating with a common oviduct, opened into the vessels themselves. (322.) From the researches of Willf, it would appear that these Acalephs are hermaphrodite, the generative apparatus consisting of elongated utricles, the testes being situated on one side and the ovaria on the other. Both sets of organs are described as having a nodulated appearance, and from the nodulated part of each passes off an excretory duct, which runs towards the mouth ; but the terminal openings of these canals have not been made out. In Professor Grant's description of Cydippe pileus, of which a figure is given above (fig. 58, l), the ovaries are said to consist of two lengthened clusters of small spherical gemmules of a lively crimson-red colour, extending along the sides of the alimen- tary canal. It is evident, therefore, that further knowledge relative to this department of the economy of the Beroes is still a desideratum in science. (323.) The Cesium Veneris (fig. 59) is nearly allied to the Beroe in the arrangement of its nutritive apparatus, notwithstanding the differ- Fig. 59. ence of form observable in these Ciliograde Medusae. In Cesium, the digestive cavity, which is exceedingly short in comparison with the * Mem. sulla steria e anatomia degli Animali senza Vertebre, torn. iv. p. 12. t Horre tergest. p. 38. tab. 1. figs. 22, 23. 124 HYDROZOA. Fig. 60. length of the animal, passes transversely across the body in a straight line from one side to the other, as represented in the engraving ; but the details of its structure, and the nature of the vessels arising from it, will be best understood by a reference to the enlarged diagram of these parts given in the next figure (fig. 60). The mouth, i, is a rhom- boidal 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, j j, which terminate in a globular cavity common to both (these would seem to constitute the digestive apparatus) ; and a straight narrow tube, o, prolonged to the opposite margin of the body to that which the mouth occupies, may be regarded as an intestine through which the residue of di- gestion is discharged. From around the oral extremity of the stomach, and from the glo- bular cavity in which the two principal canals terminate, arise vessels, t 1 1, 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 precisely correspond with the vascular rings already described in the JSeroe; and, from these, four long vessels, or branchial arteries as they might be termed, p p, q q, are pro- longed beneath the four ciliated margins all around the body. But besides these four nutritive vessels, two others, x x, arise from ^fCAWAV^ the anal ring, which run inwards towards Alimentary apparatus of ce*tum. the centre of the animal, and afterwards assuming a longitudinal direction, serve to distribute nourishment to the median portions of the animal. The casca, or blind tubes, n n, appended to the intestine, may possibly furnish some secretion useful in digestion, although perhaps we are scarcely warranted in saying decidedly that they are biliary organs*. (324.) Extraordinary as must appear the powers which the AcalephaD possess of seizing and dissolving other creatures apparently so dispro- portioned to their strength and the delicate tissues which compose their substance, 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 abundant quantity of glairy matter, which, exuding from the surface of its body, becomes diffused * Delle Chiaje, Memorie per servire alia storia degli Animali scnza Vertebre del regno di Napoli. 4to, 1 823- 1 825. PHYSOaKADA. 125 through the element around it so copiously, that it is difficult to con- ceive whence materials can be derived from which it can be elabo- rated. 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. (325.) We are equally at a loss to account for the production of the irritating secretion, in which the power of stinging seems to reside ; but it is observed that the tentacula seem to be more specially imbued with it than other parts of the body. Perhaps the most remarkable property of the Acalephae is their phosphorescence, to which the luminosity of the ocean an appearance especially beautiful in warm climates is principally due. We have more than once witnessed this phenomenon in the Mediterranean ; and the contemplation of it is well calculated to impress the mind with a consciousness of the profusion of living beings existing around us. The light is not constant, but only emitted when agitation of any kind disturbs the microscopic Medusae which crowd the surface of the ocean : a passing 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 dia- monds ; 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 Acalepha3, scarcely perceptible without the assistance of a microscope. All, however, are not equally minute : the Beroes, in which the cilia would seem to be most vividly phosphorescent, are of considerable size ; the Cestum Ve- neris, as it glides rapidly along, has the appearance of an undulating riband of flame several feet in length ; and many of the larger Pulmoni- grade forms shine with such dazzling brightness, 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 phosphorescent secretion ; but its nature and origin are quite unknown. (326.) PHYSOGKADA. In the third division of Acalephse, denominated by Cuvier " Acalephes Hydrostatiques," the body is supported in the water by a very peculiar organ, or set of organs, provided for the pur- pose. 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 ap- proach of danger. In Physalia (fig. 61), known to sailors by the name of the " Portuguese man-of-war," the swimming-bladder is single, and of great proportionate size, so that when full of air it is exceedingly buoyant, and floats conspicuously upon the waves. The top of this 126 HYDEOZOA. bladder bears a crest, c, of a beautiful purple colour, that, presenting a broad surface to the wind, acts as a sail, by the assistance of which the creature scuds along with some ra- pidity. The air-bladder is endowed with a considerable power of con- traction, and when carefully ex- amined, two orifices are observable, one at each extremity, , 6, through which, upon pressure, the contained air readily escapes a provision for enabling 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 dis- tends its float has not been accurately determined ; but it is undoubtedly a secretion furnished at pleasure when at a considerable distance from the surface, although the mode of its pro- duction is still unknown. (327.) CIKEIGEADA. The Cirri- grade Acalephaa form a very remark- able family, peculiarly distinguished by the possession of an internal solid support, or skeleton, secreted in the substance of their soft and deli- cate bodies. In Porpita (fig. 62), this consists of a flat plate of semicar- tilaginous texture (fig. 62, 2), evidently depo- sited in thin secondary laminae, which gradually increase in size as the animal advances in growth, the inferior be- ing the largest and last formed. When examined after its removal from the body, this fragile skeleton is seen to be extremely porous or cel- lular ; and the pores being filled with air, it is specifically lighter than water a circumstance that may contribute to the buoyancy of the creature even when alive. (328.) The lower surface of Porpita is furnished with numerous appendages called cirri, whereof some appear to be organs of prehension, Physalia. a b, vesicular float ; c, crest ; d, orifice ; e, nucleus; //, inferior append- ages. Porpita. CIKRIOKADA. DIPHYEA. 127 but perform also the office of oars, which, in this species, are the prin- cipal agents in progression; yet in other Cirrigrada, as Velella and Rataria, besides the horizontal lamella that 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, that 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, ap- parently of a muscular nature, by the contractions of which the sail can be depressed or elevated at pleasure. (329.) DIPHTEA. The last family of the Acalephae derives its name from the singular appearance of the creatures composing it ; each ani- mal, in fact, seems to consist of two portions (a, 6, fig. 63, 1, 2) so Fig. 63. Diphyes Bory. slightly joined together, that it is by no means easy to understand the nature of the connexion between them. (330.) The body of these strangely-organized beings is composed of two polygonal, subcartilaginous, transparent pieces placed one behind the other, the posterior division being implanted more or less deeply into the anterior. These two divisions are invariably more or less dissimilar from each other ; nevertheless they oifer this circumstance in common, that they are excavated internally by a deep cavity, which opens exter- nally with a wide orifice of regular shape, although differing in form in each division. To these details of their general appearance must be added the existence of a long filiform appendage, which issues from the upper cavity of the anterior cartilaginous portion, and which was regarded by Cuvier as the ovary. (331.) On more minute examination, there is recognizable in the anterior division a visceral mass called the nucleus, which is made up of a proboscidiform oesophagus, terminated by a sucker-like mouth, and continuous with a stomachal cavity, whereunto are appended hepatic fol- licles of a greenish colour, and sometimes a little vesicle filled with air. (332.) Besides the above structures there may be remarked, towards the lower part of the body, another glandular-looking mass, probably 128 HELMINTHOZOA. the ovary, which is connected with the long (ovigerous ?) filament above alluded to. The nucleus is contained in a proper cavity, generally distinct from the large excavation that forms the locomotive apparatus, and is connected by filaments, apparently of a vascular character, to the soft parts within the body. It has been already remarked that this latter division is excavated by a large cavity that extends nearly through- out its entire length ; from the bottom of this cavity arises a prolonga- tion, probably of a vascular character, which embraces the root of the (ovigerous ?) filament, and is apparently connected with the nucleus, from which, however, it may be detached by the slightest effort. (333.) The bodies of these strangely -constructed creatures are so extremely transparent, that their presence is discoverable with great difficulty even in small quantities of sea-water. They are generally met with at a great distance from land, abounding more especially in the seas of tropical climates. They swim with great facility, their an- terior or nuclear extremity being directed foremost ; while the water taken into their bodies, being forcibly ejected, by the contractions of their subcartilaginous parietes, through the wide apertures opening backwards, propels them through their native element. (334.) Whilst exercising this mode of locomotion, the long slender filament above alluded to is extended behind, being partially lodged in a groove excavated in the posterior division of the natatory organ. It varies considerably in length, being highly contractile, so much so, in- deed, that it is sometimes completely withdrawn into the body ; and its structure is further remarkable from the circumstance that through- out its whole length it is furnished at regular intervals with minute suckers*. But the true nature of this organ is very imperfectly known ; most probably it will be found to be analogous in its real character to the proligerous apparatus of the Salpce, to be described hereafter ; indeed, such is the evident relationship between the Diphyea and the Salpoid Tunicata, that it is very doubtful whether they ought not to be classed as members of that group. CHAPTER VII. HELMINTHOZOA. (335.) THE HELMINTHOZOA, embracing the vast class of parasitic worms, may be conveniently divided into two groups. First, those which live as parasites the EISTTOZOA, and secondly, those which are free and have an independent existence, as is the case with many of the TREMATODE WORMS and the TTJRBELLARI^E. * Quoy et Gaimarcl, Voy. de 1' Astrolabe. CCENURUS CEREBRALIS. 129 Fig. 64 (336.) 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 cir- cumstances deprived of all power of locomotion, debarred from the in^ fluences of light, and absolutely dependent upon the fluids wherein they are immersed for nutriment the Entozoa have little occasion for that elaborate organization needful to animals living in immediate communi- cation with external objects. (337.) We find, therefore, among these creatures, certain races whose structure is of the simplest character possible, in adaptation to the circumscribed powers of which they are capable. Yet, however ap- parently insignificant some of them may appear, they not unfrequently become seriously prejudicial to the animals wherein 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, sometimes prove fatal, as may be supposed from a mere inspection of the annexed figure (fig. 64), representing an Entozoon developed in the ab- dominal cavity of a fish. (338.) There are probably no races of animals which are not infested with one or more species of these parasites, from the microscopic infusoria up to man himself ; and sometimes several dif- ferent forms are met with in the same species, to which they would appear to be peculiar ; nay, in some cases the Entozoa would seem themselves to enclose other species parasitically dwelling in their own bodies. Neither is their existence con- fined 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, the cellular tissue, and, in fact, in almost all the organs of the body. (339.) The Cystiform Helminthozoa, generally known by the name of Hydatids, are the simplest in structure ; and with these, therefore, we shall commence our inquiry into the economy of these creatures. The Ccenurus cerebralis (fig. 65), one of the most common, occurs in the brain of sheep, and is the cause of a mortal disease but too well known to the farmer ; it is likewise occasionally developed in other ruminating quadrupeds, and, by partially destroying the cerebral substance, soon proves fatal. This Entozoon, represented in the figure of ordinary size, consists of a delicate transparent bladder, the walls of which, during the life of the creature, are visibly capable of spontaneous contractions on the application of stimuli. To this bladder, or common body, are Ligula simplicissima in the abdominal cavity of a Minnow. 130 HELMINTHOZOA. appended numerous heads, which are individually furnished with an apparatus of hooks and suckers (fig. 65, 2, a, 6), calculated to fix them to the surrounding tissues. Fig. 65. 1. Ccenurus cerebralis (nat. size). 2. One head magnified : a, oral circlet of hooks ; 6, suckers. Fig. 66. (340.) The Cysticerei, or common hydatids, agree in the main fea- tures of their structure with the Coenurus, but are provided with only one head or oral orifice (fig. 66, 2). These animals are found in almost all the viscera of the body, and not unfrequently, especially in pigs, exist in great numbers, 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 intrusion ; and when they abound, serious consequences frequently result from theirprescnce. (341.) CESTOIDEA. The Tcenice, or tape- worms, arc among the most 1. Cysticercus tenuicollis (nat. size). 2. Head magnified : a, circlet of hooks ; b, suckers. 131 interesting of the Sterelmintha, whether we consider the great size to which they sometimes attain, or their singular construction. 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 pro- vided with nutritious aliment, they frequently grow to enormous di- mensions, being not un- usually twenty or thirty feet in length ; and some have been met with much longer: it is therefore manifest how prejudicial their presence must prove to the health of the ani- mals in which they re- side; and we are little surprised at the emaciation and weakness to which they generally give rise. (342.) The Tcenia solium, the species most usually met with in the human subject, at least in our own country, is that selected for special description. The body of this creature consists of a great number of segments united toge- ther in a linear series (fig. 67) : the segments which immediately succeed to the head (a) are very small, and so fragile that it is rarely that this part of the animal is obtained in a perfect state ; they gradually, how- ever, increase in size towards the middle of the bodv (d}. The first Immature segment of Tania 6olium: a, J ^ ' lateral canals ; b, ovary ; c d, accessory joint of the TaBnia, generally called glands; e, lateral sucker. the head, differs materially in structure from all the rest. This segment Taenia solium : a, head ; 6, c, d, segments of the body. Fig. 68. 132 HELMINTHOZOA. in the Tcenia solium, when highly magnified, is found to be somewhat of a square shape ; in the centre is seen a pore that has been considered to be the mouth, surrounded with a circle of minute spines, so disposed as to secure its retention in a position favourable for imbibing the chyle wherein it is immersed. Around this apparatus are placed four suckers, which are no doubt additional provisions for the firm attach- ment of the head of the worm. In other Tsenise the structure of the first segment is variously modified : thus, in Tcenia lata the central pore has no spines in its vicinity ; in Bothriocephalus there are only two longitudinal sucking disks ; 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 head in a position adapted to ensure an ade- quate supply of nutritious juices. (343.) The alimentary canal seems to be represented by a double tube, which may be traced through the whole length of the body, with- out any other perceptible communication with the exterior than the minute pore in the centre of the head : at the commencement of every segment, however, there is a cross-branch, which communicates with the corresponding tube of the opposite side (fig. 68, 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. (344.) The reproductive organs in the mature segment or Proglottis of a tape-worm, each of which may be considered as an adult animal, consist of a male and of a female apparatus these two sets of organs being completely distinct from each other. The male apparatus consists of a testis, a vas deferens, and an intromittent organ, the last of which is lodged in a special sac or pouch. The testis (fig. 69, a a, b) occupies the middle of the anterior portion of the body, and is of a whitish colour, owing to the spermatozoa con- tained in its interior. It is composed essentially of a long csecal tube, folded upon itself in close convolutions, and terminating in the vas deferens (c), which reaches to the base of the intromittent organ. The penis (fig. 69, cZ), called also by authors cirrus and lemniscus, is very variable in its form in different genera ; in its real structure, however, it is merely a prolongation of the vas deferens, just as the latter tube is a continuation of the testis. * Professor Van Beneden denies the existence of the central aperture or mouth, or that the two lateral longitudinal canals with their intercommunicating trunks con- stitute an alimentary system ; on the contrary, he regards these tubes as secerning organs, the secretion of which is discharged from the terminal segment of the body through a foramen caudale. TAPE-WOEM. 133 Fig. 69. In its size it varies considerably ; it consists of two muscular coats invaginated one within the other, and unrolls itself like the finger of a glove, until it acquires its full length. The external surface, which is internal when in a state of re- pose, is covered with minute asperities or rough points ; when fully retracted, it is lodged in the pouch, e. (345.) The female generative system is composed of an ovary, which produces the germ (ger- migene), of an organ which secretes the vitelline globules (vitelligene), of ducts from these two organs, and of a matrix, a copulative vesicle, vagina, and vulva. (346.) It seems to have been by no means a rash supposition on the part of Siebold, that in some Entozoa there might exist a double set of glands for the production of the ova, one ap- propriated to the formation of the germ, the other to the secre- tion of the vitellus. In the Cestoid forms, according to Van Beneden, the proper ovary or germigene (fig. 69, i) is situated at the posterior part of the body, occupying about one-third or one-quarter of its width. This organ is double, being exactly repeated on the right and left of the median line, the two being united by a central commissural canal*: when empty, the pre- sence of this organ is discover- able with difficulty, on account of its extreme delicacy. Its appearance varies much : sometimes it is a bag surrounded with slight depressions (culs-de-sac) ; sometimes the whole viscus is divided into lobes, and has the appearance of an ordi- nary gland, whilst occasionally it is entirely made up of long ciecal tubes united together, and opening at the same point. * It is represented in the figure upon one side only, to avoid confusion. Diagram representing the fully-developed gene- rative system of the Proglottis of a Cestoid Ento- zoon (after Van Beneden). a, testis; b, com- mencement of ditto; c, vas deferens; d, penis ; e, sac of the penis ; f, orifice of vagina ; g, vagina ; k, copulative vesicle; i, germigenous organ, or ovary (represented on one side only); ?,germiduct; TO, point at which the vitelline globules enter the germiduct; n, vitelligenous organ, or vitelliduct; o o, transparent vesicles, developed at a very early period ; p, oviduct ; q, matrix ; r, longitudinal ca- nals ; *, the skin ; t, cutaneous glands. 134 HELMINTHOZOA. On the sides of the body, extending nearly its whole length, are two slender and slightly flexuous tubes (fig. 69, n n), whose presence it is difficult to detect when in an empty condition, but which generally con- tain in their interior vitelline globules closely aggregated together, which by the peristaltic movements of the tube, aided by ciliary action, are forced onward from before to behind. The two vitelligenous tubes ultimately unite to form a common canal (n), situated near the median line, through which the vitelline globules enter the germiduct at the point marked ra. On passing the opening of this canal, the germ be- ' comes suddenly invested with a layer of vitelline globules, and, being thus transformed into an ovum, is carried onwards through the flexuous canal, or proper oviduct (p), into the matrix (^), becoming invested in its passage through the oviduct with an outer covering that represents the egg-shell. The matrix (q) thus receiving a continual supply of ova, becomes gradually distended, until it occupies almost all the interior of the body, and branches out in different directions into caecal pouches at points where the least resistance is offered, until finally the skin of the proglottis becoming as tightly distended as the matrix, both are ruptured, and the ova escape in this artificial manner. The vagina (fig. 69, g g) is a large canal, having, like all the organs belonging to this apparatus, distinct parietes. It commences externally (/) in the immediate vicinity of the male organ, penetrates to the centre of the body, and, bending at an angle, makes its way backwards to the space that separates the two ovaria (germigenous organs, i). Its length is in- variably in correspondence with that of the penis of the male apparatus. At the extremity of the vagina is situated the copulative sac (h), a small vesicle with very delicate parietes, the contents of which abound in spermatozoa. Such being the anatomical arrangement of the different parts of this somewhat complex apparatus, it now remains to take a brief survey of their physiological import in the performance of the generative function. In the living Entozoa it is sometimes not difficult to see the germi- genous and the. vitelligenous organs opening into a common canal, and each of them pouring their product into its cavity ; and if a specimen is selected in which the parts are in full activity, and the compression used be such as to render the organs transparent without putting a stop to their action, the germs may be seen to arrive, one by one, at regular intervals, before the opening of the vitelligenous organ, which, contract- ing forcibly, expels a certain quantity of the vitelline substance, in which the germ becomes enveloped, having previously, on passing the orifice of the copulative sac, become impregnated by contact with the sperma- tozoa therein contained. As the vivified ovum advances onwards it receives its outward envelope, arrives in the matrix, and is there re- tained until birth is accomplished by the destruction of the animal. .REPRODUCTION OF TJENI^. 135 A question has often arisen in relation to the manner in which the act of copulation is effected in animals presenting this remarkable hermaphrodite condition of the generative system a question to which Professor Van Beneden has been able to give a satisfactory solution. In a specimen of PhyUobothrium luca, he had ocular demonstration that the individual was self-fecundating. Its penis became unrolled, and passed immediately through the vulva into the vagina, into which it was deeply inserted. Active peristaltic movements of the vaginal tube were very manifest, and spermatozoa were seen abundantly w in its in- terior, these being subsequently conveyed by peristaltic action into the copulatory pouch. The penis, after some considerable time, is with- drawn, returns into its pouch, and all the organs assume their previous condition. (347.) In studying the progressive development of the egg in the Taeniae and other Cestoid worms, it is only necessary to remember that all the ova contained in the same segment are of the same age, and that the age of the segments increases progressively, from the head to the opposite extremity of the elongated body, to enable the observer to select ova in any stage of their development in order to submit them to examination under the microscope. (348.) In their general structure, the eggs of the Taanioid Entozoa are similar to those of the other classes of Invertebrate animals ; and the segmentation and breaking-up of the yelk proceed exactly in the same manner. (349.) On arriving at maturity, however, a series of phenomena of the highest possible interest begin to develope themselves, which we will proceed to describe with as much conciseness as the subject will allow*. The worm, when it emerges from the egg, instead of being composed of a series of segments, consists simply of the first segment, or head, as it is called, of the compound worm, variously armed with hooks, suckers, or bothria, according to the genus, to which is appended a short caudal extremity, w r herein but slight traces of any internal organ are apparent. In this condition it has received the name of Scolex, and may be regarded as a sort of root from which all the rest of the animal is developed, much in the same way as the Planulce of the Acalephse are segmented off from their Hydra-like parent ( 304). In this condi- tion the " Scolex " exists for some time, and in some instances, as, for example, in the Tetrarhynchi, clothes itself in a kind of sheath formed by a mucous exudation derived from the surface of its body. (350.) We have already pointed out the similarity of structure that exists between the armature of the " head " of the Cysticercus (fig. 66) and that of the Ta3nia (fig. 67) ; but the reader, from a comparison of the two figures, would scarcely be prepared to expect that the one was * Vide " Recherches sur les vers Cestoides," par P. J. Van Beneden, Nouveaux Mem. de 1' Academic de Bruxelles for 1850. 13G HELMINTHOZOA. Fig. 70. the Scolex of the other. Siebold had, indeed, satisfied himself that the arrangement of the horny circlet of Cystwerciis fasciolaris, found in the liver of the mouse, entirely corresponded with that of Tcmia crassicollis, that inhabits the intestines of the cat. If young TaBnias and Cysticerci he carefully examined, and compared with other forms, it is satisfactorily seen that the Cysticercus is merely the Scolex from which a Trenia may be developed, and that its vesicular portion corresponds exactly with the similar vesicles of some Tetrarhynchi in a like state of development. A Taenia, says Van Beneden, might probably very well acquire its com- plete development, without assuming the vesicular form, as is proved by the Paradoxical TaBnia ; but, for that, it would be necessary that the germ should be deposited in an intestinal tube. The same is the case with the Tetrarhynchi : here also the body becomes quite out of propor- tion to the size of the head whilst the germ remains amongst the peritoneal folds of the fishes in which they are found, just as the Cysticerci do whilst imbedded in the peritoneum or amongst the muscles. The Scolex, when fully formed, has its own individual de- velopment arrested at this point ; but it now begins to give off buds, of which the body (Strobile) of the Entozoon is composed. (351.) The Scolex, therefore, in this stage of development is syn- onymous with the " head," or, as it might as well be called, the " root " of the worm ; and as long as this root, head, or Scolex remains unexpelled from the body, it will continue to give origin to fresh segments or joints, ad libitum. (352.) Gradually the tail of the Scolex, or the body of the worm, is deve- loped ; and as soon as this has attained a certain length, transverse markings begin to make their appearance, seg- ments are formed, separated from each other by slight indentations, and the internal organs appropriate to each segment are progressively evolved. When the segments have attained to maturity, or, in other words, when the gemma has grown into an adult worm (Proylottis of Van Beneden), the indentation separating each from the one preceding it increases in depth, until, being reduced to a mere pedicle, the segments are successively thrown off as so many distinct animals. From the above account, therefore, it is evident that the last, , Scolex ; bed, Strobile ; e, Proglottis. SCOLEX, STROBILE, PRO GLOTTIS. 137 or caudal, segment is always the oldest, the newly-formed segments continually pushing the others from before backwards. (353.) Most frequently the mature segment QT Proglottis is detached, as stated above, and becomes an independent worm : nevertheless this probably does not invariably happen, some apparently remaining per- manently connected together, and laying their eggs without having enjoyed a separate existence, as is the case in various forms of Asci- dians and Polyps. (354.) While the segments of the Strobile remain conjoined, they seem to enjoy a complete community of life and of movement. Some species especially may be observed to become suddenly dilated in one region and contracted in another these alternate movements, passing along the entire length of the animal, giving precisely the same ap- pearance as is witnessed in many Annelidans when they make violent efforts for progression, a circumstance which will readily explain how Taenia3 are frequently met with having their bodies tied in complicated knots a very puzzling phenomenon to the older helminthologists. (355.) The Proglottis, on becoming detached from the general com- munity, is provided with all its organs ; nevertheless its development becomes still further advanced : it even completely changes its shape ; the angles of the segment become effaced, the whole body rounded, and its movements, moreover, more extensive : nay, as Van Beneden assures us, not only does the Proglottis continue to grow, but sometimes it becomes as large as the entire Strobile a circumstance which fre- quently causes a Cestoid at this age to be mistaken for a Trematode Entozoon. (356.) Many thousands of eggs must be produced from such multi- plied sources of reproduction ; and yet, how are they preserved and replaced in circumstances favourable to their development? Fortu- nately 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 specially been the subject of our description is often called, par excellence, " the solitary worm," from this circum- stance. Yet what becomes of the reproductive germs furnished in such abundance ? Do they, as was the opinion of LinnaBus, live in a humbler form in stagnant waters and marshes, until they are casually intro- duced into the body of some animal, where, being supplied profusely with food and placed in a higher temperature, they attain to an exu- berant development ? 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 conjectures ; and, interesting as the subject is, few are more entirely involved in mystery. (357.) In some Ta3nise, as for example in T. serrata, which is found in the intestines of dogs, M. Dujardin has pointed out that the ova, 138 HELMINTHOZOA. instead of being, as Rudolph! supposed, more delicate and frail in their substance than the Entozoa themselves, are defended by envelopes so strong that, thus protected, they may be dispersed in prodigious num- bers in various situations, and escape destruction until conveyed into a nidus proper for their development. (358.) To form some idea of the number of ova furnished by a single Tsenia of this species, it must be considered that it is furnished success- ively with at least two hundred segments, which in the aggregate will produce for each Taenia 25,000,000 of eggs. The mature segments are' found loose in the intestine of the dog, and are able to move about with considerable quickness, creeping sometimes at the rate of three inches in a minute, by the contractions of which they are capable. If one of these be placed in a flask, or under a moist glass bell, they will soon begin to crawl about upon its surface, leaving a sort of milky track wherever they pass, in which, by the aid of a lens, innumerable eggs may be detected. Under these circumstances they will exist for several days, until they are entirely emptied of their ova and reduced to half their original bulk, when, their destiny being accomplished, they perish. Therefore it cannot be doubted that, when expelled naturally from the intestines of the animal in which they live, they are able to deposit their ova in a similar manner. (359.) Many interesting facts relative to the development of the in- testinal worms have been recently brought to light, and promise to lead to still more important discoveries. In 1840 M. Miescher announced, to a meeting of naturalists at Bale, the discovery that several genera of Entozoa undergo most extraordinary metamorphoses, whereby their form and character are completely changed. A Filaria, met with in a fish, became changed into a flat, oval, leaf-like worm, in fact a Planaria ; from the interior of the Planaria there subsequently issued a Tetra- rhynchus, armed with four long proboscides; and lastly, the last- mentioned form probably gave birth to a Boihriocephalus. (360.) Carrying out these observations, M. Van Beneden* has not only confirmed the doctrine, but added very materially to our know- ledge on this subject, by ascertaining that the Tetrarliynchus undergoes no fewer than four distinct phases of development. (361 .) In the first phasis of its existence, the worm is more or less vesicular in structure, being armed with four suckers, and a sort of pro- boscis in the centre. It is possessed of extraordinary contractility ; and in different species there are spots of black pigment, representing eyes. In this condition these worms have received from helminthologists the name of Scolex (Scolex polymorpTius ; Scolex Acalepharum), Sars, Tetra- stoma Playfairii, Forbes and Goodsir, Dithyridium Lacertce, &c. These are more especially found in the pyloric caeca. (362.) The second phasis is, perhaps, the most curious. In the in- * Ann. des Sc. Nat. 3 ser. x. p. 15. METAMOEPHOSES OF TETEAEHYNCHUS. 139 teriorof the Scolex there is formed a Tetrarhynchus, by a process of gemmiparous reproduction ; and from the surface of the latter a kind of viscid secretion exudes, which becomes solid, and forms a sort of sheath made up of concentric layers. (363.) In this state of development there is found a sheath formed of several layers, in the interior of which is a Trematode worm (Amphi- stoma rliopaloides, Ch. Le Blond) ; and in the interior of the latter may be perceived a Tetrarhynchus, which moves about vivaciously as soon as its prison is opened. This Tetrarhynchus has been regarded by naturalists as a parasite inhabiting the Trematode worm ; M. Yan Beneden believes it to be a moveable gemma (bourgeon mobile}. (364.) It is constantly, if not always, found in cysts formed at the expense of the peritoneum, in a great number of sea-fish cod, trigla, conger, &c. In the third state of its existence, the Tetrarhynchus is free, but in all respects resembling that which was enclosed in the Tre- matode worm ; in a short time, however, transverse lines become deve- loped upon the posterior part of its body, segments are formed, and it becomes Tcenioid. In this condition it has been named Bofhrioce- phalus, or more recently Rhynchobothrius. It is found in the intestinal canal of the skate, among the first turns of the spiral valve. (365.) In the fourth and last phase of its growth, it presents a more simple structure, the perfect animal performing the part of a tube destined to disseminate ova. In this condition it is nothing more than the last segment of the Taenioid form detached, in fact a Proglottis. In this condition it is found in the intestine of the skate, in company with the Bothriocephali : this is the mature or adult animal, provided with complete male and female sexual organs. (366.) The adult Entozoon (the Proglottis loaded with eggs) is eva- cuated together with the fasces of the skate, and with its ova serves as food to fishes of small dimensions. The ova are developed either in the intestines or the intestinal caeca of the devourer, and if the fish which contains them happens to be swallowed by another fish, the de- velopment still proceeds in its alimentary canal, or the caeca thereunto appended. When arrived at the condition of a complete Scolex, after having perhaps passed through the stomachs of several fishes that have successively devoured each other, it perforates the intestinal walls and lodges itself in the peritoneum, in which situation it forms its sheath, and produces in its interior the " moveable gemma," from which is pro- duced a Tetrarhynchus. The fishes containing the latter form are swal- lowed in turn by the voracious Rays and Sharks, and their flesh having been dissolved in the stomachs of their devourers, the Tetrarhynchus becomes free, and continues its growth in the intestines until the last forms (Proglottis'} are complete, which alone are furnished with a sexual apparatus. Thus, from the production of the egg to the completion of the mature animal, these parasites are continually passing into the ali- 140 HELMINTHOZOA. mentary canals of new fishes ; and it is only under such circumstances that they seem to attain their full development. (367.) CYSTOIDEA. Transformation of Cystiform Entozoa into Taenia?. The gradual transformation of Cysticercus pisiformis into the Tcania serrata has been established by feeding young dogs with the cystic parasites still enclosed in the cysts in which they are found in the omentum of rabbits*. The first effect produced upon the Entozoa thus enclosed in their cysts, after they have been swallowed, is the solu- tion of the cysts by the gastric juice in the dog's stomach, after which the caudal vesicle of the Cysticercus pisiformis is attacked and de- stroyed by the same digestive agent, leaving nothing of it remaining but the whitish and rounded Scolex, which, passing through the py- lorus, becomes attached to the walls of the duodenum, in which situation it has to await its subsequent growth. At the posterior end of the now tailless cyst-worm, the point at which the caudal vesicle was previously attached is distinctly indicated by a sort of cicatrix. Subsequently the growth of the Entozoon commences, its transverse wrinkles are mul- tiplied, and in the course of a few days the body becomes divided into segments, which, at first very short, elongate and soon present the marginal generative pores. After a residence of twenty-five days in the intestines of the dog, the Taenia has attained the length of from 10 to 12 inches, and in three months 20 or 30 inches or more, at which time the posterior joints appear to be sexually matured, and the last segments (Proglottides) become detached. The ova enclosed in the ripe joints are seen to be completely developed, and contain in their in- terior the mobile embryo armed with its six booklets. It must now be an important task for helminthologists to trace the further development of the embryos produced from these eggs, in order to determine the mode of origin of the Cysticercus pisiformis. (368.) The Echinococcus veterinorum, long considered as a distinct Entozoon,is in reality merely a hydatid cyst filled with the larvas (Scoleces) of Taenioid worms ; it occurs in the liver, the cavity of the abdomen, the heart, the voluntary muscles, and the ventricles of the brain of man, in the liver, lungs, &c. of the ox, sheep, goat, ape, pig, &c. The walls of the true cysts consist of numerous concentric layers or plates, re- sembling those of colloid cells. The liquid existing within them is yellowish or reddish, and albuminous. The larvaB appear to the naked eye as minute white opake specks, varying in size from about -^^" to -j-J-y " in length ; they also vary greatly in form : when the head is retracted, they appear more rounded than when it is protruded. The hooks surrounding the anterior end of the body consist of a basal por- tion, an internal transverse blunt tooth, and a curved terminal portion or claw ; they are about y-^Vo" to Win/' ^ n l en g tn - I* 1 some of the * Vide De Cysticercorum in Tsenias Metamorphosi pascendo, experimenta in In- stitute Physiologico Vratislaviensi administrata, Auctor G. Lowald : Berolini, 1852. DISTOMA HEPATICUM. 141 larvae a kind of pedicle exists, by which they are attached to the walls of the cyst. In a quite recent state the larvae have been seen swimming actively in the liquid of the cyst by means of cilia upon the surface of the body. They appear usually to be developed from the interior of the cyst ; but, as Kuhn long since showed, they are sometimes produced by external gemmation : the contents produce a slight protrusion of a part of the wall of the cyst ; the protruded portion enlarges, afterwards becoming constricted at the base, and at last probably separates from the parent, to become itself a parent in a similar manner. Hence it appears that the larvae cannot be regarded as the parasites of the cyst, but must be viewed as arising from a partial segmentation of the contents of the parent. The Echinococci do not acquire their full development into Tcenice unless they reach the alimentary canal. The cysts and their contents, including the Echinococci, undergo a kind of degeneration, becoming partially converted into fatty or calcareous matter, or the entire contents become amorphous and granular, the hooks remaining longest unaltered, but finally disappearing also*. (369.) TKEMATODA. In the fluke, Distoma (Fasciola, Linn.) hepati- cum, we have an Entozoon of more complex and perfect structure one of those forms, continually met with, which make the transition from one class of animals to another so insensible that the naturalist hesi- tates with which to associate it. (370.) 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 extremity is a circular sucker or disk 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 (fig. 71) 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 disk (a) only is perforated, the other being merely an instrument of adhe- sion. The alimentary canal (6) takes its origin from the mouth as a single tube, but soon divides into two large branches, from which rami- fications arise that are dispersed through the body, each terminating in a blind clavate extremity. These tubes, from being generally filled with dark bilious matter, are readily traced, even without preparation, or they may be injected with mercury introduced through the mouth. * Kuhn, Ann. Sc. Nat. 1 ser. xxix. p. 273 ; Siebold, Wiegmann's Archiy, 1845, and Siebold and Kdlliker's Zeitschr. iv. ; G-luge, Ann. Sc. Nat. 2 ser. viii. p. 314; Owen, Hunterian Lectures, i. p. 46 ; Dujardin, Helminthes, p. 635 ; Huxley, Ann. Nat. Hist. 2 ser. xiv. p. 379. 142 HELMINTHOZOA. (371.) Through the walls of the ventral surface of the body two nervous filaments (c) are discoverable, which, crossing over the root of the anterior sucker oracetabulum,and gra- dually 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 oeso- phagus there is a very slight ganglion, from which other nervous filaments arise to supply the suckers and the anterior part of the body. (372.) 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 re- presented 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. (373.) These animals are completely hermaphrodite, not only possessing di- stinct 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 cooperation of two individuals is requisite for mutual fecundity. (374.) To commence with the female generative system, we find the ovaria (Ji) 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, it 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 ovi- gerous canals, which, uniting on each side of the body into two principal trunks, discharge their contents into the large oviducts (g). The ovi- ducts 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 c&cal tubes (/), which opens into the uterine cavity, and no doubt furnishes some accessory secretion needful for the completion of the ova. (375.) The male apparatus occupies the centre of the body. The testes (k), in which the spermatic fluid is secreted, consist of convoluted Anatomy f Distoma. a, anterior sucker and oral orifice ; 6, alimentary canal ; c, nervous system ; d, external opening of female generative appa- ratus ; e, uterine receptacle ; f, acces- sory appendage to ditto ; g, oviduct ; li, ovary ; i, common canal, receiving k, convolutions of testis; I, vas deferens; m, capsule of the penis; w,intromittent organ. TEEMATODA. 143 vessels of small calibre, arranged in close circular folds, and so inex- tricably involved, that it is difficult to get a clear idea of their arrange- ment ; but towards the middle of the median line they become more parallel, and terminate in two larger trunks (i) (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 (I), and terminate in the root or capsule of the penis (m). The external male organ (n) is 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. (376.) Among the most interesting discoveries of modern times is the establishment of the long-suspected fact that the Trematode Entozoa undergo certain metamorphoses during their development, and those of a most extraordinary and unheard-of character, exhibiting remarkable examples of the phenomenon of alternate generation. It is to the Danish naturalist, Steenstrup*, that science is indebted for the follow- ing account of his researches. (377.) Although the best-known species of the numerous family of the Trematoda is the fluke, or liver-worm, of which the anatomical details are given above, similar forms are met with in almost all animals of the four higher classes ; and among the lower, theMollusca are equally infested by them. (378). It might almost be said that in these classes every species is infested by its own fluke ; in various animals, moreover, several different species of these parasites have been found, which inhabit either all the organs of the body indiscriminately, or are exclusively confined to one (liver, kidney, bladder), or to a definite part of an organ. Several of these Trematoda, as will be evident hereafter, when young, are not con- nected with any viscus, but enjoy the power of free locomotion in water, externally to the animal which, in their future state as Entozoa, they infest. In their free condition they are provided with a locomotive apparatus, usually a tail of moderate length, by the waving movement of which the creature propels itself through the water, like a tadpole, to which, in its external form, it is not dissimilar, though almost of microscopic dimensions. In this larval state the Trematode worms have long been known to naturalists under the generic name of Ger- caria ; but although it was well established that this form was not a permanent one, it was not until the researches of Mtzsch, Siebold, and Steenstrup revealed the true nature of the changes through which they pass, that we arrived at any satisfactory knowledge of their remarkable history. * Ueber den Generationswechsel in den Niederen Thierklassen, translated by the late Mr. Henfrey, in the publications of the Kay Society, 1842. 144 HELMINTHOZOA. Fig. 72. 1. Cercaria echinata? of Siebold. 2. Distoma-pupa, or Cercaria in the pupa state after it has cast off its tail and enclosed itself in a mucoid case. 3. The animal proceeding from the pupa a true Distoma, which has penetrated for a short distance into the body of the snail. 4. A "nurse" containing fully- developed Cercarise: v, the stomach. 5. A "parent- nurse " filled with partially-developed " nurses :" v, the stomach. (After Steenstrup.) (379.) A Cercaria, supposed by Steenstrup to be the Cercaria echi- nata of Siebold (fig. 72, l), is found by thousands in the water wherein specimens of the large fresh- water snails, Planorbis cor- nea and Limnceus staynalis, have been kept. The body of this species of Cercaria is usually of a more or less elongated- oval form, which, however, it is constantly changing, assuming, during its movements, every out- line, from the circular figure which it has in the fully contracted state, to the linear form that it presents when its body is fully extended ; it is furnished, moreover, with a triangular head, at the apex of which is situated the oral orifice, surrounded with an apparatus of spinous teeth ; and a ventral sucker is visible, situated upon the inferior surface of its body ; while inter- nally traces of viscera are discernible (as represented in the figure), the nature of which is not clearly made out. (380.) The swimming movement of these Cercarise is very charac- teristic : in performing it, the animal curves its body together into a ball, by which the head is brought near to the caudal extremity, and at the same time the elongated tail strikes out right and left into various sigmoid flexures. In this way they may be seen swarming about the water-snails in great numbers. After swimming about the snails for some time, they affix themselves, by means of their suckers, to the slimy integument of those animals, and all their movements upon it are readily perceived with a good glass. On examining, with a sufficient magnifying power, a portion of the skin of the snail with several of the Cercariae adhering to it, it will be seen that all the efforts of these creatures are directed to the inserting of themselves deeper into the mucous integument, and to the getting rid of the tail, which is no longer of any use to them as an organ of locomotion ; in this, after violent efforts, the Cercaria at length succeeds, and the now tailless animal assumes so completely the appearance of a Distoma or fluke, that it could not fail of being recognized as belonging to that genus, in case it were met with in this condition in the viscera of other animals. How- ever, it undergoes a further remarkable transformation before it becomes a true Entozoon in the common acceptation of the word. ALTEENATION OF OENEKATIONS. 145 (381.) In various Cercariae, a copious mucous secretion is observable on the surface of the body even before the loss of the tail, and this secretion apparently increases during the efforts of the animal to cast off this appendage. As soon as the tail has been got rid of, the Cercaria begins, by extending and contracting its body, to turn itself round and round in the same spot. By this sort of movement it makes for itself a circular cavity within the mucus, which gradually hardens, and forms a tough, nearly transparent case around it. This is the noted pupa-state of the Cercaria3, observed first by Mtzsch*, and afterwards by Siebold. The tailless CercariaB remain concealed under their transparent case, which is arched over them like a small, closely-shut watch-glass (fig. 72,2) . In this condition they remain some months in a quiescent and inactive state, when they present themselves with all the characters of real Flukes (fig. 72, 3), and may be found under this form lodged in the liver or appropriate viscera in the interior of the snail. (382.) Having thus seen that the Cercaria becomes an actual FluTce, it next remains to inquire what is the origin of the Cercaria. The Fluke deposits ova, from which, either within the body of the parent, or external to it, oval-shaped young proceed, which move about briskly in the fluid contained in the interior of the snail or in the surrounding water, and bear no resemblance to their parent. In what way this progeny is transformed into a Fluke, or rather, as we now know, into a Cercaria, is as yet an unexplained mystery ; but that this change can and does take place only by the intervention of several generations may be assumed as beyond doubt. (383.) The free-swimming CercariaB, afterwards converted into pupa9, as above described, have been proved, by the observations of Bojanus, to be produced from little worms of a bright yellow colour (fig. 72, 4) (" konigsgelben Wurmern "), described by him, and which occur in great numbers in the interior of snails, especially of Limnceus stagnalis and Paludina vivipara. It is consequently in these yellow worms, which are about 2 lines long, that the CercariaB, which are the Iarva9 of the actual Flukes, are developed ; and since we now know that the Flukes are perfect animals, which themselves undergo no transforma- tion and are propagated by ova, we are reduced to the conclusion that the progeny is indebted for its origin and development to creatures which, in external form, and partly in internal organization, differ from the animals into which that progeny is afterwards developed ; in other words, it may be said that we here meet with a generation of nurses, and that the yellow cylindrical worms of Bojanus, which inhabit the snail, are the nurses of the Cercaria3 and Distomataf. * Beitrag zur Infusorienkunde, oder Naturbeschreibung der Zercarien und Bacil- larien. Halle, 1817. t That the Cercarice are actually developed in the above-mentioned yellow worms, any one may be easily convinced who will take a dozen large specimens of Limn&us L 146 HELMINTHOZOA. (384.) The " ntirses " usually present the appearance of the figure given above (fig. 72, 4). The body is cylindrical, and is furnished in most instances with a spherical contracted head, which includes an oral cavity with very muscular walls and a small circular mouth. At some distance posterior to the middle of the body are situated the two cha- racteristic oblique processes, which, as well as the part of the trunk posterior to them, are simply local dilatations of the cavity of the body. Of internal organs, there is only to be seen an undivided sacculated stomach (y), very small in proportion to the size of the animal. The whole remainder of the very large body is filled with the brood of Cercarice. In the instance figured above (fig. 72, 4), all the embryos have simultaneously reached their full development, which is but seldom the case, since, in the same individual, Cercarice are found in all stages of development. (385.) Some doubt exists as to the mode in which the Cercarice quit their " nurses," since it has been observed, under the microscope, that there are two places where they come away, viz. from each side of the body, at a depression under the collar, and from the abdominal surface, between the two oblique processes : but they escape from the latter situation only when the animal has been slightly compressed between the glasses ; and from the former, on the contrary, when no pressure at all has been employed. (386.) It next remains to trace the origin of the "nurses" them- selves. Siebold (who did not regard these as independent animals, but only as living organs of generation, "germ-sacs") expresses his surprise at seeing them developed from germs which are always contained in other creatures having the same outward appearance as themselves ; and Steenstrup saw, with like astonishment, that it constantly occurred, in some of the snails taken from the same places as the others, that they harboured only Entozoa which had the outward form of the " nurses" but which, instead of Cercaria3, contained a progeny consist- ing of actual " nurses" in all stages of development. This was the case only in some, and those rather young snails, whilst all the others were inhabited by "nurses" whose progeny were true Cercarice:, it cannot, therefore, be doubted that it is normal for the " nurses" to originate in creatures of similar appearance to themselves, and which are thus the " nurses" of " nurses" These "parent-nurses" however (fig. 72, 5), notwithstanding their great resemblance, were not difficult to be distin- guished from the common ones ; the stomach, for instance, in the full- stagnalis from small stagnant pools that have been exposed to the sun ; the worms will be very readily found. They are situated not so much in the viscera themselves (the liver and reproductive organs) as in the membranes covering them, and their long bodies will be found half-floating as it were in the fluid which occupies the space between the organs, and which appears to be pure water entering through the water-canals. METAMOEPHOSES OF DISTOMA. 147 Fig. 73. grown " parent-nurses " is longer and wider than in any even of the youngest " nurses." (Compare fig. 72, 4, with fig. 72, 5.) (387.) We have thus followed the Distoma to its third stage of ascent, and, as no more stages in the generations of these animals have been detected, are not in a condition to trace the origin of the Distoma further back. Steenstrup, however, en- tertains the not unfounded supposition that the " parent-nurses" are not pro- duced from other similar creatures, but that they proceed originally from ova derived from the full-grown Fluke, a supposition which derives additional importance from observations made upon the development of other Entozoa belonging to the Trematode group. (388.) In Monostomum mutabile*, for example, which inhabits several of the cranial cavities lined with mucous membrane in certain water-birds, the young embryo is frequently hatched before or just as the ovum is expelled. The newly-hatched young (fig. 73, 1) are elongate-oval, and furnished at their ' anterior extremity with some short lobes, which the animal is able to protrude and retract; and its whole surface is covered with vibratile cilia, by the aid of which it moves readily in the water. In the anterior part of the body are two quadrangular spots, which can scarcely be regarded as anything but eyes. The posterior two-thirds of the trunk are occupied by a slightly transparent whitish body (, the sucker is shown in a still more retracted state, the con- ANATOMY OF ASTEEIAS. 175 tained fluid having been completely expelled from the muscular tube and driven back into the vesicle, which is distended to the utmost. (456.) The fluid that thus fills the suckers, and performs so import- ant a part in causing all their movements, is not secreted by the vesicles in which it is contained, but is conveyed into them by a special vas- cular apparatus (fig. 88, 2, g, /), from which branches are given off to each tube. The nature of the fluid, however, and the arrangement of the vessels through which it flows will be more properly discussed hereafter. (457.) The whole inner surface of the elaborately- constructed box that forms the skeleton as well as the integuments of the Star-fish is lined by a thin membrane, aptly enough called the peritoneum ; for, like the serous tunic so named in higher animals, it not only spreads over the walls of the body, but is reflected therefrom upon the contained viscera, so that they are completely invested by it, each viscus having a distinct mesenteric fold whereby it is supported and retained in situ. (458.) The mouth of the Asterias occupies the centre of the lower surface of the body (fig. 88, 1, a). It is usually described as being a simple orifice, entirely destitute of teeth, although it is not improbable that the osseous ring around it, and the articulated spines thereunto attached, may, to a certain extent, perform the oflice of a dental appa- ratus. (459.) The oesophagus is very muscular, and susceptible of great dilatation, its parietes being gathered into deep longitudinal folds. The stomach (fig. 88, i, b) is a wide, sacculated bag, occupying the central portion of the body, and, like the oesophagus, is evidently calculated to undergo considerable distention. There is no anal orifice ; and conse- quently, 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 that 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 voluminous ca3cum (fig. 88, 1, c). (460.) The whole of the digestive apparatus is displayed in fig. 89 : every one of the five rays contains two of the caeca! 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 casca 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 176 ECHINODEEMATA. seen in the ray g, wherein, the upper walls of the caeca having been removed, their sacculated internal structure is rendered visible. (461.) With respect to the exact office of these capacious appendages to the stomach, there exists some diversity of opinion. (462.) It is scarcely possible that they can be at all instrumental in the digestion of food, the passages whereby 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 by the receptacle into which the food is first introduced. But there is every evi- dence to prove that, al- Fig. 89. though they can have little part in digestion, the caeca are intimately connected with the ab- sorption of nutriment ; and thus, although pos- sessing 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 ma- terial usually found in them, namely, a pultaceous creamy fluid, evidently a product of diges- tion, 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 that ramify so ex- tensively 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. (463.) Those physiologists who have adopted a different view of the nature of the caecal appendages to the stomach, consider them to be adapted to the secretion of some fluid, and probably representing 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 (fig. 89, 5), and is a yellow or greenish-yellow racemose sacculus, which opens into the Digestive apparatus of Asterias : a, stomach ; b, hepatic (?) glands ; c, d, e, csecal appendages in situ if, the same unravelled ; .9, the same laid open, showing their sacculated interior. STAK-FISHES. 177 bottom of the digestive sac by a free aperture ; the contents of this organ, moreover, resemble bile both in taste and colour*. (464.) In the slender-rayed genera, such as Opliiura, the caecal appendages are not met with ; but their deficiency appears to be sup- plied by the plicated walls of the stomach itself, the numerous folds of which resemble lateral leaflets attached to the central cavity. We are unacquainted with the precise organization of the alimentary canal in Comatula ; 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. (465.) The Star-fishes, grossly considered, might be regarded as mere walking stomachs ; and the office assigned to them in the economy of nature, that of devouring all sorts of garbage and offal that would otherwise accumulate upon our shores. But, as we have already seen, their diet is by no means exclusively limited to such materials, 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 examined. Neither is the size of the prey whereon they feed so diminutive as we might suppose from a mere inspection of the orifice representing the mouth ; for not only is this 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 frequently swallowed whole ; and a living specimen of Chama antiquata, Linn., has been taken entire from the digestive cavity of an Asterias. It appears, moreover, that it is not necessary for testa- ceous mollusca to be absolutely swallowed, shells and all, to enable the AsteridaB 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 partially open, cunningly inserted one of its rays between the valves, and thus gradually insinuating itself, destroyed its victim f. Modern observations do not, as far as we are aware, fully bear out the above opinion of our ancestors as to the * Delle Chiaje. f This may be gathered from Aldrovandus, who writes as follows : " Alii ostre- arum hostes sunt Stellas marinae molli crusta intectaj,vero tarn crude! iter (ut ^Elianus, lib. ix. cap. 22, ait) inimicae ut has ipsas exedant et conficiunt. Ratio insidiarum quas eis moliuntur ejusmodi est. Cum testacea suas patefaciant conchas, cum vel refrigeratione egent, vel ut aliquid pertinens ad victum incidat ; eae, uno de suis sive cruribus sive radiis intra testas ostreae hiantis insito eas claudi prohibente, carne implentur " (Testae, lib. iii. p. 487). Thus likewise Oppian : " Sic struit insidias, sic subdola fraudes Stella marina parat, sed nullo adjuta lapillo Nititur, et pedibus scabris disjungit hiantes." 178 ECHINODERMATA. mode in which star-fishes attack oysters, although the destruction that they cause is pretty generally acknowledged. The observations re- corded by M. Eudes Deslongchamps upon this subject, however, are 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 mollusk (Mactra Stultorum, Linn.) of an inch and a half in length. The valves were invariably opened to the extent of 2 or 3 lines ; and the star-fishes were always ranged with their mouths in contact with the edges of the valves. (466.) On detaching them from the shell which they thus impri- soned, 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 distinguishable. The Mactrce were all found to be more or less devoured, some having only their adductor muscles left ; but, how- ever little they had been injured, all had lost the power of closing their 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 described escaped injury from the closing of the valves. M. Deslongchamps thinks that probably the Asterias pours into the shell a torpifying secretion, and thus ensures the death of its victim. (467.) 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 cir- culation. (468.) 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 (fig. 90, e), which likewise receives branches derived from the ovaria and other sources. (469.) The circular vein thus formed, which seems to be the common trunk of the venous system, communicates with another vascular circle * Bulletin des Sciences de M. le Baron Ferussac, vol. x. p. 296. ANATOMY OF ASTERIAS. 179 placed around the mouth (s), by means of a dilated vertical tube of communication (/), which, from its muscular appearance and great irritability, Tiedemann regards as being equivalent in function to a heart. The circle around the mouth (s) would seem to be arterial in its character, and branches are derived from it which supply the various viscera of the body. (470.) But, besides the vessels above described, apparently so dis- posed 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 ( 456) ; 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. (471.) The circular Fig. 90. vessel around the mouth, which forms the central receptacle of the vascular system, resem- bles a sinus analogous to those of the dura mater in man, and is lodged in a groove be- tween the oral circle of vertebrae and the pieces of the skeleton articu- lated therewith. Con- nected with the sinus above mentioned, and placed regularly in the interspaces between the rays, are several oval vesicles (fig. 90, & 7c), filled with a reddish- coloured transparent fluid. These vesicles, which in Asterias au- rantiaca are seventeen in number, Asterias auranfiaca opened from above : a, dorsal parietes reflected ; ft, c, d, floor of the rays, exhibiting the ambulacral vesicles ; e, dorsal circular vessel ; /, heart ; s, circular vessel surrounding the mouth ; fck, ampullae Polianse. communi- cate by distinct ducts with the central sinus, and are regarded by Delle Chiaje as reservoirs wherein 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, accord- 180 ECHINODERMATA. ing to the author last mentioned, vessels are given off that communi- cate with the ampullae connected with the ambulacral suckers, appa- rently for the purpose of supplying to them the fluid which they con- tain. These vessels are seen to run along the floor of each ray, and to give off lateral branches communicating with every vesicle, as repre- sented in the enlarged sketch (fig. 88, 2,#). By this arrangement it would seem that the contractile organs (fig. 88, 2, e) appended to the vascular sinus (/) are in reality antagonistic to the tubular structure of the feet, and serve as receptacles for fluid, which, by their contraction, they can force into the whole system of locomotive suckers whenever the feet are brought into action. (472.) The above view of the arrangement of the vascular system of Asterias is, however, by no means universally admitted to be correct. 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 ambulacral tubes become distended is neither more nor less than pure sea-water. (473.) In the Echinodermata therefore there are, 1st. The cavity of the body (i. e., the spacious interval which separates the digestive from the tegumentary system), filled with a fluid designated chylaqueous. 2nd. The protrusile suctorial feet, occupied by another class of fluid (this system constitutes the water- vascular system of Tiedemann and Miiller). 3rd. The blood-vascular system of Tiedemann, Delle Chiaje, Valentin, Agassiz, Dr. Sharpey, and Miiller. These three systems are generally regarded as distinct and independent. (474.) The mass of fluid occupying the visceral cavity of the Echino- derms (bounded on one side by the digestive system, on the other by the integuments) has been generally described as consisting purely of sea- water admitted directly from without, through the skin, for the exclusive purpose of aerating the blood proper, said to circulate in a capillary system of vessels wrought in the solid parietes circumscribing the cavity. In the Asteridae, Echinidae, Ophiuridae, and Ophiocomidae, it cannot be denied that the cavity itself is the anatomical homologue of a real perigastric cavity ; while in the Holothuridan and Sipunculidan genera it presents itself as a chamber filled with a chylaqueous com- pound, under the form of a thickly-corpusculated milky fluid organized in a high degree ; and in Sipunculus it would seem that the cephalic appendages, as well as the whole tegumentary system, are organized with especial reference to the aeration of this fluid. (475.) The skin is fenestrated ; that is, at regular intervals the mus- cular layer disappears, and an interval results, of elliptical figure, covered by only a single layer of epidermis. It is a simple musculo-membranous partition intervening between the chylaqueous fluid within and the surrounding element without ; and through this veil the two divided fluids interchange their gases. The tentacles are merely hollow musculo- CHYLAQTJEOUS FLUID. SAND-CANAL. 181 membranous appendages, lined within and without by a ciliated epithe- lium. A few proper blood-vessels reach their bases from the circular vessel ; but no trace whatever of a vascular plexus in the structure of these parts can be detected. The inference is that the tentacles are designed for the oxygenization of the chylaqueous fluid. To the genus Holothuria the same observations are strictly applicable ; but although attenuated at regular points, with a view to approximate as closely as possible the chylaqueous fluid to the external medium, no open perfora- tion anywhere exists in the tentacular or tegumentary processes. The surrounding fluid therefore cannot penetrate directly from without into the peritoneal cavity ; it is introduced through the mouth and digestive system. (476.) Before quitting this part of our subject, we must briefly men- tion 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 fig. 90, enclosed in the same sheath as the dilated vessel, /, upon the right side of which it is placed ; it communi- cates by one extremity with an isolated calcareous mass, of a rounded figure, called the madreporic plate, seen upon the exterior of the dorsal surface of the Star-fish, while by its opposite extremity it opens into the circular sinus that surrounds the mouth. The tube itself Dr. Sharpey describes* as being about the thickness of a surgeon's probe, and com- posed 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 them- selves 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 disk, appear- ing gradually to become 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 disk, by means of the calcareous tube, which will thus serve as a sort of filter to exclude impurities. (477.) Apparently with a view to ensure a continual circulation of aerated fluids through all parts of the system, the entire surface of the membrane that lines the shell, as well as that which forms the external tunic of the digestive organs, has been found to be covered with multi- tudes of minute cilia, destined by their ceaseless action to produce currents passing over the vascular membranes, and thus to keep up a * Cyclopaedia of Anatomy and Physiology, art. " ECHINODERMATA." 182 ECHINODERMATA. 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 cseca. (478.) " In Asterias rubens," says Dr. Williams, " it can be distinctly demonstrated that no open perforations exist in any part of the integu- mentary parietes. The membranous processes communicating with the visceral cavity can be proved, by injection, to be csecal at their distal extremities. . It is easy to repeat and confirm the observation of Dr. Sharpey, that the corpuscles of the visceral fluid advance to the distal end of these processes, and then return, under the impulse of ciliary agency. Nevertheless, although an injection so thick as size will not escape through these membranous processes, a thinner fluid, such as coloured water, will slowly ooze through ; it is not, therefore, impro- bable that an interchange of the fluids may to some extent occur through endosmose. The microscope renders it certain that the hollow mem- branous processes filled by the fluid of the visceral cavity, in Asterias, bear in their parietes no trace of true blood-vessels : they are lined within and without by vibratile epithelium, and composed only of in- terlacing elastic fibres ; and consequently their only office seems to be that of exposing the chylaqueous fluid to the renovating influence of the surrounding medium. In Asterias, this fluid approaches simple sea- water closely in its physical properties. It is, however, in reality a dilute, albuminous, opalescent solution. It is charged scantily with im- perfectly formed corpuscles, always the same in the same species ; and the proposition may now be confidently affirmed, that in the Echinoder- mata the chylaqueous fluid (i. e. the contents of the visceral cavity) is itself first aerated, and that by means of a machinery of soft parts it then aerates the blood proper." (479.) The organs belonging to the reproductive system in the Aste- ridce exhibit the greatest possible simplicity of structure. The ovaria (fig. 88, 1, /) are slender ca3ca, arranged in bunches around the oeso- phagus, two distinct groups being lodged at the origin of each ray. In Asterias aurantiaca (fig. 90), 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 communi- cate externally by a wide aperture that perforates the osseous circle encompassing the mouth. (480.) The generative organs of the male individuals exactly resemble those of the female, and are only distinguishable by the presence of spermatozoa in their interior. The process of reproduction f usually * See the article " CILIA," by Dr. Sharpey, in the ' Cyclopaedia of Anatomy and Physiology.' f M^moire siir le Bevel oppement des Astries, par M. Sars, Ann. des So. Nat. 1844. EMBKYOLOGY OF ASTEEIAS. 183 occurs during the spring months, at which period the ovaria of the females are found distended with eggs, wherein the vesicles of Purkinje and of Wagner are distinctly recognizable. These ova are found in the ovaria in different stages of development, and are laid in successive batches at different intervals. (481.) The newly-laid ova consist of a chorion enclosing the vitellus and a small quantity of albumen ; but the vitellus soon undergoes the usual process of segmentation, whereby it is broken up into a granular mass (fig. 91, 4, 5, 6, 7, 8). When first deposited, the ova of the Star- fishes are not at once abandoned by the parent animals, but are retained Fig. 91. Development of Star-fish. 1. EcUnaster sanguinolentm seen from below. 2. The same in pro- file : a, madreporic plate. 3. Ovarian receptacle containing ova in different states of advancement. 4, 5, 6, 7, 8. Ova exhibiting the progressive segmentation of the yelk. 9. Embryo on its first escape from the egg. 10, 11, 12. Further progress of embryo : a a, club-shaped processes ; b, cen- tral protuberance. 13, 14, 15, 16, show the gradual development of the ambulatory suckers and the assumption of the radiate form. in a kind of cavity formed by incurving the body and rays of the mother until they form a sort of chamber, beneath which the eggs are protected during the earlier part of their development (fig. 91, 2). The vitellus of the ovum is entirely employed in the construction of the foetus, which latter, at the moment of its escape from the egg, is of an ovoid or sub- spherical shape (fig. 91, 3), completely unprovided with external mem- bers, but enabled to swim vivaciously about in the surrounding water by means of the cilia with which its body is profusely covered, giving it exactly the appearance of an infusorial animalcule ; indeed, this may be called the first, or infusorial condition of the young Asterias. (482.) After the lapse of a few days certain appendages begin to 184 ECHINODEEMATA. make their appearance, sprouting, as it were, from the anterior part of the body, and ultimately appearing as four club-shaped processes (fig. 91, 10, 11, 12, 13, a a) surrounding a fifth prominent protuberance, 6, whereby the little creature fixes itself to the sides of the incubatory cavity. The body of the little Star-fish now becomes gradually flat- tened into a minute circular disk, upon one surface of which hence at once distinguishable as the ventral the rudiments of tentacula begin to be apparent, under the form of minute globular protuberances, dis- posed in ten concentric rows (fig. 91, 14, 15, c e}. (483.) If in this condition the little being is detached from the spot where it has fixed itself, it is still able to swim about in the surround- ing water by means of its ciliated surface, always keeping the organs of attachment directed forwards; but if left undisturbed, it remains perfectly still and motionless, presenting what M. Sars denominates the crinoid state of development. At this stage, the body of the young Star-fish may still be said to be bilateral ; for in all its movements the organs of attachment are directed forwards, and both sides of the body correspond exactly to each other (fig. 91, 12). But by degrees this bilateral condition is converted into the radiated form that charac- terizes the third, or perfect condition of the Asterias ; the body gradu- ally assumes a pentagonal outline, from the angles of which short blunt rays begin to project (fig. 91, 16) ; and the tentacula, now presenting the form of retractile cylinders, and completely furnished with their terminal suckers, become efficient instruments of locomotion. The red spots, regarded by Ehrenberg as the eyes, are visible at the extremities of the nascent rays ; the mouth shows itself in the centre of the ven- tral aspect of the body ; and numerous spines make their appearance. Lastly, the apparatus for attachment begins to diminish in size, and soon completely disappears, so that the young Asterias, having attained its perfect form, is ready to enter upon the duties of its station. (484.) According to the observations of Agassiz, the eggs of the Star- fish, after they are laid, are taken up by the parent animal and kept between its tubes below the mouth. The Star-fish bends itself around them, surrounds the eggs with its suckers, and moves about with them. When the eggs have been removed to some distance from the animal, it has been observed to go towards them, take them up again, and move off with them, showing that these creatures, so low in structure and apparently deprived of all instinct, really watch over their young. As the growth of the embryo commences, the external crust of the germ becomes more transparent, consisting of somewhat looser and larger granules, and the internal mass assumes a darker colour, so that two layers become distinct, between which a third is developed. On one side of the germ a protuberance now becomes visible, and the promi- nent portion separates more and more from the spherical mass, assuming by degrees the form of a peduncle. At this period there is not any EMBEYOLOGY OF ASTERIAS. 185 organ formed only changes of substance have taken place ; but now little swellings appear in five points on the sides, and the spherical portion of the germ becomes flattened by lateral dilatation. (485.) The minute animal has grown to a more hemispherical shape ; and from this time there is an upper and a lower surface to its um- brella-like disk, and a tubular part and a swollen portion to the peduncle. As soon as the peripheric part of the disk begins to spread, five small tubercles may be observed forming underneath ; and into these tubercles the peculiar aspect of the middle one extends. Soon other prominent swellings appear, two to each of the former ones, and subsequently two more. While this is going on, calcareous nets are formed by the accumulation of crystals in the cells of the germ. At first there are simply isolated crystals, formed as nuclei in the cells ; then several close together will unite and form a little irregular mass ; and they will at last combine, so as to constitute a network of solid substance, arranged very regularly, and gradually becoming more and more numerous, marking out more and more distinctly the rays of the embryo Star-fish. The tubercles of the lower surface, growing more prominent and elongated, are finally transformed into the suckers, or ambulacral tubes. With the addition of new calcareous nets, these latter become more numerous, and form, finally, rows of tentacles. Other changes have also taken place. The cells within the peduncle have under- gone alteration : some have become moveable, and a kind of circulation is going on in them. The internal space along each ray has become more transparent ; the ambulacral tubes have become hollow ; and from that time there seems to be a communication between the external water and the internal structure. "What remains of the yelk is more distinctly circumscribed in the centre of the animal, extending as a star-shaped disk into the rays. The radial portion becomes, finally, distinct from the central one ; and we have at last an internal cavity, which is the stomach, from which the cgecal appendages of the rays, with their liver-like organ, will be developed. (486.) The peduncle is reduced to a mere vesicle ; a hole is formed in the centre of the lower surface, constituting the mouth ; around this a circular thread becomes visible, answering to the nervous system, from which other threads extend towards the extremity of the rays ; and by the time the young Star-fish has attained the size of about a line in diameter, it has thus assumed the form and structure of a per- fect animal. (487.) Among the most interesting contributions to our knowledge of this group are the researches of Professor Miiller* relative to the embryonic condition of the Opliiuridce, from which it has been ascer- tained that, during the progress from the egg to the mature condition, * Ueber die Larven und Metamorphose der Ophiuren imd Seeigel. Berlin Trans. 1846. 186 ECHINODEKMATA. the individuals belonging to that family undergo a series of changes that are truly surprising in their character. (488.) The young Ophiura on leaving the egg presents itself under a most grotesque form, in which condition it has been long known to naturalists, and described under the name of Pluteus, or Easel Animal- cule, from its resemblance to a painter's easel. (489.) The Pluteus paradoxus (fig. 92) is exceedingly minute, being not more than fths of a line in length. When highly magnified, its Fig. 92. 1. Pluteus paradoxus. A A, lateral arms ; B B, inferior ditto ; C C, anterior ditto ; D D, posterior ditto ; a, mouth ; a', resophagus ; 6, stomach ; c, granular bodies, the nature of which is uncertain ; d, cfficiform appendages, which make their appearance around the oesophagus and stomach, and which are the first indications of the development of the Star-fish ; e, ciliated bands ; f, calcareous framework of the skeleton; g, zone of cilia surrounding the apex of the body; a; nervous system. 2. Further development of the caeciform appendages, d ; they begin to exhibit the appearance of the body or central disk of an Ophiura. (After Miiller.) body is seen to be somewhat of a conical shape, terminating above in a point, but dividing inferiorly into eight long processes or appendages of various dimensions, to which it owes its peculiar figure (fig. 92, l, A, B, c, D). Each of these processes is supported by an internal cal- careous framework derived from the interior of the body (fig. 92, I,/), which, branching out in different directions, forms a basis whereon the soft parts are spread out. The whole animal is perfectly transparent, its substance resembling dull glass, the apex of the body and the ex- tremities of the arms or processes being slightly tinged with orange. (490.) These singularly -formed larvce for such they are are found abundantly during the months of August and September, crowding the surface of the sea in rich profusion, swimming freely about by the aid of rows of cilia (e), with which their arms and the apex of their bodies (g) are plentifully furnished. They possess, moreover, a distinct nervous system, consisting of two little ganglia (#) situated just beneath the oral aperture, from whence delicate nervous threads may be traced in dif- ferent directions. EMBEYOLOGY OF OPHIUEA. 187 (491.) The first appearance that presents itself, indicating the com- mencement of metamorphosis, is the development of a number of csecal appendages around the stomach and oesophagus of the Pluteus (fig. 92, 1, d), which soon increase so much in number that they form a series of rows surrounding the stomachal cavity. At first these rows of cseca do not extend beyond the body of the Pluteus, remaining, as it were, concealed beneath its disk ; but soon acquiring greater develop- ment, they make their appearance externally, and begin to assume some regularity of arrangement (fig. 92, 2), in which the rudimentary form of the star-fish begins to be perceptible, and the points whence the arms are to proceed become apparent. (492.) In carrying out this part of the proceeding, it will be ob- served that the original arms or processes of the Pluteus (fig. 92, 1, A, B, c, D) have had no share. The Pluteus, in fact, stands just in the same relation to the young Ophiura as the frame does to a piece of embroi- dery ; neither has the structure of its arms anything in common with that of the rays of the future star-fish, which lies, as it were, protected beneath their shelter. As soon as the csecal appendages have arrived at this state of development and assumed so much regularity of arrangement, calcareous earth begins to be deposited in an arborescent form, which accumulates rapidly until a kind of trellis-work is formed, spreading over the entire surface of the young Echinoderm. As the caDciform appendages thus become arranged into a regular figure, the place where the mouth of the Pluteus was becomes distorted and, as it were, forcibly pushed upwards, until it remains no longer visible, its place being occupied by the central mouth of the newly-formed star- fish (fig. 93, 2). (493.) In the condition which it has now attained, the young star- fish is still much smaller than the rest of the Pluteus ; but from this point, as its growth continues, the body and processes of the latter assume more and more the appearance of being only appendages to the newly- developed animal, until by degrees they entirely disappear, the only part of the Pluteus remaining as a part of the young Ophiura being the stomach. (494.) Before, however, the arms of the Pluteus have entirely dis- appeared, the feet, or retractile suckers, have begun to show themselves, arranged in a circle around the circumference of the shield (fig. 93, 1, 2), so that it is able to creep freely about in the sea. (495.) Shortly before the disappearance of the last remnants of the Pluteus, the arms or rays of the Ophiura are already visible, projecting prominently from the margin of the shield (fig. 93, 1, 2), but consisting as yet only of the outer or terminal joint of the future ray; the moveable spines likewise begin to show themselves, and the characters of the future Echinoderm begin to be recognizable (fig. 93, l). Ultimately new segments begin to be added to the rays, making their appearance 188 ECHINODEEMATA. between the primitive segment and the margin of the disk, the original segment retaining its size and figure unaltered, while the succeeding ones differ in their shape, assuming a polygonal form, which varies in different species. The places where all new segments are formed are Fig. 93. 1. Ophiura in a still more advanced stage of development, showing the larva portion (Pluteus) in great part obliterated : first appearance of the mouth and tentacles. 2. The larva has entirely disappeared, and the feet and spines of the Ophiurus begin to develope themselves. 3. Shows the mode of growth of one of the rays : the terminal or primitive segment is easily recognizable, to which the following segments succeed in the order of their formation. (After Miiller.) in the shield itself, at points situated upon the ventral aspect, between the inter-radial spaces ; and each successive segment produced, being at the base of the ray, is of course larger than all that preceded it (fig. 93, 3, 4). (496.) In order to complete the history of the Asteridce, we have yet to mention the nervous apparatus wherewith they are furnished. This consists of a simple circular cord that runs around the mouth of the NEKYOUS SYSTEM OF ASTERIAS. 189 animal ; from this ring, three delicate filaments are given off opposite to each ray, one of which, according to Tiedemann, runs along the centre of the ambulacra! groove upon the under surface of the body, and gives off minute twigs to the locomotive suckers placed on each side of its course ; the other two filaments 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 if, as some writers assert, ganglionic enlargements are visible at the points whence the radiating nerves are given off, they are so extremely minute as not in any degree to merit the appellation of nervous centres. (497.) 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 regarded as being peculiarly the seat of sensation or perception. Nor is this inference 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 scissors 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 invariably 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 indeed seem to be a necessary consequence resulting from the deficiency of any central seat of perception whereunto sensations could be communicated. Never- theless Ehrenberg insists upon the existence of eyes in some species of Star-fish, attributing the function of visual organs to certain minute red spots, visible at the extremity of each ray, behind each of which he describes the end of the long nerve that runs along the ambulacral groove as expanding into a minute bulb. We must confess that the proofs adduced in support of such a view of the nature of the spots appear to us to be anytliing but satisfactory. The general sense of touch in the Asteridae is extremely delicate, serving not only to enable them to seize and secure prey, but even to recognize 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 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 external objects ? It would seem most probably due to some modification of the general sensibility of the body, allowing of the perception of impressions, in some degree allied to the sense of smell in higher animals, and related in character 190 ECIILNODERMATA. Fig. 94. to the kind of sensation whereby we have already seen the Actiniae and other polyps are able to appreciate the presence of light, although ab- solutely deprived of visual organs. (498.) The ECHINI, however they may appear to differ in outward form from the Asteridoe, will be found to present so many points of re- semblance 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 constructed animals that now present themselves to our notice. (499.) The Echinidce, as we have already observed, differ from the star-shaped Echinodermata in the nature of the integument that encloses their visceral cavity, as well as in the more or less circular or spheri- cal form of their bodies; so that the locomotive apparatus with which they are furnished is necessarily modified in its character and ar- rangement. (500.) The shell of an Echinus (fig. 94, l) is composed of innume- rable pieces accurately j oined together, so as to form a globular box enclosing the internal parts of the animal, but perforated at each ex- tremity of its axis by two large openings, one of which represents the mouth, and the other the anus. (501.) The calcareous plates entering into the composition of this extraordinary shell may be divided into two distinct sets, differing materially in shape, as well as in the uses to which they are subservient. The larger pieces are recognizable in the figure by hemispherical tuber- cles 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 that are situated in the neighbourhood of the mouth and anal aperture being considerably the smallest, and every succeeding plate becoming progressively larger as they approximate the central 1. Shell of Cidaris denuded of its spines. 2. A spine arti- culated with its corresponding tubercle : a, section of tuber- cle ; 6 5, capsular ligament ; c, base of spine. SHELL OF THE ECHINI. 191 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 mark- ing the degrees of longitude 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 dis- posed in pairs, each row of large pieces being united by a zigzag 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. (502.) 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, whereunto smaller spicula are appended although, from their diminutive size, these are of secondary importance in locomotion. (503.) The five large double segments that thus form the greater portion 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 support the tubercles ; hundreds of foramina, piercing these ambulacra! bands, give passage to as many tubular feet or protrusible suckers, in every respect resembling those of Asterias, and distended by a similar apparatus. (504.) It is impossible, by any verbal description at all commensurate with the limits of our present undertaking, adequately to explain 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 pre- senting 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 uniting 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, com- pact, and beautiful box, similar to that represented in the figure. The first question that naturally suggests itself, on examining a shell of this description, is concerning the object to be attained by such re- markable complexity; it would appear, indeed, at first sight, that a simple calcareous crust, had it been allowed to exude from the en- tire surface of the Echinus, would gradually have moulded itself upon the body of the creature, and thus have formed a globular shell without suture, answering every purpose connected either with support or de- fence. (505.) 