é University Libra QL 444, D3H98 “aT CORNELL | UNIVERSITY LIBRARY: *- RETURN TO ALBERT R. MANN LIBRARY ITHACA, N. Y. THE COMMON CRAYFISH Asiacus fluviatilis, Male.) Frontispiece) THE INTERNATIONAL SCIENTIFIC SERIES. THE CRA YFIS H. AN INTRODUCTION TO THE STUDY OF ZOOLOGY. BY T. H. HUXLEY, F.R.S. WITH EIGHTY-TWO ILLUSTRATIONS, RY 5. WILLIAMS: we. LO cb vavgi = YALE UNIVERSITY wa NEW HAVEN, CORN NEW YORK: D. APPLETON AND COMPANY, 1, 8, anv 5 BOND STREET. 1880. r po? I22b68 4 Aud Set py Svoxepaivery madixds THy ‘wept Tov ariporépay Gowy erick’ ey nace yap Tois puaotkols éveoti Tt Oavpacroy.”—ARISTOTLE, De Partibus, I. 5. ' “Qui enim ‘Autorum verba legentes, rerum ipsarum imagines (eorum verbis com- prehensa) sensibus propriis non abstrahunt, hi non veras Ideas, sed falsa Idola et phantasmata inania mente concipiunt....... “ Insusurro itaque in aurem tibi (amice Lector !) ut quecunque 4 nobisin hisce.... exercitalicnibus tractabuntur, ad exactam experientie trutinam pensites: fidemque iis non aliter adhibeas, nisi quatenus eadem indubitato sensuum testimonio firmissime stad liri deprehenderis.”—Harvey, Exercitationes de Generatione. Preefatio, “La seule et vraie Science est la connaissance des faits: l’esprit ne peut pas y suppléer et les faits sont dans les sciences ce qu’est l’expérience dans la vie civile.” “Le seultt le vrai moyen d’avancer la science est de travailler 4 1a description et & l'histoire des differentes choses qui en font l'objet.” — Burron,. Discowrs de la manibre d'étudier et de traiter U' Histoire Naturelle, “Ebenso hat mich auch die geniuere Untersuchung unsers Krebses gelehret, dass, so gemein und geringschitzig solcher auch den meisten zu seyn scheinet, sich an selbigem doch so viel Wunderbares findet, dass es auch den grossten Naturforscher schwer fallen sollte solches alles deutlich zu beschreiben.”—RoxrsrL v. RosENHOF, Insecten Belustigungen.—“ Der Flusskrebs hiesiges Landes mit seinen merkwurdigen Eigenschaften.” PREFACE. — In writing this book about Crayfishes it has not been my intention to compose a zoological mono- graph on that group of animals. Such a work, to be worthy of the name, would require the devotion of years of patient study to a mass of materials collected from many parts of the world. Nor has it been my ambition to write a treatise upon our English crayfish, which should in any way pro- voke comparison with the memorable labours of Lyonet, Bojanus, or Strauss Durckheim, upon the -willow caterpillar, the tortoise, and the cockchafer. What I have had in view is a much humbler, though perhaps, in the present state of science, not less use- ful object. I have desired, in fact, to show how the careful study of one of the commonest and most insignificant of animals, leads us, step by step, from every-day knowledge to the widest generalizations vi PREFACE. and the most difficult problems of zoology; and, indeed, of biological science in general. It is for this reason that I have termed the book an “Introduction to Zoology.” For, whoever will follow its pages, crayfish in hand, and will try to verify for himself the statements which it contains, will find himself brought face to face with all the great zoological questions which excite so lively an interest at the present day; he will understand the method by which alone we can hope to attain to satisfactory answers of these questions; and, finally, he will appreciate the justice of Diderot’s remark, “Tl faut étre profond dans-l’art ou dans la science pour en bien posséder les éléments.” And these benefits will accrue to the student whatever shortcomings and errors in the work itself may be made apparent by the process of verification. “‘Common and lowly as most may think the cray- fish,” well says Roesel von Rosenhof, “it is yet so full of wonders that the greatest naturalist may be puzzled to give a clear account of it.” But only PREFACE, vii the broad facts of the case are of fundamental im- portance; and, so far as these are concerned, I ven- ture to hope that no error has slipped into my statement of them. As for the details, it must be remembered, not only that some omission or mis- take is almost unavoidable, but that new lights come with new methods of investigation; and that better modes of statement follow upon the improve- ment of our general views introduced by the gradual widening of our knowledge. I sincerely hope that such amplifications and rectifications may speedily abound; and that this sketch may be the means of directing the attention of observers in all parts of the world to the crayfishes. Combined efforts will soon furnish the answers to many questions which a single worker can merely state; and, by completing the history of one group of animals, secure the foundation of the whole of biological science. In the Appendix, I have added a few notes re- specting points of detail with which I thought it vili PREFACE, unnecessary to burden the text; and, under the head of Bibliography, I have given some references to the literature of the subject which may be useful to those who wish to follow it out more fully. I am indebted to Mr. T. J. Parker, demonstrator of my biological class, for several anatomical draw- ings; and for valuable aid in supervising the execution of the woodcuts, and in seeing the work through the press. Mr. Cooper has had charge of the illustrations, and I am indebted to him and to Mr. Coombs, the accurate and skilful draughtsman to whom the more difficult subjects were entrusted, for such excellent specimens of xylographic art as the figures of the Crab, Lobster, Rock Lobster, and Norway Lobster. We day Hs Lonpon, November, 1879. CONTENTS. PREFACE . , . . . . LIST OF WOODCUTS . . . . a: oe . oe CHAPTER I Tue Natura AistorY oF THE COMMON CRAYFISH . : . 1 CHAPTER II. Tue PuysioLocy or THE ComMON CRAYFISH. THE MECHANISM BY WHIcH THE Pants or THE LiviING ENGINE ARE SUPPLIED WITH TITE MATERIALS NECESSARY FOR THEIR MAINTENANCE AND GROWTH . . . , i * . . « « 46 CHAPTER III. Tur PHYSIOLOGY OF THE CoMMON CRAYFISH. THE MECHANISM BY WHICH THE LIVING ORGANISM ADJUSTS ITSELF TO SUR- ROUNDING CONDITIONS AND REPRODUCKS ITSELF . . - 87 CHAPTER IV. Tue MorpHoLocy or THE CoMMON CRAYFISH. THE STRUCTURE AND THE DEVELOPMENT OF THE INDIVIDUAL . x e = Lav x CONTENTS. CHAPTER V. PAGE Tux CoMPARATIVE MonPHoLoGy oF THE CRAYFISH. THE STRUC- TURE AND THE DEVELOPMENT OF THE CRAYFISH COMPARED WITH THOSE OF OTHER LIVING BEINGS . 3 s ‘ « 227 CHAPTER VI. Tne DisTRIBUTION AND THE /ARTIOLOGY OF THE CRAYFISHES. . 288 NOTES. . ° . . . . . . . . - 347 BIBLIOGRAPHY . . . . Z . . . » » 3857 INDEX. . . . . . . : . . . + 363 LIST OF WOODCUTS. Frontispiece. Fic. a9 1. Astacus fluviatilis. 2 a 1 ge 10. 1}. 12. 13. 14, 15. 16. 17. 18. —_++— TuE Common CrayFisu, Astacus fluviatilis, (MALE) SIDE VIEW OF THE MALE. 3 ‘ DorsaL VIEWS OF MALE AND FEMALE VENTRAL VIEWS OF MALE AND FEMALE THE GILLS . . ‘ i DIssECcTION FROM THE DORSAL SIDE (MALE) ‘ i 5 ‘ LONGITUDINAL VERTICAL SECTION OF THE ALIMENTARY CANAL A GASTROLITH OR ‘‘CRAB’S EYE” , ATTACHMENT OF YOUNG TO SWIM- MERET OF MOTHER - anes STRUCTURE OF THE STOMACH . LONGITUDINAL SECTION OF THE STO- MACH . é 3 4 ah ne Roo¥F OF THE STOMACH, FROM WITHIN DIssECTION FROM THE SIDE (MALE). ALIMENTARY CANAL FROM ABOVE , BLoop CORPUSCLES . 7 TRANSVERSE SECTION OF THORAX THE HEART é ry ‘ STRUCTURE OF THE GILLS a @ THE GREEN GLAND . ¥ - ‘i PAGE 18 21 26 28 29 30 41 53 56 60 62 65 68 70 72 76 83 xii Fic. 19. Astacus fluviatilis. 20, 21, 22. 23. 24. ” 95. 26. 27. 28. 29. 30. 31. 32. 33. 34, 35. 36. 37. 38. ~——. 39. 40. 41. 42. 43. 44, ” ” ”» id LIST OF WOODCUTS. MuscuLak TISSUE . a ais MUSCLES OF CHELA . ‘ j ‘ ARTICULATION OF ABDOMINAL SO- MITES . é . ‘ a: MUSCULAR SYSTEM . . + . NERVE FIBRES. “ ‘ eo NERVE GANGLIA ‘ ‘i - ‘ NERVOUS SYSTEM é é e 8 OLFACTORY AND AUDITORY ORGANS AUDITORY SAC. 5 f : STRUCTURE OF EYE . f ia DIAGRAM OF EYE . FEMALE REPRODUCTIVE ORGANS . . MALE REPRODUCTIVE ORGANS STRUCTURE OF OVARY. STRUCTURE OF TESTIS . 3 5 SPERMATOZOA si % 4 es THE LAST THORACIC STERNUM IN THE MALE AND FEMALE . . TRANSVERSE SECTION OF ABDOMEN ABDOMINAL APPENDAGES . he CONNECTION BETWEEN THORAX AND ABDOMEN . - - : i CEPHALOTHORACIC STEKNA AND EN- DOPHRAGMAL SYSTEM . ess OPHTHALMIC AND ANTENNULARY §0- MITES. . . . . . THE ROSTRUM ae A SEGMENT OF THE ENDOPHRAGMAL SYSTEM . . . . LONGITUDINAL SECTION OF CEPHALO- THORAX . . . . ar TE THIRD MAXILLIPEDE, . c PAGE 91 93 97 100 102 103 104 114 117 119 123 129 130 131 182 134 136 142 144 151 153 156 157 159 162 164 ” « 45. 46. 47. 48, 49. 50. 51. 52. 53, 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 6 co LIST OF WOODCUTS. Astacus fluviatilis, THE FIRST AND SECOND MAXILLIT- PEDES . ‘i THE SECOND AMBULATORY LEG. THE MANDIBLE AND MAXILLE 95 ay TUB EYE-STALK, ANTENNULE, ANTENNA . 2 a ° 35 +“ BLooD: CORPUSCLES ‘ ‘ 53 $5 EPITHELIUM. : . . ee a CONNECTIVE TISSUE , ‘ 5 45 MuscuLaR TISSUE . . ‘ 3 PP MUSCULAR TISSUE fs ‘ 55 55 NERVE GANGLIA , ‘ F ms 55 NERVE FIBRES. . . 3 =3 CUTICULAR TISSUE . ‘ a 5a SiCTIONS OF EMBRYOS. NEWLY HATCHED YOUNG . a” ” EARLIER STAGES OF DEVELOPMENT LATER STAGES OF DEVELOPMENT . torrentium. | COMPARATIVE VIEWS OF THE CARA- +4 nobilis. PACE, THIRD ABDOMINAT, SOMITE, nigrescens. AND TELSON : : F torrentium. | COMPARATIVE VIEWS OF THE FIRST 34 nobilis. AND SECOND ABDOMINAL APPEN- nigrescens. DAGES OF THE MALE. oe Cambarus Clarkii . 7 . * ° . e 5 Parastacus brasiliensis . . . s 2 < yw Astacoides madagascarensis : : i ‘i < DIAGRAM OF THE MORPHOLOGICAL RELATIONS OF THE Astacidee - . . . . ‘ ‘ bg Homarus vulgaris. . : . : . . : Parastacus. Nephrops. |} PoDOBRAXCHIE . ‘ < Palemon. xil PAGE 166 169 171 172 176 178 179 181 182 188 189 191 208 210 216 220 233 245 248 250 251 253 258 259 XIV Fic. 69. 70. 71. 72. 73. 74, 75. 76. 77. 78. 79. 80. 81. LIST OF WOODCUTS, Nephrops norvegicus . a = - q Palinurus vulgaris. . . . . Palemon jamaicensis j F “ . Cancer pagurus . ° . . . . Peneus . F . : . Cancer pagurus. DEVELOPMENT . 5 . Astacus leptodactylis , : ‘ . Australian Crayfish s i Mar OF THE DISTRIBUTION OF CRAYFISHES Cambarus, WALKING LEG. ‘i 7 . Palemon jamaicensis : 7 . . Pseudastacus pustulosus Eryma modestiformis Hoploparia longimena . . PAGE 260 262 269 273 281 282 301 307 309 312 329 340 342 THE CRAYFISH: AN INTRODUCTION TO THE STUDY OF ZOOLOGY. CHAPTER I. THE NATURAL HISTORY OF THE COMMON CRAYFISH (Astacus fluviatilis.) Many persons seem to believe that what is termed Science is of a widely different nature from ordinary knowledge, and that the methods by which scientific truths are ascertained involve mental operations of a recondite and mysterious nature, comprehensible only by the initiated, and as distinct in their character as in their subject matter, from the processes by which we discriminate between fact and fancy in ordinary life. But any one who looks into the matter attentively will soon perceive that there is no. solid foundation for the belief that the realm of science is thus shut off from that of common sense; or that the mode of investigation which yields such wonderful results to the scientific inves- tigator, is different in kind from that which is employed 2 THE NATURAL HISTORY OF THE COMMON CRAYFISH. for the commonest purposes of everyday existence. Common sense is science exactly in so far as it fulfils the ideal of common sense; that is, sees facts as they are, or, at any rate, without the distortion of prejudice, and reasons from them in accordance with the dictates of sound judgment. And science is simply common sense at its best; that is, rigidly accurate in observation, and merciless to fallacy in logic. Whoso will question the validity of the conclusions of sound science, must be prepared to carry his scepticism a long way; for it may be safely affirmed, that there is hardly any of those decisions of common sense on which men stake their all in practical life, which can justify itself so thoroughly on common sense principles, as the broad truths of science can be justified. The conclusion drawn from due consideration of the nature of the case is verified by historical inquiry; and the historian of every science traces back its roots to the primary stock of common information possessed by all mankind. In its earliest development knowledge is self-sown. Impressions force themselves upon men’s senses whether they will or not, and often against their will. The amount of interest which these impressions awaken is determined by the coarser pains and pleasures which they carry in their train, or by mere curiosity; and reason deals with the materials supplied to it as far as that interest carries it, and no farther. Such common COMMON KNOWLEDGE AND SCIENCE. 3 knowledge is rather brought than sought; and such ratiocination is little more than the working of a blind intellectual instinct. It is only when the mind passes beyond this condition that it begins to evolve science. When simple curiosity passes into the love of knowledge as such, and the gratification of the ssthetic sense of the beauty of com- pleteness and accuracy seems more desirable than the easy indolence of ignorance; when the finding out of the causes of things becomes a source of joy, and he is counted happy who is successful in the search ; common knowledge of nature passes into what our forefathers called Natural History, from whence there is but a step to that which used to be termed Natural Philosophy, and now passes by the name of Physical Science. In this final stage of knowledge, the phenomena of nature are regarded as one continuous series of causes and effects; and the ultimate object of science is to trace out that series, from the term which is nearest to us, to that which is at the furthest limit accessible to our means of investigation. The course of nature as it is, as it has been, and as it . will be, is the object of scientific inquiry; whatever lies beyond, above, or below this, is outside science. But: the philosopher need not despair at the limitation of his field of labour: in relation to the human mind Nature is boundless; and, though nowhere inaccessible, she is everywhere unfathomable. 4 THE NATURAL HISTORY OF THE COMMON CRAYFISH. The Biological Sciences embody the great multitude of truths which have been ascertained respecting living beings; and as there are two chief kinds of living things, animals and plants, so Biology is, for convenience sake, divided into two main branches, Zoology and Botany. Each of these branches of Biology has passed through the three stages of development, which are common to all the sciences; and, at the present time, each is in these different’ stages in different minds. Every country boy possesses more or less information respecting the plants and animals which come under his notice, in the stage of common knowledge; a good many persons have acquired more or less of that accurate, but necessarily incomplete and unmethodised knowledge, which is under- stood by Natural History; while a few have reached the purely scientific stage, and, as Zoologists and Botanists, strive towards the perfection of Biology as a branch of Physical Science. Historically, common knowledge is represented by the © allusions to animals and plants in ancient literature; _ while Natural History, more or less grading into Biology, meets us in the works of Aristotle, and his continuators in the Middle Ages, Rondoletius, Aldrovandus, and their | contemporaries and successors. But the conscious at- tempt to construct a complete science of Biology hardly dates further back than Treviranus and Lamarck, at the beginning of this century, while it has received its strongest impulse, in our own day, from Darwin. COMMON KNOWLEDGE OF THE CRAYFISH. 5 My purpose, in the present work, is to exemplify the general truths respecting the development of zoological science which have just been stated by the study of a special case; and, to this end, I have selected an animal, the Common Crayfish, which, taking it altogether, is better fitted for my purpose than any other. It is readily obtained,* and all the most important points of its construction are easily deciphered; hence, those who read what follows will have no difficulty in ascertaining whether the statements correspond with facts or not. And unless my readers are prepared to take this much trouble, they may almost as well shut the book; for nothing is truer than Harvey’s dictum, that those who read without acquiring distinct images of the things about which they read, by the help of their own senses, gather no real knowledge, but conceive mere phantoms and idola. It is a matter of common information that a number of our streams and rivulets harbour small animals, rarely more than three or four inches long, which are very similar to little lobsters, except that they are usually of a dull, greenish or brownish colour, generally diversified with pale yellow on the under side of the body, and some- times with red on the limbs. In rare cases, their * If crayfish are not to be had, a lobster will be found to answer to the description of the former, in almost all points; but the gills and the abdominal appendages present differences ; and the last thoracic somite is united with the rest in the lobster. (See Chap. V.) 6 THE NATURAL HISTORY OF THE COMMON CRAYFISH. general hue may be red or blue. These are “ cray- fishes,” and they cannot possibly be mistaken for any other inhabitants of our fresh waters. PF Fic. 1.—Astaens flrviatilis.—Side view of a male specimen (nat. size) : — bg, branchiostegite ; cg, cervical groove ; 7, rostrum ; ¢, telson.— 1, eye-stalk ; 2, antennule; 3, antenna; 9, external maxillipede ; 10, forceps; 14, last ambulatory leg; 17, third abdominal ap- pendage; 20, lateral lobe of the taiJ-fin, or sixth abdominal appendage ; Xv, the first; and xx, the last abdominal somite. In this and in succeeding figures the numbers of the somites are given in Roman, those of the appendages in ordinary numerals. The animals may be seen walking along the bottom of the shallow waters which they prefer, by means of four pairs of jointed legs (fig. 1); but, if alarmed, they swim MALE AND FEMALE CRAYFISHES. 7 backwards with rapid jerks, propelled by the strokes of a broad, fan-shaped flipper, which terminates the hinder end of the body (fig. 1, #, 20). In front of the four pairs of legs, which are used in walking, there is a pair of limbs of a much more massive character, each of which ends in two claws disposed in such a manner as to constitute a powerful pincer (fig. 1; 10). These claws are the chief weapons of offence and defence of the crayfish, and those who handle them incautiously will discover that their grip is by no means to be des- pised, and indicates a good deal of disposable energy. A sort of shield covers the front part of the body, and ends in a sharp projecting spine in the middle line (r). On each side of this is an eye, mounted on a movable stalk (Z), which can be turned in any direction: behind the eyes follow two pairs of feelers; in one of these, the feeler ends in two, short, jointed filaments (2) ; while, in the other, it terminates in a single, many-jointed filament, like a whip-lash, which is more than half the length of the body (8). Sometimes turned backwards, sometimes sweeping forwards, these long feelers con- tinually explore a considerable area around the body of the crayfish. If a number of crayfishes, of about the same size, are compared together, it will easily be seen that they fall into two sets; the jointed tail being much broader, especially in the middle, in the one set than in the other (fig. 2). The broad-tailed crayfishes are the a 8 THE NATURAL HISTORY OF THE COMMON CRAYFISH. females, the others the males, And the latter may be still more easily known by the possession of four curved styles, attached to the under face of the first two rings of the tail, which are turned forwards between the hinder legs, on the under side of the body (fig. 3, A; 15, 16). In the female, there are mere soft filaments in the place of the first pair of styles (fig. 8, B; 15). ° Crayfishes do not inhabit every British river, and even where they are known to abound, it is not easy to find them at all times of the year. In granite districts and others, in which the soil yields little or no calcareous matter to the waters which flow over it, crayfishes do not occur. They are intolerant of great heat and of much sunshine; they are therefore most active towards the evening, while they shelter themselves under the shade of stones and banks during the day. It has been observed that they frequent those parts of a river which run north and south, less than those which have an easterly and westerly direction, inasmuch as the latter yield more shade from the mid-day sun. During the depth of winter, crayfishes are rarely to be seen about in a stream; but they may be found in abundance in its banks, in natural crevices and in burrowswhich they dig for themselves. The burrows may be from a few inches to more than a yard deep, and it has been noticed that, if the waters are liable to freeze, the burrows are deeper and further from the surface than otherwise. Where the soil, through TIIE FOOD OF THE CRAYFISA. 9 which a stream haunted by crayfishes runs, is soft and peaty, the crayfishes work their way into it in all directions, and thousands of them, of all sizes, may be dug out, even at a considerable distance from the banks. It does not appear that crayfishes fall into a state of torpor in the winter, and thus ‘“‘hybernate” in the strict sense of the word. At any rate, so long as the weather is open, the crayfish lies at the mouth of his burrow, barring the entrance with his great claws, and with pro- truded feelers keeps careful watch on the passers-by. Larve of insects, water-snails, tadpoles, or frogs, which come within reach, are suddenly seized and devoured, and it is averred that the water-rat is liable to the same fate. Passing too near the fatal den, possibly in search of a stray crayfish, whose flavour he highly appreciates, the vole is himself seized and held till he is suffocated, when his captor easily reverses the conditions of the anti- cipated meal. In fact, few things in the way of food are amiss to _the crayfish ; living or dead, fresh or carrion, animal or vegetable, it is all one. Calcareous plants, such as the stoneworts (Chara), are highly acceptable; so are any kinds of succulent roots, such as carrots; and it is said that crayfish sometimes make short excursions inland, in search of vegetable food. Snails are devoured, shells and all; the cast coats of other crayfish are turned to account as supplies of needful calcareous matter; and ‘the unprovcted or weakly member of the family is 10 THE NATURAL HISTORY OF THE COMMON CRAYFISH. not spared. Crayfishes, in fact, are guilty of canni- balism in its worst form; and a French observer pa- thetically remarks, that, under certain circumstances, the males “ méconnaissent les plus saints devoirs ;” and, not content with mutilating or killing their spouses, after the fashion of animals of higher moral pretensions, they descend to the lowest depths of utilitarian turpitude, and finish by eating them. In the depth of winter, however, the most alert of crayfish can find little enough food; and hence, when they emerge from their hiding-places in the first warm days of spring, usually about March, the crayfishes are in poor condition. At this time, the females are found to be laden with eggs, of which from one to two hundred are attached be- neath the tail, and look like a mass of minute berries (fig. 8, B). In May or June, these eggs are hatched, and give rise to minute young, which are sometimes to be found attached beneath the tail of the mother, under whose protection they spend the first few days of their existence. In this country, we do not set much store upon cray- fishes as an article of food, but on the Continent, and especially in France, they are in great request. Paris alone, with its two millions of inhabitants, consumes annually from five to six millions of crayfishes, and pays about £16,000 for them. The natural productivity of the rivers of France has long been inadequate to supply the’ THE ORIGIN OF THE WORD CRAYFISH. 11 demand for these delicacies ; and hence, not only are large quantities imported from Germany, and elsewhere, but the artificial cultivation of crayfish has been successfully attempted on a considerable scale. Crayfishes are caught in various ways; sometimes the fisherman simply wades in the water and drags them out of their burrows; more commonly, hoop-nets baited with frogs are let down into the water and rapidly drawn up, when there is reason to think that crayfish have been attracted to the bait; or fires are lighted on the banks at night, and the crayfish, which are attracted, like moths, to the anwonted illumination, are scooped out with the hand or with nets. Thus far, our information respecting the crayfish is such as would be forced upon anyone who dealt in cray- fishes, or lived in a district in which they were commonly used for food. It is common knowledge. Let us now try to push our acquaintance with what is to be learned about the animal a little further, so as to be able to give an account of its Natural History, such as might have been furnished by Buffon if he had dealt with the subject. There is an inquiry which does not strictly lie within the province of physical science, and yet suggests itself naturally enough at the outset of a natural history. The animal we are considering has two names, one common, Crayfish, the other technical, Astacus flu- viatilis. How has it come by these two names, and why, 12 THE NATURAL HISTORY OF THE COMMON CRAYFISH. having a common English name for it already, should naturalists call it by another appellation derived from a foreign tongue ? The origin of the common name, “ crayfish,” involves some curious questions of etymology, and indeed, of his- tory. It might readily be supposed that the word “‘cray” had a meaning of its own, and qualified the substantive “fish”—as “jelly” and “cod” in “jellyfish” and “codfish.” But this certainly is not the case. The old English method of writing the word was “ crevis” or ‘‘ crevice,” and the ‘‘cray” is simply a phonetic spelling of the syl- lable ‘‘ cre,” in which the ‘‘e” was formerly pronounced © as all the world, except ourselves, now pronounce that vowel. While ‘ fish” is the “vis” insensibly modified to suit our knowledge of the thing as an aquatic animal. Now ‘‘crevis” is clearly one of two things. Either it is a modification of the French name “ écrevisse,” or of the Low Dutch name “ crevik,” by which the crayfish is known in these languages. The former derivation is that usually given, and, if it be correct, we must refer “cray- | fish” to the same category as ‘“‘ mutton,” “ beef,” and “pork,” all of which are French equivalents, introduced by the Normans, for the ‘‘ sheep’s flesh,” ‘‘ox flesh,” and *“‘swine’s flesh,” of their English subjects. In this case, we should not have called a crayfish, a crayfish, except for the Norman conquest. On the other hand, if “ crevik” is the source of our THE TECHNICAL NAME OF THE CRAYFISH. 