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 192 ECHINODEEMATA. integument whereby 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 employment 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. Were 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 whereby, as in the bones forming the skeletons of vertebrate animals, a continual depo- sition of fresh particles could be effected, allowing of extension by inter- stitial deposit. How, therefore, could the growth of the Echinus be provided for ? How is the gradual expansion of the entire shell, thus composed of a dense and extravascular crust, to be effected and that without ever deranging the proportions of the whole fabric, or necessi- tating a loosening of its parts ? No other contrivance could apparently have been adequate to the purpose : nevertheless we see how admi- rably, by the structure adopted, the growth of these creatures proceeds in all directions ; for the living and vascular membrane that 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 arrangement, 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 still 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. (506.) The tubular suckers or retractile feet, that are protruded at the pleasure of the animal from the countless minute apertures seen in the ten rows of ambulacra! 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 whereby their motions are effected. The tubular part of each foot communicates with the in- terior of the shell by two branches passing through two apertures ; and these branches, in some species (as Echinus saxatilis), receive offsets from the vessels that run along the centre of each ambulacral groove, and convey to the feet the fluid by which their distention 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 STRUCTURE OF SPINES. 193 Fig. 95. double rows upon the inner surface of the ambulacral pieces*, by the intervention of which they are connected with the canals above men- tioned. (507.) 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 proportionate size, and even in their internal structure and mode of growth, as may be readily seen by a comparison of different species. Thus, in the flattened forms of Scu- tellce and allied genera, they are so minute as to require the employment of a microscope for their investigation ; in Echi- nus esculentus (fig. 85) they are sharp, and al- most of equal length over the entire surface of the animal ; while in the specimen repre- sented in the annexed figure (fig. 95), the shell of which we have already examined when divested of these appendages, the length of the spines that are articu- lated upon the large tubercular plates fully equals the transverse dia- meter 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, allowing of a considerable extent of motion. The structure of this articulation is exhibited in fig. 94, 2. The large tubercle (a) supports upon its apex a smaller rounded and polished eminence, perforated in the centre by a deep depression ; and the bottom of the moveable spine (c) is terminated by a smooth hemispherical cavity accurately fitted to the projecting tubercle, so that the two form com- plete 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 the round ligament found in the hip- joint, and obviously a provision for the prevention of dislocation. * Cyclopaedia of Anat. and Phys., art. " ECHINODERMATA." Cidaris. 194 ECHINODERMATA. (508.) Moreover, the whole joint is enclosed in a muscular capsule, composed of longitudinal fibres (b b) arising from the circumference 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 on the spine, produce all the move- ments whereof it is capable. (509.) The next thing to be accounted for in the history of these elaborately-constructed animals is the growth of the spines themselves : these, as we have already seen, are completely detached from the rest of the shell, to which they are secured only 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 communication between them and the body of the Echinus, would appear to be a matter of some difficulty ; and in fact, had we not already seen, in the Polyps, the amazing facility with which calcareous matter was secreted by the living textures of those animals, it would be almost impossible to conceive by what process their growth was effected. On examining one of these appendages, taken from a species wherein 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, whereof indeed the muscular capsule enclosing its articulation with the tubercle is only a thickened portion. (510.) The living covering of the spine, therefore, like the crust that 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 succes- sion of concentric laminae, of which the outer one is always the last formed. The calcareous matter thus deposited has, more or less com- pletely, 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 micro- scopical examination, as nothing can exceed the minute accuracy and mathematical precision with which each particle of every layer com- posing them appears to have been deposited in its proper place : in fact, 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 number 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 MOUTH AND TEETH OF ECHINUS. 195 most remarkable features of the history of the Echini ; it is only by a minute examination of the intimate structure of oach of these parts that the perfection of the mechanism conspicuous throughout can be properly understood. (511.) The calcareous pieces surrounding 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 movement, whereby the prehension of food is materially facilitated. The mouth itself (fig. 94, l) 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 calcu- lated to break the hard substances usually employed as food. The points of such incisor teeth, although of enamel-like hardness, would Oral apparatus of Echinus : a a a a a, pyramidal pieces forming the " lantern of Aristotle " ; b b, internal projections from shell ; c c c c c, teeth enclosed in their sockets ; d d, interposed osseous pieces ; e e, curved processes ; ff, g y, hh,ii,kk, muscular fasciculi for the movements of the jaws. nevertheless be speedily worn away by the constant attrition to which they are necessarily subjected, were there not some provision made to ensure their perpetual renewal ; like the incisor teeth of rodent quadru- peds, they are therefore continually growing, and are thus always pre- served sharp and fit for use. In order to allow of such an arrangement, as well as to provide for the movements of the teeth, jaws are provided, that are situated in the interior of the shell ; and these jaws, from their great complexity and unique structure, form, perhaps, the most ad- mirable masticating apparatus met with in the whole animal kingdom ; we must therefore entreat the patience of the student while we describe at some length the parts connected therewith. The entire apparatus o 2 196 ECHINODEEMATA. removed from the shell is represented in fig. 96, and consists of the following parts : There are five long teeth (c c), each of which is enclosed in a triangular osseous 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 (k Tc, i i), so as to form a pentagonal pyramid, having its apex in contact with the oral orifice of the shell, while its base is connected with several bony levers by means of numerous muscles provided for the movements of the whole. These parts we must now proceed to describe seriatim. The teeth (fig. 97, 1, a) resemble, at the part pro- truded from the mouth, long three-sided prisms, and at this point they are extremely hard and brittle : each tooth is fixed in a socket passing through the jaw (fig. 97, 2, e), from which it projects by its opposite extremity (fig. 97, 2, 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. 97, 2,e) being smooth, and presenting eminences provided for the attachment of muscles ; while the other two sides (fig. 97, 1, 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 po- sitions, they form a five- sided conical mass, aptly enough compared by Aristotle to a lan- tern, and frequently described by modern writers under the name of the " lantern of Ari- stotle." 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 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 fig. 97, 1, in which three of these jaws, each containing its incisor tooth, are represented in situ. Dental system of Echinus. 1. Represents three of the pyramidal pieces forming the " lantern of Aristotle " in situ : a a, cutting extremities of the incisor teeth, which are of enamel-like hardness; a' a' a', fibrous roots of the same, resembling asbestos in their texture ; 5 b, opposed flat sur- faces of the jaws; d d, arched processes. 2. An isolated pyramid: e, its external surface. Other letters as in fig. 1. "LANTERN OF AKISTOTLE." 197 (512.) The five curious jaws described above are fixed together by a set of muscles (fig. 96, Ic &), 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 (d d), arranged in a radiating manner between the bases of the different segments, with which they are con- nected by ligaments, and likewise by the pentagonal muscle (i i) that runs from one to the other. (513.) 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. (514.) The levers attached to the jaws are five long and slender pro- cesses (fig. 97, 1, d d), each arising from the central extremity of one of the radiating osseous pieces (c c), and arching outwards considerably 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 fig. 96, 66, are also five in number, and are placed around the orifice of the mouth : they are generally perforated in the centre, 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 con- traction, 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 direction, or cause the grinding surfaces of the jaws to work against each other. (515.) The antagonists to the muscles last mentioned are ten others (.9 #)> Arising from the extremities of the arches themselves, and run- ning 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 pro- cesses. 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 whereunto they are individually fixed, they will produce movements precisely opposite 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, Tc Tc) that connect the jaws to each other, and by causing the separation of the different pieces they necessarily enlarge not only the opening of the month, but all the passage leading to the oesophagus through the axis of the lantern. (516.) Yet even these are not all the muscles that act upon the 198 ECHINODEEMATA. Fig. 98. masticating apparatus : ten others (h 7i), 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 these receiving a muscle from two contiguous spaces ; and, from the length of the levers upon which these muscles act, we may well con- ceive the force wherewith they will influence the motions of the whole mass of the jaws. (517.) 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 development of a dental system, whereof not a vestige is visible in any of the preceding Echinoderm families. In others of the Echi- nida3 having the shell much depressed, the dental lantern is modified in form and proportionately flattened, but the different parts are essen- tially similar to those we have described. (518.) The oesophagus (fig. 98, d) is continued from the termina- tion of the central canal that traverses the axis of the lantern, and, after a short course, terminates in a much wider portion of the digestive tube, into which it opens on the lateral part of its ca?cal origin, in a manner precisely resembling the communica- tion between the large and small in- testines of man. (519.) The dilated alimentary tube (c) presents no separation into stomach and intestine, but is con- tinued in a winding course around the interior of the shell, which it twice encircles, and, becoming slightly con- stricted, terminates at the anal orifice (i). The walls of the intestine are extremely delicate, although they may be distinctly seen to contain muscular fibres and are covered with innumerable vascular ramifications. The external tunic of the whole canal is derived from the peritoneum, that lines the entire shell, invests the dental lantern, and forms sundry mesenteric folds as it is reflected upon the other viscera. (520.) The system of vessels provided for the circulation of the blood has been differently described by different authors a circumstance by Alimentary canal of Echinus esculenluv : a, interior of shell ; b, ambulacral fora- mina ; c c, intestinal canal ; d, commence- ment of oasophagus from the base of the " lantern of Aristotle"; e, heart; f, g, vas- cular trunks following the course of the in- testine. ALIMENTARY CANAL. CHYLAQUEOUS SYSTEM. 199 no means surprising when we consider the great difficulty of tracing such delicate and extensively-distributed canals. According to Delle Chiaje, the course of the nutritious fluid is as follows : A large vein runs along the whole length of the intestine, from the anus to 4he 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 feet, likewise opens. The intestinal vein he regards as the great agent 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 arteries are, 1st, a long vessel to the intestine, which runs along its whole length and anastomoses freely with the branches of the intestinal vein ; 2ndly, five arteries to the parts connected with the mouth ; 3rdly, five dorsal arteries that 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 Asterias, supply the vascular origins of the innumerable protractile feet. (521.) The chylaqueous system of the Echinidae, comprehending a considerable mass of fluid filling the cavity of the spherical shell, has been generally regarded as sea-water poured into the visceral cavity through perforations in certain membranous processes of the shell, which have received the name of branchial, and are distributed in groups around the circumference of the oral membranous disk. The latter, however, according to Dr. "Williams, are not connected with the suctorial or water- vascular system, but are distended by injections thrown into the open chamber of the shell, being protruded only by the force of the fluid driven into their interior ; they are consequently not perforated. (522.) In addition to the meridional rows of suctorial feet, the shell of Echinus is perforated by numerous hollow membranous processes lined within and without by vibratile cilia, and penetrated exclusively by the fluid of the visceral cavity ; they show no traces of blood-vessels, and can only subserve a respiratory purpose on the supposition that the subject of that process is the chylaqueous fluid. There is, therefore, no direct evidence to show that the external element enters through open- ings in the integuments into the peritoneal cavity of the Echinus. (523.) Nevertheless, besides this diffused respiration, Delle Chiaje regards a series of pinnated tentacula in the neighbourhood of the mouth as being in some degree capable of performing the office of branchia?. These organs, which are protruded through a row of distinct orifices placed around the oral aperture of the shell, are eminently vascular ; 200 ECHINODEEMATA. and as they present a large surface to the action of the water and receive numerous vessels from the circular trunk that surrounds the mouth, they may, no doubt, very well contribute to the complete ex- posure of the blood to the influence of the surrounding medium. (524.) Little is known concerning the nervous system of the Echini : a few delicate filaments have been observed in the neighbourhood of the oesophagus, apparently of a nervous character, communicating with a nervous ring placed in that vicinity, resembling that already described in Asterias. (525.) The Echini, like the Star-fishes, are bisexual, and in the structure of their reproductive organs display, if possible, greater sim- plicity than even the Asteridae above described. The ovaria are five delicate membranous bags, quite distinct from each other, that open externally 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 recognizable 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, distin- guished by zoological writers as the ovarian 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 distended : 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 Medi- terranean, they are eagerly sought after, when in season, by divers employed to procure them. The corresponding organs in the male sex are only distinguishable by the spermatozoa contained in their interior instead of ova. (526.) At the earliest period observed by Miiller, the larval Echinus (fig. 99, 1) had the appearance of a transparent dome-like disk, hollowed out inferiorly, and having its margin prolonged into long, slender, diverging processes supported on calcareous pieces deposited in their substance, and giving the whole animal somewhat the appearance of a timepiece standing on many legs (A, B, p, E), four of which (F, E) constitute a sort of framework surrounding the oral apparatus. (527.) The arrangement of the locomotive apparatus of these larva? is very peculiar, consisting of four epaulet-like wreaths of long cilia situated upon the dome-shaped body of the animal, and of numerous ciliated fringes spread over the arms and in the vicinity of the oral organs. (528.) The mouth is a triangular orifice (fig. 99, 1, a) furnished with broad lips, and leads immediately into the stomach (d), which is a cul- de-sac, situated in the interior of the body. (529.) In this condition the larvae are not more than half a line in length, and move freely about in the water, rowed along by the action PLUTEUS-METAMORPHOSIS OF ECHINUS. 201 of their cilia, while the marginal processes and other appendages to the body remain quite passive and motionless. The first appearance of metamorphosis is indicated by the development of a shield-like plate (fig. 99, 2, 6), which, during the months of August and September, Fig. 99. Metamorphosis of Echinus. 1. A Pluteus with thirteen arms : A A, anterior inferior lateral processes; BB, posterior inferior processes; C c, lateral processes of the vaulted disk; D, ter- minal process from the apex of the vaulted disk ; E E, anterior, and F F, posterior processes of the framework of the mouth ; Q G, posterior processes of the body : a, mouth ; a', basin-like under- lip ; I, oesophagus ; d, stomach ; e, calcareous framework of the skeleton. 2. The same in a more advanced stage of development the spines of the young Echinus beginning to make their appear- ance, covered with a transparent skin : a, remnant of the calcareous skeleton of the larva or Phiteus, which has now nearly disappeared ; 6, branched calcareous spicula belonging to the larva skeleton ; c, spines, and d, tentacles of the young Echinus. 3. The echiniform condition almost completed, only a few calcareous spicula of the larva remaining. (After Miiller.) becomes visible beneath the skin covering the dome of the body, sloping as if inclined towards its apex, and not inaptly representing the finger- 202 ECH1NODEEMATA. plate of the timepiece to which, as to its shape, the creature has been already compared. The round shield-like plate thus formed is divided by a cinquefoil-shaped figure into five compartments, and constitutes the first rudiment of the future Echinus ; as its size increases, new divisions make their appearance upon its periphery, indicating the situ- ations of the future tentacles or feet ; and soon afterwards little round tubercles begin to develope themselves, which gradually rise up into cylindrical elevations and ultimately assume the appearance and texture of the locomotive spines. (530.) The shield itself, forming the basis upon which the apparatus of suckers and spines is supported, is now seen to enclose in its sub- stance its own proper calcareous skeleton : this consists at first of minute detached triradiate spicula, w r hich, as they increase in number, arrange themselves so as to constitute a sort of network in the texture of the skin, wherein ultimately the polygonal calcareous plates of the shell make their appearance. (531.) HOLOTHUHIDJS. The name applied by naturalists to the ani- mals composing the next family of Echinoderinata is derived from a Greek word of uncertain application (6\o6ovpiov). In common language they are generally known by the appellation of " Sea-cucumbers "; and in fact, to a casual observer, the resemblance which they bear to those productions of the vegetable kingdom, both in shape and general appear- ance, is sufficiently striking. The surface of these animals is kept moist by a mucus that 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 that encase the Echini and Star-fishes ; it appears to consist of a dense fibrous cutis of consider- able thickness, covered externally with a thin epidermic layer. Be- neath the cutis is another tunic, composed of strata of tendinous fibres crossing each other in the midst of a tissue of a semicartilaginous nature, which is capable of very great distention and contraction, and serves by its elasticity to retain the shape of the body. Within this dense covering are seen muscular bands running in different directions, which by their contraction give rise to the various movements of the creature : of these muscles five strong fasciculi assume a longitudinal course, passing along the entire length of the animal from the mouth to the cloaca; and in the interspaces between these, circular and oblique muscles are readily distinguishable. The whole of this muscular case is lined with a delicate membrane or peritoneum, from which processes pass inwards to support the various viscera. (532.) But although the calcareous shell of the Echinus is thus totally lost, the locomotive suckers or feet already described are still the prin- cipal agents employed in progression. In many species, as in that represented in the annexed figure (fig. 100), these organs are distributed HOLOTHUEIA. 203 over the whole surface of the animal, and are protruded through count- less minute orifices that perforate the integument. In other cases, as in H. frondosa, they are arranged in five series, resembling the ambu- lacra of an Echinus ; and in some instances they are only found upon the middle of the ventral surface of the body, that forms a flattened disk upon which the animal creeps, somewhat in the manner of a snail. Fig. 100. Holothuria. The ambulacral feet themselves, represented on an enlarged scale at c, precisely resemble in aU 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 move- able hooks or spines (fig. 100, d\ which are likewise retractile, and most probably assist in locomotion. 204 ECHINODEEMATA. (533.) The mouth is a round aperture, as wide as the bore of a goose- quill, placed in the centre of a raised ring at the anterior extremity of the body (fig. 100, a). Around the oral orifice is placed a circle of ten- tacula, which are apparently extremely sensitive, and serve perhaps 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 that closes the mouth contracts, the tentacles are withdrawn, and become no longer visible externally ; in this state, on opening the animal (fig. 101, 6), they are found to resemble long caeca appended to the commencement of the oesophagus, and have been described by some authors as forming a salivary apparatus. (534.) The total deficiency of an external skeleton or calcareous framework 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 Echinidas 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 for the longitudinal muscles of the body, which, thus fixed around the circumference of the oral orifice, will by their contraction powerfully dilate that aperture for the purpose of taking in nourishment. (535.) The alimentary canal is of great length, but, like that of the Echinus, presents no stomachal dilatation ; from the mouth (fig. 101, 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 d), it once more passes backwards, and becoming constricted near its termi- nation, opens into a large membranous cavity (e) that 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 its entire length. The whole intestine is generally found distended with sand, wherein may be detected the debris of corals, algae, fuci, and other marine substances. (536.) In the structure of the respiratory apparatus, the Holothurida3 differ materially from the rest of the Echinodermata, and, in fact, from all other animals. In the Holothuria, the aeration of the circulating fluid is provided for by allowing the surrounding element freely to enter into the internal parts of the creature ; but instead of bathing the sur- faces of the viscera, the water is confined in a peculiar system of rami- fying canals, forming a structure of great beauty and, frqjp its singu- larity, extremely interesting in a physiological point of view. We have seen that the intestinal canal terminates in a membranous receptacle or 'EESPIEATOEY TEEE.' 205 cloaca (fig. 101, e), contained within the cavity of the abdomen, to the walls whereof 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 (/) has been passed ; and it is by this orifice that the water re- quired for the purpose of respiration is taken in, and then forced, by the muscular walls of the cloaca itself, through the whole system of respi- ratory canals whereby its distribution is effected. The organs of respi- ration commence at the upper part of the cloaca, near the termination of Fig. 101. Polian Anatomy of Holothuria : a, bristle inserted into the mouth; 5, inverted tentacula; c, ampulla Dliana; d d d, intestinal canal; e, cloacal chamber opening externally by a wide orifice, into which the bristle,/, has been passed; g g, "respiratory tree;" hh, ovarium (testis in the male) ; ii, central vascular trunk; k, intestinal vessel; I, m, vessels in relation with the " resraratorv tree." the intestine, 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 branehia? ; these extend to the oppo- site 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 ultimately terminate 206 ECHINODEKMATA. Fig. 102. in minute vesicular caeca, into which the water derived from the cloaca of course penetrates. One division of this elegant apparatus is main- tained 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, wherewith it is likewise united. Tt is this last-men- tioned division that would appear to be specially provided for the oxy- genization of the nutritive fluids. (537.) The circulation of the blood in theHolothuria, 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. Accord- ing to Tiedemann*, innumerable small veins collect the blood and nutritive products of digestion from the intestine and convey them into a large central vessel (fig. 101, i i), from whence the circulating fluid passes by other trunks (I I) to the respiratory tree ; hence it is returned by vessels (partly represented at m) to the intestinal artery (Jc), by which it is again distributed over the intestinal parietes. (538.) Delle Chiaje gives a different account of the arrangement of the vascular system in these creatures, which he seems to have investi- gated with his usual untiring perseverance. According to the last-mentioned anatomist, the blood is taken up from the intestines by a complicated system of veins, the main trunks of which are indicated in the annexed diagram (fig. 102) by the letters c, e, p p, q q ; these communicate with each other not only by the intervention of numerous 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) that convey the blood and nutri- ment absorbed from the in- testine to a vascular circle (g) placed around the com- mencement of the oesophagus, which corresponds with the circular vessel around the mouth of the Echinus. This circle Delle Chiaje regards as the centre * Anat. der Rohren-Holothurie. FoL 1816. Plan of the circulation in Holothurin, according to Delle Chiaje. EMBKYOLOGY OF HOLOTHUEIA. 207 of the arterial system, in communication with which is the contractile vesicle (/) ; and this 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 (i) ; so that these organs, be- sides being instruments of touch, from the extent of surface that they present and their great vascularity, are most probably important auxiliaries in respiration. Five other large arteries, derived from the same source (k Tc, Z), 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 Fig. 103. Embryology of Holothuria. circulates through the rest of the body. The descending arteries, thus destined to supply the integument and distend the prehensile suckers, run in the centre of each of the five longitudinal fasciculi of the mus- cular tunic of the skin as far as the cloaca, and exhibit in their distri- bution 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: in the case before us there would seem to be 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. 208 ECHINODEEMATA. (539.) The generative system of the Holothuria is essentially similar to that found in the Asteridae, consisting of long ovigerous caeca. The germs are secreted in slender ramified tubes (fig. 101, li h) ; these 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. The generative caeca at cer- tain times of the year become enormously distended, being at least thirty times as large as when not in a gravid state : if examined at this period, they are found to contain a whitish, yellowish, or reddish fluid, in which, in the female, the ova are suspended. In the male a precisely similar structure exists ; but instead of ova, the caeca contain a fluid crowded with spermatozoa during the breeding season. (540.) After their escape from the egg, the young Holothuriae have been ascertained to undergo a kind of metamorphosis scarcely less wonderful than that observed in Ophiura and Echinus. In its first or Pluteus condition, the little embryo bears no resemblance whatever to the future animal, but swims vigorously about by the agency of broad membranous-looking expansions that surround the margins of its flat- tened body, wherein the stomach and other viscera are distinguishable (fig. 103, 1, 2). In its second stage of existence it has somewhat the appearance of a polype (fig. 103, 3) ; and this ultimately becomes con- verted into a larva-like being (fig. 103, 4), surrounded with several rows of vibratile cilia, by means of which its progression is accomplished. In the interior of this larva, a set of rudimentary oral tentacula, sur- rounded at their bases by a circle Fig, 104 of calcareous spicula, is developed, an alimentary canal makes its ap- pearance, and even the ampullae Polianae are distinctly recognizable, surrounding the position of the future mouth. In its fourth stage of advancement (fig. 104), the Ho- lothurian structure is no longer doubtful, although the apparatus of vibratile cilia still exists upon the exterior of the body. The ali- mentary canal (a) may be seen to terminate in a cloacal chamber, the Polian vesicle (6) is largely increased in size, the calcareous circle (c) around the mouth is much strengthened, the tentacles (d)have assumed larger proportions, and Young Holothuria. even the appearance of the suctorial feet (e) is no longer doubtful ; the longitudinal muscular fasciculi in the integument progressively acquire FISTULAEID^. 209 strength, and the little creature is transformed into the shape and at- tains the proportions of its parent. (541.) 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 wherein they are imbedded, it is next to impossible to display their 6ourse ; most probably, as in the Echinus and Asterias, these communicate with a circular cord that embraces the oesophagus. No ganglia have as yet been discovered even in the Solothuridce ; and consequently, although the muscular actions of the body are no doubt associated by nervous filaments, the movements of these creatures appear rather to be due 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 mechanical irritation causes such powerful and uncontrollable contractions of the integu- ment, 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 circumstance, that the older anatomists were induced to suppose that, by a natural instinct, the animals, when seized, vomited their own bowels. It is, in fact, extremely difficult to obtain perfect specimens of the Holothurida3, from the constant occurrence of this accident : but, although annoying to the naturalist, such a phenomenon 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 evisceration of the creature. (542.) FISTTJLAKIDJE. In order to complete our account of the organi- zation of the Echinodermata, we have still to investigate the structure of the Fistularidce a group that, 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 accord- ance with the character both of their outward form and internal struc- ture ; 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 approximating the Annelida in all the details of their economy. We have already given a description of the outward form of a Fistularia ( 443), and seen the completely annulose condition of its p 210 EOIIINODERMATA. body, although the radiating tentacula around the mouth are evidently analogous to those of the Holothuria, already described. (543.) The Sipunculus inhabits shallow seas, concealing itself at the bottom in holes that it excavates in the sand. Having once located itself, it is seldom found to quit its concealment, but, retaining 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 water of the ocean. (544.) These animals are much sought after by fishermen, who em- ploy them as baits for their hooks ; and one species, Sipunculus edulis, is used in China as an article of food. (545.) The body is covered externally with a delicate cuticle, easily separable by maceration or simple immersion in spirit; 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 Sipunculus saccatus. (546.) The muscular investment, placed beneath the skin, is com- posed of strong fasciculi arranged in three distinct layers. The exter- nal 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, ex- tending from one extremity of the body to the other. Such an arrange- ment is evidently sufficient for the general movements of progression ; 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 in- verted and drawn inwards. (547.) 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 sensitiveness and power of contraction ; but, as we have found to be generally the case among the Echinoder- mata, 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 Sipunculi are nourished. (548.) 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 relation- ship that exists between the apodous Echinodermata and the Holo- thurida3. The oesophagus (fig. 105, 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 peculiarity to distinguish it from the rest of the intestine. In the Annelida, the digestive apparatus is invariably straight, traversing the body from one extremity to the other, SIPUNCTJLTIS. 211 Fig. 105. a circumstance that distinguishes them remarkably from the Echino- derms we are now considering ; for in Sipunculus 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 d d) nearly to the tail, where it is reflected upon itself, and mounts up again as far as the point where it com- menced ; here it again turns back, and, once more reaching the bottom of the tegumentary sac, becomes a second time directed upwards, and re-ascends as far as the point e, where the anus is situated. (549.) It is easy to account for this extreme length of the intestine when we consider the nature of the mate- rials used as food, and the small pro- portion of nutriment contained among the sand and broken shells found in the digestive canal : but the remark- able 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 exca- vation bored in the sand, from which it seldom issues), had the excrements been discharged, as in Holothuria, through a terminal orifice, their con- stant accumulation at the bottom 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 protruded from its concealment, and the excrementitious matter may be cast out without inconvenience. The intestine is retained in situ and sup- ported at all points by innumerable tendinous bands, that arise from the interior of the muscular walls of the body and form a kind of mesentery. (550.) In Sipunculus, the character of the circulating system is in all essential points strictly analogous to that of the other Echinodermata ; and moreover, from the superior concentration visible in every part, we Anatomy of Sipunculus : a, oral tentacles ; b, cesophagus ; c, stomach ; d d d, intestinal canal ; e, position of anal orifice ; f,p, ovaria ; g, external orifice of ovary; h, ampulla Poli- ana ; i, cerebral ganglia ; I, heart ; in, intes- tinal vein; n, branchial vessel; o, aortic trunk. 212 ECHINODEEMATA. have the multiplied organs of the other families exhibiting so much simplicity of arrangement, that whatever may have appeared obscure or complicated in our description of Echinus and Holothuria will receive elucidation from the diagrammatic form in which all the vessels con- nected with the circulation of the blood are represented in fig. 105. The intestinal vein (m) may be readily traced along the entire length of the alimentary canal : commencing near the anal extremity of the bowel, it follows 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 lacteals 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 oesophagus, 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 neigh- bourhood of the mouth ; the other, which we may call the aorta, dis- tributing the remainder to all parts of the tegumentary system. The branchial vessel (n) runs from the bifurcation of the intestinal vein to the base of the oral tentacles, where it forms a vascular circle around the commencement of the oesophagus, analogous to that which we have seen in Holothuria ; and in connexion with this circular vessel we find the " ampulla Poliana" (&), 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. (551.) 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 (1), 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 di- rection of the intestinal vein, it is probable that it contains an appa- ratus 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. HOMOOANGLIATA. 213 (552.) We have seen that the tentaciila are, from their vascularity, 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 neighbour- hood of the anal opening, two apertures are visible, which lead into two long sacculi (/, ^>), the entrance being guarded by muscular fibres (g) : their texture presents transverse and longitudinal striae ; and they con- tract spontaneously, even after the animal is dead ; internally they are lined with a mucous membrane. The use of these organs is not pre- cisely known ; Cuvier regarded them as belonging to the generative system, while Delle Chiaje looks upon them as respiratory organs. (553.) In this elevated form of the Echinodermata, so nearly allied to the Homogangliate type, we may naturally expect a more complete development of nervous ganglia than we have yet met with in the class ; and accordingly we find, upon the anterior part of the oasophagus, two little nervous tubercles (i) 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. (554.) We are entirely ignorant concerning the mode of reproduction in these creatures. 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 that exists in the vicinity of the tail. CHAPTER IX. HOMOaANGLIATA (Owen). ARTICULATA (Cuv.). ANNULOSA (MacLeay). (555.) THE next great division of the animal kingdom includes an immense number of living beings, adapted by their conformation to exist under a far greater variety of circumstances than any which we have hitherto had an opportunity of examining, all of which are ob- viously only adapted to an aquatic life, and accordingly are invariably found either to inhabit the waters around us, or to be immersed in the juices of living animals upon which they subsist. Even the Echino- dermata are too imperfect in their construction to admit of their en- joying a terrestrial existence, inasmuch as, possessing no nervous cen- tres adequate to give force and precision to their movements, they are incapable of possessing external limbs endowed with sufficient power 214 HOMOGANGLIATA. and activity to be capable of progression upon land ; neither are any of them furnished with organs of sense, that must be indispensable for the security of creatures exposed to those innumerable accidents to which the inhabitants of a rarer element are perpetually liable. (556.) 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 proportionate to the in- creased perfection of the animal; limbs are provided endowed with strength and energy commensurate with the development of the nervous ganglia that 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 development. (557.) The most obvious though not the most constant character that distinguishes the creatures we are now about to describe is met with in their external conformation : they are all of them composed of a succession of rings formed by the skin, or 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 lowest forms of the ARTICULATA, the body is extremely elongated, and the rings proportionately numerous ; the in- tegument, moreover, is soft and yielding, and, as a necessary conse- quence, the limbs appended to the diiferent segments are feeble and imperfect : such is the structure met with in the Worms, or ANNELIDANS, properly so called. (558.) As we advance, we perceive the tegumentary rings to become less numerous, and the skin of a denser and firmer texture, adapted to sustain 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 development we find in the MYRIAPODA, or Centipedes. (559.) In the INSECTS, the concentration of the external skeleton is still more remarkable. 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 together in those parts where great strength and firmness are necessary, and scarcely any traces are left to indicate their existence as separate pieces ; so that, instead of exhibiting that succession of similar segments seen in the Centipede, the body is appa- rently divided into three distinct portions : viz. the head, that contains the organs of the senses and the parts of the mouth ; the thorax, sus- taining the limbs, or instruments of progression ; and the abdomen, en- closing the viscera subservient to nutrition and reproduction. (560.) In a fourth division of articulated animals, namely the HOMOGANGLIATA. 215 ARACHNIDANS, or Spiders, still further 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 recognizable. (561.) Lastly, among the CRUSTACEANS we have various modifications of the outward skeleton, adapted to the habits of the different tribes. In the least perfect species, which are all aquatic, the segments 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 con- struction of the calcareous shield that covers and protects their whole body. (562.) We see, therefore, from the above rapid sketch of the different classes composing 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 more immediately connected with locomotion or the destruction of prey ; let us next examine the nature of the nervous apparatus that characterizes the HOMOGANGLIATA, and observe the relation which the outward form of the body bears to the arrangement of this primary system of the animal economy. In the humblest forms of the Annulosa, it would seem that every ring of the body contains 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 formed, passing along the whole length of the body. With the exception of the anterior pair of ganglia, or those contained in the first ring, which we may call the head, the nervous centres are arranged along the ventral region of the body, that is, beneath the alimentary canal ; but the anterior pair are invariably situated upon the dorsal aspect of the animal, and communicate with the rest by a nervous collar that embraces the commencement of the oesophagus. The nervous masses placed along the belly preside specially over the movements of the segments to which they belong, and have little to do with sensation, or the perception of external objects ; whilst the anterior or cephalic pair, from the constancy of their communi- cation with the organs of the senses, appear to be peculiarly in relation with the perceptive faculties of the creature. (563.) It may be taken as a general law, that the perfection of the nervous system of any animal may be estimated by the proportionate size of the central ganglia, upon the development of which both the energy of the actions of the body and the completeness of perception 216 ANNELIDA. depend ; and by following out this great principle, we shall be easily able to account for the progressive steps whereby the Articulata become more and more perfectly organized, as we trace them in the series above indicated. 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 energetic, we shall find the abdominal ganglia to diminish in number by becoming consoli- dated into larger masses, increasing in size and energy in accordance with the development 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. (564.) Among the Homogangliata are likewise to be detected the first traces of the sympathetic or splanchnic nervous system. This con- sists of delicate filaments which are distributed upon the alimentary canal, presenting, in their course, ganglionic enlargements, and anasto- mosing, some with the cesophageal ring, and others with the cerebral or encephalic ganglia*. (565.) These observations will suffice to introduce the student to the Homogangliate division of the animal world, and to direct his attention to those physiological points connected with the nature of their nervous system which will be more fully laid before him in the following pages. CHAPTER X. ANNELIDA. (566.) THE lowest class of articulated animals comprehends an exten- sive series of creatures generally grouped together under the common 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 frequently confounded together. But whatever may be the similarity in outward appearance between the more perfect intestinal worms and the animals belonging to the class upon the consideration 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 considerable interval in all the more modern systems of zoological arrangement. (567.) The principal characters which serve to distinguish the ANNELIDA from other forms of the animal world are readily appreciated, * Vide Brandt, Bemerkungen iiber die Mundmagen- oder Eingeweidenerven der Evertebraten. Leipzig, 1835. HIKUDO MEDICINALIS. 217 and, when once pointed out, will be found sufficient for the guidance of the most superficial observer. The body is always considerably elon- gated, and composed of a succession of rings or segments that, 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 setas, calculated 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 ; it is also generally furnished with eyes, and variously- shaped tentacula, apparently instru- ments of touch. The last segment also not unfrequently presents seti- form appendages, and occasionally a prehensile sucker, used as an organ of progression. (568.) Their blood is sometimes remarkable for its red colour, and circulates in a double system of arteries and veins ; respiration is effected either in the general cavity of the body, or by means of arborescent tufts appended to various parts of their external surface ; moreover they are almost all hermaphrodite, and generally require the congress of two individuals for mutual impregnation. (569.) ABRANCHIATA. This order comprises two distinct tribes, that differ widely in their habits and external appearance : the first com- prehends the LEECHES (Annelida suctoria), distinguished by the exist- ence of a prehensile sucker situated at each extremity ; while in the second, instruments of attachment are totally wanting, the only external appendages to the body being a number of minute and almost impercep- tible bristles, which project from the different segments and assist in progression : such are the EARTHWORMS, &c. (Annelida terricola). (570.) The common Leech (Hirudo medicinalis) affords the most interesting example of a suctorial Annelid. 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 brief notice. The body is very extensible, and divided by a great number of transverse lines into numerous rings, 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 lubricated 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 coverings or walls of the body, which form a kind of contractile bag enclosing the viscera, are 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 218 ANNELIDA. internal layer is made up of longitudinal muscles, extending from one end of the creature towards the opposite. Such an arrangement is evidently adequate to the production 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. (571.) At each extremity of the animal, the muscular coat expands into a flattened fleshy disk, 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 accom- plished by means of the terminal suckers. Supposing the posterior disk 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 contractions of the muscular integument, not precisely under- stood, assumes the appearance of a flattened band ; in this condition the Leech makes its way through the element it inhabits by successive undu- latory movements of the body, performed with much grace and elegance. (572.) The mouth of the Leech is an exceedingly complete apparatus, adapted not only to the destruction of minute aquatic animals that con- stitute 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. (573.) 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, imbedded in a strong circle of muscular fibres (fig. 106, A). Each tooth has somewhat of a semicircular form, and, when accurately examined with a microscope, is found to have its free margin surmounted with minute denticulations (fig. 106, B), so as to resemble 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 ad- herent is 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 consider- able depth and lay open the cutaneous vessels, whence 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. 106, A), will at once explain ANATOMY OF THE LEECH. 219 Dental apparatus of the Leech. A., triradiate arrange- ment of the teeth or saws ; B, a tooth magnified. the cause of the triradiate form of the incision that a leech-bite in- variably exhibits. (574.) On contemplating this singular dental apparatus found in the medicinal Leech, and con- sidering the nature of the Flg> 106 ' food upon which it usu- ally lives, it is difficult to avoid arriving at the con- clusion that such a struc- ture is rather a provision intended 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 where 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 in- dulge the instinct that 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 frequently the death of the Leech is caused by such inordinate repletion, provided the greater portion of what is taken into the body is not speedily regurgitated through the mouth. (575.) The internal digestive apparatus is evidently adapted, from the construction of all its parts, to form a capacious reservoir for the recep- tion of fluids taken in by suction : the stomach, indeed, with the nume- rous 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 natural to it in an empty state. (576.) The stomach itself (fig. 107, h, 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 that 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 condition, 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 220 ANNELIDA. Fig. 107. the simplest 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 distention ; 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 caeca with common wax injection ; and if the body be immediately removed 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 in- crease 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. (577.) The small size of the intestine (e) when compared with the capacious stomach described above is remarkable : it commences by a minute orifice at the termination of the digestive cavity, and becoming slightly enlarged, passes in a straight line, lodged be- tween the two posterior caeca, 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 m), passing between its outer walls and the muscular parietes of the body. (578.) In the Leech, the circulating system is more highly developed than in any other Annelid*. The presence or absence of a heart-like centre to this system in this class of animals is by no means the true criterion of the degree of its evolution. The amount of blood relatively to the size of the body, the degree of capillary subdivision which occurs on the periphery of the blood-system, and the proportion of the latter to the peritoneal fluid, form far more correct indications. In the Leech there exists no free space between the intestine and the integument : to * Vide Dr. Williams' s Eeport on the British Annelida, in the Reports of the British Association for the Advancement of Science for 1851. Digestive organs of the Leech (Ilirudo mediclnalis) : b, mouth; h, i, interior of the stomachal cavity, exhibiting the openings of the lateral caeca (fc) ; g, first pair of stomachal caeca; d, last pair, extending backwards on each side of the intestine e. CIRCULATORY SYSTEM OF THE LEECH. 221 this anatomical fact the highest interest will be shown to belong when explaining the mechanism of respiration in this Annelid. Here the chylous fluid, which in nearly all other Annelids occupies the general cavity of the body, like a cylindrical fluid stratum, separating the intes- tine from the integument, is transferred into the interior of the lateral diverticula of the stomach. The peritoneal chamber, being no longer required, is obliterated by the adhesion of the intestine to the integu- ment : the union of these parts is effected through the medium of a dense spongy layer of capillary blood-vessels, the contents of which are exposed internally to the influence of the fluid contained in the digestive caeca, and externally to that of the surrounding element : hence the mechanism of the respiratory process, and the power enjoyed by this and other abranchiate Annelids of dispensing with all external breathing appendages. (579.) While, however, the peripheral segment of the vascular system in the Leech exhibits proofs of great complexity, the main currents of the blood obey two leading directions. If the body of the worm be lon- gitudinally bisected by an imaginary horizontal plane into a dorsal and ventral semicylinder, then the blood in the primary trunks of the dorsal half will move from the tail towards the head, and in the ventral half from the head towards the tail : this movement prevails equally in the great longitudinal trunks of the integuments and alimentary canal. The transverse or circular movement of the blood is performed by means of branches which run between the main longitudinal vessels : this latter system is divisible into as many portions as there are rings in the body of the worm : each segment of the body under this arrange- ment has its own independent circulation, transverse and longitudinal. Thus the currents describe two excentric ellipses, cutting each other at right angles. Of course, the segmental divisions of the general system communicate with each other at every part, while the longitudinal trunks are common to all the segments. From this description it is manifestly impossible that a distinction of venous and arterial blood can exist in the circulating fluid of this Annelid ; in every part of the cir- cumference of each ring the blood is being arterialized as it is being rendered venous ; the two opposite processes proceed simultaneously in the same capillary system. The blood must be, therefore, as arterial and as venous at one and the same time in the dorsal as in the ventral trunks ; notwithstanding, the dorsal main is recipient, the ventral distributive of the blood : all the secondary currents converge upon the former, and emanate from the latter ; the blood in both is nevertheless identical in physiological properties. (580.) In addition to the main dorsal and ventral trunks, there exist in the Leech two strong and obvious lateral trunks, one on each side (fig. 108, ee). The branches exhibit in their walls a structure pre- cisely the same as that which distinguishes the vascular system in every 222 ANNELIDA. Fig. 108. other part of the body, while the primary lateral trunks are provided with remarkable muscular parietes, their fibres being of the striped kind. The fascicle of the muscle composing the walls is arranged in a manner which is quite distinctive of, and peculiar to, this vessel ; it is coiled with so much regularity as to enclose a perfect cylinder, in which the blood flows : the longitudinal fibres are all suppressed, and therefore the circular fascicles, lined within by a hyaline membrane, constitute exclusively the coat of the vessel. Such a vessel is almost unique in . structure in the animal series; but none other would perform so admirably the peculiar duties for which it is introduced into this part, obviously as a special pro- vision. Its business is to transmit with augmented force a current of blood in a transverse direction from the side to the ovario-ute- rine organs (fig. 108, //), which form a double longi- tudinal series, one on either side of the ventral mesial line in each annular seg- ment of the body. An ex- press branchfrom the latero- abdominal trunk on either side is furnished to these reproductive organs (fig. 108, 7i h)', so that the amount of blood propelled by this vessel, measured in its totality, must be very considerable, and the quan- tity during the generative season must undergo great increase in consequence of the augmentation of size which at that period these organs experience. The lateral longitudinal vessel is strikingly adapted to meet such alternation of extremes : constructed of muscle, it readily yields, under the flow of the blood-tide to the organs to whose wants it ministers, and its parietes augment by accele- rated nutrition during the periods of increased local determination of blood : formed of any other structure than muscle, such admirable adaptive alternations could not happen. (581.) It has been generally considered that, in the abranchiate An- Diagram illustrative of the circulatory apparatus in the Leech (Hirudo medicinal is). (After Dr. Williams.) , great dorsal vessel; c, ventral vessel; dd, intercom- municating vessels between dorsal and ventral trunks ; ee, lateral abdominal trunks; ///, ovario-uterine organs; g, vessels distributed over the csecal append- ages to the stomach ; h h, branches from the lateral abdominal trunks supplying the ovario-uterine appa- ratus. RESPIEATOEY SYSTEM OF THE LEECH. 223 nelidans, the organs provided for respiration are a series of membranous pouches, communicating externally by narrow ducts, or spiracles, as they have been termed, into which aerated water is freely admitted. These 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 connexion with every one of them there is a long glandular-looking appendage, represented in fig. 110, m. (582.) According to the views of M. Duges, which, previous to the appearance of Dr. Williams's interesting memoir, were received with general assent, the two lateral vessels in the Leech are appropriated to the supply of this 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* dorso -lateral, while those which join the lateral trunks to the ventral canal are the later o- abdominal 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 one of these vessels 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 currents will be seen to become completely reversed, so that an undulatory 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. (583.) On examining attentively one of the " respiratory pouches," according to the same authority, its membranous walls are seen to be covered with very fine vascular ramifications (fig. 109, /), derived from two sources : the latero-abdominal vessel (d) gives off a branch (e), which is distributed upon the respiratory sacculus ; and there is another very flexuous vascular loop (&), derived from the lateral vessel itself (a), which terminates by ramifying upon the vesicle (/) in a similar manner. The walls of the loop (6) are extremely thick and highly irritable; but on tearing it across, the internal cavity or canal by which it is per- forated is seen to be of comparatively small diameter ; so that we are not surprised that, although such appendages to the respiratory sacs were detected and well delineated by former anatomists f, their nature was unknown, and they were supposed to be glandular bodies appropriated to some undiscovered use. From the arrangement above described, M. Duges was led to believe that small circular currents of blood exist, * Ann. des Sci. Nat. vol. xv. t Delle Chiaje, op. tit. Moquin-Tandon, Monographic sur la famille des Hiru- dinees. 4to, Montpellier, 1827. 224 ANNELIDA. which are independent, to a certain extent, of the general circulation, and that opposite to each membranous bag ^S- 109- a portion of the fluid contained in the lateral vessel (a) is given off through the muscular tube (>) (which thus re- sembles a pulmonary heart), and, after being distributed over the walls of the supposed respiratory vesicle, and in this manner exposed to the influence of oxy- gen, the blood returns into the general circu- lation*. (584.) The nervous system of the Leech (fig. 110, Tc} consists of a long series of minute ganglia joined by con- necting 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 fila- ments 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 cephalic pair of ganglia that the nerves appropriated to the instruments of the senses are derived ; and we shall therefore not hesi- tate 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 analo- gous, in function at least, with the cerebral masses of more highly organized beings. * As will be seen further on, the so-called "pulmonary hearts" of M. Duges are regarded by Dr. Williams as ovario-uterine organs. Respiratory organs of the Leech, according to M. Duges : a, lateral vessel; 6, pulmonary heart; c, dorso-lateral branch ; d, latero-abdominal vessel ; e, afferent pulmonary branch; /, respiratory sacculus. NERVOUS SYSTEM AND SENSES OF THE LEECH. 225 (585.) The splanchnic system in the Leech consists of three small ganglia, situated in front of the brain, with which they are connected by delicate nervous filaments. All three send branches to the parts around the mouth and to the inferior surface of the alimentary apparatus*. (586.) When, however, we regard the minute size of these as yet rudimentary nervous centres, we cannot expect to find them associated 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 Hirudinidce we have, in addition, distinctly-formed organs of vision, exhibiting, it is true, the utmost simplicity of structure, but neverthe-^ less corresponding, in the perfection of their development, with the con- dition of the cerebral masses in relation with them. (587.) 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 sucking surface of the oral disk, a position evidently calculated to render them efficient agents in detecting the presence of food. The structure of these simple eyes, according to Professor Miillerf, does not as yet pre- sent any apparatus of transparent lenses adapted to collect or concen- trate the rays of light ; but each ocellus, or visual 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. (588.) Leeches, like the generality of the Annelida, are hermaphrodite, every one possessing two complete systems of generative organs, one subservient to impregnation, the other to the production of the ova ; nevertheless these animals are not self-impregnating, but the congress of two individuals is essential to fecundity. (589.) 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. The glands that apparently secrete the seminal fluid are about eighteen in number (fig. 110, 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 deferens, which receives the secre- tion furnished by all the testicular masses placed upon the same side of the mesial line, and conveys it to a receptacle (d), where it accumulates. The two reservoirs, or vesiculce seminales (d d), if we may so call them, communicate with a muscular bulb (c) situated at the root of the penis. * See Brandt und Eatzeburg, Med. Zool. tab. 29. t Ann. d. Sci. Nat. vol. xxii. Q 226 ANNELIDA. Fig. 110. 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 (>). 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. (590.) The ovigerous or female sexual organs of the Leech are more simple in their structure than those that 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 cf about five of the ventral rings of the body. The vulva, or external canal, leads into a pear-shaped membranous bag (fig. 110, g}, which is usually, but improperly, named the uterus. Appended to the bottom of this organ is a convoluted canal (h), that com- municates with two round whitish bodies; these are the ovaria. The germs which are formed in the ovarian corpuscles, therefore, escape through the tortuous duct (h) into the uterus () are placed in successive segments of the body from the seventh backwards ; they * Lectures on Comp. Auat. vol. iii. f Ann. des Sci. Nat. vol. xv. Plan of the circulation in an Earthworm. (After Dr. Williams.) GENERATIVE SYSTEM OF THE EARTHWORM. 235 Fig. 114. 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 bottom 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- municate by a common canal ; and the contained fluid, which, like the seminal secretion of other animals, contains spermatozoids, can readily be made to pass from one to another. (610.) The ovaria (c) are eight large white masses, of a granular texture, from which arise two deli- cate tubes or oviducts ; these have no connexion with the testes, but, running backwards, they become di- lated into two small vesicles at their termination (d), and open by two apertures or vulvae seen externally upon the sixteenth segment of the body : in these ducts, eggs have been detected as large as pins' heads. The following is Dr. Williams's account of this mysterious appa- ratus* : (611.) The generative system of the Earthworm is situated in the immediate vicinity of a thickened ring or band, bounded by abrupt limits, which implicates six or eight of the annuli of the body. This thickening, when closely examined, is found to depend upon an extra- ordinary development of the cutaneous follicles. On the abdominal aspect of this thickened portion suctorial cups are formed, by aid of which, during the congress of two individuals, mutual contact is main- tained ; but the generative segments internally have no relation with this suctorial ring of integument, nor has this latter part anything to do with the true genitalia. It is, like the thumb of the Frog, a mere provision for the mutual apposition of two individuals. The en- larged follicles of this cutaneous ring, moreover, discharge another func- tion they supply the peculiar glutinous secretion which affords a pro- tecting capsule to the ova as they escape from the body. (612.) Every ring in the body of this worm (except a few at the head and tail) contains two ' segment organs,' one on either side of the intes- * Phil. Trans. 1858. Arrangement of the sexual organs in an Earthworm. (After Duges.) 236 ANNELIDA. tine ; they are convoluted and tubular, arising from the ventral aspect of the general cavity near the median line, curving upward around the intestine, and terminating in a fan-shaped ciliated extremity, which is bridled to the septum near its dorsalmost edge. Those segmental organs which are situated anterior to the gizzard are very much larger and more distinct than those that are placed behind it. (613.) The season of the year and the state of the weather have much to do with the condition in which these organs are found. All the speci- mens upon which the following examinations were instituted were taken in the months of July and August, from a rich, loamy, highly cultivated garden soil. This fact it is material to know, since nowhere in the ordinary fields and meadows does this worm attain the same size and plumpness. The generative nisus does not seem to reach its climax until the worm has arrived at a certain period of age and fulness of growth, so that, out of a hundred specimens, only ten or fifteen may be found in that condition which is required for the successful prosecution of these researches. (614.) In Lumbricus terrestris, each ring of the body is divided from the adjacent ones by membranous partitions, which either completely or partially isolate the fluid contents of each annular space. It is pro- bable, however, that the fluid of the general cavity freely oscillates from one extremity of the body to the other, through perforations in the septa. But whether the spaces be isolated or not, the ' segmental organs' which they contain have no connexion whatever with one another. The following account is descriptive of all those which are situated posteriorly to the proventriculus and the generative region. (615.) The tube which connects the free extremity with the fixed end is extremely convoluted, and thickly intermixed with vessels. This intermediate tube is divisible into three distinct portions. First, a smooth-walled membranous part, which extends from the umbrella- like termination to the camerated or cellular portion, which is vigorously ciliated in its interior. The ciliary current sets from the free end, in the direction of the fixed end. The next division of the tube extends from the termination of the smooth membranous part to the commence- ment of the third stage : this portion is not ciliated ; the walls bulge out into lateral cells. The segmental organ in this portion is inextricably blended and matted up with blood-vessels. The fourth division of the tube stretches from the end of the celled portion to the commencement of the muscular part : this again is strongly ciliated ; its walls are thick and contractile. The last, outermost, or dilated portion, bounded ex- ternally by the attached end, is almost invariably, in the months of July, August and September, crowded with the ova and young of a Nematoid Entozoon, which might easily be mistaken for the ova and young of the Earthworm, an error into which Dr. Williams himself fell during his earlier researches. This dilated muscular portion of the segmental GENERATIVE SYSTEM OF THE EARTHWOEM. 237 organ is not ciliated internally, but its walls are capable of contracting vermicularly or peristaltically. (616.) The vascular system in connexion with this apparatus is at certain seasons extraordinarily developed. When present under the most evident circumstances, it may be described as consisting of three parts. First, of two or three large vessels, which curve upwards from the ventral to the dorsal trunks. These vessels thus form a framework by which the slender ciliated tube is held vertically in situ, they them- selves being vertically disposed. (617.) Between the primary vertical trunks extend a vast multitude of secondary branches, which further subdivide to form a dense plexus of smaller vessels, in the meshes of which the ciliated tube is entangled. At certain seasons and conditions of growth, a dense mass of florid blood is attracted to and retained in the region of the segmental organs. It is contained at these times, not in ordinary cylindrical vessels, but in capacious lateral csecal branches, terminating in large, bulbous, pear- shaped extremities, evidently constituting a special provision, the nature of which still remains a matter of conjecture. (618.) The sexual system of the Earthworm we have already found to be centred in a particular region of the body, indicated by the thickened glandular collar above alluded to, which, however, on close examination, is found to be situated several segments posterior to the generative masses contained within. (619.) Now, in studying the visceral contents of each ring within the limits of the generative region, it will be best to proceed from be- hind forwards. The dissector thus comes first upon the largest and most prominent of all the generative masses. They are testes, as may be proved by a microscopic examination of their contents. They have a white, glittering, oily colour. In figure they are intestiniform, the coils, of which there are two or three, being tied together by a kind of mesentery and enclosed in a membranous capsule. These two testi- cular masses, which lie across and fill up several rings, may be traced with perfect clearness to comparatively narrow peduncles, which, when minutely and successfully dissected, will be found to connect themselves intimately with the roots of the ciliated tubes or special segmental organs of the same annular spaces ; or, in other words, the tube and the peduncle of the testicular mass have a common outlet. This outlet cannot be detected on the external surface of the worm ; it is far too minute and closely contracted ; but its position may be clearly ascertained by the termination, internally, of the ciliated tube, proving, according to the views of Dr. Williams, that the generative gland is an outgrowth from, and organically the development of the ' segmental organ.' The two ciliated tubes, one on either side, which are contained in this tes- ticular ring, differ in no respect from those of the non-generative rings but in that of size. They are considerably larger than the latter. 238 ANNELIDA. (620.) The second generative annulus in the order mentioned is ex- clusively ovarian, and it is not difficult to see that the mass and the base of the ciliated tube run together, and become blended into one structure. The most minute dissection fails to isolate the duct which, it may be supposed, leads from the ovary ; but, as in the case of the testes behind, it is certain that the gland and the tube must have a common outlet. The ovary is considerably smaller than the testes ; its capsule is more dense and vascular, and its interior structure is much more copiously supplied with blood. (621.) In the common Earthworm the second, third, and fourth ge- nerative segments are ovarian, each being anatomically only a repetition of the other ; all are constructed upon the same plan. (622.) The fifth segment, from behind, is again testicular, exactly resembling the first, so that the first and the last segments in this re- gion are testicular, the three intermediate ones being ovarian. (623.) The ovaria of Lumbricus are much more transient in their duration than the testes ; the latter, in a certain condition, are always present at every season of the year, the former only in the summer months. The ova, while yet in the ovaria, are beautifully clear trans- parent cells. In August and September they seem to consist of nothing but germinal vesicles ; afterwards appear the germinal spots, and then the rudimentary vitellus. At a subsequent stage, just before their extru- sion from the body, they become covered with a cocoon or characteristic capsule, each capsule containing many ova. This capsule is a compound of chalk and mucus. In the median line between the ovaria there are situated two or more glandular bodies, the contents of which, under the microscope, are found to consist of nothing but carbonate of lime doubtless the source of the chalk. (624.) Both in the ovarian and testicular segments there are sacculi attached to the bases of the segmental organs, which in the former case serve as receptacles for the ova (vitellaria), and in the latter as receptacles for the semen. In one case the ova acquire their calcareous capsules; in the other the sperm-cells become developed into active spermatozoa. (625.) The eggs, when laid, are said by Duges to be two or three lines in length. In fig. 115, 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 (fig. 115, B) to permit its egress. Another remarkable circumstance observable in these eggs is that they very generally contain double yelks, and consequently two germs ; so that a couple of young ones are produced from each. (626.) It is very generally believed that the Earthworm may be multiplied by mechanical section, the separated portions reproducing such parts as are removed in the experiment, and again becoming perfect. KEPKODUCTION BY MECHANICAL SECTION. 239 Careful experiments, made to ascertain how far the statements of former authors upon this subject might be substantiated,, prove that the assertion is not entirely without foundation, al- though by no means to the extent indi- cated 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 essen- tial to existence are removed by the opera- tion, and even the course of the circulating fluids would not be materially inter- rupted by the mutilation ; but that the hinder moiety should be able to repro- duce 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 ob- servation. On cutting an Earthworm in two, the anterior portion is found, in fact, generally to survive; and the wound caused by the operation, becoming gradu- ally constricted, is soon converted into an anal orifice, rendering the animal again complete in all parts necessary for its existence. This, however, is by no means the case with the posterior portion ; for, although it will exhibit for a very long period indications of vitality, no signs of reproduction have been witnessed, and it invariably perishes. (627.) Nevertheless, although it is thus proved that the Earthworm cannot be multiplied by mechanical division, it is stated to be able to reproduce small portions of its body the removal of which does not im- plicate 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. (628.) The experiment of artificially bisecting the body of an Earth- worm, replacing the divided halves with care again in their native Eggs of the Earthworm. 240 ANNELIDA. habitats, invariably, in the hands of Dr. Williams, led to the following results : The cephalic half, by this division of the body, does not lose the power of locomotion. In a few days after the operation, it begins to grow less active and vigorous in its movements, and the annulus at the point of division begins to contract and wither ; in the course of a few more hours it dies it mortifies away. This process of dissolution creeps, in the direction of the head, from one segmental ring of the body to another, until, finally, the cephalic remnant ceases to manifest any signs of life. (629.) The tail-half immediately loses the power of advancing ; it writhes on one spot, and that only in contact with some external body ; its motions become excited, not voluntary, it never re-acquires the power of swallowing earth. The process of decay begins much sooner than in the cephalic half, and extends in the direction of the tail, im- plicating one ring after another rapidly, until the whole perishes. (630.) The little lively Naides, although terricolous in their habits like the Earthworm, are very dissimilar in organization. (631.) In Nais filiformis, so abundant in the freshwater pools of this country, the anatomist is presented with a favourable opportunity of resolving the problem of the circulation. A living specimen placed between two slips of glass, from the perfect transparency of the integu- ments, will exhibit to the eye in a perfect manner all the circulating movements both of the vessels and of the blood. In Nais, the large dorsal vessel (fig. 116, a) is first seen travelling wavingly along the dorsum of the intestine as far as the heart, which corresponds in situa- tion with the intestinal end of the oesophagus. This vessel is enveloped by the glandular peritoneal layer of the intestine, while the coats of the ventral vessel are clear and transparent : the dorsal vessel is endowed with parietes of greater strength and density than the ventral. Each of these vessels dilates into a fusiform heart (fig. 116, a', b'), situated on either side of the oesophagus. These hearts, which are joined together by transverse vessels, pulsate alternately and with exact regularity. In the dorsal vessel the blood moves forwards from the tail as far as the dorsal heart ; thence it descends into the ventral heart, by which it is now propelled, chiefly in a backward direction, partly through the main ventral trunk, and partly through the inferior intestinal. The other portion of the blood conveyed by the great dorsal vessel into the ventral heart (&') passes forwards as far as the head, where its moving power is again reinforced by a cardiac dilatation, which now impels the current from before backwards through a superior cesophageal trunk into the dorsal heart (a'), by which organ the blood received from the region of the ossophagus and coming from the head, as well as that received from the great dorsal and coming from the tail, is urged down- wards into the ventral heart, and thence, chiefly in the direction of the tail, through the ventral and intestinal trunks (f,e); this latter there- CIKCULATION IN NAIS. 241 fore is the true systemic heart. At the oesophageal end of the body, the two primary trunks, dorsal and ventral, are connected together by means of a remarkable class of vessels (g g g), which in this region pro- ceed at successive points from the dorsal oasophageal, and which may be traced in long coils, without division of the vessel, floating in the fluid of the peritoneal cavity. Posteriorly to the heart- centre these vessels emanate from the dorsal intestinal, and correspond precisely with those branches from the same vessel which in Arenicola piscatorum proceed to supply the branchial arbuscles. In Nais, therefore, partly from this analogy, but chiefly from their anatomical relations, bathed by and floating in the chylaqueous contents of the peritoneal cavity, the phy- siologist can experience no difficulty in dedicating these coiled vessels to uses very definite. First, it cannot be doubted that they absorb from this fluid the elements by which the blood-proper is formed and reple- nished ; and secondly, it is in the strongest -degree probable that the true blood is in great part aerated through the agency of these vessels upon the gaseous elements contained in the peritoneal fluid. They constitute the special branchial system (internal branchiae), while they discharge incidentally an absorbent function. In the movement of the blood, then, in Nais as in Lumbricus, there are discernible only two leading directions one forward in the primary and intestinal dorsal vessels, the other backward in the primary and intestinal ventral. It is not possible to trace the blood into the capillary parietal system of the intestine, in consequence of the transpa- rency of the stream when thus minutely sub- divided. In Nais there is also an integumentary system which intervenes between the two pri- mary (dorsal and abdominal) trunks (#,/), ramifying in the substance of the integuments, upon which, in part, a respiratory function may devolve. (632.) The generative system of the Nais, as delineated by Duges, presents a very dif- ferent arrangement to that which exists in the Earthworm. The swollen part of the body, in which the sexual organs are placed, occupies a space of five or six rings, beginning at the eleventh. On each side of the eleventh seg- ment is a minute transverse slit (fig. 117, 6) communicating with a slightly-flexuous canal which terminates in a transparent pyriform pouch or vesicle. The latter contains a clear fluid, wherein minute Plan of circulation in Nais. (After Dr. Williams.) 242 ANNELIDA. 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 arc the orifices leading to Fig. 117. the female portions of the sexual system. The ovaria (d, e) are composed of four large and several smaller masses of a granular character ; and from them proceed long and tortuous oviducts, which, just before their termination at the lateral openings (c), become thick and glandular. These animals most likely copulate like the Earthworm, and lay their eggs in a similar manner. We have already seen, in the Lumbricus terrestris, ova containing two yelks, 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 en- closing 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, sepa- rately from the ovaria, and remain distinct from each other as they pass along the tortuous canal that leads to the external opening; but at length arriving at the thick and glandular portion (c) of the ovi- gerous tube, several of them become en- closed in a common investment secreted by the walls of the oviduct, and are expelled from the body with the outward appearance of a simple egg. The account of the reproductive system of the Naides given by Dr. Williamst is as follows : (63o.) In Nais serpentina, which may be taken as the type of the genus, two of the ' segmental organs ' are present in every ring of the body, one on either side of the intestine. Each organ commences (or, if the course of the contained current be considered, ends) in a single external orifice. At its free internal extremity it is held in a determi- nate position by means of bridles of delicate threads. By the action of large and vigorous cilia, a strong current of fluid is drawn into this open mouth. From the form and structure of the trumpet- shaped extremity, '* Duges, Ann. des Sci. Nat. vol. xv. f Phil. Trans. 1858. Generative organs of Nais. (After Dugbs.) GENERATIVE SYSTEM OF NAIS. 243 as well as from the setting of the ciliary currents at the mouth and base of the tube, it seems beyond the possibility of a doubt that these organs, in this ordinary state, subserve the function of discharging externally the fluid contained in the general cavity of the body ; nevertheless, from their small diameter, it would appear that they are not capable of con- veying externally either the normal corpuscles of the chylaqueous fluid or the spermatic products. The preceding description applies only to the ' segmental organs ' distributed throughout that part of the body of the worm which is situated behind the reproductive mass. (634.) It now remains to investigate the claims of this organ to a new and hitherto unthought-of relation with the generative or reproductive structures ; for in this worm two, or possibly more, of these segmental organs on either side of the body undergo a remarkable increase of size and variation of form. (635.) In Nais it often happens that only one on either side is thus evolved. At some seasons, however, especially when the weather has continued for some time warm and dry, individuals may be found in which two on each side have undergone a marked development ; gene- rally, however, two segmental organs only, the one being developed into the male, the other into the female moiety of the reproductive system, are necessary to the generative maturity of the individual. (636.) The two segmental organs which form the basis of the repro- ductive masses are observed, even on the first view, to be very similar in general outline to those which are repeated in every ring of the body, with which indeed they are identical, differing only in size and in the shape of some of their parts. The dilated portions correspond with the equivalent parts of the other organs ; and the umbrella-like extremities, which are the same in form on the ovarian and testicular sides, are the counterpart of the free ciliated ends of the ordinary organs. The inter- mediate ciliated tubes are specially distinguished only by their greater length and thickness. It is a difficult point to settle the mode in which the generative masses are related to the ciliated tubes ; it may, however, be constantly seen that the ovarian mass moves to and fro with the dilated portion of the tube. It is impossible to detect the opening by which the ova arrive in the interior of the utriculus of the tube ; that they do get there, however, is certain. From this portion of the tube they escape by the external orifice. (637.) The testes on the other side bear the same relation to the tube as the ovary. The utriculus upon one side is represented by the ejacu- latory pouch on the other. At present it is beyond the power of science to explain why these organs, in only one or two annuli of the body, should be implicated in the sexual development, while all the rest remain in abeyance in the condition of mere ' excretionary tubes'; or why the ciliated tube of one side should be changed into the female system, and that of the other into the male. 244 ANNELIDA. (638.) Besides the ordinary mode of propagation by ova, it has long- been asserted that some of the Annelida, at least, are reproduced by spon- taneous division. Bonnet, Miiller, and Duges all agree that this is the case with certain species of Nais ; and in Nais Jiliformis the process of separation has been witnessed from its commencement to its termi- nation. The division was said to occur near the middle of the body of the animal, the posterior half remaining 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 buried itself in the mud, and no doubt there completed its develop- ment. The following is what Dr. "Williams conceives to be the interpretation of the above facts : (639.) Every Nais, as relates to its reproductive apparatus, is identi- cally constituted, and this worm is consequently androgynous. Every individual, towards the latter end of the summer, dies by the bisection of its body. It is not true, as reported by Duges, and before him by Spallanzani, that the fragments into which the body of each worm be- comes resolved are again reconstructed into a perfect whole. Although the sexual system exhibits a tendency to segmental repetition, there devolves upon the large anterior portion, described by Duges, a special function which the rest cannot perform ; and, on the contrary, a duty falls on the posterior segmental units of the system which the anterior cannot discharge. It is consequently evident that neither of the moieties into which the body is resolved during the crisis of the reproductive season can be organically perfect. Such fragmentary organism is want- ing in elements paramountly essential to individuality. (640.) These Annelids are annuals : the term of existence is completed when the organic cycle is once accomplished. They are born during the latter months of one summer, and survive the winter, attain to the maturity of growth, reproduce the species, and die by the spontaneous subdivision of the body into fragments on the arrival of the same season of the succeeding year. This brief round comprehends the history of each individual. Since these worms are monoecious, each shares the common fate. Each contributes by its own death to the multiplication of the species : the species being multiplied, the ends of its own exist- ence are accomplished. (641.) For some time before the fissure of the body occurs, the process of the maturation of the ova is proceeding. Arrived at the matured phase, they escape from the ovarian system into the free space of the peritoneal cavity, wherein they sojourn until the next phase of their growth has been attained. It is during the period marked by the presence of true ova in the chamber of the peritoneum, floating in the DOKSIBRANCHIATE ANNELIDANS. 245 contained fluid, that the division of the body of the parent animal takes place. In each fragment is therefore incubated a consi- derable number of ova. Filled still by the fluid of the peri- toneal cavity, each fragment becomes subservient to the end of hatching the young. It resists decomposition only during the period required for the accomplishment of this purpose. When the ova are committed to the sand, the fragment rapidly disappears by putrefaction. The fissure of the body, thus interpreted, becomes the last act of the parental worm, since the por- tions into which the body is subdivided by fissure never take food. With the fissure the necessity for food termi- nates. If, on the contrary, the division of the body were the first step of a real re- productive operation, charac- terized by the superaddition of new segments to the body, each fragment should grow voracious and consume extra supplies of nourishment in order to provide the necessary pabulum for the reparation of mutilated parts. As this is not the fact, the inference is clear that the division of the body is not the prelude to a series of reconstructive operations by which parts are made "wholes," or mutilations repaired. (642.) DORSIBRANCHIATA. - In the second order of the Annelidans the respiratory apparatus consists of nume- Fig. "sLUrf: Laodicoa antennatn. 246 ANNELIDA. rous vascular tufts, a pair of which are appended to the outer surface of every ring of the body, or, in some cases, only to a few of them. The organs of locomotion, which are attached to each segment, assume various forms, hut are generally composed of short moveable spines, or packets of retractile bristles, usually destined to perform the office of oars. In the annexed figure (fig. 118,1), which represents Laodicea antennata, the general form of these animals is well seen, as is the most usual arrange- ment of the branchial tufts and locomotive setae. In fig. 118, 2, showing an imaginary transverse section of one of the segments, the relative positions of the oars (c, d, e) and of the branchial appendages (b) are likewise indicated. (643.) But the organs of respiration in the Dorsibranchiate Anne- lidans are not always arborescent ; on the contrary, they are not unfre- quently spread out into thin membranous lamellae, 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. (644.) The second class of organs to be enumerated as entering into the composition of the lateral appendages are soft, fleshy, and subarti- culated processes called cirri (fig. 118, 2, c, e) ; 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 much probability, been regarded as instruments of touch. (645.) The setce (fig. 118, 2, d) are, perhaps, the most efficient agents in progression. These are long and stiff hairs disposed in bundles and implanted into strong muscular sheaths. Each packet of setae 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 powerful fins, well calculated to propel the creature through the element it inhabits. (646.) The structure of the mouth in the Dorsibranchiate Annelidans is very peculiar. The first portion of the alimentary canal, or stomach, as it is most erroneously called by some writers, is muscular ; and cer- tainly, when seen in a dead Annelid, it might easily be taken for a digestive cavity. Nevertheless, during life, this part of the alimentary apparatus is destined to a widely-different office ; for it is so constructed that, at the will of the animal, it can be completely everted, turned inside out, and, when thus protruded externally, it forms a very singular proboscis, used in seizing food, and frequently armed with powerful teeth of singular construction. The following figure (119, A), repre- senting the head of one of these worms (Goniada d chevrons, Milne- STKUCTUKE OF THE MOUTH. 247 Fig. 119. Edwards), will give a good idea of this curious organ when fully dis- played ; and in fig. 119, B, the mechanism is exhibited by which its protrusion and retraction are ac- complished. The whole appa- ratus is there seen to consist of two muscular cylinders, placed one within the other, but conti- nuous at their upper margin (B) ; or. to use a familiar illustra- tion, the proboscis may be com- pared to the finger of a glove partially inverted. It is obvious that, in this case, if the inner cylinder be drawn inwards that is, into the mouth, the whole structure becomes short- ened, 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 Mouth of Goniada claviffera , drawn forwards. The internal surface of this remarkable proboscis, moreover, is variously modified in its structure, 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. 