13 word, it may have come to us straight from the Angle and Saxon contingent of our mixed ancestry. As to the origin of the technical name; dorakds, astakos, was the name by which the Greeks knew the lobster ; and it has been handed down to us in the works of Aristotle, who does not seem to have taken any special notice of the crayfish. At the revival of learning, the early naturalists noted the close general similarity between the lobster and the crayfish; but, as the latter lives in fresh water, while the former is a marine animal, they called the crayfish, in their Latin, Astacus fluviatilis, or the ‘ river-lobster,” by way of distinction; and this nomenclature was re- tained until, about forty-five years ago, an eminent French Naturalist, M. Milne-Edwards, pointed out that there are far more extensive differences between lobsters and crayfish than had been supposed; and that it would be advisable to mark the distinctness of the things by a corresponding difference in their names. Leaving Astacus for the crayfishes, he proposed to change the technical name of the lobster into Homarus, by latin- ising the old French name ‘‘ Omar,” or “ Homar” (now Homard), for that animal. At the present time, therefore, while the recognised technical name of the crayfish is Astacus fluviatilis, that of the lobster is Homarus vulgaris. And as this nomencla- ture is generally received, it is desirable that it should not be altered; though it is attended by the inconvenience, that Astacus, as we now employ the name, does not 14 THE NATURAL HISTORY OF THE COMMON CRAYFISH. denote that which the Greeks, ancient and modern, signify, by its original, astakos; and does signify something quite different. Finally, as to why it is needful to have two names for the same thing, one vernacular, and one technical. Many people imagine that scientific terminology is a needless burden imposed upon the novice, and ask us why we cannot be content with plain English. In reply, I would suggest to such an objector to open a conversation about his own business with a carpenter, or an engineer, or, still better, with a sailor, and try how far plain English will go. The interview will not have lasted long before he will find himself lost in a maze of unintelligible technicalities. Every calling has its technical termin- ology; and every artisan uses terms of art, which sound like gibberish to those who know nothing of the art, but are exceedingly convenient to those who practise it. In fact, every art is full of conceptions which are special to itself; and, as the use of language is to convey our conceptions to one another, language must supply signs for those conceptions. There are two ways of doing this: either existing signs may be combined in loose and cumbrous periphrases; or new signs, having a well-understood and definite signification, may be in- vented. The practice of sensible people shows the advantage of the latter course ; and here, as elsewhere,’ science has simply followed and improved upon common! sense. THE USE OF THE BINOMIAL NOMENCLATURE. 15 Moreover, while English, French, German, and Italian artisans are under no particular necessity to discuss the processes and results of their business with one another, science is cosmopolitan, and the difficulties of the study of Zoology would be prodigiously increased, if Zoologists of different nationalities used different tech- nical terms for the same thing. They need a universal language ; and it has been found convenient that the lan- guage shall be the Latin in form, and Latin or Greek in origin. What in English is Crayfish, is Eerevisse in French; Flusskrebs, in German; Cammaro, or Gambaro, or Gammarello, in Italian: but the Zoologist of each nationality knows that, in the scientific works of all the rest, he shall find what he wants to read under the head of Astacus fluviatilis. But granting the expediency of a technical name for the Crayfish, why should that name be double? The reply is still, practical convenience. If there are ten children of one family, we do not call them all Smith, because such a procedure would: not help us to dis- tinguish one from the other; nor do we call them simply John, James, Peter, William, and so on, for that would not help us to identify them as of one family. So we give them all two names, one indicating their close relation, and the other their separate individuality —as John Smith, James Smith, Peter Smith, William Smith, &. The same thing is done in Zoology; only, in accordance with the genius of the Latin language, 16 THE NATURAL HISTORY OF THE COMMON CRAYFISH. we put the Christian name, so to speak, after the sur- name. There are a number of kinds of Crayfish, so similar to one another that they bear the common surname of Astacus. One kind, by way of distinction, is called fuwiatile, another slender-handed, another Dauric, from the region in which it lives; and these double names are rendered by—Astacus fluviatilis, Astacus leptodactylus, and Astacus dauricus; and thus we have a nomenclature: which is exceedingly simple in principle, and free from confusion in practice. And I may add that, the less attention is paid to the original meaning of the sub- stantive and adjective terms of this binomial nomen- clature, and the sooner they are used as proper names, the better. Very good reasons for using a term may exist. when it is first invented, which lose their validity: with the progress of knowledge. Thus Astacus fluviatilis. was a significant name so long as we knew of only one kind of crayfish ; but now that we are acquainted with a | number of kinds, all of which inhabit rivers, it is meaning- less. Nevertheless, as changing it would involve endless confusion, and the object of nomenclature is simply to have a definite name for a definite thing, nobody dreams of proposing to alter it. Having learned this much about the origin of the names of the crayfish, we may next proceed to consider those points which an observant Naturalist, who did not THE SKELETON EXTERNAL AND CALCIFIED. 17 care to go far beyond the surface of things, would find to notice in the animal itself. . Probably the most conspicuous peculiarity of the cray- fish, to any one who is familiar only with the higher animals, is the fact that the hard parts of the body are outside and the soft parts inside; whereas in ourselves, and in the ordinary domestic animals, the hard parts, or bones, which constitute the skeleton, are inside, and the soft parts clothe them. Hence, while our hard framework is said to be an endoskeleton, or internal skeleton; that of the crayfish is termed an exoskeleton, or external skeleton. It is from the circumstance that the body of the crayfishes is enveloped in this hard crust, that the name of Crustacea is applied to them, along with the crabs, shrimps, and other such animals. Insects, spiders, and centipedes have also a hard exoskeleton, but it is usually not so hard and thick as in the Crustacea. If‘a piece of the crayfish’s skeleton is placed in strong vinegar, abundant bubbles of carbonic acid gas are given off from it, and it rapidly becomes converted into a soft laminated membrane, while the solution will be found to contain lime. In fact the exoskeleton is composed of a peculiar animal matter, so much impregnated with carbonate and phosphate of lime that it becomes dense and hard. It will be observed that the body of the crayfish is naturally marked out into several distinct regions. There Fig. 2.—Astacus flwwiatilis.—Dorsal or tergal views (nat. size). A, male; B, female :—bcg, branchio-cardiac groove, which marks the boun- dary between the pericardial and the branchial cavities ; eg, cervical groove ; these letters are placed on the carapace ; 7, rostrum ; ¢, ¢’, the two divisions of the telson ; 1, eye-stalks; 2, antennules ; 3, antenne; 20, lateral lobes of tail-fin ; Xxv-xx, somites of the abdomen. THE EXOSKELETON. 19 is a firm and solid front part, covered by a large con- tinuous shield, which is called the carapace ; and a jointed hind part, commonly termed the tail (fig. 2). From the perception of a partially real, and partially fanciful, analogy with the regions into which the body is divided in the higher animals, the fore part is termed the cepha- lo-thoraz, or head (cephalon) and chest (thorax) com- bined, while the hinder part receives the name of abdomen. Now the exoskeleton is not of the same constitution throughout these regions. The abdomen, for example, is composed of six complete hard rings (fig. 2, xv-xx), and a terminal flap, on the under side of which the vent (fig. 8, a) is situated, and which is called the telson (fig. 2, t, ¢’). All these are freely moveable upon one another, inasmuch as the exoskeleton which connects them is not calcified, but is, for the most part, soft and flexible, like the hard exoskeleton when the lime salts have been removed by acid. The mechanism of the joints will have to be attentively considered by-and-by; it is sufficient, at present, to remark that, wherever a joint exists, it is produced in the same fashion, by the exo- skeleton remaining soft in certain regions of the jointed part. The carapace is not jointed ; but a transverse groove is observed about the middle of it, the ends of which run down on the sides and then turn forwards (figs. 1 and 2, cg). This is called the cervical groove, and it marks off 20 THE NATURAL HISTORY OF THE COMMON CRAYFISH. the region of the head, in front, from that of the thorax behind. The thorax seems at first not to be jointed at all; but if its under, or what is better called its sternal, surface is examined carefully, it will be found to be divided into as many transverse bands, or segments, as there are pairs of legs (fig. 8); and, moreover, the hindermost of these segments is not firmly united with the rest, but can be moved backwards and forwards through a small space (fig. 8, B; xiv). Attached to the sternal side of every ring of the abdomen of the female there is a pair of limbs, called swimmerets. In the five anterior rings, these are small and slender (fig. 83, B; 15, 19); but those of the sixth ring are very large, and each ends in two broad plates (20). These two plates on each side, with the telson in the middle, constitute the flapper of the crayfish, by the aid of which it executes its retrograde swimming movements. The small swimmerets move together with a regular swing, like paddles, and probably aid in propelling the animal forwards. In the breeding female (B), the eggs are attached to them; while, in the male, the two anterior pairs (A; 15,16) are converted into the peculiar styles which distinguish that sex. The four pairs of legs which are employed for walking purposes, are divided into a number of joints, and the foremost two pairs are terminated by double claws, arranged so as to form a pincer, whence they are said to errr TTT TT yO Fic. 3.—Astacus fluviatilis, Ventral or sternal views (nat. size). A, male; B, female :— a, vent; gg, opening of the green gland; Jb, labrum; mé, metastoma or lower lip; od, opening of the oviduct; vd, that of the vas deferens. 1, eye-stalk; 2, antennule; 3, antenna; 4, mandible; 8, second maxillipede ; 9, third or external maxillipede ; 10, forceps ; 11, first leg; 14, fourth leg; 15, 16, 19, 20, first, second, fifth, and sixth abdominal appendages ; x., x1., x1v., sterna of the fourth, fifth, and eighth thoracic somite ; xv1., sternum of the second abdominal somite. In the male, the 9th to the 14th and the 16th to the 19th appendages are removed on the animal’s left side: in the female, the antenna (with the exception of its basal joint) and the 5th to the 14th appendages on the animal's right are removed ; the eggs also are shown atlached to the swimmerets of the left side of the body. 22 THE NATURAL HISTORY CF THE COMMON CRAYFISH. be chelate. The two hindermost pairs, on the other hand, end in simple claws. In front of these legs, come the great prehensile limbs (10), which are chelate, like those which im- mediately follow them, but vastly larger. They often receive the special name of chele; and the large terminal joints are called the “hand.” We shall escape confusion. if we call these limbs the forceps, and restrict the name of chela to the two terminal joints. All the limbs hitherto mentioned subserve locomotion and prehension in various degrees. The crayfish swims by the help of its abdomen, and the hinder pairs of ab- dominal limbs ; walks by means of the four hinder pairs . of thoracic limbs ; lays hold of anything to fix itself, or to assist in climbing, by the two chelate anterior pairs of these limbs, which are also employed in tearing the food seized by the forceps and conveying it to the mouth; while it seizes its prey and defends itself with the forceps. The part which each of these limbs plays is termed its function, and it is said to be the organ of that function; so that all these limbs may be said to be organs of the functions of locomotion, of offence and defence. In front of the forceps, there is a pair of limbs which have a different character, and take a different direction from any of the foregoing (9). These limbs, in fact, are turned directly forwards, parallel with one another, and with the middle line of the body. They are divided into a number of joints, of which one of those near the base THE FOOT-JAWS AND THE JAWS. 23 is longer than the rest, and strongly toothed along the inner edge, or that which is turned towards its fellow. It is obvious that these two limbs are well adapted to crush and tear whatever comes between them, and they are, in fact, jaws or organs of manducation. At the same time, it will be noticed that they retain a curiously close general resemblance to the hinder thoracic legs; and hence, for distinction’s sake, they are called outer foot- jaws, or external mazillipedes. If the head of a stout pin is pushed between these external maxillipedes, it will be found that it passes without any difficulty into the interior of the body, through the mouth. In fact, the mouth is relatively rather a large aperture; but it cannot be seen without forcing aside; not only these external foot-jaws, but a number of other limbs, which subserve the same function of manducation, or chewing and crushing the food. We may pass by the organs of manducation, for the present, with the remark that there are altogether three pairs of maxillipedes, followed by two pairs of somewhat differently formed mazille, and one pair of very stout and strong jaws, which are termed the mandibles (4). All these jaws work from side to side, in contradistinction to the jaws of vertebrated animals, which move up and down. In front of, and above the mouth, with the jaws which cover it, are seen the long feelers, which are called the antenne (8); above, and in front of them, follow the small feelers, or antennules (2) ; and over them, again, lie 24 THE NATURAL HISTORY OF THE COMMON CRAYFISH. the eye stalks (1). The antenne are organs of touch ; the antennules, in addition, contain the organs of hear- ing; while, at the ends of the eyestalks, are the organs of vision. Thus we see that the crayfish has a jointed and segmented body, the rings of which it is composed being very obvious in the abdomen, but more obscurely trace- able elsewhere; that it has no fewer than twenty pairs of what may be called by the general name of ap- pendages; and that these appendages are turned to different uses, or are organs of different functions, in different parts of the body. The crayfish is obviously a very complicated piece of living machinery. But we have not yet come to the end of all the organs that may be discovered even by cursory inspection. Every one who has eaten a boiled crayfish, or a lobster, knows that the great shield, or carapace, is very easily separated from the thorax and abdomen, the head and the limbs which belong to that region coming away with the carapace. The reason of this is not far to seek. The lower edges of that part of the carapace which belongs to the thorax approach the bases of the legs pretty closely, but a cleft-like space is left; and this cleft extends forwards to the sides of the region of the mouth, and backwards and upwards, between the hinder margin of the carapace and the sides of the first ring of the abdo- men, which are partly overlapped by, and partly overlap, that margin. If the blade of a pair of scissors is care- THE BRANCHIAL CHAMBER AND THE GILLS. 25 fully introduced into the cleft from behind, as high up as it will go without tearing anything, and a cut is then made, parallel with the middle line, as far as the cervical groove, and thence following the cervical groove to the base of the outer foot-jaws, a large flap will be removed. This flap of the carapace is called the branchiostegite (fig. 1, bg), because it covers the gills or branchie (fig. 4), which are now exposed. They have the appear- ance of a number of delicate plumes, which take a direc- tion from the bases of the legs upwards and forwards behind, upwards and backwards in front, their summits converging towards the upper end of the cavity in which they are placed, and which is called the branchial chamber. These branchie are the respiratory organs ; and they perform the same functions as the gills of a fish, to which they present some similarity. _ If the gills are cleared away, it is seen that the branchial cavity is bounded, on the inner side, by a sloping wall, formed by a delicate, but more or less calcified layer of the exoskeleton, which constitutes the proper outer wall of the thorax. At the upper limit of the branchial cavity, the layer of exoskeleton is very thin, and turning out- wards, is continued into the inner wall or lining of the branchiostegite, which is also very thin (see fig. 15, p. 70). Thus the branchial chamber is altogether outside the body, to which it stands in somewhat the same relation as the space between the flaps of a man’s coat and his waistcoat would do to the part of the body enclosed by the Fic. 4.—Astacus fiwiatilis.—In A, the gills, exposed by the removal of the branchio- stegite, are seen in their natural position ; in B, the podobranchiz (see p. 75) are re- moved, and the anterior set of arthrobranchie turned downwards (x 2): 1, eye-stalk ; 2, antennule ; 3, antenna; 4, mandible ; 6, scaphognathite ; 7, first maxillipede, in B the epipodite, to which the line points, is partly removed ; 8, second maxillipede ; 9, third maxillipede ; 10, forceps ; 14, fourth ambulatory leg; 15, first abdominal appendage ; xv., first, and xv1., second abdominal somite; arb. 8, arb. 9, arb. 18, the posterior arthrobranchiz of the second and third maxillipedes and of the third ambulatory leg ; arb’. 9, arb’. 13, the anterior arthrobranchie of the third maxillipede and of the third ambulatory leg ; pbd. 8, podobranchie of the second maxillipede ;_ pod. 13, that of the third ambulatory leg; plb. 12, plb. 13, the two rudimentary pleurobranchiz ; plb. 14, the functional pleurobranchia ; r, rostrum. THE BREATHING APPARATUS. 27 waistcoat, if we suppose the lining of the flaps to be made in one piece with the sides of the waistcoat. Or a closer parallel still would be brought about, if the skin of a man’s back were loose enough to be pulled out, on each side, into two broad flaps covering the flanks. It will be observed that the branchial chamber is open behind, below, and in front ; and, therefore, that the water in which the crayfish habitually lives has free ingress and egress. ‘Thus the air dissolved in the water enables breathing to go on, just as it does in fishes. As is the case with many fishes, the crayfish breathes very well out of the water, if kept in a situation sufficiently cool and moist to prevent the gills from drying up; and thus there is no reason why, in cool and damp weather, the crayfish should not be able to live very well on land, at any rate among moist herbage, though whether our common crayfishes do make such terrestrial excur- sions is perhaps doubtful. We shall see, by-and-by, that there are some exotic crayfish which habitually live on land, and perish if they are long submerged in water. With respect to the internal structure of the crayfish, there are some points which cannot escape notice, how- ever rough the process of examination may be. Thus, when the carapace is removed in a crayfish which has been just killed, the heart is seen still pulsating. It is an organ of considerable relative size (fig. 5, h), which is situated immediately beneath the Fic. 5.—Astacus fluviatilis—A male specimen, with the roof of the carapace and the terga of the abdominal somites removed to show the viscera (nat. size) :—aa, antennary artery ; ag, anterior gastric ‘ muscles; amm, adductor muscles of the mandibles; cs, cardiac portion of the stomach ; gg, green glands ; h, heart; hg, hind gut, © or large intestine ; 7’, liver; oa, ophthalmic artery ; pg posterior « gastric muscles ; sac, superior abdominal artery ; ¢, testis ; ed, vas deferens. THE “CRABS’-EYES,” 29 middle region of that part of the carapace which lies behind the cervical groove; or, in other words, in the dorsal region of the thorax. In front of it, and therefore in the head, is a large rounded sac, the stomach (fig. 5, cs; fig. 6, cs, ps), from which a very delicate intestine (figs. 5 and 6, hg) passes straight back through the thorax and abdomen to the vent-(fig. 6, a). Fig. 6.—Astacus fluviatilis.—A longitudinal vertical section of the ali- mentary canal, with the outline of the body (nat. size) :—a, vent; ag, anterior gastric muscle ; bd, entrance of left bile duct; cg, cervical groove; e@, cecum; cpr, cardio-pyloric valve ; cs, cardiac portion of stomach; the circular area immediately below the end of the line from cs marks the position of the gastrolith of the left side; fg, hind-gut; 72, labrum; 7, lateral tooth of stomach ; m, mouth ; mg, mid-gut ; mt, median tooth ; &, cesophagus; pe, pro- cephalic process ; yg, posterior gastric muscle ; ps, pyloric portion of stomach; 7, annular ridge, marking the commencement of the hind-gut. In summer, there are commonly to be found at the sides of the stomach two lenticular calcareous masses, which are known as ‘‘ crabs’-eyes,” or gastroliths, and were, in old times, valued in medicine as sovereign remedies for all sorts of disorders. These bodies (fig. 7) are smooth and flattened, or concave, on the side which is turned towards 30 THE NATURAL HISTORY OF THE COMMON CRAYFISH. the cavity of the stomach; while the opposite side, being convex and rough with irregular prominences, is some- thing like a “‘ brain-stone ” coral. Moreover, when the stomach is laid open, three large below ; C, from one side (all x 5); D, in vertical section (x 20). reddish teeth are seen to project conspicuously into its interior (fig. 6, lé, mt); so that, in addition to its six pairs of jaws, the crayfish has a supplementary crushing mill in its stomach. On each side of the stomach, there is a soft yellow or brown mass, commonly known as the THE GROWTH OF THE: CRAYFISH. 3l liver (fig. 5, Lr); and, in the breeding season, the ovaries of the females, or oreaus in which the eggs are formed, are very conspicuous from the dark-coloured eggs which they contain, and which, like the exoskeleton, turn red when they are boiled. The corresponding part in a cooked lobster goes by the name of the “ coral.” Beside these internal structures, the most noticeable are the large masses of flesh, or muscle, in the thorax and abdomen, and in the pincers; which, instead of being red, as in most of the higher animals, is white. It will further be observed that the blood, which flows readily when a crayfish is wounded, is a clear fluid, and is either almost colourless, or of a very pale reddish or neutral tint. Hence the older Naturalists thought that the crayfish was devoid of blood, and had merely a sort of ichor in place of it. But the fluid in question is true blood ; and if it is received into a vessel, it soon forms a soft, but firm, gelatinous clot. The crayfish grows rapidly in youth, but enlarges more and more slowly as age advances. The young animal which has just left the egg is of a greyish colour, and about one quarter of an inch long. By the end of the year, it may have reached nearly an inch and a half in length. Crayfishes of a year old are, on an average, two inches long; at two years, two inches and four-fifths ; at three years, three inches and a half; at four years, four inches and a half nearly; and at five years, five inches. They 32 THE NATURAL HISTORY OF THE COMMON CRAYFISH. go on growing till, in exceptional cases, they may attain | between seven inches and eight inches in length; but at what degree of longevity this unusual dimension is reached is uncertain. It seems probable, however, that the life of © these animals may be prolonged to as much as fifteen or twenty years. They appear to reach maturity, so far as the power of reproduction is concerned, in their fifth or, more usually, their sixth year. However, I have seen a female, with eggs attached under the abdomen, only | two inches long, and therefore, probably, in her second year. The males are commonly larger than females of the same age. The hard skeleton of a crayfish, once formed, is incapable of being stretched, nor can it increase by in- terstitial addition to its substance, as the bone of one of the higher animals grows. Hence it follows, that the enlargement of the body, which actually takes place, involves the shedding and reproduction of its invest- ment. This might be effected by insensible degrees, and in different parts of the body at different times, as we shed our hair; but, as a matter of fact, it occurs periodi- cally and universally, somewhat as the feathers of birds are moulted. ‘The whole of the old coat of the body is thrown off at once, and suddenly; and the new coat, which has, in the meanwhile, been formed beneath — the old one, remains soft for a time, and allows of a rapid increase in the dimensions of the body before it THE SHEDDING OF THE SKIN, 33 hardens. This sort of moulting is what is technically termed ecdysis, or exuviation. It is commonly spoken of as the “shedding of the skin,” and there is no harm in using this phrase, if we recollect that the shed coat is not the skin, in the proper sense of the word, but only what is termed a cuticular layer, which is secreted upon the outer surface of the true integument. The cuticular skeleton of the crayfish, in fact, is not even so much a part of the skin as the cast of a snake, or as our own nails. For these are composed of coherent, formed parts of the epidermis; while the hard investment of the crayfish con- tains no such formed parts, and is developed on the out- side of those structures which answer to the constituents of the epidermis in the higher animals. Thus the cray- fish.grows, as it were, by starts ; its dimensions remaining stationary in the intervals of its moults, and then rapidly increasing for a few days, while the new exoskeleton is in the course of formation. The ecdysis of the crayfish was first thoroughly studied a century and a half ago, by one of the most accurate observers who ever lived, the famous Réaumur, and the following account of this very curious process is given nearly in his words.