122, 6, c, d) f and the surface of the exserted proboscis (c, d) is covered with deli- cate transverse rugae, evidently so arranged as to give tenacity to its gripe. In Goniada it supports two distinct sets of horny teeth, provided for very different 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 (fig. 119, A, d), and probably forms a kind of file, or rather a scraper, wherewith the animal excavates the subterranean galleries in which it lives. The other set does not make its appearance till the proboscis is Fig. 120. Mouth of Phyllodoce laminosa. more completely expanded, and is evidently an instrument of pre- 248 ANNELIDA. hension, formed by two horny hooks (fig. 119, B, a, 6) placed upon an elevated ridge near the entrance of the oesophagus, so as to take a secure hold of any victim seized by this curious mouth. (647.) In Phyllodoce laminosa the teeth form a circle of semicarti- laginous beads encompassing the extremity of the proboscis when that organ is pushed out to its full length (fig. 120, 6), an arrangement well adapted to hold and, perhaps, to crush their prey. (648.) But the most formidable jaws are met with in some of the Nereidiform species, as in Laodicea antennata, of which a figure is given above (fig. 118). When the proboscis of one of these creatures is slightly everted, the extremities of three pairs of strong horny plates Fig. 121. A Jaws of Laodicea antennata. emerge from the mouth : of these, one pair terminates by forming a powerful hooked forceps, while the others 'present strong denticulated margins (fig. 121, A, a, 6, c). The 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 con- nexions. (649.) 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 Nereidce it is provided with numerous lateral pouches, somewhat resembling those of the Leech. In Aphrodite these lateral ca3ca are very long, slender, and branched at their extremities, so that they have been thought by some to be secreting organs, representing the liver. In Arenicola we find, at the termination of the oesophagus (fig. 128, /), two large csecal appendages (e), of unknown office, while the rest of the tube(c) is entirely covered with minute saccoli, the walls of which are decidedly glandular and secrete a fluid of a greenish-yellow colour. (650.) In the majority of the Annelids, observes Dr. Williams, the alimentary system constitutes a cylindrical tube, which bears a general resemblance of outline to the integumentary, this latter forming, with CIRCULATION IN AMPHINOME. 249 respect to the former, an exterior concentric or embracing cylinder. These two cylinders are in no instance in agglutinated contact : a space intervenes, varying in capacity in different species, to designate which the term ' peritoneal/ or splanchnic, may be used with perfect propriety. This space is occupied by a vital or organized fluid charged with cor- puscles, which exhibit, under the microscope, characters distinctive of species. Independently of its physiological uses, this fluid enacts me- chanical functions indispensable to the well-being of the animal : on it, as upon a pivot, the vermicular motions of the intestinal cylinder are performed. (651.) Although, as a whole, forming a cylinder, in no instance does the alimentary canal of the Annelida present the figure of a smooth- walled tube. The parietes are invariably sacculated, and often super- ficially multiplied in the most elaborate manner. In the Lumbriciform species, each segment of the body has its own independent stomach. Those of contiguous segments communicate through an opening con- siderably more contracted in diameter than that portion of the intes- tine from which it leads. Thus the intestine of the Errant Annelids, especially, may be compared to a line of pears, the apex of each suc- cessive pear being applied to the base of its predecessor in the series : if these bases were prolonged on each side, the stomach of the Leech would be the result ; if compressed, that of those species in which the tube is nearly straight. (652.) 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. The annexed figure (122) of an elaborate dissection of an Amphinome (A. capillata}, copied from one of the beautiful drawings contained in the Hunterian Collection *, affords an example of a circulating system in which the propulsion of the blood is effected entirely by vessels, without the intervention of any muscular cavities or heart. In this animal the respiratory organs are penniform appendages placed along the back, and these external vascular tufts communicate 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 (qqq)\ and of these, one marked q' has been turned aside. The blood and nutritious fluids derived from the whole alimentary track are col- lected by the large ventral intestinal vein (n n 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 connexions may be better seen. Besides the blood and nutriment thus derived from the intestine, the branchial plexuses receive the circulating fluid from all * Descriptive and Illustrated Catalogue of the Physiological Series of Compara- tive Anatomy in the Mus. Roy. Coll. Surg. of England, vol. ii. pi. 14. 250 the segments of the muscular en- velope by separate veins (p p) ; and thus the blood from all parts is brought to the gills and exposed to the influence of oxygen. (653.) After un- dergoing respira- tion, the blood is collected from the branchial plexuses by the lateral veins (rrr), from which, through commu- nicating vessels (s s s), it passes into the aorta or great dorsal vessel (t t t) to be dis- tributed through the body. From the aorta large trunks (v v) are given off to form the intestinal ar- tery (w w), which, ramifying over the intestine, commu- nicates with the in- testinal vein (n n) and thus com- pletes the vascular circle*. (654.) In Eunice sanguinea the cir- culatory apparatus consists of a short but capacious dor- sal vessel (fig. 123, l') t which ANNELIDA. Fig. 122. Circulatory system of Amphinome capillata. 7 / ' The parts indicated in the drawing by letters not referred to in the text are the CIKCULATION IN EUNICE SANOUINEA. 251 Fig. 123. rests upon the pharyngeal portion of the digestive tube, without, how- ever, being adherent thereunto, and com- municates posteriorly with a vascular circle that surrounds the stomach and receives the blood from the intestinal parietes through two longitudinal trunks (fig. 123, I) situated upon the dorsal aspect of the alimentary tube. (655.) In its course towards the head, the single dorsal vessel (l') } which is a continuation of the two dorso-intestinal veins (Z), receives several branches, some derived from the digestive canal, others proceeding from the muscles and integu- ments of the neighbouring part of the back. These last-named branches com- municate with a slender cutaneous me- dio-dorsal vessel that runs along the entire length of the body and receives from each segment numerous cutaneous' ramusculi (a?). Lastly, the dorsal vessel gives off from its anterior extremity various branches to the cephalic segments, as well as others which are directed out- wards, as in the Terebella described here- after ; but these, instead of supplying branchial organs, take a backward course, and are either distributed to the parts in the vicinity of the pharynx, or their ra- mifications anastomose with those of the ventral vessel, immediately to be de- scribed. (656.) The ventral vessel (fig. 123, q 1 ) gives origin, opposite each segment, to a pair of lateral branches ; but the confor- mation of these branches, as well as the functions to which they are destined, are very different. Immediately after its origin, each lateral branch becomes con- siderably dilated and bends back sud- denly upon itself, so as to resemble, when superficially examined, an 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 ; h, the anus ; *, k k, the bases of the dorsal and ventral oars, with their surrounding muscles ; 1 1, the dorsal longitudinal muscular bands ; m m, the ventral longitudinal muscular bands. Circulatory and respiratory appara- tus of Eunice sanguined : a, b, c, the antennae ; e, the first segment of the body ; f, lateral appendages, or rudi- mentary feet ; g, pharynx; g', mandibu- lar muscles ; i, intestine ; I', vessel per- forming the functions of an aortic or systemic heart ; I, superior intestinal vessels ; s, their lateral branches (or branchial veins) ; g', ventral vessel ; f, its lateral branches ; t', contractile ampullae, performing the functions of branchial hearts ; u, branchiae ; x, sub- cutaneous vessels of the back. (After Milne-Edwards.) 252 ANNELIDA. ovoid vesicle, or ampulla ; it then runs outwards, furnishing an ascend- ing branch to the alimentary canal, and on arriving at the base of the feet, or locomotive appendages, gives off several small anastomosing branches, forming a sort of vascular network, whence vessels are supplied to the corresponding branchial filaments. (657.) The blood, after being subjected to the influence of oxygen in the branchial appendages, is returned by other transverse vessels which run along the interannular septa to the alimentary canal, where they ultimately discharge themselves into the large median trunks (Z) situ- ated upon the dorsal aspect of the intestine. (658.) Considered generally*, it will be perceived that the distribu- tion of the vascular trunks in the Eunices is pretty much the same as that which exists in the Terebellaa ; but when their functions are con- sidered, and the relations in which they stand relatively to the respi- ratory apparatus, very important differences are at once apparent be- tween the two genera. (659.) In Eunice, it will be seen (fig. 123) that Professor Milne-Ed- wards has described and figured the branchial vessels as ampullated soon after the origin of each from the common trunk, the ampulla? being designed to fulfil the function of branchial hearts. These vessels there- fore, according to the representations of Milne-Edwards, are, in Eunice, the analogues of those remarkable cardiac vessels (pulmonary hearts) de- scribed by Duges in the Leech. The existence of these latter vessels has already been found to be extremely problematical ( 583). According to the observations of Dr. Williams, these vessels in Eunice present nothing approaching to the ampulla? figured in the illustrations of Milne-Edwards. The pouched dilatations are produced by the dissec- tion and exposure to atmospheric stimulus, just as in the Earthworm the moniliform character of the descending vessel was shown to be caused by the stretching. In Eunice the lateral large segmental branches are relatively large at first, but soon divide into three lesser branches, of * The following axioms may be laid down relative to the circulation of the blood: 1st. In all Annelids, the blood flows in the great dorsal trunk from the tail towards the head. 2nd. In all Annelids, the blood flows in the great ventral trunk from the head towards the tail. 3rd. In the whole integumentary system of vessels, the blood moves from the great ventral towards the great dorsal trunk : this movement constitutes the annular, or transverse circulation. The main current of the blood in the ventral trunk pursues a longitudinal course until exhausted by successive lateral deviations. 4th. In Annelids, the intestinal system of vessels consists of four longitudinal trunks : one dorsal, which may be called dorso-intestinal ; one ventral, which may be distinguished as the sub-intestinal ; and two lateral. These several trunks are joined together by circularly-disposed branches bearing a dense glandular capillary system. In the inferior intestinal system the general movement of the blood is from before backwards ; in the circular branches, from the ventral towards the dorsal trunk. REPRODUCTION BY GEMMATION. 253 which one goes to the feet, the other to the intestine, and the third to the branchiae, from which the blood returns into the dorsal vessel, which in this worm, accordingly, carries arterial blood. The sudden- ness of this division favours the imprisonment of a drop of blood in the first stage of the vessels, the drop thus enclosed occasioning a bulged enlargement ; but this appearance is altogether accidental. The blood is admitted into and returned from the branchiae by alternate movements of contraction and dilatation : these movements are not simultaneous in all the branchiae, but various and independent in each individually, the afflux into one being synchronous with the efflux of blood from those contiguous. This contractile power is by no means peculiar to these vessels. The motion of the blood in the vessels of every part of the body of the Annelid is effected, not through the agency of uniformly- travelling undulatory contractions of their coats, but by complete con- tractions and relaxations of successive portions of the tube ; so that, during the instant of contraction, the cylinder of the vessel in the part contracting is completely emptied of blood, the sides collapsing and meeting in the axis ; and during the period of dilatation the same por- tion of the vessel becomes densely distended with blood : and this is the true mechanism of the circulation in those species, even, in which a central propulsive organ exists for example, in Neds and Arenicola* In no part of the system, therefore, is the superadded contractile bulb required as an agent of circulation, since this contractile power re- sides in every part of every vessel in virtue of the muscularity of its parietes. (660.) A general survey of the circulation in Eunice will suffice to satisfy the physiologist that no part of the system contains pure arterial, and no part pure venous blood. Into the double dorsal trunk arterial blood is poured from the branchiae ; but to the same trunk the intestinal branches contribute venous blood : the mingling of these two classes of currents in the same trunk must result in blood of an intermediate quality. It is, then, manifest that the great subneural trunk, which in this worm is both systemic and branchial, must distribute blood of com- position intermediate between venous and arterial. No part of the circulatory apparatus therefore contains pure arterial blood except the efferent branches of the branchiae. (661.) In some Annelidans we still find that gemmation performs a very important part in the reproductive process : the multiplication of the individual segments of the body depends entirely upon this mode of increase. But this is not all : it not unfrequently happens that when these animals have attained their full growth, a constriction becomes apparent near the posterior extremity, immediately behind which a proboscis and eyes are developed, forming the head of a new animal subsequently to become separated by spontaneous fissure (fig. 124) ; and even as many as six of these strangely-formed offsets have been 254 ANNELIDA. Fig. 124. counted by Milne-Edwards in continuity with each other*. The process of division is represented in fig. 124 : the hinder part of the body, including about seventeen seg- ments, is seen to be gradually separated from the anterior or larger portion ; and moreover, at the point of separation, a new head, with eyes and tentacular cirri, is distinctly formed. " In one case," says Miiller, " I found a mother to which three fetuses of different ages adhered 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 seven- teen segments and a head. (662.) From various observations, it would seem that similar phenomena present themselves during the develop- ment of other Annelidans ; proving that the bodies of these animals grow by the successive formation of new segments, or zonules, which sprout from those already in existence, in accordance with a funda- mental plan, and become arranged in regular sequence, one behind the other. (663.) It is likewise evident that the two extreme portions of the body, namely the oral and anal segments, are first formed, and that it is in the space between these that all the segments of the trunk, how- ever numerous, have their origin, their development being carried on in a single series, which is progressively extended from before, backwards, by the continual addition of new segments, which are so disposed that the relative age of each ring is indicated by the position which it occupies. Each newly-formed zonule is invariably interposed between the last- developed segment and the anal ring ; so that it becomes natural to inquire from which of these two it more immediately derives its origin, a question that at first might appear of difficult solution, but which seems to be set at rest by the following considerations. From the ob- servations of M. de Quatrefagesf, it appears that in some genera, at a * The phenomena of fissiparous generation in these Annelidans will be better un- derstood by reference to Mr. Newport's important discoveries relative to the growth of the Myriapoda. Vide post, 732. t Ann. des Sci. Nat. 1844. Gemmiparous reproduction of Cirrhatula. ARENICOLA PISCATORUM. 255 certain period of their growth, a new individual, entirely appointed for sexual reproduction, is developed from the posterior part of the body, from which it separates itself after having remained for some time adherent thereunto. Now these young Annelidans, thus formed by a process of gemmation, are developed precisely in the same position as the new segments of the larvse, that is to say, between the anal ring and the last zonule of the trunk ; but they are not all formed at the same time, and, from the different stages of development at which they had each arrived in the individual described above ( 661), it was evident that the youngest were placed nearest to the trunk of the parent animal. The first-formed young one, therefore, had been, as usual, primarily situated between the terminal segment of the trunk of the adult Annelid and its anal seg- ment, which, being consequently pushed backwards, had ceased to be- long to the parent stock and had become a constituent part of the newly-formed young one; the second must, consequently, have been developed between that first formed and the same terminal joint of the trunk of the parent, and could have had no relation whatever with the original caudal segment, and so on for the third and fourth, &c., proving that the penultimate segment of the body, and not the caudal ring, con- stitutes the point from which development emanates. The gemmiparous production of a new individual resembles, therefore, to a certain extent, the formation of the new zonules in the body of the larva : only, in the latter case, the reproductive ring loses its creative power as soon as it has given birth to a new segment, with which it becomes intimately connected, and which, in its turn, assumes the reproductive faculty ; whilst, on the contrary, in the process of multiplying individuals by gemmation, the product becomes, to a certain extent, separated from the economy of the parent animal, and the reproducing segment retains its gemmigerous faculty and gives origin to a series of new beings, one after the other, the last-formed pushing their predecessors further back as they are successively developed. (664.) Some curious speculations have been entertained by conti- nental writers relative to this mode of propagation. The tail of the original Nereis is still the tail of its offspring ; and however often the body may divide, still the same tail remains attached to the hinder por- tion, so that this part of the animal may be said to enjoy a kind of im- munity from death. (665.) In Arenicola piscatorum (fig. 125), a worm met with abun- dantly upon our own coasts, and eagerly sought after as a bait by fisher- men, who dig it from the holes that it excavates in the sand, the branchi (6) are confined to the central portion of the body, where they form on each side a series of bunches, remarkable during the life of the creature for their beautiful red colour, derived from the crimson blood that cir- culates copiously through them. (666.) Respiration is performed in Arenicola by means of naked A 256 ANNELIDA blood-vessels, projecting, in the adult worm Fig. 125. (fig. 125, 6), at the root of the setiferous pro- cesses upwards and outwards one-fourth of an inch from the surface of the body. They are limited in number and distribution to the fourteen or sixteen middle annuli or segments. These external branchiae are commonly de- scribed as forming simply arborescent tufts ; the division of the vessels is, however, found, on more minute examination, to be regulated in accordance with a fixed principle. When fully injected with blood, the vessels of each branchia form a single flattened plane, which rises obliquely above and across the body im- mediately behind each brush of setse. In the adult animal each gill is composed of from twelve to sixteen primary branches arising from a single trunk that proceeds from the great dorsal vessel': the vessels in the bran- chial tuft describe zigzag outlines, the se- condary branches projecting from the salient point or the outside of each angle of the zig- zags, and the tertiary from similar points on the secondary branches. This mode of division, occurring in one place and in all the smaller branches, results in a plexus of vessels of great beauty of pattern or design. Each branchial tuft and each individual vessel pos- sesses an independent power of contraction : in the contracted state the tuft almost dis- appears, so completely effected is the empty- ing of the vessels. The contraction or systole in any given tuft occurs at frequent but irre- gular intervals : this movement does not take place simultaneously in all the branchia3, but at different periods in different tufts. The vessels have the appearance of being quite naked; and if examined in the living state, each ramuscule seems to consist of only a single trunklet : if this were really the case, it would of course resolve itself into a tube end- ing in a cul-de-sac, and the blood-movement would be a flux and reflux ; but by injection it is easy to show that the finest division of the branchial arbuscule contains a double Amiicoia piscator CIECULATION IN THE DOKSIBRANCHIATA. 257 Fig. 126. vessel enveloped in a common muscular (although extremely dia- phanous) sheath. That these vascular sheaths, which are only fine pro- ductions of the integuments, are furnished with voluntary muscular fibres, is proved by the rapid and simultaneous retraction of all the branchiae into the interior of the body which follows when the animal is touched. In Arenicola, as in all Annelida in which the vessels of these organs are naked, the branchiae are destitute of vibratile cilia ; and it will be found that under such circumstances, namely when the branchial vessels occur as naked projections from the external surface, the description here given of these organs in Arenicola will apply in every respect to all other Annelida so furnished. It will be observed that in all the dorsibranchiate genera furnished with branchiae such as those described above, the true blood, circulating in its proper vessels, is found to be exclusively the seat and subject of the respiratory pro- cess ; the fluid in the peri- toneal cavity, abundant in quantity and highly organized though it be, does not in the least degree participate in this great function. The Dor- sibranchiate Annelids may, however, be divided into two great groups, of which one would comprehend those ge- nera in which the function of breathing devolves exclusively on the true blood, while the other would be characterized by the fact that the branchiae are constructed so as to permit more or less completely the exposure, in conjunction with the blood-proper, of the chylaqueous fluid of the visceral cavity to the influence of the surrounding aerating ele- ment. Thus it will be seen that when the branchial apparatus is penetrated by two separate and distinct fluids, coordinate probably in organic properties, the vascular system of the body generally will be found so much the less developed, in proportion as the peritoneal fluid supplants the blood in the branchiae. In those races of Dorsibranchiate worms possessing both these kinds of circulation, naked unciliated blood-vessels no longer form exclusively the branchial organs; loose and large-celled tissue (fig. 126, a a} is superadded to the proper blood- vessels, which are far less in relative size than those in the former variety of branchiae ; into the cells of this tissue the fluid of the visceral cavity insinuates Itself, its course being marked by a slow motion. There exists, however, another point of structural difference between the 258 ANNELIDA. Fig. 127. branchial organs of this group and those of the former, viz., that wherever the fluid of the peritoneal cavity is admitted into the interior of the branchial organs, the latter are invariably supplied more or less pro- fusely with vibratile cilia. (667.) In the- branchial structure of worms thus constituted, the branchial appendages are found, instead of being composed of naked vessels, to present the appearance of round or laminated organs, into which the fluid of the visceral cavity freely penetrates. (668.) The blood-system is more concentrated in Arenicola than in any known Annelid. A large dor- sal trunk (fig. 127, a; fig. 128, i), at the anterior three-fourths of the body receiving exclusively the efferent vessels of the branchiae, proceeds forwards from the tail and empties itself into the cardiac cavities, of which one is situated on either side of the oesophagus (fig. 127, b V ; fig. 128, b b). An- other vessel, proceeding from the head towards the heart, empties it- self into the same cavity with the former. The blood then enters a second cavity (fig. 127, c' c'), more ventrally situated, by which it is partly propelled forwards into the suboesophageal trunk, but princi- pally backwards into the great lon- gitudinal trunks of the alimentary canal. The blood returning from the intestinal system of vessels reaches the dorsal intestinal (g) (lying in the median line under- neath the dorsal trunk), from which the current diverges laterally at right angles into the branchias (//). This conformation differs from that prevalent in all other Dorsibranchiate Annelidans, in which the great ventral trunk is the source of the branchial arteries. But the typical plan of the circulation is observed in the system of Arenicola at the posterior half of the bran- chial division of the body, where the afferent vessels of the branchiae emanate from the ventral trunk. It may be necessary to explain that the motion of the blood in that part of the circulating system which is anterior to the heart is the reverse of that posterior to this centre. The Plan of the circulation in Arenicola. (After Dr. Williams.) CIRCULATION IN THE DORSIBRANCHIATA. 259 I 1 -i ventral cBSOphageal carries the blood forwards, and the dorsal backwards towards the heart. (669.) The independent contractile Fi g- 128 (ergo circulating) power of each indi- vidual vessel may be very completely proved by an examination of the branchiae of a living Arenicola. A single ramuscule in the branchial tuft may contract and empty itself while the surrounding branches are expanding diastolically. There is no synchronism in the circula- tory movements of these vessels. Both the afferent and efferent vessels of the branchiae are long and tortuous, but discover no cardiac ampulla? in any part of their course. In Arenicola the peri- toneal chamber is filled with a highly corpusculated fluid, the basis of which consists of sea- water, and the presence and movements of which are indispen- sable to the circulation of the blood- proper. By this remarkable mass of fluid, the slender tortuous vessels are shielded from injurious pressure. (670.) In Arenicola piscatorum* the generative apparatus consists of six late- ral pouches, which during the months of July and August are in a condition of extreme vascularity. They are quite visible to the naked eye by their bright red colour. The minute structure of these pouches can be studied by carefully dissecting them from their attachments to the abdominal wall of the cavity and placing them under the microscope. One of them thus viewed will be seen to con- sist of a sac or bag, the fundus of which is csecal. The interior cavity throughout the upper three-fourths is a single un- divided space. The attached extremity is formed into two very distinctly marked channels or tubes, the interior of which is formed of cilia that beat in opposite directions. In the glandular tube, the ciliary current sets towards the cavity of the organ ; in the * Dr. Williams, Phil. Trans. 1858, p. 118. s2 Circulation in Arenicola. 260 ANNELIDA. simple tube it sets out from this cavity. In the fundiis the current leads up on one side, and down towards the outgoing limb on the other. The two limbs by which the organ is tied to the walls of the perigastric chamber are not similarly formed. The ingoing limb ex- hibits a more glandular character, and its walls are considerably thicker and more richly supplied with blood, than the other. About the middle of its course it enlarges into a round gland-like body, the axis of which is perforated by the tube. The ingoing limb commences in an ex- ternal orifice on the abdominal surface of the animal ; so that the in- going current which it serves to convey can only consist of sea-water, which being thus introduced into the cavity, is driven out again, in whole or in great part, by outwardly-acting cilia through the limb, which also opens externally : none penetrates into the cavity of the body. Nevertheless Dr. "Williams has no doubt, although he has never been able to demonstrate the fact, that the outgoing leg of the looped organ not only communicates directly with the exterior, but also, by a lateral process, opens into the cavity of the body ; and by means of this last process, the ova in the female and the sperm-cells in the male reach the perigastric chamber. The ova in the female and the sperm- cells in the male are abundantly found in the fluid of the general cavity ; and the ova may also be actually seen in large crowds in the outgoing limb. The ova at this point consist of clear, pellucid germinal vesicles : the vitellus has not yet appeared ; after they have sojourned for some time in the general cavity, the latter begins to show itself. According to Dr. Williams, then, the segmental organ is the true ovary in the female and the true testis in the male. The ingoing limb of the organ is a highly glandular structure : its vessels are densely packed and spe- cially arranged ; its walls are thick and stromatous ; and at its mid-point is a notable glandular development. From the vascular system of this gland proceeds the great vascular organ. (671.) To the one side of this great vascular system there are ap- pended peculiar ca3cal pouches, from the other a dense capillary plexus. This vascular appendage is the morphological equivalent of the blood- system connected with the ovogenetic limb of the segmental organ of the Leech, and of the botryoidal apparatus of vessels connected with the segmental organ of Lumbricus, and must either be the receptacle of an extra supply of blood to an organ susceptible of periodical expansion, or it excretes something from the blood-proper into the cavity of the segmental organ which is essential to the further development of the generative products. It is quite certain that the ova and sperm-cells pass through the last stage of their development in the perigastric chamber. How they escape out of this chamber has never yet been proved. (672.) Rathke and Grube have argued that Arenicola is androgynous. De Quatrefages, however, from his knowledge of the development of the POWEE OF SPONTANEOUS DIVISION. 261 spermatic particles, has long recognized the existence of separate sexes* ; and long before this, Stannius had concluded that the sexes were sepa- rate, from the fact that in different individuals the contents of the general cavity of the body were different f. Stannius also observes that the parent sperm- cells leave the segmental organs (his testes) before the formation of the spermatozoa, which are found only in the cavity of the body. Krohn, Frey, and Leuckart assert that the ova and spermatozoa are developed free in the general cavity of the body. (673.) In relation to the spontaneous division of the Annelida, Dr. Williams observes : It is true that, towards the latter end of every summer, certain species of worms are multiplied by a cutting across the body at one or more points. If the fissure occurs at more than one point, the animal, of course, becomes divided into more than two pieces. This circumstance seldom happens. The fissure in Arenicola generally occurs somewhere within the middle third of the body, securing a few branchial tufts for each fragment. The tail, however, is sometimes detached, and sometimes the division happens very near the head. This process, in Arenicola, happens during July and August. The ce- phalic and caudal pieces continue for some time to writhe in the sand, somewhat further down in the soil from the surface than the perfect individuals. Towards September, the fragments (both that attached to the head and that belonging to the tail) dissolve away, ring by ring, and finally disappear by decomposition. If the fragment examined be that of the tail, it will be observed at the point of separation to exhibit an eversion of the edges, placing the alimentary canal exteriorly ; and a very evident increase of size in the vessels also occurs, accompanied by a tumefied state of all the structures of the part. From this latter fact, it is easy to be misled into the idea that the vessels can become enlarged for no other purpose than that of repairing the injury done by the fissure, or, perchance, of reproducing the part detached by that process. Such would naturally be the meaning which a physiologist would attach to the swollen appearance of the blood-vessels. But such is not the conclusion to which the careful practical observer is conducted by the study of the actual phenomena of the process. It is, of course, in- disputable that nature accomplishes some adequate object by the fissure of the body of the worm ; but that_object, whatever else it be, is unques- tionably not that of multiplying the species. The tail-fragment never, as can be proved by easy observation, produces a single new ring or segment of the body. If this be true, how completely improbable must be the statement that the headless piece is capable of producing a new head ! In Arenicola, Dr. Williams can confidently declare that such reproductive properties as those implied in the re-formation, and that too by a remnant of an integral part of the body, do not exist. It is equally inaccurate to maintain that a new tail is formed by the cephalic * Comptes Eendus, xvi. 1843. t Miiller's Archiv, 1840. 262 ANNELIDA. Fig. 129. fragment. This half of the divided worm, like the former, gradually presents evidences of decay: it becomes less and less irritable, the muscles and integuments begin to decompose, the blood-vessels of the branchi become black, and the whole disappears by the dissolution of the structures. (674.) The APHRODITACE^:, or Sea-mice, are remarkable for the long hairy tufts with which their pedal appendages are generally furnished (lig. 129). Nothing can exceed the splendour of the colours that 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 efful- gence only comparable to that which adorns the breast of the humming-bird. But it is not for their dazzling beauty merely that these seta3 are re- markable ; they are not un- frequently important weapons of defence, and exhibit a com- plexity of structure far be- yond anything to be met with in the hair of higher animals. In the Aphrodite aculeata, for example (fig. 129, A), they are perfect harpoons ; the point of each being provided with a double series of strong barbs (fig. 129, 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. (675.) But here we cannot help observing an additional provision, rendered necessary by the construction of these lance-like spines. We have before noticed that the bundles of setae are all retractile, and can be drawn into the body by the muscular tube from whence 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 beautiful as it is simple. Every barbed Aphrodite aculeata. KESPIEATION OF APHEODITACE^. 263 spine is furnished with a smooth horny sheath (fig. 129, c, a, 6), com- posed of two blades, between which it is lodged ; and these, closing upon the barbs when they are drawn inwards, effectually protect the neighbouring soft parts from laceration. (676.) In the Aphrodite we have an additional appendage developed from the upper part of each lateral oar, in the shape of a broad membra- nous scale, which, arching inwards over the back (fig. 130, c), forms with its fellows a series of imbricated plates, or elytra, as they are techni- cally named (fig. 129, A). Each of the elytral scales is formed by a dou- ble membrane, between the laminae of which at certain seasons the eggs Segment of Aphrodite. are found to be deposited, a situation evidently adapted to ensure the exposure of the ova to the influence of the surrounding element, and thus to provide for the respiration of the embryo. (677.) The Aphroditaceae, indeed, constitute a group of Annelids to which the term 'dorsibranchiate' by no means correctly applies ; that is, in the majority of species embraced in this order, no branchial append- ages exist, either on the dorsum or any other part of the body. Respi- ration is performed on a novel principle, of which no illustration occurs in any other family of worms. In all the Aphroditacea3 the blood is colourless. The blood-system is in abeyance, while that of the chyl- aqueous is exaggerated. Although less charged with organic elements than that of other orders, the fluid of the peritoneal cavity in this famiy is unquestionably the exclusive medium through which oxygen is ab- sorbed. The true Aphrodite type of respiration occurs in Aphrodite aculeata (fig. 129, A). In this species, the tale of the real uses of the elytra or scales is plainly told. Supplied with a complex apparatus of muscles, they exhibit periodical movements of elevation and depression. Overspread by a coating of felt, readily permeable to the water, the space beneath the scales during their elevation becomes filled with a large volume of filtered water, which during the descent of the scales is forcibly emitted at the posterior end of the .body. It is important to remark that the current thus established laves only the exterior of the dorsal region of the body. It nowhere enters the internal cavities; the latter are everywhere shut out by a membranous partition from that spacious exterior enclosure bounded above by the felt and elytra. The complex and labyrinthic appendages of the appendiculated stomach lie floating in this fluid and in the chambers which divide the roots of the feet. From this relation of contact between the peritoneal fluid and the digestive caeca, which are always filled by a dark-green chyle, 264 ANNELIDA. it is impossible to resist the conclusion that the contained fluid is really a reservoir wherein the oxygen of the external respiratory current becomes accumulated. From the peritoneal fluid the aerating element extends in the direction of the ca3ca, and imparts to their contents a higher degree of organization. These contents, thus prepared by a sojourn in the caaca of the stomach, become the direct pabulum for replenishing the true blood which is distributed in vessels over the parietes of these chylous repositories. The sequence of events now indicated will convey to the mind of the physiologist a clear idea of the mechanism of the processes both of respiration and sanguification, by an arrangement strikingly analogous with what we have already seen in the Asterida3 amongst Echinoderms . (678.) In order to arrive at a correct knowledge of the reproductive system of this Annelid, specimens of both sexes should be examined in the spring, and again in the autumn. A good example of a female Aphro- dite being obtained, the dissection should be thus proceeded with : Pin the animal down to the trough with its back upwards. Open it by a longitudinal incision extending from the tail to the head. The in- cision should cut through the scales, felt, and integuments, in order to lay open the spacious perigastric chamber. The integuments should now be carefully stretched, and pinned down to the sides, so as to expose the interior. Let the dissection be then gently floated in salt water, and the parts will present the appearance here described. A network of minute tubes or threads will be seen to twine round and embrace the diverticula of the alimentary canal. (679.) The entire alimentary system must next be taken away, and with it necessarily a considerable portion of the reproductive network. A view will thus be obtained of the attached ends or roots of the branched segmental organs. These roots will be found to equal the ali- mentary ca?ca in number, and therefore that of the feet which are situated posteriorly to the proboscidiform oesophagus. They appear under the character of pyriform tubuli, commencing or ending in a single external orifice. Internally they are lined with ciliated epithelium, the cilia being large, dense, and acting with great force and vigour. The current raised by these cilia sets up on one side and down on the other. The ciliated epithelium ceases at the point where the primary branches divide. All the rest of the organ is unciliated, and filled with the re- productive products. This portion is elaborately branched, the branches twining round the diverticula of the stomach. No microscopic object can be more beautiful than a portion of this tubular network. The individual tubes are bridled on one side and glandular on the other. A similar structure is exhibited by the male tubes*. (680.) TUBICOLA. The two preceding orders of Annelidans are erratic ; but in the third we find creatures inhabiting a fixed and per- * Dr. Williams, Phil. Trans. 1858, p. 134. SEEPULA CONTOETUPLICATA. 265 manent residence that 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 as grains of sand, small shells, or fragments of various materials, by means of a secretion that exudes from the surface of the body and hardens into a tough membranous substance ; such is the case of Terebella Medusa (fig. 132). In other cases, as in the Serpula contor- tuplicata (fig. 131), the tube is homogeneous in its texture, formed of calcareous matter resem- bling the shells of certain bivalve mollusca, and ap- parently secreted in a similar manner. These tubes are generally found incrusting the surface of stones or other bodies that have been immersed for any length of time at the bottom of the sea ; they are closed at one end, and from the oppo- site extremity the head of the worm is occasionally protruded in search of nourishment. It must be evident that, in animals thus en- cased, the character of the respiratory apparatus must be considerably modified*; instead, therefore, of the numerous branchiaB appended to * M. de Quatrefages gives the following resume of the various modifications met with in the respiratory apparatus of the Annelidans : ' 1. Eespiration general and entirely cutaneous (Lwnbriconereis). ' 2. Eespiration cutaneous, but confined to certain segments ( Cfuetopterus). ' 3. Eespiration cutaneous, but confined to certain points of each segment (Nereis). ' 4. Eespiratory organs taking the form of a simple caecum or bladder ( Grlycera}. ' 5. The branchiae characterized more and more by the formation of a canal in connexion with larger or smaller lacunae. ' 6. These branchiae may be distributed all along the body (Eunice sanguined}. ' 7. They may be confined to a certain number of segments placed towards the in ddle of the body (Arenicola, Hermella). " 8. They may all be placed at the extremity of the body, so as to form a double tuft." " The breathing is accomplished," says Dr. Williams, " in every species, the Earth- worm not excepted, in strict conformity with the aquatic principle. No known Anne- lid respires on the atmospheric model. In every Annelid the blood, though variable Serpula contortuplicata. 266 ANNELIDA. the segments of the body which we have found in the Dorsibranchiate order, the respiratory tufts are all attached to the anterior extremity of the creature, where they form most elegant arborescent appendages, generally tinted with brilliant colours, and exhibiting, when expanded, a spectacle of great beauty. In some species, as in that represented in fig. 131, there is a remarkable provision made for closing the entrance 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 operculum that accurately fits the orifice of the shell, and thus forms a kind of door, well adapted to prevent intrusion or annoyance from external enemies. (681.) The curious habitation of the Terebella Medusa is constructed by cementing together minute shells and other small bodies (fig. 132). In neither case is there any muscular connexion between the worm and its abode ; so that the creature can be readily drawn out from its residence in order to examine the external appendages belonging to the individual segments of its body. When thus displayed (fig. 133), the modifications conspicuous in the structure of the lateral oars are at once seen to be in relation with their circumscribed movements, and offer a wide contrast to the largely-developed spines, setae, and tentacular cirri met with in the Dorsibranchiata. In the upper part of the body, rudimentary pro- tractile bunches of hairs are still discernible, but so feebly developed that their use must evidently be restricted to the performance of those motions by which the protrusion of the head is effected ; while upon the posterior segments even these are obliterated, the only organs attached to the rings being minute foot-like processes adapted to the same office. The tentacular cirri, which were likewise distributed along the entire length of the Dorsibranchiate order, are here transferred to the head, where they form long and delicate instruments of touch, and, most probably, assist materially in distinguishing and seizing prey. The branchiae, likewise, are no longer met with upon the segments enclosed within the tegumentary tube, but are placed only in the immediate in colour, is non-corpusculated. The converse is true of the chylaqueous fluid, which in every instance abounds in regularly and determinately organized floating cells." " In the Annelida the function of respiration is discharged under two remarkably distinct conditions. Under the first, the chylaqueous fluid alone is submitted to this process; under the second, the blood-proper fulfils the office. The mechanical organs subservient to this function under the former are constructed on a plan dia- metrically different from that of those provided under the latter circumstances. In the Annelid, the true blood and chylaqueous fluid, though coexistent in the same organism, constitute two perfectly distinct and independent fluid-systems. There is between them no direct communication of any sort ; they are, physically, very dissi- milar fluids. An order of branchial organs must therefore be recognized in which, in equal or unequal proportions, the chylaqueous fluid and the blood-proper, either in the same or in distinct appendages, participate in the process of respiration." Ann. & Mag. Nat. Hist. 1853, vol. xii. p. 393. TEREBELLA MEDUSA. 267 Fig. 132. Fig. 133. Tcrebella Medusa. 268 ANNELIDA. vicinity of the neck, where they form fanlike expansions, or ramified tufts, so arranged 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 upper lip, but is unpro- vided with any dental structure. The alimentary canal is generally a simple and somewhat capacious tube that traverses the axis of the body ; but in some species, as in Sabella pavonina, it assumes a spiral course, making close turns upon itself from the mouth to the anal aper- ture, which is always terminal. (682.) The branchial organs, in the genus Terebella*, appear under the form of blood-red tufts, proceeding from three separate root-vessels on either side of the occiput. The vessels divide for the most part dichotomously, forming an arborescent bunch of florid blood-vessels ; each ramusculus is enclosed in a delicate cuticular envelope perfectly destitute of cilia, and is, moreover, double that is, composed of an afferent and efferent vessel. Although extremely transparent and attenuated, the cuticular structure embracing these branchial blood- vessels must include some retractile fibres, since each separate ramus- culus may be emptied and rendered bloodless by the compression of the parietes, a provision which frequently exists in many parts of the cir- culating system of the Annelida. (683.) The cephalic tentacles of the Terebellae present a problem interesting alike to the physiologist and the mechanician. Prom their extreme length and vast number, they expose an extensive aggregate surface to the agency of the surrounding medium. They consist, in Terebella nebulosa, of hollow, flattened, tubular filaments, furnished with strong muscular parietes. Each of these hollow band-like tentacula may be rolled longitudinally into a cylindrical form, so as to enclose a hollow semicircular space if the two edges of the band meet, or a semi- cylindrical space if they only imperfectly meet. This inimitable mecha- nism enables each filament to take up and firmly grasp, at any point of its length, a molecule of sand, or, if placed in a linear series, a row of molecules. But so perfect is the disposition of the muscular fibres at the extreme end of each filament, that it is gifted with the twofold power of acting on the sucking and on the muscular principle. When the tentacle is about to seize an object, the extremity is drawn in, in consequence of the sudden reflux of fluid in the hollow interior ; by this movement a cup-shaped cavity is formed, in which the object is securely held by atmospheric pressure: this power, however, is immediately aided by the contraction of the circular muscular fibres. Such are the marvellous instruments by which these peaceful worms construct their habitations, and probably sweep their vicinity for food. (684.) The inferior aspect of each of these tentacles is profusely clothed with cilia, and this side is thinner than the dorsal. The peri- * Dr. Williams, loc. cit. p. 194. TEEEBELLA NEBULOSA. 269 toneal fluid, which is so richly corpusculated, and which freely enters the hollow axes of all these tentacles, is thus brought into contact with the surrounding water. (685.) In addition to the two important uses already assigned to the tentacles in the Terebellee, they constitute also the real agents of loco- motion. They are first outstretched by the forcible ejection into them of the peritoneal fluid, a process which is accomplished by the undulatory contraction of the body from behind forwards ; they are then fixed like so many slender cables to a distant surface ; and then, shortening in their lengths, they haul forwards the helpless carcass of the worm. (686.) In the Terebellae, in consequence of the concentration of the tentacles and branchise around the head, the blood- system at this extre- mity of the body discovers a great increase of development. The peri- toneal fluid in this genus is very voluminous and densely corpusculated ; the system of the blood-proper is, notwithstanding, elaborate and full- formed. The chamber of the peritoneum is one undivided space, the segmental partitions of the Earthworm and the Leech being here re- placed by limited bands proceeding from the intestine to the integu- ment, tying together these two cylinders so, however, as to permit one to move within the other with remarkable freedom. (687.) The great dorsal vessel in Terebdla nebulosa is limited to the anterior of the body (fig. 134, a). It emanates chiefly from a large circular vessel (6) embracing the oesophagus, and which receives all the blood from the intestinal system. In this species, therefore, the primary and intestinal dorsal trunks over the whole intestinal region are united, or the former vessel is superseded by the latter. (688.) On the dorsal view of the oesophagus, a large, pulsatile, fusi- form vessel () is displayed on the first laying open of the integument in a longitudinal direction. Slightly attached to the structure on which it rests, it appears as if suspended in the fluid of the peritoneal cavity. Advancing to the occipital ring, it breaks out into six branches (d), of which three proceed to the branchiae of each side, while the reduced continuation of the original trunk furnishes minute ramuscules to the tentacles, in the hollow axes of each of which an afferent and efferent vessel is contained, surrounded by the peritoneal fluid, which penetrates to the remotest ends of these exquisite organs. Both from the ten- tacles and branchiae the blood now returns into the great ventral trunk (c\ which to the posterior extremity of the body is distinct from, and independent of, the intestinal system (f). From this trunk branches are detached on either side of the median line for the supply of the feet and integument. (689.) At the point corresponding with the circular vessel (fig. 134, 6), the primary ventral sends off a considerable division for the supply of the intestinal system. The current, therefore, entering the glandular parietes of the intestine is purely arterial in this genus ; for it is un- 270 ANNELIDA. Fig. 134. mixedly composed of blood returning from the tentacles and branchiae, by both of which the function of respiration is performed. Here, again, there exist but two principal directions in which the blood circulates, viz. longitudinally and transversely, or circu- larly, the former currents being connected with the latter. The circular vessel (fig. 134, 6) acts like an auricle ; it receives the blood from the intestinal system and delivers it into the great dorsal (a). The alimentary canal is embraced in this genus, as in all Annelids, by a framework of longitudinal and transverse vessels (/), in which the blood moves backwards below, and forwards above. (690.) In the dissection of Terebella nebu- losa, figured by Milne-Edwards, a large vessel (fig. 135, 1) is readily distinguishable towards the anterior part of the animal, running along the median line of the back, and situated imme- diately beneath the integuments. This vessel rests upon the alimentary canal, and exhibits irregular contractile movements whereby the blood contained in its interior is propelled from behind forwards, and consequently performs the functions of a heart ; and if we would compare it with what exists in the higher animals, it might be considered as physiologically representing a pulmonic ventricle, seeing that the vessels that convey the blood to the branchiae for the pur- poses of respiration take their origin from its an- terior extremity. (691.) By its posterior extremity, the great dorsal trunk receives the blood which it is ap- pointed to propel through the branchial organs, from several large veins which are, for the most part, adherent to the walls of the intestine (fig. 134, /), from which they receive a multitude of branches derived from the rich vascular network distributed over the intestinal walls. The principal veins, however, that communicate with this tubular heart are two large transverse trunks which form a ring around the digestive canal, beneath which they unite and become continuous with a large median trunk (fig. 135, h) that runs along the under surface of the in- testine, from which, in the same manner as the dorsal veins, it receives numerous lateral branches derived from the vascular network already mentioned. Lastly, there is a small median trunk, situated upon the internal surface of the integuments of the back (fig. 135, m\ into which open the veins derived from the different segments of the body, and Kespiratory and circula- >aratus in Terebella. TEKEBELLA NEBULOSA. 271 Fig. 135. which likewise communicates with the dorso-intestinal vessel by nume- rous anastomosing ramifications. (692.) The vessels above enumerated may be considered as consti- tuting the general venous system of the body ; and the blood which they convey to the dorsal trunk is, by the contractions of that vessel, in great part distributed to the branchiae through three pairs of branchial arteries derived imme- diately from the dorsal heart. Still, however, all the blood thus moving from behind forwards is not con- veyed to the branchial organs, since a certain portion finds its way through a small median vessel to the labial organs and cephalic ten- tacula. (693.) After having passed through the branchial organs, the renovated blood is received by ves- sels which unite to form a median trunk (fig. 135, o) that runs be- neath the alimentary tube and im- mediately above the ventral chain of nervous ganglia. This ventral trunk is continued along the whole length of the body, and gives oft 7 opposite to each ring a pair of trans- verse vessels, which, after having supplied branches to the integu- ment and locomotive organs, bend upwards, to be distributed over the walls of the intestine, where their ramifications contribute to form the vascular network above alluded to. (694.) The ventral vessel and its ramifications fulfil, therefore, the functions of an arterial system ; and consequently the branchiae them- selves must be regarded as the agents employed in propelling the blood through the systemic circulation. These organs, indeed, may be ob- served, at intervals, to contract with considerable energy, and thus materially to assist in urging the blood through the arterial rami- fications. (695.) M. de Quatrefages observes* that both in the Erratic andTubi- colous Annelidans the sexes are separate, and states that the generative * " Memoire sur les Hermelliens," Ann. des Sci. Nat. 3 ser. 1848. Arrangement of the vascular system in Terebella. (After Milne-Edwards.) 272 ANNELIDA. apparatus, both in the males and females, is restricted to the abdominal portion of the body. According to this distinguished anatomist, the testicle consists of a kind of areolar web of extreme delicacy, which, arising from a median aponeurosis, adheres to the internal and inferior surface of the general cavity, rising as high as the middle of the di- gestive canal. The tenuity of this tissue is such that it is impossible to procure more than small fragments for microscopic examination. (696.) The ovary is in every respect similar to the testicle : perhaps its texture may be rather firmer, but not sufficiently so to be adapted for satisfactory histological distinction. (697.) In the males as well as in the females, but more especially in the latter, during the period of reproduction a pigment is secreted in great abundance, which lines the generative organs ; but in proportion as the ova or zoosperms become developed, the amount of this pigment diminishes. Both the ovary and testicle are evidently temporary organs, no traces of them being distinguishable in the generality of specimens ; and moreover, in proportion as their products become developed in the general visceral cavity, they become gradually atrophied. When the male secretion is at maturity, a jet of water washes away the sperma- tozoids, and no trace of the testicle is left ; on the contrary, when the sperm is immature, washing still leaves behind a delicate web almost resembling a light cloud. (698.) The segmental organs in the genus Terebella, in all essential particulars, agree in their general structure with those of Arenicola. They diifer in number in different species : thus, in Terebella nebu- losa there are sixteen pairs, in T. conchilega only six, and in T. multi- setosa twenty-four. The testes in the male Terebella, according to Dr. Williams, are the lateral pouches or true segmental organs ; in the female, these are the ovaria : the generative products in both sexes are early introduced into the general cavity, in the fluid of which they become rapidly developed. There exists, however, in the TerebellaB a large glandular mass extending from the head, along the median line, to some distance in the direction of the tail. This glandular-looking organ coincides internally with the smooth, foot-like, dense tegument- ary structure observable on the thoracic half of the abdominal aspect of the body externally, and has been described by Cuvier, Milne - Edwards, De Quatrefages, Grube, Stannius, and others as the testes. It would seem however, from the researches of Dr. Williams, that they have nothing to do with the generative system. They are present alike in the male and in the female. They consist of follicles filled with large fatty particles; and their office seems to be to supply the lubricating and cementing fluid by which the animal forms and moulds its tube. (699.) In these Erratic Annelidans, according to M. de Quatrefages *, the eggs, as well as the spermatozoids, which exist in a very rudimentary * " Sur le Sang des Ann&ides," Ann. des Sci. Nat. 1846. GENERATIVE SYSTEM OF THE ANNELIDANS. 273 condition in the ovary or the testicle, break loose into the abdominal cavity, where, insulated from all the solid parts, and without any con- nexion with the vascular system, they undergo all the principal phases of their development. " It appears," says M. de Quatrefages, " that the liquid which thus bathes them on all sides must be vitalized, and that it is from it that they receive the materials necessary to enable them to grow to ten times their original size ; in fact, this fluid acts the part of an ovary and of a testis to them. The liquid enclosed in the general cavity of the body of the Annelidans is therefore in some respects a fluid organ." (700.) The spermatogenous masses floating in the fluid contained in the general cavity of this Annelid are irregularly ovoid, and present themselves, as is usual, in different stages of development. At first they are perfectly diaphanous, smooth, and manifestly homogeneous, without any trace of an enveloping membrane. The dimensions attained by them in this condition reach to as much as y^-th of a millimetre in length, and J^rd of a millimetre in breadth. (701.) At this epoch they may be seen to exhibit two grooves, cross- ing each other at a right angle, and whose direction does not appear to present any constant relation with the form of the mass itself. The number of grooves soon increases, and they become more marked and deeper, and the mass, after having presented a surface subdivided into large irregular lobes, assumes a mulberry-like aspect, and ultimately becomes completely granular. During the time that these phenomena are being manifested, the mass continues to increase in volume, and in its ultimate condition it is sometimes -jL-th of a millimetre long by nearly of a millimetre broad. (702.) The masses, when a little further advanced, split up, and the tail of the spermatozoids is then apparent. The spermatozoids continue to adhere together for some time longer by their bodies, as well as to the granulations not yet transformed : ultimately they are gradually separated. (703.) At the moment when the spermatozoids separate themselves from the minute masses of which they constitute a part, their body is almost fusiform, and perhaps not more than yjjoth of a millim. long and ^Jj-yth of a millim. thick ; but they grow during the time they remain in the fluid that bathes them: the body and the tail elongate, and, besides this, the former increases considerably in its transverse dia- meter. Among spermatozoids quite mature, some will have attained to a length of g^th of a millim., and a breadth of -j-T^yth of a millim. (704.) According to Dr. Williams*, here, as elsewhere throughout the class of Annelidans, the segmental organs are to be regarded as the primary parts of the reproductive system. Throughout the Nereid group, the ciliated, horseshoe-shaped segmental organ exists. It con- sists of a tube, highly ciliated, both ends of which communicate with the exterior : the ingoing limbs are situated in the immediate vicinity of * Phil. Trans. 1858, p. 124. T 274 ANNELIDA. each dorsal foot ; the outgoing limbs, considerably longer and more tubular than the former, open externally to the median side of the root of each ventral foot. The cilia with which this horse-shoe tube is lined are highly vigorous, and capable of supporting a powerful current, which arises externally and terminates externally ; but, as Dr. Williams asserts, the ova in the female and the sperm-cells in the male escape, although in some undetermined mode and by some undemonstrated passage, from this organ into the complexly areolated tissue which fills the chamber of the pedal appendages, which is a development of the segmental organ, and in size and vaseularity is proportionate to the stage at which the contained germinal elements have arrived. The following observations of M. Sars*, relative to the embryogenesis of these worms, are extremely interesting and important : (705.) In Polynoe cirrata the months of February and March are the period of propagation, when the body assumes a pale rose colour, arising from a numberless quantity of eggs, which fill the abdominal cavity, with the exception of about the first anterior fourth, and appear everywhere through the skin. When the animal is opened, the eggs appear to hang together in masses by means of a connecting tenacious mucus. In other individuals the eggs occur on the top of the back of the mother, beneath the dorsal scales, in immense numbers, surrounded by a tena- cious mucus. The heaps of eggs cover the whole hinder half of the back, but, more anteriorly, only the sides above the bases of the feet. It would seem that the eggs pass out through a very small aperture just above the feet, as Rathke found in the case of Nereis pulsatoria. Here, protected beneath the dorsal plates, the eggs remain until the young escape. In the meantime the yelk undergoes the usual process of mul- berry-fission, until it becomes finely granular. The ova become slightly oval ; and the foetus (into which the entire yelk is converted, without any part whatever separating) is smooth, greyish white, and more or less narrowly enclosed in chorion. A peculiar kind of motion was now ob- servable on placing the separated ova under the microscope, the ova turning round and round: this was effected by a very short fringe, which is seen now and then to move slowly and curve in a worm-like form, drawing the egg with it backwards and forwards. The foetus itself, which gradually acquires a white greyish-green colour, is still without motion in most of the ova : only, in a few a circle of extremely minute projecting and vibrating cilia was perceptible, which surrounds horizontally the centre of the body of the foetus, at an equal distance from the two poles of the ovum. At last the foetus arrives at maturity, and the mother now carries on her back many thousands of young ones, which gradually come forth from the mucus surrounding the eggs, leave their mother, and swim freely about in the water, visible to the naked eye as very minute greenish-grey points (^th of a millimetre in size) * Sars, Wiegm. Archiv, 1845, part 1. EMBEYOLOGY OF THE ANNELIDANS. 275 endowed with lively motion. They are extremely unlike their parent both in form and structure, being short, oval, cylindrical, and devoid of segmentation, furnished with a circle of long cilia around the centre of the body, but otherwise without external organs. The portion of the body situated anterior to the ciliary circle is somewhat narrower than the hinder one, and bears two eyes : this is the head ; and the young one always swims with this extremity in front. Frequently these young animals revolve, during swimming, around their longitudinal axis. Their sight is distinctly developed ; for they avoid each other with adroitness, and always swim towards the light. The time from the extrusion of the young to the laying of the eggs may probably amount to a couple of weeks. (706.) Many interesting particulars relative to the development of various genera belonging to the class under consideration have been ascertained by Milne-Edwards*. In the Terebellce (fig. 136), according to the observations of this distinguished naturalist, the young, on leaving the egg, have no resemblance whatever to the adult animal, insomuch indeed that it would be difficult to guess, a priori, the class to which they really belonged. On their first appearance upon the stage of active existence they might be mistaken for the ciliated larvae of certain Polyps or Medusa?, presenting no traces of the annulose type of structure (fig. 136, l) : in the course of a short time, however, their bodies be- come elongated, and they begin to assume somewhat of a bilateral or symmetrical form, the body of the young Terebella becoming distin- guishable, divided into four zones or rudimentary segments, the pos- terior of which is still provided with a ciliary apparatus (fig. 136, 2). Shortly after this, a fifth ring (fig. 136, 3, cT) begins to make its ap- pearance in the space situated between the penultimate and terminal, and rudiments of a mouth and alimentary canal become distinguishable. The growth of the young Annelidan now begins to advance rapidly ; and its body is rendered more worm-like as new segments are progressively added to its length, these all successively making their appearance in the space between the last-formed ring and the anal or terminal joint of the body ; so that the relative position of the newly developed seg- ments is precisely in accordance with their respective ages, except in the case of the last segment, which is persistently terminal. Meantime the larva ceases to be, as it was at first, completely apodous : simple subulate setae, supported upon minute fleshy tubercles, begin to show themselves on both sides of the body, the development of these loco- motive appendages being accomplished in the same order of sequence as that of the segments, namely from before backwards. (707.) At this period of their growth the young Terebellce present the appearance of minute subcylindrical worms (fig. 136, 4), and the * " Kecherches Zoologiques faites pendant un Voyage sur les Cotes de la Sicile, par M. Milne-Edwards," Ann. des Sci. Nat. for 1844. 276 ANNELIDA. different viscera in the interior of the hody become very clearly de- nned. (708.) The digestive apparatus is now distinctly perceptible : ante- riorly it presents a kind of fleshy bulb (fig. 136, 4, p) ; then a short cylindrical oesophagus, followed by a capacious ovoid stomach (r), the contents of which appear to be still saturated with the coloured sub- stance of the vitellus, and an intestine (s), which commences at about the posterior third of the body. The glandular structures near the anterior part of the animal now become apparent, and the subcutaneous muscles clearly distinguishable ; still it is remarkable that, even in the most transparent portions of the creature, no traces of a vascular system can be detected. (709.) In the course of three or four days more, the cilia have com- pletely disappeared from the surface of the body, which now presents all the characters of one of the erratic Annelids, but in no respect re- sembles the tubicolous genera to which the creature really belongs. The young larva, in short, is furnished with a distinct head, an antennary organ, eyes, and feet armed with subulate seta3 ; while the adult Tere- bellse are acephalous, being destitute both of antenna and eyes, and having feet provided with hook-like appendages. (710.) After the larva has been furnished with one or two additional pairs of feet, the head begins to be changed in its shape (fig. 136, 5), a Fig. 136. Development of Terebella nebulosa, (After Milne-Edwards.) transverse constriction makes its appearance at a little distance in front of the eyes, and its anterior lobe, which thus becomes distinctly defined, is seen to be studded near its free margin with a series of stinging vesicles, some of which are armed with little spine-like filaments. The post-cephalic ciliated collar becomes at the same time much narrower, NEKVOUS SYSTEM OF THE ANNELIDANS. 277 and forms a prominent ridge underneath the head, that constitutes a kind of upper lip. In the course of two or three days more, the anterior cephalic lobe (136, 5, a) becomes perfectly distinct from the oculiferous segment, and is much elongated, taking a cylindrical form, and consti- tuting a very flexible median appendage, having all the characters of an antenniform organ. Its axis is occupied by a canal that communicates with the general cavity of the body ; and a fluid may be seen to circu- late in its interior. The natatory cilia have almost entirely disap- peared both from the neck and from the posterior extremity of the body ; and the young Terebella in this condition presents itself exhibit- ing all the characters of an Annelid belonging to the erratic group- not, as yet, at all resembling any of the tubicolous genera, of which it is a member. (711.) Having become deprived of the locomotive cilia with which they were previously furnished, the larvae now cease swimming and begin to enclose themselves in a kind of mucous substance, which gra- dually solidifies, so as to form a cylindrical tube open at both extremi- ties. The first period of their existence, during which they lead an erratic life, then closes, and they begin to assume the habits of their parents. The ventral oars, with their armature of terminal booklets, are successively developed in a regular series from before backwards, as additional segments are added to the length of the body. The ten- tacular appendages next begin to be developed from the sides of the head. But it is not before the body has acquired thirty-eight or forty pairs of feet that the branchial apparatus makes its appearance, under the form of two simple tu- bercles developed from the lateral regions of the neck ; these, how- ever, rapidly enlarge, and soon assume the functions which, in the adult animal, they are destined to perform. (712.) The structure of the nervous system in the Annelida conforms, in its arrangement, with the general type common to the articulated classes. A considerable supra-cesophageal mass (fig. 137, a a) represents the encephalon, in front of which are situated minute ganglia (6, c, d, e), from whence nerves are derived to supply the prin- cipal instruments of sensation connected with the cephalic portion of Plan of the nervous system in the Dorsibranchiate Annelidans. (After Quatrefages.) 278 ANNELIDA. the animal. The circum-oesophageal ring (nri) is strongly marked, communicating on each side with the ventral series of ganglia (o, p} that extends throughout the entire length of the body, giving off nerves to supply the different segments. Communicating with the posterior aspect of the encephalic ganglia are several small ganglionic masses (i, fc, I, m), which are joined together by delicate filaments, and apparently represent the sympathetic system, inasmuch as from them are derived filaments supplying the alimentary canal and the principal viscera. (713.) In Torrea vitrea (an Annelid the Fig. 138. transparency of which is such that, when plunged into sea-water, its pre- sence is only distinguish- able from the bright red colour of its eyes and a double line of violet- coloured spots that ex- tend along its back), M. de Quatrefages* was enabled to examine the structure of the organs of vision in a very satis- factory manner. The eyes in this species are only two in number; and, indeed, they constitute by far the larger part of the creature's head, forming two very con- siderable prominences that are almost con- joined in the mesial line of the body. The in- tegument, which is here extremely thin and per- Structure of the eye in Torrea vitrea, and of the supposed auditory apparatus in Arenicola (after Quatrefages). 1. a a, integument passing in front of the eye, and forming a transparent cornea ; b, c, granular cellular tissue enclosing the globe of the eye ; d, external surface of reticular pig- mental membrane ; f, internal surface of the same, seen through the pupillary aperture ; e, the iris ; g, the crystal- line lens ; g', optic nerve ; h, sheath of ditto, derived from the dura mater ; i, k, vascular trunks forming a circle around the base of the eyeball. 2. Auditory apparatus of an Arenicola : a, acoustic nerve ; b, c, cellular tissue investing the auditory capsule ; d, otolithic masses. 3. Auditory ap- paratus of AmpMcoryne :. a, cellular tissue; b, auditory capsule; c, otolith. fectly diaphanous, passes over the ocular globe, and evidently in this case performs the functions of a transparent cornea (fig. 138, 1, a). A thick fibrous stratum, repre- senting the sclerotic (d), encloses the eye, and becomes continuous with the sheath, likewise fibrous (h), of the optic nerve (g'). The colourless sclerotic presents upon one side a large irregularly-rounded aperture that is partly closed by a sort of choroid of a brownish colour (6), in the centre of which is an almost circular pupil surrounded with a border of * Ann. des Sci. Nat. 1850. SENSES OF THE ANNELIDANS. 279 a deep blue colour. Through the pupillary opening it may be perceived that the interior of the eye is lined by the choroid, and that the whole interior of the ocular capsule is filled up with a vitreous humour so ab- solutely transparent that the crystalline lens situated in its centre seems to be in connexion with nothing. On the outside of the eye the optic nerve can be plainly seen arriving at the eyeball and expanding to form the retina. The eyes of other Annelidans are, however, when present, by no means so easily examined; they may, however, from the re- searches of Miiller*, "Wagner f, EathkeJ, and Siebold, be briefly stated to consist of a round transparent medium or lens enclosed in a layer of pigment, and provided posteriorly with a retinal expansion. (714.) An apparatus to which the functions of an organ of hearing have been attributed by several eminent anatomists is met with in some Annelidans. Grube and Stannius || first announced a very remark- able structure in Arenicola, the existence of which has been confirmed by subsequent observers, that certainly resembles very closely in its conformation an organ common among the Mollusca, to which a similar function has been generally conceded :. this consists of a transparent membranous capsule (fig. 138, 2 & 3, a, >, c) enclosing a fluid, wherein one, or sometimes several minute bodies, having every appearance of otoliths, are suspended. M. de Quatrefages describes these auditory capsules as being situated in the first or second segment of the body, one on each side of the opening of the oesophagus, and observes that a nerve of considerable size is distinctly traceable in them. (715.) Many of the smaller marine Annelids are luminous ; their luminosity, however, is not a steady glow like that of the glow-worm or fire-fly, but a series of vivid scintillations (strongly resembling those produced by an electrical discharge through a tube spotted with tin-foil) that pass along a considerable number of segments, lasting for an instant only, but capable of being repeatedly excited by any irritation applied to the body of the animal. These scintillations may be observed even in separated segments if they be subjected to the irritation of a needle- point or of gentle pressure ; and it has been ascertained by M. de Quatre- fages that they are given out by the muscular fibres in the act of con- traction^". * Ann. des Sci. Nat. t. xxii. t Icones Physiologicse, pi. 28. J De Bopyro et Nereide, pi. 2. figs. 4 & 5. Lehrbuch der vergleichenden Anatomic, p. 200. I Lehrbuch der vergleichenden Anatomie von Siebold und Stannius, p. 201. *[[ See his Memoirs on the Annelida of La Manche, in Ann. des Sci. Nat. ser. 2. t. xix. and ser. 3. t. xiv. 280 MYEIAPODA. CHAPTER XI. MYEIAPODA*. (716.) THE Annelidans examined in the preceding chapter, with the singular exception of the Earthworm, are only adapted to an aquatic life : the soft integument which forms their external skeleton, and the setiform and tentacular organs appended to the numerous segments 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. Supposing, however, that, as a mere matter of speculation, it was inquired by what means animals of similar form could be rendered capable of assuming a terrestrial exist- ence, 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 indicate 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 firm- ness to the tegumentary skeleton, to allow of more powerful and accu- rately-applied muscular force by diminishing the number 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 instruments of locomotion fitted for progression upon the ground. Yet all these changes would be in- efficient without corresponding modifications 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 circumstances ; the small encephalic brain would be incompetent to correspond with more exalted senses ; so that, as a necessary consequence of superior organization, the nervous centres must be all increased in their proportionate development to adapt them to higher functions. (717.) 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 MYRIAPODA, upon the consideration of which we are now entering : 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. * fivpia, ten thousand, i. e. many ; irovs, a foot. JULID^E. 281 (718.) The body of a Myria/pod consists of a consecutive series of segments of equal dimensions, but, unlike those of the generality of the Annelida, composed of a dense semicalcareous or else of a firm cori- aceous substance ; and to every segment is appended one or two pairs of articulated legs, generally terminated by simple points. (719.) 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 an- tennae, generally regarded as appropriated to the sense of touch, but which probably are connected with other perceptions less intelligible to us. (720.) 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 rami- fying tubes or tracheae to all parts of the system. (721.) The number of segments, and consequently of feet, increases progressively with age, a circumstance which remarkably distinguishes the Hyriapoda from the entire class of Insects, properly so called. (722.) The MYRIAPODA may be divided into two families, originally indicated by Linnaeus : the Julidce, or millepedes, and the Scolopen- dridce, or centipedes, each of which will require our notice. (723.) JULIDCE. The lowest division, which derives its name from the Julus, or common millepede, is most nearly allied to the Annelidans, both in external form and also in the general arrangement of its different organs; this therefore we shall first examine, and select the Julus ter- restris, one of the species most fre- quently met with, as an example of the rest. These animals (fig. 139, A) are generally found concealed under stones, or beneath the bark of decayed timber, where they find subsistence by devouring decomposing animal and vegetable substances. The body is long and cylindrical, composed of be- tween 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 mesial line upon the under or ventral surface ; but these feet, although distinctly articulated (fig. 139, c, p), are as yet extremely small in comparison with the bulk of the animal, and are Fig. 139. Julus terrestris. A, in the act of pro- gression; B, the same rolled up in a spiral form; C, segments of the body magnified, showing the mode of attachment of the feet (i, p) on each side of the mesial line (r) of the abdomen. 282 MYBIAPODA. evidently but mere rudiments of the jointed legs developed in more highly organized forms of homogangliate beings ; the movements of the Julus are, consequently, 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 (fig. 139, B), the feet being concealed in the concavity of the spire, and thus protected from injury. (724.) The mouth resembles in structure that of the larvae of some insects, and is furnished with a pair of stout horny jaws, moving hori- zontally, and provided at their cutting edges with sharp denticulations, so as to render them effective instruments in dividing the fibres of rotten wood, or the roots and leaves of vegetable substances usually employed as food ; and the alimentary canal, which is straight and very capacious, is generally found filled with materials of this description. (725.) In most points of their internal organization, the Myriapoda resemble insects ; and we should only anticipate the observations that will be more conveniently made hereafter did we enter into any minute description of their anatomy : we shall therefore, in this place, simply confine ourselves to the notice of those peculiarities observable in the animals under consideration whereby they are distinguished from in- sects and entitled to rank as a distinct class. We have seen that, in such of the Annelida as have been most carefully investigated, the ori- fices 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 Julidce still present analogies with the red-blooded worms ; for in them the external openings of the male parts are situ- ated immediately 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. (726.) In the female also, the sexual orifices are advanced very far 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. (727.) The male generative organs of Julus are two elongated and partially convoluted tubes, placed side by side beneath the alimentary canal. The excretory ducts, or terminations of these tubes, run towards the anterior part of the body, where they terminate in two minute in- tromittent organs, situated at the under surface of the seventh segment, immediately behind the seventh pair of legs. As they pass backwards, the secerning tubes, or testes, gradually separate from each other, and have developed from their sides, at short distances from each other, numerous small glandular caeca, which doubtless constitute the se- creting portions of the apparatus, or proper testes. The two efferent GEOWTH OF JULUS TEEEESTEIS. 283 ducts, whereby the secretion of these caeca is conveyed out of the hody, intercommunicate freely by means of short transverse canals, and, from the sacculated appearance that they present towards their termination, appear likewise to perform the office of reservoirs for the seminal fluid. (728.) In the female Julus, the organs of reproduction are as simple in their structure as those of the male. They consist of a single elon- gated bag or oviduct, covered on its exterior surface with a very great number of ovisacs or cseca of various sizes, each of which secretes but a single ovum. This oviduct extends backwards beneath the alimentary canal from the vaginal outlet, which is double, and situated in the fourth segment of the body, behind the second pair of legs. In the pregnant female the oviduct appears smooth externally, being distended with the ova that have passed into it from the ovisacs where they were formed, and which are retained in readiness to be deposited immediately after intercourse with the male. (729.) The ova, when fully developed, are found to present all the structures belonging to a perfectly formed egg, the yelk, the germinal vesicle with its macula, the membrana vitelli, the albumen, and likewise the shell, lined by the membrana externa, or chorion, being all distinctly recognizable. (730.) Another important distinction between these animals and insects properly so called, is met with in the mode of their growth and development. Insects (as we shall more fully explain hereafter) 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, that constitute 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 per- fect development 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 advance 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 trans- itions from their immature to their fully-developed state has been well observed by De Geer* and Savif ; and the result of their observations is here given, in order that the reader may compare the different steps of the process with what we shall afterwards meet with in the t more highly organized Articulata. (731.) The eggs (fig. 140, A), which are very minute, are deposited * Memoires pour servir a 1'Histoire des Insectes. 7 vols. 4to. Stockholm, 1778. f Osservazioni per servire alia storia di una specie di Julus comunissima. Bologna, 1817. 284 MYEIAPODA. - 14 - in the earth or vegetable mould in which the Julus is usually met with. When first hatched, the young Myriapod is of course exceedingly dimi- nutive ; 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. 140, B, c). Some days subsequent to it first moult, the skin is'again cast, and the millepede, ac- quiring larger dimensions, is seen to possess seven pairs of ambulatory extremities, which are, however, still placed only upon the anterior segments (fig. 140, 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 an- terior pairs are used in pro- gression. At the fourth moult the number of legs is in- creased to thirty- six pairs ; and at the fifth, at which time the body becomes composed of thirty segments, there 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 reproduction. (732.) The development of the young Julus has been traced more recently by Mr. Newport with great care ; and the result of that gentle- man's observations relative to this part of the history of the Myriapods is of extreme interest, both to the physiologist and in an entomological point of view. (733.) The embryo, when it first becomes distinguishable in the in- terior, of the ovum, is entirely destitute of limbs, or of any appearance of segmental division, and even at the moment of its escape from the egg, which is effected by the laceration of the egg-shell, but very faint traces of segmentation are discernible. After its extrusion, however, its growth advances with considerable rapidity, a.nd it soon becomes visibly divided into eight distinct segments, including the head (fig. 141, A) the ninth Growth of young Julus terrestris. (After De Geer.) DEVELOPMENT OF JULUS TEKEESTEIS. 285 or anal segment (d) being still indistinct. The four thoracic segments, moreover, now exhibit on their ventral surface little nipple-shaped pro- tuberances, three of which on each side are the rudiments of future legs. No internal viscera are as yet distinguishable, the whole embryo being still a congeries of vesicles, or cells, in the midst of which some faint traces of a future alimentary canal seem to be indicated. In this state the body of the embryo is completely enclosed in a smooth and perfectly transparent membrane (fig. 141, A, e), which seems to contain a clear Fig. 141. Development of the embryo in Julus terrestris. (After Newport.) fluid. This membrane Mr. Newport regards as the analogue of the amnion the vitelline or investing membrane of the embryo in the higher animals, and identical with the membrana vitelli, or proper membrane of the yelk. It is a shut sac that completely invests the embryo, except at its funnel-shaped termination at the extremity of the body (fig. 141, A, d), where it is constricted, and, together with another membrane (which in the unburst egg is external to this and lines the interior of the shell), assists to form the cord or proper funis (d) that enters the body of the embryo at the posterior part of the dorsal surface of the future ante- penultimate segment, where the mucro or spine exists in the adult animal. (734.) A new process is now about to commence the development of new segments. Up to the present period the posterior part of the body remains less distinctly divided into segments than the anterior, the first five segments being the most distinctly marked ; the sixth and seventh now become more defined. It is in the membrane (fig. 141, c,/) that connects the seventh with the eighth segment (at the posterior margin of which last the funis (d) enters, and which is permanent as the penulti- mate segment throughout the life of the animal) that the formation of new segments is taking place. At this period it is only a little, ill- MYEIAPODA. defined space, that unites the seventh and eighth segments into one mass ; but in proportion as the anterior parts of the body become developed, this part is also enlarged, not as a single structure, but as a multiplica- tion or repetition of similar structures. (735.) About the seventeenth day the little embryo is ready to leave the amnion in which it has been hitherto enveloped. Its body is found to have become considerably elongated, the increase of length being mainly occasioned by the growth of the posterior segments, but more especially by the development of new ones, which now begin to make their appearance in the antepenultimate space (fig. 141, c, /), which is, in fact, the proper germinal space or germinal membrane, whereby the production of all the future segments is effected. The seven anterior segments, including the head, are now greatly enlarged, and the hitherto minute penultimate and anal segments (8, 9) become much enlarged, and rapidly acquire the form they afterwards retain through the life of the animal. This latter fact shows that it is not merely by an elongation and division of the terminal segment that the body of the Julus is de- veloped, but that it arrives at its perfect state by an actual production of entirely new segments, the formation of which is in progress long before they are apparent to the eye, and that the original segments of the ovum into which the animal is first moulded are permanent. (736.) The manner in which new legs are produced is equally curious. Up to the present period the animal is furnished with only three pairs (fig. 141, c, 6 c), but four additional pairs are nevertheless in progress of formation. These, at present, exist only as eight minute nipple- shaped prominences on the under surfaee of the sixth and seventh seg- ments (fig. 141, c, 6, 7), four on each, covered by the common integu- ment, which, as in the larval condition of insects, is a deciduous mem- brane. The newly-formed legs, however, go on rapidly increasing in size until about the twenty-sixth day, when, throwing off the skin in which it has hitherto been encased, the young Julus presents itself with seven pairs of legs and a body consisting of fifteen segments (fig. 141, E). (737). In this condition the body of the animal still continues to elongate, not by the division of the already-formed segments into others, but always by the formation of new ones in the germinal membrane that extends from the posterior margin of the antepenultimate segment to the penultimate, which last segment, as well as the anal, undergoes no change ; and it may likewise be observed that that segment of the newly-formed portion of the body is always furthest advanced in growth which is immediately posterior to the last segment which possesses legs, and then, the next in succession, until we arrive at the terminal ones (the penultimate and the anal), that never have legs appended to them. (738.) On again casting its skin, the new segments of the body pro- SCOLOPENDEID^E. 287 Fig. 142. duced at the former change, from the eighth to the twelfth inclusive (fig. 141, E, 8-12), are hecome of the same size as the original ones, and each has developed from it two additional pairs of legs, so that the whole number becomes increased to thirty-four; and thus at each change of skin the number of new segments and of additional legs is increased, by development from the germinal membrane, until the full complement is acquired. (739.) SCOLOPENDEID^J. In the second family of Myriapoda we have a very striking illustration of the manner in which the de- velopment 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 locomotion are in relation with their vegetable diet and retiring habits. But in the predaceous and carnivorous Scolopendra (fig. 142), which, although it lurks in the same hiding-places as the Julus, obtains its food by pursuing and devouring insects, far greater activity is indispensable; and accordingly we find the segments of the body, and the extremities appended to them, exhibiting a perfection of structure adapted to greater vivacity and more ener- getic movements. (740.) This is at once evident upon a mere inspection of their outward 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 presents a quadrangular outline. In order to give greater flexibility to the animal, instead of the semicrustaceous hard substance which forms the rings of the Julus, the integument is here composed of a tough and horny sub- stance, 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 calcu- lated to give the greatest possible freedom of motion, and thus to enable the Scolopendra to wind its way with serpent-like pliancy through the tortuous passages wherein it seeks its prey. (741.) 