* A few hours before the process of exuviation com- * See Réaumur’s two Memoirs, “Sur les diverses reproductions qui se font dans les écrevisses, les omars, les crabes, etc.,” “ Histoire de l’Académie royale des Sciences,’ année 1712 ; and “ Additions aux ob- servations sur la mue des écrevisses données dans les Mémoires de 1712.” Ibid. 1718. 3 384 THE NATURAL HISTORY OF THE COMMON CRAYFISH, mences, the crayfish rubs its limbs one against the other, and, without changing its place, moves each separately, throws itself on its back, bends its tail, and then stretches it out again, at the same time vibrat- ing its antenne. By these movements, it gives the various parts a little play in their loosened sheaths. After these preparatory steps, the crayfish appears to become distended; in all probability, in consequence of the commencing retraction of the limbs into the interior of the exoskeleton of the body. In fact, it has been remarked, that if, at this period, the extremity of one of the great claws is broken off, it will be found empty, the contained soft parts being retracted as far as the second joint. The soft membranous part of the exo- skeleton, which connects the hinder end of the carapace with the first ring of the abdomen, gives way, and the body, covered with the new soft integument, protrudes; its dark brown colour rendering it easily distinguishable from the greenish-brown old integument. Having got thus far, the crayfish rests for a while, and then the agitation of the limbs and body recommences. The carapace is forced upwards and forwards by the pro- trusion of the body, and remains attached only in the region of the mouth. The head is next drawn backwards, while the eyes and its other appendages are extracted from their old investment. Next the legs are pulled out, either one at a time, or those of one, or both, sides together. Sometimes a limb gives way and is left behind in its sheath. THE SHEDDING OF THE SKIN, 35 The operation is facilitated by the splitting of the old integument of the limb along one side longitudinally. When the legs are disengaged, the animal draws its head and limbs completely out of their former covering ; and, with a sudden spring forward, while it extends its abdomen, it extracts the latter, and leaves its old skele- ton behind. The carapace falls back into its ordinary position, and the longitudinal fissures of the sheaths of the limbs close up so accurately, that the shed integu- ment has just the appearance the animal had when the exuviation commenced. The cast exoskeleton is so like the crayfish itself, when the latter is at rest, that, except for the brighter colour of the latter, the two cannot be distinguished. After exuviation, the owner of the cast skin, ex- hausted by its violent struggles, which are not unfre- quently fatal, lies in a prostrate condition. Instead of being covered by a hard shell, its integument is soft and flabby, like wet paper; though Réaumur remarks, that if a crayfish is handled immediately after exuviation, its body feels hard; and he ascribes this to the violent con- traction which its muscles have undergone, leaving them in a state of cramp. In the absence of the hard skeleton, however, there is nothing to bring the contracted muscles at once back into position, and it must be some time before the pressure of the internal fluids is so distributed as to stretch them out. When the process of exuviation has proceeded so far 36 THE NATURAL HISTORY OF THE COMMON CRAYFISH. that the carapace is raised, nothing stops the crayfish from continuing its struggles. If taken out of the water in this condition, they go on moulting in the hand, and even pressure on their bodies will not arrest their efforts. The length of time occupied from the first giving way of the integuments to the final emergence of the animal, varies with its vigour, and the conditions under which it is placed, from ten minutes to several hours. The chitinous lining of the stomach, with its teeth, and the ‘‘ crabs’-eyes,” are shed along with the rest of the cuti- cular exoskeleton ; but they are broken up and dissolved in the stomach. The new integuments of the crayfish remain soft for a period which varies from one to three days ; and it is a curious fact, that the animal appears to be quite aware of its helplessness, and governs itself accordingly. An observant naturalist says: ‘‘I once had a do- mesticated crayfish (Astacus fluviatilis), which I kept in a glass pan, in water, not more than an inch and a half deep, previous experiment having shown that in deeper : water, probably from want of sufficient aération, this - animal would not live long. By degrees my prisoner became very bold, and when I held my fingers at the edge of the vessel, he assailed them with promptness and energy., About a year after I had him, I perceived, as I thought, a second crayfish with him. On examination, I found it to be his old coat, which he had left in a most perfect state. My friend had now lost his heroism, and > THE REPRODUCTION OF LIMBS. 37 fluttered about in the greatest agitation. He was quite soft ; and every time I entered the room during the next two days, he exhibited the wildest terror. On the third, he appeared to gain confidence, and ventured to use his nippers, though with some timidity, and he was not yet quite so hard as he had been. In about a week, how- ever, he became bolder than ever; his weapons were sharper, and hé appeared stronger, and a nip from him was no joke. He lived in all about two years, during which time his food was a very few worms at very uncer- tain times ; perhaps he did not get fifty altogether.’’* It would appear, from the best observations that have yet been made, that the young crayfish exuviate two or three times in the course of the first year; and that, afterwards, the process is annual, and takes place usually about midsummer. There is reason to suppose that very old crayfish do not exuviate every year. It has been stated that, in the course of its violent efforts to extract its limbs from the cast-off exoskeleton, the crayfish sometimes loses one or other of them; the limb giving way, and the greater part, or the whole, of it remaining in the exuvie. But it is not only in this way that crayfishes part with their limbs. At all times, if the animal is held by one of its pincers, so that it cannot get away, it is apt to solve the difficulty by casting off * The late Mr. Robert Ball, of Dublin, in Bell’s “ British Crustacea,” p. 239. 388 THE NATURAL HISTORY OF THE COMMON CRAYFISU, the limb, which remains in the hand of the captor, while the crayfish escapes. This voluntary amputation is always effected at the same place; namely, where the limb is slenderest, just beyond the articulation which unites the basal joint with the next. The other limbs also readily part at the joints; and it is very common to meet with crayfish which have undergone such mutilation. But the injury thus inflicted is not permanent, as these animals possess the power of reproducing lost parts to a marvellous extent, whether the loss has been inflicted by artificial amputation, or voluntarily. Crayfishes, like all the Crustacea, bleed very freely when wounded ; and if one of the large joints of a leg is cut through, or if the animal’s body is injured, it is very likely to die rapidly from the ensuing hemorrhage. A cray- fish thus wounded, however, commonly throws off the limb at the next articulation, where the cavity of the limb is less patent, and its sides more readily fall together ; and, as we have seen, the pincers are usually cast off at their narrowest point. When such amputation has taken place, a crust, probably formed of coagulated blood, rapidly forms over the surface of the stump ; and, eventually, it becomes covered with a cuticle. Beneath this, after a time, a sort of bud grows out from the centre of the surface of the stump, and gradually takes on the form of as much of the limb as has been removed, At the next ecdysis, the covering cuticle is thrown off along with the rest of the exoskeleton; while the rudi- THE REPRODUCTION OF THE SPECIES, 39 mentary limb straightens out, and, though very small, acquires all the organization appropriate to that limb. At every moult it grows; but, itis only after a long time that it acquires nearly the size of its uninjured and older fellow. Hence, it not unfrequently happens, that crayfish are found with pincers and other limbs, which, though alike useful and anatomically complete, are very unequal in size. Injuries inflicted while the crayfish are soft after moulting, are apt to produce abnormal growths of the part affected; and these may be perpetuated, and give rise to various monstrosities, in the pincers and in other parts of the body. In the reproduction of their kind by means of eggs the co-operation of the males with the females is necessary. On the basal joint of the hindermost pair of legs of the male a small aperture is to be seen (fig. 3, A; vd). In these, the ducts of the apparatus in which the fecundating substance is formed terminate. The fecundating material itself is a thickish fluid, which sets into a white solid after extru- sion. The male deposits this substance on the thorax of the female, between the bases of the hindermost pairs of thoracic limbs. The eggs formed in the ovary are conducted to apertures, which are situated on the bases of the last pair of ambula- tory legs but two, that is, in the hinder of the two pair which are provided with chelate extremities (fig. 3, B; od), 40 THE NATURAL HISTORY OF THE COMMON CRAYFISH. After the female has received the deposit of the spermatic matter of the male, she retires to a burrow, in the manner already stated, and then the process of laying the eggs commences. These, as they leave the apertures of the oviducts, are coated with a viscid matter, which is readily drawn out into a short thread. The end of the thread attaches itself to one of the long hairs, with which the swimmerets are fringed, and as the viscid matter rapidly hardens, the egg thus becomes attached to the limb by a stalk. The operation is repeated, until sometimes a couple of hundred eggs are thus glued on to the swimmerets. Partaking in the movements of the swimmerets, they are washed backwards and forwards in the water, and thus aérated and kept free of impurities ; while the young crayfish is formed much in the same way as the chick is formed in a hen’s egg. The process of development, however, is very slow, as it occupies the whole winter. In late spring-time, or early summer, the young burst the thin shell of the egg, and, when they are hatched, present a general re-— semblance to their parents. This is very unlike what takes place in crabs and lobsters, in which the young leave the egg in a condition very different from the parent, and undergo a remarkable metamorphosis before they attain their proper form. For some time after they are hatched, the young hold on to the swimmerets of the mother, and are carried about, protected by her abdomen, as in a kind of nursery. NEWLY-HATCHED CRAYFISHES. Al That most careful naturalist, Roesel von Rosenhof, says of the young, when just hatched :— ‘At this time they are quite transparent ; and when Fic. 8.—Astacus fluviatilis —A, two recently hatched crayfish attached to one of the swimmerets of the mother (x 4). pr, protopodite ; en, endopodite ; and ex, exopodite of the swimmeret ; ec, ruptured egg-cases. B, chela of a recently hatched crayfish (x 10), such a crayfish [a female with young] is brought to table, it looks quite disgusting to those who do not know 42° THE NATURAL HISTORY OF THE COMMON CRAYFISH. what the young are; but if we examine it more closely, especially with a magnifying-glass, we see with pleasure that the little crayfish are already perfect, and resemble the large one in all respects. When the mother of these little crayfish, after they have begun to be active, is quiet for a while, they leave her and creep about, a short way off. But, if they spy the least sign of danger, or there is any unusual movement in the water, it seems as if the mother recalled them by a signal; for they all at once swiftly return under her tail, and gather into a cluster, and the mother hies to a place of safety with them, as quickly as she can. A few days later, however, they gradually forsake her.” * Fishermen declare that ‘‘ Hen Lobsters” protect their young in a similar manner.t Jonston,t who wrote in the middle of the seventeenth century, says that the little crayfish are often to be seen adhering to the tail of the mother. Roesel’s observations imply the same thing; but he does not describe the exact mode of adherence, and I can find no observations on the subject in the works of later writers. It has been seen that the eggs are attached to the swimmerets by a viscid substance, which is, as it were, smeared over them and the hairs with which they are * “Der Monatich-herausgegeben Insecten Belustigung.” Dritter Theil, p. 336. 1755. + Bell’s “ British Crustacea,” p. 249. + “Joannis Jonstoni Historie naturalis de Piscibus et Cetis Libri quinque. TomusIV. ‘De Cammaro seu Astaco fluviatili, ” NEWLY-HATCHED CRAYFISHES. 43 fringed, and is continued by longer or shorter thread-like pedicles into the coat of the same material which invests each egg. It very soon hardens, and then becomes very firm and elastic. When the young crayfish is ready to be hatched, the egg case splits into two moieties, which remain attached, like a pair of watch glasses, to the free end of the pedicle of the egg (fig. 8, A; ec). The young animal, though very similar to the parent, does not quite ‘‘resemble it in all respects,” as Roesel says. For not only are the first and the last pairs of abdominal limbs wanting, while the telson is very different from that of the adult; but the ends of the great chele are sharply pointed and bent down into abruptly in- curved hooks, which overlap when the chele are shut (fig. 8, B). Hence, when the chele have closed upon anything soft enough to allow of the imbedding of these hooks, it is very difficult, if not impossible, to open them again. Immediately the young are set free, they must instinc- tively bury the ends of their forceps in the hardened egg-glue which is smeared over the swimmerets, for they are all found to be holding on in this manner. They exhibit very little movement, and they bear rough shaking or handling without becoming detached; in consequence, I suppose, of the interlocking of the hooked ends of the chele imbedded in the egg-glue. Even after the female has been plunged into alcohol, the young remain attached. I have had a female, with young affixed in this manner, under observation for five 44 THE NATURAL HISTORY OF THE COMMON CRAYFISH. days, but none of them showed any signs of detaching themselves ; and I am inclined to think that they are set free only at the first moult. After this, it would appear that the adhesion to the parent is only temporary, The walking legs are also hooked at their extremities, but they play a less important part in fixing the young to the parent, and seem to be always capable of loosing their hold. I find the young of a Mexican crayfish (Cambarus) to be attached in the same manner as those of the English crayfish; but, according to Mr. Wood-Mason’s recent observations, the young of the New Zealand crayfishes fix themselves to the swimmerets of the parent by the hooked ends of their hinder ambulatory limbs. Crayfishes, in every respect similar to those found in our English rivers, that is to say, of the species Astacus fluviatilis, are met with in Ireland, and on the Continent, as far south as Italy and northern Greece; as far east as western Russia; and as far north as the «shores of the Baltic. They are not known to occur in Scotland; in Spain, except about Barcelona, they are either rare, or have remained unnoticed. There is, at present, no proof of the occurrence of Astacus fluviatilis in the fossil state. Curious myths have gathered about crayfishes, as about other animals. At one time “crabs’-eyes” were CRAYFISHES AND PIGS. 45 collected in vast numbers, and sold for medicinal purposes as a remedy against the stone, among other diseases. Their real utility, inasmuch as they consist almost entirely of carbonate of lime, with a little phos- phate of lime and animal matter, is much the same as that of chalk, or carbonate of magnesia. It was, for- merly, a current belief that crayfishes grow poor at the time of new moon, and fat at that of full moon; and, perhaps, there may be some foundation for the notion, considering the nocturnal habits of the animals. Van Helmont, a great dealer in wonders, is responsible for the story that, in Brandenburg, where there is a great abundance of crayfishes, the dealers were obliged to transport them to market by night, lest a pig should run under the cart. For if such a misfortune should happen, every crayfish would be found dead in the morning: “ Tam exitialis est porcus cancro.” Another author improves the story, by declaring that the steam of a pig-stye, or of a herd of swine, is instantaneously fatal to crayfish. On the other hand, the smell of putrifying crayfish, which is undoubtedly of the strongest, was said to drive even moles out of their burrows. CHAPTER II. THE PHYSIOLOGY OF THE CRAYFISH. THE MECHANISM BY WHICH THE PARTS OF THE LIVING ENGINE ARE SUPPLIED WITH THE MATERIALS NECESSARY FOR THEIR MAIN- TENANCE AND GROWTH. Aw analysis of such a sketch of the ‘“‘ Natural History of the Crayfish” as is given in the preceding chapter, - shows that it provides brief and general answers to three questions. First, what is the form and structure of the animal, not only when adult, but at different stages of its growth? Secondly, what are the various actions of which it is capable? Thirdly, where is it found? If we carry our investigations further, in such a manner as to give the fullest attainable answers to these questions, the knowledge thus acquired, in the case of the first question, is termed the Morphology of the crayfish; in the case of the second question, it constitutes the Physiology of the animal; while the answer to the third question would represent what we know of its Distribu- tion or Chorology. ‘There remains a fourth problem, which can hardly be regarded as seriously under dis- cussion, so long as knowledge has advanced no further than the Natural History stage; the question, namely, TELEOLOGY AND PHYSIOLOGY. 47 how all these facts comprised under Morphology, Physi- ology, and Chorology have come to be what they are ; and the attempt to solve this problem leads us to the crown of Biological effort, Aitiology. When it supplies answers to all the questions which fall under these four heads, the Zoology of Crayfish will have said its last word. As it matters little in what order we take the first three questions, in expanding Natural History into Zoology, we may as well follow that which accords with the history of science. After men acquired a rough and general knowledge of the animals about them, the next thing which engaged their interest was the discovery in these animals of arrangements by which results, of a kind similar to those which their own ingenuity effects through mechanical contrivances, are brought about. They observed that animals perform various actions ; and, when they looked into the disposition and the powers of the parts by which these actions are performed, they found that these parts presented the characters of an apparatus, or piece of mechanism, the action of which could be deduced from the properties and connections of its constituents, just as the striking of a clock can be deduced from the properties and connections of its weights and wheels. Under one aspect, the result of the search after the rationale of animal structure thus set afoot is Teleology; | ‘or the doctrine of adaptation to purpose. Under another 48 THE PHYSIOLOGY OF THE COMMON CRAYFISH. aspect, it is Physiology; so far as Physiology consists in the elucidation of complex vital phenomena by deduction from the established truths of Physics and Chemistry, or. from the elementary properties of living matter. We have seen that the crayfish is a voracious and indiscriminate feeder ; and we shall be safe in assuming that, if duly supplied with nourishment, a full-grown crayfish will consume several times its own weight of food in the course of the year. Nevertheless, the increase of the animal’s weight at the end of that time is, at most, a small fraction of its total weight; whence it is quite clear, that a very large proportion of the food taken into the body must, in some shape or other, leave it again. In the course of the same period, the crayfish absorbs a very considerable quantity of oxygen, supplied by the atmosphere to the water which it inhabits ; while it gives out, into that water, a large amount of carbonic acid, and a larger or smaller quantity of nitrogenous and other ex- crementitious matters. From this point of view, the crayfish may be regarded as a kind of chemical manu- factory, supplied with certain alimentary raw materials, which it works up, transforms, and gives out in other shapes. And the first physiological problem which offers itself to us is the mode of operation of the apparatus contained in this factory, and the extent to which the products of its activity are to be accounted for by reasoning from known physical and chemical principles. THE PROCESS OF FEEDING, 49 We have learned that the food of the crayfish is made up of very diverse substances, both animal and vegetable ; but, so far as they are competent to nourish the animal permanently, these matters all agree in containing a peculiar nitrogenous body, termed protein, under one of its many forms, such as albumen, fibrin, and the like. With this may be associated fatty matters, starchy and sac- charine bodies, and various earthy salts. And these, which are the essential constituents of the food, may be, and usually are, largely mixed up with other substances, such as wood, in the case of vegetable food, or skeletal and fibrous parts, in the case of animal prey, which are of little or no utility to the crayfish. The first step in the process of feeding, therefore, is to reduce the food to such a state, that the separation of its nutritive parts, or those which can be turned to account, from its innutritious, or useless, constituents, may be facilitated. And this preliminary operation is the subdivision of the food into morsels of a convenient size for introduction into that part of the machinery in which the extraction of the useful products is performed. The food may be seized by the pincers, or by the anterior chelate ambulatory limbs; and, in the former case, it is usually, if not always, transferred to the first, or second, or both of the anterior pairs of ambulatory limbs. These grasp the food, and, tearing it into pieces of the proper dimensions, thrust them between the external maxillipedes, which are, at the same time, 50 THE PHYSIOLOGY OF THE COMMON CRAYFISH. worked rapidly to and fro sideways, so as to bring their toothed edges to bear upon the morsel. The other five pairs of jaws are no less active, and they thus crush and divide the food brought to them, as it is passed between their toothed edges to the opening of the mouth. As the alimentary canal stretches from the mouth, at one end, to the vent at the other, and, at each of these limits, is continuous with the wall of the body, we may conceive the whole crayfish to be a hollow cylinder, the cavity of which is everywhere closed, though it is traversed by a tube, open at each end (fig. 6). The shut cavity between the tube and the walls of the cylinder may be termed the perivisceral cavity; and it is so much filled up by the various organs, which are inter- posed between the alimentary canal and the body wall, that all that is left of it is represented by a system of irregular channels, which are filled with blood, and are termed blood sinuses. The wall of the cylinder is the outer wall of the body itself, to which the general name of integument may be given; and the outermost layer of ‘this, again, is the cuticle, which gives rise to the whole of the exoskeleton. This cuticle, as we have seen, is extensively impregnated with lime salts; and, moreover, in consequence of its containing chitin, it is often spoken of as the chitinous cuticula. Having arrived at this general conception of the dis- position of the parts of the factory, we may next proceed to consider the machinery of alimentation which is con- THE MACHINERY OF ALIMENTATION, 51 tained within it, and which is represented by the various divisions of the alimentary canal, with its appendages; by the apparatus for the distribution of nutriment; and by two apparatuses for getting rid of those products which are the ultimate result of the working of the whole organism. And here we must trench somewhat upon the province of Morphology, as some of these pieces of apparatus are complicated ; and their action cannot be comprehended without a certain knowledge of their anatomy. The mouth of the crayfish is a longitudinally elongated, parallel-sided opening, in the integument of the ventral or sternal aspect of the head. Just outside its lateral boundaries, the strong mandibles project, one on each side (fig 3, B.; 4); their broad crushing surfaces, which are turned towards one another, are therefore completely external to the oral cavity. In front, the mouth is over- lapped by a wide shield-shaped plate termed the upper lip, or labrum (figs. 8 and 6, 1b); while, immediately be- hind the mandibles, there is, on each side, an elongated fleshy lobe, joined with its fellow by the posterior boundary of the mouth. These together constitute the metastoma (fig. 8, B; mt), which is sometimes called the lower lip. A short wide gullet, termed the ceso- phagus (fig. 6, 0c), leads directly upwards into a spacious bag, the stomach, which occupies almost the whole cavity of the head. It is divided by a constriction into a large anterior chamber (cs), into the under face of which the 52 THE PHYSIOLOGY OF THE COMMON CRAYFISH. gullet opens, and a small posterior chamber (ps), from which the intestine (hg) proceeds. In a man’s stomach, the opening by which the gullet communicates with the stomach is. called the cardia, while that which places the stomach in communication with the intestine is named the pylorus ; and these terms having been transferred from human anatomy to that of the lower animals, the larger moiety of the crayfish’s stomach is called the cardiac division, while the smaller is termed the pyloric division of the organ. It must be recollected, however, that, in the crayfish, the so-called cardiac division is that which is actually furthest from the heart, not that which is nearest to it, as in man. The gullet is lined by a firm coat which resembles thin parchment. At the margins of the mouth, this strong lining is easily seen to be continuous with the cuticular exoskeleton; while, at the cardiac orifice, it spreads out and forms the inner or cuticular wall of the whole gastric cavity, as far as the pylorus, where it ends in certain valvular projections. The chitinous cuticle which forms the outermost layer of the integument is thus, as it were, turned in, to constitute the innermost layer of the walls of the stomach; and it confers upon them so great an amount of stiffness that they do not collapse when the organ is removed from the body. Furthermore, just as the cuticle of the integument is calcified to form the hard parts of the exoskeleton, so is the cuticle of the stomach calcified, or otherwise hardened, to give rise, in the first THE STOMACH OF THE CRAYFISH. 53 place, to the very remarkable and complicated apparatus which has already been spoken of, as a sort of gastric mill Fia. 9.—Astacus fluviatilis._-A, the stomach with its outer coat removed, seen from the left side ; B, the same viewed from the front, after removal of the anterior wall ; C, the ossicles of the gastric mill separated from one another; D, the prepy- lorie ossicle and median tooth, seen from the right side; E, transverse section of * the pyloric region along the line zy in A (all x 2). ¢, cardiac ossicle; epv, cardio- pyloric valve ; lp, lateral pouch ; Zt, lateral tooth, seen through the wall of the stomach in A; mg, mid-gut; mé, median tooth, seen through the wall of the stomach in A; ws, esophagus; p, pyloric ossicle; pe, pterocardiac ossicle ; pp, prepyloric ossicle; uc, uro-cardiac process ; t, convexities on the free surface of its hinder end ; v!, median pyloric valve ; zc, zygocardiac ossicle. or food-crusher; and, secondly, to a filter or strainer, whereby the nutritive juices are separated from the in- nutritious hard parts of the food and passed on into the intestine. 54 THE PHYSIOLOGY OF THE COMMON CRAYFISH. The gastric mill begins in the hinder half of the cardiac division. Here, on the upper wall of the stomach, we see a broad transverse calcified bar (figs. 9-11, c) from the middle of the hinder part of which another bar (uc), united to the first by a flexible portion, is continued backwards in the middle line. The whole has, therefore, somewhat the shape of a cross-bow. Behind the first- mentioned piece, the dorsal wall of the stomach is folded in, in such a manner as to give rise to a kind of pouch; and the second piece, or what we may call the handle of the crossbow, lies in the front wall of this pouch. The end of this piece is dense and hard, and its free surface, which looks into the top of the cardiac chamber, is raised into two oval, flattened convex surfaces (¢). Con- nected by a transverse joint with the end of the handle of the crossbow, there is another solid bar, which ascends obliquely forwards in the back wall of the pouch (pp). The end which is articulated with the handle of the cross- bow is produced into a strong reddish conical tvoth (mt), curved forwards and bifurcated at the summit; conse- quently, when the cavity of the stomach is inspected from the fore part of the cardiac pouch (fig. 9, B), the two- pointed curved tooth (mt) is seen projecting behind the convex surfaces (¢), in the middle line, into the interior of that cavity. The joint which connects the handle of the crossbow with the hinder middle piece is elastic ; hence, if thetwo are straightened out, they return to their bent dis- position as soon as they are released. The upper end of - THE GASTRIC MILL. 55 the hinder middle piece (pp) is connected with a second flat transverse plate which lies in the dorsal wall of the pyloric chamber (p). The whole arrangement, thus far, may be therefore compared to a large cross-bow and a small one, with the ends of their handles fastened together by a spring joint, in such a manner that the handle of the one makes an acute angle with the handle of the other ; while the middle of each bow is united with the middle of the other by the bent arm formed by the two handles. But, in addition to this, the outer ends of the two bows are also connected together. A small, curved, calcified bar (pc) passes from the outer end of the front crosspiece downwards and outwards in the wall of the stomach, and its hinder and lower extremity is articulated with another larger bar (zc) which runs upwards and backwards to the hinder or pyloric crosspiece, with which it articulates. Internally, this piece projects into the cardiac cavity of the stomach as a stout elongated reddish elevation (id), the surface of which is produced into a row of strong sharp, transverse ridges, which diminish in size from before backwards, and constitute a crushing surface almost like that of the grinder of an elephant. Thus, when the front part of the cardiac cavity is cut away, not only are the median teeth already mentioned seen, but, on each side of them, there is one of these long lateral teeth. There are two small pointed teeth, one under each of the lateral teeth, and each of these is supported by 56 THE PHYSIOLOGY OF THE COMMON CRAYFISH. a broad plate, hairy on its inner surface, which enters into the lateral wall of the cardiac chamber. ‘There are various other smaller skeletal parts, but the most im- PP: Fig. 10.— Astacus fuviatilis,—Longitudinal section of the stomach (x4), e, cardiac ossicle; ce, cecum ; ¢.p.v, cardio-pyloric valve; cs, cushion- shaped surface ; hg, hind-gut ; hp, aperture of right bile duct ; Jp, lateral pouch ; /¢, lateral teeth ; mg, mid-gut ; mt, median tooth ; @s, cesophagus ; 7, pyloric ossicle ; ye, pterocardiac ossicles ; pp, prepy- loric ossicle ; we, urocardiac process ; v!, median pyloric valve; v’, lateral pyloric valve; x,position of gastrolith; ze, zygocardiac ossicle. portant are those which have been described; and these, from what has been said, will be seen to form a sort of hexagonal frame, with more or less flexible joints at the angles, and having the anterior and the posterior sides THE GASTRIC MILL. 57 connected by a bent jointed middle bar. As all these parts are merely modifications of the hard skeleton, the apparatus is devoid of any power of moving itself. It is set in motion, however, by the same substance as that which gives rise to all the other bodily movements of the crayfish, namely, muscle. The chief muscles which move it are four very strong bundles of fibres. Two of these are attached to the front crosspiece, and proceed thence, upwards and forwards, to be fixed to the inner face of the carapace in the front part of the head (figs. 5, 6, and 12, ag): The two others, which are fixed into the hinder crosspiece and hinder lateral pieces, pass upwards and backwards, to be attached to the inner face of the carapace in the back part of the head (pg). When these muscles shorten, or contract, they pull the front and back crosspieces further away from one another; consequently, the angle between the handles becomes more open and the tooth which is borne on their ends travels downwards and forwards. But, at the same time, the angle between the side bars becomes more open and the lateral tooth of each side moves inwards till it crosses in front of the middle tooth, and strikes against this and the opposite lateral tooth, which has undergone a corresponding change of place. The muscles being now relaxed, the elasticity of the joints suffices to bring the whole apparatus back to its first position, when a new contraction brings about a new clashing of the. teeth. Thus, by the alternate con- traction and relaxation of these two pair of muscles, the 4 58 THE PHYSIOLOGY OF THE COMMON CRAYFISH. three teeth are made to stir up and crush whatever is contained in the cardiac chamber. When the stomach is removed and the front part of the cardiac chamber is cut away, the front cross-piece may be seized with one pair of forceps and the hind cross-piece with another. On slightly pulling the two, so as to imitate the action of the muscles, the three teeth will be found to come together sharply, exactly in the manner described. Works on mechanics are full of contrivances for the conversion of motion; but it would, perhaps, be difficult to discover among these a prettier solution of the problem; given a straight pull, how to convert it into three simul- taneous convergent movements of as many points. What I have called the filter is constructed mainly out of the chitinous lining of the pyloric chamber. The aper- ture of communication between this and the cardiac chamber, already narrow, on account of the constriction of the walls of the stomach at this point, is bounded at the sides by two folds; while, from below, a conical tongue- shaped process (figs. 6, 10, and 11, cpv), the surface of which is covered with hairs, further obstructs the opening, In the posterior half of the pyloric chamber, its side walls are, as it were, pushed in; and, above, they so nearly meet in the middle line, that a mere vertical chink is left be- tween them ; while even this is crossed by hairs set upon the two surfaces. In its lower half, however, each side wall curves outwards, and forms a cushion-shaped surface (fig. 10, cs) which looks downwards and inwards. Tf the THE FILTERING APPARATUS. 59 floor of the pyloric chamber were flat, a wide triangular passage would thus be left open in its lower half. But, in fact, the floor rises into a ridge in the middle, while, at the sides, it adapts itself to the shape of the two cushion- shaped surfaces; the result of which is that the whole cavity of the posterior part of the pyloric division of the stomach is reduced to a narrow three-rayed fissure. In transverse section, the vertical ray of this fissure is straight, while the two lateral ones are concave upwards (fig. 9, #). The cushions of the side walls are covered with short close-set hairs. The corresponding surfaces of the floor are raised into longitudinal parallel ridges, the edge of each of which is fringed with very fine hairs. As everything which passes from the cardiac sac to the intestine must traverse this singular apparatus, only the most finely divided solid matters can escape stoppage, so long as its walls are kept together. Finally, at the opening of the pyloric sac into the intestine, the chitinous investment terminates in five symmetrically arranged processes, the disposition of which is such that they must play the part of valves in preventing any sudden retura of the contents of the intestine to the stomach, while they readily allow of a passage the other way. One of these valvular processes is placed in the middle line above (figs. 10 and 11, v}). It is longer than the others and concave below. The lateral processes (v?,) of which there are two on each side, are triangular and flat. 60 THE PHYSIOLOGY OF THE COMMON CRAYFISH. ‘The cuticular lining which gives rise to all the com- plicated apparatus which has just been described, must Fig. 11.—Astacus fluviatilis—View of the roof of the stomach, the ventral wall of which, and of the mid-gut, is laid open by a longi- tudinal incision ( x 4). On the right side (the left in the figure), the lateral tooth is cut away, a8 well as the floor of the lateral pouch. -The letters have the same signification as in fig. 10. not be confounded with the proper wall of the stomach, which invests it, and to which it owes it origin, just as the cuticle of the integument is produced by the soft FORE-GUT, MID-GUT, AND HIND-GUT. 61 true skin which lies beneath it. The wall of the stomach is a soft pale membrane containing variously disposed muscular fibres ; and, beyond the pylorus, it is continued into the wall of the intestine. It has already been mentioned that the intestine is a slender and thin-walled tube, which passes straight through the body almost without charige, except that it becomes a little wider and thicker-walled near the vent. Immediately behind the pyloric valves, its surface is quite smooth and soft (figs. 9, 10, and 12, mg), and its floor presents a relatively large aperture, the termination of the bile duct (fig. 12, dd, fig. 10, hp.), on each side. The roof is, as it were, pushed out into a short median pouch or cecum (ce). Behind this, its character suddenly changes, and six squarish elevations, covered with a chitinous cuticle, encircle the cavity of the intestine (r). From each of these, a longitudinal ridge, corresponding with a fold of the wall of the intestine, takes its rise, and passes, with a slight spiral twist, to its extremity (hg). Each of these ridges is beset with small papille, and the chitinous lining is continued over the whole to the vent, where it passes into the general cuticle of the integu- ment, just as the lining of the stomach is continuous with the cuticle of the integument at the mouth. The alimentary canal may, therefore, be distinguished into a fore and a hind-gut (hg), which have a thick internal lining of cuticular membrane; and a very short mid- gut (mg), which has no thick cuticular layer. It will be of ‘OpIs 4JeT ouy Jo sesepuedde peurmopqe WIXIs ‘og pue ‘puooss ‘oy “asm ‘ez ‘sdeogoy 4Joq “OF { epedtyIxeuL peureyxe Jo] ‘G ¢ e[QIP -uem 440] ‘Y { ofnuUaque 4ySIr ‘G | suaTeyap seA 4JoT Jo oIngIede ‘pa { suaTejEp SA qJOT “pa ! stqseq ‘(qrvey 94} ream) 2 £ wostey “(9JoT oY 01) 2 $ A1O47e TeUTMOpge aoITedns ‘wus { L10478 yeurays “vs { yowutoys Jo womszod optoyéd ‘sd ‘ uowazesut 941 04 Avme yno AUS oyy ‘sepsnut olyses rorraysod ‘6d ‘snseqdosa ‘a ‘ Axrogie ormpeysydo ‘vo ‘qn8-prut ‘hw : reat, yoy ‘uz {qreoy Jo erngiode peroqey yqysta ‘v7 { 10478 TeuTMLOpge JOIZesUT ‘vv, § qn8-pury ‘Fy $ Ax047e oneday ‘ny ‘qreeq “y $ Uolsues peurmopge 4suy ‘gy ‘ws $ UOITsuBs [eeseqdosa-qus ‘gz ‘wA £ wos -uwed peoseqdoseowrdns ‘7 ‘wh ¢ £r0q7e o1sysed ‘vf { uatMOpge Jo saposnur ToxeR ‘ws § uowoOpge Jo soposnu iostezxe “wa ‘Yowuloys Jo uoT4I0d ovtpxvo ‘sa * ofOsnUT epoLte appre qystr ‘wud $ mmoae ‘2 $ PORULOJS JO SATOSNUL LOFOLIASUOD “wo ! app 9114 WYSIt Jo orngrode ‘pq { woryrosut s4¢ 0} ABA QNO FSI at ‘soposnu o1y4ses Ioyteqzue ‘hy £ qz10Y8 quo ‘K1947e AreuUEyUR ‘wv { snue ‘Dv ‘(oats "qvU) Opts qYsI1 9y} Wor outpoeds e[eU B Jo MOMoossIP Y—'s7p7MLANY snonisy>—ZT “DI aL PA VOS mg Nee THE DIGESTION OF FOOD, 63 importance to recollect this distinction by-and-by, when the development of the alimentary canal is considered. If the treatment to which the food is subjected in the alimentary apparatus were of a purely mechanical nature, there would be nothing more to describe in this part of the crayfish’s mechanism. But, in order that the nutritive matters may be turned to account, and undergo the chemical metamorphoses, which eventually change them into substances of a totally different cha- racter, they must pass out of the alimentary canal into the blood. And they can do this only by making their way through the walls of the alimentary canal; to which end they must either be in a state of extremely fine division, or they must be reduced to the fluid condition. In the case of the fatty matters, minute subdivision may suffice; but the amylaceous substances and the insoluble protein compounds, such as the fibrin of flesh, must be brought into a state of solution. Therefore some sub- stances must be poured into the alimentary canal, which, when mixed with the crushed food, will play the part of a chemical agent, dissolving out the insoluble proteids, changing the amyloids into soluble sugar, and convert- ing all the proteids into those diffusible forms of protein matter, which are known as peptones. The details of the processes here indicated, which may be included under the general name of digestion, have only quite recently been carefully investigated in the crayfish; and we have probably still much to learn about G+ THE PHYSIOLOGY OF THE COMMON CRAYFISH. them; but what has been made out is very interesting, and proves that. considerable differences exist between crayfishes and the higher animals in this respect. The physiologist calls those organs, the function of which is to prepare and discharge substances of a special character, glands; and the matter which they elaborate is termed their secretion. On the one side, glands are in relation with the blood, whence they derive the materials which they convert into the substances characteristic of their secretion; on the other side, they have access, directly or indirectly, to a free surface, on to which they pour their secretion as it is formed. Of such glands, the alimentary canal of the crayfish is provided with a pair, which are not only of very large size, but are further extremely conspicuous,- on account of their yellow or brown colour. These two glands (figs. 12 and 138, Jv) are situated beneath, and on each side of, the stomach and the anterior part of the intestine, and answer in position to the glands termed liver and pancreas in the higher animals, inasmuch as they pour their secretion into the mid-gut. These glands have hitherto always been re- garded as the liver, and the name may be retained, though their secretion appears rather to correspond with the pancreatic fluid than with the bile of the higher animals. Each liver consists of an immense number of short tubes, or ceca, which are closed at one end, but open at the other into a general conduit, which is termed their duct. The mass of the liver is roughly divided into Fig, 18.—Astacus fluviatilis.—The alimentary canal and livers seen from above (nat. size). dd, bile-duct ; cw, cecum ; ¢s, cardiac portion of stomach, the line pointing to the cardiac ossicle ; Ag, hind-gut ; mg, mid-gut ; ye, pterocardiac ossicle ; ys, pyloric portion of stomach, the line pointing to the pyloric ossicle ; 7, ridge separating mid-gut from hind-gut ; zc, zygocardiac ossicle. 66 THE PHYSIOLOGY OF THE COMMON CRAYFISH. three lobes, one anterior, one lateral, and one posterior ; and each lobe has its main duct, into which all the tubes composing it open. The three ducts unite together into a wide common duct (bd), which opens, just behind the py- loric valves, into the floor of the mid-gut. Hence the aper- tures of the two hepatic ducts are seen, one on each side, in this part of the alimentary canal ,when it is laid open from above. Every cecum of the liver has a thin outer wall, lined internally by a layer of cells, constituting what is termed an epithelium; and, at the openings of the hepatic ducts, this epithelium passes into a layer of some- what similar structure, which lines the mid-gut, and is continued through the rest of the alimentary canal, beneath the cuticula. Hence the liver may be regarded as a much divided side pouch of the mid-gut. The epithelium is made up of nucleated cells, which are particles of simple living matter, or protoplasm, in the midst of each of which is a rounded body, which is termed the nucleus. It is these cells which are the seat of the manufacturing process which results in the formation of the secretion ; it is, as it were, their special business to form that secretion. To this end they are constantly being newly formed at the summits of the ceca. As they grow, they pass down towards the duct and, at the same time, separate into their interior certain special products, among which globules of yellow fatty matter are very conspicuous. When these products are fully formed, what remains of the substance of the cells dissolves away, and THE DIGESTION OF FOOD. 67 the yellow fluid accumulating in the ducts passes into the mid-gut. The yellow colour is due to the globules of fat. In the young cells, at the summit of the cxca, these are either absent, or very small, whence the part appears colourless. But, lower down, small yellow granules appear in the cells, and these become bigger and more numerous in the middle and lower parts. In fact, few glands are better fitted for the study of the manner in which secretion is effected than the crayfish’s liver. We may now consider the alimentary machinery, the general structure of which has been explained, in action. The food, already torn and crushed by the jaws, is passed through the gullet into the cardiac sac, and there reduced to a still more pulpy state by the gastric mill. By degrees, such parts as are sufficiently fluid are drained off into the intestine, through the pyloric strainer, while the coarser parts of the useless matters are probably rejected by the mouth, as a hawk or an owl rejects his casts. There is reason to believe, though it is not certainly known, that fluids from the intestine mix with the food while it is undergoing trituration, and effect the transforma- tion of the starchy and the insoluble protein compounds into asoluble state. At any rate, as soon as the strained-off fluid passes into the mid-gut it must be mixed with the secretion of the liver, the action of which is probably 68 THE PHYSIOLOGY OF THE COMMON CRAYFISH. similar to that of the pancreatic juice of the higher animals. The mixture thus produced, which answers to the chyle of the higher animals, passes along the intestine, and the greater part of it, transuding through the walls of the alimentary canal, enters the blood, while the rest accumulates as dark coloured feces in the hind gut, and Fig. 14.—Astacus fluviatilis—The corpuscles of thé blood (highly mag- nified). 2-8 show the changes undergone by a single corpuscle during a quarter of an hour; 9 and 10 are corpuscles killed by magenta, and having the nucleus deeply stained by the colouring matter. 7, nucleus. is eventually passed out of the body by the vent. The fecal matters are small in amount, and the strainer is so efficient that they rarely contain solid particles of sensible size. Sometimes, however, there are a good many minute fragments of vegetable tissue. The blood of which the nutritive elements of ‘the food THE BLOOD AND ITS CORPUSCLES. 69 have thus become integral parts, is a clear fluid, either colourless, or of a pale neutral tint or reddish hue, which, to the naked eye, appears like so much water. But if subjected to microscopic examination, it is found to con- tain innumerable pale, solid particles, or corpuscles, which, when examined fresh, undergo constant changes of form (fig. 14). In fact, they correspond very closely with the colourless corpuscles which exist in our own blood; and, in its general characters, the crayfish’s blood is such as ours would be if it were somewhat diluted and deprived of its red corpuscles. In other words, it resembles our lymph more than it does our blood. Left to itself it soon coagulates, giving rise to a pretty firm clot. The sinuses, or cavities in which the greater part of the blood is contained, are disposed very irregularly in the intervals between the internal organs. But there is one of especially large size on the ventral or sternal side of the thorax (fig. 15, sc), into which all the blood in the body sooner or later makes its way. From this sternal sinus passages (av) lead to the gills, and from these again six canals (bev), pass up on the inner side of the inner wall of each branchial chamber to a cavity situated in the dorsal region of the thorax, termed the pericardium (p), into which they open. The blood of the crayfish is kept in a state of con- stant circulating motion by a pumping and distributing machinery, composed of the heart and of the arteries, with laa. Fig. 15.—Astacus fluviatilis—A diagrammatic transverse section of the thorax through the twelfth somite, showing the course of the circulation of the blood (x 3). arb. 12, the anterior or lower, and arb’. 