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 becomes Scolopendra. 288 MYEIAPODA. thus fitted to direct the movements of more perfect limbs. The legs, therefore, as a necessary consequence, are now proportionately 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 concentration 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) con- verted into the carnivorous and powerful Scolopendra, well able to wage successful war with the strongest of the insect tribes, and not unfre- quently formidable, from its size, even to man himself. (742.) 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 armed with a tremendous pair of massive and curved fangs ending in sharp points, and perforated near their termination by a minute aperture, through which a poisonous fluid is most probably instilled into the wound inflicted by them. It is to this structure that the serious consequences which in hot climates not unfrequently result from the bite of one of these animals must no doubt be attributed. (743.) In their internal anatomy the Scolopendridce 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 systems, so far as they are understood, seem to correspond with what we shall afterwards find to exist in the larvae of insects. (744.) In the Scolopendridce, as we learn from the researches of Mr. Newport*, the heart is enclosed in a distinct membranous covering, which may be regarded as a true pericardium, consisting of a loose deli- cate membrane, between which and the sides of each chamber of the heart there is a slight interspace. The heart itself is a long pulsating organ, corresponding in its general structure and position with the dorsal vessel of insects ; it is situated immediately beneath the integu- ments, and runs along the mesial line of the dorsal region of the body, consisting of a series of chambers, twenty-one in number, that com- municate with each other and extend through the entire length of the animal from the tail to the cephalic segment. (745.) The minute structure of the heart is exceedingly interesting. This organ is composed of two distinct contractile tunics, one external and the other internal, each being covered by its proper serous mem- brane. The external tunic is a very thick muscular layer, the fibres of which are loosely interwoven with each other. The internal tunic is * Phil. Trans. 1843. SEXUAL OKGANS OF SCOLOPENDBA. 289 Fig. 143. composed of two sets of muscular fibres, of which the inner stratum is disposed longitudinally, while the external one is formed of numerous short, broad, transverse muscular bands, very much resembling in ap- pearance the cartilaginous rings of the trachea in vertebrated animals. They do not completely encircle the longitudinal ones, but pass only half- way round on each side, leaving a space between those of the two sides, both upon the upper and under surface. (746.) Prom each compartment of the heart proceed the systemic arteries, which supply nearly the whole of the blood to the viscera and lateral portions of each segment. The anterior pair of these systemic arteries, however, instead of being distributed like the rest, form a vas- cular collar, which, after surrounding the cesophageal tube (to which, and to the different parts belonging to the cephalic segment, it gives off numerous branches), unites beneath the oesophagus to form the great supra-ganglionic vessel or aortic trunk,extendingbackwards along the middle line of the body, immediately above the centres of the nervous system (which it supplies plentifully with blood), as far as the terminal ganglion in the last segment, giving off in its course numerous arterial canals, which ramify extensively in the surrounding structures. The return of the blood from the various viscera to the dorsal ves- sel is effected, as in insects, by lacunar or interstitial channels, as will be explained in the next chapter. (747.) In the position and ar- rangement of the sexual organs, the Scolopendridee complete the transition between the Anne- lidans 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 Scolopendra they are removed to the tail. The structure of the male organs (fig. 143, 2) is remarkable. The testes are seven in number ; and on opening the posterior segments of the animal, they are found closely u 1. Female, and 2. Male generative system of Scolopendra. 290 INSECTA. 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 are hollow, arises a narrow duct ; so that there are four- teen 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 termination, the common ejaculatory duct com- municates with five accessory glands (fig. 143, 2, c, d d, e e), four of which are intimately united until unravelled, while the fifth is a simple caecum of considerable length*. (748.) The ovarian system of the female Scolopendra is a single tube (fig. 143, l), without secondary ramifications, but receiving near its termination the ducts of accessory glands, as represented in the figure. (749.) Some Scolopendrae (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 inadvertently seizes it. CHAPTER XII. INSECTA. (750.) THE word Insect has at different times been made use of in a very vague and indeterminate manner, and applied indiscriminately to various articulated animals f. In the restricted sense in which we now use it, we include under this title only such of the HOMO- GANGLIATA as in their perfect or mature state are recognizable by the following characters, by which they are distinguished from all other creatures. (751.) The body, owing to the coalescence of several of the segments which compose their external skeleton, is divided into three principal portions the Head, the Thorax, and the Abdomen. The Head contains the oral apparatus and the instruments of the senses, including the antenna? 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 fre- quently wanting. The Abdomen is destitute of legs, and contains the viscera connected with nutrition and reproduction. (752.) But insects, before arriving at that perfect condition in which they exhibit the above-mentioned characters, undergo a series of changes, * Vide Cyclop, of Anat. and Phys., art. " GENERATION, ORGANS OP" (Comp.Anat.). f The word Insect, derived from the Latin word Insecta, simply means divided into segments. CLASSIFICATION OF INSECTS. 291 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 butterflies, but caterpillars animals with elongated worm- like bodies divided into numerous segments, and covered with a soft coriaceous integument (fig. 143, A). The head of the caterpillar is pro- vided 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 part of the body, are soft and membranous. The caterpillars, or larvae*, 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 external limbs, and almost incapable of the slightest motion resembling rather a dead substance than a living creature ; it is then called a chrysalis, nymph, or pupa ^ (fig. 148, B). (753.) 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 converted into a gay and active denizen of the air (fig. 148, c). (754.) 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 under nearly the same form as they will keep through life ; these form the Insecta Ametabolat 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 re- cognizable by being possessed of these organs in an undeveloped or rudimentary state : an example of this is seen in the House-cricket (fig. 145), in which A represents the imago ; B, the pupa ; c, the full- grown larva ; D, the young just hatched ; and E, the eggs. (755.) The extensive class of INSECTS has been variously arranged by different entomologists, and distributed into numerous orders . Among * 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. f The first two of these names are purely fanciful : the last is derived from pupa, a baby wrapped up in swaddling bands. | d, without ; fierajSoXij, change. The classification of Insects here given is that of Burmeister, which we select u2 292 INSECTA. the different systems which have been given, we select the following as best calculated to render the reader acquainted with the transforma- tions, as well as the principal forms, to which allusion will be made in subsequent pages. (756.) I. INSECTA AMETABOLA. The larva resembles the perfect insect, but is without wings. The pupa? 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 setas lying in a sheath. (757.) 1st Order, ffemiptera*. In such insects of this order as possess wings, which when present are always four in number, the an- terior 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. 144), is a familiar example : c and D Fig. 144. Notonecta. represent immature, and F mature, larvae. The pupa (G, H) differs little in outward form from the perfect insect (E), but possesses only the rudi- ments of wings. /3. Having mouths furnished with jaws, or distinct mandibles and maxillae. (758.) 2nd Order. Orthoptera^. In this order the perfect insect possesses four wings, the posterior pair being the largest ; and when at 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.) * ijfjiiffvs, half ; irrepbv, a wing. f opObs, straight ; Trrtpov. CLASSIFICATION OF INSECTS. 293 rest, these are folded both in a transverse and longitudinal direction. The anterior wings are of a denser texture, resembling leather or parch- ment. To this order belongs the common House-cricket (Gryllus domes- ticus), of which, as well as of its eggs, larae, and pupa, figures are here given (fig. 145). (759.) 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. Fig. 145. Gryllus domesticus. (760.) II. INSECTA METABOLA. The larva is a worm either with or without legs. The pupa is quiescent ; or if it moves, it does not eat. (761.) 4th Order. Neuroptera-f. Insects having four equally large or equally long wings with reticulated nervures, and mouths provided 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 (fig. 146), equally remarkable for its beau- tiful form, powerful flight, and carnivorous habits, is among the most formidable tyrants of its class ; while the Iarva3, which abound in our ditches and stagnant pools, are eminently destructive to their aquatic companions. The larva (fig. 147, B) possesses six articulated legs ; while the pupa (A), which certainly forms an exception to the general rule given above, is not only furnished with rudimentary wings, but is eminently rapacious, and possesses in the structure of its mouth, to be s, reticulated ; Trrepbv, a wing, f I'evpov, a nerve ; 294 INSECTA. described hereafter, peculiar facilities for gratifying its bloodthirsty disposition. (762.) In other orders, the wings are always unequal, the posterior, and sometimes both pairs, not unfrequently being wanting. Fig. 146. Dragon-fly- a. Mouth adapted to sucking. (763.) 5th Order. Diptera*. Instead of posterior wings, we find in this order pedunculated appendages called halteres or poisers. The mouth contains a soft proboscis, and is usually armed with several setae and provided with a pair of palpi: of such the common House-fly affords a familiar instance. (764.) 6th Order. Lepidoptera-f. The insects belonging to the Lepidopterous order are possessed of four wings, which are generally covered with microscopic scales, frequently exhibiting the most beau- tiful colours. The larva? are provided with feet and a distinct head ; the mouth of the perfect insect is a long spiral proboscis. (765.) The Butterflies, so conspicuous for their beauty, are well-known diTrre/oos (Sis, Trrepov), with two wings. AeTris, a scale ; Trrepov. CLASSIFICATION OF INSECTS. 295 Fig. 147. representatives of this order ; and the usual forms of these insects in the larva, pupa, and imago state are familiar to all (fig. 148, A, B, c). ft. Mouths with distinct biting jaws. (766.) 7th Order. Hymenoptera*. Possessing four naked wings traversed by ramose nervures. Larvae generally without head or feet, but sometimes with both. Wasps, Bees, &c. (767.) 8th Order. Coleoptera . In this last order, the anterior wings are converted into dense horny cases or elytra, beneath which the posterior pair, adapted to flight, are folded up when the insect is at rest. The larvae pos- sess a head, and are sometimes provided with feet, but not always. (768.) The Coleopterous divi- sion of the insect world embraces the extensive tribe of Beetles, both terricolous and aquatic ; of the former we have an example in the common Cockchafer (Melolontha), of which a figure is here given, as well as of the different stages of its development (fig. 149, A, B, c, D, E)f. (769.) Having thus introduced the reader to the chief orders com- posing the vast class of Insects, our next object must be to examine more in detail the principles upon which these animals are con- structed, 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 * >>p.r}v -evos, a membrane ; irrepbv, a wing. f It would be foreign to our present purpose to do more than enumerate other O rders of Insects which have been formed by different authors. Of these, the following are the most important : Dermaptera (Leach), Sepfia, skin ; trrepov. Earwigs (Forficula). Triclioptera (Leach), Opi%, rpixbs, hair ; irrepov. May-flies (Phryganea). Aphaniptera (Kirby), a^arr/s, invisible ; trrepov. Fleas (Pulex). Aptera, aTrrepos, without wings. Wingless insects. Parasita (Latreille). Lice (Pediculus). Thysanoura (Latreille), Ovtrav-ovpos, bushy-tailed. Spring-tails (Lepismenze). Metamorphoses of Dragon-fly. INSECTA. relative to the metamor- phoses to which they are subject for subsequent consideration. (770.) Insects, exa- mined generally, differ from all other articu- lated beings in one re- markable circumstance : they are capable of flight, and can maintain themselves in the air by means of wings. It is true, indeed, that some species are met with, in all the orders described above, which are apte- rous, being destitute of such organs ; but these form exceptions, to be noticed hereafter. Such Fig. 148. Metamorphoses of Lepidoptera. a mode of progression, through so rare a medium as that of the atmo- sphere, necessarily demands an exercise of muscular power of the most Fig. 149. Metamorphoses of Melolontha. EXTERNAL FRAMEWORK OF INSECTS. 297 vigorous and active description, and a correspondent strength and firm- ness in the skeleton upon which the muscles act. It is sufficient 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 ener- getic as would be indispensable to wield the instruments used in flying, or raise the body above the surface of the ground. (771.) Similar changes, therefore, to those which we found requisite in order to convert the aquatic Annelid 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 reduced ; 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 gathered into masses of increased power, suffi- cient to animate the more vigorous muscles with which they are in relation. (772.) Such changes are precisely those which are most remarkable when we compare the external appearance of a centipede with that of a winged insect : the entire number of segments, and consequently 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 con- nected by a moveable joint. The three anterior segments 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 rings only, the organs of locomotion are ap- pended. 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 possible, the lightness of the animal in parts where strength is not required. Here, then, is an annulose skeleton adapted to flight ; dense and unyielding where support is required for the at- tachment of the locomotive organs, but thin and flexible elsewhere. (773.) 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, inas- much as very different degrees of motion are required between the in- dividual rings. In the Annelida and Myriapoda a very simple kind of 298 INSECTA. junction was sufficient ; for in them the segments were all united by the mere interposition of a thinner coriaceous membrane extending be- tween their contiguous margins ; but in insects several kinds of articu- lation are met with in the construction of the trunk, adapted to the mobility of different regions. (774.) The first mode of connexion 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 moving the wings and legs derive extensive origins. (775.) A second means whereby the pieces of the thorax are fastened together is by sympTiysis, in which a somewhat soft membrane is inter- posed between two plates, so as to admit of a slight degree of motion. {776.) 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 development of the eggs after impregnation. The rings of the abdomen are therefore united by a membrane passing from one 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. (777.) But in other regions there is an absolute necessity for a mode of communication intermediate in character between the two kinds men- tioned 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 seg- ment is admitted into a smooth cavity excavated in the corresponding margin of the other, and secured in this position by muscles and an ex- ternal 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. (778.) The legs of insects, as we have already stated, are invariably six in number, one pair being attached to each of the three thoracic segments. Considered separately, every leg may be seen to consist of LEGS OF INSECTS. 299 several pieces, connected together by articulations of different kinds, which require our notice. The first division of the leg, or that in im- mediate connexion 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 degrees of motion in different insects, is called the hip (coxa) ; and upon this, as upon a centre, the movements of the limb are performed. To the extremity of the coxa a small moveable piece is attached, 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 recognize ; they are for the most part united together by articulations so constructed as to allow simply of flexion and extension, which will be best under- stood by inspecting, in some large insect, the junction between the femur and the tibia, or the knee-joint, as we might term it. Upon the upper extremity of the tibia the observer will find on each side a precise semi- circular 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. (779.) 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 distributed, and the variety of situations in which insects live, we are led to expect corre- sponding adaptations in the construction of their instruments of loco- motion ; and in this our expectations will not be disappointed. (780.) In the generality of terrestrial species, the last segment of the tarsus or foot is provide^ 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. (781.) 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. 300 INSECTA. 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. (782.) 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 substances which, from their polished surfaces, could afford no hold to any appa- ratus of forceps or hooklets. In the common flies (Muscidce), the exer- cise of this faculty is of such everyday occurrence, that, wonderful as it is, it scarcely attracts the attention of ordinary observers. The foot of the House-fly, nevertheless, is a very curious piece of mechanism ; for, in addition to the recurved hooks possessed by other climbing species, it is furnished with a pair of minute membranous flaps (fig. 150, c), which, Fig. 150. Feet of insects : A, F, Dytiscus. B, Bibiofebrilis. c, Musca domestica. r, Cimbex lutea. E, Abyssinian Grasshopper. under a good microscope, are seen to be covered with innumerable hairs of the utmost 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 progression denied to insects of ordinary construction. (783.) In Bibio febrilis (fig. 150, B), the sucking disks appended to the foot are three in number, but in other respects their conformation is the same. LOCOMOTION OF INSECTS. 301 (784.) In Cimbex lutea (fig. 150, D) the arrangement of the suckers is different, one large and spoon-shaped disk 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 provided with a sucking disk, while the two together form a strong prehensile forceps. (785.) 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 of securely hold- ing the female during sexual union, than as being specially connected with locomotion. (786.) In the anterior legs of the male Dytiscus the first three 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 number of sucking-cups (fig. 150, r), two or three being much larger than the rest ; but they form collectively a wonderful instrument of adhesion. (787.) The middle pair of legs of the same beetle (fig. 150, A) exhibit a somewhat similar structure ; but in this case the disk upon which the sucking apparatus is placed is much elongated, and the suckers are all of small dimensions. (788.) In the female Dytiscus (fig. 152, 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 intercourse of the sexes in these powerful aquatic beetles. (789.) 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 irritans) (fig. 153), 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 familiar instance. In such insects (fig. 145, 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 disposed to leap, such insects bend each hind-leg, so as to bring the tibia into close contact with the thigh, which has often a longitudinal farrow, 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 obviating any jar when the animal alights from its lofty leaps f, or else they may * Kirby and Spence, Introduction to Entomology. 4 vols. 8vo. f Sir E. Home, Phil. Trans. 1816. 302 INSECTA. act like firm cushions, adapted, by their elasticity, to give greater effect to the spring which raises the insect from the ground. In the magni- fied view of the tarsus of an Abyssinian Grasshopper (fig. 150, E) the arrangement of these organs is well exhibited. (790.) 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 (Oryllotalpa vulgaris) (fig. 151). In this creature, the anterior Fig. 151. Gryllotalpa vulgaris. segment of the thorax, whereunto the fore -legs are appended, is won- derfully 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 obliquely 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 first two 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*. (791.) Similar examples of adaptation in the mechanical structure of the legs of insects might be multiplied indefinitely ; we shall, however, * Kirby and Spenrc, Introd. to Ent. volii. p.3G2. DYTISCUS MAKOINALIS. 303 select but one other illustration before leaving this part of our subject, namely the conversion of these organs into instruments for swim- ming, whereby, in aquatic insects, they become adapted 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 ex- amining 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 marginalia (fig. 152, c), the two anterior pairs of legs, that could be of small service Fig. 152. Metamorphoses of Dytiscus. as instruments 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 development of the hinder segment of the thorax, in order to approximate their origins to the centre of the body ; and the individual segments composing them are broad and compressed, so as to present to the water an extensive surface, which is still further enlarged by the presence of flat spines appended to the end of the tibia, 304 INSECTA. 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. (792.) The same principles are carried out even more perfectly in the construction of the swimming-legs of the Water -boatman (Notor- necta), a kind of water-bug. The resemblance of this creature (fig. 144, G, H) to a boat with its oars cannot escape the most inattentive exa- miner ; 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. (793.) The wings of insects, when present, are invariably attached to the two posterior segments of the thorax, which, as we have already seen, are strengthened in every possible manner, so as to afford a sup- port of sufficient density and firmness to sustain the violent exertions of the muscles inserted into the organs of flight. (794.) In the most perfectly organized families the wings are four in number, as in the Neuroptera (fig. 146), the Hymenoptera (fig. 173), the Orthoptera (fig. 145), the Dictyoptera, the Hemiptera (fig. 144), the Lepidoptera (fig. 148), and the Coleoptera (fig. 149). (795.) In the Dipterous insects there are only two wings, which are fixed upon the central segment of the thorax ; while, in the position usually occupied by the posterior pair, we find a pair of pedunculated globular bodies, generally named the halteres or poisers, as in the Gnat (CWaO (fig. 177, F). (796.) But, in every one of the orders above enumerated, there are certain families which, throughout the whole period of their existence, are never provided with wings at all ; and these by many entomologists have been formed into an order by themselves, under the name of Apte- rous 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 dissimilar kinds. In proof of this, he ad- duces the fact that, in the same family, we not unfrequently meet with both winged and apterous species, nearly related to each other ; and in many cases the males possess wings, while the females of the same insect are entirely 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. 153) the wings never become apparent, and in consequence the thorax, even in the imago state, does not exhibit that development and consolidation of its parts invariably met with in * Manual of Entom. p. 623. WINGS OF INSECTS. 305 winged 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 footless 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, Fig. 153. Pulex irritans. Caving 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. (797.) The wings of insects differ much in texture. In the Neuro- ptera, by far the most powerful fliers met with in the insect world, all four wings are of equal size, and consist of a thin membranous expansion of great delicacy and of a glassy appearance, supported at all points by a horny network (fig. 146). Few things are met with in nature more admirable than these structures ; they present,, indeed, a combination of strength and lightness absolutely unequalled by anything of human in- vention ; 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 in which he 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 menagerie about a hundred feet long, and apparently their powers were fairly tested. The swallow was in full pursuit ; but the little creature flew with such asto- nishing 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 f, " such is the power of the long wings by which the Dragon-flies are distinguished, and such the * Epist, 6, Mart. 1717. t Kirby and Spence, op. cit. vol. ii. p. 351. x 300 INSECTA. 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." (798.) In Hymenopterous insects (figs. 171 and 173) 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 numerous. (799.) In several orders the anterior pair of wings are converted into shields for the protection of the posterior ; such is the case in the Ortho- ptera, 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 de- veloped to such a size as to present a very extensive surface (fig. 149, 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. (800.) The above observations relate only to the general disposition and connexion of the different parts of the skeleton, and locomotive 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. (801.) The hard covering of an insect, like the skin of vertebrate animals, consists of three distinct layers. The outer stratum, or epi- dermis, is smooth, horny, and generally colourless, so that it forms a dense inorganic film spread over the whole surface of the body. Imme- diately beneath the epidermis is a soft and delicate film, the rete mu- cosum, 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 generally 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 here- after, are imbedded and nourished. (802.) The wings are mere derivations from this common covering, and are composed of two delicate films of the epidermis, stretched upon a strong and netlike framework. Every membranous wing is, in fact, a delicate bag formed by the epidermic layer of the integument, 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 between them, distends the organ until APPENDAGES TO THE SKELETON OF INSECTS. 307 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 framework upon which they are extended, that they seem to form a single membranous expansion. (803.) The ribs, or nervures, whereby the two plates of the wing are thus supported, are slender hollow tubes, filled with a soft parenchyma : in the interior of some, Burmeister detected an air-vessel (recognizable by the texture of its walls) and a minute nervous filament. (804.) We have still, in order to complete our description of the ex- ternal 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 di- stinct structures, all these are essentially very nearly related to each other. (805.) The spines are horny processes developed from the epidermis ; and sometimes, especially in the Coleopterous order, as in some lamel- licorn Beetles, exhibit considerable dimensions. These spines are some- times bifurcated or branched ; but, whatever their shape or size, they never grow from bulbs implanted in the cutis, but are mere prolonga- tions of the exterior layer of the integument. (806.) The hairs appear to resemble those of quadrupeds in their mode of growth, inasmuch as they are secreted from roots imbedded in the substance of the cutis or true skin : they are fine horny cylinders, 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. (807.) 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 insensible trans- itions ; and, moreover, they grow from bulbs of precisely similar con- struction. 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 de- rived from this source must not be confounded with the iridescent tints for which they are not unfrequently 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 microscope, to be marked with regular parallel striaB 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. * Heusinger, System der Histologie, 2. Heft. Burmeister, op. cit. p. 224. x2 308 INSECTA. (808.) The muscular system of insects has always excited the wonder and astonishment of the naturalist, in whatever point of view he exa- mines 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 dimensions " minims of nature," . !.. . " that wave their limber fans For wings, and smallest lineaments exact, In all the liveries deck'd of summer's pride ;" their presence, indeed, around us is only remarked as conferring addi- tional life and gaiety to the landscape ; and except when, by some in- ordinate increase in their numbers, they make up by their multitude for their diminutive size, the ravages committed by them are trifling and insignificant. Far otherwise, however, would it be if they attained to larger growth, and still possessed the extraordinary 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. (809.) 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, " calculates that in its ordinary flight the common House-fly (Musca domestica) makes with its wings about six hundred strokes, which carry it 5 feet, every second ; but if alarmed, he states their velocity can be increased six- or seven-fold, or to 30 or 35 feet in the same period. In this space of time a race-horse could clear only 90 feet, which is at the rate of more than a mile 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 creature 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*." * Kirby and Spence, op. cit. vol. ii. p. 358. MUSCULAR SYSTEM OF INSECTS. 309 (810.) 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 ? (811.) 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. (812.) 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 horny septa 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. (813.) All the muscles of an insect may be arranged in two great divisions : the first including those that unite the different segments of the body ; the second, those appropriated to the movements 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. (814.) 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 segment, 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. (815.) 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 fig. 154, representing the muscles of the leg of a Cockchafer (Melo- lontha vulgar is), as they are depicted by Straus-Durckheim*. In the thigh, for example, there are two muscles, one of which bends, the other straightens, the tibia. The flexor (fig. 154, a) arises from the lining membrane of the femur, and is inserted by a tendon into a process of the tibia in such a manner as to flex the leg upon the thigh ; while its antagonist (6), attached to a process derived from the other side of * Considerations generates sur 1'Anatomie comparee des Animaux articules, aux- quelles on a joint 1'Anatomie descriptive du Hanneton. 1 vol. 4to. Paris, 1828. 310 INSECTA. Fig. 154. the joint, has an opposite effect, and by its contraction extends the leg. In the tibia there are likewise two muscles, so disposed as to move the entire tarsus and foot. The extensor (/) of the tarsns 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 : bnt the flexor of the foot (c), arising 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 accessoiy muscle (d). The extetisor of the claw (0) is likewise placed in the penultimate tarsal segment, and strikingly exhibits, by its small comparative size, the feebleness of its action when compared with that of the flexors of the same joint. (816.) 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 arrangement of the muscular system, not in insects only, but in all the ARTICULATA pro- vided with jointed extremities. (817.) The substances employed as food by insects are various, in proportion to the exten- sive distribution of the class. Some devour the leaves of vegetables, or feed upon grasses and succulent plants ; others destroy timber, and the bark or roots of trees ; while some, more deli- cately organized, are content to extract the juices of the expanding buds, or sip the honeyed fluids from the flowers. Many tribes are carnivo- rous 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 appointed 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, innumerable modifi- cations 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 con- Muscles of the leg of a Cockchafer. TARTS OF THE MOUTH OF INSECTS. 311 trivance 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 construct- ing for themselves edifices of inimitable workmanship. (818.) Parts of the mouth. The mouths of insects may be divided 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, constituting the suctorial, 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 describing the different pieces of which a perfect mouth consists, viz. an upper and an under lip, and four horny jaws. We select the mouth of the Dragon-fly (fig. 155, A) as an example. The upper lip (labrum, B) is a somewhat convex corneous plate, placed transversely across the upper margin of the cavity wherein the jaws are Fig. 155. Parts of the mouth of a Dragon-fly. lodged, so that, when the mouth is shut, it folds down to meet the under lip (labium), and these two pieces more or less completely con- ceal the proper jaws, which are lodged between them. (819.) The upper pair of jaws (mandibulce) are two hard and powerful hooks (fig. 155, c), placed immediately beneath the upper lip, and so articulated with the cheeks that they move horizontally, opening and shutting like the blades of a pair of scissors. Their concave edge is 312 INSECTA. armed with strong denticulations of various kinds, sometimes furnished with cutting edges that, like sharp shears, will clip and divide the hardest animal and vegetable substances ; sometimes they form sharp and pointed fangs, adapted to seize and pierce their victims ; and not unfre- quently they constitute a series of grinding surfaces, so disposed (like the molar teeth of quadrupeds) as 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 carnivorous 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, pickaxes, saws, scissors, and knives, as the necessity of the case may require. (820.) Beneath the mandibles is situated another pair of jaws, of similar construction, but generally smaller and less powerful ; these are called the maxillce (fig. 155, F). (821.) The lower lip, or labium (fig. 155, 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 membranous or somewhat fleshy organ, reposing upon the chin internally, and called the tongue (lingua) of the insect (D). (822.) 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 entomologist in the determination and distribution of species. For more minute details concerning them, the reader is necessarily referred to authors who have devoted their attention specially to this subject ; we must not, however, omit to mention certain appendages or auxiliary instruments inserted upon the maxillce and the labium, usually named the palpi, or feelers, and most probably constituting special organs of touch, adapted to faci- litate the apprehension and to examine the nature of the food. The maxillary feelers (palpi maxillares) are attached to the external margin of the maxillaB by the intervention of a small scale and very pliant hinge, and consist of several (sometimes six) distinct but extremely minute pieces articulated with each other. The labial feelers (palpi labiales) are inserted into the labium close to the tongue, or occasionally upon the chin (mentum) itself. The joints in the labial palpi are gene- rally fewer than in the maxillary, but in other respects their structure and office appear to be the same. (823.) In the suctorial orders of insects we have the mouth adapted COMPOSITION OF THE MOUTH IN INSECTS. 313 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 Savigny* that the parts composing a suctorial mouth are fundamentally the same as those met with in the mouths of mandibulate insects, but transformed in such a manner as to form a totally different apparatus. (824.) According to the distinguished authors of the < Introduction to Entomology t/ there are five kinds of imperfect mouth adapted to suction, each of which will require a separate notice. (825.) The first is met with among the Hemiptera, and is formed 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. 156. First, there is a long jointed sheath (d), which is in fact the lower lip (labium) considerably elongated, and composed of three or four parts articulated together ; secondly there is a small conical scale covering the base of the sheath last mentioned, and representing the upper lip ; and between these are four slender and rigid bristles or lancets (scalpetta) (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 mandibles and maxillae strangely altered in their form and excessively length- Mouth of Notonecta . ened, so as not merely to become efficient piercing instruments, but so disposed as to form by their union a suc- torial tube, through which animal or vegetable fluids may be imbibed. This kind of mouth, when not employed, is usually laid under the thorax, between the legs, in which position it is easily seen in most Hemiptera. 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. (826). The second kind of mouth is that met with among the Diptera ; and from its construction in some tribes, we may well understand 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 (Tabanus) seems to at- tain its maximum of development. 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 ; * Memoires sur les Animaux saris Vertebres. 8vo. Paris, 1816. t Kirby and Spence, vol. iii. p. 463. 314 INSECTA. in other cases, as in the common Flesh-fly, soft and muscular, and folds up when at rest in such a manner as to form two angles, repre- senting 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 (e.ultelli, or knives) represent the mandibles of a perfect mouth, the two lower ones (scalpella, the lancets) are the maxillaB, the fifth or middle piece (glossa- rium) is the tongue ; and between them all is the oral opening. The Fig. 157*. Head of a Flea. strength of the above piercing instruments varies greatly : in the Gnat 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 * Head of the Flea, as represented by the Solar microscope in Canada balsam ; dedicated by permission to the President and Members of the Entomological Society, by W. Lens Aldous. MOUTH OF THE BUTTERFLY. 315 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. (827.) 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 evident on in- specting the figure in the preceding page, reduced from a beautiful drawing by Mr. W. Lens Aldous. (828.) In this insect the piercing organs are two sharp and razor- like instruments (fig. 157, 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 (a a) and a pair of triangular plates (b b) complete this re- markable apparatus. (829.) 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 stores are lodged. When unfolded, the apparatus in question represents a long double whip-lash (fig. 158, a, 6, c, d) ; and if carefully examined under the microscope, each division is found to be made up of Fi S- 158- innumerable rings connected together, and moved by a double layer of spiral mus- cular fibres, that wind in opposite 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 examined. Each of the two long filaments composing this trunk (which, in fact, are the representa- tives of the maocillce exces- sively lengthened) is then seen to be tubular ; and, when they are placed in contact, it is found that their edges lock to- gether 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 maxillse, that fluids are imbibed. Burmeister, however, asserts that the cavities contained in each division Mouth of a Butterfly. 316 INSECTA. likewise communicate with the commencement of the oesophagus, so that the Lepidoptera have, as it were, two mouths, or rather two separate methods of imbibing nourishment one through the common canal formed by the junction of the whip-like jaws, the other through the cavities of the filiform maxillae themselves ; such an arrangement, how- ever, which would be quite anomalous, may reasonably be doubted. In this mouth, therefore, all the parts, except the maxillae, 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 man- dibles, of the lower lip, as well as of the labial and maxillary palpi, be distinctly demonstrated. (830.) The last kind of mouth to which we shall advert is that met with in the Louse tribe (Pediculi) ; but, from the extreme minuteness of the parts composing it, the details of its structure are only 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 barbed piercer is denuded and plunged into the skin, where it is retained until a sufficient supply of nourishment has been obtained. (831.) 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 consequently 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 substances used as food is accomplished partly by mechanical and partly by chemical agency. 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 will, however, 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 observations must be of general application, and such as will enable the reader to assign its proper function to any organ which may present itself to his notice. The first part of the digestive apparatus is disposed in the same manner in all insects, and is a slender canal arising from the mouth and passing straight through the thorax into the cavity of the abdomen ; this por- ALIMENTARY CANAL OF INSECTS. 317 tion represents the oesophagus (fig. 159, a a ; fig. 160, o). 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, especially if herbivorous, the intestine becomes much elongated and winds upon itself in various convolutions; nevertheless, however tor- tuous 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 connexion, and one which is better adapted to the wants of these animals. (832.) We must 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 some- times 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. (833.) The crop, or sucking -stomach, as it is called by some writers, is only met with in Hymenoptera, Lepidoptera, and Diptera insects which have no gizzard*. In Bees, Wasps, and other Hymenoptera, it is a simple bladder-like distention of the oesophagus (fig. 159, b) ; in Butterflies and Moths it forms a distinct bag, that opens into the side of the gullet (fig. 160, v v) ; while in the Diptera it is a de- tached vesicle, appended to the oesophagus by the intervention of a long thin duct. This organ, which in Bees is usually called the honey- bladder, 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 llamdohrt and Meckel consider it, but as being a sucking instru- ment for imbibing liquids, by be- coming distended, as he expresses it, and thus, by the rarefaction of the air contained within it, facili- tating the rise of the fluids in the proboscis and oesophagus. It must, however, be confessed that there is something very anomalous in the * Burmeister, op. tit. p. 125. Treviranus, Vermischte Scliriften. t Kamdohr, Ueber die Verdauungswerkzcugo der Insekten. Halle, 1811. Alimentary canal of the Honey Bee, Apia melliftca. a a, resophagus ; b, the crop or suck- ing-stomach ; c, d d, the stomach proper; e e, small intestine;/, large intestine; g, anal ori- fice ; A h, biliary vessels ; i i, auxiliary glands. 318 INSECTA. idea of a delicate bag having the power of distending itself : its mus- cular walls might indeed contract ; but that a thin sacculus should forci- bly expand itself would be a fact new to physiology. (834.) 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 its intro- duction into the digestive stomach. In order to effect this, it is lined internally with a dense cuticular membrane, and occasionally studded with hard plates of horn, or strong hooked teeth, adapted to crush or tear in pieces whatever is submitted to their action. (835.) When bruised in the gizzard, the food passes on into the proper stomach, which is generally a long intestiniform organ (fig. 159, d d), extending from the crop or gizzard to the point where the biliary vessels discharge themselves into the intestine. The size and shape Fig. 160. of this organ will vary, of course, with the nature of the food. Thus, in the Butterfly, which scarcely eats at all, or sparingly sips the honey from the flowers, it is very minute (fig. 160, 6) ; but in insects which live upon coarse and indigestible materials, it is proportionately elongated and capacious. (836.) The stomach gene- rally ends in the small in- J but this is occasionally en- tirely wanting, so that the stomach seems to terminate immediately in the colon or large intestine, which is the terminal portion of the ali- mentary canal: when much developed, the small intestine is sometimes divided by a constriction into two parts, to which the names of duodenum and ilium have been applied by ento- mological writers. The colon (fig. 159, / ; fig. 160, Te) is separated from the small intestine by a distinct valve ; and in connexion with its com- mencement a wide blind sacculus or caecum is often met with. Alimentary canal of a Butterfly : a a, proboscis ; p, mouth ; m m, pharynx ; sp, s p, salivary glands ; o, oeso- phagus ; v v, crop, communicating by a canal (c,/) with the stomach proper (b, z) ; i, small intestine ; k, large intestine ; g g g, hepatic vessels ; h, n, their termina- tions in the vicinity of the pylorus. ASSISTANT CHYLOPOIETIC VISCEKA OF INSECTS. 319 (837.) We may now notice the secerning organs that pour fluids into different parts of the digestive apparatus, beginning with those which open into the ossophagus in the vicinity of the mouth, and examining them in the order of their occurrence as we proceed backwards. (838.) The first are the salivary vessels, which terminate in the neighbourhood of the mouth itself, into which they seem to pour a se- cretion analogous to 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. 160, 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 excessively acrid and irritating, acting as a kind of poison when infused into a puncture made by the mouth : this is especially remarkable in many bugs and gnats, and is the chief cause of the pain and inflammation frequently occasioned by their bite. (839.) Besides the proper salivary vessels, there are other glands, or rather caeca, which open into the stomach itself, occasionally covering that organ over its entire surface, as is the case in some water-beetles (Hydropliilus} : these, no doubt, secrete a fluid subservient to digestion ; but whether of a peculiar description, or allied to saliva in its proper- ties, is unknown. (840.) The third kind of auxiliary vessels connected with the in- testinal canal of insects are supposed to furnish a secretion analogous to the bile of other animals, and consequently to represent the liver. These bile-vessels (fig. 159, hh; fig. 160, #(7) 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 termi- nate in the neighbourhood of the pylorus (fig. 160, h, ri), 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. (841.) 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 kidneys 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 recognizable until we arrive at much more elevated forms of life than the insects we are now 320 INSECTA. considering. There is, however, another reason for rejecting the opinion that these accessory vessels secrete urine ; and that is, that they are only met with in a few heetles and some species of Orthoptera a circum- stance that alone would be sufficient to disprove such a supposition. (842.) In the vertebrate animals, as the reader is well aware, the nutritious products of digestion are taken up by a system of absorbing vessels that ramify extensively over the coats of the intestine ; and the nutriment is thus conveyed into the mass of the circulating 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 ab- sorbents nor veins, a different arrangement 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 immediately mixed up with the blood. This transudation has indeed been actually witnessed by Ramdohr and Bengger*, and even analysed by the last-mentioned physiologist, who found it to consist almost entirely of albumen. (843.) The respiratory organs of the Insecta, as well as their circu- latory apparatus, are constructed upon peculiar principles, and are evi- dently in relation with the capability of flying which distinguishes 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 apparatus of arteries and veins for conveying the blood to and fro for the purpose of purifying it by securing its exposure to the influence of air. Ey 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 state ; for in the larva and pupa conditions, where flight is not possible, various additional organs, frequently of consider- able bulk, are provided, that we shall speak of in another place. If we examine the external skeleton of any large insect (a beetle, for example), we shall find, between the individual segments of the body, minute apertures or pores (spiracles} through which the air is freely admitted : * Physiologische Untersuchungen iiber die thierische Haushaltung der Insekten. 8vo. 1817. RESPIKATOKY SYSTEM OF INSECTS. 321 these openings, ten in number on each side of the body, are situated ^n 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 Tig. 161. 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 admis- sion. In many insects, indeed, especially in bee- tles which crawl upon the dusty ground, an additional provision is necessary to prevent the entrance of foreign mat- ter; and in such cases the spiracles are seen to be covered with a dense investment of minute and stiff hairs, so dis- posed as to form a sieve of exquisite fineness, a beautiful contrivance by which the air is fil- tered, as it were, before it is allowed to pass into the breathing-tubes, and thus freed from all pre- judicial particles. From every spiracle is derived a set of extremely deli- cate tubes (trachece),tha,t pass internally, and be- come divided and sub- ~~ - : ---~ divided to an indefinite Respiratory apparatus of Melolonthavulgaris. 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 322 INSECTA. Fig. 162. tubes of exquisite delicacy ; in other cases they present a beaded or vesi- cular structure ; and in many insects they are dilated at intervals into capacious cells or receptacles, wherein air is retained in great abundance. The beautiful figure given in the preceding page (fig. 161), taken from Straus-Durckheim's elaborate work upon the anatomy of the Cock- chafer, will illustrate this arrangement. The spiracles, situated at the points respectively marked by the letters a, b, c, d, e, f, g, Ti, 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 enlargement, plunge at once into different textures, and supply the viscera and internal organs. The muscular system, the legs, the wings, the ali- mentary 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. (844.) There is one circumstance connected with the tracheae which is specially deserving of admiration, whether we consider the obvious design of the contrivance, or the remarkable beauty of the struc- ture employed. It is evident that the sides of canals so slender and delicate as the tracheao of insects would inevitably collapse and fall together, so as to obstruct the passage of the air they are de- stined to convey; and the only plan which would seem calculated to obviate this would appear to be, to make their walls stiff and inflexible. Inflexibility and stiff- ness, however, would never do in this case, where the vessels in question have to be distributed in countless ramifications through so many soft and distensible vis- cera ; and the problem therefore is, how to maintain them perma- nently open, in spite of external pressure, and still preserve the perfect pliancy and softness of their walls. The mode in which this is effected is as follows : Between the two thin layers of which each air-vessel consists, an elastic spiral thread is interposed (fig. 162, a), so as to form by its revolutions a firm cylinder of sufficient strength to ensure the calibre of the vessel from being diminished, but not at all Trachea! tube of an insect, highly magnified, showing, at a, the elastic spiral thread. TEEMINATIONS OF THE TEACHEAL TUBES. 323 interfering 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. (845.) There is a limit, observes Dr. Williams*, different in different structures, at which the spiral thread ceases ; and at this point the mem- branous trachea begins. It is not the external covering which ceases, but the spiral which, growing less and less visible, graduates insensibly into a continuous tube. The diameter of the " spiral" trachea constantly decreases as it divides ; that of the membranous observes, throughout its entire course, whether it multiply into a network, or wavy brushes, or into the muriform plexus which exists in the substance of muscles, a uniformity which can compare only with that of the true blood- capillaries of the vertebrate animal. The tracheae terminate differently, and form different plexuses, in different organs, according to the varying mechanical arrangements of the ultimate parts of the latter. They are evidently air-tubes throughout, even to their final extremes. (846.) The primary, secondary, and tertiary air-tubes divide and sub- divide arborescently, the branches never uniting, but the ultimate rami- fications dividing and subdividing in the same profuse retiform manner as the blood- capillaries of the vertebrate animal, supplying the muscles, the glands, the mucous membranes, the brain, and every other viscus. The large air-tubes which travel along the axes of the spacious blood- channels detach from their sides here and there minute wavy branches which float in the fluid, and appear to be expressly intended to aerate the fluids. (847.) In all the transparent structures of insects, such as the wings, antennae, branchiae, &c., the blood- currents travel in the same passages as the tracheae. On closer scrutiny it will be seen that a channel, such as that of the nervure of the wings, bearing in its centre a large tracheal tube, exhibits on one side a current going in one direction; on the other, another bearing in an opposite course. These are afferent and effe- rent, arterial and venous blood-streams. They are bounded by separate walls. The afferent current is circumscribed by its own proper coats, the efferent by its own ; and the trachea is placed intermediately, having parietes quite distinct from, although contiguous with, those of the blood-channels. This coincidence between the tracheae and the blood- currents can be traced in the wings nowhere beyond the limits of the nervures into the scaly spaces that they circumscribe. The returning of the corpuscles at a certain point renders this fact quite unquestion- able. Beyond this limit, only the fluid elements, not the corpuscles of the blood, penetrate. In this extra- vascular region it is cyclosis, not circulation, which governs the movements of the nutritive fluid. If, says Dr. Williams, everywhere the blood and the air travelled toge- * Loc. cit. T2 324 INSECTA. ther, the inference would be that the sole design of the tracheal appa- ratus of the insect consisted in aerating the fluids. Since, however, the blood returns much before the tracheae reach their remote pene- tralia, it is evident that the tracheal system in the insect fulfils some other function. What can be the meaning of those incomparable pneumatic plexuses veritable retia mirabilia which embrace imme- diately the very ultimate elements of the solid organs of the body ? those microscopic air-tubes, which carry oxygen in its gaseous form, unfluidi- fied by any intervening liquid, to the very seats of the fixed solids which constitute the fabric of the organism? The intense electrical and chemical effects developed by the immediate presence of oxygen at the actual scene of all the nutritive operations of the body, fluid and solid, give to the insect its vivid and brilliant life, its matchless nervous activity, its extreme muscularity, its voluntary power to augment the animal heat. Such contrivance, subtle and unexampled, reconciles the paradox of a being, microscopic in corporeal dimensions and remarkable for the relative minuteness of the 'bulk of its blood, sustaining a frame graceful in its littleness, yet capable of prodigious mechanical results. (848.) We must now consider 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 con- tinually performing movements of expansion and contraction that suc- ceed each other at regular intervals, 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 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 vibration caused by the air stream- ing 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 accident, or by the simple application of any greasy fluid to the exterior of their body, speedy death, produced by suffocation, is the inevitable result. (849.) A moment's reflection upon the facts above stated concerning the respiration of insects will suggest other interesting views connected with the physiology of these little creatures. It is evident, 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 become deteriorated. The perfection of their muscular power, their great strength and indomitable * Sorg, Disquisitiones Physiologies circa Kespirationem Insectorum et Vermium- STKUCTTJKE OF THE DOESAL VESSEL. 325 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, endowing 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 in- fluence 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 propulsion of the vitiated blood through pulmonary or branchial organs are no longer requisite ; so that, by dispensing with the complicated structures usually provided for this purpose, the body is considerably lightened. The cir- culation 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 examine 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. (850.) 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 abdo- minal 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. (851.) The structure of this remarkable heart has been fully in- vestigated by Straus-Durckheim*, and is extremely curious : it con- sists, in the Cockchafer, of eight distinct compartments, separated 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 direction. (852.) 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 refer- * Op.dt. 326 INSECTA. Fig. 163. ence to the accompanying figure (fig. 163), representing a magnified view of the interior of a portion of the heart of the Cockchafer, as depicted by the celebrated entomotomist before alluded to. The organ has been divided longi- tudinally, so that one-half only is represented in the figure, upon a very large scale. The compart- ments (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 mem- branous valves, admit blood from the cavity of the abdomen, but effectually prevent its return. (853.) Let us now consider the movements of the circulating fluids produced by the contrac- tions of this apparatus. The chyle or nutritive material extracted by the food exudes, 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 surface 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 aper- tures ; and as it cannot return through that open- ing on account of the valves (c) that guard the entrance, nor escape into the posterior divisions of the heart by reason of the valves (d), the contrac- tion 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 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 Iarva3 that are readily to be met with. When any of the limbs of these larvae are examined under a powerful microscope, continual currents of minute oat-shaped 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 resemble those of the sap in Chara, and other transparent vegetables, in which the circulation of that fluid is visible under a microscope. (854.) The organs appropriated to furnish the different secretions met Internal view of a por- tion of the dorsal vessel of a Cockchafer : aaa,bb, muscular walls of the compartments; d d, in- tercompartmental valves ; c, valve defending one of the orifices communicat- ing with the general cavity of the abdomen. NEBVOUS SYSTEM OF INSECTS. 327 with in the economy of insects are modified in their structure to cor- respond 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 envelope of the caterpillar, are all derived from the same source, and in some mysterious manner elabo- rated 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. (855.) In the nervous system of the INSECTA, we have many interest- ing illustrations of that gradual concentration of the parts composing it, and consequently of increased proportionate development 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 supra-cesophageal 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 additional nervous apparatus, apparently representing the sympathetic system of vertebrate animals, which is distributed to the viscera appro- priated to digestion. Each of these divisions will therefore require a separate notice. (856.) The brain, or encephalic ganglion (fig. 164, l), is a nervous mass of considerable size placed above the gullet : it consists essentially of two ganglia united into one mass ; and from it all the nerves appro- priated to the special instruments of the senses are derived ; so that it may naturally be regarded as the chief seat of sensation and intelligence. The nerves originating from this common sensorium are seen upon an enlarged scale in fig. 165 : they are the optic (fig. 165, a), supplying the eyes, and the antennal (fig. 165, e\ which run to the special instruments of touch, or antennce organs of a very singular character, that we shall examine more minutely hereafter. Two other cords of variable length (fig. 165,# g) are given off from the inferior aspect of the brain, and serve to connect it with the anterior ganglion of the ventral chain (fig. 165, Ti), 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 con- nected with the mandibles, maxillce, and other organs of the mouth. (857.) The rest of the ventral chain of ganglia forms a continuous series (fig. 164, 2, 3, 4, 5, 6, 7, 8) of nervous centres arranged in pairs and 328 INSECTA. 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 proportionate size, inasmuch as they furnish the nerves that supply the muscles of the Fig. 164. Anatomy of Melog: a, the stomach; 6, hepatic vessels; c, intestine; d d, ovaria; e t section of ovary showing the internal cavity;/, vagina; g, spermatheca; h, i, gluten-secretors : 1, supra- oasophageal ganglion of the nervous system; 2,3,4,5, 6,7, 8, ventral ganglia; 99, nervus vagus; 10, cephalic nerves ; 11, optic ganglion. wings and legs ; the succeeding ganglia give branches to the abdominal segments ; and the last, which is commonly of considerable bulk, sup- plies the sexual organs and the extremity of the colon. (858.) It is the general opinion of modern physiologists, that the NEEVUS VAGUS. 329 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 Magendie demonstrated the existence of distinct columns or tracts in the spinal axis of vertebrate animals, various anatomists have endeavoured to show that corresponding parts may be pointed out in the ventral chain of articulated animals. There can, indeed, be no doubt that this portion of the nervous system of an insect corresponds in function with the medulla spinalis ; and if, in the one case, the nerves which preside over the general muscular move- ments 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 reasonably suppose a similar arrangement to exist in the struc- ture 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 exist in the central axis of insects ; but, from the extreme minuteness of the different parts, it is not easy satisfactorily to demonstrate them sepa- rately. In the larger ARTICTTLATA, however, as for example in the CRUSTACEANS, two distinct columns of nervous matter are readily de- tected ; 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 con- clusions from the comparison. (859.) The last division of the nervous apparatus, which we have already mentioned as being the re- presentative of the sympathetic sy- stem, 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 * Biblia Nature. Fig. 165. Nervus vagus and sympathetic system of an Insect : a a, optic nerves ; d d, su- pra-cesophageal ganglion; I, oesophagus; b b, origins of the recurrent nerve ifk; g g, nerves surrounding the oesophagus, and communicating between the supra- cesophageal ganglion, d d, and the first pair of ventral ganglia, h; c c, 1 1, sympathetic ganglia. 330 INSECTA. nerve, arises (fig. 165, b b) by two roots from the opposite extremities of the brain, close to the origins of the antennal nerves. The nervous cords thus derived soon unite to form a minute central ganglion (fig. 165, i), from which proceeds a single nerve (fig. 165,/fc) that runs with the gullet beneath the brain, and spreads in delicate ramifi- cations upon the oesophagus as far as the muscular stomach (fig. 164, 9 9), or to the gizzard when that organ exists. (860.) The sympathetic system properly so called consists of four small ganglia (fig. 165, cc, 1 1), 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 ceso- phagus, and supply it with minute nerves. (861.) Various are the conjectures entertained by different authors 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 ; the nature of their sensations has been a fruitful subject of inquiry ; and some physio- logists have even gone so far as to deny the correspondence 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 conjec- tures promulgated from various sources relative 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, specu- lations respecting them are not at all calculated to lead to satisfactory conclusions. We must from necessity take our own senses as the standard of comparison, limiting our inquiries to examining 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, investigating the structure of the organs by which they are exercised. (862.) The sense of touch is indubitably bestowed upon all insects ; and, to judge from the perfection of the edifices which they build, and the precision of their usual operations, this must be extremely delicate. It is sufficient, however, to look at the external construction of the skeletons of ARTICTTLATA to perceive that the hard and insensible inte- gument spread over the entire surface of their bodies is but little calcu- lated to receive tactile impressions. The antennae, or feelers as they are popularly called, have been very generally regarded as being pecu- liarly instruments of touch ; and whoever watches the proceedings of an insect in which these appendages are largely developed will, we appre- hend, easily convince himself that they are employed to investigate surrounding objects by contact. Straus-Durckheim regards the feet as being specially appropriated to the sense of feeling ; but this opinion SENSES OF INSECTS. 331 seems quite inadmissible. Burmeister places the exercise of touch exclusively in the palpi attached to the maxillae and labium, and ob- serves 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 rami- fications. (863.) 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 assign- ing the function of tasting to other parts are purely conjectural. (864.) 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 antennae and the palpi have each had the power of smelling assigned to them, but without much plausi- bility. 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 rami- fications of the tracheae 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 connexion with the mouth, to which they attri- bute 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. (865.) 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 connected 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. Burmeister f 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 ap- pendages, 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 * Vermischte Schriften, vol. ii. t Op. cit. p. 296. 332 INSECTA. vibratory movements communicated to the antennae by the sonorous undulations of the atmosphere. (866.) 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 antennae, spread them- selves out ; but this apparatus has not been detected in other insects. (867.) The eyes of insects are of two kinds, simple and compound, 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. (868.) Some insects, as the Dictyotoptera and Thysanoura, only possess simple eyes ; others, as for example the Coleoptera, have only com- pound eyes ; but in general both kinds exist together. In the Sireae gigas (fig. 171), 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. (869.) The structure of the eyes has been most minutely investigated by several distinguished entomotomists ; and the labours of Marcel de Serrest, Joh. MiillerJ, Straus-Durckheim , and Duges|| have done much to dispel the mistaken notions entertained by preceding anato- mists. (870.) The simple eyes consist of a minute, smooth, convex, trans- parent 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 distinct 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 compound eyes of insects ; for these are con- structed upon principles so elaborate and complex, that we feel little surprise at the amazement expressed by early writers who examined them, although their ideas concerning their real structure came far short of the truth. (871.) 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 hemi- spherical. 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 corner (for * Annalen der Wetterau. Gesel. f. d. ges. Naturk. rol. i. 1809. f Mem. sur les Yeux composes et les Yeux lisses des Insectes. Montpellier, 8vo, 1813. } Zur vergleichenden Physiologic des Gesichtssinnes. 8vo, 1826. Ann. des Sci. Nat, torn, xviii. || Ibid. torn. xx. STRUCTURE OF COMPOUND EYES. 333 Fig. 166. such, in fact, they are) varies in different genera : thus, in the Ant (Formica) there are 50 ; in the common House-fly (Musca domesticd), 4000 ; in some Dragon-flies (Libellula), upwards of 12,000. In Butter- flies (Papilio) 17,355 have been counted ; and some Coleoptera (Mor- della) possess the astonishing number of 25,088 distinct corneae. (872.) 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, pro- vided with a perfect nervous apparatus, and exhibiting its peculiar lens, iris, and pupil ; thus being completely entitled to be considered a distinct instrument of vision. (873.) By attentively examining the annexed figure, representing a section of the eye of the Cockchafer (Melolonfhd), as displayed by Straus- Durckheim, the whole structure of the organ will be readily understood. The optic nerve (fig. 166, a}, derived immediately from the supra- cesophageal mass of nervous matter, swells soon after its origin into a rounded gan- glion nearly half as large as the brain itself. 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 bril- liant red colour, and is placed concentrically with the con- vex outer surface of the eye. Behind this membrane (called by Straus - Durckheim the common choroid), the second- ary optic nerves (b) unite to form a membranous expan- sion of nervous matter (c), which may be denominated the general retina. 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 towards the individual eyes whereunto they are respectively destined, and the structure of which we must now proceed to examine. In fig. 166, B, a portion of the circumference of the compound eye is represented upon II I Structure of the eye of a Cockchafer. A, sectional view : a, optic ganglion ; 6, secondary nerves ; c, ge- neral retina, in front of which is a layer of pigment, d ; e, proper optic nerves, supplying the individual facets of the compound eye. B, a group of ocelli, much magnified : /, bulb of optic nerve ; g, layer of pigment ; h, vitreous humour ; i, cornea. 334 INSECTA. a very large scale, in order to show the construction of the hexagonal ocelli that enter into its composition. Each cornea (?') is a double con- vex lens, adapted by its shape to bring to a focus the rays passing through it. Behind every lens so constituted is placed a hexahedral transparent prism (Ji), which from its office may be compared to the vitreous humour of the human eye ; and it is upon the posterior extre- mity of these prisms that the proper optic nerves (fig. 166, 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 accounts given of them by different anatomists. Straus-Durckheim represents every optic nerve as terminating in a minute pyriform bulb (fig. 166, B, /), and points out a dark layer of pigment (g), which forms a choroid tunic proper to each ocellus ; while, according to Miiller 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 (^o56.) In Spiders, the organization of the mouth is altogether dif- ferent. The mandibles (fig. 186, 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 inflicted ; such, indeed, is the malignity of this poisonous secretion that its effects in destroying the life of a wounded insect are almost 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. (957.) The palpi connected with the maxillaa 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. 185, B), the terminal part of the palpus presents a club-like dilatation, which, however, on close inspection will be found to consist of several pieces (fig. 185, A, a, b, c, d, e), connected with each other by articulations, and capable of being opened out in the manner represented in the figure. Fi &- 185> This strange instrument was for- merly imagined to be the penis of the male Spider, and was thought to contain the terminations of the seminal ducts : this supposition, however, has been proved to be erroneous ; for the palpus is im- . . , .. Palpus of male Spider. perforate, and the sexual apertures of the male are situated elsewhere ; but the organ in question is never- theless apparently used in the process of impregnation, in a manner to be explained hereafter. (958.) Both in Scorpions and Spiders the alimentary canal is exceed- ingly 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 material. (959.) In the Scorpionidae there is no stomachal dilatation whatever : a straight intestine passes directly from the mouth to the anus, situated at the extremity of the abdom n ; and the insertion of the biliary DIGESTIVE SYSTEM OF SPIDEE. 371 vessels forms the only distinction between its ventricular and intestinal divisions. Five delicate caeca are derived from each side of the ven- tricular portion, and plunge into the centre of a fatty substance in which the alimentary canal is imbedded. In Spiders, likewise, caeca are appended to the commencement of the digestive apparatus ; and a slight enlargement (fig. 186, b) may be said to represent the stomach, from which a slender intestine (g) is continued to the anus. As in the Scorpion, a large quantity of fat (h) surrounds the nutrient organs and fills up a great propor- tion of the cavity of the abdomen. Like the fat-mass of the larvae of insects, this substance must, no doubt, be regarded as a reservoir of nutriment ; and when the habits of these animals are considered, the precarious supply of food, and the frequent necessity for long-pro- tracted fasts when a scarcity of insects deprives them of their ac- customed prey, such a provision is evidently essential to their preser- vation. (960.) One peculiarity connected with the arrangement of the chy- lopoietic 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 termi- nation 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. 186 & 188,/) joins the intestine, into which the branched tubes (fig. 188, o o, & fig. 186, s) empty themselves. This circumstance has long been a subject of interesting inquiry to the comparative physio- logist. 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 production. Are the caeca appended to the stomach biliary organs ? If so, the apparatus in question may be of a totally distinct character, and its product only furnished to be expelled from the system. In conformity with the last supposition, many anato- mists have been induced to regard these vessels as being analogous to Digestive system of the Common Spider : c, poison-fang; oo, the jaws, with their ap- pended poison-glands; a a a, caecal append- ages to d, the commencement of the alimen- tary canal, with which the muscle (e) is con- nected ; 6, stomachal dilatation ; g, intestine ' h h, accumulation of fat ; f, sacculus receiving the terminations of the secreting-tubes (). 372 AKACHNIDA. Fig. 187. 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 investiga- tions is far more advisable than rashly to assign a definite func- tion to a part the real nature of which is a matter of specu- lation. (961.) The respiratory sy- stem of the Pulmonary Arach- nidans is constructed upon very peculiar principles, being neither composed of gills adapted to breathe water, nor of lungs like those of other air-breathing animals, but presenting a com- bination of the characters of both. Thepulmonibranchice are, in fact, hollow viscera resem- bling bags, the walls of which are so folded and arranged in laminae, that a considerable sur- face is presented to the influence of oxygen. It is, indeed, highly pro- bable that these organs are intermediate in function as well as in struc- ture between an aquatic and air-breathing respiratory apparatus ; for, as both the pedi- palp 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 pulmonibranchia opens externally by a distinct orifice resembling the spiracle of an insect, and is closed in a similar manner by moveable horny lips. In the Scorpion (fig. 184) 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 re- markable organs (repented in the figure), resembling a pair of combs, which are ap- minal saccuius receiving the ter- ., 11-11 i .-, minations of the secerning vessels parently adapted to keep the spiracular ( 00 ). Digestive and circulatory apparatus of the Harvest Spider : a, the stomach, with its lateral caeca, on which is situated the dorsal vessel ; b 6, vascular sinuses. Fig. 188. Termination of the alimentary CIECULATOEY SYSTEM OF SPIDER. 373 orifices free from dirt, and thus prevent any obstruction to the free ingress and egress of the air. (962.) In the Araneidae, the form and arrangement of the spiracles are somewhat different. According to Treviranus, there are four pairs on each side of the cephalothorax, 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. (963.) In order to understand the manner in which respiration takes place inpulmonibranchice 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 condi- tion. 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 a general 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 opi- nions entertained at the present day relative to the means whereby the circulation of Arachnidans is accomplished. (964.) According to Treviranus, spiders are provided with a long contractile vessel (fig. 189, a), which runs along the mesial line of the Fig. 189. Plan illustrating the circulatory system in a Spider: a, dorsal vessel; b, suspensory muscle ; c,the ocelli; d, poison-gland; e, palpus ; f, pulmonibranchial organ; g, poison-fang; A, cephalo- thorax; i, caecal appendices to the stomach; I, vascular trunks derived from the dorsal heart, running to the pulmonibranchise ; m, abdomen ; n, spinnerets. back, and resembles in form the dorsal vessel of insects, although in structure it is widely different. In insects, it will be remembered, the dorsal vessel communicated freely with the abdominal cavity by nume- rous valvular apertures, and neither arteries nor veins were necessary for diffusing the blood through the system ; but in the Pulmonibranchiate Arachnidans numerous vascular trunks (II) are given off from both sides of the dorsal heart and are dispersed in all directions. All the branches proceeding from the sides of the dorsal vessel are presumed to be of an arterial character, with the exception of a few large canals 374 AKACHN1DA. 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 pulmonibranchice (/) 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 diffused circulation seen in the larva of an insect, than that of a creature 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 con- clusions relative to the circulation of Arachnidans : The pulmoni- branchice 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 distribution 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 contraction drives the blood through numerous arterial canals to the periphery of the system ; the blood so distributed gradually finds its way into capa- cious 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 the heart drives a portion of the cir- culating fluid into the pulmonibranchice 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. (965.) In the Scorpions, the circulatory system resembles that of the Myriapoda, but it is more completely organized ; the heart, which, as in the Scolopendra, is divided into compartments, is elongated at its posterior extremity into a long caudal artery, and gives off from each chamber a pair of systemic arteries, which are distributed among the viscera, and also send their principal divisions to supply the muscles of the inferior and lateral regions of the body, as well as the pulmonary sacs. At the anterior part of the abdomen, the heart assumes the character of an * Audouin, Cyclop, of Anat. and Phys., art. "ARACHNIDA." CIKCULATOKY SYSTEM OF SCORPION. 375 aorta, descending suddenly into the thorax, and dividing immediately behind the brain into a number of large vessels, that supply the head and the locomotive organs. The posterior of these form a vascular collar around the oesophagus, which gives origin to the great arterial trunk, or supra-ganglionic vessel, whereby the blood is conveyed to the posterior part of the body, as in the Myriapoda (vide 746). This vessel passes beneath the transverse arch of the thorax, with which it is slightly connected by fibrous tissue, and then runs backwards, gradu- ally diminishing in size, until it reaches the terminal ganglia of the tail, where it divides into branches that accompany the nerves. In addition to the above arrangement, Mr. Newport has discovered a fibrous structure, from which are given off two pairs of vessels, to be distributed to the first pair of branchial organs, as also a little vessel which, passing backwards, anastomoses with the spinal artery, to form the sub-spinal vessel. This latter takes its course beneath the chain of nervous ganglia, communicates directly by means of short branches with the supra- ganglionic artery, and, at intervals, gives off from its under surface large vessels, which, uniting together, convey the blood which has circulated in the abdominal segments directly to the branchiae, whence it is returned to the heart by a great number of slender canals, which, emanating from the posterior aspect of each branchial organ, unite to form larger trunks, that run along the walls of the segments, to pour their contents into the valvular orifices situated upon the dorsal aspect of the heart. (966.) The heart of the Scorpion* is a strong muscular organ ex- tending along the middle of the back, from its continuation with the great caudal artery in the last segment of the abdomen to the com- mencement of the aorta. In the dorsal part of its course the heart is divided into eight separate chambers, which are wider and stronger in proportion to their length than in the highest of the Myriapoda. They are more muscular and compact in proportion to the greater quantity of blood to be transmitted through them, and the force with which it is necessarily propelled. The form of each chamber is somewhat heart- shaped, being slightly contracted in its middle portion and enlarged at its posterior. Each chamber has two. auricular openings for the passage of the blood, placed very close to the median line of the heart on its dorsal surface ; and it gives off at its inferior lateral angles a pair of large arterial vessels (the systemic arteries), which distribute the blood downwards to the viscera and to the dorsal and lateral surfaces of the body. (967.) Each chamber is provided at its sides, as in the Myriapoda and Insects, with two sets of muscles, the alee cordis. The anterior and larger pair of muscles are attached to the anterior part of each segment, and pass diagonally forwards, and the posterior (the proper * Newport, Phil. Trans. 1843. 376 AEACHNIDA. retractor muscles of the chamber) to its posterior angle, and pass backwards, leaving between the two sets of muscles a passage for the (968.) The structure of the chambers internally differs considerably from that of the chambers in the Melolontha, as described by Straus- Durckheim ( 852). Each valve, or division between them, is formed by a reduplication of the whole muscular structure of the dorsal surface of the organ. This reduplication, which is chiefly on the upper and lateral surfaces, is veiy imperfect on the under, and in some of the chambers is entirely absent on the under surface. The reason for this imperfect structure of the valves may perhaps be explained by the fact that the blood is distributed from the heart, in the Scorpion, in opposite directions partly backwards towards the tail, but chiefly forwards to- wards the head and sides ; and hence it may be necessary that a reflux of the blood should not be entirely prevented, as may be required in those instances in which the whole current is in one direction. The structure of the heart is exceedingly thick, opake, and muscular ; it is formed of two layers of fibres, longitudinal and circular in each layer, the most powerful of which are the latter. On its internal surface it is smooth, and lined by an exceedingly delicate membrane, through which the strong circular fibres are distinctly marked. It is by means of these that its most powerful contractions are effected, the auricular action being chiefly the result of the relaxation of these fibres, assisted by the reactions of the lateral muscles. (969.) The aorta arises from the anterior extremity of the heart or dorsal vessel. It is short, thick, and smooth on its external surface, without lateral muscles or internal divisions into chambers. It descends obliquely forwards and downwards, and after passing beyond the great median arch of the thorax, to which many of the muscles of this region of the body are attached, it gives off the vessels to the head, to the organs of locomotion, and to form the great supra-spinal artery, which, as in the Myiiapoda, represents the aortic trunk, or rather the aorta descendens, which, running above the chain of nervous ganglia, supplies the neighbouring parts in this region of the body, as well as branches to the alimentary canal and to the liver. (970.) The portal system of vessels is situated chiefly below the nervous cord, on the ventral surface of the body, and is the means by which the blood is collected and conveyed to the branchiaB, from which it seems to be returned to the system, after circulating through the organs, by means of a large sinus or vessel at their posterior superior angles. Behind the bony arch of the thorax there is a hollow fibrous structure, that closely surrounds the cord and nerves as in a sheath. It seems to form a kind of sinus, from the posterior part of which a small vessel passes backwards, which, joined by anastomoses from the supra- spinal artery, forms the commencement of the sub -spinal vessel ; and STKUCTUEE OF PULMONIBRANCHI^E. 377 it gives off two pairs of vessels at its sides. The first and second pairs of these efferent vessels, covered hy the thick peritoneal lining of the abdomen, send the blood in a diagonal direction backwards to the first pair of abdominal branchiae. The first pair of these vessels originate close to the folds of the diaphragm. They pass backwards and ontwards into the abdomen, and are joined in their course by numerous small vessels from the sides of the segments, and immediately anterior to the first pair of abdominal branchiae are each divided into two branches, which are again divided and subdivided into a multitude of anasto- mosing vessels before they are distributed on the branchiae. These branchiae likewise receive the second pair of efferent vessels, which, like the first, pass diagonally backwards from the fibrous structure to the inner side of the branchiae, on approaching which they are divided like the other pair into two branches, which are subdivided, and anastomose with the divisions of the first pair. The whole form a most intricate web of anastomosing pulmonic capillary vessels before they are distri- buted on the anterior part of the branchiae. We have thus a complete distribution of the blood to the pulmonibranchiae in the anterior part of the abdomen. There is a similar but less perfect distribution in the posterior. (971.) Professor Miiller* has accurately described the pulmoni- branchiae as formed of a multitude of closely- approximated, thin, double lamellae, which communicate, by a small orifice in each, with the ex- ternal air admitted into a common cavity through the spiracle on the surface of the body. The blood, distributed through these lamellae, is brought into contact with the air in their interior through their mem- branous structure. The minute anatomy of these lamellae, and the manner in which they are permeated by the blood, afford some points of interest. Each side of these double lamellae is formed of an exceedingly delicate and apparently structureless double membrane, which includes within it a parenchymatous tissue formed of single vesicles or cells. The convex margin of each lamella is bounded by a delicate but distinct vessel, which seems to form the means of intercommunication with the anastomosing network of vessels distributed over the branchiae, since the delicate evanescent vessels traced into the lamellae are derived from those which bound their convex margin. (972.) At the posterior part of the inner side of the branchiae, where the lamellae are covered by the thick membrane and peritoneum that lines the common cavity of the branchiae, there are several small orifices, the commencement of vessels which afterwards, when collected together, form the larger channels that convey back the blood to the heart. These vessels form delicate trunks or sinuses, which pass around the sides of the body in the posterior part of each segment, and, be- coming gradually enlarged by communicating with other vessels in their * Meckel's Archiv, 1828. 378 AEACHNIDA. progress, pour their contents into the heart at the auricular orifices upon its dorsal aspect. (973.) The following, then, will be the course of the circulation in the Macrourous Arachnidans. The blood received by the veins from the branchiae is conveyed to the heart round the sides of the segments, receiving accessions from other vessels in the segments in its course, and enters the heart at the posterior part of each chamber through the orifices of Straus-Durckheim. The auriculo-ventricular cavity, dilated by the influx of blood, begins first to contract by the action of the circular fibres at the posterior part of each chamber. By this contraction, part of the blood is at once propelled laterally, through the systemic arteries, to the interior and sides of the body, while the remaining and chief portion is forced onwards, through the valves and body of the chamber, by the successive contraction of the circular fibres, into the next chamber. A fresh accession of blood enters the heart at the auricular orifices in the short interval of time that elapses between the contractile actions of the two chambers, which interval is probably occasioned by the reaction of the lateral muscular appendages of the organ. These con- tractions are carried gradually onwards through the whole of the suc- ceeding segments ; so that before a third chamber has contracted, the first is again filled and ready to be emptied, thus occasioning, by their alter- nate movements, those pulsatory motions which are so well known in the dorsal vessel of Insects. The blood, propelled by these successive contractions through the aorta, is distributed to the organs in the head and thorax and the organs of locomotion. Part of it is also sent round the aortic arches through the supra-spinal artery backwards into the ab- domen, giving off its minute currents for the nourishment of the nervous ganglionic cord, while another portion, intermingled with that collected in the portal vessels, is sent to the branchiae. But its principal current still flows in the supra-spinal artery, along the upper surface of the cord to the terminal ganglion of the tail, where it divides into four streams, two of which go out at the sides of the ganglion to nourish the segment, while the other two, now greatly reduced in size, proceed backwards along the terminal nerves of the cord, and, becoming more and more subdivided in the last segment of the tail, are diffused through the sur- rounding structures. These form minute anastomoses with numerous small vessels, which, gradually collecting in separate trunks on the under surface of the last segment, form the origin of the caudal portion of the sub -spinal vessel which conveys the returning blood forwards from the tail to the abdomen, to be aerated in the branchiae before it is again transmitted to the heart. In like manner, the blood that has already circulated through the organs of locomotion, the cephalothorax, and abdomen, appears to be collected in the veins which transmit it to the branchiae before it is again employed in the circulation. Throughout the whole of its course along the artery in the tail, the blood is passed NEKVOUS SYSTEM OF AEACHNIDANS. 