12, the posterior or upper arthrobranchia of the twelfth somite ; av, afferent branchial vessel ; dev, branchio-cardiac véin ; bg, branchiostegite; em, extensor muscles of abdomen; ep, epi- meral wall of thoracic cavity; ev, efferent branchial vessel; fm, flexor muscles of abdomen ; fp, floor of pericardium ; gn. 6, fifth thoracic ganglion ; /, heart ; ig, hind-gut ; iaa, inferior abdominal artery, in cross section ; Ja, lateral valvular apertures of heart ; 7, liver ; mp, indicates the position of the mesophragm by which the sternal canal is bounded laterally ; », pericardial sinus ; pdb. 12, _podobranchia, and plb. 12, pleurobranchia of the twelfth somite ; sa,sternal artery ; saa, superior abdominal artery ; sc, sternal canal ; t, testis; XII, sternum of twelfth somite. The arrows indicate the direction of the blood flow. THE HEART AND THE ARTERIES, 71 their larger and smaller branches, which proceed from it and ramify through the body, to terminate eventually in the blood sinuses, which represent the veins of the higher animals. When the carapace is removed from the middle of the region which lies behind the cervical groove, that is, when the dorsal or tergal wall of the thorax is taken away, a spacious chamber is laid open which is full of blood. This is the cavity already mentioned as the. peri- cardium (fig: 15, DP); though, as it differs in some respects from that which is so named in the higher animals, it will be better to term it the pericardial sinus. The heart (fig. 15, h}, lies in the midst of this sinus. It is a thick muscular body (fig. 16), with an irregularly hexa- gonal contour when viewed from above, one angle of the hexagon being anterior and another posterior. The lateral angles of the hexagon are connected by bands of fibrous tis- sue (ac) with the walls of the pericardial sinus. Otherwise, the heart is free, except in so far as it is kept in place by the arteries which leave it and traverse the walls of the peri- _cardium. One of these arteries (figs. 5, 12, and 16, saa), starting from the hinder part of the heart, of which it is a sort of continuation, runs along the middle line of the abdomen above the intestine, to which it gives off many branches. A second large artery starts from a dilatation, which is common to it with the foregoing, but passing directly downwards (figs. 12 and 15, sa, and fig. 16, st. a), either on the right or on the left side of the intestine, 72 THE PHYSIOLOGY OF THE COMMON CRAYFISH. traverses the nervous cord (figs. 12 and 15), and divides into an anterior (fig. 12, sa) and a posterior (taa) branch, both of which run beneath and parallel with that cord. Fig 16.—Astaeus fluviatilis.—The heart (x 4). A, from above ; B, from below ; C, from the left side. aa, antennary artery ; ac, ale cordis, or fibrous bands connecting the heart with the walls of the peri- cardial sinus ; 2, bulbous dilatation at the origin of the sternal artery ; ha, hepatic artery ; Ja, lateral valvular apertures ; oa, oph- thalmic artery ; s.a, superior valvular apertures ; s.a.a, superior abdominal artery ; st.a, sternal artery, in B cut off close to its origin. A third artery runs, from the front part of the heart, forwards in the middle line, over the stomach, to the eyes and fore part of the head (figs. 5, 12, and 16, oa); and two others diverge one on each side of this, and sweep THE ACTION OF THE HEART, 73 round the stomach to the antenne (aa). Behind these, yet two other arteries are given off from the under side of the heart, and supply the liver (ha). All these arteries branch out and eventually terminate in fine, so-called capillary, ramifications. In the dorsal wall of the heart two small oval aper- tures are visible, provided with valvular lips (fig. 16, sa), which open inwards, or towards the internal cavity of the heart. There is a similar aperture in each of the lateral faces of the heart (Ja), and two others in its inferior face (ta), making six in all. These apertures readily admit fluid into the heart, but oppose its exit. On the other hand, at the origins of the arteries, there are small valvular folds, directed in such a manner as to permit the exit of fluid from the heart, while they prevent its entrance. The walls of the heart are muscular, and, during life, they contract at intervals with a regular rhythm, in such a manner as to diminish the capacity of the internal cavity of the organ. The result is, that the blood which it contains is driven into the arteries, and necessarily forces into their smaller ramifications an equivalent amount of the blood which. they already contained; whence, in the long run, the same amount of blood passes out of the ultimate capillaries’ into the blood sinuses. From the disposition of the blood sinuses, the impulse thus given to the blood which they contain is finally conveyed to the blood in the branchiz, and a proportional quantity of that 74. THE- PHYSIOLOGY OF THE COMMON CRAYFISH. blood leaves the branchiz and passes into the sinuses which connect them with the pericardial sinus (fig. 15, bev), and thence into that cavity. At the end of the contraction, or systole, of the heart, its volume is of course diminished by the volume of the blood forced out, and the space between the walls of the heart and those of the pericardial sinus is increased to the same extent. This space, how- ever, is at once occupied by the blood from the branchie, and perhaps by some blood which has not passed through the branchiz, though this is doubtful. When the systole is over, the diastole follows; that it to say, the elasticity of the walls of the heart and that of the various parts which connect it with the walls of the pericardium, bring it back to its former size, and the blood in the pericardial sinus flows into its cavity by the six apertures. Witha new systole the same process is repeated, and thus the blood is driven in a circular course through all parts of the body. It will be observed that the branchie are placed in the course of the current of blood which is returning to the heart; which is the exact contrary of what happens in fishes, in which the blood is sent from the heart to the branchiz, on its way to the body. It follows, from this arrangement, that the blood which goes to the branchie is blood in which the quantity of oxygen has undergone a diminution, and that of carbonic acid an increase, as compared with the blood in the heart itself. For the THE ORGANS OF RESPIRATION. 75 activity of all the organs, and especially of the muscles, is inseparably connected with the absorption of oxygen and the evolution of carbonic acid; and the only source from which the one can be derived, and the only recep- tacle into which the other can be poured, is the blood which bathes and permeates the whole fabric to which it is distributed by the arteries. The blood, therefore, which reaches the branchie has lost oxygen and gained carbonic acid; aud these organs constitute the apparatus for the elimination of the inju- rious gas from the economy on the one hand, and, on the other, for the taking in of a new supply of the needful ‘* vital air,” as the old chemists called it. It is thus that the branchie subserve the respiratory function. The crayfish has eighteen perfect and two rudimentary branchiz in each branchial chamber, the boundaries of which have been already described. f Of the eighteen perfect branchie, six (podobranchie) are attached to the basal joints of the thoracic limbs, from the last but one to the second (second maxillipede) inclusively (fig. 4, p. 26, pdb, and fig. 17, A, B); and eleven (arthro- branchie) are fixed to the flexible interarticular mem- branes, which connect these basal joints with the parts of the thorax to which they are articulated (fig. 4, arb, arb’, fig. 17, C). Of these eleven branchie, two are attached to the interarticular membranes of all the ambulatory legs but the last, (=6) and to those of the pincers and of the external maxillipedes, (=4) and one to that of the Fic. 17.—Astacus fluviatilis.—A, one of the podobranchiz from the outer side ; B, the same from the inner side ; C, one of the arthro- branchie ; D, a part of one of the coxopoditic seta ; E, extremity of the same seta ; F, extremity of a seta from the base of the podo- branchia ; G, hooked seta of the lamina; (A—C, x 3; D—G, highly magnified). 6, base of podobranchia; cs, coxopoditic sete; exp, coxopodite ; J, lamina, pl, plume, and st, stem of podobranchia ;- t, tubercle on the coxopodite, to which the sete are attached. ARTHROBRANCHLE AND PODOBRANCHIA. 77 second maxillipede. The first maxillipede and the last ambulatory limb have none. Moreover, where there are two arthrobranchie, one is more or less in front of and external to the other. These eleven arthrobranchie are all very similar in structure (fig. 17, C). Each consists of a stem which con- tains two canals, one external and one internal, separated by a longitudinal partition. The stem is beset with a great number of delicate branchial filaments, so that it looks like a plume tapering from its base to its summit. Each filament is traversed by large vascular channels, which break up into a net-work immediately beneath the surface. The blood driven into the externat canals of the stem (fig. 15, av) is eventually poured into the inner canal (ev), which again communicates with the channels (bev) which lead to the pericardial sinus (p). In its course, the blood traverses the branchial filaments, the outer investment of each of which is an excessively thin chitinous membrane, so that the blood contained in them is practically separated by a mere film from the aérated water in which the gills float. Hence, an exchange of gaseous constituents readily takes place, and as much oxygen is taken in as carbonic acid is given out. The six podobranchie, or gills which are attached to the basal joints of the legs, play the same part, but differ a good deal in the details of their structure from those which are fixed to the interarticular membranes. Each con- sists of a broad base (fig.17, Aand B; b) beset with many 78 THE PHYSIOLOGY OF THE COMMON CRAYFISH. fine straight hairs, or sete (F), whence a narrow stem (st) proceeds. At its upper end this stem divides into two parts, that in front, the plume (pl), resembling the free end of one of the gills just described, while that behind, the lamina (J), is a broad thin plate, bent upon itself longi- tudinally in such a manner that its folded edge lies for- wards, and covered with minute hooked sete (G). The gill which follows is received into the space included between the two lobes or halves of the folded lamina (fig. 4, p. 26). Hach lobe is longitudinally plaited into about a dozen folds. The whole front and outer face of the stem is beset with branchial filaments ; hence, we may compare one of these branchie to one of the preceding kind, in which the stem has become modified and has given off a large folded lamina from its inner and posterior face. The branchie now described are arranged in sets of three for each of the thoracic limbs, from the third maxillipede to the last but one ambulatory limb, and two for the second maxillipede, thus making seventeen in all (8 x 5 + 2=17); and, between every two there is found a bundle of long twisted hairs (fig. 17, A, cz.s; D and Ii), which are attached to a small elevation (¢) on the basal joint ofeach linb. These coxopoditic sete, no doubt, serve to prevent the intrusion of parasites and other foreign matters into the branchial chamber. From the mode of attachment of the six branchiz it is obvious that they must share in the movements of the basal joints of the PLEUROBRANCHLE, COMPLETE AND RUDIMENTARY. 79 legs; and that, when the crayfish walks, they must be more or less agitated in the branchial chamber. The eighteenth branchia resembles one of the eleven arthrobranchie in structure; but it is larger, and it is attached neither to the basal joint of the hindermost ambu- latory limb, nor to its interarticular membrane, but to the sides of the thorax, above the joint. From this mode of attachment it is distinguished from the others as a pleuro- branchia (fig. 4, plb. 14). Finally, in front of this, fixed also to the walls of the thorax, above each of the two preceding pairs of ambulatory limbs, there is a delicate filament, about a sixteenth of an inch long, which has the structure of a branchial filament, and is, in fact, a rudimentary pleurobranchia (fig. 4, plb. 12, plb. 13). The quantity of water which occupies the space left in the branchial chamber by the gills is but small, and as the respiratory surface offered by the gills is relatively very large, the air contained in. this water must be rapidly exhausted, even when the crayfish is quiescent; while, when any muscular exertion takes place, the quan- tity of carbonic acid formed, and the demand for fresh oxygen, is at once greatly increased. For the efficient performance of the function of respiration, therefore, the water in the branchial chamber must be rapidly renewed, and there must be some arrangement by which the supply of fresh water may be proportioned to the demand. In many animals, the respiratory surface is 80 THE PHYSIOLOGY OF THE COMMON CRAYFISH. covered with rapidly vibrating filaments, or cilia, by means of which a current of water is kept con- tinually flowing over the gills, but there are none of these in the crayfish. The same object is attained, however, in another way. The anterior boundary of the branchial chamber corresponds with the cervical groove, which, as has been seen, curves downwards and then forwards, until it terminates at the sides of the space occupied by the jaws. If the branchiostegite is cut away along the groove, it will be found that it is attached to the sides of the head, which project a little beyond the anterior part of the thorax, so that there is a depression behind the sides of the head—just as there is a depression, behind a man’s jaw, at the sides of the neck. Between this depression in front, the walls of the thorax internally, the branchiostegite externally, and the bases of the for- ceps and external foot-jaws below, a curved canal is in- cluded, by which the branchial cavity opens forwards as by a funnel. Attached to the base of the second maxilla there is a wide curved plate (fig. 4, 6) which fits against the projection of the head, as a shirt collar might do, to carry out our previous comparison ; and this scoop- shaped plate (termed the scaphognathite), which is con- cave forwards and convex backwards, can be readily moved backwards and forwards. If a living crayfish is taken out of the water, it will be found that, as the water drains away from the branchial cavity, bubbles of air are forced out of its anterior opening. THE RESPIRATORY CURRENT, 81 Again, if, when a crayfish is resting quietly in the water, a little coloured fluid is allowed to run down towards the posterior opening of the branchial chamber, it will very soon be driven out from the anterior aperture, with considerable force, in a long stream. In fact, as the scaphognathite vibrates not less than three or four times in a second, the water in the funnel-shaped front passage of the branchial cavity is incessantly baled out; - and, as fresh water flows in from behind to make up the loss, a current is kept constantly flowing over the gills. The rapidity of this current naturally depends on the more or less quick repetition of the strokes of the scaphognathite; and hence, the activity of the respira- tory function can be accurately adjusted to the wants of the economy. Slow working of the scaphognathite answers to ordinary breathing in ourselves, quick working to panting. A further self-adjustment of the respiratory apparatus is gained by the attachment of the six gills to the basal joints of the legs. For, when the animal exerts its muscles in walking, these gills are agitated, and thus not only bring their own surfaces more largely in contact with the water, but produce the same effect upon the other gills. The constant oxidation which goes on in all parts of the body not only gives rise to carbonic acid; but, so far as it affects the proteinaceous constituents, it produces 5 82 THE PHYSIOLOGY OF THE COMMON CRAYFISH. compounds which contain nitrogen, and these, like other waste products, must be eliminated. In the higher animals, such waste products take the form of urea, uric acid, hippuric acid, and the like; and are got rid of by the kidneys. We may, therefore, expect to find some organ which plays the part of a kidney in the crayfish; but the position of the structure, which there is much reason for regarding as the representative of the ‘kidney, is so singular that very different interpretations have been put upon it. , On the basal joint of each antenna it is easy to see a small conical eminence with an opening on the inner side of its summit (fig. 18). The aperture (x) leads by a short canal into a spacious sac, with extremely delicate walls (s), which is lodged in the front part of the head, in front of and below the cardiac division of the stomach (cs). Beneath this, in a sort of recess, which corresponds with the epistoma, and with the base of the antenna, there isa discoidal body of a dull green colour, in shape somewhat like one of the fruits of the mallow, which is known as the green gland (gg). ‘The sac narrows below like a wide funnel, and the edges of its small end are continuous with the walls of the green gland; they surround an aperture which leads into the interior of the latter structure, and conveys its products into the sac, whence they are excreted by the opening in the antennary papilla. The green gland is said to contain a substance termed guanin (so named because it is found in the guano which is the accumulated THE RENAL ORGAN. 83 excrement of birds), a nitrogenous body analogous in some respects to uric acid, but less highly oxidated ; Td. Fig. 18.—Astacus fluviatilis.—A, the anterior part of the body, with the dorsal portion of the carapace removed to show the position of the green glands ; B, the same, with the left side of the carapace removed; OC, the green gland removed from the body (all x 2). ag, left anterior gastric muscle; ¢, cireumcesophageal commis- sures; es, cardiac portion of stomach; gy. green gland, exposed in A on the left side by the removal of its sac; ima, inter- maxillary or cephalic apodeme ; @s, cesophagus seen in transverse section in A, the stomach being removed ; s, sac of green gland ; az, bristle passed from the aperture in the basal joint of the antenna into the sac. and if this be the case, there can be little doubt that the green gland represents the kidney, and its secretion 84 THE PHYSIOLOGY OF THE COMMON CRAYFISH. the urinary fluid, while the sac is a sort of urinary bladder. Restricting our attention to the phenomena which have now been described, and to a short period in the life of the crayfish, the body of the animal.may be regarded as a factory, provided with various pieces of machinery, by means of which certain nitrogenous and other matters are extracted from the animal and vegetable substances which serve for food, are oxidated, and are then delivered out of the factory in the shape of carbonic acid gas, guanin, and probably some other products, with which we are at present unacquainted. And there is no doubt, that if the total amount of products given out could be accurately weighed against the total amount of materials taken in, the weight of the two would be found to be identical. To put the matter in its most general shape, the body of the crayfish is a sort of focus to which certain material particles converge, in which they move for a time, and from which they are afterwards expelled in new combinations. ‘The parallel between a whirlpool in a stream and a living being, which has often been drawn, is as just as it is striking. The whirlpool is permanent, but the particles of water which constitute it are in- cessantly changing. Those which enter it, on the one side, are whirled around and temporarily constitute a part of its individuality ; and as they leave it on the other side, their places are made good by new comers. THE WHIRLPOOL OF LIFE. 85 Those who have seen the wonderful whirlpool, three miles below the Falls of Niagara, will not have forgotten the heaped-up wave which tumbles and tosses, a very embodiment of restless energy, where the swift stream hurrying from the Falls is compelled to make a sudden turn towards Lake Ontario. However changeful in the contour of its crest, this wave has been visible, approxi- mately in the same place, and with the same general form, for centuries.past. Seen from a mile off, it would appear to be astationary hillock of water. Viewed closely, it is a typical expression of the conflicting impulses generated by a swift rush of material particles. Now, with all our appliances, we cannot get within a good many miles, so to speak, of the crayfish. If we could, we should see that it was nothing but the constant form of a similar turmoil of material molecules which are constantly flowing into the animal on the one side, and streaming out on the other. The chemical changes which take place in the body of the crayfish, are doubtless, like other chemical changes, accompanied by the evolution of heat. But the amount of heat thus generated is so small and, in consequence of the conditions under which the crayfish lives, it is so easily carried away, that it is practically insensible. The crayfish has approximately the temperature of the sur- rounding medium, and it is, therefore, reckoned among the cold-blooded animals. If our investigation of the results of the process of 86 THE PHYSIOLOGY OF THE COMMON CRAYFISH. alimentation in a well-fed Crayfish were extended over a longer time, say a year or two, we should find that the products given out were no longer equal to the materials taken in, and the balance would be found in the inerease of the animal’s weight. If we inquired how the balance was distributed, we should find it partly in store, chiefly in the shape of fat ; while, in part, it had been spent in increasing the plant and in enlarging the factory. That is to say, it would have supplied the material for the animal’s growth. And this is one of the most remark- able respects in which the living factory differs from those which we construct. It not only enlarges itself, but, as we have seen, it is capable of executing its own repairs to a very considerable extent. CHAPTER III. THE PHYSIOLOGY OF THE CRAYFISH—THE MECHANISM BY WHICH THE LIVING ORGANISM ADJUSTS ITSELF TO SURROUNDING CONDITIONS AND REPRODUCES ITSELF. Ir the hand is brought near a vigorous crayfish, free to move in a large vessel of water, it will generally give a vigorous flap with its tail, and dart backwards out of reach; but if a piece of meat is gently lowered into the vessel, the crayfish will sooner or later approach and devour it. If we ask why the crayfish behaves in this fashion, every one has an answer ready. In the first case, it is said that the animal is aware of danger, and therefore hastens away; in the second, that it knows that meat is good to eat, and therefore walks towards it and makes a meal. And nothing can seem to be simpler or more satisfactory than these replies, until we attempt to con- ceive clearly what they mean ; and, then, the explanation, however simple it may be admitted to be, hardly retains its satisfactory character. For example, when we say that the crayfish is ‘‘ aware of danger,’ or ‘‘ knows that meat is good to eat,” what 88 THE PHYSIOLOGY OF THE COMMON CRAYFISH. do we mean by “being aware” and ‘ knowing”? Certainly it cannot be meant that the crayfish says to himself, as we do, “‘ This is dangerous,” ‘That is nice ;” for the crayfish, being devoid of language, has nothing to say either to himself or any one else. And if the cray- fish has not language enough to construct a proposition, itis obviously out of the question that his actions should be guided by a logical reasoning process, such as that by which a man would justify similar actions. The crayfish assuredly does not first frame the syllogism, “Dangerous things are to be avoided; that hand is dangerous ; therefore it is to be avoided; ”’ and then act upon the conclusion thus logically drawn. But it may be said that children, before they acquire the use of language, and we ourselves, long after we are familiar with conscious reasoning, perform a great variety of perfectly rational acts unconsciously. A child grasps at a sweetmeat, or cowers before a threatening gesture, before it can speak ; and any one of us would start back. from a chasm opening at our feet, or stoop to pick upa jewel from the ground, “without thinking about it.” And, no doubt, if the crayfish has any mind at all, his mental operations must more or less resemble those which the human mind performs without giving them a spoken or unspoken verbal embodiment. If we analyse these, we shall find that, in many cases, distinctly felt sensations are followed by a distinct desire to perform some act, which act is accordingly performed ; THE CRAYFISH MIND. 89 while, in other cases, the act follows the sensation with- out one being aware of any other mental process; and, in yet others, there is no consciousness even of the sensa- tion. As I wrote these last words, for example, I had not the slightest consciousness of any sensation of hold- ing or guiding the pen, although my fingers wére caus- ing that instrument to perform exceedingly complicated movements. Moreover, experiments upon animals have proved that consciousness is wholly unnecessary to the carrying out of many of those combined movements by which the body is adjusted to varying external conditions. Under these circumstances, it is really quite an open question whether a crayfish has a mind or not; more- over, the problem is an absolutely insoluble one, inas- much as nothing short of being a crayfish would give us positive assurance that such an animal possesses con- sciousness ; and, finally, supposing the crayfish has a mind, that fact does not explain its acts, but only shows that, in the course of their accomplishment, they are accompanied by phenomena. similar to those of which we are aware in ourselves, under like circumstances. So we may as well leave this question of the crayfish’s mind on one side for the present, and turn to a more profitable investigation, namely, that of the order and connexion of the physical phenomena which intervene between something which happens in the neighbourhood of the animal and that other something which responds to it, as an act of the crayfish. 90 THE PHYSIOLOGY OF THE COMMON CRAYFISH. Whatever else it may be, this animal, so far as it is acted upon by bodies around it and reacts on them, is a piece of mechanism, the internal works of which give rise to certain movements when it is affected by particular external conditions ; and they do this in virtue of their physical properties and connexions. Every movement of the body, or of any organ of the body, is an effect of one and the same cause, namely, muscular contraction. Whether the crayfish swims or walks, or moves its antenne, or seizes its prey, the imme- diate cause of the movements of the parts which bring about, or constitute, these bodily motions is to be sought in a change which takes place in the flesh, or muscle, which is attached to them. The change of place which constitutes any movement is an effect of a previous change in the disposition of the molecules of one or more muscles; while the direction of that move- ment depends on the connexions of the parts of the skeleton with one another, and of the museles with them. The muscle of the crayfish is a dense, white substance ; and if a small portion of it is subjected to examination it will be found to be very easily broken up into more or less parallel bundles of fine fibres. Each of these fibres is generally found to be ensheathed in a fine trans- parent membrane, which is called the sarcolemma, within which is contained the proper substance of the muscle. When quite fresh and living, this substance is soft and THE STRUCTURE OF MUSCLE. 