379 Fig. 190. in small currents into the sub -spinal vessel, thus intermingling the venous and arterial blood, precisely as occurs in the abdomen. But the circulation in the caudal prolongation of the heart yet remains to be explained. We have already seen that the great dorsal artery in the tail, above the colon, forms direct vascular communications around its sides with the sub-spinal vessel upon the ventral surface, in which the course of the blood is propelled forwards to the abdomen. It is certain, therefore, that the action of the great chamber of the heart must impel the blood at once in every direction, chiefly forwards, and laterally, but also in part backwards through the caudal artery ; otherwise it would be impossible for this structure to form its anastomoses with the sub- spinal vein without occasioning two opposing currents in the same vessel. (974.) In the nervous system of Spiders we observe that progressive 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 essential to its position ; that it should excel its prey in cunning and sagacity is likewise a necessary conse- quence; and by following out the same principles which have already been so often insisted upon, concerning the in- separable connexion that exists between the perfection of an animal and the cen- tralization of its nervous ganglia, we find in the class before us an additional con- firmation of this law. In Scorpions, in- deed, the nervous masses composing the ventral chain of ganglia are still widely separated, especially those situated in the segments of the tail : in the cephalo- thorax they are of proportionately larger dimensions, and moreover exhibit this re- markable peculiarity, that, instead of being united by two cords of communication, there are three interganglionic nerves connecting each division. It is in Spiders that the concentration of the nervous system reaches its climax ; for in them we find the whole series of ganglia (ence- phalic, thoracic, and abdominal) aggregated together, and fused, as it were, into one great central brain, from whence nerves radiate to all Nervous system of the Spicier : a a, encephalic ganglia, from which are derived the optic nerves; ee, thoracic ganglia, forming by their coalescence the central mass (c) ; n n, nerves supplying the abdomen. 380 AEACHNIDA. Fig. 191. parts of the body. The extent to which centralization is here carried will be at once appreciated by reference to the annexed figure (fig. 190) : the encephalic masses (a a), whence the optic nerves distributed to the ocelli are derived, are in close contact with the anterior part of a large ganglion (c), that represents all the abdominal ganglia collected into one mass ; and from the posterior part of this, nerves (n n), destined to supply the parts contained in the abdomen, derive their origin. The thoracic ganglia (ee) 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. (975.) The ocelli or eyes of Arachnidans have been minutely investi- gated by Miiller*, and seem to present a type of structure very far supe- rior to that of insects. In the Scorpion, this distinguished anatomist succeeded in detecting most of the parts which enter into the construc- tion 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. (976.) The sexual organs of the male and female Arachnidans exhibit very great simplicity in their structure. The testes, or secreting ves- sels, of the male Spider are two long caeca (fig. 191, b), lodged in the abdomen, and termi- nating by simple orifices at the ventral surface. No external intromittent organ is perceptible ; and it was on this account that the peculiar apparatus above referred to, situated at the ex- tremity of the maxillary palpus, was so long con- sidered as giving passage to the impregnating secretion. The singular instrument already de- scribed ( 957) would seem, indeed, to "be in some manner really siibservient to the fecundating process, being used most probably as an exciting- agent preparatory to the intercourse between the sexes. (977.) The ovigerous system of the female is equally devoid of com- plication, and, like the male testes, consists of two elongated membra- nous 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 accomplishment 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 (fig. 192, A, B), would infallibly devour * Ann. des Sci. Nat. torn. xvii. Generative organs of male Spider: a a, pul- monibranchiae ; b, testes ; c, cephalothorax. SPINNING OEOANS OF AKANEID.E. 381 him, either before or after the consummation of his purpose, did he not exercise the most guarded caution and circumspection in making his advances. Fig. 192. A. Female Spider. B. Male of the same species. C. Arrangement of the eyes. (978.) One peculiar characteristic of the Araneidae is the possession of a spinning apparatus, whereby the threads composing their web are manu- factured. The instruments employed for this purpose are situated near the posterior extremity of the abdomen (fig. 194 a , h), and consist exter- nally of four spinnerets and two palpiform organs (fig. 193, A, B). Each Fig. 193. Spinning apparatus of the Spider. spinneret, when highly magnified, is found to be perforated at its extre- mity by innumerable orifices of extreme minuteness (fig. 193, c), through which the filaments are drawn ; so that, unlike the silk of the Cater- pillar, the thread of the Spider, delicate as it is, is composed of hundreds 382 ARACHNIDA. of smaller cords, sometimes woven together by zigzag 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 constructed, is secreted in a set of glands represented in the subjoined engraving Fig. 194. Silk-secreting glands of the Spider. (fig. 194). The secerning extremities of the glandular tubes are com- posed of branched cceca (s), whence arise long and tortuous ducts (a a a), that become dilated 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 different species of the Araneidae convert the delicate threads thus produced. Some construct 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 filaments 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 (fig. 192) ; 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. (979.) A few only of the most remarkable applications of this deli- cate material can be noticed in this place. The Mason-spiders (Mygale) 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 observa- tion. The construction of these singular abodes has long excited the admiration of the naturalist : a deep pit is first dug by the Spider, often to the depth of 1 or 2 feet, which, being carefully lined throughout with VAKIOUS USES OF THEIR THREAD. 383 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 Fig. 194 a . mouth of the hole, but which is attached to the margin of the aperture by one point only of its circumference, this point, of course, forming the hinge. The Spider then pro- ceeds 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 per- fected, the concealment af- forded 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. (980.) Another Spider (Clo- tho Durandii) constructs a dwelling equally artificial and ingenious a kind of tent, in which it lives and rears its young. This tent is com- posed of several superposed sheets of the finest taffeta, and its contour presents seven or eight prominent angles, which are fixed to the sur- face 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 continually 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 concealment ; but, within, everything is beautifully clean and white. The most admirable part of the contrivance, how- ever, is the perfect safety afforded to the young when the parent leaves Anatomy of Mygale : a, centralized ganglia of the nervous system; b, termination of the ganglionic cord ; c, respiratory stigmata ; d, anterior pulmoni- branchial organ of the left side, displayed ; e, poste- rior pulmonibranchial organ, partially covered by a large abdominal muscle (i); /.ovary; g, section of integument ; A, spinnerets and anal aperture. 384 AEACHNIDA. her tent in search of food : some of the superposed sheets are fastened together at their edges ; others are simply laid upon each other ; and as the parent herself alone possesses the secret which enables her to Fig. 194 b . 1. Female of Hydrachne globulus, represented of the natural size just previous to ovipo- sit Ion. 2. The same magnified, and seen from below, showing the mouth or beak furnished with two palpi, and the eight legs appended to four separate pairs of coxae ; between the hinder pair may be seen the heart-shaped genital scale, and a little posterior to this a round orifice, which is the anus. 3. Newly-hatched larva of Hydrachne globulvs. 4. Hinder part of an aquatic insect (Nepa cinerea), to which numerous nymphae of Hydrachne indifferent stages of growth are attached. 5. One of these nymphs magnified, exhibiting the head-like sucker pro- vided with a pair of palpi, immediately behind which are seen two of the epidermic cases of two of the old legs of the larva (the four others have fallen off), and posterior to these, five pairs of little sprouts, which are the rudiments of antennae and legs : the median circle indi- cates the position of the genital organs. 6. Represents the same nymph in its last stage of development, seen in profile : the integuments are supposed to be transparent, so as to show the young Hydrachne within, ready to escape. 7. Ventral surface of young Hydrachne, showing the arrangement of the coxae. 8. Secondary nymph making the transition from the second to the third phasis of the creature's existence : the perfect animal, ready for its escape, is repre- sented encased in the integuments of the preceding, the epidermic sheaths of the limbs still remaining adherent to the exuvium. raise those layers hy which entrance is to be obtained, no other animal can find its way into her impenetrable abode. (980 a .) The development and mode of exuviation of some of the Acaridans offer several very curious and interesting phenomena. The CKUSTACEA. 385 Hydrachne, or Water-mites, for example, at their birth present them- selves under the form represented in fig. 194 b , 3, being at that time pro- vided with only six locomotive limbs and a very remarkable proboscis*. These larvae at first swim at large in the water, but at length contrive to fix themselves to the body of some aquatic insect, in which position they pass into the state of nymphs (fig. 194 b , 4) ; the hinder part of their bodies becomes remarkably elongated, and at length assumes the form of a pear, in which all resemblance to its former state is lost. Nevertheless, during this remarkable increase in size, the proboscis and the legs undergo but little alteration ; for as soon as the body begins to elongate, the legs and the palpi are withdrawn from their original integument, and, retiring with the body into the pear-shaped sac formed by the distended skin, nothing is left behind but the exu- viated sheaths of the old legs, which are then easily broken off by the slightest violence. A nymph, formed in the interior of its own skin, has replaced the larva ; but it is a nymph which continues to nourish itself, and to increase in size, until it assumes the appearance shown at fig. 194 b , 5, in which the rudiments of a new set of legs are clearly perceptible through the transparent envelope : the eyes of the contained animal are distinctly traceable, and may even be seen to abandon their former corneae, and to recede in the same proportion as the limbs from the old case, which at last, rending transversely into two portions, allows the new animal to escape, which immediately begins to swim vivaciously about, under a form closely resembling that of the parent. It has, however, still another moult to go through before it can be pronounced adult ; for, after having lived some weeks in this third condition, and visibly increased in size, these immature individuals fix themselves to some water-plant, to which they hold firmly by means of their beak and claws, and, becoming motionless, again exuviate (fig. 194 b , 8), and are ready to reproduce their kind. CHAPTER XIV. CKTJSTACEA. (981.) 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 con- sequently the waters of the ocean would be utterly untenanted by cor- responding forms of Articulata, were there not a class of beings belong- * Duges, Ann. des Sci. Nat 2 ser. torn. i. p. 165. 2c 386 CEUSTACEA. Fig. 195. ing 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 Crusta- ceans, upon the consideration 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. (982.) 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 ac- quires in the larger species great density and hardness. (983.) As regards the mechanical ar- rangement of the skeleton, we shall find the same general laws in operation as we have observed throughout all the Annulose orders a continual centralization and progres- sive coalescence of the different rings or elements composing the external integu- ment, and a strict correspondence between the degree to which this consolidation is carried and the state of the nervous system within. (984.) In the lowest forms of the Crus- tacea we have, in fact, a repetition of the condition of the skeleton met with in the Myriapoda, or in the larva state of many insects, the whole body being composed of a series of similar segments, to which are appended external articulated members of the simplest construction (fig. 195). (985.) The number of rings or segments composing the body varies in different species ; but such variation would seem, from the interesting researches of Milne -Edwards and Audouin concerning the real organi- zation 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 integu- ment 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 segments. (986.) The normal number of these elements Milne-Edwards con- EXTERNAL SKELETON OF CRUSTACEA. 387 siders 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 abdo- minal region of the body. (987.) 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 eifected. (988.) In Talitrus (fig. 196) the cephalic elements are completely united, their existence being only j?j g jgg indicated by the several pairs of appendages one pair, of course, belonging to each ring. The first ring of the cephalic region, in this instance, has no external articu- lated member; but in higher orders the eyes are supported upon long peduncles connected with this element of the skeleton, Talitrus. that may be regarded as the representatives of those limbs which take different names in different regions of the body. The second and third lings support jointed organs, here called antemice; while the several pairs of jaws appertaining to the mouth indicate the existence of so many elements united together in the composition of the head. (989.) 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, and have natatory extremities developed from the five posterior rings. (990.) In the Lobster (Astacus marinus) we find not only the ce- phalic segments anchylosed together, but those of the thorax also ; and although the lines of demarcation between them are still recognizable upon the ventral aspect of the body, superiorly the entire thorax and head are consolidated into one great shield (cephaloihorax), the abdo- minal segments only remaining distinct and moveable. (991.) 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 lead- ing a terrestrial existence, as in the case of the Land-Crab of the "West India Islands. The abdominal segments, however, still remain 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. (992.) In the King-Crab (Limulus Polyphemus, fig. 197) even the divisions of the abdomen are obliterated, the whole body being covered by two enormous shields, and the tail prolonged into a formidable serrated spine, of such density and sharpness that in the hands of 2c2 388 CEUSTACEA. Limuius Polyphemus. savages it becomes a dreadful weapon, and is used to point their spears either for the chase or war. (993.) The reader will at once Fig. 197. perceive the strict parallelism that may be traced between the changes which occur during the metamor- phoses of Insects, and those observable as we thus advance from the lowest to the most highly organized Crus- tacean genera; and even the steps whereby we pass from the Annelidan to the Myriapod, and from thence to the Insect, the Scorpion, and the Spider, seem to be repeated, as we thus review the progressive develop- ment of the class before us. (994.) Having thus found that the annuli, or rings, which compose the annulose skeleton may be detected even in the most compactly formed Crustacea, it remains for us to in- quire, in the next place, what are the principal modifications observable in the articulated 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 developments 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 , Fig. 198. the case may render needful. In the lower, or more completely an- nulose forms (figs. 195 and 198), these mem- bers are pretty equally developed from all the segments of the body, and are subservient to locomotion, being ge- . nerally 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 scarcely to be recognized as consisting of similar elements modified only in their forms and relative proportions. AETICULATED APPENDAGES TO SEGMENTS. 389 To notice all the varieties which occur in the extensive class before us, would be to weary the reader with tedious and unnecessary details ; we shall therefore select the Decapod* division of these animals, as abundantly sufficient for the illustration of this part of our subject. This division, which includes the most highly organized forms, has been divided by writers into three extensive families: 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, although entitled to be considered as jaws so far as their use would indicate the name be- longing to them, are no less obviously merely modifications of articu- lated feet ; and the term foot-jaws has now, by common consent, become the appellation by which they are distinguished. (995.) 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 (chelce) ; 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 Lobster holds fast by some submarine fixed object, and thus prevents itself from being tossed about in an agitated sea ; the other is apparently a cutting instrument for tearing or dividing prey. (996.) To the chelce succeed four pairs of slender legs, scarcely at all serviceable for the purposes of locomotion ; but the two anterior being terminated by feeble forceps, they become auxiliary instruments of prehension. (997.) The articulated appendages belonging to all the abdominal segments are so rudimentary that they are no longer recognizable 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 development of large organs in this position would materially impede the progress of animals presenting 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 beneath her abdomen. * So called from the circumstance of their having five pairs of limbs BO largely developed as to become ambulatory or prehensile organs. 390 CKUSTACEA. (998.) 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 lamella, 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 distance of eighteen or twenty feet by one sweep of this remarkable locomotive instrument. (999.) If we now pass on to the consideration of the Anomourous Decapods, we find that the external organs above enumerated, although existing in precisely similar situations, are so far modified in their con- struction 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 conforma- Fig. 199. Hermit Crab. tion. Instead of being encased in a hard coat of mail, as in the Macroura, the hinder part of the body is soft and coriaceous, possessing only a few DERMO-SKELETON OF THE HERMIT CRAB. 391 detached calcareous pieces analogous, it is true, to those found in the Lobster, but strangely altered in structure. (1000.) These animals (fig. 199), usually known by the name of Soldier Crabs, or Hermit Crabs, frequent level and sandy shores, and, from their defenceless condition, are obliged to resort to artificial pro- tection. This they do by selecting an empty turbinated shell of propor- tionate size, deserted by some gasteropod mollusk, into which they in- sinuate their tail, and, retreating 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 observer. The chelce, 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 Fig. 200. Swimming Crab. only crawl, but drag after it its heavy habitation. Behind these loco- motive legs are two feeble pairs, barely strong enough to enable the Soldier-Crab to shift its position in the shell it has chosen ; and the false feet attached to the abdomen are even still more rudimentary in 392 CEUSTACEA. their development. But the most singularly altered portion of the skeleton is the fin of the tail, which here becomes transformed into a kind of holding apparatus, by which the creature retains a firm grasp upon the bottom of its residence. (1001.) In the Brachyura, or Crabs, we have at once, in the concen- tration observable in all parts of the skeleton, an indication of its being formed for progression on land, or at least for creeping at the bottom of the sea. The tail, the great instrument of locomotion in the Lobster, is here reduced to a rudiment, and the fin at its extremity entirely obliterated ; the chelae 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 terrestrial in their habits, or else, in the Swimming Crabs, the posterior pair become expanded into flattened oars useful in natation (fig. 200). (1002.) 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 development 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. (1003.) 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 unfet- tered 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. (1004.) "We are indebted to Eeaumur*, who watched the process in the Cray-fish (Astacus fluviatilis) , for the first account of the mode in which this change of shell is effected. In the animal above mentioned, towards the commencement of autumn, the approaching moult is indi- cated by the retirement of the Cray-fish into some secluded position, where it remains for some time without eating. While in this condi- tion, 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, by various contortions of its body, seems to be employed in loosening * M&n. de 1'Acad. des Sciences, 1718. EXUVIATION OF ASTACUS FLUVIATILIS. 393 thoroughly every part of its worn-out covering from all connexion with the recently-secreted investment. This being accomplished, it remains to extricate itself from its imprisonment an operation of some diffi- culty ; and when the nature of the armour to he 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 cephalothorax has be- come 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, also after much struggling ; 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 renovation of the external skeleton are so unimaginable, that it is really extra- ordinary how little has been done towards elucidating the nature of the operation. The first question which presents itself is, how are the limbs liberated from their confinement ? for, wonderful as it may appear, the joints even of the massive chelae, 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 explaining 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 muscles 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 exuviae ; even the singular dental apparatus situated in the stomach, of which we shall speak hereafter, is cast off and re-formed ! And yet, how is all this accomplished ? 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 inter- esting subjects of inquiry be pointed out to those whose opportunities enable them to prosecute researches connected with their elucidation*. * Since writing the above, I have been fortunate in procuring a very good speci- men of Astacus fluviatilis, 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 exuvise were connected together by the old articulations, and accurately represented the external form of the complete animal, the carapace, or dorsal shield of the cephalothorax, alone being detached, having been thrown off in one piece. The pedicles of the eyes and external corneas, as well as the antennae, remaind in situ, the corresponding parts 3 so Disposed as to allow the blood to pass from the sinus into the heart, but prevent its return in an opposite direction. Such is the apparatus provided in the Lobster for the circulation of the blood. Our next inquiry must be concerning the course that it pursues during its circuit through the body. (1022.) MM. 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 propelling the blood through the body, but having nothing to do with the branchial circula- tion ; they conceived that the circulating fluid, having been collected in the venous sinuses, was brought to the roots of the branchia?, 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 Eoyal College of Surgeons of England f, would seem to give great reason to doubt the accuracy of the conclusions arrived at by the eminent natu- ralists referred to, and to show that the heart, instead of being purely * " Eecherches Anatomiques et Physiologiques sur la Circulation dans les Crus- taceV' Ann. des Sci. Nat. torn. ii. t Catalogue of the Physiological Series of Comparative Anatomy contained in the Museum of the Eoyal College of Surgeons of England, vol. ii. 400 CEUSTACEA. 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 (fig. 201, Jc) is conveyed 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 (fig. 204, g) subdivide into secondary trunks (h h h), which ramify Transverse section of the body of a Lobster, just behind the heart: a, cut edge of the shell of the back ; 6, the under surface or sternal aspect ; c, the posterior end of the heart ; d d, two orifices of veins entering the heart ; e, cut end of the superior caudal artery ; /, the trunk of the large artery (sternal) going to the legs and gills (fc in fig. 201) ; g, trunk of an artery going to supply one tier of gills ; A h A, the branches going to each gill; t, artery of the leg; kkkk, the internal vessels or veins from each gill ; I, the common trunk or branchial vein ; m w, the gills ; n n, the fiabella, or laminae subservient to the movement and renewal of the respired medium ; o o, basal joints of the legs. through the individual branchiae 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 Tc Jc), represented on the opposite side of the body, and conveyed by the large vessel (Z) to the dorsal sinus (fig. 203, 6), 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 (fig. 204, d d), to recom- mence the same track. (1023.) In the Crustacea, as in the class of Insects, the blood* occupies * Milne-Edwards, loc. cit. NERVOUS SYSTEM. 401 all the interspaces left between the various viscera, as well as the still smaller lacunae situated among the muscular fibres or underneath the skin : but the heart, instead of opening immediately into this system of intercommunicating cavities, as among the true insects, is continuous with a special system of tubes, the walls whereof are well defined, and of which the peripheral branches ramify in the substance of all the organs of the body, thus constituting a very complete arterial system, although, by their ultimate ramifications, the centrifugal vessels thus formed become continuous with and lost amongst the interstitial lacunae of the body, which in their turn communicate with more considerable cavities situated between the viscera ; so that the blood ejected by the heart and arteries, arriving in the last ramifications of those tubes, escapes into the general interstitial lacunary system, by the intermedium of which it returns towards the heart. Thus the circulating fluid is brought into direct contact with all the viscera, and fills up the abdominal cavity, so that not until after it has passed through the respiratory apparatus does it again find itself enclosed in vessels properly so called. (1024.) As might be anticipated from an examination of the external configuration of the different families comprised in the extensive class we are now considering, the nervous system is found to pass through all those gradations of development which we have found gradually to present themselves as we have traced the Homogangliata 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, theoretically, might have been expected to exist, but has only been distinctly recognized in the class before us. (1025.) "We have all along spoken of the nervous centres of the Articulata as arranged in symmetrical pairs, although in no example which has 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-o3sophageal ganglion are found united into one brain in the humblest forms of annulose animals ; and even in the ganglia form- ing the ventral series, although we might presume each to be composed of two symmetrical halves, the divisions are most frequently so inti- mately 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 subject ; and the results of 402 CKUSTACEA. their investigations are of such great physiological importance*, thae the following condensed account 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 (thirteen 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. (1026.) In Oniscus Asellus, a concentration of the elements com- posing 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 transverse. 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 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 ^ 2Q5 direction, and gradu- ally the whole chain becomes shorter by the fusion of several pairs into larger and more powerful masses. (1027.) In the Crab (which, from its ter- restrial habits, holds a position among the Crustacea equivalent to that which the Spiders occupy among other Ar- ticulata), this centrali- zation is carried to the utmost extent, and all the abdominal and thoracic ganglia become agglomerated into one great centre, from which nerves radiate to the parts of the mouth and in- struments of locomotion (fig. 205). * " Itecherches Anatomiques sur le Systems Nerveux des Crustacea," Ann. des Sci. Nat. t. xiv. Nervous system of the Crab : from the ventral aspect. NERVOUS SYSTEM. 403 (1028.) But this change in the condition of the nervous system is not only observable as we proceed from species to species, as they rise higher in the scale of development ; similar phenomena are met with in watching the progress of any individual belonging to the more perfect families, as it advances from the embryo to its mature condition. Thus, in the Cray-fish (Astacus fluviatilis), Eathke* 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 first six pairs then unite transversely, so as to form as many single masses, from which the nerves of the man- dibles 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 ganglia, 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 distinct. The reader will at once recognize the resemblance between 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 f. (1029.) From a review of the above facts, Milne-Edwards and Audouin arrived at the following conclusions : 1st. That the nervous 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. 2nd. That all the modifications encountered, whether at different periods of the development or in different species of the series, depend especially on the more or less complete approximation of these nuclei, and on an arrest of development in some of their number. 3rd. That approximation takes place from the sides towards the mesian line, as well as in a longitudinal direction. (1030.) In the Crab, the distribution of the nerves is briefly as follows : The encephalic mass, or brain, which still occupies its position above the oesophagus, and joins the abdominal centre by two long cords of connexion (fig. 206), gives off nerves to the eyes, and muscles con- nected with them, as well as to the antennae and neighbouring parts. (1031.) 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 important branch, apparently the representative of the neruus vagus of insects. This, after * " Untersuchungen iiber die Bildung des Flusskrebses," Ann. des Sci. Nat. t. xx. t For a minute account of the arrangement of the nervous system in these ani- mals, the reader is referred to the Cyclopaedia of Anatomy and Physiology, art. " CRUSTACEA," by Dr. Milne -Edwards. 404 CEUSTACEA. ramifying largely upon the coats of the stomach, joins that of the oppo- site side, and, assuming a ganglionic structure, is ultimately lost upon the intestine. (1032.) The nerves of the extremities, derived from the central ab- dominal ganglion, are represented in the annexed figure (fig. 206), which requires no explanation*. Fig. 206. Nervous system of the Crab : from the dorsal aspect. (1033.) We have already ( 858), when describing the nervous system of insects, hinted at the probable existence in the HOMOGAN- GLIATA of distinct tracts of nervous matter in the composition of the central chain of ganglia, and in the filaments whereby they are con- nected with each other : reasoning therefore from analogy, it seems fair to presume that, if this be the case, such tracts correspond with the sensitive and motor columns which have been distinctly 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 factf; and the accuracy of his observations is readily demonstrable by a careful exa- mination of the ganglionic chain of the Lobster and other large Crus- tacean species. Each ganglionic enlargement is, upon close inspection, clearly seen to consist of two portions : first, of a mass of cineritious nervous substance forming the inferior aspect of the ganglion, and of a cord of medullary or fibrous matter which passes over the dorsal or * Fide Swan, Illustrations of the Comparative Anatomy of the Nervous System. London, 4to. f Phil. Trans. 1834. FEELING OF PAIN. 405 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 the subject is derived from an examination of the manner in which the nerves given off from the central axis take their origin ; 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 connexion with the grey matter, but arise at some distance from the ganglionic mass: 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. (1034.) 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 consideration, appa- rently of little importance, has given rise to a variety of curious specu- lations 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. (1035.) A more interesting inquiry connected with this part of our subject is, concerning the extent to which the AKTICTJLATA are suscept- ible of pain. Is it really true in philosophy, as it has become a stand- ing axiom in poetry, that " The poor beetle, that we tread upon, In corporal sufferance feels a pang as great As when a giant dies" ? (1036.) 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 beneficence 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 sup- posed 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. (1037.) The medulla spinalis, which, as we shall see hereafter, corresponds to the ventral chain of ganglia in articulated animals, can 406 CBUSTACEA. perceive external impressions and originate motions, but not feel pain ; hence we may justly conclude that, in the Homogangliata likewise, the supra-cesophageal ganglia (the representatives of the 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 conclusion, namely, that the perception of pain depends upon the development 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 intelligence of an animal, by which it can escape from pain, depend upon the perfection of the brain, so does the perception of torture depend upon the condition of the same organ. How far the feeling of pain is acutely developed in the animals we are now considering 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 larvaB of the Ichneumon, hatched in its body, devour its very viscera; and in the Crustacea before us, of so little importance is the loss of a leg, that the Lobster will throw off its claws if alarmed by the report of a cannon. (1038.) "We learn from Dr. Williamson* that the shell of the Deca- pods, in its most complete form, consists of three strata: namely, 1. a horny structureless layer, covering the exterior ; 2. a cellular stratum ; and 3. a laminated tubular substance. The innermost, and even the middle layers, however, may be altogether wanting. Thus, in PJiyllo- soma (Glass-crab) the envelope is formed of the transparent horny layer alone ; and in many of the small Crabs belonging to the genus Portuna, the whole substance of the carapace beneath the horny investment is made up of hexagonal thick-walled cells. It is in the large thick- shelled Crabs that we find the three layers most differentiated. Thus, in the common Cancer pagurus we may easily separate the structureless horny covering after a short maceration in dilute acid ; the cellular layer, in which the pigmentary matter of the coloured parts of the shell is contained, may then be brought into view by grinding away as flat a piece as can be selected from the inner side (having first cemented the outer surface to the glass slide), and by examining this with a magni- fying power of 250 diameters, driving a strong light through it ; whilst the tubular structure of the thick inner layer may be readily demon- strated by means of sections parallel and perpendicular to its surface. This structure, which very strongly resembles dentine, save that the tubuli do not branch, but remain of the same size through their whole course, may be particularly well seen in the black extremity of the claw, which is much denser than the rest of the shell, the former having * Microscopic Journal. POWEB OF KEPEODTJCINQ CAST-OFF LIMBS. 407 almost the semitransparency of ivory, whilst the latter has a chalky opacity. In a transverse section of the claw, the tuhuli may be seen to radiate from the central cavity towards the surface, so as very strongly to resemble their radiation in a tooth; and the resemblance is still further increased by the presence, at tolerably regular intervals, of minute sinuosities corresponding with the laminations of the shell, which seem, like the secondary curvatures of the dentinal tubuli, to indicate successive stages in the calcification of the animal basis. This inner layer rises up, through the pigmentary layer of the Crab's shell, in little papillary elevations ; and it is from the deficiency of the pigmentary layer at these parts that the coloured portion of the shell derives its minutely speckled appearance. Many departures from this type are presented by the different species of Crustacea. Thus, in the Prawns there are large stellate pigment- cells, the colours of which are often in remarkable conformity with those of the rock-pools frequented by these creatures ; whilst in the Shrimps there is seldom any very distinct trace of the cellular layer, and the calcareous portion of the skeleton is dis- posed in the form of concentric rings, an approach to which arrangement is seen in the papilla of the surface of the deepest layer in the Crab's shell. (1039.) The singular power of breaking off their own limbs, alluded to in paragraph 1037, 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 were there not some contrivance to remedy the other- wise unavoidable results of such a catastrophe. 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 stanched*. * Mr. Spence Bate gives the following account of this remarkable process: " When a limb is injured, all Crustaceans have the power of rejecting it, except the wound be below the last joint. This is done by a violent muscular contraction, finish- ing with a blow from another limb, or against some foreign body. The amputation is the work of a few seconds, except when they have but recently cast their exuviae ; at such times the wounded limb will sometimes remain for half an hour or longer before it is rejected. "The new limb is formed within the shell, where it lies folded up until the next moult, when it appears as a part of the new skeleton, the sac-like membrane which protected it being cast off with the old shell ; and the restored member is larger or smaller, in accordance with the length of time which may exist between the amputation of the limb and the shedding of the skin. The condition in which the limb is at that time remains permanent until the next moult, when the whole 408 CRUSTACEA. (1040.) 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 predecessor. A beau- tiful example of this curious mode of reproducing a lost organ is pre- served in the Museum of Comparative Anatomy 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 process of reproduction is as follows : The broken extremity of the second joint skins over, and pre- sents 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, indi- cating the position of the articulation ; but the whole remains unpro- tected by any hard covering until the next change of shell, after which it appears in a proper case, being, however, still considerably smaller than the corresponding claw on the opposite side of the body, although equally perfect in all its parts. (1041.) Mr. H. D. S. Goodsir has shown* that in the Lobster this regenerative faculty does not reside at any part of the claw indifferently, but in a special locality, situated at the basal end of the first joint of each of the legs. This joint is almost filled by a mass of nucleated cells surrounded by a fibrous and vascular band ; and other nucleated cells intervene between this vascular band and the outer crust. The vessels of the band pass onwards for about half an inch, and return upon them- selves, forming loops. When a claw is broken, or otherwise injured or disabled, the Lobster, or Crab, by a violent muscular effort casts it off at the transverse ciliated chink, or groove, which indents the reproductive segment. The new claw is developed by the multiplication of cells, which soon become divided into five groups, answering to the five joints of the future limb ; these nascent joints are folded upon each other in the Crab, but extended in the Lobster ; in both, they are at first en- veloped in a sac formed by the distended cicatrix ; the budding limb ultimately bursts this cicatrix, and its growth is rapidly completed. A great proportion of the reproductive cells contained in the basal ex- tremity of the injured limb is made use of in the production of the new limb ; but a mass of them is retained unchanged at the basal joint, and is ready to renew the reproductive process when needed. In the lower Crustaceans such groups of cells are found at more numerous joints. (1042.) The observations made in a former chapter relative to the organs by which the senses of touch, taste, and smell are exercised in creature again advances in size, but the new limb more rapidly than the remainder of the animal, until it attains its full relative proportions." Ann. and Mag. Nat. Hist. 1851, vol. vii. p. 300. * Vide Owen on Parthenogenesis, p. 48. STEUCTUEE OF THE EYE AND EAR. 409 insects, are equally applicable to the animals composing the class before us ; for in the Crustacea, although we are compelled to admit the pos- session of the above faculties, we are utterly ignorant of the mode in which they are exercised ; 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 present state of our knowledge, to be deduced. (1043.) The eyes of Crustaceans are of three kinds simple, agglome- rated, and compound. . (1044.) The simple eyes (ocelli, stemmata) resemble those of Spiders, and, like them, are said to consist of a cornea, a spherical lens, a gela- tinous vitreous humour, a retina, and deeply- coloured choroid, all occupying their usual relative positions. These eyes never exceed two or three in number. (1045.) In the agglomerated eyes, such as those of Daphnia (fig. 212), 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. (1046.) The compound eyes appear to be constructed upon the same principles as those of insects. The cornese are extremely numerous, and in general 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. The com- pound eyes of Crustaceans have not, however, as yet been examined with the same patient diligence as those of the Cockchafer ; 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 external skeleton, and thus they may be turned in various directions without moving the whole body at the same time. This pro- vision 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 exceedingly limited, and inadequate to the security of creatures exposed to such in- numerable enemies. (1047.) It is in the higher Crustacea that we, for the first time, in- dubitably 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 410 CEUSTACEA. joints of the second pair of antennae. On looking carefully, in this situation the student will find a prominent tubercle formed by the shell, the top of which is perforated by a small circular opening covered with a tense membrane. Behind 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. (1048.) The function of this organ in the Lobster is contested by Dr. Arthur Farre, who observes that it is situated not far from the mouth, and is directed downwards ; it is by far the most sensitive part of the body, since, while the mechanical irritation of any other parts excites only slight movements in the limbs of the animal, the touching of this part is immediately followed by violent and almost spasmodic flappings of the tail. These circumstances, together with the situation of the organ, appear to Dr. Farre to point it out as intended possibly for the purpose of testing the quality of the food as, in fact, an organ of smell, evidently endowed with an exquisite sensibility*. This, however, is evidently merely a matter of conjecture, more especially as in the gene- rality of the Crustaceans such an apparatus is altogether wanting. (1049.) The true organ of hearing, according to Dr. Farre, is situated in the base or first joint of the lesser pair of antennae its precise seat being indicated externally by a tough membrane, which covers an oval aperture in the upper surface of this joint. Towards the inner and anterior margin of this membrane there exists a small round aperture, into which a bristle can be easily passed. On removing the membrane, together with a portion of the surrounding shell, the internal organ is brought into view, completely imbedded in the muscular structure of the antennae. (1050.) This organ, the vestibular sac, nearly fills the cavity of the joint, is somewhat sacciform in its shape ; and its walls present a delicate horny structure of the consistence of a thin quill, being so transparent as to admit of its contents being seen through the parietes. These are found to consist of numerous minute particles of siliceous sand, which are loosely contained in the interior of the sacculus. The walls of the ves- tibular sac are furnished with several rows of minute ciliated processes which, when highly magnified, are seen to be hollow and to be covered with a fine down of hairs of exquisite delicacy, while in their interior are contained numerous minute granules, which are apparently nerve- granules. These processes are dilated at their base so as to form a globular swelling, where they are articulated to corresponding circular apertures in the walls of the sac, from which they spring in immediate * " On the Organ of Hearing in Crustaceans," by Dr. Arthur Farre, Phil. Trans. 1843, p. 234. MALE QENEEATIVB SYSTEM. 411 apposition with a plexus of the auditory nerve, which has a separate and distinct origin from the supra-oasophageal ganglion. (1051.) The existence of this singularly- constructed apparatus is by no means universal even among the Macrourous Decapods, and in the Brachyura it seems to be altogether wanting. We recognize, however, in its structure all the essential parts of an organ of hearing in its primitive form, viz. a distinct acoustic nerve, terminating in a plexus, which is expanded upon a vestibular sac. The remarkable arrange- ment of ciliated processes immediately overlying this plexus, with each process filled with nerve-granules, exhibits an apparatus for extending the extremities of the nerves in such a manner as to render them sen- sitive to the most delicate vibration of the fluid with which the sac is filled. But to heighten the effect of this, the grains of sand are added, thus forming adventitious otoliths, which, moving freely in the fluid contents of the sac, doubtless considerably increase the vibration of that fluid. (1052.) In the Brachyura, or Crabs, the membrane covering the ex- ternal orifice of the ear is converted into a moveable calcareous lamella, from which, in some genera, a furcate process is continued internally ; so that the whole, when removed by maceration, has no very distant re- semblance to the stapes of the .p. 2Q7 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. (1053.) One of the first cir- cumstances calculated to at- tract the notice of the anato- mist who turns his attention to the structure of the genera- tive system, both in male and female Decapod Crustacea, is the complete separation which exists between the organs be- longing to the two sides of the body ; for not only are the internal secreting viscera for the most part perfectly di- stinct from each other, but even the external sexual ori- fices are equally separate and unconnected. (1054.) Beginning with the parts observable in the male, we will Male generative apparatus of Astacus fluviatilis ' a a, 6, testicular mass ; c c, vasa deferentia, forming by their convolutions a kind of epididymus, d d;f, their external orifices. 412 CRUSTACEA. take the Cray-fish (Astacus fluviatilis) as a standard of comparison, and briefly notice the principal variations from the type of structure ob- servable in that species which are met with in other genera. (1055.) In the Cray-fish, and also in the Lobster, the secerning organs or testes, when examined in situ, are found to occupy the dorsal region of the thorax, lying upon the posterior part of the stomach. (1056.) Examined superficially, the testes would seem to form but one mass, consisting of three lobes (fig. 207, act, 6); 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 epididymus (d), 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 externally, so as to become effi- cient in directing the course of the fecundating fluid. (1057.) 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 ab- dominal surface of the last thoracic ring itself. (1058.) In the male Brachyura^ the so-called false feet constitute the external sexual organs ; and Mr. Bate has several times taken Carcinus mcenas in the act of copulation, under which circumstances he distinctly saw these styliform processes inserted within the vulvae of the female. These false feet consist of two pairs, the largerbeing anterior, and attached to the first abdominal ring, the less, or posterior, to the second ring. In all, except the edible Crab, the second pair is very small, apparently rudimentary, and lie with their extremities inserted posteriorly into the larger pair. But in Cancer pagurus, though slight, they are equally long with the first pair, and have a joint, peculiar to this Crab, situated near their centre, in addition to one, common to others, attached to the basal joint. The orifice of this pair is slightly frilled : it lies posteriorly against the first pair, which are the most important. These latter are styliform, and attached by a hinge to a calcareous continuation of the dermal membrane of the abdomen. From the first joint of the fifth pair of legs a membranous tube (the vas deferens) passes out and enters at the second joint of the so-called false feet, continuing through, and termi- nating at the apex in an oval slit. Internally the tube is derived im- mediately from the testicle. (1059.) Mr. C. Spence Bate believes that Crabs have more than one * Cyclop, of Anat. and Phys., art. " CRUSTACEA." f " Notes on Crustacea," by C. Spence Bate, Ann. & Mag. Nat. Hist., 2 ser. vi. p.109. FEMALE GENEEATITE OEOANS. 413 Fig. 208. brood to a single impregnation by the male, and that the male can only impregnate the female immediately after the shedding of the exuviae. " For days previously the male may be seen running about and hiding himself under stones, holding the female by one or more of his legs, the carapace being pressed against the sternum of the male. In this relative position they continue until the female throws off her calcareous clothing, when connexion immediately follows, and continues perhaps for a day or two." (1060.) 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 fluviatilis, the ovaria (fig. 208, 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. (1061.) In Crabs, an important addition is made to the female generative system: prior to the termination of each oviduct it is found to communicate with a wide sacculus, the function of which is apparently analogous to that of the spermatheca of insects ( 886), inasmuch as it seems to form a receptacle for the fecundating secretion of the male, in which the seminal fluid remains, ready to impregnate the ova as they successively pass its orifice during their expulsion from the body. (1062.) 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. (1063.) Many of the Decapod Crustaceans, as, for example, the Cray- fish (Astacus fluviatilis), do not 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 certain that great metamorphoses take place in the ex- ternal appearance of the young animals. Cavolini long since announced Female generative organs of Astavus fiu- viatilis: a,bb, ovaria; c, oviduct; d, external termination of the right oviduct; e, escaped ovum. 414 CRUSTACEA. that the embryo of Cancer depressus exhibited at birth a singular and uncouth appearance, of which he gave a very tolerable representation*; Fig. 209. Metamorphosis of Crab. Fig. 210. and Mr. Thompson has rendered it certain that, even in the develop- ment of the common Crab, so dif- ferent is the outward form of the newly-hatched embryo from that of the adult, that the former has been described as a distinct species, and even grouped among the EN- TOMOSTBACA, under the name of Zoea pelagica. On leaving the egg, the young Crab presents a curious and grotesque figure (fig. 209) : its body is hemispherical, and its back prolonged upwards into a horn-like appendage; the feet are scarcely visible, with the exception of the last two pairs, which are ciliated like those of a Branchiopod, and formed for swimming. The tail is longer than the body, possesses no false feet, and the terminal joint is crescent-shaped and co- vered with long spines. The eyes are very large, and a long beak projects from the lower surface of the head. (1064.) In a more advanced stage of growth the creature assumes a totally different shape (fig. 210), under which form it has been known to naturalists by the name of Megalopa. The eyes become peduncu- lated, the cephalothorax rounded, the tail flat and provided with false feet, and the chelae and am- bulatory extremities well developed. * Sulla Generazione dci Pesci e del Granchi. 4to. Naples, 1787. Metamorphosis of Crab. METAMORPHOSES. BRANCHIOPOD A. 415 (1065.) 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 recognizable (fig. 211). Fig. 211. not Metamorphosis of Crab. (1066.) Notwithstanding the diversity of form under which the young Crab presents itself at different phases of its growth, examples of which we have here figured, it would seem, from the observations of Mr. C. Spence Bate*, that the progress made towards the mature con- dition is not by any sudden metamorphosis, but by a series of moult- ings similar to those which take place in the adult, and that with each successive moult there is a corresponding degree of progress in its development. But the amount of change at each moult is so little that it gives to the animal but a very small degree of difference in its general appearance ; and it is only by a comparison of the earliest form with the last, and that without any consideration of the intermediate stages in its growth, that the idea of a true metamorphosis in Decapod Crus- tacea has arisen. There are, in fact, six or seven well-marked stages or forms that the growing animal passes through in its progress to maturity ; and each of these is linked to the preceding as well as to that which follows by a succession of changes that are but just ap- preciable. (1067.) BRANCHIOPODA. In the Branchiopod Crustacea (so called from this circumstance), the legs used in swimming would appear to be converted into broad-fringed lamellae, so thin that they perform the office of branchiae, and render needless the existence of other instruments of respiration. In Daphnia, for example (fig. 212), a creature common in every stagnant pool, the body is contained, as it were, between two corneous plates, open along their inferior edge. Through this trans- parent envelope the legs may be perceived in constant movement ; and * Phil. Trans. 1848. 416 CRUSTACEA. from the extreme delicacy of the covering that invests them, they evi- dently present to the surrounding medium a surface of sufficient extent for the purpose of exposing the blood to its action, thus rendering them efficient substitutes for branchiae, while, at the same time, their move- ments ensure a perpetual renovation of the water in contact with them ; so that, as a necessary consequence, the respiratory process will be accomplished with greater completeness in proportion as the exertions of the animal become more vigorous. In the Crustacea, indeed, we have many interesting and beautiful examples of the connexion between the respiratory and locomotive organs. The amount of respiration must necessarily be equivalent to the expenditure of muscular energy ; and a more elegant manner of ensuring an exact correspondence between the one and the other, than that adopted, could scarcely be imagined ; for, by appending the branchiae to the locomotive agents themselves, the more actively the latter are employed, the more freely will the former receive the influences of the aerated water in which they are immersed. Fig. 212. Daphnia. (1068.) In the Squilla, which swims by means of the movements of its broad tail, it is the false feet beneath the abdominal segments that become branchial organs ; and these, being expanded into broad and vascular lamellae, perform the office of gills. In the Squilla, therefore, and similarly-formed genera, the free movement of the tail ensures the full and complete exposure of the respiratory structures to the surround- ing element. (1069.) The more minute Branchiopods, or Entomostraca, as they are called by zoologists, offer, in their mode of reproduction, several remark- able variations from what has been described above ; and a brief account of their most interesting peculiarities is therefore still wanting to com- plete 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 resemble so nearly, that they are still EEPEODUCTION. 417 confounded together by many authors. The female Entomostraca fre- quently 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 receptacles 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 therefore could not by any contrivance be developed internally without bursting the crustaceous covering that invests the mother. Jurine*, Ramdohrf, 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 undoubt- edly 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; and such is the prodigious fertility of these little beings that a single female will, in the course of three months, produce ten suc- cessive families, each consisting of from thirty to forty young ones. (1070.) In the genus Apus, another plan is resorted to for the protection of the ova: the eleventh pair of legs, called by Schaffer " 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. (1071.) In Daphnia (fig. 212), the ovaria are easily distinguished through the exquisitely transparent shell, especially when in a gravid state ; and the eggs, after extrusion, are lodged in a cavity situated between the shell and the exterior of the body, where they remain until the embryo attains its full growth. (1072.) 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 concurrent testi- mony of numerous observers who have witnessed it in many different genera (Cyclops, Daphnia^ &c.), it might still be admitted with suspi- cion. 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 generations. * Histoire des Monocles. 1 vol. 4to. Geneve, 1820. f Materiaux pour 1'Histoire de quelques Monocles allemands. 4to. 1805. 2E 418 CRUSTACEA. (1073.) In many species there is a double mode of reproduction, the sexual and the non-sexual. The former takes place at certain seasons only, the males disappearing entirely at other times ; while the latter continues at all periods of the year, so long as warmth and food are supplied, and is repeated many times, so as to give origin to many successive broods. Further, a single act of impregnation serves to fertilize not merely the ova which are then mature, or nearly so, but all those subsequently deposited by the same female, even at consider- able intervals. In these two modes the multiplication of these little creatures is carried on with great rapidity, the young animal speedily coming to maturity and beginning to propagate, so that, according to the computation of Jurine, founded upon data ascertained by actual observation, a single fertilized female of the common Cyclops might be the progenitor of 4,442,189,120 young. (1074.) The eggs of some Entomostraca are deposited free in the water, or are carefully affixed in clusters to aquatic plants ; but they are more frequently carried for some time in special receptacles developed from the posterior part of the body, and in many instances they are retained there until the young are ready to come forth. In Daphnia the eggs are received into a large cavity between the back of the animal and the shell, and there the young undergo almost their whole deve- lopment. Soon after their birth a moult or exuviation takes place, and the egg-coverings are got rid of with the cast shell. In a very short time afterwards another brood of eggs is seen in the cavity, and the same process is repeated. At certain times, however, the Daphnia may be seen with a dark opake substance within the back of the shell, which has been called the ephippium, from its resemblance to a saddle. This, when carefully examined, is found to be of dense texture, and to be composed of a mass of hexagonal cells ; and it contains two oval bodies, each consisting of an ovum covered with a dense horny casing, enve- loped in a capsule, which opens like a bivalve shell. Prom the recent observations of Mr. Lubbock*, it appears that the ephippium is really only an altered part of the carapace, its outer walls being a part of the outer layer of the epidermis, and its inner valve the correspond- ing part of the inner layer. The development of the ephippial eggs takes place at the posterior part of the ovaries, and is accompanied by the formation of a greenish-brown mass of granules ; from this situation the eggs pass into the receptacle formed by the new carapace, where they become included between the two layers of the ephippium. This is cast, in process of time, with the rest of the skin, from which, how- ever, it soon becomes detached, and continues to envelope the eggs, generally floating on the surface of the water until they are hatched with the returning warmth of spring. This curious provision is ob- viously destined to afford protection to the eggs which are to endure * Proc. of Roy. Soc., Jan. 29, 1857. EPHIPPIAL OVA. 419 the severity of the winter's cold. There seems a strong probability, from the observations of Mr. Lubbock, that the ephippial eggs are true sexual products, since males are to be found at the time when the ephippia are developed, whilst it is certain that the ordinary eggs can be produced non-sexually, and that the young that spring from them can reproduce their race in like manner. It has been ascertained by Dr. Baird that the young produced from the ephippial eggs have the same power of continuing the race by non-sexual reproduction as the young developed under ordinary circumstances. In most Entomostraca, the young, when first hatched, have only the thoracic portion of the body as yet developed, and possess but a small number of locomotive organs. The eyes, too, are at first frequently wanting. The process of development goes on with great rapidity, the animal at each successive moult (which process takes place at intervals of a day or two) present- ing some new parts and becoming more and more like its parent, the females laying eggs before they have acquired their full size. (1075.) The cast shell carries with it the sheaths not only of the limbs and plumes, but of the most delicate hairs and setae connected with them. If the animal have previously sustained the loss of a member, it is gradually renewed at the next moult, as in the higher Crustacea. (1076.) Some authors have supposed, from the circumstance of all the individuals which have been met with belonging to certain 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. (1077.) The last point which we have to notice, in connexion with the history of the Crustacea, is the progress of their development from the embryo condition to the mature state. This is a subject 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 configuration, while others assert that they undergo changes so important as only to be comparable with the metamorphoses of insects. (1078.) Among the Entomostraca such changes have been again and again witnessed, and the appearances observed during their growth care- fully 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 development. In Cyclops, for example, the newly-hatched embryo possesses only four legs, and its body is round, having as yet no appear- ance of caudal appendages : of young animals in this condition Miiller 2E2 420 CECJSTACEA had formed a distinct genus, Anymene* ; in about a fortnight they get another pair of legs, and form the genus Nauplius of the same author. Fig. 213. Fig. 214. : Artcmia salinus. Fig. 215. Metamorphoses of Artemia. They then change their skin for the first time, and present the form of * Latreille, Eegne Animal, vol. iv. METAMOEPHOSES OF ENTOMOSTEACA. 421 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. Fig. 216. Fig. 217. Metamorphoses ofArtemia. (1079.) The Salt-marsh Shrimp (Artemia salinus) affords a good ex- ample of these remarkable exuviations. This animal is especially in- teresting, as being, perhaps, the nearest approach in existing nature to the extinct forms of Trilobites so abundantly met with in certain geo- logical strata ; and we have accordingly given, upon an enlarged scale, accurate drawings of its external organization, both in the male and female (figs. 213 & 214). Who, however, would recognize, in the em- bryo of this Crustacean on its first quitting the egg (fig. 215), any re- semblance to the adult creature, or even, in its second condition (figs. 216 & 217), be able to identify it as belonging to the same species as that depicted above so completely are all its parts remodelled in their structure before arriving at the mature state ? (1080.) EPIZOA. An extensive group of animals closely resembling the Crustaceans have been 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. (1081.) These parasites are commonly found to infest fishes and other inhabitants of fresh and salt water, generally fixing themselves in positions where an abundant supply of animal juices can be readily 422 EPIZOA. obtained, and where, at the same time, the water in which they are im- mersed is perpetually renewed for the purpose of respiration. The gills of fishes, therefore, offer an eligible situation for their development, as do the branchiae of other animals ; 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. (1082.) The least-elaborately organized of these animals exhibit ex- ceedingly grotesque and singular shapes, resembling Fig. 218. imperfect embryos rather than mature beings, the first buddings of external limbs, in the earlier period of foetal development, imi- tating not very remotely the appearance of the ru- dimentary appendages re- presented in the annexed figure* (fig. 218). (1083.) A great number of species of these para- sites, generally described under the name of Ler- neans, have been observed by authors, and it would seem, moreover, that each is peculiar to a particular Lerneans. kind of fish. The variety exhibited in their outward forms is, of course, exceedingly great ; but the examples depicted in the figure, namely the Lerncea gobina, found in the branchiae of Coitus Gobio, and Lerncea radiata, which infests the mouth of Coryphwna rupestris, will make the reader sufficiently ac- quainted with their general appearance and external structure. In the former parasite, of which a posterior and an anterior view are given in fig. 218 (a, b), the appendages seen upon the head and sides of the body answer the purpose of hooks or grappling organs, whereby the creature retains 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 detached. In the second example (c, cZ), 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 * Miiller (O. F.), Zoologia Danica, 1788. ACHTHERES PERCARUM. 423 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. (1084.) These examples, however, are taken from the most imper- fectly 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 zoologist remains in doubt whether the more elaborately-constructed ought not to be admitted among the Crustacean families which they most resemble. (1085.) The Achiheres Percarum (fig. 219) is one of those species most nearly allied to the AHTICTJLATA ; 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 group. (1086.) TheAchtTieres is found to infest the Perch (Perca fluviatilis), adhering firmly to the roof of the mouth, to the tongue, or sometimes 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. (1087.) The female, represented in the figure, is about 2 lines in length ; the male, which differs materially from the other sex in many points, is con- siderably smaller. (1088.) The outer covering of the body of these little creatures is at once seen to have assumed a horny hardness ap- proximating to the density of the cover- ings of the Entomostraca ; and indica- tions are even perceptible of a division into segments : the distinction^ moreover, between the trunk (cephalothorax), to which the limbs are appended, and the abdomen, wherein the viscera are lodged, is obvious. (1089.) Instead of the rude and imperfect limbs we have seen in the Lerneans, the legs are visibly more perfect in their entire construction ; and in the female, the posterior pair of these appendages are converted into a most singular instrument of attachment, whereby the Acliiheres fixes itself to the gums of the fish. The hinder pair of extremities alluded * Mikrographische Beitrage zur Natiirgeschichte der wirbcllosen Tliiere. Berlin, 1832. Adheres Percarum: a, adhesive disk; 6 6, posterior pair of limbs ; c, stomach ; d d, ovaria ; e, anal ori- fice ; ff, ovisacs ; o, antennae. 424 EPIZOA. to (fig. 219, b 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 at fig. 220, l) is of a cartilaginous hardness, resembles a little bowl the inside of which is studded with sharp teeth, and is not only calculated to act as a powerful sucker, but, from the hooks within its cavity, is capable of taking a most tenacious hold upon the lining membrane of the mouth. (1090.) The other members (fig. 219, o) are much less developed, but are nevertheless so constructed as to assist materially in fixing the Epizoon ; they are represented upon a very large scale in fig. 220, 2, where the outer pair (a a) are seen to exhibit, in the transverse lines indented upon their surface, the first indication of articulated limbs, 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 efiecting the same object. (1091.) The mouth itself (fig. 220, 2, c) is formed upon similar prin- Fig. 220. Details of the structure of Achtheres. I. Adhesive disk on an enlarged scale : e e, conjoined ex- tremities of the hinder pair of legs. 2. Structure of the mouth and parts adjacent: a a, anterior limbs; b b, antennae ; c, mouth, furnished with rudimentary jaws. ciples, the external orifice being surrounded with a circle of minute re- curved spines, well calculated to ensure its firm application to the 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 Achtheres, the sucking-bowl possessed by the female does not exist, the prehensile organs being merely four stout articulated extremities, armed at the end with strong prehensile hooks. METAMOKPHOSES OF ACHTHERES. 425 (1092.) 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 same figure) terminates in a straight digestive canal (fig. 221, a), which passes through the centre of the abdomen ; but no separation between stomach and intestine is visible. The entire tube, from the Kg. 221. 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 (6), situated at the posterior extremity of the body. (1093.) Near the termination of its course, the alimentary canal passes through a loop formed by transverse bands (n n), and, more- over, seems to be retained in its position by radiating fibres, appa- rently of a ligamentous character, but which have been described as representing a biliary apparatus. (1094.) The muscular system of this animal is far more perfect in its arrangement than in the preceding classes, and the delicate fasciculi which move the rudimentary limbs are visible through the transparent integument (fig. 219). In the abdomen, the muscles form longitudinal and transverse bands that intersect each other at right angles (fig. 221, d) an arrangement not very different from what we shall soon meet with in the rotiferous animalcules. (1095.) The nervous system appears to consist prnicipally of two long filaments (fig. 221, c) that run beneath the alimentary canal : but it is extremely probable that these communicate with some minute ganglia in the neighbourhood of the head ; at least, the perfect structure of the oral apparatus and the development of the limbs would seem to indicate such a type of structure. (1096.) The generative organs in the female Achtheres consist of two parts the ovaria, wherein the eggs are formed, contained in the abdo- minal cavity (fig. 219, d (T) and of two external appendages, or egg- sacs (fig. 219, //), which are attached to the posterior extremity of the body, for the purpose of containing the eggs until their complete de- Viscera of Achtheres Percarum: a, alimentary canal ; 6, anal orifice ; c, nervous filaments ? ; d, muscular bands ; e, unimpregnated, and f, im- pregnated ovary ; g g, external openings of the ovaria. 426 EPIZOA. velopment is accomplished : this arrangement we have already had an opportunity of examining in the Entomostracous Crustaceans. (1097.) The internal ovaria (fig. 221, /), 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 saccu- lated walls, opening externally by an orifice (g g) through which the ova are expelled into the egg-sacs, where their development is completed. (1098.) 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 metamorphoses, 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. (1099.) On first quitting the egg, the young Acfitheres 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. Before, however, the first change is effected, another set of feet may be per- ceived 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 the 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. (1100.) In Lamproglena pulchella we have a still more decided exemplification of the Crustacean type of structure, and the rudimentary 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 whereunto this parasite attaches itself, namely the gills of the Chub (Cyprinus Jeses), in which situation it is most usually found. The two anterior pairs (fig. 222, 6, c) are far more largely developed than those which are placed upon the posterior parts of the animal, and are apparently strengthened by a cruciform cartilaginous framework, seen through the transparent integument. The first pair of these holding-feet consist of two robust and powerful hooks, termi- nated by simple horny points ; whilst the second, which are likewise unciform, terminate in trifid prongs, and are evidently equally adapted to prehension. The four pairs of members that succeed to these are mere rudiments, and can be of little service as organs of attachment ; but, to make up for their imperfection, we find at the posterior extre- mity of the body, between the orifices of the ovaria (y), a pair of carti- laginous suckers, well calculated to fix this part of the animal. LAMPKO&LENA PULCHELLA. 427 222 - (1101.) 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 structure of the creature, we must imagine the existence of muscles provided for the movements of each articulated member, although, from their ex- treme minuteness, they escape detection. (1102.) The opening of the mouth is placed in the centre of the space bounded by the four anterior prehensile hooks ; and the alimentary 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 network, that probably con- stitutes a biliary apparatus. (1103.) In a creature thus highly organized we may well expect to find senses of propor- tionate 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 regarded as special instruments of touch, are well developed, and, both in number and position, resemble those that characterize the Crustacean orders, to which we are thus conducted by almost imperceptible gradations. (1104.) The reproductive organs are entirely similar to those of Achfheres, already described. Those of the female, represented in the figure, consist of sacciform ovaria, wherein 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. (1105.) One of the most singular and anomalous forms of the Epizoa is found in the Nicoihoe Astaci, a creature met with in great abundance at certain seasons, attached to the gills of the Lobster, from which it derives its supply of nourishment. This remarkable animal (fig. 223), which is free, and gifted with energetic powers of locomotion during the first periods of its existence, and constructed, at its first appearance from the egg, in perfect accordance with the normal type belonging to its class, ultimately selects for its domicile the branchial chamber of a Lobster, where, fixing itself permanently to the branchial lamella, it undergoes a complete metamorphosis : its external Lamproglena puicheiia. 428 EPIZOA. form is entirely changed ; its senses, and means of relation with the ex- ternal world, become atrophied ; singularly-formed excrescences sprout Nicothoe Astaci. from its sides ; and thus transformed, it is content to live beneath the shell of the Lobster, without further intercourse with the external world than is necessary to supply it with the blood which it sucks for food. (1106.) The mouth of the Nicothoe is a sort of membranous proboscis, armed near its extremity with styliform points, with which it is enabled to pierce the branchial membrane. Instead of the ordinary more or less flexuous tube which constitutes the alimentary canal in other forms of Entomostraca, the digestive apparatus of Nicothoe consists of two wide sacculi, united together in the median line, in the shape of a horse- shoe, from the centre of which a narrow canal proceeds towards the mouth, constituting the oesophagus (fig. 223, 5), whilst, derived from the opposite side, another tube of similar calibre runs backwards to the termination of the tail, forming the intestine (c). The stomach, therefore, is constituted by the two great lateral ca3ca (g, h), in the inte- rior of which alimentary substances undergo their principal modifica- tions ; so that these caeca are evidently analogous to what will be ob- served in the Pycnogonidae ( 1112), with this difference, that in those animals the caeca have penetrated into the interior of the ambulatory claws. The thickness of the walls of these stomachal caeca is uniform throughout; they are exceedingly delicate, only exhibiting in their texture some small reddish cells, and are apparently connected to the parietes of the body, in which they are loosely suspended by delicate muscular fraena. (1107.) One very remarkable circumstance presented by the alimentary NICOTHOE ASTACI. 429 apparatus of Nicothoe is the peristaltic action of its parietes, which is continued even after its removal from the body, and which here is evi- dently in relation with the " phlebenterism" exhibited in the arrange- ment of the digestive system. No proper respiratory organs exist in these simply-organized beings ; the diffusion of the blood through the interior of the body, subservient alike to respiration and nutrition, seems to be entirely effected by the contractions of the intestinal walls; and the proper chylific viscera themselves perform the duties of the lacteal, circulatory, and respiratory apparatus of the higher animals. (1108.) As is the case with the generality of the Lerneans, the male of the Nicothoe was, until recently, unknown to naturalists a circum- stance attributable to two causes: in the first place, the individuals of the male sex are very diminutive in all the genera belonging to this group, insomuch that from their size they seem rather like parasites on the female ; and secondly, because in some of them, which have been more particularly studied, a phenomenon is observable analogous with what occurs in the Aphides among insects there occur whole genera- tions of fertile females, and most probably, also, gemmiparous (nursing) races, during a certain portion of the year, as is the case with Limnadia* and Daphnia-^ ( 1072). (1109.) The male of Nicothoe is represented in fig. 223, 2, magnified in the same proportion as the female. The generative apparatus of the female is largely developed : it is situated principally in the lateral ap- pendages of the body, of which it occupies a considerable part, being lodged by the side of the digestive caeca. The ovarium (fig. 223, 1, h) is very irregular in its shape ; anteriorly it is bifurcate, and its whole surface has a sacculated appearance. The oviducts (i) become conjoined near the mesial line, and then bend downwards to terminate at the vulva. These canals are frequently filled with ova throughout their whole length. From the oviducts the eggs pass directly into the enor- mous ovisacs (/), suspended from each side of the caudal portion of the body, between the lateral appendages, in which they are contained until sufficiently mature for exclusion, when the ovisac, bursting, gives issue to hundreds of minute Mcothoes hatched in its interior. (1110.) On emerging from its prison this little creature is exceedingly active, and presents exactly the form of a Cyclops (fig. 223, 3) ; neither would any one ever suspect it to be the same creature which vegetates upon the branchiae of the Lobster. However, no sooner has it fixed itself in that situation than its body begins to swell out laterally, in the shape of two tubercles that sprout from the sides of the body just behind the third thoracic segment, into which the viscera are seen to penetrate ; and as these tubercles enlarge, the animal becomes gradually * Fide Ad. Brongniart, " M&n. sur le Limnadia," M,)t is attached to the extremity of a long flexible tail (o), wherein the muscular fibres destined for its motions are distinctly visible. (1122.) The cilia, whose action produces the appearance of wheels turning upon the anterior part of the body ; are variously disposed ; and from their arrangement Ehrenberg * has derived the characters whereon * Abhandlungen der Koniglichen Akademie der Wissenschaften zu Berlin for 1833. 2F Brachionus urceolaris (after Ehrenberg) : o, b, c, rotatory apparatus and marginal teeth of the shell ; d, " calcar," or tubular prolongation of the shell, communicating with the visceral cavity; e, oculi- form spot ; f, gizzard with its enclosed masticatory apparatus ; g, stomachal cavity; h h, caecal append- ages to the stomach; k, common outlet; I, lateral canals, to which the vibratory organs are attached in, contractile vesicle ; , ovaries ; o, flexible tail, in which the muscular bands are distinguishable; p, terminal forceps. 434 KOTIFERA. he bases the division 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 conceive what could be the nature of the connexion 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. (1123.) With respect to the agents employed in producing the ciliary movement in the Rotifera, we are as much in ignorance as we are con- cerning the cause of the same phenomenon in the Polygastrica. Ehren- berg describes the cilia as arising from a series of lobes, as represented in Notommata clavulata (fig. 230, a) ; these he regards as being mus- cular, and capable of producing by their contractions the rapid vibra- tions of the fibrillae attached to them. We confess, however, that such lobes, even were 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 undulations, to which the appearance of rotation is due, can be produced by organs of this description. (1124.) The observations of Dr. Arthur Farre* concerning the ciliary movements 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 com- pletely upon itself, the intervening space being completed by others in every degree of extension, so as to present 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 travel onward, whilst the cilia never change their position in this direction, having, in fact, no lateral motion. (1125.) The whole of the ciliary movements are so evidently under the control of the animal as to leave not the slightest doubt in the 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 * Phil. Trans, for 1837. EXPLANATION OF WHEEL-LIKE MOVEMENT. 435 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. (1126.) M. Dujardin's explanation of this phenomenon is based upon the fact, that if equal and parallel lines placed at equal distances from each other become bent at regular intervals, so as to overlap the neigh- bouring lines, they produce dark intersections somewhat resembling the teeth of a saw, instead of a uniform surface (fig. 226). In this manner Fig. 226. onmll i kyf edciaorunliiJty/ edcittonrolJaiJi>y/edci a, the vibratile cilia, being arranged parallel to each other and separated by similar interspaces, would equally intercept the light, so that none would appear more conspicuous than others ; but if, in consequence of a general movement propagated along such a row of cilia, some of them, by being momentarily bent down, are placed in juxtaposition with the neighbouring cilia, the light being more intercepted, a darker or more obscure line will be the consequence. It is easy, then, to conceive that, when all the cilia thus bend themselves in regular succession, numerous intersections of this kind will occur, apparently progressively advancing in the direction of the propagation of the movement ; consequently, if each of these intersections whilst in motion preserves the same form, as being formed by the same number of equal lines the inclination of which is similar as respects each other, it will give to the eye the appearance of a solid body of a definite shape, such as the teeth of a saw, or of a wheel in uniform movement. In this way it is easy to understand how the circular rows of cilia in the Eotifera produce the appearance of a dentated wheel in motion*. * To render intelligible the production of this wheel-like appearance by ciliary movement, we annex M. Dujardin's figure representing the position of a row of cilia at a given moment. In this, it is to be supposed that the straight cilia, which are parallel and equidistant from each other, are susceptible of successive oscillations, like the cilium A B, the first of the series, each capable of describing by a uniform movement the angle B A c, of which the apex is at the point of attachment, bj changing its position from the perpendicular, A B, till it attains the position A c, and 2r2 436 EOTIFEEA. (1127.) 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 examination 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, 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 steamboat, carry it rapidly along with an equable and gliding movement. (1128.) 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 fasciculi in a longitudinal direction, to be inserted into the lobules whereon the cilia are arranged (fig. 230, m m). (1129.) But, besides these retractor muscles, other fasciculi of mus- cular fibres are distinctly seen to run transversely, crossing the former at right angles : these are, most probably, the agents provided for the extrusion of the wheel-like apparatus ; for, arising, 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 then returning with the same rapidity of motion to its first condition, A B, repeating continually similar movements in both directions. Now, as the other cilia of the series only commence this movement one after the other, each being in advance of the preceding one on the left hand by a fourteenth part of the space occupied by the entire wave, and the same distance from that which succeeds it on the right hand, at every fourteenth interval the cilia present themselves in the same state of flexure, and a row of cilia in motion presents, for the instant, the appearance represented in the figure, in which, at spaces of from fourteen to fifteen cilia, there is a shaded inter- section, which advances with a uniform movement from left to right as each cilium successively assumes the position of that which follows it on the right-hand side. Suppose, now, the duration of each oscillation divided into fourteen instants, a given cilium will occupy successively the positions A B, or A o, A n, A m, A I, A Jk, A i, Ah, AC, in the space B A c, during the first half of the oscillation, the movement taking place from left to right. The other positions during the second half of the oscillation, the movement being from right to left, are, Aff, Af, Ae, A d, A c, A b, A a, the position A a' being the same as A B, or A o, constituting the limit of the second half of that oscillation and the commencement of a new one. In this manner the intersections, having the appearance of the teeth of a saw, will appear to advance with a uniform motion in the direction of the movement of oscil- lation, giving the appearance of a chain or row of pearls in motion in the case of a rectilinear row of cilia, or of a toothed wheel if the cilia are disposed in a circle. Vide Dujardin, Hist. nat. des Infusoires. DIGESTIVE SYSTEM. 437 towards the orifice of the shell, it will, of course, push before it the wheels, so as to evert the tegumentary membrane connecting them with the shell, by unrolling it like the finger of a glove, and thus causing the rotatory organs to protrude at the pleasure of the animal. (1130.) "We have already described the means whereby the Hotifera procure a supply of food, namely by exciting currents in the surround- ing water ; the materials so obtained pass at once into a pharynx, the relative capacity of which varies 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 gradually constricted in its diameter, terminates at the caudal extremity of the body. (1131.) The usual arrangement of the digestive apparatus will be readily understood on reference to the annexed figures : thus, in Stepha- noceros EicTihornii (fig. 224), the pharynx (a) is very capacious, receiving readily the materials brought into it by the ciliated arms ; the gizzard (0) 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 perceptible division into stomach and intestine, extends from the gizzard to the anal aperture. (1132.) In Brachionus urceolaris (fig. 225) the pharynx or ossophagus (e) is less capacious; the gizzard (/) exhibits through its transparent coats the peculiar dental organs placed within it ; and the stomach ( 228> their inner extremities, where they are at- tached to the plates, whilst at their oppo- site ends they are united with the others of the same side by a curved connecting bar (fig. 228, c c), from the outer sides of which are given off various loops and processes. The three uppermost of these bars are the Gastric dental apparatus of largest, the rest gradually diminishmg in *** *?* Size as Well as Strength, till the inferior Ones work; oi>, an animal. t Ann. des Sci. Nat. for Sept. 1828 and July 1836. f Symbol Physic. Zoological Besearches and Illustrations, Memoir 5. Cork, 1830. || Phil. Trans., Part 2. for 1837. 464 POLYZOA. of the anatomical details of these creatures in a manner which leaves few points of their economy unknown. Fig. 238. (1196.) We shall se- lect an individual, named by Dr. Farre Bower- banJcia densa, as an il- lustration of the general structure of the POLYZOA, partly from the complete manner in which its or- ganization has been de- veloped in the memoir alluded to, and partly because we have had frequent opportunities of verifying the accuracy of the observations re- corded. (1197.) The tentacula of SowerbanJcia (fig. 238) during the expanded state of the animal are kept quite straight and mo- tionless, as represented in the drawing. Each tentacle is provided upon its outer aspect with a series of stiff and im- moveable spines, proba- bly serving to keep off any foreign bodies that by their proximity might interfere with the ciliary movements immediately to be described. (1198.) Besides the stiff spines, the tentacula are covered with an immense number of vibrating cilia, which, at the will of the animal, are thrown into most rapid movement, so as to produce strong and con- tinuous currents in the surrounding fluid, whereby particles floating in the neighbourhood are hurried along with great velocity. Prom the direction of the streams produced by the cilia, namely towards the mouth, we at once perceive the utility and beauty of the contrivance, compensating to a great extent for the fixed condition of the Polyzoon : animalcules floating in the vicinity no sooner come within the influence of the currents so produced than they are forced towards the mouth, situated in the centre of the tentacular zone, and, being at once seized, are immediately swallowed. (1199.) The tentacula themselves, notwithstanding their immobility Anatomy of Bowerbankia densa (after Farre). a, The animal with its tentacula expanded : 1, pharynx ; 2, oesophagus; 3, the gizzard; 4, the stomach; 5, the pylorus ; 6, the intestine ; 7, the anal aperture, b re- presents the Bryozoon retracted into its cell : 1, 2, 3, muscular fasciculi, c, An imperfect gemma before the opening of the cell: 1, stomachal cavity, d, A gemma sprouting from the common stem. DIGESTIVE SYSTEM. 