91 semi-fluid, but it hardens and becomes solid immediately after death. Examined, with high magnifying powers, in this Fie. 19.—Astacus fluviatilis.—A, a single muscular fibre ; transverse diameter ;},th of an inch; B,a portion of the same more highly magnified; C, a smaller portion still more highly magnified ; D and E, the splitting up of a part of fibre into fibrille,; F, the connexion of a nervous with a muscular fibre which has been treated with acetic acid. w, darker, and 0, clearer portions of the fibrillee ; , nucleus of sarcolemma; nv, nerve fibre; s, sarcolemma; t, tendon; 1—5, successive dark bands answering to the darker ‘portions, a, of each fibrilla. 92 THE PHYSIOLOGY OF THE COMMON CRAYFISH. condition, the muscle-substance appears marked by very regular transverse bands, which are alternately opaque and transparent ; and it is characteristic of the group of animals to which the crayfish belongs that their muscle- substance has this striped character in all parts of the body. A greater or less number of these fibres, united into one or more bundles, constitutes a muscle ; and, except when these muscles surround a cavity, they are fixed at each end to the hard parts of the skeleton. The attachment is frequently effected by the intermediation of a dense, fibrous, often chitinous, substance, which constitutes the tendon (fig. 19, A; ¢) of the muscle. The property of the living muscle, which enables it to be the cause of motion, is this: Every muscular fibre is capable of suddenly changing its dimensions, in such a manner that it shortens and becomes proportionately thicker. Hence the absolute bulk of the fibre remains practically unchanged. From this circumstance, muscular contraction, as the change of form of a muscle is called, is radically different from the process which commonly goes by the same name in other things, and which involves a diminution of bulk. The contraction of muscle takes place with great force, and, of course, if the parts to which its ends are fixed are both free to move, they are brought nearer at the moment of contraction: if one only is free to move that is approximated to the fixed part; and if the muscular MUSCLE AS THE SOURCE OF MOTION. 93 fibre surrounds a cavity, the cavity is lessened when the muscle contracts. This is the whole source of motor power in the crayfish machine. The results produced by the exertion of that power depend upon the manner Fic. 20.—Astacus fluviatilis.—The chela of the forceps, with one side cut away to show, in A, the muscles, in B, the tendons (x 2). ep, carpopodite ; prp, propodite ; dp, dactylopodite ; m, adductor muscle; m’, abductor muscle; ¢, tendon of adductor muscle; 7, tendon of abductor muscle ; x, hinge. in which the parts to which the muscles are attached are connected with one another. One example of this has already been given in the curious mechanism of the gastric mill. Another may be found in the chela which terminates the forceps. If the 94 THE PHYSIOLOGY OF THE COMMON CRAYFISH. articulation of the last joint (fig. 20, dp) with the one which precedes it (prp) is examined, it will be found that the base of the terminal segment (dp) turns on two hinges (2), formed by the hard exoskeleton and situated at opposite points of the diameter of the base, on the penultimate segment; and these hinges are so disposed that the last joint can be moved only in one plane, to or from the produced angle of the penultimate segment (prp), which forms the fixed claw of the chela.. Between the hinges, on both the inner and the outer sides of the articulation, the exoskeleton is soft and flexible, and allows the terminal segment to play easily through a certain arc. It is by this arrangement that the direction and the extent of the motion of the free claw of the chela are determined. ‘The source of the motion lies in the muscles which occupy the interior of the enlarged penul- timate segment of the limb. Two muscles, one of very great size (m), the other smaller (m’), are fastened by one end to the exoskeleton of this segment. The fibres of the larger muscle converge to be fixed into the two sides of a long flat process of the chitinous cuticula, on the inner side of the base of the terminal segment, which serves as a tendon (é); while those of the smaller muscle are similarly attached to a like process which proceeds from the outer side of the base of the terminal seg- ment (é’). It is obvious that, when the latter muscle shortens it must move the apex of the terminal seg- ment (dp) away from the end of the fixed claw; while, MOTION DIRECTED BY JOINTS. 95 when the former contracts, the end of the terminal segment is brought towards that of the fixed claw. A living crayfish is able to perform very varied move- ments with its pincers. When it swims backwards, these limbs are stretched straight out, parallel with one another, in front of the head; when it walks, they are usually carried like arms bent at the elbow, the ‘“‘ forearm” partly resting on the ground; on being irritated, the crayfish sweeps the pincers round in any direction to grasp the offending body; when prey is seized, it is at once conveyed, with a circular motion, towards the region of the mouth. Nevertheless, these very varied actions are all brought about by a combination of simple flexions and extensions, each of which is effected in the exact order, and to the exact extent, needful to bring the chela into the position required. The skeleton of the stem of the limb which bears the chela is, in fact, divided into four moveable segments ; and each of these is articulated with the segments on each side of it by a hinge of just the same character as that which connects the moveable claw of the chela with the penultimate segment, while the basal segment is similarly articulated with the thorax. If the axes ofall these articulations * were parallel, it is obvious that, though the limb might be moved as a whole through a considerable arc, and might be bent in various * By axis of the articulation is meant a line drawn through the pair of hinges which constitute it, 96 THE PHYSIOLOGY OF THE COMMON CRAYFISH. degrees, yet all its movements would be limited to one plane. But, in fact, the axes of the successive articula- tions are nearly at right angles to one another; so that, if the segments are successively either extended or flexed, the chela describes a very complicated curve; and by varying the extent of flexion or extension of each segment, this curve is susceptible of endless varia- tion. It would probably puzzle a good mathematician to say exactly what position should be given to each segment, in order to bring the chela from any given position into any other; but if a lively crayfish is incautiously seized, the experimenter will find, to his cost, that the animal solves the problem both rapidly and accurately. The mechanism by which the retrograde swimming of the crayfish is effected, is no less easily analysed. The apparatus of motion is, as we have seen, the abdomen, with its terminal five-pointed flapper. The rings of the abdomen are articulated together by joints (fig. 21, x) situated a little below the middle of the height of the rings, at opposite ends of transverse lines, at right angles to the long axis of the abdomen. Each ring consists of a dorsal, arched portion, called the tergum (fig. 21; fig. 86, p. 142, t. XIX), and a nearly flat ventral portion, which is the sternum (fig. 86, st. XIX). Where these two join, a broad plate is sent down on each side, which overlaps the bases. of the abdominal appendages, and is known as the pleuron (fig. 86, pl. XIX). THE JOINTS OF THE ABDOMEN. 97 The sterna are all very narrow, and are connected together by wide spaces of flexible exoskeleton. When the abdomen is made straight, it will be found that these intersternal membranes are stretched as far as they will yield. On the other hand, when the abdomen \\ i HH yi Hi ‘Fig. 21.—Astacus fluviatilis.—Two of the abdominal somites, in vertical section, seen from the inner side, to show x, x, the hinges by which they are articulated with one another (x 3). The anterior of the two somites is that to the right of the figure. is bent up as far as it will go, the sterna come close together, and the intersternal membranes are folded. The terga are very broad; so broad, in fact, that each overlaps its successor, when the abdomen is straightened or extended, for nearly half its length in the middle line; and the overlapped surface is smooth, convex, and 98 THE PHYSIOLOGY OF THE COMMON CRAYFISH. marked off by a transverse groove from the rest of the tergum as an articular facet. The front edge of the articular facet is continued into a sheet of flexible cuti- cula, which turns back, and passes as a loose fold to the hinder edge of the overlapping tergum (fig. 21). This tergal interarticular membrane allows the terga to move as far as they can go in flexion; whilst, in extreme exten- sion, they are but slightly stretched. But, even if the in- tersternal membranes presented no obstacle to excessive extension of the abdomen, the posterior free edge of each tergum fits into the groove behind the facet in the next in such a manner, that the abdomen cannot be made more than very slightly concave upwards without breaking. Thus the limits of motion of the abdomen, in the vertical direction, are from the position in which it is straight, or has even a very slight upward concavity, to that in which it is completely bent upon itself, the telson being brought under the bases of the hinder thoracic limbs. No lateral movement between the somites of the abdomen is possible in any of its positions. For, when itis straight, lateral movement is hindered not only by the extensive overlapping of the terga, but also by the manner in which the hinder edges of the pleura of each of the four middle somites overlap the front edges of their successors: The pleura of the second somite are much larger than any of the others, and their front edges overlap the small pleura of the first'abdominal somite; and when the abdomen is much flexed, these pleura even THE EXTENSORS AND FLEXORS OF THE ABDOMEN. 99 ride over the posterior edges of the branchiostegites. In the position of extension, the overlap of the terga is great, while that of the pleura of the middle somites is small. As the abdomen passes from extension to flexion, the overlap of the terga of course diminishes; but any de- crease of resistance to lateral strains which may thus arise, is compensated by the increasing overlap of the pleura, which reaches its maximum when the abdomen is completely flexed. It is obvious that longitudinal muscular fibres fixed into the exoskeleton, above the axes of the joints, must bring the centres of the terga of the somites closer together, when they contract; while muscular fibres attached below the axes of the joints must approximate the sterna. Hence, the former will give rise to extension, and the latter to flexion, of the abdomen as a whole. Now there are two pairs of very considerable muscles disposed in this manner. The dorsal pair, or the exten- sors of the abdomen (fig. 22, em), are attached in front to the side walls of the thorax, thence pass backwards into the abdomen, and divide into bundles, which are fixed to the inner surfaces of the terga of all the somites. The other pair, or the flexors of the abdomen (fm) consti- tute a very much larger mass of muscle, the fibres of which are curiously twisted, like the strands of a rope. The front end of this double rope is fixed to a series of processes of the exoskeleton of the thorax, called apode- mata, some of which roof over the sternal bluod-sinuses 100 THE PHYSIOLOGY OF THE COMMON CRAYFISH, and the thoracic part of the nervous system ; while, in the abdomen, its strands are attached to the sternal exoske- leton of all the somites and extend, on each side of the rectum, to the telson. . When the exoskeleton is cleaned by maceration, the ene add.m Fig. 22.—Astacus. fluviatilis.—A longitudinal section of the body to show the principal muscles and their relations to the exoskeleton (nat. size). a, the vent; add.m, adductor muscle of mandible ; em, extensor, and fm, flexor muscle of abdomen ; @s, cesophagus ; pep, procephalic process; ¢,t’, the two segments of the telson ; xv—xx, the abdominal somites ; 1—20, the appendages; x, x, hinges between the successive abdominal somites. abdomen has a slight curve, dependent upon the form and the degree of elasticity possessed by its different parts ; and, in a living crayfish at rest, it will be observed that the curvature of the abdomen is still more marked. , Hence it is ready either for extension or for flexion. A sudden contraction of the flexor muscles instantly increases the ventral curvature of the abdomen, and THE INFLUENCE OF NERVE ON MUSCLE. 101 throws the tail fin, the two side lobes of which are spread out, forwards; while the body is propelled back- wards by the reaction of the water against the stroke. Then the flexor muscles being relaxed, the extensor muscles come into play; the abdomen is straightened, but less violently and with a far weaker stroke on the water, in consequence of the less strength of the extensors and of . the folding up of the lateral plates of the fin, until it comes into the position requisite to give full force toa new downward and forward stroke. The tendency of the extension of the abdomen is to drive the body forward ; but from the comparative weakness and the obliquity of its stroke, its practical effect is little more than to check the backward motion conferred upon the body by the flexion of the abdomen. Thus, every action of the crayfish, which involves motion, means the contraction of one or more muscles. But what sets muscle contracting? A muscle freshly removed from the body may be made to contract in various ways, as by mechanical or chemical irritation, or by an electrical shock; but, under natural conditions, there is only one cause of muscular contraction, and that is the activity of a nerve. Every muscle is supplied with one or more nerves. These are delicate threads which, on microscopic examination, prove to be bundles of fine tubular filaments, filled with an apparently structureless substance of gelatinous consistency, the nerve jibres 102 THE PHYSIOLOGY OF THE COMMON CRAYFISH. (fig. 28). The nerve bundle which passes to a muscle breaks up into smaller bundles and, finally, into separate fibres, each of which ultimately terminates by becoming continuous with the substance of a muscular fibre fig. 19, F.) Now the peculiarity of a muscle nerve, or motor nerve, as it is called, is that irritation of the nerve fibre at any part of its length, however distant from the muscle, FIG. 23.— Astacus fluviatilis—Three nerve fibres, with the connective tissue in which they are imbedded. (Magnified about 250 dia- meters.) , nuclei. brings about muscular contraction, just as if the muscle itself were irritated. A change is produced in the mole- cular condition of the nerve at the point of irritation; and this change is propagated along the nerve, until it reaches the muscle, in which it gives rise to that change in the arrangement of its molecules, the most obvious effect of: which is the sudden alteration of form which we call muscular contraction. If we follow the course of the motor nerves in @ NERVE FIBRES AND NERVE CELLS. 103 - direction away from the muscles to which they are dis- tributed, they will be found, sooner or later, to terminate in ganglia (fig. 24 A. gl.c; fig. 25, gn. 1—13.) A gan- glion is a body which is in great measure composed of Fic. 24.— Astacus fluviatilis. —A, one of the (double) abdominal gan- glia, with the nerves connected with it (x 25); B,w nerve cell or ganglionic corpuscle (x 250). a, sheath of the nerves; c, sheath of the ganglion ; co, co’, commissural cords connecting the ganglia with those in front, and those behind them. gi.c. points to the ganglionic corpuscles of the ganglia; , nerve fibres. nerve fibres ; but, interspersed among these, or disposed around them, there are peculiar structures, which are termed ganglionic corpuscles, or nerve cells (fig. 24, B.) These are nucleated cells, not unlike the epithelial cells which have been already mentioned, but which are larger Fig. 25.—Astacus fluviatilis—The central nervous system seen from above (nat. size). a, vent; an, antennary nerve; a’n, antennulary nerve; ¢, circumcesophageal commissures ; gn. 1, supracesophageal ganglion; gn. 2, infracesophageal ganglion; gn. 6, fifth thoracic ganglion ; gn. 7, last thoracic ganglion ; gn. 13, last abdominal gang- lion ; &s, cesophagus in cross section ; on, optic nerve; sa, sternal artery in crosssection ; sgn, stomatogastric nerve, THE CHAIN OF GANGLIA. 105 and often give off one or more processes. These pro- cesses, under favourable circumstances, can be traced into continuity with nerve fibres. The chief ganglia of the crayfish are disposed in a longitudinal series in the middle line of the ventral aspect sct. of the body close to the integument (fig. 25). In dhe Buslomien, for example, Six ganglionic. masses are somite, Ciimncatal by longitudinal bands of nerve fibres, and giving off ' branches to the muscles. On careful ex- amination, the longitudinal connecting bands, or com- missures (fg. a4, Bole are seen to be double, and each six, larger, double’ ree masses, likewise, connected by double commissures ; and the most anterior of these, which is the largest (fig. 25, gn. 2), is marked at the sides by notches, as if it were made up of several pairs of ganglia, run together into one continuous whole. In front of this, two commissures (¢) pass forwards, separating widely, to give room for the gullet (ws), which passes between them; while in front of the gullet, just behind the eyes, they unite with a transversely elongated mass of ganglionic substance (gn. 1), termed the brain, or cerebral ganglion. All the motor nerves, as has been said, are traceable, directly or indirectly, to one or other of these thirteen sets of ganglia; but other nerves are given off from the ganglia, which cannot be followed into any muscle. In 6 106 THE PHYSIOLOGY OF THE COMMON CRAYFISH. fact, these nerves go_ either to the saat or to the organs of sense, and they are termed senso res. When a musele is is connected by its motor nerve with a ganglion, irritation of that ganglion will bring about the contraction of the muscle, as well as if the motor nerve itself were irritated. Not only so; but if a sensory nerve, which is in connexion with the ganglion, is is irritated, the same effect.is produced ; moreover, the sensory nerve itself need not be excited, but the “same. e_ result, will take place, if ‘the organ to which it is distributed is stimulated. Thus the nervous system is fundamentally an apparatus by which two separate, and it may be dis- tant, parts of f the body, are are brought nto relation with one another; and this relation is of such a nature, that a change of state arising in the one part is followed by the propagation of changes along the sensory nerve to the ganglion, and from the ganglion to the other part; where, if that part happens to be muscle, it produces contraction. If one end of a rod of wood, twenty feet long, is applied to a sounding-board, the sound of a tuning-fork held against the opposite extremity will be very plainly heard. Nothing can be seen to happen in the wood, and yet its molecules are certainly set vibrating, at the same rate as the tuning-fork. vibrates; and when, after travelling rapidly along the wood, these vibrations affect. the sounding-board, they give rise to vibrations of the molecules of the air, which reaching the ear, are converted into an audible note. So in the nerve tract: THE CO-ORDINATION OF MOVEMENTS. 107 no apparent change is effected in it by the irritation at one end; but the rate at which the molecular change produced travels can be measured ; and, when it reaches the muscle, its effect becomes visible in the change of form of the muscle. The molecular change would take place just as much if there were no muscle connected with the nerve, but it would be no more apparent to ordinary observation than the sound of the tuning-fork is audible in the absence of the sounding-board. If the nervous system were a mere bundle of nerve fibres extending between sensory organs and muscles, every muscular contraction would require the stimulation of that special point of the surface on which the appro- priate sensory nerve ended. The contraction of several muscles at the same time, that is, the combination of movements towards one end, would be possible only if the appropriate nerves were severally stimulated in the proper order, and every movement would be the direct result of ex- ternal changes. The organism would be like a piano, which may be made to give out the most complicated harmonies, but is dependent for their production on the depression of a separate key for every note that is sounded. But it is obvious that the crayfish needs no such separate impulses for the performance of highly complicated actions. The simple impression made on the organs of sensation in the two examples with which we started, gives rise to a train of complicated and accurately co- ordinated muscular contractions. To carry the analogy 108 THE PHYSIOLOGY OF THE COMMON CRAYFISH. of the musical instrument further, striking a single key gives rise, not to a single note, but to a more or less elaborate tune; as if the hammer struck not a single string, but pressed down the stop of a musical box. It is in thé ganglia that we must look for the analogue of the musical box. A single impulse conveyed by a sensory nerve to a ganglion, may give rise to a single muscular contraction, but more commonly it originates a series of such, combined to a definite end. The effect which results from the propagation of an impulse along a nerve fibre to a ganglionic centre, whence it is, as it were, reflected along another nerve fibre to a muscle, is what is termed a reflex action. As it is by no means necessary that sensation should be a concomitant of the first impulse, it is better to term the nerve fibre which carries it afferent rather than sensory; and, as other phenomena besides those of molar motion may be the ultimate result of the reflex action, it is better to term the nerve fibre which transmits the reflected im- and the first abdominal ganglia are cut, or if the thoracic ganglia are destroyed, the crayfish is no longer able to control the movements of the abdomen. If the forepart of the body is irritated, for example, the animal makes no effort to escape by swimming backwards. Never- theless, the abdomen is not paralysed, for, if it be irri- tated, it will flap vigorously. This is a case of pure INVOLUNTARY RHYTHMICAL MOVEMENTS. 109 reflex action. The stimulus is conveyed to the abdo- minal ganglia through afferent nerves, and is reflected from them, by efferent nerves, to the abdominal muscles. But this is not all. Under these circumstances it will be seen that the abdominal limbs all swing backwards and forwards, simultaneously, with an even stroke; while the vent opens and shuts with a regular rhythm. Of course, these movements imply correspondingly regular alternate contractions and relaxations of certain sets of muscles; and these, again, imply regularly recurring efferent impulses from the abdominal ganglia. The fact that these impulses proceed from the abdominal ganglia, may be shown in two ways: first, by destroying these ganglia in one somite after another, when the move- ments in each somite at once permanently cease; and, secondly, by irritating the surface of the abdomen, when the movements are temporarily inhibited by the stimula- tion of the afferent nerves. Whether these movements are properly reflex, that is, arise from incessant new afferent impulses of unknown origin, or whether they depend on the periodical accumulation and discharge of nervous energy in the ganglia themselves, or upon periodical exhaustion and restoration of the irritability of the muscles, is unknown. It is sufficient for the present purpose to use the facts as evidence of the peculiar co-ordinative function of ganglia. The crayfish, as we have seen, avoids light; and the slightest touch of one of its antenne gives rise to active motions of the whole body. In fact, the animal’s posi- 110 THE PHYSIOLOGY OF THE COMMON CRAYFISH. tion and movements are largely determined by the in- fluences received through the feelers and the eyes. These receive their nerves from the cerebral ganglia; and, as might be expected, when these ganglia are extirpated, the crayfish exhibits no tendency to get away from the light, and the feelers may not only be touched, but sharply pinched, without effect. Clearly, therefore, the cerebral ganglia serve as a ganglionic centre, by which the afferent impulses derived from the feelers and the eyes are transmuted into efferent impulses. Another very curious result follows upon the extirpation of the cerebral ganglia. Ifan uninjured crayfish is placed upon its back, it makes unceasing and well-directed efforts to turn over; and if everything else fails, it will give a powerful flap with the abdomen, and trust to the chapter of accidents to turn over as it darts back. But the brainless crayfish behaves in a very different way. Its limbs are in incessant motion, but they are “all abroad ;” and if it turns over on one side, it does not seem able to steady itself, but rolls on to its back again. If anything is put between the chele of an uninjured crayfish, while on its back, it either rejects the object at once, or tries to make use of it for leverage to turn over. In the brainless crayfish a similar operation gives rise to a very curious spectacle.* If the object, whatever it be a My attention was first drawn to these phenomena by my friend Dr. M. Foster, F.R.S., to whom I had suggested the desirableness of an experimental study of the nerve physiology of the crayfish. THE ACTIONS OF BRAINLESS CRAYFISHES. 