465 during the process of watching for prey, are highly irritable, and sen- sible of the slightest contact. No sooner does an animalcule impinge 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 cooperate in seizing and retaining it. (1200.) The existence of these cilia upon the tentacula would seem to be characteristic of the Polyzoa, 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 unciliated polyps; for in the class before us, besides the stomach, there is a distinct intestinal tube and anal outlet. In the specimen under consideration the organization of the alimentary organs is rendered even more elabo- rate than is usual in the class, by the addition of a gizzard or cavity wherein the food is mechanically bruised before its introduction into the proper stomach. The mouth is placed in the centre of the space en- closed by the tentacula : it appears to be a simple orifice, incapable of much distention, through which the particles of food brought by the ciliary action pass into a capacious oesophagus (fig. 238, a, 1,2); this, gradually contracting its dimensions, ends in a globular muscular organ, to which the name of gizzard has been applied (3). The walls of this viscus are composed of fibres that 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 tessellated pavement. The contractions of the gizzard are vigorous; and, from the structure of its interior, its office cannot be doubtful. (1201.) To the gizzard succeeds a stomach (fig. 238, a, 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 ossophagus 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 that contains a transparent fluid and encloses distinct muscular fasciculi, to be described 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. (1202.) 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 466 POLYZOA. pharynx, are swallowed by a vigorous contraction of its parietes, and carried rapidly down the oesophagus and through the cardia to the giz- zard, that 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. (1203.) 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 operations, it is returned to the stomach. Here it is rolled about by the contraction of its parietes, and at its upper part is frequently submitted to a rotating motion. This rotation of particles is chiefly near the pyloric orifice ; and a mass may be occasionally seen projecting 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, the cilia, by which this rotation is eifected, were distinctly perceptible, surrounding the orifice. (1204.) 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 pel- lets, that are rapidly pushed, by the contraction of the intestine, towards the anal orifice, through which they are expelled from the body. (1205.) The tube or cell inhabited by this Polyzoon is of exquisite structure, and the mechanism concerned in the protrusion and retraction of the animal of great simplicity and beauty. (1206.) The inferior two-thirds of the cell in the species under con- sideration 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 whereby 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 setae, which are arranged 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 that connects them ; this membrane is evidently analogous to the infundi- bular termination of the cells of polyps already described. (1207.) When the Polyzoon retires into its abode, the setae and soft termination of the cell are gradually folded inwards, in the manner exhibited in the annexed figures (fig. 239) representing the various stages of the process. The oesophagus, surmounted by its tentacula, descends first, whilst the integument of the upper part of the body OPEKCULAK APPAKATUS. 467 begins to be inverted at tbe point where it has its insertion around the base of the tentacles (c). As the descent of the tentacula proceeds, the inversion of this membrane continues ; and when the extremities of the arms have reached the level of the extremities of the setae, 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 setae are now brought together in a bundle (fig. 239, 2, a) Fig. 239. 432 Bowerbankia, showing the opercular apparatus. and are gradually drawn inwards, inverting around them the rest of the flexible portion of the cell, until they form a close fasciculus (fig. 239, 3 & 4, a) occupying the axis of the opening of the tube, constituting a complete protection against intrusion from without. (1208.) 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. (1209.) The muscles may be divided into two sets: one for the retraction of the alimentary apparatus ; the other acting upon the setae 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. 238, a, 8) ; the other passes upwards along the side of the oesophagus (fig. 238, 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. (1210.) The muscles that 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 setae (fig. 239, d d). 2n2 468 POLYZOA. (1211.) The mode in which the protrusion of the tentacula is effected is 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 ali- mentary 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. (1212.) The ELUSTBJE and ESCHARS are intimately allied to Bower- bankia in all the details of their structure, as we are assured by the re- searches of Dr. Milne-Edwards concerning these singularly aggregated forms of marine Polyzoa*. (1213.) The cells of the Flustrce and Escharce are disposed side by side upon the same plane, so as to form a common skeleton of a cori- aceous or horny texture. The individual cells, which are extremely minute, vary in shape in different species ; and the orifice of each is generally defended by projecting spines, or sometimes by a moveable operculum, or lid, that 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. (1214.) The facts observed by Milne-Edwards relative to the forma- tion 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, whence it can be removed with facility by means of extremely dilute muriatic acid. When so treated, a brisk effervescence is produced ; 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 denned as it was before, but the membranous cell appears continuous with the tentacular sheath. "We see, therefore, that in these creatures the cell is an integral part of the animal itself not a mere 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 movement. It is evident, likewise, that what is called the * "Eecherches Anatomiques, Physiologiques, et Zoologiques sur les Eschares" (Ann. des Sci. Nat. for 1836). ORGANIZATION OF CELL. 469 body of the Polyzoon constitutes, in fact, but a small portion of it, principally consisting of the digestive apparatus. (1215.) 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 remains sufficiently soft and flexible to obey the action of the muscles inserted into it. (1216.) The tegumentary sac, deprived of its carbonate of lime, seems to be formed of a tomentose membrane, covered, especially upon its outer side, with a multitude of cylindrical filaments disposed perpen- dicularly 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. (1217.) But the above are not the only arguments adduced by Milne- Edwards in confirmation of this view of the mode in which these skeletons are held in vital connexion with the animal. On examining the cells at different ages, it is found that they undergo material changes of form. (121 8.) This examination is easily made, since in many species the young spring from the sides of those first formed, and do not separate 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 posi- tion 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 extremities of the latter. "When examined in this manner, not only is it seen that the general con- figuration of the cells changes with age, but also that these changes are principally produced upon the external surface. For instance, in the young cells of Eschar a 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 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 presents the appearance of a stony mass, in which the aper- tures of the cells only are visible. (1219.) It appears evident therefore that there is vitality in the sub- stance composing the 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. 470 POLYZOA. (1220.) The anatomy of these Polyzoa differs slightly from that of Bowerbankia. The crown of ciliated tentacula is inserted into the ex- tremity 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 tentacles, and serving to enclose them when the animal retires into its abode. These appendages, thus retracted, are not bent upon themselves, but perfectly straight and united into a fasciculus, the length of which is nevertheless much shorter than that of the same organs when expanded. (1221.) By the opposite extremity to that fixed to the margin of the opening of the cell, the tentacular sheath unites with a tolerably capa- cious tube, the walls of which are exceedingly soft and delicate ; 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 transversely, and are evidently mus- cular ; their use cannot be doubted. When the animal wishes to expand itself, the membranous sheath above alluded to becomes rolled out- wards, 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. (1222.) 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 extremely 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. (1223.) Beneath the first enlargement, the digestive apparatus be- comes 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 becomes attached to an organ of a soft and membranous texture, having the appearance of a caBcum, and which seems to be continuous superiorly with the digestive tube ; the latter continues its progress towards the upper part of the cell, and ultimately terminates by a distinct anal aperture upon the upper aspect of the ten- tacular sheath. (1224.) The operculum which closes the cell in Flustrce and Escharce is moved by two muscular fasciculi inserted into the internal face of this valve by the intermedium of two filaments analogous to tendons ; 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, VARIOUS MODES OF REPRODUCTION. 471 and the mouth of the cell thus opened, they, by their contraction, can close it like a door. (1225.) The existence of nervous ganglia has been satisfactorily de- tected in many genera of the Polyzoa : it consists of a nervous ganglion, situated immediately above the oesophagus, from each side of which pro- ceeds a nervous cord forming a collar around that tube, as well as other filaments distributed to the muscular system. (1226.) No organ of special sensation has been discovered in any animals of this class, either in their adult state, or during the earlier periods of their development. (1227.) From what is known concerning the propagation of the Polyzoa, it would appear that their reproduction is effected in several different ways. (1228.) The most ordinary is by the development of gemmae, or buds, that sprout from the parent stem in the branched species, or, as in the Flustrce and Eschar ce, are derived from the sides of contiguous cells. (1229.) In Pedicellina Belgica, the phenomena attending the gemmi- parous mode of reproduction are the following* : First, there sprouts from the common stem of the Polyzoon, without any determinate situ- ation, a minute tubercle, which is simply a prolongation from the stem itself; this tubercle gradually extends outwards, becomes more promi- nent, and soon swells into a vesicle, which is the first appearance of the new individual. Up to this period the interior of the vesicle is organized precisely in the same manner as the stem itself, of which it is only an extension; but now a cellule becomes visible in its centre, which forms the point of departure whence the development of the embryo proceeds. (1230.) Around this primitive cell a series of other very small cellules soon group themselves, which seem to constitute the parietes of the primitive vesicle, or blastoderm, the original cell representing the vitel- line cavity. The bud now enlarges ; and as its growth proceeds, the internal tissue becomes thickened, so as to fill it completely ; subse- quently an indentation becomes apparent on each side of the little cavity, separating the embryo into two halves, the inferior of which will form the stomach, the superior the intertentacular chamber. (1231.) In Layuncula repens, the reproductive gemmse sprout from the creeping stems which connect the individual animals, appearing at first as a slight prominence that soon expands into a rounded tubercle, which is the commencement of a new cell. On close inspection, each gemma is found to consist of a transparent envelope, that is, in fact, a continuation of the general investment of the animal, lined throughout with a soft membrane, having its inner surface studded with minute * VanBeneden, "Recherclies sur 1'Anatomie, la Physiologie, etla Developpeinent des Bryozoaires qui habitent la Cote d'Ostende" (Bulletin de PAcad. Roy. de Bruxelles, toni. xix.). * 472 POLYZCU. globules, by the accumulation of which the polyp is ultimately formed. The bud itself is hollow, and communicates with the parent stem : it therefore has nothing in its composition resembling that of an egg; neither distinct vesicle nor vitellus. The newly-formed cell soon grows taller, and its lining membrane becomes thicker, indicating the first ap- pearance of the intestinal canal, which is at first a simple cavity bounded by the thickened lining of the cell. This cavity once formed, the development of the different organs proceeds rapidly. First there appears a longitudinal fold, resembling two lips, that, as they approach each other, divide the cavity of the body into an anterior and posterior compartment. The two lips, which have a valvular appearance, become very regularly indented along their margins, and are soon recognizable as the rudiments of the tentacular circle. (1232.) At this epoch, it must be remarked, the polyp presents two cavities distinct from each other : there is a space between the walls of the body and the parietes of the future alimentary canal, the interspace being in communication with the stem of the parent polyp, and filled with a fluid that is analogous to the blood of higher animals ; superiorly this cavity likewise passes into the tentacles, and the fluid which bathes the exterior of the alimentary canal thus finds admission even to the extremities of those organs. The second cavity, which is the intestinal canal, has as yet no communication with the external world. As the formation of the tentacles proceeds, the portion which is situated in front of them will become the sheath, and the other part the intestine. As the tentacula are formed by the prolongation of the tubercles, which were their first rudiments, the cavity of the stomach and the rest of the intestinal tube gradually become apparent ; and at the same time some globules are visibly disposed around the cut de sac of the former viscus, which gradually become arranged into fibrillse, and constitute the re- tractor muscles. (1233.) When the cell has nearly reached its full development, its parietes become softened, and an opening is formed, which brings the young polyp into communication with the surrounding element. The Polyzoon has now attained its complete form, and can expand its ten- tacula ; but, as yet, there are no traces of a generative apparatus, which seems to be matured at a subsequent period. (1234.) Reproduction is likewise effected, in the Polyzoa, by means of true ova. The ovary in which these are developed is situated immedi- ately above the stomach, and is generally found containing eggs in different stages of growth. In the same vicinity is situated another viscus, regarded by Van Beneden as the testes, his opinion being founded on the fact that, when a mature specimen of the animal is placed between two plates of glass and gently compressed, so as to rupture its parietes and cause the escape of the viscera, spermatozoa are easily discoverable in its interior. DEVELOPMENT OF EMBRYO. 473 (1235.) The spermatozoa exhibit considerable vivacity in their move- ments, have a disk-like body and a caudal filament, and are propor- tionately of large size. Around them may be seen multitudes of free cellules without caudal appendages, which are apparently young sper- matozoa. In some individuals the spermatozoa are so numerous that the intestinal canal appears completely enveloped by them, and the whole periintestinal cavity seems alive with their movements. (1236.) In the mature ovary may be seen ova in different states of development, in each of which the vesicles of Wagner and Purkinje are distinctly visible. In ova approaching their complete maturity, an external vitelline membrane, or chorion, and a vitellus are perceptible, but the two vesicles above-mentioned have disappeared. (1237.) When arrived at the proper term, the ova break from their envelope, or ovisac, and escape into the general cavity of the body, where they move freely about, surrounded on all sides by spermatozoa. At length the eggs accumulate in the interior of the parent, near the base of the tentacula ; and their expulsion is ultimately accomplished in the following manner, through a special orifice in the immediate vicinity of the anus : When an ovum is thus about to escape, its external mem- brane is first seen to protrude partially through the aperture, constituting a sort of hernia ; the vitellus then gradually flows from the still enclosed portion of the egg into that which is external ; and when the vitellus has thus entirely passed out, the egg is found separated from the parent animal, and falls into the surrounding water. These eggs are entirely destitute of external cilia, and are carried off by any casual current to attach themselves where chance may bring them ; they are also re- markable for the irregularity of their shape, their form seeming to depend upon the pressure they have been subjected to in the interior of their parent. (1238.) In PediceUina, Professor Van Beneden has witnessed the escape of upwards of twenty eggs from a single individual. They are of a pyriform shape, and are enclosed in a pellucid membrane, by the intervention of which they adhere to each other, so that, in the interior of the body of the parent Polyzoon, they have a racemose appearance, and when extruded spontaneously are generally united together in pairs. Between the vitellus and the envelope of the egg there is always a small quantity of a transparent whitish fluid, which doubtless repre- sents the albumen, while the pellucid external membrane itself is the chorion. (1239.) The vitellus breaks up into granules, at first of large size, and afterwards, by subdivision, of smaller and smaller dimensions, giving a tuberculated appearance, like that of a raspberry, to the mass. This division seems to be accomplished exactly as in the ova of the higher animals, the yelk first separating into two, then into four, after which its breaking up proceeds- rapidly. 474 POLYZOA. (1240). The embryo enclosed within the egg at first presents a rounded form, but soon becomes divided by an indentation into an anterior and posterior moiety, and vibratile cilia become apparent upon the anterior extremity. That portion upon which the cilia have made their appearance next insensibly enlarges, and assumes the shape of a funnel, while the long cilia with which it is fringed begin to keep the particles suspended in the water around in rapid motion. The margins of the funnel rapidly extend themselves; the body exhibits frequent contractions, and at the end of about two hours little tubercles are apparent upon its anterior extremity, which subsequently become de- veloped into the tentacula. Professor Yan Beneden thinks that when the tentacula have become developed and furnished with their proper vibratile apparatus, the original cilia disappear. The formation of the tentacula at once indicates which are the two extremities of the body, and the point by which the embryo will subsequently attach itself. (1241.) The embryo, when mature, is quite free, and strikingly re- sembles some forms of Infusoria ; but after a while a pedicle is formed, whereby it proceeds to fix itself to some foreign body, and thus per- manently assumes the aspect of its race. The pedicle seems to be formed from a cell, developed below the stomach, which grows directly outwards, and thus completes the organization of the young Polyzoon. (1242.) A third form of reproduction is that by ciliated gemmules, common in Halodactylus diaphanus and other similar species having soft and fleshy or gelatinous poly- paries. These are readily seen in spring, when they appear as minute whitish points imbedded in the sub- stance of the mass ; some- times, however, they are of a dark-brown colour, and exceedingly numerous, ap- pearing to occupy almost the entire substance of the po- lypary (fig. 240). If one of these points bo carefully turned out with a needle and examined, it is found to consist of a transparent sac, in which are contained generally from four to six of the gemmules, which, as soon as the sac is torn, escape, and swim about with the greatest vivacity*. * Dr. A. Farrc, Phil. Trang. 1837, p. 140. Thin transverse section of Halodactylus diaphanus. The centre occupied by cellular tissue and water ; the circumference formed by cells in close approximation ; the brown bodies scattered through the substance, a a, Position of the gemmules enclosed in their sac ; 6, one of the gemmules escaped during the section of the central tissue. Sometimes they FLUVIATILE POLYZOA. 475 simply rotate upon their axis, or they tumhle over and over ; or, selecting a fixed point, they whirl round it in rapid circles, carrying every loose particle with them ; others creep along the bottom of the watch-glass upon one end, with a waddling gait ; but generally, after a few hours, all motion ceases, and they are found to have attached them- selves to the bottom of the glass. At the expiration of forty-eight hours, the rudiments of a cell are observable, extending beyond the margin of the body ; but any account of their further development is still a desideratum. (1243.) We have hitherto only spoken of those Polyzoa whose habitat is the sea ; besides the marine genera, there are, however, many individuals belonging to this class that abound in fresh water. The polyparies of the FLTJVIATILE POLYZOA are met with in ponds and streams, adherent to any foreign bodies which may be casually sub- merged*. Thus, they are found attached to stones at the bottom of the water ; upon the shells of Anodon, Unio, and other freshwater mollusca; upon leaves, more especially those of the Water-lily (Nymphceo) and of the Bistort (Polygonum amphibium) ; upon floating wood ; upon the stems of Arundo phragmites and of various other plants. Some genera (Alcyonella and Fredericella) frequently agglomerate into masses of con- siderable size, such as might be mistaken for spongillge. The Paludi- cellce often form an inextricable interlacement of filaments, spread out over shells and stones. Cristatella and Lophopus are generally met with upon the stem of some aquatic plant, such as the Brook-lime ( Vero- nica beccabunga), resembling, when examined by the naked eye, a layer of fluid albumen, which might easily be mistaken for the eggs of Lim- nceus stagnates. In order to examine these animals in a living state, it is necessary to leave the leaf to which they are attached for some time undisturbed in a glass of clear water, when they will soon be seen spread- ing forth their beautiful tentacula as they protrude from their delicate cells. By frequently changing the water, more especially if it is rich in Naviculce and Bacillarice, they may be kept alive for months, affording objects of continual interest for the microscope. (1244.) In the freshwater Polyzoa the structure of the external enve- lope is similar to that of the marine species, except that in no instance are the fluviatile genera known to possess a calcareous polypary. (1245.) In Cristatella mucedo^ (fig. 241, 3) the polypary or external envelope (d) is membranous and slightly cordiform, its surface is tuberculated, and it is incapable of contraction. In this outer covering several individuals are contained; but although produced from one * " Recherches stir les Bryozoaires fluviatiles de Belgique," par P. J. Van Beneden (Nouv. Mem. de 1'Acad. de Bruxelles, 1847). t M. Turpin, " Etude microscopique de la Cristatella mucedo, espece de polype d'eau douce " (Ann. des Sci. Nat. for 1837). Also, another memoir upon the same subject by M. P. Gervais (ibid.}. 476 POLYZOA. another, they are only aggregated, being lodged in distinct tubular cells. The body of each animal appears to consist of a digestive canal constricted once or twice in its course, and terminated by an anal orifice. When these creatures are extended, the upper part of the body pro- trudes from the cell, the tentacular appara- tus being supported on a kind of neck, whereon the mouth (a) is easily and near it the Cristatella mucedo. 1. Egg, natural size. 2. Egg, magnified. 3. Animal after its escape from the egg : a, the mouth ; b, open- ings of cell; c, the stomach; d, shell; e,f, ciliated tentactda. anus. (1246.) 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, 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. (1247.) The three individuals that thus inhabit the same general covering are produced at two distinct generations the two lateral being the offspring of the central one, derived from it by a process of gemma- tion ; but, when complete, they are evidently quite separate from and independent of their parent. (1248.) The number of the tentacular appendages varies very con- siderably in different genera : in Paludicella and Fredericella, which have the fewest, there are about twenty, while in Alcyonella, Plumatella, and Cristatella (fig. 241) there are as many as sixty, or even more. In Paludicella the arrangement of the tentacula is infundibular ; but in Lophopus, Alcyonella, Plumatella, and Cristatella (fig. 241) they assume the shape of a horse-shoe. In Fredericella (fig. 242) they are united together for one-half of their length by means of a delicate membrane. (1249.) The digestive apparatus in all these different genera consists of an oesophagus, of a stomach, which forms a cul de sac, and intestinal tube. The intestine is always straight, and without convolutions. Its cavity is separated from that of the stomach by a pyloric valve that completely closes the aperture ; whilst the oesophagus is in like manner provided with a fold, situated sometimes near its middle, sometimes at its lower part, that performs the office of a cardiac valve. CHYLAQUEOTJS FLUID. 477 (1250.) The aliments,before admission into the stomach, accumulate in a cavity formed at the commencement of the digestive tube (fig. 242,/), which in most genera is defended by a largely developed lip (d) that opens and shuts like a valve ; this lip is densely covered with cilia, the action of which is very energetic. (1251.) The anus (j) is always situated at the base of the tentacular zone. (1252.) The food of the fluviatile Polyzoa consists of Infusorial animalcules and the microscopic Desmidieae which abound in the waters they frequent, and whose remains Fig. 242. are distinguishable both in their stomachs and in the contents of the intestine. They are likewise easily made to swallow carmine, sepia, and other colouring substances. (1253.) There seems to be no doubt relative to the nature of the circulation in these animals. The place of blood seems to be altogether supplied by the chylaqueous fluid. This fluid is not contained in vessels, unless the cavities of the tubular tentacula be considered as such, but moves freely in all directions around the parietes of the digestive canal. There is consequently neither heart nor any vascular system, the chyl- aqueous fluid, which thus repre- sents the blood, being kept in con- tinual movement in the periintes- tinal cavity by the action of the cilia that cover the exterior of the intestinal apparatus. It is therefore ciliary action that determines the course of the aliment in the interior of the alimentary apparatus, and of the fluid external to its walls the cilia thus answering the pur- pose of a heart as well as of the muscular coat of the intestines. (1254.) All the viscera of the body being thus bathed by the chyl- aqueous fluid that surrounds the intestinal canal, they receive directly, through the intermedium of that fluid, both the materials for nourish- ment and the means of respiration. (1255.) Reproduction among the freshwater Polyzoa is accomplished in two ways by gemmation and by true ova. The first of these modes resembles exactly what has been described as existing among the marine genera; but as regards the process of oviparous reproduction, there are some remarkable points of difference that require notice. Fredericella sultana, : d, mouth ; f, pharynx ; .;', anal orifice. 478 POLYZOA. (1256.) It is now generally understood that, wherever oviparous reproduction occurs, there is a formation of spermatozoa ; and modern observations have proved the existence of these distinguishing products of the male sex in most genera of the ciliobrachiate polyps. Frequently both the sexes are conjoined, so that there is a complete hermaphro- ditism ; but in some cases the sexes are separate, and the number of female individuals is greater than that of the males. Seeing that among these compound animals the blood, or its representative fluid, is common to an entire group, and that the ova as well as the spermatozoa are dif- fused through this liquid before they are evacuated, a single male indi- vidual may, strictly speaking, suffice for the fecundation of the eggs of a whole colony. (1257.) But with regard to the ova themselves, a remarkable difference is observable between those of the fluviatile and of the marine Polyzoa. Among the Alcyonellce and other genera there exist two sorts of eggs the one covered with vibratile cilia, capable of swimming freely about exactly like Infusorial animalcules, and the other enclosed in a hard shell, having somewhat the appearance of the seeds of some plants. The first sort, without a shell, is also met with among marine species ; but the second seems peculiar to the freshwater Polyzoa. In Crista- tella mucedo, for example, the ova are of this latter description, being enclosed in a dense horny shell, the exterior of which is covered over with sharp booklets, giving them an appearance strikingly like some of the Desmidieai (fig. 241, 2). This shell is probably intended to preserve the ova during the winter season from being destroyed by the freezing of the ponds in which they occur, while the marine polyps, being sub- jected to no such changes of temperature, can dispense with such a covering. It is on this account apparently that these ova are met with sometimes naked, and sometimes provided with a shell ; and in the same way, in the genus Paludicella (in which ova have not been detected), the gemma? become invested on the approach of winter with a horny covering. (1258.) As there are thus two modes of reproduction, so are there two kinds of embryogenic development observable among the fluviatile Polyzoa ; that is to say, the polyp which is produced from an egg is formed in a different manner from that which is produced by the pro- cess of gemmation. In the mature ovum, both the germinal spot and the germinal vesicle are distinctly perceptible ; but in the nascent gemma the existence of neither of these elements is to be detected. According to the gemmiparous mode of propagation, the young individual is formed by direct extension from the tissues of the parent. In the formation of the embryo from an egg, there is, from the first, a complete isolation of the newly-formed progeny : a vesicle or cell is formed, which, pre- vious to its conversion into a new individual, requires the cooperation of another cell, or, in other words, the ovum remains unproductive TUNICATA. 479 unless brought in contact with the male fluid containing spermatozoa, whereas in gemmiparous reproduction such a concurrence is by no means necessary ; neither germinal vesicle nor any male apparatus is required. CHAPTER XIX. TUNICATA*. (1259.) 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 by the texture of the external investment 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 coriaceous 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. (1260.) Yarious are the forms under which these animals present themselves 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 connected with the structure and habits of an Ascidia belonging to one of the most per- fectly organized families ; and after examining this attentively, our descriptions of allied genera will be rendered more simple and intelli- gible. The Ascidians are abundantly met with upon the shores of the ocean, especially at certain seasons of the year. In their natural con- dition 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 individuals are simply agglomerated, without organic union. Incapable of locomotion, and deprived of any external organs of sense, few animals seem more helpless or apathetic than these apparently shapeless beings, and the anatomist is surprised to find how remarkably the beauty and delicacy of their interior contrasts with their rude external appearance. In the species selected for special description (Phallusia nigrd), the external envelope (fig. 243, a a a) is soft and gelatinous in its texture, fixed at its base to a piece of coral (I), and exhibiting at its opposite extremity two orifies (&, /), placed upon prominent portions of the body. Through the most elevated of these orifices (h) the water required for respiration and the materials used * Tunicatus, clad in a tunic. 480 TUNICATA. Fig. 243. as food are taken in, while the other (/) gives egress to the ova and excrementitious matter. The soft outer covering is permeated by blood- vessels, which ramify extensively in it ; it is moreover covered externally with an epidermic layer, and lined within by a serous vascular mem- brane, which, in the neighbourhood of the two orifices, is reflected from it on 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 reflexion of the peri- toneal membrane above mentioned. (1261.) On removing a portion of the exterior tunic, that in reality re- presents the shells of a bivalve Mol- lusk, the soft parts of the Ascidian are displayed. The body is seen to be covered with a muscular invest- ment (the mantle) (fig. 243, 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 be de- scribed, in a thin continuous stream, sometimes projected to a distance of many inches. (1262.) Respiration is effected in an apparatus of very peculiar con- trivance, to the examination of which we must now request the at- tention of the student. A consider- able 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 introduced in the dissec- tion, represented in the figure (fig. 243) ; its walls are seen to be com- posed of a thin but very vascular membrane (d d d), that has been partially turned back, so as to display the interior of the respiratory sac. The membrane (fig. 243, d d d; fig. 244, e), when examined Structure of Phallusia nigra : a a a, ex- ternal envelope ; b b, the mantle ; c, mantle reflected so as to display d d, the membrane lining the respiratory sac ; e e, alimentary canal ; f, excretory orifice ; g, orifice of ovi- duct ; h, oral aperture ; /, a piece of coral to which the animal is fixed; n, the anus. (After Hunter.) HEART AND CIRCULATORY SYSTEM. 481 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 discovered to be densely studded with vibratile cilia, whose rapid action constantly diffuses fresh supplies of water over the whole vascular mem- brane. The respiratory cavity has but one orifice for the admission of water (fig. 244, a), and this is guarded by a fringe of delicate and highly sensitive tentacula (fig. 244, 6) ; 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. (1263.) The heart itself presents the simplest possible form, being generally a delicate elongated contractile tube, receiving at one extre- mity 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 circu- lating fluid and disperses it through the system. (1264.) The heart, above described, is extremely thin and trans- parent, and is lodged in a distinct pericardium, which separates it from the other viscera. (1265.) Notwithstanding this apparently simple arrangement of the vascular system in the Ascidians, the nature of the circulation of the blood, throughout the class, is extremely curious, the action of the heart being completely reversed at brief intervals, and the course of the blood entirely changed a phenomenon which is easily witnessed in any of the smaller and more transparent species, when placed under the mi- croscope. The contractions of the heart succeed each other with regu- larity ; but they are sluggish, not extending at once through the whole organ. The systole commences at one extremity, and is propagated by an undulatory movement towards the opposite end by a sort of peri- staltic action. For some time the contractions succeed each other with rapidity, passing on in the same course, when they suddenly cease, and, after a pause, recommence from the other end of the viscus. The blood, thus impelled alternately from behind forwards, and then in the contrary direction, ascends towards the branchial apparatus ; nevertheless it docs not appear to be conducted there by closed vessels, but seems to be diffused between the inner tunic of the abdomen and the viscera, where it flows in currents that vary in their direction as the movements of the animal, or any other mechanical causes, affect their passage. The chief portion of the blood, however, ascends by the dorsal or the ventral 2i 482 TUNICATA. surface of the abdomen, and, after having bathed the surface of the viscera, gains the base of the branchial sac. When the contractions of the heart are directed forwards, the ascending current of blood passes along the anterior wall of the abdominal cavity and enters a capacious sinus, situated in front of the respiratory chamber, which gives origin on each side to a series of large transverse vessels ; and these intercom- municating with each other by means of innumerable branches disposed vertically, a rich vascular network is formed, that, after spreading all over the walls of the branchial cavity, pours its blood into another vertical sinus situated at the opposite side of the thoracic cavity, into which is likewise poured the vitiated blood derived immediately from the system. Lastly, the circulating fluid, again diffusing itself between the viscera, descends along the dorsal region of the abdomen and again reaches the heart. "Were the circulation constant in the above direc- tion, as Milne-Edwards observes, it would somewhat resemble that of other Acephalous Mollusca. The heart might then be compared to an aortic ventricle, and the anterior thoracic sinus to a branchial vein. But, owing to the contrary directions of the currents of blood, due to the changing action of the heart, the vessels that during one minute perform the functions of veins, are in the next converted into arteries. (1266.) When we consider the fixed and immoveable condition of an Ascidian, and its absolute deprivation of all prehensile instruments adapted to seize prey, it is by no means evident, at first sight, how it is able to subsist, or secure a supply of nourishment adequate to its support ; neither is the structure of the mouth itself, or the strange position which it occupies, at all calculated to lessen the surprise of the naturalist who enters upon the consideration of this part of its 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 (fig. 243, and fig. 244, g). It is obvious, then, that, whatever materials are used as aliment, they must be brought into the body with the water required for respiration ; but even when thus introduced into the branchial cavity, the process by which they are conveyed to the mouth and swallowed 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 * Phil. Trans, for 1834, p. 378. DIGESTIVE APPAEATUS. 483 Fig. 244. to them in regular and quick succession, which produces the appearance of waves ; and each wave answers here to a tooth. (1267.) 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 downward stream of similar materials ; and they proceed together, receiving accessions from both sides, and enter at last the oesophagus, placed at the bottom (fig. 244, #), which carries them, without any effort of swallowing, towards the stomach. (1268.) The oesophagus (fig. 244, h) is short, and internally gathered into longitudinal folds. The stomach (i) is simple, moderately dilated, and has its walls perforated by several orifices, through which the biliary secretion en- ters its cavity. The liver is a glandular mass intimately adherent to the exterior of the stomach and the intestinal canal (fig. 243, e e), of variable length, and, more or less convoluted in different spe- cies, after one or two folds terminates in the rectum, which, emerging from the peritoneal investment covering the intes- tine, has its extremity loosely floating in the cavity communicating with the second orifice (/) : into the latter a bristle is introduced in the figure, having its ex- tremity 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 ele- vated 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 occa- sionally met with in the branchial chamber, 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. * Cuvier, Memoire sur les Ascidies, p. 14. 2i 2 Diagram of an Ascidian, showing the position of the viscera: a, the oral orifice ; b, tentacula guarding it ; c, d, mach and intestine ; o, ovary ; p, termi- nation of oviduct ; q, common excretory orifice. 484 TUNICATA. (1269.) The reproductive system in these humble forms of Mollusca presents the utmost simplicity of parts, being composed of an ovarian nidus, in which the germs of their progeny are elaborated, and a duct, through which their expulsion is accomplished. (1270.) The researches of Milne-Edwards*, however, relative to this part of their economy, tend to show that the structure of these creatures is more complex than was previously supposed ; and in Amaroucium Argus, one of the compound Ascidians, that indefatigable observer also succeeded in recognizing the presence of a male apparatus. This con- sists of a largely-developed testicular gland, that occupies almost all the lower part of the post-abdomen, and communicates with the common excretory cavity by means of a long filiform canal that was regarded by Savigny as the oviduct. This gland is made up of a multitude of whitish vesicles, which, at first sight, have much the appearance of rudimentary eggs, but which, in reality, are found to swarm with sper- matozoa, thus revealing the true nature of the organ. A similar ar- rangement has since been detected in numerous other genera ; so that its existence throughout the entire class is now no longer doubtful. The ovarium is situated in close juxtaposition with the testes. In all the Polyclinian group it is lodged in the post-abdomen, and posteriorly is hardly distinguishable from the male apparatus, but towards the upper part becomes easily recognizable in consequence of the size and colour of the ova contained within it. The eggs, of which a few only are developed at a time, as they issue from the ovigerous organ pass immediately into the cloaca, or even become lodged in the lateral part of the thoracic chamber, between the proper tunic of this cavity and the branchial sac, where they remain for some time exposed to the influence of the surrounding aerated medium. (1271.) Whilst the ova remain enclosed in the upper part of the post-abdomen, they increase considerably in size and assume a spherical form ; the yelk acquires a deep-yellow colour ; and the vesicle of Pur- kinje, which was distinctly visible during the commencement of their development, disappears, and is replaced by a cloudy spot, which appears to be the blastoderm, or proligerous layer, from which proceeds the embryo of the young Ascidian. (1272.) During incubation the egg acquires more of an oval form, the vitelline mass seems to contract, and its surface, growing denser, appears to become organized into a membrane which is distinct from the more deeply-seated substance of the yelk, and gradually the whole becomes moulded into something like the shape of a tadpole (fig. 245, A), the anterior extremity of which is surmounted with a sort of tentacular apparatus ; and on the bursting of the egg, this embryo, by means of its long tail, swims about in the water with con- siderable vivacity ; after the lapse of a few hours, however, the little * " Observations sur les Ascidies composees," Mem. de 1'Acad. torn, xviii. DEVELOPMENT OF EMBEYO. 485 creature, in size not yet larger than the head of the smallest pin, fixes itself to some foreign object by means of one of the little suckers situated on the anterior extremity of its body, and permanently loses all capability of locomotion. (1273.) Having thus fixed itself for life, the larval Ascidian soon begins to change its form (fig. 245, B). The anterior extremity of its body becomes expanded, the tentacular appendages disappear, the central portion of the tail becomes retracted into the central mass, Fig. 245. Larva and progressive stages of metamorphosis in an Ascidian Mollusk. and, lastly, the tail itself, which was at first such an important loco- motive agent, gradually withers away, until no traces remain of such an organ having existed (fig. 245, c, D). (1274.) From the above description of the development of the young Ascidians, it appears that during the first part of their existence they are solitary and isolated animals, although at a later period they are found united into numerous colonies, either connected together by means of a creeping common stem, or associated into a compact mass by a tegumentary tissue wherein the entire colony is arranged after a certain order, or regular pattern, which is constant in each species ; and the manner in which this is effected thus presents itself as a problem possessing considerable interest. (1275.) Savigny, while prosecuting his dissections of the Botrylli, had remarked, situated upon the borders of the stellate groups formed by the association of individuals belonging to that genus, a multitude of membranous tubes, slightly dilated at their extremities, to which he gave the name of the marginal tubes, at the same time pointing out their existence in other families, but without entering into any details con- cerning the relations existing between them and the associated Ascidians contained in the tegumentary mass. Milne-Edwards, however, ascer- tained, by the examination of transparent groups of these creatures whilst in a living state, that each of these canals is at first a little tubercle, developed from the surface of the abdominal portion of the 486 TUNICATA. inner tunic of an adult Ascidian. This tubercle becomes elongated by growth into a tube, the extremity of which is closed, but free, while its internal cavity communicates freely by the opposite end with the abdominal cavity of the Ascidian from which it originally sprouted ; so that the blood circulating in the latter easily penetrates into the caacal appendage, wherein an active circulation is kept up. Generally speak- ing, in proportion as these marginal tubes advance into the common tegumentary tissue around them, they divide into several branches, and the extremity of each of them becomes inflated and claviform; soon there appears, towards the summit of each terminal swelling, a small granular mass wherein the forms of an Ascidian gradually develope themselves, and which in time becomes a new animal, resembling those already existing in the common mass, of which it is destined itself to become a new inhabitant. Ultimately the communication between the parent and the young individual becomes obliterated; but still the newly-formed animals, thus derived from the same parent, remain for some time united by their pedicle ; and, apparently, to this circumstance their mode of arranging themselves in groups is due. (1276.) The ovary is a whitish glandular mass, imbedded with the liver among the folds of the intestine ; its position in fig. 243 is indi- cated by the letter m ; and at o, fig. 244, it is seen separated from the surrounding structures. The oviduct, which is occasionally very tor- tuous, accompanies the rectum, and terminates near the anal aperture (fig. 243, m, fig. 244,^>) ; so that the ova ultimately escape through the common excretory orifice. (1277.) Since the publication of the former editions of this work, important additions have been made to our knowledge relative to the generative system of the class under consideration, and a distinct male apparatus, the existence of which was formerly denied, has been satis- factorily described by many skilful observers. The arrangement of these organs, as they exist in Cynthia ampulla, dissected by Yan Beneden, is shown in the accompanying figure (fig. 246, A, B). In this species the sexual parts appear at first sight to form but a single organ, imbedded in a fold of the alimentary canal (a, 6) ; but by the assistance of a micro- scope, this is readily seen to consist of two portions one male, and the other female. The testicle (c) surrounds the base of the ovary, and is distinguishable by its milky- white colour; its substance is entirely made up of innumerable short convoluted caeca, visible to the naked eye, and resembling the seminiferous tubes of many of the higher animals. Three or four glandular prolongations (/) arise from the surface of this organ, which are hollow internally, and contain a milky fluid which is poured into the cloaca, and which the microscope reveals to be almost entirely composed of spermatozoa with disciform heads and filamen- tary tails. The ovary is of a dark colour, and is imbedded, as it were, in the testes ; its oviduct (e) opens into the cloaca by the side of the anus. SALP.E. 487 (1278.) In Ascidia grossularia, the eggs, as seen through the walls of the ovary, are of a fine red colour, and are contained in separate sacs ; so that the ovary when distended resembles a bunch of grapes. By the side of the ovary is another series of sacculi (fig. 246, B, 6), the contents of which abound with spermatozoa, intimating their identity with the male apparatus above described. Fig. 246. B A Generative system of Cynthia ampulla. (1279.) Deprived as these animals are of any of the higher organs of sense, and almost cut off from all relation with the external world, we can look for no very great development of the nervous centres. 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. (1280.) In the seas of tropical latitudes, many forms of Tunicated Mollusca are met with abundantly which, although allied to Ascidians in the main points of their economy, present certain peculiarities of struc- ture that require brief notice in this place. These, grouped by authors under the general name of Salpce, 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 posterior 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 chan- nel ; 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 back- ward direction. The branchial chamber of Ascidia is consequently 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 singular kind of branchial organ is placed 488 TUNICATA. 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 per- petually renovated, and afterwards distributed, by a heart resembling that met with in the genus last described, to all parts of the body. (1281.) The viscera, which occupy comparatively a very small space, are lodged in a distinct compartment between the membranous respira- tory 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 respiratory surface bring into it a sufficient supply of nutritive molecules. 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 posterior 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 ; these, no doubt, are the ovaria, and form a reproductive system as devoid of complication as that of the sessile Ascidians. (1282.) A very remarkable feature in the history of these animals is that many species are found swimming together in long chains, appa- rently 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 Chamissof, that such aggregated animals give birth to insulated individuals of very different appearance, which in their turn reproduce concatenated forms resembling their progenitors ; so that the alternate generations are quite dissimilar both in conforma- tion and habits. (1283.) The observations of Chamisso have in later times been substantiated and carried out by the researches of Krohn, Steenstrup, Eschricht, Milne- Ed wards, and others ; and the phenomena connected with the process are so interesting that it will be necessary to lay before the reader a brief abstract of the result of their labours. (1284.) TheJSalpce are all viviparous, and each species is propagated by an alternate succession of generations most dissimilar from each other in their forms, habits, and mode of increase. The concatenated Salpse produce but a single egg apiece, which is distinctly visible in the interior of their transparent bodies ; they seem, moreover, to be bisexual, having, as it would appear, two generative functions to per- * For excellent drawings representing the anatomy of various Salpce, the reader is referred to the Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy contained in the Museum of theKoyal College of Surgeons of England, vol. i. plates 6 & 7. t Dissert, de Salpa, Berlin, 1830. AGGREGATED ASCIDIANS. 489 form the one to produce a new being, the other to fecundate a future generation of animals similar in all respects to themselves. But whilst the isolated Salpians are thus produced from eggs, their progeny are produced by a process of gemmation, springing like buds from the sur- face of a most remarkable organ, the stolon prolifer, the existence of which is to be detected in the isolated Salpae even while contained in the body of their concatenated parent ; it then appears a slender fila- ment, derived by one extremity immediately from the heart of the embryo Tunicary ; after birth, however, its growth increases apace in proportion to the development of the continual succession of progeny to which it gives origin. The reason of the immediate connexion between the " stolon proliferum " and the maternal heart appears to be this, that the newly-formed offspring being entirely dependent for support upon the blood of the parent, it is so situated in order to secure a free supply of the vital fluid, which is thus injected into its vessels imme- diately by the heart's action. Two vessels traverse it throughout its entire length, one derived from the anterior extremity of the maternal heart, and the other from its opposite end; so that the blood supplied to one of these vessels by the contraction of the heart is returned by the other ; and when the contractions of the heart become reversed, as we have seen is continually the case by this arrangement, the circulation in these vessels is readily adapted to the change. It is from the surface of the stolon that the generation of concatenated Salpae sprout, in two parallel rows, appearing in rapid succession like so many little buds, which, as their growth advances, gradually assume a similar form and structure ; and as successive groups become mature, detaching themselves from the nidus where they had their birth, they swim away united in long chains, the links of which are joined together after the fashion of the species. On examining one of these chains of concatenate Salpae, the individuals composing it are found to be united to each, not by any organic coalescence, but by special organs and facets of attachment, frequently, but improperly, described as suckers, the position of which varies in different species in accordance with their mode of aggregation. It would appear, from the observations of M. Krohn, that the concate- nated Salpas cannot spontaneously detach themselves from each other, and that, when individuals are met with swimming free, their separation from the chain is always to be ascribed to accidental violence ; he even thinks that concatenation is so essential to the existence of the animals that they soon perish if separated from the rest. (1285.) The last families of TUNTCATA which we have to notice would seem to constitute a connecting link between the ASCIDIANS and the POLYZOA, which latter in many points of their anatomy they much resemble. These animals generally are exceedingly minute, and indi- vidually present an organization analogous to that of Ascidians. At first it would appear that they are detached from each other, and, like 490 TUNICATA. Salpce, 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 community of action. They are arranged by Cuvier* in three principal groups, distinguished by the following characters. In the first (BotryUus^}, 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 centre. If the external orifice is irritated, the animal to which it belongs alone con- tracts ; but if the centre be touched, they all shrink at once. (1286.) 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 fee described as consisting of a great number of stars of Botrylli piled one above the other, the whole mass remaining free and capable of locomo- tion. 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. (1287.) In all other forms of these aggregated Mollusca, which are designated by the general name of Polydinum, as in ordinary Asci- dians, 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 disk resembling a flower or an Actinia ; but, whatever the general arrangement of the common mass, it is composed of numerous asso- ciated individuals, every one of them corresponding more or less closely, as regards their internal structure, with the description above given of the organization of Salpae and Ascidians. * Begne Animal, vol. iii. p. 168. t /3orpus, a bunch of grapes. | trvp (-os), fire ; in this case single, is likewise represented in situ, suspended from the roof of the cavity that contains it. (1410.) In fig. 275 the roof of the respiratory cavity (00} has been reflected, and the three rows of branchial fringes (n) suspended therefrom are well seen. (1411.) A sixth order of Gas- teropods has been formed by Cuvier under the name of TUBTJLI- BRANCHTATA, 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 por- tion twisted into a few spiral curves. To this order belongs Vermetus (fig. 268), the shells of which, agglomerated into masses, might be taken for those of cer- tain Serpulce. As locomotion is here out of the question, owing to the immoveable condition of the habitations of such genera, the foot would seem at first to be altogether deficient, but upon close inspection it is found to be converted into a fleshy organ that bends forward and projects beyond the head, where its extremity expands into a disk 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 branchiae are pectiniform, forming a single row attached to the roof of a branchial chamber. (1412.) The SCUTIBEAXCHIATA likewise have pectinated gills disposed 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. (1413.) An eighth division of this extensive class takes the name of Vermetus. 534 GASTEROPODA. CYCLOBRANCHIATA, because the branchiae form a fringe around the body of the animal, between the edge of the body and the foot (fig. 260, c ; fig. 263, <>: (1414.) Lastly, a distinct order has been established to embrace certain families in which the foot is so much compressed as to constitute a vertical muscular lamella, that presents merely a remnant of the ventral sucker so characteristic of the entire class, and which can only be serviceable in performing the office of a fin used in swimming ; hence these mollusks have been called HETEROPODA. Their branchiae are placed upon the back (fig. 269, d), and resemble small detached tufts. The form of these heteropod Gasteropoda the reader will gather from Fig. 269. Pterotrachea. an inspection of the accompanying figure, representing a species of Pterotrachea ; but the details connected with their anatomy, therein delineated, will be explained hereafter. (1415.) It would be useless to weary the student by describing the course of the blood-vessels in all the orders we have just enumerated ; their distribution necessarily varies with the changes observable in the position of the branchiaB ; 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 purpose. In the Pectinibranchiata, as for instance in Buccinum (fig. 275), 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 (fig. 275, q), that communicates with the auricle (a). The con- COUESE OF THE CIRCULATION. 535 traction of the auricle forces the circulating fluid into the ventricle (r), which in turn drives it into the aortic or arterial system of vessels. The aorta, in the case hefore us, divides into two principal trunks, of which one (n) is directed forwards to supply the foot and anterior part of the body, while the other (t) winds among the mass of viscera con- tained 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 reconveyed to the branchiae, there to begin again the circuit we have described. (1416.) 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 branchiaB to the heart by capacious veins which run beneath each branchial fringe and collect it from the nume- rous respiratory tufts ; or if, as in Doris (fig. 266), the branchiae encircle 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. (1417.) In Aplysia, one of the tectibranchiate Gasteropods, the branchiae (fig. 270, a, 6) consist of delicate lamellae minutely subdi- vided ; 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 pene- trate 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 organiza- tion of these Mollusca led him to the following important conclusions, which are no doubt extensively applicable to the GASTEROPODA gene- rally : 1. That in Aplysia there are no other vessels appointed to con- vey the blood to the branchiae than the two above described. 2. That all the veins of the body terminate in these two canals. Now, as their communication with the abdominal cavity is evident and palpable, whether we call them venae, cavce, or cavities analogous to a right ventricle, or branchial arteries, for it is manifest 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 * Cuvier, Memoire sur le Genre Aplysia. 536 GASTEROPODA. the mass of the blood and thus conveyed to the branchiae, and that the veins perform the office of absorbent vessels. (1418.) This extensive communication is undoubtedly a first step towards 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 nutritive 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. (1419.) The vein appointed to convey the renovated blood from the branchiae to the heart, when slit open (fig. 270, d), exhibits the orifices Fig. 270. Vena cava of Aplysia laid open. of the smaller vessels derived from the respiratory lamina arranged 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 (y) by a valve (/), whereby any retrograde movement of the blood is prevented. (1420.) In Aplysia, the arterial blood, having been distributed throughout the body by means of the heart and aortic vessels, is received into a capillary system, which forms a rich network composed of minute vessels, the walls of which are perfectly distinct ; but these capillaries are found not to be continuous with any system of recurrent vessels, but gradually resolve themselves into little lacuna) formed amongst the interstices which occur between the bands of cellular membrane and the fibres of various tissues. These vacuoles communicate in their turn with larger lacunae, situated beneath the common integuments, or occu- pying the interspaces between the muscular fasciculi of the foot of the mantle, and of other parts of the body. The result of this arrangement is the formation of a vast system of venous cavities, dispersed through- out the abdominal parietcs. In the foot and in the lobes of the mantle LACUNAE. 537 those lacunse are very dilatable, and afford space for a considerable accumulation of fluid ; in the dorsal region, on the contrary, they are small, and more densely congregated. It is this structure which con- stitutes the aquiferous system of Delle Chiaje ; but it has no commu- nication with the exterior of the body. The membrane, which imper- fectly lines the abdominal cavity, separates this structure from the visceral chamber, but does not cut off the communication that exists between them ; on the contrary, the peritoneal tunic is itself of a spongy texture, and is perforated with numerous apertures, whereby a free passage is established between the subcutaneous lacunse and the interior of the abdomen. In this way it happens that, when a coloured fluid is injected into the visceral cavity, the whole lacunary system becomes filled; and on throwing injections, even of coarse materials, into the muscular interstices of the foot or mantle, they are seen at once to diffuse themselves through the abdominal cavity. (1421.) From the above and similar facts, Milne-Edwards has satis- factorily established the following important conclusions : 1st. That no complete vascular system exists in any of the Mollusca. 2nd. That throughout a greater or less extent of the circulatory circle veins are entirely wanting, their functions being performed through the medium of lacunse, or by the great cavities of the body. 3rd. That frequently the veins are wanting altogether, and that in such cases the blood distributed through the body by the arterial system can only return to the respiratory surface by the intervention of the interstitial lacunae above described. (1422.) Professor Huxley, in a letter addressed to Professor Milne- Edwards*, relative to the circulation of the blood, expresses himself very decidedly upon this important point in the anatomy of the Mol- lusca. In Firola, one of the Heteropod division, he observes that, owing to the perfect transparency of the body of this mollusk whilst alive, nothing is more easy than to observe the circulation of the blood throughout its entire course. In this creature no veins whatever are observable. The globules of the blood may be seen to issue in crowds from the open termination of the arteries of the foot, through the substance of which they immediately become diffused, and may likewise be observed to pass from the mass of the mouth, in which the aorta terminates, directly into the peri-intestinal cavity, in which they may be seen to return gently, frequently stopping in their course towards the heart. Occasionally some of them may be traced directly into the auricle, passing through the interspaces between the network of muscu- lar fibres composing its walls f, in the meshes of which they may some- * Ann. des Sci. Nat. 1850. f In Firola, Professor Huxley assures us, the walls of the auricle of the heart are composed of a kind of lacework made up of striated and ramified muscular fibres, between which large open spaces are observable. 538 GASTEKOPODA. times be observed to stop for a short period. When the animal begins to grow weak, and the circulation becomes enfeebled, it is even possible to follow with the eye any given globule during its passage through the peri-intestinal cavity, and through the heart into the aorta. (1423.) In studying the anatomy of Haliotis, Milne-Edwards* ob- served that, although injections thrown into the heart were easily made to fill the general arterial system, so as to exhibit the arteries supplied to the liver, to the stomach, and internal viscera generally, and also to render visible even the capillary vessels, in the head he invariably found the injection extravasated so as to fill a great cavity, in which were lodged the brain, the salivary glands, the pharynx, and all the muscles belonging to the oral apparatus. At first it was supposed that this extensive extravasation was caused by some rupture of the vascular parietes ; but after many unsuccessful attempts it was at last discovered that, on attempting to follow the course of the aorta into the head, it was impossible to find any trace of that vessel beyond the point where this extravasation invariably began to show itself : at this place, indeed, the walls of the great artery entirely disappeared, or, rather, became confounded with the membranous septum that here separates the abdo- men from the cephalic cavity : neither could any continuity be traced between the arterial trunk, after its entrance into this extensive cavity, and the arteries proceeding from it to ramify in the fleshy portion of the foot, although these latter were invariably well filled with the coloured injection employed ; and it soon became evident, from numerous observations, that in this Gasteropod a free communication is normally established between the great arterial trunk of the body and the cephalic cavity, wherein are lodged the principal nervous centres and the whole anterior portion of the digestive apparatus, and that this cavity, in the living animal, is filled with arterial blood. In fact, the aorta having reached the spot where the digestive canal curves downwards to descend from the upper aspect of the pharyngeal bulb into the abdominal cavity, it plunges directly into a wide space or lacuna which surrounds the pharynx and occupies all the front part of the head, taking the place of the cephalic portion of the aorta ; and the arterial blood poured by that vessel into this space directly bathes the brain, the muscles of the pro- boscis, and all the anterior part of the alimentary canal, after which it goes to supply the muscles of the foot and the cephalic appendages. (1424.) But there is one circumstance connected with this arrange- ment which appears even still more strange, namely that, while a por- tion of the general cavity of the body thus completes the vascular appa- ratus, the aorta to a certain extent acts as an abdominal cavity ; for in its interior there is lodged a part of the digestive apparatus. (1425.) To ascertain this fact it is only necessary to slit open the * "Observations sur la Circulation chez les Mollusques," par M, Milne-Edwards (Ann. des Sci. Nat. 1847). BLOOD-VESSELS OF HALIOTIS. 539 aorta, the calibre of which is in this part as wide as a goose-quill ; it is then seen that the large subcylindrieal basis of the tongue, which pro- Fig. 271 Circulation of Haliotis (after Milne-Edwards). A, the head; B, the foot; C C, the two lobes of the mantle ; D, mucus-secreting organ ; E E, 'the two branchiae ; r, the anus. Beneath the rectum, that terminates at this outlet, is seen the orifice of the urinary apparatus ; and a little further back, situated above the intestine, is the orifice of the generative apparatus. G, fold of intestine, which is lodged in a special compartment of the abdominal cavity, separated from that containing the stomach by a fibrous septum. H, the stomach, of which the anterior portion has been in a great measure removed. I, pharyngeal cavity laid open. J, abdomen. a, aortic ventricle surrounding the rectum. 6, the left auricle, into which opens the efferent vessel of the corresponding branchia, a portion of which is shown at E. The right auricle is seen immediately beneath the ventricle, and the corresponding branchia has been raised in order to show throughout its entire length the branchial vein or efferent canal, E, which runs along the adherent margin of the branchia, and brings arterialized blood from that organ to the heart. c, the great aorta, which arises from the posterior extremity of the ventricle and runs forward between the stomach and the intestine to discharge itself into the cephalic cavity. d, the abdominal artery, or posterior aorta, which arises from the commencement of the aorta and foil ows the convolutions of the intestine, to which, as well as to the liver, it furnishes branches. e, arterial sinus, into which the aorta empties itself. This is a great cephalic lacuna, limited above by the parietes of the pharynx, in front by the integuments and muscles of the head, and posteriorly by fibro-cellular bands. On injecting the animal by this cephalic chamber, the whole arterial system is immediately filled. f, the great artery of the foot, which arises from the cephalic sinus, and soon divides into four branches, which extend towards the hinder part of the foot. g, one of its lateral branches. h, afferent vessel of the left branchia. A little in front of the heart is seen the transverse canal, or common venous reservoir of the branchia, which unites this vessel with its fellow, and which receives the veins from the rectum. i i, veins of the two lobes of the mantle in communication with a capillary network that extends along the base of the branchiae, and is proceeding to anastomose with the branchio-cardiac vessels. fc, efferent vessels from the urinary gland opening into the common venous reservoir of the branchiae. I, venous canal of the shell-membrane or partition that extends from the walls of the abdomen to the margin of the shell. TO, hepatic veins proceeding to open directly into the free space which surrounds the intestine, and which is continuous with the rest of the abdominal cavity. On the posterior part of the foot are seen veins which open into a system of lacunae situated upon the median line, and communicating with the abdominal cavity. 540 GASTEKOPODA. jccts from the posterior margin of the pharyngeal mass, is entirely enclosed within it. This organ, indeed, protrudes to a considerable distance into the interior of the arterial tube ; and it is from the portion of the aorta which thus forms a sheath for the lingual apparatus that several arteries take their origin, the branches of which are distributed to the intestine and abdominal parietes, the orifices of which are discover- able when the tongue is withdrawn from its aortic sheath. (1426.) The inferior condition of the circulatory system in the Haliotis is, however, not indicated only by the singular arrangements described above. In that portion of the mantle which is adherent to the shell, and which forms a sort of border to the posterior and lateral parts of the body, arterial vessels seem to be altogether wanting, the whole circulation being apparently carried on by vessels which receive venous blood, derived immediately from the abdominal cavity, to which they partially return it, but at the same time convey a portion thereof into the branchio- cardiac vessels in the immediate vicinity of the heart. The fibrous tissue wherein these vessels are enclosed seems but little calculated to perform the functions of an accessory respiratory appa- ratus ; so that it would appear, from this anatomical arrangement, that all the blood in progress towards the heart is not submitted to the in- fluence of the air, and that it is a mixture of venous and arterial blood that is distributed by the heart throughout the arterial system. (1427.) Lastly, it may be noticed that in the cephalic region, where the diiferent organs are in immediate contact with the arterial blood, no traces are discernible either of veins or of Iacuna3 serving to return the blood thus effused to the respiratory apparatus, whereas in other parts of the body venous canals are met with, the disposition of which is very remarkable : these all communicate freely with the abdominal cavity, as is the case in other Gasteropod Mollusca ; but in the liver, the generative glands, and more especially in the urinary apparatus, they nevertheless form true vessels, the ramifications of which are ex- tremely numerous. (1428.) In Patella, or the Limpet, the size of the cephalic sinus that receives blood from the aorta is even more remarkable than in the Haliotis : in the Patella, indeed, the tongue is not itself lodged in the aorta, as in the former case, but is enclosed in a membranous sheath ; the sheath, however, in its turn becomes part of an arterial chamber, into which the aorta empties itself. The aorta itself gives off very few branches, while from the lingual sheath arise all the principal arteries of the body. (1429.) The arterial blood fills not only the sheath of the tongue, but is likewise diffused throughout the whole cephalic cavity, where it bathes the muscles and nerves in the same manner as in Haliotis ; but the extent of the sanguiferous sinus is much more considerable than in that mollusk. If, indeed, the capacity of these sinuses be estimated, WATER-CANALS. 541 they will be found to contain more blood than all the rest of the arterial system put together. (1430.) 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 COISTCHIFERA, slight modifications are met with. Thus, in Chiton (fig. 263), 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 branchia? ; and, what is still more remarkable, each auricle would seem to communicate with the ventricle by two distinct orifices. In Haliotis, Fissurella, and others of the Scutibran- chiate 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. - (1431.) In Pterotrachea (fig. 269), the branchiae (d) 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 (wi), which traverses the body as far as the head. (1432.) Independent of the ordinary vascular system, Delle Chiaje discovered the existence in most Gasteropods of a system of water- vessels largely distributed throughout the substance of the foot and other parts of the body. Thus, in the anterior part of the foot of the Muricidae*, there are to be seen certain holes or antra, which are the apertures to as many little cavities lying underneath, and which per- meate the interior substance of the foot. There are, besides, between these cavities slender canals communicating with the same orifices, by means of which the whole are connected and inosculated together. The water entering the body through the siphuncle is thus, at the will of the animal, driven into the substance of the foot, which is in this way rendered turgid and firm ; and when necessary, by a strong pressure, the fluid is ejected, or is spontaneously discharged after death, when the foot becomes flaccid and extenuate. Opinions relative to the use of the water thus freely admitted into the body of the Mollusca are various ; its principal object, however, seems to be to enlarge and moisten the structures over which it is distributed. (1433.) 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 . * Delle Cliiaje, Anim. senza Vert. d. Nap. ii. p. 204. 542 GASTEROPODA. most important modifications, it would be easy to overwhelm the most patient reader with accumulated details. (1434.) 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 de- scribed ( 1380). (1435.) The second form of mouth that, for instance, of Pleuro- brancJiiis (fig. 267, a) and of Pterotrachea (fig. 269, b) consists of a simple muscular proboscis, or fleshy tube, which is capable of consider- able elongation and contraction : such an oral apparatus is entirely devoid 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. (1436.) 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 Hombergii, re- presented in the annexed figure (fig. 272), whereof Cuvier gives the following graphic de- Flg> ^ scription*. In this animal the mouth forms a large oval and fleshy mass enclosing the jaws and 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 ; 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 Mouth of Tritonia Hombergii. differ, however, in the following particulars : 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. (1437.) 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. (1438.) 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. (1439.) The fourth and most complicated form of mouth is found * Memoire sur le Tritonia. PROBOSCIS OF BTJCCINUM. 543 Fig. 273. 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 descrip- tion of this unique apparatus*. (1440.) The proboscis of Bucdnum is organized with marvellous artifice. It is not simply provided, like that of the elephant, with the means of flexion and extension, joined with a limited power of contrac- tion and elongation, but it can be entirely retracted into the body by drawing itself into itself in such a manner that the 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 remains more or less folded upon itself. : tf (1441.) It may be represented as being composed of two flexible cylinders, one contained within the other, as shown in the annexed figure (fig. 273), the upper edges (i i) of the two cylinders being continuous in such a manner that by drawing out the inner cylinder (b b) it becomes elongated at the expense of the other, and on pushing it in again it be- comes shorter, while the outer cylinder (&) is lengthened by adding to its upper margin. (1442.) The reader must now imagine a multitude of longitudinal muscles (d d), all very much divided at both their extremities, and attached 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 pro- boscis (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. (1443.) When thus retracted, a great part of the inner surface of the internal cylinder (6) will necessarily become a portion of the external surface of the outer cylinder (fc), and the contrary when the proboscis is protruded. It is in consequence of this that the insertions of the muscles (d d) vary in position. (1444.) The protrusion of this proboscis is effected by the action of the circular muscles that form its walls. (1445.) When the proboscis is extended, the retractor muscles * " Memoire sur le grand Buccin (Bucdnum undatum}, et sur son Anatomic." Proboscis of Bucdnum. 544 GASTEROPODA. 274 - (fig. 273, d d), if they do not act altogether, serve to bend it in any direction, thus becoming the antagonists to each other. (1446.) In the internal cylinder are contained the tongue, with all its apparatus (e e), the salivary ducts (/), and the greater part of the oesophagus () is of considerable 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 joins the canal leading from the uterine cavity to the exterior of the body, which likewise receives the secretion of two small glandular sacs (&) apparently destined to furnish some investment to the eggs prior to their expulsion. (1470.) The male parts are situated in the general cavity of the body, quite apart from the female apparatus. The testicles seem to be repre- sented by two wavy casca (fig. 269, i), which terminate at the root of a small intromittent organ (s) placed at a short distance behind the open- ing of the vulva. (1471.) All the Tectibranchiata, Infer obranchiata, 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 composing these extensive orders would scarcely answer any useful purpose, even were it practicable within the limits of this work. (1472.) In the male Patella, the testicle is situated upon the right side of the body, between the visceral mass and the external envelope. It is of a pale-yellow colour, with a slight pinkish tint, and seems to be entirely made up of minute tubes, many times folded upon themselves, and imbedded in a granular-looking substance. On cutting into the substance of the testicle, there flows out a milky fluid, which the micro- scope reveals to contain innumerable spermatozoa, whose movements are very active as long as the seminal secretion is fresh. (1473.) The ovary of the female occupies nearly the same situation EMBEYO OF NUDIBEANCHIATA. 551 as the male testis ; but all attempts to trace the excretory duct of either have as yet proved futile. (1474.) When the ova of the Nudibranchiate Mollusca are placed under the microscope soon after the extrusion of the spawn, each is seen to consist of a thin transparent case-membrane*, with a round smooth and opake body in its centre (the ovum proper), which is chiefly composed of minute cells enclosed in a vitelline membrane. These ova vary in size from -^ro^h to -g-g-g-th of an inch in diameter. A few hours after the extrusion of the spawn the yelk divides by progressive segmentation until the end of the fifth day, when the division of the cells appears to have reached its utmost limit and the vitelline mass has changed its shape, having become broader at one end, narrower at the other (fig. 276, 2). At the end of the sixth day no additional change takes Fig. 276. Development of the embryo of a Nudibranchiate Mollusk. 1. Gelatinous coil, in which the ova are imbedded. 2. A portion of the same, magnified. 4, 5. Embryos in different stages of growth. 6. Mature embryo when newly hatched, enclosed in a minute nautiloid shell. place in the external form of the ovum, but the cells into which it has divided continue to coalesce, and minute cilia become apparent on the upper surface of the broad extremity. On the eighth day it assumes the form represented at fig. 276, 3, its circumference becomes more or less translucent, and the external layer of cells seems to separate from the rest to form the commencement of the shell (fig. 276, 4), the cilia on the broad extremity become larger and more active in their movements, and traces are observed of the division of this end into ciliated disks : it is now entitled to the name of embryo. * This case-membrane is not the homologue of the ordinary egg-shell, seeing that it sometimes encloses two, three, four, or even five ova. 552 PTEEOPODA. (1475.) From the ninth to the eleventh day the ciliated disks become more developed, more separated from each other, and more moveable ; the largest of the four lobes of the body has arranged itself into stomach and intestine, in which occasional contractile movements may be seen. (1476.) The case-membrane previous to the escape of the embryo becomes gradually thinner, and at last either entirely disappears or is reduced to shreds, probably by the incessant strokes upon its inner sur- face of the long cilia of the ciliated disks during the active revolutions of the embryo round its interior. The embryo at the time of its libera- tion is provided with a shell somewhat resembling that of the Nautilus, from which it can protrude the anterior part of its body and retract it at pleasure (fig. 276, 6), swimming about actively in the surrounding water by means of its ciliated disks. (1477.) The spawn of other Gasteropods is deposited under diverse forms. In the marine species it is usually found attached to the surface of stones, shells, or sea-weed, the ova being connected with each other in long ribands or delicate festoons, which are sometimes extremely beautiful and curious. The Doris and Tritonia deposit their ova in long gelatinous bands, resembling beautiful frills of rich lace. In Aplysia the spawn resembles long strings of jelly, in which the ova are seen, varying in tint, so as to give different colours to different parts of the thread. In Helix and Bulimus the eggs are protected by a hard shell ; whilst in Buccinum, Valuta, Mure.v, and other marine species the ova are enveloped in membranous capsules, agglomerated together in large bunches. These capsules have been sometimes erroneously regarded as the eggs themselves ; they are, however, merely coriaceous envelopes, answering the purpose of the gelatinous coating that encloses the eggs of other species. Many of the Gasteropoda are exceedingly prolific : a single Doris will lay 50,000 eggs at a birth ; and when we take into account that all the individuals are prolific (the sexes being combined), and that each will produce spawn two or three times in a season, it is evident how vast must be the number of their progeny. CHAPTER XXIII. PTEEOPODA* (Cuvier). (1478.) NEARLY allied to the Gasteropods in their internal organiza- tion, but differing from them remarkably in the character and position of their locomotive apparatus, are the PTEBOPODA a class of mollusks of small dimensions, but met with in astonishing quantities, at certain seasons, in various parts of the ocean. So numberless, indeed, are * irTfpbv, a wing ; TTOVS, nodus, the foot. CLIO BOREALIS. 553 these little beings in those regions where they are common, that the surface of the sea seems literally alive with their gambollings ; 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, engulfs multitudes of such tiny victims, and hence derives the materials for its subsistence. (1479.) Several distinct genera of Pteropoda have been established by zoologists, and some important modifications have been detected in their organization, although in all of them the lateral ala3 form the in- struments of progression. (1480.) The Clio borealis, anatomized by Cuvier*, and more recently and completely investigated by Professor Eschricht of Copenhagen f, is one of the species best known, as well as most abundantly met with ; it is therefore by a description of this Pteropod that we shall proceed to introduce the reader to the general facts connected with the history of the animals under consideration. Fig. 277. Clio borealis ; represented at A in a state of repose, while B, C exhibit the various external appendages fully protruded : a a, wing-like oars ; b, hood retracted ; g, bladder-like organ ; A, penis ; k, tentacle ; o, globular protuberances ; s s, conical appendages. (1481.) The body of the Clio is about an inch in length, of an oblong shape, and terminating posteriorly in a point; while at the opposite extremity there is a little head supported upon a short neck, and * Memoire sur le Clio borealis. t Anatomische Untersuchungen iiber die Clione borealis, von D. F. Eschricht. Kopenhagen, 1838, 4to. 554 PTEROPODA. furnished with delicate retractile tentacles, apparently instruments of touch. The locomotive organs, as the name of the class imports, con- sist of two delicate wing-like appendages (fig. 277, 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 apparatus exactly comparable to the double-barrelled oar with which the Greenlander so dexterously steers his kajac, or canoe, through the very seas inhabited by the little Clio we are de- scribing. (1482.) The head of one of these animals is surmounted by various 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 (fig. 277, c, s), that to a superficial examiner might appear 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 further 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 ex- amined, a transparent cylinder, resembling the cell of a polyp, and containing within its cavity about twenty pedunculated disks, which may be protruded from the orifice of their sheath (fig. 278, 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 prehension perhaps unparalleled in the creation. (1483.) 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. 277, A). (1484.) 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. 277, c, fc), somewhat resembling the feelers of a Snail, are protruded at the will of the animal ; and by CLIO BOEEALIS. 555 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. (1485.) The mouth itself is described by Cuvier as being a simple triangular opening, resembling the wound inflicted by a trocar ; and in the solitary specimen at his disposal he did not succeed in detecting any dental structures. Eschricht, however, with superior opportunities, was more successful in displaying the oral organs, and found the Clio to possess jaws of very singular conformation, and a tongue covered, as in many other Mollusca, with sharp horny spines. (1486.) One of the jaws removed from the body, and magnified twenty-eight diameters, is represented in the subjoined figure (fig. 278, A). It consists of a series of sharp horny teeth of unequal length, Fig. 278. B A A, one of the jaws of Clio borealis. B, the tongue, with its recurved spines. C, cylinder enclosing the prehensile suckers. fixed to the sides of a lateral pedicle in such a manner that their points are all nearly at the same level. The teeth themselves 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, appears to be made up of a multitude of regularly-disposed fibres that cross each other in two principal directions. (1487.) 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. 556 PTEEOPODA. (1488.) The manner in which the Clio uses these dental organs is obvious from their anatomical position. The curved muscular cylinders 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. (1489.) 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 upper surface of which is seen, when highly magnified, to be covered with regular rows of spiny booklets, all directed backwards, and evi- dently intended to assist in deglutition (fig. 278, B). (1490.) The structure of the alimentary canal is extremely simple. The oesophagus (fig. 279, t) gradually dilates into a wide stomachal cavity that is surrounded on all sides by the mass of the liver ; while the intestine (v), 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 satisfactorily 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. (1491.) With respect to the real nature of the respiratory apparatus in Clio, 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 micro- scope, present a network of vessels so regular, so close, and so delicate, that it is not possible to doubt that they are intended to perform the functions of a respiratory apparatus, adding, moreover, that their con- nexion with the internal vessels and the heart confirms this view of the nature of these membranes. (1492.) 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. (1493.) 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 (fig. 279, m), and gives off at one extremity a large vessel (m), 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. (1494.) The nervous system of this mollusk is easily distinguished, not only on account of the large proportionate size of the ganglia, but CLIO BOEEALIS. 557 from the circumstance of the nerves being of a pale-red colour. The ganglia form a ring placed around the oesophagus 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. (1495.) 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 addition to these, organs of vision are provided, apparently of a very complete cha- racter. 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. (1496.) The generative system of Clio resembles in all essential particulars that of the most highly organized Fig. 279. Gasteropoda, and, as in them, is composed of a complete set of male organs as well as of ovi- gerous viscera. Accord- ing to the views which Cuvier was led to enter- tain from the dissection of a single specimen, he supposed that the ovary (fig. 279, n) gave off a slender oviduct (o) ter- minating in a thick glan- dular canal, the testicle (&), which, beginning by a csecal prolongation, and gradually diminish- ing 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 Viscera of Clio borealis: mm, the heart, giving off a large vessel ; t, oesophagus ; v, intestine ; w, salivary glands ; n, ovary ; o, oviduct ; k, testicle ; p, its excretory canal ; q, bladder-like organ. 558 PTEROPODA. French anatomist pointed out another vesicle (r), which he compared to the bladder (spermaiheca) of Gasteropod Mollusks. The more complete researches of Professor Eschricht have, however, rendered considerable modifications of the above description requisite, inasmuch as that gentle- man has succeeded not only in detecting a testis quite distinct from the ovigerous canal, but also a very complete intromittent apparatus. The testis, in fact, in a fresh specimen is so large as to occupy a great por- tion 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 (fig. 279, i) which he thought to be entirely made up of the liver. The duct from this testis communicates with the re- ceptacle (q) ; so that the glandular canal (&) must be regarded as a part of the oviduct analogous to what has been called the uterus in the Snail. (1497.) Another important discovery for which science is indebted to the Danish Professor is that the Clio possesses a long and sin- gularly-formed penis (fig. 277, c, h) , lodged, when retracted, in the interior of the head of the Pteropod, but which, together with the bladder (#), 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. (1498.) 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 semitransparent skin (fig. 279, /) of soft texture ; and within this is a second covering (#), thicker than the first, and exhibiting very distinct muscular fibres, principally distributed in a longitudinal direction, so that their action would seem to shorten the animal and make its shape more spherical. (1499.) What fills up the space that intervenes between the mus- cular 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. (1500.) The other genera included in this class agree in their general form, and in the arrangement of their digestive and reproductive organs, with Olio above described, but present a few important modifications in the disposition of their branchiae, and other minor circumstances. (1501.) In Hyalcea the mantle contains a shell composed of two un- equal plates, one of which is dorsal, and the other ventral ; and the branchiaB, which are here distinctly recognizable, form a circle 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. CEPHALOPODA. 559 (1502.) In Pneumodermon, again, the branchiae occupy a totally dif- ferent 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, and our space warns us that we have yet to encounter forms of life widely different from any that have hitherto fallen under our notice. CHAPTER XXIV. CEPHALOPODA* (Cuvier). (1503.) WE now arrive at the highest order of MOLLTTSCA, composed of animals distinguished by most strange and paradoxical characters, and exhibiting forms so uncouth, that the young zoologist 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. (1504.) Let him conceive an animal whose body is a closed bag, con- taining the viscera connected with digestion, circulation, and reproduc- tion, 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 loco- motive 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. (1505.) The Octopus vulgaris, or common Poulpe, represented 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 delineated, his curiosity would doubtless be excited to learn something of its habits and economy. (1506.) Yet not only can the Poulpe walk in the manner exhibited in the subjoined figure (fig. 280), but it is well able to swim, if occasion 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 progres- sion; for, by vigorous flappings of this extensive organ, the animal actively impels itself through the water in a backward direction and shoots along with wonderful facility. (1507.) The feet or tentacula appended to the head are not, however, exclusively destined to effect locomotion : they are used, if required, as * Kea\ri, the head ; TTOVS, TTO^OS, the foot. 560 CEPHALOPODA. agents in seizing prey, and are of so terrible a character, that, armed with these formidable organs, the Poulpe becomes 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 the lower surface of every one of the eight flexible arms. If the Fig. 280. The Poulpe (Octopus vulgaris). Poulpe but touch its prey, it is enough : once a few of these tenacious suckers get firm hold, the swiftness of the fish is unavailing, as it is soon trammelled on all sides by the firmly-holding tentacula 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. (1508.) In the genus Octopus the arms are only eight in number, and nearly of equal length; but to the Calamaries (Loligo) and other genera STRUCTURE OF TENTACLES. 561 Fig. 281. an additional pair is given, which, being prolonged considerably beyond the rest, are not merely useful for seizing prey at a distance, but become convertible to other purposes, and may be employed as cables whereby the Cephalopods so furnished ride securely at anchor in a tempestuous sea, the suckers being placed upon an expanded disk, situated (fig. 282) 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 (fig. 282), evidently adapted to assist in sculling the animal along. (1509.) Wonderful as are the provisions above described for ensuring food and safety to these formidable inhabitants of the sea, it is only by an attentive examination of the individual suckers, so numerously dis- tributed over the tentacula, that the reader will fully appreciate the mechanism we are so inadequately describing. Machines of human con- struction 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 com- parison are merged in the superlative ; everything is best, completest, perfect. (1510.) Examine any one of these thousand suckers : it is an admirably arranged pneumatic apparatus an air-pump. The adhesive disk (fig. 281, A) is composed of a muscular membrane, its circumference 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 (B, 6), at the bottom of which is placed a prominent piston (c), that may be retracted by muscular fibres provided for the pur- pose. No sooner therefore is the cir- cumference of the disk placed in close and air-tight contact with the surface of an object, than the muscular piston is strongly drawn inwards, and, a vacuum being thus produced, the ad- hesion of the sucker is rendered as firm as mechanism could make it. (1511.) Yet even this elaborate and wonderful system of prehensile organs would seem, in some cases, to be insufficient for the purposes of 2o Structure of the tentacular suckers in the Cephalopoda. 562 CEPHALOPODA. nature. In the powerful and rapacious Onychoteuihis (fig. 282), the cupping-glasses which arm the extremities 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. Fig. 282. I I A B Onychoteuthig, showing the structure of the arms. Nor is this all that claims our admiration in the organization of the arms of Onychoteuihis : at the base of each fleshy expansion that supports the tenacious and fanged suckers above described is a small group of single adhesive disks, by the assistance of which the two arms can be AKGONAUT. 563 locked together (fig. 282, A), and thus he made to cooperate in dragging to the mouth such powerful or refractory prey as, singly, the arms might be unahle to subdue an arrangement which has been rudely imitated in the construction of the obstetric forceps*. (1512.) The Argonaut constitutes the type of another family of the CEPHALOPODA, and is remarkable as being the inhabitant of a shell of Fig. 283. Argonaut (Argonauta Argo). (After Poli.) 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. (1513.) 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 * Cyclopaedia of Anatomy and Physiology, art. CEPHALOPODA. 2o2 564 CEPHALOPODA. the little barque in which the Argonaut was supposed to skim over the waves, hoisting a pair of sails to the breeze, and steering its course by the assistance of oars provided for the purpose. (1514.) The figure annexed (fig. 283), given by Poli in his magnifi- cent work already referred to*, was in perfect accordance with the generally-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 Argonaut, as we are told, draws in his arms, lowers his sail, and, settling to the bottom of his shell, disappears beneath the waters. (1515.) It is a thankless office to dispel the pleasant dreams of imagi- nation ; yet such becomes our disagreeable duty upon this occasion. M. Sander Rang, in a recently-published memoir upon this subject f, has, from actual observation, apparently established the following facts : 1st, that the belief, more or less generally entertained 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 mem- branes have no other function than that of enveloping the shell in which the animal lives, and that for a determinate object to be ex- plained 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 infundibuliform disk, formed by the junction cf the arms at their base, and presenting (alas !) the appearance of a Gastero- pod mollusk. (1516.) 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 reputa- tion of being an active and skilful navigator, it has been very generally stigmatized 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 residence precisely corresponding in dimen- * Testacea utriusque Siciliae. f Gue"rin's ' Magasin de Zoologie.' Translated in the Magazine of Natural History, vol. iii. new series, p. 521. NAUTILUS POMPILIUS. 565 sions with those of the possessor. The apparent want of resemblance between the outward form of the animal (fig. 285) and that of its fragile covering, together with the absence of any muscular connexion between the two, were looked upon as furnishing sufficient evidence of its parasitical habits. The careful observations of Madame Jeannette Power, to be noticed more at length hereafter, and those of M. Sander Eang, 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 satisfac- torily understood. (1517.) A still more interesting group of CEPHALOPODS, and one which in former periods of the world has been extensively disseminated, in- habited chambered shells. But of all the varied forms of these crea- tures, whose remains are so abundantly met with in a fossil state, and known by the names of Ammonites, Belemnites, Nummulites, &c., two Fig. 284. Animal of the Nautilus Pompilius. (After Owen.) 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 * For this invaluable addition to zoological knowledge, science is indebted to George Bennett, 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 566 CEPHALOPODA. subject of a monograph by Professor Owen, who has most completely investigated its general organization and relations with other families of the Cephalopoda. The shell of the Pearly Nautilus (N. Pompilius) is extremely common, and may be met with in every conchological collection, notwithstanding the extreme rarity of the mollusk that inhabits it a circumstance, perhaps, to be explained by the fact that the 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 (fig. 284, s s), in the last of which the body of the animal is situated. A long tube or siphuncle (h h), partly calcareous and partly membranous, passes through all the compartments 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 communication with the exterior. (1518.) 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 or rarefied. Should this suppo- sition 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 contained 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. (1519.) 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 infundibulum (i) the ova and excrementitious matters are expelled from the body. The most re- not far distant from the ship, and resembling, as the sailors expressed it, a dead tortoiseshell 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 sinking when caught." (Dr. Bennett's Journal.) * Philosophical Experiments and Observations. 8vo, 1726. EXO-SKELETON. ENDO-SKELETON. 567 markable feature, however, exhibited in the external conformation of Nautilus is the conversion of the sucker-bearing arms of other Cepha- lopoda into an elaborate apparatus of tentacular organs appended to the head (o o) ; but these, as well as the eye (m), will be more minutely described as we proceed. (1520.) 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. 285, a), through which, as through a fleshy pipe, the products of excretion, as well as the eggs or seminal fluid, are ejected, is formed 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. (1521.) In those natatory species which, like Loligopsis, or Onycho- teuthis (fig. 282), 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. (1522.) One important circumstance observable in the class before us must not be forgotten in connexion with this portion of the history of the Cephalopoda. We may remind the student that, in the verte- brate division of animated nature, to which these creatures immediately lead us, the locomotive system is supported 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, extravascular shells, formed by successive depositions of layers of calcareous material, or jointed cuticular armour equally incapable of growth, having as yet repre- sented the skeleton, and formed the only levers upon which the muscular system could act in producing the movements connected with loco- motion. (1523.) Having, however, already had abundant opportunities of seeing how gradually nature proceeds in effecting the development 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 568 CEPHALOPODA. animals possessed of an internal bony framework ; and our expectations 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 system 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 gan- glionic 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 muscular system in different regions of the body. (1524.) 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 embraces the oesophagus with a cartilaginous ring, encases the brain, affords passage to the optic nerves, and gives off orbital plates for the protection of the eyes. The cranial cartilage likewise gives a firm origin to the muscles of the locomotive tentacola 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, therefore, it claims to be con- sidered 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 append- ages of that description, offer, undoubtedly, the rudiments of those por- tions of the skeleton that sustain the locomotive limbs of quadrupeds. (1525.) 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. (1526.) On slitting up the mantle of a Calamary (Loligo) along the mesial 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 of the gladius as the growth of the animal proceeds. Several of these plates may be produced in suc- cession ; 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 connexion whatever with CUTTLE-BONE. 509 the soft parts of the Calamary, to which, in fact, they are so little ad- herent that they fall out as soon as the sac wherein they are secreted is laid open. (1527.) In the Cuttle-fish (Sepia officinalis) the dorsal plate (os Sepice) is found in the same situation as the gladius of the Calamary, from which, however, it differs remarkably both in texture and com- position. 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 assistance might be derived from lightness and buoyancy. Did a creature so apparently destitute of natatory organs possess a swimming-bladder like that of a fish, to assist in supporting it in the water, we should conceive such an ap- paratus to be far more adapted to its predatory habits than a shell so bulky as that which it is destined to carry. (1528.) We have, however, already seen, in the case of the Nautilus, 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 vertical laminae of the same substance, having, from the tortuosity of their meanderings, the appearance of millions 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 Sqoia, having no connexion whatever with the sides of the cavity wherein it is placed, being so loosely adherent that it readily falls out on opening the sac. (1529.) The cuttle-bone is formed in the same manner as other 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 resemble 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. (1530.) We now come to consider the long-disputed question relative 570 CEPHALOPODA. to the nature of the shell of the Argonaut. The Poulpe that inhabits the elegant abode represented in a preceding figure (fig. 283), when removed from its testaceous covering, has the general form of an Oc- topus. Its body (fig. 285) is enclosed in an ovoid muscular sac (d) ; and the head is surmounted by eight long sucker-bearing arms, of which six (e, f) taper gradually from their origins to their extremities, while the other two, formerly regarded as sails, and which we shall continue to designate by their ordinary name, vela, expand into broad membranes (b). Fig. 285. Animal of the Argonaut out of its shell: a, the siphon; 6, the so-called vela; c, the head; d, the body ; e,f, locomotive tentacula. (After Poli.) (1531.) M. Sander Rang, who, during a residence at Algiers, had ample opportunity of studying the living Argonaut, ascertained that in the figure copied from Toll, which we have given in a preceding page, SHELL OF THE ARGONAUT. 571 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 (fig. 286), 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 retro- grade motion, which is sometimes very rapid. Its usual movements are, however, confined to crawling at the bottom with its head down- wards ; and in this way it creeps, carrying its shell upon its back. (1532.) The reader will obtain a better idea of the real appearance of the Argonaut in its shell by inspecting the annexed copy of M. Hang's figure than from any verbal description, and we borrow that gentle- man's own account of its general appearance *. The membranous por- Fig. 286. Argonaut. (After M. Sander Eang.) tions of the expanded arms, dilated beyond anything 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, * For more ample details upon this subject, the reader is referred to an excellent translation of M. Eang's paper contained in Mr. Charlesworth's Magazine of Natural History, new series, vol. iii. 572 CEPHALOPODA. and consequently the keel. The application of these membranes is direct, and without 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 and their membranes, so as partially to uncover the shell in front, as is repre- sented in the figure (fig. 286). (1533.) There is little doubt that the vela of the Argonaut, which thus envelope its abode, are the organs employed in constructing the brittle fabrics, and the agents whereby fracture and wounds in the shell are repaired and filled up. (1534.) The positive experiments of Madame Power* leave no doubt upon the subject ; for not only did that lady, by rearing young Argo- nauts 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 accom- plished, 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 lamella had become thickened to a certain degree and more opake; 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 mending the fractures, the Argonaut applied its vela to the exte- rior 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. (1535.) In order to understand the manner in which the remarkably- 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 suc- cessive layers to the margin 6f 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 (fig. 284, {) is situated, the gradually- expanding shell naturally revolves around an excentric axis. While the growth of the shell continues, the animal is constantly advancing forwards, and thus leaves the first-formed portions of the * Magazine of Natural History, April 1839, " Observations on the Poulpe of the Argonaut," by Madame Jeannette Power. MOUTH OF THE CUTTLE-FISH. 573 Fig. 287. shell unoccupied. At intervals, as the Nautilus thus removes itself further and further from the bottom of its abode, that portion of its mantle which covers the general surface of its visceral sac (fig. 284, a) secretes floors of shelly substance behind it ; and thus the septa (s s) 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. 284, , 6, 7, 8, 9, nerves supplying the retrac- tile tentacula; 10, nerves supplying olfactory organ ; 11, 12, 13, nerves supplying the muscular integument ; 14, 15, 16, nerves representing the sympathetic system. nerves, those supplying the external labial tentacles being derived im- mediately from the brain (fig. 291, 6 6), while those distributed to the SENSE OF SMELL. 589 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 (V). (1578.) 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 individual suckers appended to them, are highly sensitive to tactile impressions. Every one of the arms receives a large nerve, derived from the same portion of the O3sophageal 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. During this course the nerve becomes slightly dilated at short distances, and gives off from each enlarge- ment numerous small nervous twigs which penetrate into the fleshy substance of the foot. Immediately after entering the arm and under- going the dilatation 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 contiguous 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. (1579.) There is little doubt, from the character of the soft and papillose membrane which forms a considerable portion of the surface of the tongue, that both in the Nautilus and in the Dibranchiate Cepha- lopods the sense of taste is sufficiently acute far superior, 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. (1580.) 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 per- ception of odours, well deserving the attention of the physiologist. We may here premise that the exercise of this function in creatures con- tinually immersed in water must depend upon conditions widely differ- ing 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 * Cuvier, Memoire sur la Poulpe, p. 36. 590 CEPHALOPODA. inspiration, along the nasal passages, and, being thus examined with a minuteness 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 par- ticles must necessarily be extremely slow, and the power of perceiving their presence comparatively of little importance, 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. (1581.) In Nautilus, the part indicated by Professor Owen* as ap- propriated to the sense of smell consists of a series of soft membranous laminae (fig. 289, I \ fig. 291, h) compactly arranged in a longitudinal direction, and situated at the entry of the mouth, between the internal labial processes. These laminaa 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 towards the sides. They are supplied by nerves (fig. 291, 10) from the small ganglia (8) which are connected with the ventral extremities of the anterior suboesophageal ganglia, and from which the nerves of the internal labial tentacula are likewise given off. (1582.) The structure of the eyes in the two divisions of the CEPHA- LOPODA differs remarkably, and in both is so entirely dissimilar from the usual organization met with in other classes of animals, that we must invite the special attention of the reader to this portion of their economy. (1583.) In the TETBABBANCHIATA, of which the Nautilus is the only example hitherto satisfactorily investigated, according to Professor Owen's observations t the eye appears to be reduced to the simplest condition that an organ of vision can assume without departing alto- gether 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 obsctira, and a nerve of ample size is appropriated to receive the impression, yet the parts which regulate 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 OwenJ, the Nautilus approximates the Gasteropods, numerous genera of which, and especially the PECTINIBBANCHIATA of Cuvier, present ex- * Mem. on Nautilus, p. 41. t Loc. cit. p. 39 et seq. J Op. tit. p. 51. ORGANS OF VISION. 591 amples analogous 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 locomotive instruments of the Cephalopods, the anatomist whose re- marks we quote hence deduces the more immediate principle of their reciprocal inferiority with respect to their visual organ, observing 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. (1584.) The eyes of Nautilus (fig. 284, m) are not contained in orbits, but are attached each by a pedicle to the side of the head, im- 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 naturally 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 pro- bably 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 direc- tions ; 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 extended to the utmost by enlarging the pupillary aperture. (1585.) The principal tunic of the eye is a tough exterior membrane or sclerotic (fig. 291), 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 whatever 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 592 CEPHALOPODA. consequence freely admitted the preserving liquid into the interior of the globe. (1586.) However much is still left to be ascertained by future ob- servations, 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 every indication of inferiority of construction when compared with that of the Dibranchiate tribes. Encased in no orbital cavity, and conse- quently unprovided with any other muscular apparatus than the fleshy pedicle whereby it is connected with the head unprotected by eyelids and devoid of lacrymal appendages without either transparent cornea, aqueous humour, iris, or crystalline lens and, moreover, coated inter- nally 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 ob- server who studies on a large scale this part of the animal economy. (1587.) The eyes of the Dibranchiate Cephalopoda are not less re- markable in their construction than those of the Nautilus, and from their greater complexity will require a more elaborate description. 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 common Cuttle-fish (Sepia officinalis), first, the orbit ; secondly, the globe of the eye ; thirdly, the chamber of the optic ganglion ; and fourthly, the muscles of the visual organ. (1588.) The orbit differs from that of all other classes of animals, 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 common fleshy integument of the body (fig. 292, 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 connexion with the eye-ball itself. Beneath the cornea the integument again becomes opake, and forms a thickened fold (a), which might be considered as the rudiment of an under eyelid. The orbit therefore forms a complete capsule, en- closing the whole of the apparatus of vision. (1589.) 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 prominent * Descriptive and Illustrated Catalogue of the Physiological Series of Compara- tive Anatomy contained in the Museum of the Koyal College of Surgeons of England, vol. iii. part 1. pi. 52. EYE OF THE CUTTLE-FISH. 593 surface of the crystalline lens (o) is found quite naked beneath it, neither an aqueous humour, nor an iris properly so called, being pre- sent. The outer coat of the eye (g g, i) represents the sclerotic tunic in Man : it is tough, fibrous, and of a silvery lustre ; perforated ante- riorly by a large round aperture, representing that which contains the cornea in the human eye, and pierced posteriorly by numerous foramina, through which the multitudinous branches derived from the optic gan- glion (&) enter. Fig. 292. Anatomy of the eye of the Cuttle-fish. (After Cuviev.) (1590.) 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 off from the optic ganglion (&), having penetrated into the interior of the eye through the cribriform sclerotic, immediately expand into a thick nervous mem- brane which lines the sclerotic tunic, and is continued forward to a deep groove in the substance of the crystalline 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. (1591.) 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 presented by this hitherto incomprehensible and anomalous arrangement, 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 retina of vertebrate animals. " In the eyes of 594 CEPHALOPODA. different Sepice which we had immersed in alcohol preparatory to dis- section, we have, however, invariably found, between the pigment and the hyaloid coat, a distinct layer of opake white pulpy matter, of suffi- cient consistence to be detached in large flakes and easily preserved and demonstrated in preparations. We confess, however, that we can discover no connexion between this layer and the thick nervous expan- sion behind the pigment ; but nevertheless we cannot but regard it as being composed of the fine pulpy matter of the optic nerve, and as con- stituting a true pra3-pigmental retina*." (1592.) 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 Yertebrata, is of short focus and great power, being, in fact, not merely, as it is generally de- scribed, a double convex lens, which is the usual shape of this im- portant 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 opake matter. The lens of the Cuttle-fish is in like manner divided into two parts of un- equal size (fig. 292, 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 pene- trates deeply into the vitreous humour : the latter, enclosed in a deli- cate hyaloid membrane, fills up, as in Man, the posterior part of the eyeball ; while the small space that intervenes between the posterior surface of the crystalline and the back of the ocular chamber suffi- ciently attests the shortness of the focus of so powerful a lens. (1593.) 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 the great ganglion of the optic nerve (fc), imbedded in a mass of soft white sub- stance. This supplementary chamber is formed by a separation of the sclerotic into two layers, of which one, already described (i), forms the posterior boundary of the eyeball, while the other (h), passing back- wards, circumscribes the cavity in question. On entering the compart- ment thus formed, the optic nerve (q) dilates into a large reniform ganglion (k\ almost equal in size to the brain itself ; and from the peri- * Cyclopaedia of Anatomy and Physiology, art. CEPHALOPODA. EYE OP THE CUTTLE-FISH. 595 phery of the optic ganglion arise the numerous nervous filaments which, after perforating the posterior part of the globe of the eye, expand into the post-pigmental retina. (1594.) Between the globe of the eye (g) and the cornea (f) is a capacious serous cavity, which extends to a considerable distance to- Fig. 293. Loligopsis Verani. wards 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, evidently forming an 2Q2 596 CEPHALOPODA. arrangement for facilitating the movements of the eye. The serous membrane which lines this cavity, after investing 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 eyeball, 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 even passes over the anterior surface of the crystalline. This serous mem- brane Cuvier, very improperly, named the "conjunctiva"; but, as Pro- fessor Owen has suggested*, it is evidently rather analogous to the membrane of the aqueous humour, here excessively developed in con- sequence 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 intervention of a minute orifice visible in the transparent tegumentary cornea. (1595.) 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 prolonga- tions of the cranial cartilage, and are inserted into the sclerotic tunic. (1596.) It is always interesting to the physiologist to observe the earliest appearance of a new system of organs, and witness the gradual development 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 struc- ture of the human ear are not a little curious. In the simplest aquatic forms the central portion of the internal ear alone exists, imbedded in the as yet cartilaginous cranium. Gradually, as in fishes, semicircular canals, prolonged from the central part, increase the auditory surface, but still have no communication with the exterior of the body. In rep- tiles 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 collecting and con- veying sound to the parts within, complete the most complex and per- fect form of the acoustic instrument. (1597.) 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 condition of a localized apparatus adapted to receive sounds. (1598.) In the anterior and broadest part of the cartilaginous cranium t, where its walls are thickest and most dense, are excavated * Cyclopaedia of Anatomy and Physiology, loc. cit. p. 552. t Cuvier, M6moire sur la Poulpe, p. 41. AUDITOEY APPARATUS. 597 two nearly spherical cavities (fig. 294, d), which in themselves repre- sent the osseous labyrinth of the ears. A vesicle or membranous Fig. 294. Brain and auditory apparatus of the Cuttle-fish : a, 6, brain ; c, auditory apparatus ; d, cavity in which it is lodged; e,f, g, the eye. 1, 2, 3, otolith. sacculus (c), likewise Fig. 295. nearly of a spherical form, is suspended in the centre of each of these cartilaginous cells by a great number of filaments, that are pro- bably minute vessels. The two auditory nerves derived from the euce- phalon enter these ca- vities through special canals ; and each, divi- ding into two or three branches, spreads out over the vesicle to which it is destined. The au- ditory vesicle itself is filled with a transparent glairy fluid, and con- tains, attached to its posterior part, a minute otolith (1, 2, 3), of vari- able shape in different genera, the oscillations of which doubtless in- crease the impulses whereupon the production of sound depends. Generative organs of the female Cuttle-fish. (After Cuvier.) CEPHALOPODA. B (1599.) Such is the simplest form of an ear ; and if the reader will compare the organ above described with that possessed by the highest Articulata, as, for example, the Lobster ( 1047), the similarity of the arrangement will be at once manifest. (1600.) All the CEPHALOPODA are dioecious ; and the structure of the sexual organs both of the males and females is remarkable, inasmuch as it is peculiar to the class. (1601.) In the females, the ovarian receptacle is lodged at the bottom of the visceral sac (fig. 290, p, q), 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 bodies, attached by short vascular pedicles to a circumscribed Fig. 296. portion of its internal sur- face (fig. 295, a). These vesicles, the ovisacs or ca- lyces, as they are called by comparative anatomists, are, in fact, the nidi wherein the ova are secreted ; and if examined shortlybefore ovi- position commences, every one of them is seen to con- tain an ovum in a more or less advanced stage of de- velopment. In this condi- tion the walls of the ovi- sacs are thick and spongy ; and their lining membrane, which constitutes the vas- cular surface that really secretes the egg, presents a beautiful reticulate appear- ance. (1602.) If the contained ova be examined when nearly ripe for exclusion, each is found to be com- posed of a yelk 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 im- mediately with the interior of the ovarium by a wide orifice, the dimen- A. Generative organs of the female Cuttle-fish. B. A bunch of eggs. FEMALE GENEEATIVE SYSTEM. 599 Fig. 297. sions 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 ovigerous 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 men- tioned that secretes the ex- ternal horny covering of the egg a defence which seems to be deposited in successive layers upon the outer surface of the previously existing chorion, and, when com- pleted, forms a thick flexible case made up of concentric lamellae of a dark-coloured corneous substance. (1603.) After extrusion, the ova of the different fa- milies of CEPHALOPODA are found agglutinated and fas- tened together into masses of very diverse appearance. The eggs of the common Cuttle-fish, frequently found upon the shore, are not in- aptly 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 aggregated in large clusters, and fas- tened by long pedicles either to each other or to some foreign body (fig. 296). The Argonaut carries its eggs, which are compara- tively of small size, securely lodged in the recesses of its shell ; while the ova of the Calamary, encased in numerous long gela- tinous cylinders that conjointly contain many hundreds of eggs, are fixed to various submarine substances, and thus protected from casualties. 1. Male organs of the Cuttle-fish (Sepia officinalis), seen from before: a, tunic enveloping the testis; b, body of the testicle ; c, convolutions of the vas deferens; e, commencement of the Needhamian canal ; d, pouch of Needham. In fig. 4 the same parts are represented as seen from behind, the testicle being removed in order to show the commencement of the vas deferens. 2 and 3 represent the spermatophores of the same animal, much magnified : a, the external sheath; 6, inner cylinder containing the spermatic fluid; c, the ejaculatory apparatus. In fig. 3 the spermatophore is shown in the act of discharging its contents. GOO CEPHALOPODA. - 298 - 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. (1604.) Cuvier remarks that the male Poulpes must be less nume- rously met with than the female, as among the numerous specimens dissected by him scarcely one-fifth were of the former sex. (1605.) The various parts of the male generative apparatus are re- markably similar, both in structure and arrange- ment, to the correspond- ing portions of the sexual organs of the female. The testicle strikingly resembles the ovary both in its outward form and internal arrangement : like that viscus, it con- sists of a capacious mem- branous sac (fig. 298, 6); and on opening this there is found, attached to a small portion of its inner surface, a large bundle of branched ca3ca (a), in which no doubt the seminal fluid is ela- borated. These strangely disposed seminigerous casca 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 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 seminalis, the interior of which is divided by imperfect septa ; and, its texture being apparently muscular, this part of the excretory apparatus may possibly, by its con- tractions, expel the spermatic fluid from the body. On issuing from the seminal vesicle, the semen passes the extremity of an oblong gland (/), which Cuvier denominates the prostate : its structure is compact and granular; and it seems to be destined to furnish some accessory fluid Generative organs of the male Cuttle-fish. (After Cuvier.) MALE GENEEATIVE SYSTEM. 601 subservient to impregnation. Having passed the prostate, the ejacu- latory duet communicates with a large muscular sacculus (#), the con- tents 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 fila- ments when taken out, even long after the death of the Cephalopod, exhibit, when moistened, various contortions, and by some have been regarded as Etitozoa. (1606.) These remarkable spermatic filaments (the famous "filament machines " of JSTeedham) present, in fact, a very complicated structure. Their form varies in different species ; but in their essential composition they are all found to consist of a long tubular sheath (fig. 297, 2, 3), composed of two membranes, and enclosing a long tube, convoluted upon itself like an intestine, which is filled with an opake white fluid, in which are contained millions of zoosperms ; and the apparatus to which it is attached anteriorly constitutes an ejaculatory instrument, by the aid of which the spermatic secretion is forcibly ejected. These " spermato- phores" as they have been named by Milne-Edwards, serve as vehicles for the conveyance of the seminal fluid into the generative system of the other sex, notwithstanding the absence of any copulatory apparatus. (1607.) A most extraordinary modification of the male sexual organs is met with in the males of the Argonaut, Tremoctopus, and probably of other kindred genera, in which one of the arms is so strangely modified, both in its shape and structure, that Cuvier mistook it for a parasite, de- scribing it, under the name of ffectocotyles, as " a long, parenchymatous worm, compressed at the an- terior extremity, where the Fi g- 299 - mouth is situated, having its inferior surface furnished with suckers, from sixty to a hundred in number, arranged in pairs, and furnished with a SaCCuluS, Situated at the i, 2. Male Argonaut, of the natural size, repre- n/iafarW f^rtrPTYiitv of its sented in front and in profile, showing the sacculus posterior extrei in wMch the Hecfcocotylus is contained; from a body, which is filled With the specimen preserved in spirit. 3. Sacculus, in which ,,,, -jo.)? the Hectocotylus may be distinguished through folds Ot the OVldllCt. its fcran8parent walla . The specimen from which (1608.) The HectOCOtyluS this figure was taken having been preserved in , , . spirit, is, of course, contracted in all its dimen- Argonautce*, as this strange sions> appendage is still called, is, in fact, a portion of the Argonaut itself, developed in a remarkable sac, which supplies the place of the left arm of the third pair. The male Argonaut * Henri Miiller, Ann. des Sc. Nat, 1851. 602 CEPHALOPODA. (fig. 299, l, 2) is of very small size as compared with the female, being not more than an inch in length, and has no shell ; moreover the upper pair of arms are not, as in the female (fig. 283), expanded into velae, but are Fig. 300. 1. Sacculus of Argonaut laid open, showing the abnormal arm (^Hectocotylus) folded up in its interior ; magnified two diameters. 2. A portion of the Hectocotyliform arm, still further magnified: a, the sacculus laid open; 6, the arm, showing the commencement of the "lash," and also the suckers, nervous ganglia, &c. pointed. The sac above alluded to, on being opened, is invariably found to contain a solitary Hectocotylus, the dilated portion of which is attached at its base, whilst the rest of this remarkable organ is free and rolled up towards that side upon which the suckers are situated ; but as soon as the sac is opened, or when it is ruptured by the movements of the con- tained Hectocotylus, the latter unfolds itself (fig. 301, a), assuming so exactly the appearance of a parasitic animal that the mistake committed by Cuvier is by no means surprising. The Argonaut itself possesses a well- developed testis, according in its structure with that of ordinary Cuttle-fishes, and which contains spermatozoids in different stages of development; but its excretory duct terminates in the Hectocotylus, which is evidently nothing more than one of the arms of the Argonaut thus strangely developed. But, what is stranger still, this arm, arrived at maturity, detaches itself from the Argonaut, and from that moment enjoys an independent existence. It lives adherent to a female Argonaut, which it impregnates by a real coitus ; so that in this respect, as well as by its movements, and by the length of its life after its detachment, it might be mistaken for a complete male animal. Still it cannot be re- garded as an independent being, seeing that it has no alimentary appa- ratus ; neither is it the organ whereby the seminal fluid is produced, but simply a vehicle whereby the male secretion is transported. Well indeed does it deserve the epithet bestowed upon it by Cuvier, who, ignorant of its real nature, pronounced it " un ver bien extraordinaire !" (1609.) From the pouch of Needham a short canal leads to the penis (h), a short, hollow, muscular tube, through which the fecundating fluid is expelled. DEVELOPMENT OF EMBEYO. 603 (1610.) Although we mean to defer any minute account of the de- velopment of the embryo in ovo until an examination of the eggs of ovi- parous Vertebrata shall afford more ample materials for elucidating 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 Fig. 301. Fig. 302. A male Tremoctopus, seen from the ventral aspect. The visceral sac has been laid open and the left half of the mantle turned aside, to show the branchia and the opening of the generative apparatus during the expulsion of the spermatophore. a, sacculus containing the Hectoco- tylus ; b, branchia of the left side, with its branchial heart, c; d, the " bottle," from which the spermato- phore (e) is in progress of expul- sion; //, the mantle. Tremoctopus carina (male), showing the Hectocotylus (a) in its ordinary position. 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 preservation : 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. 303, 1), slowly make their appearance ; so that be- fore birth the little creature presents most of the peculiarities which characterize the species to which it belongs. But the most prominent feature that strikes the attention of the physiologist is the remarkable position of the duct communicating between the yelk 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 604 CEPHALOPODA. imperfect Sepia. This communication, which 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 oc- Fig. 303. 1234 Embryo of Cuttle-fish. cupies a very unusual situation, being inserted into the head, through which it penetrates, by an aperture situated in the front of the mouth, to the oesophagus, where it terminates (fig. 303, 3). (1611.) Among the many interesting phenomena presented by the CEPHALOPODA, few are more remarkable than the extraordinary power which these animals possess of continually changing their colour in con- formity with the varying tints of surrounding objects, affording a means of defence almost as efficient for concealment as the ejaculation of their inky fluid. It is indeed an extremely beautiful sight to witness the flickering hues of one of these animals, that seem to succeed each other with astonishing rapidity. In order to explain the cause of this very curious faculty, it is only necessary to examine with the microscope a portion of integument removed from the living animal, when it becomes at once apparent that the coloured layer of the skin is studded with innumerable pigment-cells (chromatojphores), filled with colours that exactly correspond with the hues of the creature's body, and which in- dividually possess a remarkable power of changing their shape and size, and thus modifying, by their contraction and expansion, the coloration of the integument*. (1612.) Interesting as the subject is, our space will permit us to ad- vert but very briefly to the important light which our comparatively recently acquired knowledge of the anatomy of the Cephalopod Mollusca has thrown upon the history of innumerable races of similarly con- structed beings long extinct, the remains of which are extensively distributed through a variety of geological strata. The fossil Ammonites for example, the nature of which was previously inexplicable, were at once rendered intelligible by the discovery and description of the Nau- * This wonderful faculty of changing their colours, dependent on a similar organi- zation of the rete mucosum, is possessed by fishes, frogs, and many other animals, as may be demonstrated by simply placing a few small trouts, gudgeons, or minnows in differently coloured earthen pans filled with clear water ; and the phenomenon is rendered still more conspicuous by suddenly transferring them from the lighter to the darker coloured vessels, and vice versa. AMMONITES, BELEMNITES, &c. 605 tilus Pompilius, to which in their structure they were evidently nearly related. The countless petrified remains known by the names of Hamites, Lituites, Orthoceratites, Cirthoeeratites, and other allied forms are still representatives of the existing Spirula (fig. 304), which, although its structure even at the present day is but imperfectly understood, is mani- festly a Cuttle-fish provided with a camerated shell constructed much after the same plan as that of the Nautilus ; and even the Belemnites, so long a puzzle and a mystery to geologists, when restored by the labours of Professor Owen, as represented in fig. 305, unmistakeably confesses its relationship to the Cuttle-fishes we have been describing. Fig. 305. Fig. 304. 1 . Animal of Spirula, exhibiting the shell in situ. 2. Transverse section, and, 3, lateral view of the shell re- moved from the body. Belemnite restored : a, ce- phalic tentacles ; b, siphon ; c, ink-bag ; d, e, section of shell. The outline exhibits the fin-like expansions of the integument. (1613.) 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 dif- ferent type, embracing the most highly gifted and intelligent occupants of the planet to which we belong. 606 VERTEBKATA. CHAPTER XXV. VERTEBRATA. (1614.) THE fifth division of the animal kingdom is composed of four great classes of animals, closely allied to each other in the grand fea- tures of their organization, and possessing in common a general type of structure clearly recognizable in every member of the extensive 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 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 occupied our attention, and gradually rising higher and higher in their attributes, until they conduct us at last to Man himself. PISHES, restricted by their organization to an aquatic life, are connected by amphibious beings, that present almost imperceptible gradations of development, with terrestrial and air-breathing REPTILES : these, pro- gressively 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 MAMMALIA is effected with equal faci- lity ; 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. (1615.) 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, nourished by blood-vessels and supplied with nerves, capable of growth, and undergoing a per- petual renovation by the removal and replacement of the substances that enter into its composition. (1616.) 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 COMPOSITION OF THE SKELETON. 607 in the interstices of the cartilaginous substance, and, in proportion as these accumulate, additional firmness is bestowed upon the skeleton, until at length it assumes hardness and solidity proportioned to the quantity of 'the contained earthy matter, and becomes converted into perfect bone. (1617.) Phenomena precisely similar are observable in tracing the formation and development of the osseous system, even in those genera possessed, when arrived at maturity, of the most completely organized skeletons. (1618.) 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. (1619.) The complete skeleton of a vertebrate animal may be con- sidered as being composed of several sets of bones employed for very different purposes, consisting of a central portion, the basis and sup- port of the rest, and of various appendages derived from or connected with the central part. The centre of the whole osseous fabric is gene- rally 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 employed in different animals for various and distinct purposes, present 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 ele- ments 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 development of others in particular races that we must ascribe all the endless diversity of form and mechanism so conspicuously met with in this division of the animal world. (1620.) 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 of animal organization, it is found to have been 608 VERTEBRATA. constructed upon principles the most aberrant and remote from those which an extensive investigation of the lower animals has revealed to the physiologist. (1621.) A skeleton, described generally, is made up of the following portions : first, 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, acting as prominent levers, either serve to give insertion to the muscles which move the spine, or afford additional security to the articulations between the vertebral pieces. Those ver- tebrae which defend the posterior portions of the nervous axis, usually called the spinal cord, 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, are by the human anatomist distinguished as the cranium or skull. (1622.) 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 vertebrae, 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, how- ever, waste the time of the student by considering in this place the as yet unsettled and vague opinions of transcendental anatomists upon this subject ; it is sufficient for our purpose merely to indicate the facial bones as appendages to the cranial vertebras, avoiding for the present further discussion concerning them. (1623.) 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. (1624.) 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 por- tion 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 con- nexions with th'e 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. COMPOSITION OF THE SKELETON. 609 (1625.) 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 interesting 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 appended to the extremities of the transverse processes of the vertebrae. 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 vertebral column by a very complete joint, but at the opposite extremity they are loose and uncon- nected. In proportion, however, as the hyoid bones, with the larynx, of which they form an important part, become converted into a vocal apparatus (as they gradually do), the ribs, assuming more complete de- velopment 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. (1626.) The next additions required to complete the skeleton are two pairs of locomotive limbs, representing the legs and arms of Man. In- finitely diversified as are these members both in form and office, they are, when philosophically considered, found to be constructed after the 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 respect- ively 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 evidently 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 said, 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 consist of similar pieces, five in number, called the metacarpal or metatarsal bones, and of the phalanges, or joints of the fingers and toes. 2B 610 VERTEBRATA. (1627.) 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 frame- work ; 4, the ribs ; 5, a sternal system of bones, constituting, in con- junction with some of the ribs, a thorax ; and 6, 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 con- sidered as modifications of one grand and general type. (1628.) We must, however, proceed one step further in this our pre- paratory 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 parts or elements, all or only a part of which may be developed in any given portion of the osseous system. (1629.) In order to simplify as much as possible this important sub- ject, we will select, first, what is generally considered as a single bon# one of the most complex vertebrae of a Fish for instance and examine its real composition. (1630.) This bone (fig. 306, A) is found to consist of a central por- tion (a), and of sundry processes derived there- from, 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 recognizes in the la- teral processes (d d) the analogues of the transverse processes of the human spine ; but here his knowledge fails him, inasmuch 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. (1631.) It is evident, therefore, that the human vertebrae are imper- fectly-developed bones, and do not possess all the parts or elements met with in the corresponding portion of the skeleton of a Fish. Fig. 306. Elements of a vertebra. ELEMENTS OF THE VERTEBRAE. 611 (1632.) 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. (1633.) Taking the example above given as a specimen of a fully- formed vertebra, it has been found to be divisible into 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 (fig. 306, B). They are, 1st, the centre or body of the bone ; 2ndly, two elements (b b), which embrace the spinal marrow ; 3rdly, the superior process (c) ; 4thly, the two transverse processes (d) ; 5thly, two ele- ments forming the inferior arch, and enclosing the principal blood-vessels (e) ; and 6thly, an inferior spinous process (g\ (1634.) 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 vertebra is totally different : near the tail the vertebrae consist of the body (a), the superior arch (b) and spinous process (c), and the in- ferior arch (e) and spinous process (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, there- fore, that the form of a vertebra may be modified to any extent by the simple arrest of the development of certain elements and the dispropor- tionate expansion of others, until at length it becomes scarcely recog- nizable as constituting the same piece of the skeleton. (1635.) Who would be prepared to expect, for example, that the occipital 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 proportionate length of the trans- verse processes (d), we arrive at the form of a human vertebra, which exhibits precisely similar elements. Enlarge the arches (b b) that sur- round 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 im- perceptibly at the occipital bone of the human cranium, the main dif- ferences 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. (1636.) One other illustration of this interesting subject. What bones compose a completely-formed thorax ? In Man we find, as every 612 VEBTEBRATA. tyro knows, 1st, the dorsal vertebrae ; 2ndly, the ribs, with their carti- lages ; and 3rdly, the sternum. But it is not in Man that we must expect a perfectly-developed thoracic framework ; it is in the Birds, which 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 vertebrae ; 2ndly, the dorsal ribs, firmly arti- culated on each side both with their bodies and transverse processes ; 3rdly, the sternal ribs, extending from the ribs last mentioned to the sternum ; and lastly, the sternum, composed, as we shall afterwards see, of various elements not found in the human body. If we prosecute our survey a little further, we shall find this portion of the skeleton offering 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 repre- sented by the costal cartilages ; and thus a thorax of every required description is constructed by adding or taking away, expanding or con- tracting certain elements, all of which a typical skeleton might be sup- posed to contain developed in a medium condition. (1637.) Comparison of the skeleton of a Fish with those of the higher animals demonstrates that the natural arrangement of the parts of the endoskeleton is in a series of segments succeeding each other in the axis of the body. These segments are not, indeed, composed of the same number of bones in any class, or throughout any individual animal ; but certain parts of each segment do maintain such constancy in their existence, relation, position, and offices, as to enforce the conviction that they are homologous parts, both in the constituent series of the same individual skeleton, and throughout the series of vertebrate animals. Each of these primary segments of the skeleton is designated a " ver- tebra," but with as little reference to the primary signification of the word as when the comparative anatomist speaks of a sacral vertebra. A vertebra is defined by Professor Owen as " one of those segments of the endoskeleton which constitute the axis of the body and the protecting canals of the nervous and vascular trunks ; " such a segment may also support diverging appendages. (1638.) A vertebra consists, in its typical completeness, of the ele- ments or parts represented in the following diagram : STRUCTURE OF TYPICAL VERTEBRA. 613 Tig. 307. Neural spine. Zygapophysis- - Neurapophysib. Heemapophysis. Zygapophysis. Haemal spine. (1639.) The names in the above diagram printed in Roman type signify those parts which, being usually developed from distinct and in- dependent centres, have been named autogenous elements. The italics denote the parts more properly called processes, which shoot out as con- tinuations from some of the preceding elements, and are termed exogenous. (1640.) The autogenous "elements generally circumscribe holes about the centrum, which in the chain of vertebrae form canals. The most constant and extensive canal is that formed above the centrum (fig. 307) for the lodgment of the main trunk of the nervous system (neural axis) by the elements thence termed neurapopliyses. The second canal, below the centrum (fig. 307), is in its entire extent more irregular and inter- rupted ; it lodges the central organ and large trunks of the vascular system (haemal axis), and is usually formed by the laminae which are therefore called Hcemapophyses. At the sides of the centrum, most com- monly in the cervical region, a canal is circumscribed by the pleurapo- pliysis, or costal process, by the parapopliysis, or lower transverse pro- cess, and by the diapopliysis t or upper transverse process, which canal includes a vessel, and often also a nerve. (1641.) Thus, a typical or perfect vertebra, with all its elements, pre- sents four canals or perforations around a common centre ; such a ver- tebra we find in the thorax of Man, and most of the higher classes of vertebrates, also in the neck of many birds. In the tails of most reptiles and mammals the inferior are articulated or anchylosed to the under part of the central elements, space being needed there only for the caudal artery and vein. But where the central organ of the circulation has to be lodged, an expansion of the haemal arch takes place, constituting a thorax. Accordingly, in order to construct the thoracic cavity, the pleurapophyses (fig. 307) are much elongated, and the haemapophyses (fig. 307) are removed from the centrum, and are articulated to the distal ends of the pleurapophyses, the bony hoop being completed by the inter- 614 VERTEBKATA. calation of the haemal spine (fig. 307) between the ends of the haemapo- physes. And this spine is here sometimes as widely expanded (in the thorax of Birds and Chelonians for example) as is the neural spine (parietal bone or bones) of the middle cranial vertebra of Mammals. In both cases also it maybe developed from two lateral halves ; and a bony intermuscular crest may be extended from the mid-line, as in the skull of the Hyena and the breast-bone of Birds. (1642.) The ossified parts of the abdominal vertebrae of osseous Pishes answer to the centrum, the neurapophyses, the neural spine, the parapophyses, the pleurapophyses, and certain appendages to be here- after noticed. (1643.) In the air-breathing Vertebrata, in which the heart and breathing-organs are transferred backwards to the trunk, the corre- sponding osseous segments of the skeleton are in most instances de- veloped in their typical completeness, in order to encompass and protect those organs. The thoracic haemapophyses in the Crocodiles are par- tially ossified, and in Birds completely so, in which class the haemal spines of the thorax coalesce together, become much expanded laterally, and usually develope a median crest downwards, to increase the sur- face of attachment for the great muscles of flight. This speciality is indicated by the name " sternum," applied to the confluent elements in question. (1644.) The typical thoracic vertebrae in Birds support diverging ap- pendages, either anchylosed, as in most, or articulated, as in the Penguin and Apteryx, to the posterior border of the pleurapophysis. The func- tion of such appendages in this form of typical vertebra is to connect one haemal arch with the next in succession, so as to associate the two in action, and to give firmness and strength to the whole thoracic frame. (1645.) The diverging appendages are, as might be expected, of all the elements of the vertebral segment, the least constant in regard to their existence, and the subjects of the greatest amount and variety of modi- fication. Simple, slender spines or styles in Pishes (fig. 311, 13) simple plates retaining long their cartilaginous condition in Crocodiles short, flat, slightly-curved pieces in most Birds, such, with one exception, is the range of the variety of form to which these parts are subject in the segments of the trunk. But that exception is a remarkable one, inas- much as we are enabled to trace the diverging appendage of that ver- tebral segment of the body, which from its form and character consti- tutes the pelvic arch, through various progressive phases of development from that of a simple, articulated, solitary ray, such as exists in the Lepidosiren, through innumerable modifications, whereby it is adapted for swimming, steering, balancing, and anchoring for exploration, for burrowing, creeping, walking, and running for leaping, seizing, climb- ing, or sustaining erect the entire frame of the animal under the general appellation of the posterior or pelvic limb. CKANIAL VEKTEBR.E. 615 (1646.) Any given appendage, however, as Professor Owen * justly observes, might have been the seat of such developments as convert that of the pelvic arch into a locomotive limb ; and the true insight into the general homology of limbs enables us to point out many potential pairs in the typical endoskeleton. The possible and conceivable modifications of the vertebrate archetype are far from having been exhausted in the forms that have hitherto been recognized, from the primaeval fishes of the palaeozoic ocean of this planet up to the present time ; or, in other words, it would be by no means contrary to the general laws of osteogenic development, however different from the ordinary course of nature, were vertebrate animals to occur possessed of more than the two pairs of locomotive extremities usually conferred ; so that such beings as hippogriffs and other winged quadrupeds, however fabulous, would be by no means monstrous productions. (1647.) As the segments approach the tail in the air-breathing verte- brates, they are usually progressively simplified, first by the diminution, coalescence, and final loss of the pleurapophysis, next by the similar diminution and final removal of the ha3mal and neural arches, and some- times also by the coalescence of the remaining central elements, either into a long osseous style, as in the Progs (fig. 333), or into a shorter flattened disk, as in many Birds. In Fishes, however, the seat of the terminal degradation of the vertebral column is first and chiefly in the central elements, which in the Homocercals, i. e. in those genera which, like the Perch (fig. 311), have a symmetrical bilobed tail, are commonly blended together, and shortened by absorption, whilst both neural and haemal arches remain with increased vertical extent, and indicate the number of the metamorphosed or obliterated centrums. (1648.) The anterior vertebrae of the spinal series are modified in their form and dimensions in proportion to the increased development of the anterior part of the cerebro- spinal axis, and that to such an extent, more especially in the mammiferous races, that their real nature and character are completely masked from ordinary observation, neverthe- less guided by the principles above laid down, that the bones of the cranial portion of the spinal column conform in their essential arrange- ment with what has been observed in the rest of the vertebral series, and that the skull is in reality made up of the same elemental parts, modified, it is true, to a very remarkable extent, yet still recognizable, in accordance with just principles of philosophical induction, as the homo- logues of those described above. (1649.) The cranial bones, when examined by any unprejudiced ob- server, readily resolve themselves into four distinct vertebrae, which may be named, reckoning them from behind forwards, the * " On the Archetype and Homologies of the Vertebrate Skeleton." London : John Van Voorst. 616 VEKTEBRATA. Occipital, or Epencephalic ; Parietal, or Mesencephalic ; Frontal, or Prosencephalic Nasal, or Bhinencephalic. (1650.) The OCCIPITAL YERTEBRA, in the higher vertebrates, is repre- sented by the occipital bone, in which all the vertebral elements are consolidated into one piece ; in the Beptilia, however, it is by no means difficult to identify the several parts which enter into its composition. They are as follows : Centrum Basioccipital .... 5* Neurapophyses . . . Exoccipital .... 9 Spine Supraoccipital ... 8 Pleurapophyses . . . Paroccipital . . . .10 The composition of the PARIETAL VERTEBRA is Parietal centrum . . Basisphenoid . . . ^ , 6 Neurapophyses . . . Alisphenoid. Spine Parietal 7 Pleurapophyses . . . Mastoid 12 The FRONTAL YERTEBRA consists of , , f Prosphenoid and Frontal centrum . . 1 * [ Entosphenoid, Neurapophyses . . . Orbitosphenoid . . .14 Spine Frontal 1 Pleurapophyses . . . Postfrontal .... 4 The NASAL YERTEBRA is composed of Nasal centrum . . . Yomer 16 Neurapophyses . . . Prefrontal .... 2 Spine Nasal . . . .. / ' .. 20 (1651.) Thus far we are enabled to identify the cranial bones as being modified representatives of the spinal column, by allowing for that increased development of the neural elements here rendered neces- sary by the inordinate magnitude of the ganglionic centres of the cerebro- spinal axis ; but when we come to turn our attention to the constitution of the other portions of the chain, representing the lateral and inferior arches, or, in other words, the parapophysial and the haemapophysial elements, together with the diverging appendages derived therefrom, the task becomes much more difficult. The labours of Professor Owen upon this interesting subject, unparalleled for depth of research, and exhibit- ing a grasp of philosophical argument rarely to be met with, have, how- ever, satisfactorily revealed their real nature, and established beyond a * These numbers correspond with those that indicate the individual bones of the cranium in subsequent figures. AECHETYPE OF THE SKELETON. 617 II ijlJfiffffiilfft**/ Ill"-5l^if ill'Ilalll ^ -3 * -jjj |,g |^| 1 1 1 *5J "*l iHSi? g a 3^.3 fl? FJrfji^'J2i lllIlliMlf? *"!'' T "gSs^-^-s 1 1 u^ s I H %% > .a .s .s 1 1 51 !^.31| I 111! Mil till i|||ll s||.^| |ll|1f|| i:& 3 2'3* c3 iiifftl-B s I.TSSj.S**'! Illfli 1 *'!:! is*> n is tt 1'jlilli], ilJil lilJIPill t*-DD^S5uS!c3 O^ r ^'^ 3 S ( 15 F p8*Sdfe ^-SY^^^ sJi.-i-els-S fl g-g^ 4lry 8 1411lll- 9 i &3 n2 W> ry^PffUfiiJf! Iff ire fill it^|l &t- I* |l| f 5 i U I ! " !'lE 8* I 09 tfa^^ "^ " ~ & ^ " if filial. 618 VERTEBRATA. doubt the alliances which exist between the elaborate structures in question, and the arches which exist under simpler conditions appended to the vertebral segments of the trunk. (1652.) From an extended survey of the organization of the skeleton throughout the vertebrate series, it is easy to perceive that, however diversified in adaptation to external circumstances, there is a general agreement between the various parts of the osseous framework, sufficient to convince us that all have been constructed in accordance with an ideal plan or archetype, from which Metamorphoses of the os hyoides in the Tadpole. (After Dr. St.-An S e.) 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 be- comes directed outwards, and it loses the little cartilaginous teeth previously appended to it; the os hyoides thus assumes the simple form represented in fig. 346, c. Lastly, the cartilage 6 disappears, and the complex branchial apparatus of the tadpole becomes con- verted into the permanent and com- paratively simple os hyoides of the Salamander, depicted in fig. 346, D. (1971.) The branchial arches 2, 3, 4, Dr.St.-Ange remarks, are absorbed 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 di- latation of the anastomotic vessels (fig. 345, e e e) and the enlargement of the pulmonary arteries (&). 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 Fig. 347. Course of the circulation in Lepido- siren. (After Owen.) HEART OF LEPIDOSIREN. 703 atrophy of the branchial capillaries, and subsequently of the whole bran- chial apparatus, is produced. (1972.) We must in the last place, before leaving the consideration of the circulating system of the B-EPTILIA, describe that of the Lepido- siren, a creature so exactly intermediate 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. (1973.) The heart resembles that of a fish, and consists of a single auricle (fig. 347, a), a ventricle (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 (m m), passes along as far as the auriculo- ventricular opening, where it empties its contents into the ventricle by a distinct orifice, pro- tected by a cartilaginous valvular tubercle. (1974.) It is, therefore, only necessary in this case to dilate the pul- monary 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 dioecious 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. (1975.) The aorta, or, rather, the bulbus arteriosus (), called the proventriculus, or bulbus glandulosus, in which the food undergoes further preparation. The walls of the proventriculus 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 maceration in the juices of the crop, and become subsequently saturated with the gastric fluid, that constitutes so import- ant an agent in digestion, alimentary substances are at length received into the gizzard (c), where further preparation is necessary. (2049.) 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. 3A2 Gizzard of a bird. 724 AVES. Its cavity is very small, and lined with a dense, coriaceous cuticular 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. 358, c), situated upon the opposite sides of the viscus. The action of these lateral muscles will obviously grind and crush with great force whatever is placed in the central cavity, a pro- cess that is materially expedited by the presence of hard and angular pebbles, swallowed for the purpose, by the assistance of which the con- tained food is speedily comminuted. (2050.) 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 this living mill, and retain the material to be ground within the influence of the crushers until it is properly prepared, when other fibres, acting the part of a pylorus, allow it to pass on into the duodenum (e). (2051.) 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 (fig. 359, d h) 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 continued 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, ca3cal append- ages, which communicate with the cavity of the gut at no great distance from its cloacal extremity. (2052.) In Birds, the auxiliary secretions subservient to the digestive process are the salivary, the gastric, the hepatic, and the pancreatic. (2053.) The salivary apparatus varies much in structure and dispo- sition 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 character. 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 mouth ; and their secre- tion 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. (2054.) In the Woodpeckers the glands that secrete the fluid whereby * Cuvier, 1*90118 d'Anat. Comp. torn. iii. p. 221. AUXILIAKY SECRETIONS. 725 the tongue is lubricated are of very considerable size. They pass further back than the angle of the lower jaw, extending even to be- neath the occiput ; and their secretion, which is viscid and tenacious, enters the mouth by a single orifice situated under the point of the tongue. (2055.) 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. (2056.) 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 chylopoietic viscera, namely, the liver, the pancreas, and the spleen. (2057.) The liver is a viscus of considerable magnitude, consisting of two principal lobes, and firmly suspended in situ by broad ligaments and membranous processes. The vena portce, 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 the renal and sacral veins, another proof, if any were wanting, that no arrangement by which the decarbonization of the blood can be facilitated has been omited in the organization of the class before us. The hepatic arteries and the hepatic veins present nothing remarkable in their disposition; but the course of the bile from the liver into the intestine merits our notice. Two sets of ducts are provided for this purpose : the first (fig. 359, i) carries the bile from the liver into the gall-bladder (#), 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 wanted im- mediately, 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. (2058.) The pancreas (fig. 359, e e) is a conglomerate gland of con- siderable size, situated in the elongated loop formed by the duodenum : it generally consists of two portions more or less intimately 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 726 AVES. 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. Fig. 359. Digestive apparatus of a Fowl. (2059.) The spleen (fig. 359,/) is of very small size in aU birds ; it is situated near the anterior extremity of the pancreas, and is loosely connected to the side of the proven triculus (6). The distribution of its vessels and its general structure are the same as in Mammalia. (2060.) 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 termi- nate 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. (2061.) 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 stated RESPIRATOKY SYSTEM. _ concerning the heat and purity of the blood in these creatures, , prepared to find presenting the highest possible condition of ment. 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 Keptiles are, but rather resemble spongy masses, of extreme vascularity, firmly bound down in contact with the dorsal aspect of the thorax, their posterior 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 contraction; so that inspiration and expiration must be provided for by a mechanism specially adapted to the emergency. From an examination of fig. 360, the arrangement Fig. 360. B Inferior larynx and lungs of a bird. adopted will easily be understood. The bronchi derived from the bifur- cated inferior extremity of the trachea plunge into the anterior face of 728 AVES. the lungs (c c), and by innumerable canals distribute air throughout their spongoid substance ; but the main trunks of the bronchial tubes, passing right through the pulmonary organs, open by wide mouths, represented 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 inter- communicating cells, all of which are freely permeated by the atmo- spheric 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. (2062.) 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 ex- panded 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. (2063.) 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 rarefied, 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 leverage and firmer purchase ; so that their efforts are materially favoured. And, lastly, it is owing to the capacity of the air-cells that the Singing Birds are enabled to prolong their notes to that extent which renders them pre-eminent among the vocalists of creation. (2064.) In connexion, therefore, with the respiratory system of the feathered races, it will be advisable, in the next place, to consider 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. (2065.) The trachea is of very great proportionate length in corre- spondence 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 (fig. 360, 1 1}. The trachea of Birds is composed of cartilaginous rings, which are very generally ossified, each ring, with the exception of two or three imme- diately beneath the upper larynx, forming a complete circle (fig. 361, A) surrounding the tracheal tube : these rings are enclosed between the soft VOCAL OEGANS. 729 Fig. 361. Cartilages of the superior larynx of a bird. membranes of the trachea, and thus keep the air-passages constantly permeable to the atmosphere. (2066.) 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- stance the object of which has not as yet been satisfactorily explained ; and, what is still more inexplicable, in some genera, and those too with the longest necks, as, for example, 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 muscles whereby the rings may be approximated ; and thus the length of the tube is considerably modified: these muscles (fig. 360, A, B,^,) arise from the sternum, and sometimes also from the furcula, and are continued along the sides of the windpipe throughout its whole length. (2067.) 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 (fig. 361, A, B, /), corresponding with the arytenoid cartilages of Mammalia ; these, however, in the Bird are not connected with chordae, vocales, but simply, as they are separated or approximated, open or close the fissure of the glottis. When, there- fore, we compare the framework of this organ with the cartilaginous pieces found in the larynx of Mammalia, considerable difference is per- ceptible, insomuch that it is not easy positively to recognize the analo- gous portions, more especially as in the Bird the cartilages are more or less completely ossified. If the broad anterior plate (fig. 361, 6) be considered as the thyroid cartilage, we must suppose the cricoid to be represented by three distinct ossicles, two of which (cc) are lateral, while the third or central portion (e) supports the arytenoid bones (//), which are moveably articulated with its anterior margin. The aryte- noid 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 super- ficial pair (fhyro-arytenoidei) (fig. 362, B) serving to pull asunder, while the more deeply-seated (constrictores glottidis) (fig. 362, A) bring toge- ther the lips of the glottis. (2068.) 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 730 AVES. Fig. 362. Muscles of the superior larynx of a bird. 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 capable. (2069.) In the Insessorial Birds, by far the most accomplished songsters, 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 (fig. 360, A, B, n, o, z, s, ft). In the Parrots, three pairs only are met with*; some of the Natatores have two ; other Natatorial birds, as well as the Rasores and Gral- latores, only one ; and in a few, as the King of the Vultures and the Condor, the vocal muscles are quite deficient. (2070.) Not only is the respiration of these highly- gifted Vertebrata thus abundantly provided for, but, as an im- mediate 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 auricle and of a strong ventricular chamber. The right side of the heart receives the vitiated blood from all parts of the system, which is poured into the corre- sponding auricle by three large veins, viz. one inferior and two superior vence cavce. The contraction of this auricle drives the blood into the right ventricle, the auriculo-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. (2071.) The aerated blood is then returned from the lungs by two veins, which pour it into the left auricle ; and the left ventricle, now entirely 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. (2072.) In the nervous system of Birds there is a very perceptible improvement when compared with that of Reptiles, more especially in the increased proportional development of the cerebral hemispheres : still, however, there are no convolutions seen upon the surface of the cerebrum, neither are those extensive communications between the * Vide Yarrell on the Organs of Voice in Birds (Linn. Trans, vol. xvi.). BRAIN AND ORGANS OF SENSE. 731 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 assumed its complete development, pre- senting only the central portion ; 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 recognizable as distinct elements of the cerebral mass, and the origins of the nerves strictly conform to the arrangement already described in the brain of Reptiles. The rest of the cerebro-spinal axis presents no peculiarity worthy of special notice ; and the general distribution of the cerebral and spinal nerves is so similar in all the Vertebrata, that it would be useless again to describe them in this place. (2073.) The sympathetic system in Birds is well developed, and its arrangement 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 reception of the vertebral artery, and are thus securely protected, in spite of the unusual length and slenderness which the neck not unfrequently exhibits. (2074.) But if in the general arrangement of the nervous system of the feathered races there is little to arrest our notice, we shall find in the construction of the organs of their senses many circumstances of con- siderable interest to the physiological reader ; and consequently these will require a more extended description. (2075.) 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 ; never- theless in some tribes the beak is undoubtedly extremely sensitive, 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. (2076.) As we advance from the lower to the more highly gifted races of the animal creation, taste is evidently one of the last indulgences granted ; 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 732 AVES. 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. (2077.) 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 ; and the nasal Fig. 363. Olfactory apparatus in a Goose. 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 consi- derable extent, which individually communicate with the pharynx (fig. 363, c) ; and, upon the outer wall of each compartment, three convo- luted laminaB, 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 (fig. 363, 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, insomuch that Scarpa con- sidered that the comparative powers of smell possessed by different birds might be estimated by the development of this portion of the olfactory organ. The olfactory nerves (fig. 363, 6), 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. (2078.) The eye of a Bird is an optical instrument of such admirable 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 in- dulge in lengthened details relative to the adaptation and uses of its various parts. If we contrast the Bird with the Reptile, or more espe- cially with the Fish, and consider the totally different circumstances under which these animals exercise the sense of vision, we may well expect extraordinary modifications in the structure of their organs of STKUCTTJKE OF THE EYE. 733 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 soaring into regions where that air is still further rarefied, must survey a 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 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 as much 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 eyeball. (2079.) A glance at fig. 365, 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 humours of the eye occupy the same relative positions, and ,, . . , .,. r ,, ,, . , Fig. 364. the iris and ciliary folds exist, as in the human subject. De- scending from generalities, however, he will find many points in the organization of a bird's eye eminently deserv- ing separate examination; and it is to these we would speci- ally invite his notice. First, the shape of the eyeball is peculiar : it is not spherical, as in Man, nor flattened an- teriorly, as in Fishes and J) m Eye of the Owl. aquatic Reptiles ; but, on the contrary, the cornea is rendered extremely prominent, and the antero- posterior axis of the eye considerably lengthened. This is remarkably 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 * Rees's Cyclopaedia, art. BIRDS. 734 AYES. 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 pro- duce a greater convergence 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. (2080.) But it is evident that, in order to retain this conical shape of the eyeball, some further mechanical arrangements are necessary, which in the spherical form of the human eye are not requisite. In Fishes, where the eyeball is constructed upon entirely opposite prin- ciples, being compressed anteriorly, cartilaginous supports are found im- bedded 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 the cornea ; and this latter plan is again adopted in Birds, to maintain their eyes in a shape precisely the con- verse of the former. In the Owls these ossicles are most largely deve- loped ; in such birds they form a broad zone (fig. 364), extending from the margin of the cornea, embracing the anterior conical portion of the eye, and imbedded between two fibrous layers of the sclerotic. The figure which is thus given to the eye, from the increased space obtained, is evidently 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 correspondence 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 eyeball : 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 re- cedes 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. (2081.) But the most beautiful Section of the eye of an Owl. * Vide Cyclop, of Anat. and Phys. p. 304. EYELIDS. MEMBRANA NICTITANS. 735 piece of mechanism, if we maybe pardoned 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 ensure the clearest possible 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 orpecten, which is lodged in the posterior part of the vitreous humour (fig. 365, a). This organ is composed of folds of a membrane resembling the choroid coat of the eye, and, being in like manner covered with pigment, might easily be mis- taken for a process derived from that tunic, with which, in fact, it has no connexion, being attached to the optic nerve just at the point where it expands into the retina. Its substance seems to be made up of erectile tissue, and it is most copiously supplied with blood derived from an arterial plexus formed by the arteria centralis retince * ; 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 eyeball ; 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. (2082.) Birds have three eyelids : an upper and a lower, resembling 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 especially appropriated to its motions, so as to sweep over the entire cornea, which it then covers like a curtain. (2083.) The upper and the lower eyelids differ but little in their structure 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 eyelashes attached to the palpebral margins ; and secondly, the lower eyelid is the most moveable of the two, and not only contains a distinct arsal cartilage, but is provided with a special depressor muscle, which arises from the bottom of the orbit, like the levator palpebrce superioris of the human subject : the elevator of the upper eyelid and orbicularis palpebrarum are likewise well developed. (2084.) The third eyelid, or nictitating membrane, is represented in fig. 366, A, e (the upper and lower eyelids having been divided through the middle, and turned back to display it) : it is necessarily, to a cer- tain extent, transparent ; for birds sometimes look through it, as for instance when the Eagle looks at the sun f : it is therefore of a mem- branous texture, and a most admirable and peculiar muscular appa- * Vide Barkow, in Meckel's Archiven, Band xii. t Cuyier, Le9ons d'Anat. Comp. torn. ii. p. 431. 736 AVES. Fig. 366. ratus is given, by which its movements are effected. This is placed at the back of the eyeball, and may easily be displayed by turning aside the recti and obliqui muscles, as in fig. 366, 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 quadratics membrance nictltantis, arising from near the upper aspect of the eye, de- scends towards the optic nerve ; but instead of being inserted into any- thing, as muscles usually are, it terminates in a most remarkable man- ner, ending in a tendinous sheath or pully, through which the tendon of the next muscle passes as it winds around the optic nerve. The second muscle (d), called the pyramidalis memb. nictitantis, arises from the inner aspect of the eye- ball ; and its fibres are col- lected into a long slender tendon, which, as it turns round the optic nerve, passes through the tendinous sheath formed by the qua- dratus, 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 eyelid, into which it is inserted. The reader will at once perceive how beautifully these two muscles, acting simulta- neously, cause the nicti- tating membrane to sweep over the cornea, which returns again into the inner canthus of the eye by its own elasticity. (2085.) Being thus provided with moveable eyelids, a lacrymal appa- ratus 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, apparently destined to facilitate the movements of the membrana nictitans. (2086.) The lacrymal gland is situated, as in Man, at the outer angle of the eye, and its duct pours the lacrymal secretion upon the eyeball near the external canthus. The lacrymal canal, whereby the tears, after moistening the cornea, are discharged into the nose, commences by two orifices (fig. 366, A, e) situated just behind the internal commis- Muscles of the nictitating membrane. STEUCTUEE OF THE EAE. 737 sure of the eyelids, and is continued into the nasal cavity, where it terminates in front of the representative of the middle turbinated bone. (2087.) The second gland, the glandula Harderi, seems to supply the place of the Meibomian glands of the human eyelids : it forms a considerable glandular mass, situated behind the conjunctiva, at the nasal angle of the eyelids ; and through its excretory duct, which opens behind the nictitating membrane, the lubricating secretion that it fur- nishes is poured out. (2088.) 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, however, is poured into the nose by one or more ducts, and thus serves copiously to moisten the Schneide- rian membrane. (2089.) The auditory apparatus of a Bird is almost precisely similar 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 fea- thers around the ear are so disposed as to collect faint impressions of sound; and in the Owls, besides possess- ing a broad opercular flap, Organ of hearing in the Owl. that forms a kind of external ear, there are sinuosities, external to the membrana tympani, which resemble, not very distantly, those found in the ear of Man. (2090.) Entering into the composition of the organ of hearing in the class before us, we have the membrana tympani (fig. 367, a), and tym- panic cavity, from which a wide Eustachian tube (d) leads to the poste- rior nares. The labyrinth presents the vestibule (c), the semicircular canals (6), and the rudimentary cochlea (e) ; all of which so exactly correspond in structure with what has already been described when speaking of the ear of Reptiles ( 1997 et seqq.), as to render repetition needless. A single trumpet-shaped bone, the representative of the stapes, communicates immediately between the membrana tympani and the fenestra ovalis ; but two or three minute cartilaginous appendages, connected with the membranous drum of the ear, are regarded as being the rudiments of the malleus, incus, and os orbiculare met with in the next class. (2091.) The kidneys in the Bird (fig. 368, e e e) are very large ; they SB 738 AVES. 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. Fig. 368. Generative organs of the Cock. (2092.) From tne manner in which the kidneys are imbedded, the ureters are necessarily derived from their anterior aspect. After re- ceiving 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 MALE GENERATIVE ORGANS. 739 ureters, and also, as we shall immediately see, of the sexual passages : 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 ; these, moreover, are 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 (fig. 368, m m) contains the orifices of the ureters and generative canals ; the latter is therefore generally distinguished by the name of the urethra -sexual portion of the cloaca, and is in truth a remnant of the allantois, and a rudiment of a bladder for the accumulation of the urine. (2093.) An unctuous secretion, peculiar to the class under considera- tion, 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 coc- cygis; from this source the bird distributes the oily material thus afforded to all parts of its plumage. (2094.) The male generative organs in Birds are fully as simple in their structure as those of the Reptilia. The testes are two oval bodies (fig. 368, #), invariably situated in the lumbar region, lying upon the anterior portion of the kidney. In their intimate structure they con- sist of contorted and extremely slender tubes, wherein the semen is elaborated, contained in a strong capsule. The sperm-secreting tubules of each testis terminate in a slightly flexuous v as deferens (h, f), 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 vas- cular papilla} at the termination of the vasa deferentia constituting the entire intromittent apparatus ; so that copulation between the male and female must, in the generality of species, be effected by a simple 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 descrip- tion extracted from Cuvier*. (2995.) 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 proportioned 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 * Le9ons d'Anat. Comp. torn. v. p. 108. 3B2 740 AVES. 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 continued 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 com- pletely encloses the canal of the urethra, while the two others represent the corpus cavemosum. The whole apparatus, when not in use, is drawn into the cloaca by two pairs of retractor muscles. (2096.) In Geese, Ducks, and many wading birds, such as the Stork, the structure of the male intromittent organ is totally different. 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 por- tions, 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 transverse grooves and folds. This latter portion during erection unrolls itself outwards like a glove ; and at the same time, the half first mentioned, introducing itself into the hollow cylinder formed by the 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 direction being oblique, they prevent it from stretching out in a straight line, but oblige it to assume a corkscrew appearance. A deep groove runs along the whole length of this singular organ ; and it is into the com- mencement of this groove that the vasa deferentia pour the seminal secretion. (2097.) The females of species whose males possess a large penis are provided with a rudimentary clitoris of similar construction. (2098.) The female generative system in the feathered tribes offers a remarkable exception to what we have as yet seen in the vertebrate Ovipara. Instead of being symmetrically developed upon the two sides of the body, the right oviduct, and most frequently the corresponding ovarium, remain permanently atrophied, and, although they do exist in a rudimentary condition, they never arrive at such dimensions as to allow them to assist in the reproductive process. (2099.) The fertile ovarium presents in all essential circumstances the same organization as those of the Reptilia, and is in the same way attached by folds of peritoneum in the vicinity of the spine (fig. 369,/). 