111 —a bit of metal, or wood, or paper, or one of the ani- mal’s own antenne—is placed between the chele of the forceps, it is at once seized by them, and carried back- wards; the chelate ambulatory limbs are at the same time advanced, the object seized is transferred to them, and they at once tuck it between the external maxilli- pedes, which, with the other jaws, begin vigorously to raasticate it. Sometimes the morsel is swallowed; sometimes it passes out between the anterior jaws, as if deglutition were difficult. It is very singular to observe that, if the morsel which is being conveyed to the mouth by one of the forceps is pulled back, the forceps and the chelate ambulatory limbs of the other side are at once brought forward to secure it. The movements of the limbs are, in short, adjusted to meet the increased resistance. All these phenomena cease at once, if the thoracic ganglia are destroyed. Itis in these, therefore, that the simple stimulus set up by the contact of a body with, for example, one of the forceps, is translated into all the sur- prisingly complex and accurately co-ordinated movements, which have been described. Thus the nervous system of the crayfish may be regarded as a system of co-ordi- nating mechanisms, each of which produces a certain action, or set of actions, on the receipt of an appropriate stimulus. When the crayfish comes into the world, it possesses in its neuro-muscular apparatus certain innate poten- 112 THE PHYSIOLOGY OF THE COMMON CRAYFISH. tialities of action, and will exhibit the corresponding acts, under the influence of the appropriate stimuli. A large proportion of these stimuli come from without through the organs of the senses. The greater or less readiness of each sense organ to receive impulses, of the nerves to transmit them, and of the ganglia to give rise to combined impulses, is dependent at any moment upon the physical condition of these parts; and this, again, is largely modified by the amount and the condition of the blood supplied. On the other hand, a certain number of these stimuli are doubtless originated by changes within the various organs which compose the body, including the nerve centres themselves. When an action arises from conditions developed in the interior of an animal’s body, imasmuch as we cannot perceive the antecedent phenomena, we call such an spontaneous;”’ or, when in ourselves we are aware that it 1s accompanied by the idea of the action, and the desire to perform it, we term the act ‘‘ volun- tary.” But, by the use of this language, no rational person intends to express the belief that such acts are uncaused or cause themselves. “‘ Self-causation” is a contradiction in terms; and the notion that any pheno- action ‘ menon comes into existence without a cause, is equivalent to a belief in chance, which one may hope is, by this time, finally exploded. In the crayfish, at any rate, there is not the slightest reason to doubt that every action has its definite physical SENSORY ORGANS. 113 cause, and that what it does at any moment would be as clearly intelligible, if we only knew all the internal and external conditions of the case, as the striking of a clock is to any one who understands clockwork. The adjustment of the body to varying external con- ditions, which is one of the chief results of the working of the nervous mechanism, would be far less important from a physiological point of view than it is, if only those external bodies which come into direct contact with the organism * could affect it; though very delicate influences of this kind take effect on the nervous apparatus through the integument. It is probable that the sete, or hairs, which are so generally scattered over the body and the appendages, are delicate tactile organs. They are hollow processes of the chitinous cuticle, and their cavities are continuous with narrow canals, which traverse the whole thick- ness of the cuticle, and are filled by a prolongation of the subjacent proper integument. As this is supplied with nerves, it is likely that fine nerve fibres reach the bases of the hairs, and are affected by anything which stirs these delicately poised levers. * It may be said that, strictly speaking, only those external bodies which are in direct contact with the organism do affect it—as the vibrating ether, in the case of luminous bodies; the vibrating air or water, in the case of sonorous bodies; odorous particles, in the case of odorous bodies: but I have preferred the ordinary phraseology to a pedantically accurate periphrasis. 114 THE PHYSIOLOGY OF THE COMMON CRAYFISH. There is much reason to believe that odorous bodies affect crayfish ; but it is very difficult to obtain experi- Fig. 26.—Astacus fluviatilis,—A, the right antennule seen from the inner side(x 5); B, a portion of the exopodite enlarged ; C, olfactory appendage of the exopodite ; a, front view; b, side view (x 300); a, olfactory appendages ; au, auditory sac, supposed to be seen through the wall of the basal joint of the antennule ; 3, sete ; en, endopo- dite ; ea, exopodite ; sp. spine of the basal joint. mental evidence of the fact. However, there is a good deal of analogical ground for the supposition that some peculiar structures, which are evidently of a sensory THE OLFACTORY ORGANS. 115 nature, developed on the under side of the outer branch of the antennule, play the part of an olfactory apparatus. Both the outer (fig. 26 A. ex) and the inner (en) branches of the antennule are made up of a number of delicate ring-like segments, which bear fine sete (b) of the ordinary character. The inner branch, which is the shorter of the two, pos- sesses only these sete; but the under surface of each of the joints of the outer branch, from about the seventh or eighth to the last but one, is provided with two bundles of very curious appendages (fig. 27, A, B, C, a), one in front and one behind. These are rather more than 1-200th of an inch long, very delicate, and shaped like a spatula, with a rounded handle and a flattened somewhat curved blade, the end of which is sometimes truncated, sometimes has the form of a prominent papilla.. There is a sort of joint between the handle and the blade, such as is found between the basal and the terminal parts of the ordinary sete, with which, in fact, these processes entirely correspond in their essential structure. A soft granular tissue fills the interior of each of these pro- blematical structures, to which Leydig, their discoverer, ascribes an olfactory function. It is probable that the crayfish possesses something analogous to taste, and a very likely seat for the organ of this function is in the upper lip and the metastoma; but if the organ exists it possesses no structural pecu- liarities by which it can be identified. 116 THE PHYSIOLOGY OF THE COMMON CRAYFISH. There is no doubt, however, as to the special recipients of sonorous and luminous vibrations; and these are of particular importance, as they enable the nervous ma- chinery to be affected by bodies indefinitely remote from it, and to change the place of the organism in relation to such bodies. Sonorous vibrations are enabled to act as the stimulants of a special nerve (fig. 25, a’n) connected with the brain,. by means of the very curious auditory sacs (fig. 26, A, au) which are lodged in the basal joints of the antennules. Each of these joints is trihedral, the outer face being con- vex ; the inner, applied to its fellow, flat; and the upper, on which the eyestalk rests, concave. On this upper face there is a narrow elongated oval aperture, the outer lip of which is beset with # flat brush of long close-set sete, which lie horizontally, over the aperture, and effectually close it. The aperture leads into a small sac (au) with delicate walls formed by a chitinous continuation of the general cuticula. The inferior and posterior wall of the sac is raised up along a curved line into a ridge which projects into its interior (fig. 27, A,r). Each side of this ridge is beset with a series of delicate sete (as), the longest of which measures about ;';th of an inch; they thus form a longitudinal band bent upon itself. These auditory sete project into the fluid contents of the sac, and their apices are for the most part imbedded in a gelatinous mass, which contains irregular particles of sand THE EAR OF THE CRAYFISH. 117 and sometimes of other foreign matter. A nerve (n n’,) is distributed to the sac, and its fibres enter the bases of the hairs, and may be traced to their apices, where they end in peculiar elongated rod-like bodies (fig. 27, C). Here is an auditory organ of the simplest description. Fig. 27.—Astacus fluviatilis. A, the auditory sac detached and seen from the outside (x 15) ; B, auditory hair ( x 100) ; C, the distal ex- tremity of the same more highly magnified. a, aperture of sac; as, auditory sete; b, its inner or posterior extremity; n’, nerves ; r, ridge. It retains, in fact, throughout life, the condition of a simple sac or involution of the integument, such as is that of the vertebrate ear in its earliest stage. 118 THE PHYSIOLOGY OF THE COMMON CRAYFISH. The sonorous vibrations transmitted through the water in which the crayfish lives to the fluid and solid contents of the auditory sac are taken up by the delicate hairs of the ridge, and give rise to molecular changes which traverse the auditory nerves and reach the cerebral ganglia. The vibrations of the luminiferous ether are brought to bear upon the free ends of two large bundles of nerve fibres, termed the optic nerves (fig. 25, on), which proceed directly from the brain, by means of a highly complex eye. This is an apparatus, which, in part, sorts out the rays of light into as many very small pencils as there are separate endings of the fibres of the optic nerve, and, in part, serves as the medium by which the luminous vibrations are converted into molecular nerve changes. The free extremity of the eyestalk presents a convex, soft, and transparent surface, limited by an oval contour. The cuticle in this region, which is termed the cornea, (fig. 28, a), is, in fact, somewhat thinner and less dis- tinctly laminated than in the rest of the eyestalk, and it contains no calcareous matter. But it is directly con- tinuous with the rest of the exoskeleton of the eyestalk, to which it stands in somewhat the same relation as the soft integument of an articulation does to the adjacent hard parts. The cornea is divided into a great number of minute, usually square facets, by faint lines, which cross it from side THE EYE OF THE CRAYFISH. 119 to side nearly at right angles with one another. A longi- tudinal section shows that both the horizontal and the vertical contours of the cornea are very nearly semicir- cular, and that the lines which mark off the facets merely arise from a slight modification of its substance between the facets, The outer contour of each facet forms part Fig. 28.—Astacus fluviatilis.—A, a vertical section of the eye-stalk (x 6); B, a small portion of the same, showing the visual ap- paratus more highly magnified ; a, cornea ; 6, outer dark zone ; ¢, outer white zone; d, middle dark zone; e, inner white zone; J, inner dark zone ; cr, crystalline cones ; g, optic ganglion ; op, optic nerve ; sp, striated spindles. of the general curvature of the outer face of the cornea ; the inner contour sometimes exhibits a slight deviation 120 THE PHYSIOLOGY OF THE COMMON CRAYFISH. from the general curvature of the inner face, but usually nearly coincides with it. When a longitudinal or a transverse section is taken through the whole eyestalk, the optic nerve (fig. 28, A, op) is seen to traverse its centre. At first narrow and cylindrical, it expands towards its extremity into a sort of bulb (B, g), the outer surface of which is curved in correspondence with the inner surface of the cornea. The terminal half of the bulb contains a great quantity of dark colouring matter or pigment, and, in section, appears as what may be termed the inner dark zone (f). Outside this, and in connection with it, follows a white line, the inner white zone (e), then comes a middle dark zone (d); outside this an outer pale band, which may be called the outer white zone (c), and between this and the cornea (a) is another broad band of dark pigment, the outer dark zone (6). When viewed under a low power, by reflected light, this outer dark zone is seen to be traversed by nearly parallel straight lines, each of which starts from the boundary between two facets, and can be followed inwards through the outer white zone to the middle dark zone. Thus the whole substance of the eye between the outer surface of the bulb of the optic nerve and the inner surface of the cornea is marked out into as many segments as the cornea has facets; and each segment has the form of a wedge or slender pyramid, the base of which is four-sided, and is applied against the inner surface of THE VISUAL PYRAMIDS. 121 one of the facets of the cornea, while its summit lies in the middle dark zone. Each of these visual pyramids consists of an axial structure, the visual rod, invested by a sheath. The latter extends inwards from the margin of each facet of the cornea, and contains pigment in two regions of its length, the intermediate space being devoid of pigment. As the position of the pigmented regions in relation to the length of the pyramid is always the same, the pigmented regions necessarily take the form of two consecutive zones when the pyramids are in their natural position. The visual rod consists of two parts, an external crystalline cone (fig. 28, B, er), and an internal striated spindle (sp). The crystalline cone consists of a trans- parent glassy-looking substance, which may be made to split up longitudinally into four segments. Its inner end narrows into a filament which traverses the outer white zone, and, in the middle dark zone, thickens into a four- sided spindle-shaped transparent body, which appears transversely striated. The inner end of this striated spindle narrows again, and becomes continuous with nerve fibres which proceed from the surface of the optic bulb. The exact mode of connection of the nerve-fibres with the visual rods is not certainly made out, but it is pro- bable that there is direct continuity of substance, and that each rod is really the termination of a nerve fibre. Eyes having essentially the same structure as that of 122 THE PHYSIOLOGY OF THE COMMON CRAYFISH. the crayfish are very widely met with among Crustacea and Insecta, and are commonly known as compound eyes. In many of these animals, in fact, when the cornea is re- moved, each facet is found to act as a separate lens; and when proper arrangements are made, as many distinct pictures of external objects are found behind it as there are facets. Hence the notion suggested itself that each visual pyramid is a separate eye, similar in principle of construction to the human eye, and forming a picture of so much of the external world as comes within the range of its lens, upon a retina supposed to be spread out on the surface of the crystalline cone, as the human retina is spread over the surface of the vitreous humour. But, in the first place, there is no evidence, nor any probability, that there is anything corresponding to a retina on the outer face of the crystalline cone; and secondly, if there were, it is incredible that, with such an arrangement of the refractive media as exists in the cornea and crystalline cones, rays proceeding from points in the external world should be brought to a focus in cor- respondingly related points of the surface of the supposed retina. But without this no picture could be formed, and no distinct vision could take place. It is very probable, therefore, that the visual pyramids do not play the part of the simple eyes of the Vertebrata, and the only alterna- tive appears to be the adoption of a modification of the theory of mosaic vision, propounded many years by Johannes Miller. THE THEORY OF MOSAIC VISION. 123 Each visual pyramid, isolated from its fellows by its coat of pigment, may be supposed, in fact, to play the part of a very narrow straight tube, with blackened walls, one end of which is turned towards the external world, while the other incloses the extremity of one of the nerve fibres. The only light which can reach the latter, under these circum- stances, is such as proceeds from points which lie in the Fig. 29.—Diagram showing the course of rays of light from three points a, y, z, through the nine visual rods (supposed to be empty tubes) A—I of a compound eye ; a—i, the nerve fibres connected with the visual rods. direction of a straight line represented by the produced axis of the tubes. Suppose A—1 to be nine such tubes, a—i the corre- sponding nerve fibres, and « y z three points from which light proceeds. Then it will be obvious that the only light t 124 THE PHYSIOLOGY OF THE COMMON CRAYFISH. from x which will excite sensation, will be the ray which traverses B and reaches the nerve-fibre b, while that from y will affect only e, and that from x only h. The result, translated.into sensation, will be three points of light on a dark ground, each of which answers to one of the luminous points, and indicates its direction in reference to the eye and its angular distance from the other two.* The only modification needed in the original form of the theory of mosaic vision, is the supposition that part, or the whole, of the visual rod, is not merely a passive transmitter of light to the nerve-fibre, but is, itself, in someway concerned in transmuting the mode of motion, light, into that other mode of motion which we call nervous energy. The visual rod is, in fact, to be re- garded as the physiological end of the nerve, and the instrument by which the conversion of the one form of motion into the other takes place; just as the auditory hairs are instruments by which the sonorous waves are converted into molecular movements of the substance of the auditory nerves. It is wonderfully interesting to observe that, when the so-called compound eye is interpreted in this manner, * Since the visual rods are strongly refracting solids, and not empty tubes, the diagram given in fig. 29 does not represent the true course of the rays, indicated by dotted lines, which fall obliquely on any cornea of a crayfish’s eye. Such rays will be more or- less bent towards the axis of the visual rod of that cornea ; but whether they reach its apex and so affect the nerve or not will depend on the curvature of the cornea; its refractive index and that of the crystalline cone; and the relation between the length and the thickness of the latter. DO CRAYFISHES HEAR AND SEE? 125 the apparent wide difference between it and the verte- brate eye gives place to a fundamental resemblance. The rods and cones of the retina of the vertebrate eye are extraordinarily similar in their form and their relations to the fibres of the optic nerve, to the visual rods of the arthropod eye. And the morphological discrepancy, which is at first so striking, and which arises from the fact that the free ends of the visual rods are turned towards the light, while those of the rods and cones of the vertebrate eye are turned from it, becomes a confir- mation of the parallel between the two when the develop- ment of the vertebrate eye is taken into account. For it is demonstrable that the deep surface of the retina in which the rods and cones he, is really a part of the outer surface of the body turned inwards, in the course of the singular developmental changes which give rise to the brain and the eye of vertebrate animals. Thus the crayfish has, at any rate, two of the higher sense organs, the ear and the eye, which we possess our- selves; and it may seem a superfluous, not to say a frivolous, question, if any one should ask whether it can hear and see. But, in truth, the inquiry, if properly limited, is a very pertinent one. That the crayfish is led by the use of its eyes and ears to approach some objects and avoid others, is beyond all doubt; and, in this sense, most indubit- ably it can both hear and see. But if the question 126 THE PHYSIOLOGY OF THE COMMON CRAYFISH. means, do luminous vibrations give it the sensations of light and darkness, of colour and form and distance, which they give to us? and do sonorous vibrations produce the feelings of noise and tone, of melody and of harmony, as in us ?—it is by no means to be answered hastily, perhaps cannot be answered at all, except in a tentative, probable way. The phenomena to which: we give the names of sound and colour are not physical things, but aré states of con- sciousness, dependent, there is every reason to believe, on the functional activity of certain parts of our brains. Melody and harmony are names for states of conscious- ness which arise when at least two sensations of sound have been produced. All these are manufactured arti- cles, products of the human brain; and it would be exceedingly hazardous to affirm that organs capable of giving rise to the same products exist in the vastly simpler nervous system of the crustacean. It would be the height of absurdity to. expect from a meat-jack the sort of work which is performed by a Jacquard loom; and it appears to me to be little less preposterous to look for the production of anything analogous to the more subtle phenomena of the human mind in something so minute and rude in comparison to the human brain, as the insignificant cerebral ganglia of the crayfish. At the most, one may be justified in supposing the existence of something approaching dull feeling in our- selves ; and, tu return to the problem stated in the begin- THE MORTALITY OF CRAYFISHES, 127 ning of this chapter, so far as such obscure consciousness accompanies the molecular changes of its nervous sub- stance, it will be right to speak of the mind of a crayfish. But it will be obvious that it is merely putting the cart before the horse, to speak of such a mind as a factor in the work done by the organism, when it is merely a dim symbol of a part of such work in the doing. Whether the crayfish possesses consciousness or not, however, does not affect the question of its being an engine, the actions of which at any moment depend, on the one hand, upon the series of molecular changes excited, either by internal or by external causes, in its neuro- muscular machinery; and, on the other, upon the dispo- sition and the properties of the parts of that machinery. And such a self-adjusting machine, containing the im- mediate conditions of its action within itself, is what is properly understood by an automaton. Crayfishes, as we have seen, may attain a considerable age; and there is no means of knowing how long they might live, if protected from the innumerable destructive influences to which they are at all ages liable. It is a widely received notion that the energies of living matter have a natural tendency to decline, and finally disappear; and that the death of the body, as a whole, is the necessary correlate of its life. That all living things sooner or later perish needs no demonstration, but it would be difficult to find satisfactory grounds 128 THE PHYSIOLOGY OF THE COMMON CRAYFISH. for the belief that they must needs doso. The analogy of a machine that, sooner or later, must be brought to a standstill by the wear and tear of its parts, does not hold, inasmuch as the animal mechanism is continually renewed and repaired ; and, though it is true that indi- vidual components of the body are constantly dying, yet their places are taken by vigorous successors. A city remains, notwithstanding the constant death-rate of its inhabitants ; and such an organism as a crayfish is only a corporate unity, made up of innumerable partially independent individualities. Whatever might be the longevity of crayfishes under imaginable perfect conditions, the fact that, notwithstand- ing the great number of eggs they produce, their number remains pretty much the same in a given district, if we take the average of a period of years, shows that about as many die as are born; and that, without the process of reproduction, the species would soon come to an end. There are many examples among members of the group of Crustacea to which the crayfish belongs, of animals which produce young from internally developed germs, as some plants throw off bulbs which are capable of reproducing the parent stock; such is the case, for example, with the common water flea (Daphnia). But nothing of this kind has been observed in the crayfish; in which, as in the higher animals, the reproduction of the species is de- pendent upon the combination of two kinds of living THE OVARY AND THE TESTIS. 129 matter, which are developed in different individuals, termed males and females. These two kinds of living matter are ova and sperma- tozoa, and they are developed in special organs, the ovary and the testis. The ovary is lodged in the female; the testis, in the male. The ovary (fig. 30, ov) is a body of a trefoil form, which is situated immediately beneath, or in front of, the heart, between the floor of the pericardial sinus and the alimentary canal. From the ventral face of this Fig. 30.—Astacus fluviatilis.—The female reproductive organs (x 2) ; ov, ovary ; od, oviduct ; od’, aperture of oviduct. organ two short and wide canals, the oviducts (od), lead down to the bases of the second pair of walking limbs, and terminate in the apertures (od’) already noticed there. The testis (fig. 831, ¢) is somewhat similar in form to the ovary, but, the three divisions are much narrower 130 ‘HE PHYSIOLOGY OF THE COMMON CRAYFISH. and more elongated: the hinder median division lies under the heart; the anterior divisions are situated between the heart behind, and the stomach and the liver in front (figs. 5 and 12, ¢). From the point at which the Fig. 31.—Astacus fluviatilis. —The male reproductive organs (x 2); t, testis ; vd, vas deferens ; vd’, aperture of vas deferens. three divisions join, proceed two ducts, which are termed the vasa deferentia (fig. 81, vd). These are very narrow, long, and make many coils before they reach the apertures upon the bases of the hindermost pair of walking limbs, by which they open externally (fig. 81, vd’, and fig. 85, vd). Both the ovary and the testis are very much larger THE OVARY AND THE EGGS. 131 during the breeding season than at other times; the large brownish-yellow eggs become conspicuous in the ovary, Fig. 32.