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. FEMALE GENERATIVE SYSTEM. 741 Fig. 369. (2100.) The oviduct (fig. 369, d, e) commences by a wide funnel- shaped aperture, and soon assumes the appearance of a convoluted intes- tine. Its lining membrane varies in texture in different parts : near the infundibular orifice it is thin and smooth ; further down it becomes thicker and corru- gated ; and at last, near the termination of the canal, where the egg is completed by the calcification of its outward covering (g\ it presents a villose texture. The oviduct ultimately opens into the cor- responding side of the urethro- sexual compartment of the cloaca. (2101.) We must in the next place proceed to de- scribe, with as much brevity as is consistent with the im- portance 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 addi- tions made to the ovulum as it passes through the oviduct; and lastly, the phenomena that take place during the de- velopment of the embryo by incubation. (2102.) If the ovarium of a bird be examined whilst in functional activity, such of the pedunculated ovisacs (calyces, fig. 369, /) 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 zone (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 per- haps little plexuses of vessels. Within this ovisac the basis of the future egg (ovulum) is formed. (2103.) 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 * Vide Purkinje, Symbolic ad ovi Avium historiam ante incubationem. 4to. Lipsise, 1830. Generative apparatus of the Hen. 742 AVES. (rtiembrana vitelli} : this is the yelk of the future egg. Upon the sur- face of the yelk there is visible a slightly-elevated opaque spot (clca- tricula), wherein is lodged the reproductive 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 Pur- Jcinje," or the germinal vesicle. (2104.) The phenomena attending conception are therefore simply these : The membranes of the ovisac are gradually thinned by absorp- tion ; and being embraced and squeezed by the infundibular commence- ment of the oviduct, the transparent zone or stigma gives way, allowing the ovulum, covered only by its membrana vitelli, to escape into the ovi- ductus. The rent ovisac is soon removed by 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 Purkinje supposes, ruptured by the violence to which it has been subjected. (2105.) 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 chalazce, the membrana putaminis, and the calcareous shell. (2106.) 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 chalazce, which, being- twisted by the revolutions of the yelk, as it is pushed forward in the oviduct, is gathered into two delicate and spiral cords (fig. 371, c c), whereby the yelk is retained in situ after the egg is completed. (2107.) The ovulum, now covered with a thick coating of albumen, and furnished with the chalazae,at length approaches the terminal extre- mity of the oviduct, where a more tenacious material is poured out : it is here that the whole becomes encased in a dense membrane resembling very thin parchment, called " membrana putaminis ; " and ultimately, on arriving in the last, dilated portion of the canal (fig. 369, g), the lining membrane of which secretes cretaceous 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 in- terstices are left between them for the purpose of transpiration. (2108.) Thus, as the oviduct is traced from its infundibular com- mencement, 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 chalaza3 ; in the next tract it furnishes the membrana putaminis ; and in the last place, the shell ; after which, the complete egg is expelled through the cloaca. ANATOMY OF THE EGG. 743 (2109.) The anatomy of the egg prior to the commencement of incu- bation 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 (fig. 370, a, b), called the vesicula aeris ; we may further notice that the chamber so formed is filled with air con- taining an unusual proportion of oxygen, destined to serve for the respi- ration of the future embryo. Enclosed in the membrana putaminis the student next finds the albumen and chalazas (fig. 371, c) ; and lastly, the yelk, enclosed in its proper membrane (fig. 370, c), the membrana vitelli. (2110.) We must dwell a little more at length, however, upon the composition of the yelk. The cicatricula (fig. 370, g} is made up of a Fig. 370. Anatomy of the egg. 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. (2111.) Immediately over the blastoderm the membrana vitelli is slightly thickened (fig. 370, h) ; and beneath it is a canal (e), which leads to a chamber (d) placed in the centre of the yelk ; this cavity is filled with a whitish granular substance. (2112.) 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 development of the embryo is accomplished in the following manner. (2113.) No sooner has incubation* commenced, than the blastoderm * Dr. Karl Ernst v. Baer, iiber Entwickelungsgeschichte der Thiere. Beobachtung und Reflexion. 4to. 1837. 744 AVES. becomes distinctly separate from the yelk and the membrana vitelli, and, as it begins to spread, assumes the form of a central pellucid spot, surrounded by a broad dark ring (fig. 371, g, h) ; it at the same time becomes thickened and prominent, and is soon separable into three layers ; of these, the exterior (fig. 372, c) is a serous layer ; the internal, Fig. 371. Egg at commencement of incubation. or that next the yelk (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 importance, as from the first- mentioned all the serous structures, from the second all the mucous structure, and from the third the entire vascular system of the embryo originate. Fig. 372. Earliest appearance of embryo. (2114.) Towards the close of the first day of incubation the blasto- derm has already begun to change its appearance, and two white fila- ments are apparent in the middle of the central pellucid circle. Sup- posing a longitudinal section of it at this period, the membrana vitelli DEVELOPMENT OF THE EMBKYO. 745 will be found to have become more prominent where it passes over the germinal space (fig. 372, i, D). 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. (2115.) At the commencement of the second day (fig. 372, 2), the anterior portion of the embryo is dilated, and bent down so as to inflect the three membranes of the blastoderm at this point. (2116.) At the conclusion of the second day this inflection is carried still further, and from the vascular layer a single pulsating cavity (fig. 372, 3, h), the punctum saliens (the first appearance of a heart), has become developed ; so that considerable advance is already made towards that disposition of the foetus and its membranous investments repre- sented in the next figure, to which we now beg the reader's attention. (2117.) The serous membrane (fig. 372, c) has at the third day become reflected to a considerable distance over the back of the foetus ; at one extremity investing the head with a serous covering, while at the oppo- site it in like manner covers the tail : it is this reflection of the serous layer which forms the amnion, as will be observed in fig. 373, where the amniotic sac (c) is completed. Fig. 373. Embryo in a more advanced stage. (2118.) The mucous layer (A) is now seen to line the as yet open space which is to form the abdominal cavity, and by its inflections gives birth to the rudiments of the abdominal viscera. (2119.) From the vascular layer (B) has been developed the heart, now composed of two chambers (a, >), and the branchial arteries (c), which join to form the aorta (m), exactly as in the Menopoma (fig. 343). The allantois (p), the uses of which will be described hereafter, like- wise begins to make its appearance*. (2120.) At the fifth day (fig. 374) the lineaments of the viscera become tolerably distinct. The sac of the amnion (c) is completed ; the liver (i) and the lungs (e) begin to show themselves ; and the bag of the allantois (p) is largely developed ; still, however, the heart (a, b) is that of a fish, and the aorta (m) formed by the union of the branchial * Des Branchies et des Vaisseaux branchiaux dans les Embryons des Animaux vortebres, par Prof. Ch. Ernst r. Baer (Ann. des Sci. Nat. torn. xv.). 746 AVES. 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. Fig. 374. Embryo about the fifth day of incubation. (2121.) On the third day of incubation there exist four vascular arches (fig. 373, c) on each side, having a common origin from the bulb (&), which obviously represents the bulbus arteriosus of Fishes and Reptiles, before described ; these encircle the neck, and join on arriving in the dorsal region to form the aorta, which commences by two roots, each made up of the union of the four branchial vessels of the corre- sponding 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 meantime 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. (2122.) At this period three fissures are perceptible between the branchial 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. (2123.) 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 cellular 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 recognizable, 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. (2124.) The second arch then becomes diminished in size, insomuch FOKMATION OF THE VASCULAR SYSTEM. 747 that the third and fourth receive the greater part of the blood, while in the meantime a fifth arch makes its appearance behind the fourth ; so that in this way there are still four permeable arches. (2125.) 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. (2126.) 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 left. The remainder of the metamorphosis seems to depend principally upon changes that occur in the bulbus arteriosus (fig. 373, >), 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 gradu- ally more and more separated and bent upon themselves. The separa- tion of the bulbits arteriosus into two vessels is, in the opinion of Pro- fessor Baer, owing to the circumstance that the ventricles gradually become separated by a septum, which, as it grows 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 on the course impressed upon the cur- rents derived from the two ventricles. Each current becomes more and more distinct ; and at last each is provided with a proper channel, form- ing the trunks of the future pulmonary artery and of the future aorta. (2127.) It will be seen that as yet the real aorta does not exist ; for at this period of the metamorphosis all the blood passes through the vascular arches that remain into the dorsal vessel (fig. 374, m), which is formed in the same manner as the aorta of Fishes, by the union of the branchial vessels. (2128.) While the branchial fissures penetrated into the pharyngeal cavity, the branchial vessels were contained in the corresponding bran- chial arches ; but as soon as these fissures disappear, the vascular trunks abandon the neighbourhood of the pharynx and begin to assume the character they afterwards present. (2129.) The most posterior arch of the left side gradually disappears, and on the seventh day of incubation is no longer recognizable ; whilst in the meantime the current of blood from the right ventricle is directed 748 AYES. 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. (2130.) 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 commencement of the aorta towards the head. Thus, one part of the primitive root of the aorta becomes the trunk of the carotid artery. (2131.) 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. (2132.) 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 respective 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. (2133.) 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 narrower than the root of the aorta on the right side. On the right side, in fact, the middle arch now becomes of great importance, and really constitutes the commencement of the descending aorta, receiving the other communications as sub- ordinate parts. (2134.) 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 distinct circulations are thus established one proceeding from the right side of MEMBEANES OF THE OVUM. 749 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 right side of the heart. Fig. 375. Fig. 376. Membranes of the ovum. (2135.) 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-ce- phalic 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 con- verted 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 transforma- tion of the single cavity of the original " bulbus arteri- osus" into two distinct canals, and thus this wonderful me- tamorphosis is accomplished. (2136.) About the one hundred-and-twentieth hour from the commencement of incubation, the vascular layer of the blasto- 750 AVES. Fig. 377. derm has spread extensively over the yelk (fig. 375, b) ; and as the vessels formed by it become perfected, they are found to converge to the 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 foetus, and forming a vascular circle surrounding the yelk. The omphalo-mesenteric arteries (fig. 375, 6, 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 supe- rior vena cava of the young chick. (2137.) As soon as the in- testinal system of the embryo bird is distinctly formed, the membrane enclosing the yelk (vitellicle) is seen to commu- nicate with the intestine by a wide duct (ductus vitello- intestinalis), whereby the nu- tritive substance of the yelk enters the alimentary canal to serve as food, and the mucous membrane lining the vitellicle becomes thrown into close wavy folds, so as to pre- sent a very extensive surface. Gradually, as growth ad- vances, the yelk diminishes in size ; and at length, before the young bird is hatched, the remains of it are entirely withdrawn into the abdominal cavity (figs. 378, 379), 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. (2138.) While the above phenomena are in progress, another im- portant system of vessels, provided for the respiration of the bird in ovo, is developed, and obliterated before the egg is hatched. (2139.) At ^ about the period represented in fig. 374, the sides of the abdominal cavity, which is still open anteriorly, are occupied by transi- tory secreting organs, named corpora Wolfiana ; these, apparently, are the rudiments of the genito-urinary system ; and, to receive their secrc- Ves8els of the allantoi^. DEVELOPMENT OF THE EESPIEATOEY APPAEATUS. 751 tion, a bladder is developed, called the allantoid sac, a viscus which is moreover destined to play an important part in the economy of the em- bryo, and soon becomes its principal respiratory organ. The allantois first makes its appearance as a delicate bag (fig. 374, p}, derived from the anterior surface of the rectum ; but it expands rapidly, and soon occupies a very considerable portion of the interior of the egg (fig. 375, c), until at last it lines nearly the whole extent of the membrana putaminis, and becoming thus extensively exposed to the influence of the air that pene- trates the egg-shell, it ultimately takes upon itself the respiratory func- tion. When fully developed (fig. 376), it is covered with a rich network of arteries and veins (a, 6) spread upon its surface. The arteries (fig. 377, a) are derived from the common iliac trunks of the embryo, and of course represent the umbilical arteries of the human foetus ; 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 condi- tion, into the inferior cava, as does the umbilical vein of Mammalia. (2140.) About the nineteenth day of incubation, the air-vessel at the large extremity of the egg (fig. 376, c) is ruptured, and the lungs begin Fig. 378. to assume their function, by breathing the air that this vesicle contains. The circu- lation through the allantois then gradually diminishes, and it is slowly obliterated, until merely a ligamentous remnant, called the urachus, is left. In Reptiles, how- ever, as we have already seen, a portion of the allantoid bag remains even in the adult creature (fig. 340, q) ; and in Birds, in that compartment of the cloaca in which the genital and urinary passages terminate, are vestiges of the same organ. (2141.) Although the above description is intended to give a general view of the process of oviparous generation in its most perfect and consequently most complex form, the reader, in applying it to the development of the ovum in the inferior OVIPAHA, must bear in mind the following important differences: 1st, That in the air-breathing REPTILIA the white of the egg is almost, if not entirely, wanting ; but the other phenomena are similar to those witnessed in the Bird. 2ndly, That in FISHES not only is there no white formed, but, for obvious reasons, Position of the chick in ovo. 752 AVES. the allantoid apparatus is not developed. The egg in these lower tribes contains only the yelk and the cicatricula ; it swells from absorbing the surrounding water, and the fetus is developed upon the surface of the yelk, the latter, which, as in Birds, communicates with the intestine, being slowly received into the abdominal cavity. Fig. 379. Viscera of mature chick. (2142.) 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 foetus, and will be described in the next chapter. MAMMALIA. 753 CHAPTER XXIX. MAMMALIA. (2143.) 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 nourishment. 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 Yertebrata ; but it is to the Mammalia alone, the most sagacious and intelligent of all the inha- bitants of this world, that the Creator has permitted the full enjoyment of paternal and maternal love, has thrown the' offspring absolutely help- less 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. (2144.) The grand circumstance whereby the entire class of beings generally designated under the name of QUADRUPEDS may be distin- guished from all other members of the animal kingdom is, that the females of every species are furnished with mammary glands secern- ing 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 suffi- ciently 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 muscular 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. (2145.) The MAMMALIA, as we might be prepared to anticipate 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 bloodthirsty warfare against animals inferior to them- selves 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 3c 754 MAMMALIA. the fishes even in their own element ; and the gigantic Whales, wallow- ing upon the surface of the sea, " tempest the ocean" in their fury. (2146.) With habits so diverse, we may well expect corresponding 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 Mammiferous skeleton, and then, as we arrange the Mammalia under the various orders into which they have been distri- buted, speak of the most important aberrations from the given type. (2147.) The vertebral column of all Mammals, with the remarkable 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. (2148.) The cervical vertebra 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 vertebra ; the researches of Professor Bell, however, show that even this animal conforms to the general law. The distinguished naturalist referred to has demonstrated *, " that the posterior two of these verte- bra3 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 vetebras, modified into a cervical form and function suited to the peculiar wants of the animal." Professor Bell further observes that " the object of the in- creased number of vertebrae in the neck of the Sloth is evidently to allow of a more extensive 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 extraordinary 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 modifying the two superior dorsal vertebra), and giving them the office of cervical, rather than by infring- ing on a rule which is thus preserved entire without a single known exception." (2149.) The occipital bone articulates with the atlas by two lateral condyles, instead of by a single central articulating surface, a circum- * Cyclop, of Anat. and Phys. art. EDENTATA. OSTEOLOGY. 755 stance which depends upon the greatly-increased development of the encephalon, and the consequent expansion of the cranium. (2150.) The number of dorsal vertebra 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 vertebra may be counted. The lumbar and sacral verte- bras will likewise be more or less numerous in different genera ; and in the number of pieces composing the coccyx, or tail, there is every variety, from four to five-and-forty. (2151.) The thorax is enclosed by ribs, which in structure, and in their mode of connexion with the dorsal vertebra, resemble those of Man. At its dorsal extremity each rib is articulated by its head to the bodies of the vertebras, and to the intervertebral substance ; while its tubercle, or the representative of the second head of the rib of a Bird, is move- ably connected with the corresponding 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 con- nexion does not exist. The anterior ribs are therefore called true ribs, and the posterior, false, or floating ribs, precisely as in the human skeleton. (2152.) The sternum is composed of several narrow pieces, placed in a line behind each other along the middle of the breast. These pieces are generally consolidated : by their lateral margins they give attach- ment anteriorly to the clavicles, if these bones be present, and, behind these, to the costal cartilages of the true ribs. (2153.) From the whole arrangement of the thorax, it is evident that the ribs are capable of extensive movements of elevation and depression, 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. (2154.) The anterior extremity is appended to a broad scapula, gene- rally 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 element 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 ana- tomist 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 for the present. (2155.) In the posterior extremity there is equal dissimilarity in the construction of the distal portions of the limb ; but the pelvis, although 3c2 756 MAMMALIA. much modified in form, consists of the same pieces as in the human subject, and in like manner has the pubic arch and foramina fully completed. (2156.) 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 Yertebrata. Their de- velopment in the facial region is large, in proportion 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 development of the whole series. (2157.) 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. 380, 18), and the intermaxillary (17). These Fig. 380. Skull of the Pig. bones, moreover, bound extensively the cavity of the nose, and, together with the palatine process of the palate bone (fig. 381, 22), constitute the bony palate, or roof of the mouth. The nasal bones (fig. 380, 20, 20) com- plete the upper part of the face ; and, being in contact along the mesial line, arch over the nasal chamber. (2158.) The orbit is bounded anteriorly by the lacrymal bone (c), and ihejugal or malar bone (6). Its posterior boundary is generally want- ing, as the external angular processes of the jugal and frontal bones do not meet. (2159.) The orbital cavity is principally formed by processes derived BONES OF THE SKULL. 757 from the os frontis, the sphenoid, the lacrymal, and the malar bone ; the ethmoid and the palatine rarely entering into its composition. (2160.) The os eihmoides, the vomer, and the turbinated bones will be described minutely when we speak of the olfactory apparatus, which they contribute to form. (2161.) The inferior maxilla in Mammals is characterized by two circumstances, 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. (2162.) 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. (2163.) These marks, identifying the mammiferous lower jaw, ought to be well remembered by the palaeontologist. (2164.) We shall hereafter have occasion to describe the teeth that arm the jaws of the different tribes of quadrupeds; and therefore we now proceed to examine their cranial cavity, and the bones that enter into its formation. (2165.) The frontal bones (figs. 380, 381, 1, l) 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. Fig. 381. Section of the skull of the Pig. (2166.) The parietal bones (figs. 380, 381, 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. (2167.) The occipital bone consists primarily of the same pieces as 758 MAMMALIA. in the Reptile ; but in the Mammifer these are at an early period con- solidated 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. (2168.) 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 body, including the posterior clinoid processes, and of the greater alee and pterygoid processes (fig. 381, 25). The anterior half is formed by the anterior clinoid pro- cesses and alae minores (fig. 381, 11). These two halves may therefore be called, respectively, the anterior and posterior sphenoids. (2169.) Lastly, we have the temporal lone, exhibiting but one piece, although made up of all the parts which in the Reptile were so ob- viously distinct elements. The petrous portion, wedged into the base of the cranium, still encloses the internal car. The tympanic element (fig. 380, a) supports the membrana tympani. The mastoid process (fig. 381, 12) is the homologue of the mastoid bone of the Crocodile ; and, lastly, the squamous element, with which the lower jaw is articu- lated (fig. 380, 23), in the Reptilia was visibly a distinct bone. Even to these may be added the zygomatic process, which Professor Owen regards as an independent elemental part. (2170.) Reviewing, therefore, all that has been said relative to the composition of the skull in the differen^ classes of Yertebrata, the fol- lowing deductions may be arrived at */ 1. That as we advance from lower to higher forms, the proportionate 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 ptery- goid and tympanic bones, which even in Birds are separate 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 following : 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. (2171.) The bones of the face are : two superior maxillary, two inter- * Meckel, Traite G6nerale d'Anatomie Comparee, torn. iii. 2 de part., p. 195. VEETEBEJS. 759 maxillary, two nasal, two lacrymal, the vomer, two inferior turbinated bones, two palate bones, two jugal bones, and, lastly, the two halves of the lower jaw. (2172.) It is true that some slight exceptions occur : thus, forexample, in the Cetacea the pterygoid bones remain detached ; in the Rodentia the occipital is divided into a superior and inferior portion ; but in the latter, the two frontal and the two parietal become consolidated into one bone. (2173.) In Man the bones of the cranium become much less nume- rous, 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 cranium, and fourteen in the face. (2174.) Even this number is not the smallest ; for in some Monkeys the nasal bones unite and become consolidated into one piece. (2175.) 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 continuations of the spinal chain of bones, or real vertebrae modified in form and proportions 1 conformity with the increased volume of the nervous masses they are gained to enclose. "We must premise, however, that it is by no means 2 itention to adopt unreservedly the theoretical opinions of those mental writers who find vertebral elements in the bones of the face, i even in the nasal cartilages ; still, without overstraining the facts, .G is easy to demonstrate very satisfactorily that the cranial pieces that immediately enclose the cerebral masses are strictly vertebra, and pre- sent the same essential structure as those of the spinal region. (2176.) 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 Mam- miferous cranium, where, from the enormous proportionate size of the encephalon, the cranium is most distorted, it is not difficult to perceive the relationship. (2177.) 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. (2178.) The occipital vertebra (fig. 382, A) has for its body the basilar 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 defends, forms the posterior portion of the skull. (2179.) The body of the second or parietal vertebra (B) is the body of the sphenoid that is, more properly speaking, the posterior sphenoid bone, whose large alae, curving upwards, meet the parietal, and thus an arch is formed of sufficient span to cover the middle lobes of the cerebrum. 760 MAMMA Fig. 382. (2180.) The anterior, or frontal vertebra (c) has for its body the an- terior sphenoid (alee minores) ; its arch being completed by the cavity of the os frontis, which encloses anteriorly the cribriform plate of the ethmoid bone. (2181.) From this analysis of the composition of the cranium, it is appa- rent that the temporal bones, although in Man they assist so materially in com- pleting the cranial cavity, are only in- tercalated between the real vertebral elements ; as indeed might almost have been anticipated, seeing how differently the pieces belonging to this bone are ar- ranged in different classes of Vertebrata. (2182.) Such is the general organi- zation of the Mammiferous skeleton. Let us now proceed to consider the os- teology of the different orders into which the Mammalia have been distributed, and observe in what respects they in- dividually differ from each other. (2183.) The transition from Birds to Quadrupeds, remotely separated as they might appear to be, is effected by gentle gradations of structure ; and the MONO- TKEMATA, 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. (2184.) 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 pas- sages, and the sexual organs, all discharge themselves into a common cloacal chamber; so that there is still but a single vent a circumstance from which the name of the order is derived. (2185.) Even their skeleton, in many points, presents a very close affinity to that of a Bird, as will be evident on examining the osseous system of the Orniihorhynclius paradoxus (fig. 383). (2186.) The mouth of this quadruped, indeed, resembles that of a Cranial vertebrae. MAESUPIALIA. 761 Duck, whence the name of " Duck-bill," whereby it is usually distin- guished. It has, moreover, a distinct furcular bone in addition to what Fig. 383. Skeleton of Ornithorynchus paradoxus. would seem to be the ordinary clavicles ; but in reality these are the coracoid bones still largely developed. Moreover, the anterior 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. (2187.) 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. (2188.) The Marsupial quadrupeds bring forth their young alive, but in such an imperfect condition, that at the period of their birth scarcely the rudiments 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 pro- ductions, 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 Mammifer is, the existence of two peculiar bones attached to the anterior margin of the pubis, which in the living animal are imbedded 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 MOITOTREMATA, that are evidently nearly allied to the proper MARSUPIALIA, although no pouch is met with even in the female sex, the bones alluded to are found connected with the pubis. (2189.) This great section of the Vertebrate creation, which perhaps 762 MAMMALIA. ought rather to be regarded as a elass by itself, is composed of numerous families, of diverse forms and very opposite habits. The Opossums (DidelpJiys) 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 grasping the boughs, whence they are called Pedimanes; their tail is likewise prehensile. Others are terrestrial in their habits, wanting the prehensile thumb. Fig. 384. Skeleton of the Kangaroo Rat. (2190.) The Kangaroo Eat, or Potoroo (Hypsiprymnus), of whose skeleton we have given a drawing (fig. 384), 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 furnished with two claws. Such legs are well adapted to make strong and vigorous leaps over a level plain ; and in the Kangaroos (Macropus) the extraordinary development of the posterior extremities is even yet more wonderful. In other respects, the skeletons of the Marsupialia conform to the general description already given. (2191.) All other MammiferousVertebrata produce their young alive, and not until they have attained a considerably advanced state of deve- lopment during their mtra-uterine existence. The connexion between the maternal and foetal systems in these orders is maintained during the latter periods of gestation by the development 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. PLACENTAL MAMMALIA. 763 (2192.) The lowest order of PLACENTAL MAMMALIA comprises those forms which, although they breathe air by means of lungs, and have hot blood like ourselves, are ap- pointed 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 enor- mous proportions; there is no neck appa- rent externally, the head and trunk, as in fishes, appearing continuous. The anterior extremities are converted 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 vertical dorsal fin ; but this, too, is entirely soft and cartilaginous, so that in the skeleton no vestiges of it are apparent*. (2193.) In the Whalebone- Whale (Ba- Icena mysticetus) the peculiarities of the Cetaceous skeleton are well exhibited. In this gigantic animal (fig. 385), which some- times measures upwards of a hundred feet from the snout to the tail, the head forms nearly a fourth part of the entire length of its stupendous carcass ; so enormously de- veloped are the bones of the face that form the upper and the lower jaws. The cranial cavity, wherein the brain is lodged, of course does not participate in this excessive * It is interesting to see these fins still formed by the skin (exosJcdeton], where the osseous system could not enter into their composition without de- viating altogether from the Mammiferous type. Fig. 385. Skeleton of the Whalebone Whale. 764 MAMMALIA. dilatation, but corresponds to the size of the brain lodged within it. It, however, presents one point of physiological interest, serving to prove still more demonstratively 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 connexion. This remarkable arrange- ment is, no doubt, intended to prevent the stunning noises that would else be conveyed from every side to the ear, by cutting off all imme- diate communication between the auditory apparatus and the osseous framework of the head. (2194.) The cervical vertebrae, in conformity with the shortness of the neck, are exceedingly thin ; and some of them are not unfrequently anchylosed into one piece. (2195.) The thorax is composed in the ordinary manner ; but the posterior ribs are only fixed to the transverse processes of the corre- sponding vertebras. Behind the thorax the whole spine is flexible, its movements being untrammelled 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 expedition, than if it had been placed vertically, as it is in fishes. (2196.) 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 generation : the caudal vetebrae, however, are 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. (2197.) In the Herbivorous Cetacea, as the Manatus and Dugong, 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. (2198.) 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 SKELETON OF HIPPOPOTAMUS. 765 bones, the bulky head, and even the variable and irregular teeth that arm the ponderous jaws are all again conspicuous in the PACHYDERM ATA ; and the river and the marsh, the localities frequented by the latter, as obviously indicate the intermediate position which these animals occupy between the aquatic and the terrestrial Mammalia. (2199.) The skeleton of the Hippopotamus (fig. 386) offers a good example 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, form, as it were, pillars upon which the trunk is raised. Fig. 386. Skeleton of Hippopotamus. (2200.) The most important differences observable between the various genera of Pachydermatous Mammalia are found in the structure of their feet, and in the number 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 externally. In the Hippopotamus, above delineated, 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. (2201.) In the SOLIDUNGTJLA, or SOLIPEDS, regarded by Cuvier as a family belonging to the order last mentioned, we have a tribe of ani- 706 MAMMILTA. Fig. 387. mals quite peculiar as relates to the construction of their locomotive extremities. (2202.) In the Horse, for example (a creature obviously formed to be an assistant to the human race), so completely has every other con- sideration been sacrificed in order to ensure the utmost possible strength and solidity in the structure of the foot, that all the toes appear exter- 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 effi- cient to carry additional burdens, or to draw heavy loads in the service of mankind. (2203.) In the anterior extremity of a Soliped (fig. 387) the shoulder consists only of the scapula, there being no clavicle to connect it with the sternum. The hu- merus is short and very strong : the radius and ulna are partially consolidated toge- ther, so that all movements of pronation and supination are impossible. The carpus 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-meta- carpal articulation being looked upon as the " knee." Lastly, the foot consists of three great phalanges ; whereof the proxi- mal 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 re- sembling osseous splints attached to each side of the metacarpus or cannon bone. (2204.) In the posterior limbs of the Horse the same peculiarities are observable, in the construction both of the leg and foot. (2205.) The RTJMINANTIA constitute an- other order of quadrupeds 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 purpose of undergoing a second process of mastication. They Fore leg of the Horse. SKELETON OF THE STAG. 767 Fig. 388. 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 delineation (fig. 388). (2206.) The most remarkable feature observable in the Ruminant order of quadrupeds is, that, with the exception of th^ Camel tribe and the Musk-deer, the males, and sometimes the females, are provided with two horns attached to the os frontis, appen- dages not met with in any other Yertebrata. In some, as the Giraffe, these horns consist merely of a bony pro- tuberance developed from each frontal bone, which is coated with a hairy skin de- rived from the com- mon integument of the head. In others, as in the Ox, Goat, Antelope, ), over which the air is made to pass in its progress to the lungs before it arrives at the posterior nares (c). The whole of this complication of bony lamella is covered with a delicate and highly-lubricated mucous membrane, wherein the olfactory nerves terminate; 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 Carnivora. (2363.) With this perfection of the olfactory sense a corresponding mobility of the outer nostrils is permitted to the Mammiferous races. In the Reptiles and Birds the external apertures leading to the nose were merely immovable perforations in the horny or scaly covering of the upper mandible ; but now the nostrils become surrounded with moveable cartilages and appropriate muscles, adapted to dilate or con- tract the passages leading to the nose, or even to perform more im- portant and unexpected duties, as, for example, in the proboscis of the Elephant. (2364.) 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 crea- tures do, they are compelled to breathe atmospheric air. Are they, then, to smell through the intervention of an aquatic or aerial medium ? 803 MAMMALIA. 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 organization. To smell in air would be useless to the Whale ; and moreover its nasal passages are required for another function, with which the exercise of smell would apparently be incompatible. (2365.) Thus circumstanced, we find the whole nasal apparatus com- pletely metamorphosed, and so disposed as to answer two important purposes: viz. first, to allow the Cetacean to breathe air whilst its mouth is immersed in water ; and second, to provide an outlet whereby the water that is necessarily taken into the mouth may escape without being swallowed. (2366.) 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 (fig. 407, 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 ? (2367.) 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 repre- sented in the figure ; and, being firmly embraced by a sphincter muscle whilst in that situation, the air is admitted into the trachea through the passages (a, 6), without ever entering the oral cavity. Fig. 407. Blowing apparatus of the Porpoise. (2368.) 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 accom- OPTIC LOBES OF THE BRAIN. plished. The two canals forming the posterior nares (6) 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 com- munication is cut off between the posterior nares and the capacious cavities placed above them. (2369.) 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 fall, the walls are distended so as to form capacious oval receptacles. Externally these chambers are enve- loped by a very strong expansion of muscular fibres, by which they can be violently compressed. (2370.) 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 posterior nares (6), 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 Fig. 408. do this, the valve between the pouches and the posterior nares being firmly closed, the sacs are forcibly com- pressed by the muscles that embrace them, and the water is then spouted up through the "blow-holes," or nos- trils, to a height corresponding to the violence of the pressure. (2371.) It must be evident that it would be impossible that a nose, through which salt water is thus con- tinually 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 Brain of the Eabbit. of the brain, are totally deficient. (2372.) The second pair of ganglia entering into the composition 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 Keptile these were at once recognizable as primary elements of the brain ; but in the Mammifer, owing to the excessive development of the surround- ing parts, they are quite overlapped and concealed by the hemispheres. * Cuvier, Leqona d'Anat. Comp. ii. p. 673. 810 MAMMALIA. Nevertheless the tubercula quadrigemina (fig. 408, d d) occupy the same relative position as in the Tortoise (vide fig. 348, 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. (2373.) The two optic nerves, before passing to their final destina- tion, 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. 4 (2374.) Minutely to describe the construction of the eyeball in the Mammalia would be quite superfluous, seeing that in every essential particular it exactly corresponds with that of Man. The disposition of the sclerotic and choroid coats, the structure of the cornea, the arrange- ment of the humours and of the retina, the organization of the iris in short, the whole economy of the eye is the same throughout the entire class. Nevertheless there are a few points of secondary importance deserving our attention, whereby the organ is adapted to peculiarities of circumstances in which different tribes are placed. (2375.) In the Cetacea, and also in the amphibious Carnivora that catch their prey in the water, the shape of the lens is nearly spherical as in Fishes ; and the an tero -posterior dia- meter of the eye is in consequence consider- Fig. 409. ably diminished by the extraordinary thick- ness of the sclerotic at the posterior aspect of the eyeball, an arrangement approaching very nearly to that already described ( 1809). (2376.) Instead of the dark-brown paint which lines the choroid of the human eye, in many Mammals the Euyschian tunic secretes a pigmentum, of various brilliant hues, that shines with metallic splendour. This mem- brane, called the " tapetum" partially lines the bottom of the eyeball; but its use has not as yet been satisfactorily pointed out. (2377.) The shape of the pupil likewise varies in different quadrupeds : for the most part, indeed, the pupillary aperture is round, as it is in Man ; but in Ruminants, and many other Herbivora, it is transversely oblong. In the Cats (Felidce), that hunt in the gloom, structure of the eye . 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 smaUer genera ; for in those Feline Carnivora that surpass the Ocelot in size, such as the Leopard, the Lion, and the Tiger, the pupil again assumes a round form. (2378.) The eyes of Mammalia are lodged in bony orbits, as in the MUSCLES OF THE EYEBALL. 811 oviparous Vertebrata, and in like manner are supported in their move- ments by a quantity of semifluid fat, with which the orbital cavities are filled up. In Man, as in Birds, Reptiles, and Pishes, six muscles are appropriated to the movements of each eyeball, viz. four recti and two obliqui. The four recti muscles have the same disposition in Mammalia as in Birds ; that is, they arise from the margin of the optic foramen, and run forward to be inserted opposite to each other upon the superior, inferior, and lateral surfaces of the sclerotic coat. The inferior oblique likewise offers a similar arrangement in all the Yertebrata, 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 supe- rior oblique, in the class before us, takes a very peculiar course. Arising 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. 410, c), and turns back again to be inserted into the external and posterior aspect of the eyeball. Fig. 410. Muscles of the eyeball. (2379.) In addition to the six muscles appointed for the movements of the eye in Man and the Quadrumana, other Mammalia have a seventh, called the choanoid or funnel-shaped muscle. This likewise arises from the borders of the optic foramen, and, gradually expanding, forms a hollow cone interposed between 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 divided into four portions, in which case the animals so pro- vided would seem to have eight recti muscles. (2380.) The eyelids of Mammalia resemble the human in every 812 MAMMALIA. respect, excepting that in the lower orders a remnant of the nictitating membrane is still met with ; but it is of small dimensions, and unpro- vided with muscles. (2381.) 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 eyelids. The lacrymal ducts, likewise, whereby the tears are conveyed into the nose, so nearly resemble the human as to require no particular descrip- tion. The carunculce lacrymales are also met with at the inner canthus of the eyelids. 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 eyelid. (2382.) In Whales, as might be expected from their aquatic habits, no vestige of a lacrymal apparatus is to be seen. (2383.) Behind the optic lobes of the encephalon the nervous centres, from whence the other cerebral nerves take their origin, are so inti- mately blended together that the anatomist is no longer able to distin- guish them from each other. They form, in fact, the " medulla ob- fangata" and are the commencement of that long series of sentient and of motor ganglia that forms the spinal cord. (2384.) All the nerves derived from the medulla oblongata and from the spinal cord are throughout the Mammiferous class exactly com- parable to those met with in our own species, and therefore will require but brief notice. (2385.) The third, fourth, and sixth pairs are destined to the muscles of the eye, and their distribution is the same as in Man. (2386.) 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, allowance of course being made for the varying form of the jaws, and for the proportionate size of the different organs connected with mastication. (2387.) The seventh, or facial nerve, as also the glosso-pharyngeal, the pneumogastric, and the lingual, have the same origin and general distribution throughout the whole class. (2388.) The eighth pair of nerves are here, as in all the Vertebrata, devoted to the sense of hearing, which in the Mammifera attains its highest development and perfection. The sensitive portion 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 subjects, it is by no means easy to display its different parts. (2389.) 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 ORGAN OF HEARING. 813 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 communicate with the vestibule by five orifices. The vestibule itself is small, and no longer contains any chalky concretions : it communicates 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 development and perfection. (2390.) In Reptiles and Birds, as the reader will remember, the cochlea was a simple canal bent upon itself (fig. 367, 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 scalae of the cochlea are considerably 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 * . (2391.) 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 Mammiferous ear and that of the Bird are still more striking and decided. (2392.) 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 membrana tympani, the vibrations of which are to be conveyed to the labyrinth. (2393.) 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 malleus, the incus, the os orUculare, and the stapes, intervenes between the labyrinth and the membrana tympani : these ossicles, both in their disposition and con- nexions, 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. (2394.) However remote the structure of the tympanic chain of ossicles in the Mammal may appear to be from that of the simple columnella of the Bird, it is interesting to see how gradually the trans- ition is effected from one class to another even in this particular of their economy ; for in the Ornithorhynchus, the Echidna, and the Kangaroo, * 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 Eodent quadrupeds, as for example in the Guinea Pig, the Cavy, and the Porcupine, there are as many as three turns and a half. 814 MAMMALIA. 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 perfect quadrupeds. (2395.) 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 ( 2089) are scarcely 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 into various sinuosities, so disposed, no doubt, as to concentrate sonorous impressions. In Man, as the ana- tomist is aware, numerous small muscles act upon the auricular carti- lages ; but in quadrupeds 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. (2396.) More minutely to describe the structure of the auditory apparatus in the Mammiferous class would be foreign to our present purpose ; nevertheless we must not omit to notice one most remarkable provision whereby the Whales, strangely circumstanced as those crea- tures are, are permitted to hear either through the medium 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 conveyed through the denser element, could never appreciate the more delicate vibrations of the air ; and the ordi- nary 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 ? (2397.) 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 hydro- phonic apparatus, and is all that is exposed for the reception 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. (2398.) So far, as the student will have perceived, the different portions of the encephalon to which we have adverted correspond most * Vide Sir Anthony Carlisle " On the Physiology of the Stapes," Phil. Trans, for 1805. STRUCTUKE OF THE BEAIN. 815 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 Hammi- ferous brain is peculiarly characterized ? The peculiarities of the brain of a Mammal are entirely due, first, to the increased proportional deve- lopment of the cerebral hemispheres ; and secondly, to the existence of lateral cerebellic lobes, in connexion with both of which additional structures become requisite. (2399.) In those Marsupial tribes that form the connecting links between the Oviparous and Placental Yertebrata, the brain still ex- hibits a conformation nearly allied to that of the Bird, and the great commissures required in the more perfect encephalon are even yet defi- cient ; but in the simplest brain of a Placental Mammifer the charac- teristic differences are at once apparent. (2400.) In the Eabbit, for example (fig. 408), the cerebral hemi- spheres (6) are found very ma- terially to have increased in Fig. 411. their proportionate dimensions ; and although, even yet, con- volutions upon the surface of the cerebrum are scarcely in- dicated, additional means of in- tercommunication between the hemispheric masses become in- dispensable. The corpus cal- losum, therefore, or great trans- verse commissure of the hemi- spheres (fig. 408, c), is now superadded to those previously in existence ; while other me- dullary layers, called by various ridiculous names, bring into unison remote portions of the cerebral lobes. (2401.) In proportion as in- Brain of the Lion. telligence advances, the surface of the cerebral hemispheres, becoming more extensive, is thrown into numerous convolutions separated by deep sulci ; until at length in the Carnivora, as, for instance, in the Lion (fig. 411), the cerebrum (e e) attains such enormous dimensions that the other elements of the en- cephalon are, as it were, hidden among its folds. (2402.) But, in addition to this increased complexity of the cere- brum, the cerebellum likewise has assumed a proportionate importance. In the Oviparous races this important element of the brain consisted only of the mesial portion, so that no cerebellic commissure was requi- site : but in the Mammal it exhibits in addition two large lateral lobes 816 MAMMALIA. (fig. 411, c c); and coexistent with these thepons Varolii (d) makes its appearance, embracing the medulla oblongata and uniting the opposite sides of the cerebellum. (2403.) 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; neither does the ana- tomical distribution of the individual nerves derived 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. (2404.) 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 im- pressions ; so that, in a class so extensively distributed as that before us, we need not be surprised to find a special apparatus of touch deve- loped in very different and remote parts adapted to particular exigen- cies. Thus the whiskers of the Seal 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 instru- ments, and exercise the sense in question with the utmost delicacy. (2405.) 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 development, 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 Spallan- zani, 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. (2406.) The sympathetic system of the Mammifera differs in no im- portant particular from the human, the arrangement of the ganglia and the distribution of the plexuses being in all respects the same. (2407.) In the conformation of the genito-urinary apparatus in Mammalia the physiologist will find many circumstances of extreme interest. (2408.) Even in Birds, as the reader will remember, the secretions 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, TJKINAKY APPAKATTJS. 817 grooved, but imperforate penis, even where that organ was most fully developed, was sufficient for the purpose of impregnation. (2409.) 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 secretions 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 per- mitted to accumulate in considerable quantities, prior to its expulsion through the urethra the excretory duct common to both the urinary and generative organs. (2410.) Not less remarkable are the corresponding changes observable 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 mam- mary 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 development step by step, and gradually ascend from the Ovipa- rous type up to the most complete forms of the genito-urinary system. (2411.) 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. (2412.) 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 returned to the circulation by the emulgent veins that empty themselves into the inferior cava. (2413.) As relates to their intimate structure, the kidneys of all quadrupeds are essentially similar to those of our own species, each of these organs being composed of uriniferous tubules of extreme tenuity 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, becoming exceedingly flexuous, are inextricably intervolved among each other, so that the entire cortex is composed of their gyrations. At last all the uriniferous vessels terminate in blind extremities, and, according to Muller*, have no immediate communication with the vascular system. * De Glandularum Structure,, p. 102. 3a 818 MAMMALIA. (2414.) 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 foetus the kidneys present 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 become entirely obliterated. In many genera, however, this division into lobes remains permanent during the whole lifetime of the creature : such, for example, is re- markably the case in amphibious Carnivora, 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 com- position, 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. (2415.) 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 *. (2416.) We have preferred laying before the reader the above general view of the urinary system of Mammalia to noticing in detail those varieties that occur in the disposition of the bladder and urethra of some of the lower tribes, in conformity with the different types of organization presented by their sexual organs; these, however, must not be lost sight of in following out the development 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. (2417.) The oviparous Yertebrata lay eggs, and their young are per- fected 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 in- dependent 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. (2418.) The student, however, who has followed us thus far through * The Lemur and the Mole form remarkable exceptions ; for in these creatures the female urethra traverses the clitoris precisely as in the other sex. OKNITHOEHYNCHUS PARADOXUS. 819 Fig. 412. the long series of living beings that have successively presented them- selves to our notice must naturally expect that, between animals so dissimilar in their economy as the Bird and the Mammal, intermediate types of organization must occur, and that the transition from one to the other is here, as elsewhere, gradually accomplished. (2419.) In this respect his expectations will be by no means disap- pointed. The Ornithorhynchus paradoxus and the Echidna, animals met with only in the continent of New Holland, are most obviously connecting links between these two grand classes ; and, therefore, it is with the history of these strange animals that we must commence our examination of the Mammiferous generative system. (2420.) The Ornithorhynchus paradoxus well deserves the specific epithet 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 de- stitute of prominent nipples ; so that the mode in which the young animal obtains the milk provided for it is even yet a puzzling question. Does the Ornithorhyn- chus lay eggs? or produce living young ones ? This is a query that has not been satisfactorily 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. (2421.) 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 viviparous creature, as will be evident from the following details respecting them. (2422.) The penis of the male Ornithorhynchus is perforated by a ureiJiral canal, through which the semen passes, but not the urine; its extremity, moreover, is terminated by two tubercles, giving it almost a bifid appearance. This penis, when in a relaxed state, is lodged in a Male generative organs of Ornithorhy nchus paradoxus. 820 MAMMALIA. Fig. 413. little pouch in the floor of the cloaca, from which it projects when erected. (2423.) The cloaca! cavity, as in birds, gives passage to the faeces and to the urine. The testes (fig. 412, a) and the vasa deferentia (b) re- semble those of an oviparous animal ; but, on the other hand, there is a complete urinary bladder (c), and moreover a pair of auxiliary (CWper's) glands (d d), organs never met with except in the Mammiferous class. (2424.) The anatomy of the female organs is not less singular. The ovaria (fig. 414, a a) are large and racemose, like those of a bird ; while the two oviducts or uteri (fig. 413, 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 (6). (2425.) It is to Professor Owen that science is indebted for all that is known relative to the anatomy of the female Ornithorhynchus when in a gravid state; and his re- searches upon this subject ap- pear to establish the follow- ing interesting particulars : First, that the ovaria, not- withstanding their racemose appearance, exhibit all the essential characters of the Mammiferous type of struc- ture ; and corpora lutea were formed where the reproduc- tive germs had escaped from them. Secondly, that the eggs contained in the uterine cavities (fig. 414, c, e) had no connexion whatever with the walls of the uterus. Thirdly, that each ovum exhibited the usual parts of an egg, viz. the cortical membrane, the albumen, and the yelk ; and that upon the latter a membrana mtelli and the blastoderm or germinative mem- brane were plainly perceptible. Fourthly, that the uterine walls assume an increased thickness when in an impregnated 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 * " On the Ova of the Ornithorhynchus paradoxus," by Richard Owen, Esq., Phil. Trans, pt. ii. for 1834, p. 663. Generative organs of female Ornithorhynchus. OENITHOEHYNCHUS PAKADOXTJS. 821 reasoning as to the probability of the Ornithorhynchus being a vivipa- rous mammal. The form, the structure, and the detached condition of the ova, observes Professor Owen, may still be regarded 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 supposed either that the parietes of this cavity, after having secreted the requisite quantity of soft material, suddenly assume a new function, and complete the ovum by providing it with the calcareous covering necessary to enable it to sustain the superincumbent 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 abdominal glands poured out upon the ovum after its exclusion. Fig. 414* Ovaria of Ornithorhynchus parddoxus. (2426.) 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 * 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 822 MAMMALIA. Fig. 415. organization which appear to be essential to successful incubation, viz. a voluminous yelk 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 development of the foetus requires that all the necessary nutritive material be accumulated 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 con- tents, and unyielding from its necessary defensive covering : but, what- ever affinities of structure may exist in other parts of the Ornitho- rhynchus, it is most important to the question of its generation to bear in mind that it manifests no resemblance to the Bird in the disposition of its pubic bones. (2427.) From the above considerations it is therefore probable that the young Ornithorhynchi 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 connecting forms of Vertebrata. (2428.) But if from these arguments, derived from the anatomical construction of the female parts, it is allowable to conjecture that the Orni- thorhynchus is ovoviviparous (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 development, we have yet to learn how the foetus 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 mar- supial pouch in which the prema- turely-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. (2429.) In this state of uncertainty, the anatomy of the young Orni- thorynchus, examined at as early a period as possible, becomes a subject of extreme interest ; and fortunately Professor Owen has been enabled to add observations upon this subject to his other valuable researches Young Ornithorhynchus. MAKSUPIAL GENEKATION. 823 relative to the generation of these creatures *. The annexed figure (fig. 415) is a portrait 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 development. It was as yet blind ; and the situation of the eyes was 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 eyeball anteriorly : its skeleton, moreover, was quite in a cartilaginous condition ; and it was obviously in every respect helpless, and still dependent upon its mother for sustenance. (2430.) 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 foetal 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 placenta had existed. The ilium was carefully examined, but there was no appearance of the pedicle of the vitelline vesicle ; nevertheless the other vestiges of foatal 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 anterior margin of the suspensory ligament to the liver. It was reduced to a mere filamentary tube filled with coagulum. From the same cicatrix the remains of the umbilical arteries extended downwards, and near the urinary bladder were contained within a duplicature 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 embryo of the Bird and that of the ovo viviparous Eeptile have an allantois and umbilical vessels developed, no certain inference can be drawn from the above appearances as to the oviparous or viviparous nature of the generation of the Ornithorhynchus. (2431.) 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 understood ; and to these we must now beg the attention of the student. (2432.) The MARSUPIILIA, from the variety of their forms and exten- sive distribution, constitute a most important section of Mammiferous quadrupeds, distinguished by the peculiarities that occur in the organi- zation 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 reproduction is concerned, from all the Mammiferous inhabitants of the Old World, that they might even be * Owen on the Young of the Ornithorhynchus paradoxus, Trans. Zool. Soc. vol. i. 824 MAMMALIA. regarded as forming quite a distinct and separate group in the animal creation, serving to accomplish another step in that grand transition by which the physiologist is conducted from the oviparous to the placental Vertebrata. (2433.) The MARSTJPIALIA are, strictly speaking, ovoviviparous ; that is to say, the uterine ovum never forms any vascular connexion with the maternal system, but after a very brief intra-uterine gestation the em- bryo is expelled in a very rudimentary and imperfect condition, even its extremities being as yet but partially developed ; and in this helpless state the foetus is conveyed from the uterus into a pouch or marsu- pium, 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. (2434.) We may naturally expect, therefore, that, with habits so re- markable, the structure of the generative apparatus, both in the male and female Marsupial, will oifer important peculiarities; and these accordingly first present themselves for description. (2435.) 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. (2436.) 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 placental 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 embraced by a bulky and conical pros ta te, to which succeed three pairs of large secreting organs (Cowper's glands), each enveloped in a musculo-membranous sheath, apparently intended to compress their substance, and thus effi- ciently discharge their secretion into the canal of the urethra, there to be mixed up with the seminal fluid. (2437.) But perhaps the most decided peculiarities that characterize the males of Marsupial quadrupeds are met with in the construction 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 in a powerful muscular envelope. The bulbous portion of the urethra is GENERATIVE ORGANS OF THE KANGAROO. 825 likewise double, and embraced by powerful muscles. In the Kangaroo, moreover, the spongy erectile tissue that encloses the urethra passes with that canal through the centre of the body of the penis, formed by the corpora cavernosa, so that a glans can scarcely be said to exist ; but in other Marsupials, as, for example, in the Opossums (Diddpkys), the extremity of the introinittent organ is bifid, thus forming another ap- proximation to the oviparous type. (2438.) 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 subjoined figure (fig. 416). The ovaria Fig. 416. Generative organs of the female Kangaroo. (a a) are now reduced to comparatively small dimensions when com- pared with those of the Ovipara a circumstance that depends upon the 826 MAMMALIA. reduced size of the ovarian ovules, which no longer present the bulky yelks peculiar to oviparous 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 (b, c), in which the first part of gestation is accomplished. The two uteri open by two orifices (e, f) into the two vaginae (gg}, which remain quite distinct from each other from their com- mencement to their termination in the wrethro-sexual canal (h) a kind of cloaca into which both the vaginae and the urethra empty themselves. (2439.) Such being the arrangement of the generative apparatus of the female Kangaroo, we are prepared, in the next place, to consider 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 de- velopment of the foetus is ultimately completed. (2440.) The ovary of a Marsupial animal, as has been already ob- served, resembles that of ordinary Mammalia, and presents the same dense structure. But the ovarian ovules, although characterized by the paucity of yelk as compared with the oviparous classes, yet have a larger proportion than exists in the placental Mammalia. "When impregna- tion is effected in the Marsupial animal, the Graafian 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 yelk. The development of the embryo from the blastoderm or germinal membrane is, no doubt, accomplished in the same manner in all Mam- malia 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 yelk is required to contribute to the nourishment of the newly-formed being, in the Mam- mifera, where no adequate supply of yelk 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 ordinary Mammal a placenta is developed, forming a means of vascular communication between the mother and the foetus. (2441.) The important investigations of Professor Owen upon 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 foetus was met with that had apparently arrived very nearly at the term of its intra -uterine existence ; and the following is a sum- mary of its anatomy at this period. (2442.) The ovum (fig. 416, c) was lodged in one of the uterine cavities, and the foetus was about an inch and four lines in length. The * " On the Generation of Marsupial Animals, with a Description of the Impreg- nated Uterus of the Kangaroo," by Kichard Owen, Esq., Phil. Trans. 1834. EMBEYO OF KANOAKOO. 827 walls of the gravid uterus were obviously dilated, and its parietes varied in thickness from one to two lines, being in the unimpregnated state about half a line ; but this increase was not in the muscular coat, but in the lining membrane, which was thrown into irregular folds and wrinkles. There was, however, not the slightest trace of any vascular connexion 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 (chorion) exhibited not the slightest trace of vascularity, even under the microscope, and seemed in Fi g- 417. every respect to resemble the membrana putaminis that lines the egg-shell. (2443.) The body of the foetus itself was immedi- ately enclosed in a trans- parent membrane (>), the amnios. (2444.) Between the chorion (a) and the amnios (6) was an extensive vas- cular membrane (c, dd,ee): its figure seemed to have been that of a cone, of which the apex was at the umbilicus of the foetus. (2445.) Three vessels COUld be distinguished dl- verging from the umbilical cord, and ramifying over it. Two of these trunks contained coagulated blood ; while the third was smaller, empty, and evidently the arterial trunk. No trace of any other membrane could be seen' extending from the foetus, besides the three above mentioned the chorion (a), the amnios (6), and the interposed vascular membrane, the nature of which becomes the next subject of inquiry. (2446.) On tracing the three vessels above alluded to as ramifying 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, penetrated the liver : these, consequently, were the representatives of the omphalo -mesenteric or vitelline vein of the embryo Bird ( 2136). The third vessel passed between the convolutions of the small intestine along the mesentery to the abdominal aorta, corresponding to an omphalo -mesenteric or vitelline artery. The membrane, therefore, upon which they ramified answers Embryo of Kangaroo. 828 MAMMALIA. to the vascular layer of the germinal membrane which spreads over the yelk in the oviparous animals, or to the vitelline vesicle of the embryo of ordinary Mammalia. (2447.) A filamentary pedicle connected this membrane to the in- testine near the termination of the ilium, 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 important point : the contents of the vitelline sac, in the Marsupials, although doubtless intended to aiford nourishment to the embryo animal, and thus representing the yelk of the Bird's egg, differ from it in one very essential circumstance. The yelk of the oviparous ovum is ready-formed in the ovary and exists prior to conception ; but in the Mammal, where the ovarian yelk 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. (2448.) In the Marsupial ovum the vascular membrane of the vitel- licle is doubtless sufficient for the respiration of the little creature up to the time of its birth ; and accordingly the allantoic system ( 2139) 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 weeks of uterine gestation the urinary bladder is pro- longed beyond the umbilicus so as to form a small allantois destined to receive the renal secretion, which becomes more abundant as the little foetus increases in size and completeness *. (2449.) In the mammary foetus of a Kangaroo a fortnight old, Pro- fessor Owen detected both a 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 themselves into a new system, whereby the communication between the mother and her offspring is still more effectually provided for. (2450.) When we consider the very early period at which the young Kangaroo is born, namely, at about the thirty-ninth day after concep- tion, it is only reasonable to suppose that the organs most immediately connected with the vital actions are precociously matured ; and accord- ingly, even in the embryo above delineated (fig. 417), 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. (2451.) This rapid development of the viscera connected with circu- lation and respiration is, in truth, essentially requisite ; for no sooner * See Proceedings of the Zoological Society for August 1837. FCETAL KANGAKOO. 829 - 418t Foetal Kangaroo. has the embryo arrived at the size represented in the next figure (fig. 418, A), the limbs being still in a most rudimentary condition, than 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 ob- tained until it has acquired sufficient strength and size to leave the strange portable nest in which its foetal growth is accomplished and procure food adapted to a maturer condition. (2452.) A very beautiful provision is met with in the construction of the respi- ratory passages of the young Marsupial, intended to obviate the possibility of suf- focation consequent upon the admission 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 referred 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 sustenance therefrom by its own unaided efforts. The mother, as Pro- fessor Geoffrey f and Mr. Morgan J 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 injecting the milk from the nipple into the mouth of the adherent foetus. Now it can scarcely be supposed that the foetal efforts of suction should always be coincident with the maternal act of injection ; and if at any time this should not be the case, a fatal accident might happen from the milk being forcibly injected into the larynx. Professor Geoffrey first de- scribed 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 foetuses of the Kangaroo for the especial purpose of showing the relation of the larynx to the posterior nares . * P.Z.S., August 1837, p. 348. f Memoires du Musee, torn. xxv. p. 48. J Trans. Linn. Soc. vol. xvi. p. 61. "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." 830 MAMMALIA. 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 (fig. 418, B, a), which projects, as in the CETACEA, into the poste- rior nares, 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 reference to each other's peculiar condition, and affording therefore the most irrefragable evidence of Creative foresight, the feeble offspring 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 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 occasional 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 foetus may have been de- posited in the pouch ; for the latter, as we have seen, attaches itself to a different nipple from the one which had previously been in use." (2453.) Thus therefore are we conducted by the Ovovivipara, as the MABSFPIALIA are properly called, to the most perfect or placental type of the generative system. (2454.) Commencing our account of the reproductive organs of VIVI- PAROUS MAMMALIA by examining those of the male sex, we have another striking example of the insufficiency of the nomenclature employed by the anatomist who confines his studies to the human body, when it becomes necessary to describe corresponding organs even in animals organized after the same type. (2455.) 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. (2456.) It is not, however, our business here to criticize the labours of authors upon this subject ; we must content ourselves with selecting 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. (2457.) The annexed figure (fig. 419, A) represents the generative PLACENTAE MAMMALIA. 831 viscera of the male Hedgehog. The rectum (a) and the neck of the bladder (&) 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. (2458.) The testes (b 6) present the same structure in all the class, and consist essentially of an immense assemblage of extremely delicate tubuli seminiferi enclosed in a dense albugineous tunic, from which septa pass internally, whereby the seminiferous tubes are divided into several fasciculi : after piercing the proper fibrous tunic of the testes, the sperm-se- creting tubes are col- lected into an ex- tremely tortuous duct, that by its convolu- tions forms the epi- didymis, as in Man, and is then continued, under the name of vas defer ens, to the com- mencement of the ure- thra, into which the two ducts open (B, b 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. (2459.) In their situation the testes of placental Mammals are found to offer very striking differences. In the Cetacea, the Elephant, and the Seal, they remain permanently in the abdomen, bound down by a process of the peritoneum. In Man, and most quadrupeds, on the con- trary, they pass out of the abdominal cavity through the inguinal rings, and are suspended in a scrotal pouch formed by the skin, and a cre- master muscle, and lined by a serous prolongation of the peritoneal sac. The spermatic 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 Male generative apparatus of the Hedgehog. 832 MAMMALIA. enter the abdomen through an inguinal canal. Still, from their hori- zontal posture, quadrupeds are but little liable to hernia, even where the inguinal passages are much more open than in the human subject. (2460.) The quantity of the seminal fluid furnished by the testes is very small, as must be evident from the extreme narrowness of the duct through which it passes into the urethra. Nevertheless, as the impreg- nation 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 dif- ferent 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. (2461.) The vesiculce seminales are the first of these accessory se- creting organs that require our notice. In Man, the seminal vesicles, as they are erroneously termed, resemble two membranous reservoirs, situ- ated 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 elabo- rate to the secretion of the testes. (2462.) But notwithstanding their apparent importance in the human species, these organs do not exist at all in by far the greater number of Carnivora ; neither are they found in the Ruminants, nor in the ceta- ceous Mammals. (2463.) In other quadrupeds, on the contrary, they are found, and their proportionate size is extremely remarkable. This is specially the case in the Rodent tribes and among the Insectivora. In the Hedge- hog, for example, their bulk is enormous. In this creature they form two large masses (fig. 419, A, cc), each composed of four or five bundles of long and tortuous secerning vessels folded upon themselves in all directions, and pouring the product of their secretion into the urethra by two ducts (fig. 419, B, c c), quite distinct from the vasa deferentia. (2464.) The prostates are the next succenturiate glands, superadded to the essential generative organs of the placental Mammals ; and so diverse is their structure in different tribes, that it is not always easy to recognize them under the varied forms that they assume. (2465.) In Man the prostate is a solid glandular mass, that embraces PLACENTAL GENERATIVE ORGANS MALE. 833 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. (2466.) 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 accumulates in the interior of the gland, from whence it is conveyed into the urethra by appropriate excretory canals. (2467.) In most of the Rodentia, 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" (2468.) In the Hedgehog, the prostate is replaced by two large masses (fig. 419, A, d d), each composed of parallel, flexuous, and branched tubes, all of which unite into ducts common to the whole group, whereby the fluid elaborated is conveyed into the urethra through minute orifices (fig. 419, B,^). (2469.) 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 (fig. 419, A,/) they are obviously composed of convoluted tubes, and their ducts open by distinct apertures (B, g g) into the floor of the urethra. (2470.) 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 apparatus. It extends from the neck of the bladder to the extremity of the penis ; but in this course, owing to its relations with the surrounding parts, it will be necessary to consider it as divisible into two or three distinct por- tions, 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 " prostatic 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. (2471.) 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 ge- nerative secretions are poured from their respective ducts (fig. 419,B,6, c, e f g, h). Externally this division of the urethra is enclosed by strong muscles (fig. 419, A, i i), which by their convulsive contractions forcibly ejaculate the different fluids concerned in impregnation, and thus secure an efficient intromission of the seminal liquor into the female organs. 834 MAMMALIA. (2472.) 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 commencement 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 secre- tion 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. (2473.) The body of the penis in the Mammalia, as in all other Verte- brata 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 en- close the canal of the urethra in a thick erectile sheath, and, moreover, to form the glans, or most sensitive part of the intromittent apparatus. (2474.) The corpora cavernosa are now securely fixed to the bones of the pelvis by two roots or crura ; and even in the Cetacea, where no pelvis is met with, the ossa ischii exist, apparently, only for the purpose of giving firm support to the origin of the parts in question. 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 excite- ment ; but in some tribes an additional provision is required to ensure ade- quate firmness. Thus in Monkeys, Bats, the Carnivora, the Rodentia, and the Balcenidce among Cetaceans, a bone is imbedded 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. (2475.) 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 bulbous origin that embraces the urethra, and it accompanies that canal quite to the extremity of the penis, where it dilates into the glans. (2476.) 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 importance 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 struc- ture met with in some of the Rodent tribes, whereby the penis is ren- PLACENTAL GENERATIVE ORGANS FEMALE. 835 dered 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. (2477.) Thus, in the Guinea-pig tribe (Cavia, Illig.), 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 (fig, 420, d) are protruded externally to a considerable length. Fig. 420. Both the erected pouch (b) and the entire sur- face of the glans are, moreover, covered dense- ly with sharp spines or booklets ; 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 Penis of the Agouti. saws (c c) appended to the sides of the organ. From this terrible armature of the male Cavies, 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. (2478.) We have, in the last place, to examine the generative system of the female placental Mammalia, and thus to trace the development of this important system to its most complete and highest form. (2479.) In the Marsupialia, as the reader will remember, there were still two distinct uteri, that were obviously the representatives of the oviducts of the Oviparous classes. In the Human female, on the con- trary, 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 vivipa- rous Mammals offer in the anatomical structure of the generative system of the female so many intermediate gradations of form, that we are almost insensibly conducted even from the divided uteri of the Ornitho- rhynchus up to the most elevated and concentrated condition that the uterine apparatus ultimately attains in our own species. (2480.) In the female Eabbit, for example, we have a placental Mammal that in every part of the organization of its reproductive organs testifies its near affinity to the Marsupial type. The ovaria (fig. 421, k, I), 3n2 83G MAMMALIA. 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. Fig. 421. Uterus of the Babbit. (2481.) The oviducts (n, o), or the Fallopian tubes as we must now call them, are reduced in their diameter to very small dimensions, and testify by their tenuity how minute must be the ovule to which they give passage. To these succeed the uteri (e, /), still entirely distinct from each other throughout their whole extent, and even opening into the vagina (g) by separate orifices, into which the probes (i, Ti) have been introduced. As far as its anatomy is concerned, such a uterine apparatus might belong to a marsupial Mammifer ; and even in the rest of the sexual parts, obvious relations may be traced between the rodent we are describing and the ovoviviparous quadrupeds. (2482.) It is true there are no longer two vaginae terminating in a single cloaca! cavity ; but let the reader observe how nearly the vagina of the Rabbit (fig. 421, 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 con- joined with the aperture of the vulva, that the anatomist is almost in doubt whether the external opening might not be described as common both to the vagina and intestine. Advancing from this lowest form of a placenta! 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 vagina by a single passage, named the os tincce ; still, however, the cornua uteri, especially in those tribes that PLACENTAL GENERATIVE ORGANS FEMALE. 837 are most remarkable for their fecundity, become during gestation far more capacious than the mesial portion of which they appear to be prolongations. It is, in fact, in the cornua that the numerous progeny 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. 422), where the cornua uteri, (c c) are of remarkable dimensions. Fig. 422. Uterus of the Sow. (2483.) As we ascend from the more prolific inferior races to the Quadrumana 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. (2484.) In every other part of the generative system we shall like- wise find the characters of the type at length completely established. 838 MAMMALIA. The ovaria (fig. 422, 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 remnant of the yelk, has become almost inappreciable, and the little ovarian ovules are enclosed in a dense parenchymatous substance enveloped by a smooth albu- gineous tunic. The Fallopian tubes (6) correspond, in the smallness of their diameter, with the minuteness of the globules they are destined to convey from the ovaries into the uterine receptacle ; and lastly, the excretory canal of the bladder (d) becomes quite separated from the vagina (e), and the anal and generative apertures are found completely distinct from each other. (2485.) After the above brief sketch of the anatomy of the organs of generation in the higher Mammalia, it now remains for us to trace the development of the germ from the moment of impregnation to the birth of the foetus, and observe in what particulars placental generation differs from the oviparous and ovoviviparous types already described. In the viviparous or placental Mammifer the effect of impregnation is the bursting of one or more of the Graafian 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 mem- branes composing them, when once ruptured, are speedily removed by absorption ; but in the Mammal this is not the case, and a cicatrix remains permanently visible upon the surface of the ovary, indicating where the rupture has occurred : such cicatrices are known by the name of corpora lutea. (2486.) On the rupture of the ovarian ovisac, the vesicle of Purkinje, or the essential germ, accompanied only by a most minute quantity of granular fluid, or yelk, is taken up by the fimbriated extremity of the Fallopian tube, and conveyed into the interior of the uterus, where its development commences. Observations are wanting to teach us pre- cisely 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 exactly comparable to what occurs in the egg of the Bird, already minutely described in the last chapter (2110 etseq.}, 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, tobe afterwards nourished in the pouch of its mother from materials derived from the breast. (2487.) But precisely at that point of development where the Mar- supial embryo is expelled from the uterus of its parent, namely, when the functions both of the vitellicle and of the allantoid apparatus become no longer efficient either for nutrition or respiration, a third system of organs is developed in the placental Mammifer, whereby a vascular FCETUS IN UTEKO. 839 intercommunication is established between the fo3tus and the uterine vessels of the mother, forming what has been named by human embryo- legists the placenta. (2488.) In the ovum of a Sheep, at that period of the growth of the foetus which nearly corresponds with the end of uterogestation in the prematurely-born Kangaroo, all the three systems alluded to are co- existent and easily distinguishable, as will be seen in the accompanying figure (fig. 423). The foetus (a), enclosed in its amniotic membrane (6), has its limbs as yet but very imperfectly formed, exhibiting pretty- nearly the condition of a nascent Marsupial (vide fig. 418) ; but here it will be seen that the umbilical systems exhibit very striking differences in the two races. The vitdlide (/), with its pedicle (e) } are of very small dimensions ; the allantoid sac (g), on the contrary, is of con- siderable bulk, and, having ceased to act as a respiratory organ, becomes adapted to receive the urinary secretion through the canal of the Fig. 423. Embryo of the Sheep. urachus. The most important feature, however, is the rapid extension of the umbilical vessels (d), which in Birds and Marsupials were dis- tributed only to the attantois, but in the placental Mammals these vessels rapidly spread over the chorion (Ji), and, coming in contact with the vascular surface of the womb, they soon form a new bond of com- munication between the mother and the foetus, constituting the placenta; and thus the offspring is nourished until, its intra-uterine growth being accomplished, it is born in an advanced condition of development, and becomes the object of maternal care during that period in which it is dependent upon the breast of its mother for support. (2489.) 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 (fig. 423, i i\ that interdigitate with correspond- ing processes derived from the maternal womb ; in the Mare it covers the whole surface of the chorion ; but in the greater number of Mammals, 840 MAMMALIA. and in the Human female, it forms a single vascular cake, whence is derived the name appropriated by anatomists to this important viscus. (2490.) After the development of the placental system, it is obvious that the arteries derived from the common iliac trunks of the foetus, which at first were distributed only to the allantois, as in the case of the Bird ( 2139), on the development of the placenta become trans- ferred to the latter viscus, and form the umbilical arteries of the navel- string. The vein, likewise, notwithstanding its prodigiously-increased ex- tent of origin after the placenta has been formed, takes the same course on entering the umbilicus of the foetus 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. (2491.) In order to complete our history of foetal development up to the full establishment of the permanent double circulation that charac- terizes all the hot-blooded Vertebrata after birth, it only remains for us to notice the changes that occur in the vessels of the foetus, whereby, on the cessation of the functions of the placenta, the pulmonary circu- lation is at length brought into action. (2492.) Up to the period of birth the arrangement of the foetal cir- culation remains essentially that of a Eeptile, 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 imperfectly separated chambers of the heart. Under these circumstances 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 foetal exist- ence it is obvious that in that viscus all the blood derived from the placenta, from the venous system of the foetus, and also from the as yet inactive lungs, is mingled together prior to its distribution through the arterial system. The two auricles communicate freely with each other through 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 veiy small proportion only finding its way into the pulmonary arteries. Such a heart, there- fore, supplies to the foetal system a mixed fluid, 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. (2493.) 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 MAMMAEY GLANDS. 841 vessels become obliterated. The ductus venosus is no longer permeable, so that the portal system and that of the vence cavce 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 expanded lungs, and be returned through the pulmonary veins to the left side of the heart. Thus the pulmonary and systemic circulations 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 circulatory apparatus is fully established. (2494.) After birth the mammary glands supply the first nutriment 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 mammce are situated beneath the abdomen or in the inguinal region. Their structure, however, is similar throughout the entire class ; each gland consisting of innumerable 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 lactiferous canals terminate by numerous orifices upon the extremity of the nipple ; but where the nipples are of large size, they generally contain a wide cavity wherein the milk accumulates in considerable 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 conferred the en- dearments of maternity where He has bestowed intelligence to appre- ciate 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 thus obtains no slight addition to the bounteous table spread for his enjoyment. THE END. PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET. Post 8vo, 544 pp., with Eight Coloured Plates, each with many figures, price 18s. THE AQUARIAN NATURALIST: A MANUAL FOE THE SEA-SIDE. BY PKOFESSOE T. RYMER JONES, F.R.S. " Of all popular books upon the Natural History of our shores, it is the most complete and the most scientific, while of scientific books it is the most popular. 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