— Astacus fluviatilis—A, a two-thirds grown egg contained in its ovisac (x 50); B, an egg removed from the ovisac (x 10); C, a portion of the wall of an ovisac with the adjacent portion of the contained egg, highly magnified; cp, epithelium of ovisac ; gs, germinal spots; gv, germinal vesicle; m, membrana propria ; v, vitellus ; vm, vitelline membrane ; 1, stalk of ovisac. and the testis assumes a milk-white colour, at this period. The walls of the ovary are lined internally by a layer of 132 THE PHYSIOLOGY OF THE COMMON CRAYFISH. nucleated cells, separated from the cavity of the organ by a delicate structureless membrane. The growth of these cells gives rise to papillary elevations which project into the cavity of the ovary, and eventually become globular a Fic. 33.—Astaeus fluviatilis.—A, a lobule of the testis, showing a, acini, springing from 3, the ultimate termination of a duct (x 50). B, spermatic cells ; a, with an ordinary globular nucleus n ; 6, with a spindle-shaped nucleus ; c, with two similar nuclei; and d, with a nucleus undergoing division (x 600). bodies attached by short stalks, and invested by the struc- tureless membrane as a membrana propria (fig. 82, m). These are the ovisacs. In the mass of cells which be- comes the ovisac, one rapidly increases in size and occupies the centre of the ovisac, while the others THE OVA AND THE SPERMATOZOA, 133 surround it as a peripheral coat (ep.). This central cell is the ovum. Its nucleus enlarges, and becomes what is called the germinal vesicle (g.v.). At the same time numerous small corpuscles, flattened externally and convex internally, appear in it and are the germinal spots (g.s.)e The protoplasm of the cell, as it enlarges, becomes granular and opaque, assumes a deep brownish- yellow colour, and is thus converted into the yelk or vitellus (v.). As the egg grows, a structureless vitelline membrane is formed between the vitellus and the cells which line the ovisac, and incloses the egg, as in a bag. Finally, the ovisac bursts, and the egg, falling into the cavity of the ovary, makes its way down the oviduct, and sooner or later passes out by its aperture. When they leave the oviduct, the ova are invested by a viscous, transparent substance, which attaches them to the swimmerets of the female, and then sets; thus each egg, inclosed in a tough case, is firmly suspended by a stalk, which, on the one side, is continued into the substance of the case, while, on the other, it is fixed to the swimmeret. The swimmerets are kept constantly in motion, so that the eggs are well supplied with aérated water. The testis consists of an immense number of minute spheroidal vesicles (fig. 38, A, a), attached like grapes to the ends of short stalks (6), formed by the ultimate ramifications of the vasa deferentia. The vesicles may, in fact, be regarded as dilatations of the ends and sides Fic. 34.—Astacus fluviatilis._A—D, different stages in the development of a sperma- tozoon from a seminal cell ; E, a mature spermatozoon seen from the side; F, the same viewed en face (all x 850); G, a diagrammatic vertical section of the same. THE PROCESS OF FERTILIZATION. 135 of the finest branches of the ducts of the testis. The cavity of each vesicle is filled by the large nucleated cells which line its walls (fig. 33, B), and, as the breeding season approaches, these cells multiply by division. Finally, they undergo some very singular changes of form and internal structure (fig. 84, A—D), each becom- ing converted into a flattened spheroidal body, about aHoath of an inch in diameter, provided with a number of slender curved rays, which stand out from its sides (fig. 834, E—G). These are the spermatozoa. The spermatozoa accumulate in the testicular vesicles, and give rise to a milky-looking substance, which traverses the smaller ducts, and eventually fills the vasa deferentia. This substance, however, consists, in addition to the spermatozoa, of a viscid material, secreted by the walls of the vasa deferentia, which envelopes the spermatozoa, and gives the secretion of the testis the form and the consistency of threads of vermicelli. The ripening and detachment of both the ova and the spermatozoa take place immediately after the com- pletion of ecdysis in the early autumn; and at this time, which is the breeding season, the males seek the females with great avidity, in order to deposit the fertilizing matter contained in the vasa deferentia on the sterna of their hinder thoracic and anterior abdominal somites. There it adheres as a whitish, chalky-looking mass ; but the manner in which the contained sperma- tozoa reach and enter the ova is unknown. The analogy 136 THE PHYSIOLOGY OF THE COMMON CRAYFISH. of what occurs in other animals, however, leaves no doubt that an actual mixture of the male and female ele- ments takes place and constitutes the essential part of the process of impregnation. Ova to which spermatozoa have had no access, give rise to no progeny; but, in the impregnated ovum, the young crayfish takes its origin in a manner to be described below, when the question of development is dealt with. Fic, 35.—-Astacus fluviatilis.—The last thoracic sternum, seen from behind, with the proximal ends of the appendages, A, in the male, B, in the female,(x 3). am, articular membrane; cap, coxopo- dite ; st XIV, last thoracic sternum ; vd, aperture of vas deferens. CHAPTER IV. THE MORPHOLOGY OF THE COMMON CRAYFISH: THE STRUC- TURE AND THE DEVELOPMENT OF THE INDIVIDUAL. In the two preceding chapters the crayfish has been studied from the point of view of the physiologist, who, regarding an animal as a mechanism, endeavours to dis- cover how it does that which it does. And, practically, this way of looking at the matter is the same as that of the teleologist. For, if all that we know concerning the pur- pose of a mechanism is derived from observation of the manner in which it acts, it is all one, whether we say that the properties and the connexions of its parts account for its actions, or that its structure is adapted to the performance of those actions. Hence it necessarily follows that physiological pheno- mena can be expressed in the language of teleology. On the assumption that the preservation of the indi- vidual, and the continuance of the species, are the final causes of the organization of an animal, the exist- ence of that organization is, in a certain sense, explained, when it is shown that it is fitted for the attainment of those ends ; although, perhaps, the importance of de- 138 THE MORPHOLOGY OF THE COMMON CRAYFISH. monstrating the proposition that a thing is fitted to do that which it does, is not very great. But whatever may be the value of teleological ex- planations, there is a large series of facts, which have as yet been passed over, or touched only incidentally, of which they take no account. These constitute the sub- ject matter of Morphology, which is related to physiology much as, in the not-living world, crystallography is related to the study of the chemical and physical pro- perties of minerals. Carbonate of lime, for example, is a definite compound of calcium, carbon, and oxygen, and it has a great variety of physical and chemical properties. But it may be studied under another aspect, as a substance capable of assuming crystalline forms, which, though extraordinarily various, may all be reduced to certain geometrical types. It is the business of the crystallographer to work out the relations of these forms ; and, in so doing, he takes no note of the other properties of carbonate of lime. In like manner, the morphologist directs his attention to the relations of form between different parts of the same animal, and between different animals; and these relations would be unchanged if animals were mere dead matter, devoid of all physiological properties—a kind of mineral capable of a peculiar mode of growth. A familiar exemplification of the difference between teleology and morphology may be found in such works of human art as houses. TELEOLOGY AND MORPHOLOGY. 139 A house is certainly, to a great extent, an illustration of adaptation to purpose, and its structure is, to that extent, explicable by teleological reasonings. The roof and the walls are intended to keep out the weather; the foundation is meant to afford support and to exclude damp; one room is contrived for the purpose of a kitchen; another for that of a coal-cellar; a third for that of a dining-room; others are constructed to serve as sleeping rooms, and so on; doors, chimneys, windows, drains, are all more or less elaborate contrivances directed towards one end, the comfort and health of the dwellers in the house. What is sometimes called sanitary architec- ture, now-a-days, is based upon considerations of house teleology. But though all houses are, to begin with and essentially, means adapted to the ends of shelter and comfort, they may be, and too often are, dealt with from a point of view, in which adaptation to purpose is largely disregarded, and the chief attention of the architect is given to the form of the house. A house may be built in the Gothic, the Italian, or the Queen Anne style; and a house in any one of these styles of architecture may be just as convenient or inconvenient, just as well or as ill adapted to the wants of the resident therein, as any of the others. Yet the three are exceedingly different. To apply all this to the crayfish. It is, in a sense, a house with a great variety of rooms and offices, in which the work of the indwelling life in feeding, breath- ing, moving, and reproducing itself, is done. But the 140 THE MORPHOLOGY OF THE COMMON CRAYFISII, same may be said of the crayfish’s neighbours, the perch ‘and the water-snail; and they do all these things neither better nor worse, in relation to the conditjons of their existence, than the crayfish does. Yet the most cursory inspection is sufficient to show that the “styles of archi- tecture” of the three are even more widely different than are those of the Gothic, Italian, and Queen Anne houses. That which Architecture, as an art conversant with pure form, is to buildings, Morphology, as a science conversant with pure form, is to animals and plants. And we may now proceed to occupy ourselves exclusively with the morphological aspect of the crayfish. As I have already mentioned, when dealing with. the physiology of the crayfish, the entire body of the animal, when reduced to its simplest morphological expression, may be represented as a cylinder, closed at each end, ex- cept so far as it is perforated by the alimentary aper- tures (fig. 6); or we may say that it is a tube, inclosing another tube, the edges of the two being continuous at their extremities. The outer tube has a chitinous outer coat or cuticle, which is continued on to the inner face of the inner tube. Neglecting this for the present, the outermost part of the wall of the outer tube, which answers to the epidermis of the higher animals, and the innermost part of the wall of the inner tube, which is an epithelium, are formed by a layer of nucleated cells. A continuous layer of cells, therefore, is everywhere to ENDODERM, MESODERM, AND ECTODERM. 141 be found on both the external and the internal free sur- faces of the body. So far as these cells belong to the proper external wall of the body, they constitute the ectoderm, and so far as they belong to its proper internal wall, they compose the endoderm. Between these two layers of nucleated cells lie all the other parts of the body, composed of connective tissue, muscles, vessels, and nerves; and all these (with the exception of the ganglionic chain, which we shall see properly belongs to the ectoderm) may be regarded as a single thick stratum, which, as it lies between the ectoderm and the endoderm, is called the mesoderm. If the intestine were closed posteriorly instead of opening by the vent, the crayfish would virtually be an elongated sac, with one opening, the mouth, affording an entrance into the alimentary cavity: and, round this cavity, the three layers just referred to— endoderm, mesoderm, and ectoderm —would be disposed concen- trically. We have seen that the body of the crayfish thus com- posed is obviously separable into three regions—the cephalon or head, the thorax, and the abdomen. The latter is at once distinguished by the size and the mobility of its segments: while the thoracic region is marked off from that of the head, outwardly, only by the cervical groove. But, when the carapace is removed, the lateral depression already mentioned, in which the 142 THE MORPHOLOGY OF THE COMMON CRAYFISH. scaphognathite lies, clearly indicates the natural boundary between the head and the thorax. It has further been observed that there are, in all, twenty pairs of ap- pendages, the six hindermost of which are attached to the abdomen. If the other fourteen pairs are carefully removed, it will be found that the six anterior belong to the head, and the eight posterior to the thorax. The abdominal region may now be studied in further detail. Lach of its seven movable segments, except the telson, represents a sort of morphological unit, the repe- tition of which makes up the whole fabric of the body. If the abdomen is divided transversely between the SEXIX. , Fig. 36.—Astacus fluviatilis. A transverse section through the nine- teenth (fifth abdominal) somite (x 2). ¢.m., extensor muscles; f.m., flexor muscles ; gn. 12, the fifth abdominal ganglion ; h.g., hind-gut ; i.a.a., inferior abdominal artery ; s.a.a, superior abdominal artery ; pl. XIX, pleura of the somite; st. XIX, its sternum ; ¢. XX, its tergum ; ep. XIX, its epimera ; 19, its appendages. , SOMITES AND APPENDAGES. 143 fourth and fifth, and the fifth and sixth segments, the fifth will be isolated, and can be studied apart. It constitutes what is called a metamere ; in which are distinguishable a central part termed the somite, and two appendages (fig. 36). In the exoskeleton of the somites of the abdomen several regions have already been distinguished; and although they constitute one continuous whole, it will be convenient to speak of the sternum (fig. 36, st. XIX), the tergum (t. XIX), and, the pleura (pl. XIX), as if they were separate parts, and to distinguish that portion of the sternal region, which lies between the articulation of the appendage and the pleuron, on each side, as the epimeron (ep. XIX). Adopting this nomenclature, it may be said of the fifth somite of the abdomen, that it consists of a segment of the exoskeleton, divisible into tergum, pleura, epimera, and sternum, with which two appendages are articulated; that it contains a double ganglion (gn. 12), a section of the flexor (fm) and extensor (em) muscles, and of the alimentary (hg) and vascular (s.a.a, t.a.a) systems. The appendage (fig. 36, 19), which is attached to an articular cavity situated between the sternum and the epimeron, is seen to consist of a stalk or stem, which is made up of a very short basal joint, the coxopodite (fig. 37, D and E, ex.p), followed by a long cylindrical second joint, the basipodite (b.p), and receives the name of pro- topodite. At its free end, it bears two flattened narrow Fig. 37.—Astacus fluviatilis.—Appendages of the left side of the abdo- men(x 8). A, the posterior face of the first appendage of the male ; B, the same of the female ; C, posterior. and C’, anterior faces of the second appendage of the male; D, the third appendage of the male ; E, the same of the female ; F, the sixth appendage. a, the rolled plate of the endopodite ; , the jointed extremity of the same ; dp., pasipodite ; car.p., coxopodite ; en.p., endopodite ; ex.p., exopodite. SOMITES AND APPENDAGES. 145 plates, of which one is attached to the inner side of the extremity of the protopodite, and is called the endopodite (en.p), while the other is fixed a little higher up to the outer side of that extremity, and is the exopodite (ex.p). The exopodite is shorter than the endopodite. The endopodite is broad and is undivided for about half its length, from the attached end; the other half is narrower, and is divided into a number of small segments, which, however, are not united by definite articulations, but are merely marked off from one another by slight constric- tions of the exoskeleton. The exopodite has a similar structure, but its undivided portion is shorter and nar- rower. The edges of both the exopodite and the endo- podite are fringed with long sete. In the female crayfish, the appendages of this and of the fourth and third somites are larger than in the male (compare D and E, fig. 87). The fourth and fifth somites, with their appendages, may be described in the same terms as the third, and in the sixth there is no difficulty in recognising the corresponding parts of the somite; but the appendages (fig. 87, F), which constitute the lateral portions of the caudal fin, at first sight appear very different. In their size, no less than in their appearance, they depart widely from the appendages of the preceding somites. Nevertheless, each will be found to consist of a basal stalk, answering to the protopodite (cx.p), which how- ever is very broad and thick, and is not divided into two \ 146 THE MORPHOLOGY OF THE COMMON CRAYFISH. joints ; and of two terminal oval plates, which represent the endopodite (en.p) and the exopodite (ex.p), The latter is divided by a transverse suture into two pieces ; and the edge of the larger or basal moiety is beset with short spines, of which two, at the outer end of the series, are larger than the rest. The second somite is longer than the first (fig. 1); it has very broad pleura, while those of the first somite are small and hidden by the overlapping front margins of the pleura of the second somite. In the female, the appendages of the second somite of the abdomen are similar to those of the third, fourth, and fifth somites ; but in those of the first somite (fig. 87, B), there is a considerable variation. Sometimes, in fact, the appendages of this somite are altogether wanting ; sometimes one is present, and not the other; and sometimes both are found. But, when they exist, these appendages are always small; and the protopodite is followed by only one imperfectly jointed filament, which appears to represent the endopodite of the other ap- pendages. In the male, the appendages of the first and second somites of the abdomen are not only of relatively large size, but they are widely different from the rest, those of the first somite departing from the general type further than those of the second. In the latter (C, C’) there is a protopodite (cx.p, bp) with the ordinary structure, and it is followed by an endopodite (en.p) and an exopodite SOMITES AND APPENDAGES. 147 (ex.p); but the former is singularly modified. The un- divided basal part is large, and is produced on the inner side into a lamella (a), which extends slightly beyond the end of the terminal jointed portion (b). The inner half of this lamella is rolled upon itself, in such a manner as to give rise to a hollow cone, something like an extinguisher (C’, a). The appendage of the first somite (A) is an unjointed styliform body, which appears to represent the proto- podite, together with the basal part and the inner pro- longation of the endopodite of the preceding appendage. The terminal half of the appendage is really a broad plate, slightly bifid at the summit, but the sides of the plate are rolled in, in such a manner that the anterior half bends round and partially incloses the posterior half. They thus give rise to a canal, which is open at each end, and only partially closed behind. These two pairs of curiously modified appendages are ordinarily turned forwards and applied against the sterna of the posterior part of the thorax, in the interval be- tween the bases of the hinder thoracic limbs (see fig. 8, A). They serve as conduits by which the spermatic matter of the male is conveyed from the openings of the ducts of the testes to its destination. If we confine our attention to the third, fourth, and fifth metameres of the abdomen of the crayfish, it is obvious that the several somites and their appendages, and the various regions or parts into which they are 148 THE MORPHOLOGY OF THE COMMON CRAYFISH. divisible, correspond with one another, not only in form, but in their relations to the general plan of the whole abdomen. Or, in other words, a diagrammatic plan of one somite will serve for all the three somites, with insignificant variations in detail. The assertion that these somites are constructed upon the same plan, in- volves no more hypothesis than the statement of an architect, that three houses are built upon the same plan, though the fagades and the internal decorations may differ more or less. In the language of morphology, such conformity in the plan of organisation is termed homology. Hence, the several metameres in question and their appendages, are homologous with one another; while the regions of the somites, and the parts of their appendages, are also homologues. When the comparison is extended to the sixth meta- mere, the homology of the different parts with those of the other metameres, is undeniable, notwithstanding the great differences which they present. To recur to a previous comparison, the ground plan of the building is the same, though the proportions are varied. So with regard to the first and second metameres. In the second pair of appendages of the male, the difference from the ordinary type of appendage is comparable to that pro- duced by adding a portico or a turret to the building ; while, in the first pair of appendages of the female, it is as if one wing of the edifice were left unbuilt; HOMOLOGY AND HOMOLOGUES. 149 and, in those of the male, as if all the rooms were run into one. It is further to be remarked, that, just as of a row of houses built upon the same plan, one may be arranged so as to serve as a dwelling-house, another as a warehouse, and another as a lecture hall, so the homologous appendages of the crayfish are made to subserve various functions. And as the fitness of the dwelling-house, the warehouse, and the lecture-hall for their several purposes would not in the least help us to understand why they should all be built upon the same general plan; so, the adaptation of the appendages of the abdomen of the crayfish to the dis- charge of their several functions does not explain why those parts are homologous. On the contrary, it would seem simpler that each part should have been constructed in such a manner as to perform its allotted function in the best possible manner, without reference to the rest. The proceedings of an architect, who insisted on con- structing every building in a town on the plan of a Gothic cathedral, would not be explicable by considera- tions of fitness or convenience. In the cephalothorax, the division into somites is not at first obvious, for, as we have seen, the dorsal or tergal surface is covered over by a continuous shield, distin- guished into thoracic and cephalic regions only by the cervical groove. Even here, however, when a transverse section of the thorax is compared with that of the abdo- 150 THE MORPHOLOGY OF THE COMMON CRAYFISH. men (figs. 15 and 86), it will be obvious that the tergal and the sternal regions of the two answer to one another ; while the branchiostegites correspond with greatly de- veloped pleura; and the inner wall of the branchial chamber, which extends from the bases of the appendages to the attachment of the branchiostegite, represents an immensely enlarged epimeral region. On examination of the sternal aspect of the cephalo- thorax the signs of division into somites become plain (figs. 3 and 39, A). Between the last two ambulatory limbs there is an easily recognisable sternum (XIV.), though it is considerably narrower than any of the sterna of the abdominal somites, and differs from them in shape. The deep transverse fold which sepa Oe hinder- most thoracic sternum from the rest of the one wall of the cephalothorax, is continued upwards the inner or epimeral wall of the branchial cavity; and thus the sternal and the epimeral portions of the posterior thoracic somite are naturally marked off from those of the more anterior somites. we The epimeral region of this somite presents a very curious structure (fig. 838). Immediately above the ar- ticular cavities for the appendages there is a shield- shaped plate, the posterior, convex edge of which is sharp, prominent, and setose. Close to its upper boundary the plate exhibits a round perforation (plb.), to the margins of which the stem of the hindermost THE CEPHALOTHORAX. 151 * pleurobranchia (fig. 4, plb. 14) is attached; and in front of this, it is connected, by a narrow neck, with an elongated triangular piece, which takes a vertical direction, and lies in the fold which separates the posterior thoracic somite from the next in front. The base of this cpe. t.Xv. Fig. 38.— Astacus fluviatilis—The mode of connexion between the last thoracic and the first abdominal somites (x 3). a, L-shaped bar ; cpe, carapace ; cap. 14, coxopodite of the last ambulatory leg ; plb., place of attachment of the pleurobranchia ; st. XV, sternum, and t. XV, tergum of the first abdominal somite. piece unites with the epimeron of the penultimate somite. Its apex is connected with the anterior end of the horizontal arm of an L-shaped calcified bar (fig. 88, a), the upper end of the vertical arm of which is firmly, but moveably, con- nected with the anterior and lateral edge of the tergum of the first abdominal somite (¢. XV.). The tendon of one 152 THE MORPHOLOGY OF THE COMMON CRAYFISH. of the large extensor muscles of the abdomen is attached .close to it. The sternum and the shield-shaped epimeral plates constitute a solid, continuously calcified, ventral element of the skeleton, to which the posterior pair of legs is attached; and as this structure is united with the somites in front of and behind it only by soft cuticle, except where the shield-shaped plate is connected, by the intermediation of the triangular piece, with the epimeron which lies in front of it, it is freely movable backwards and forwards on the imperfect hinge thus constituted. In the same way, the first somite of the abdomen, and, consequently, the abdomen as a whole, moves upon the hinges formed by the union of the L-shaped pieces with the triangular pieces. In the rest of the thorax, the sternal and the epimeral regions of the several somites are all firmly united together. Nevertheless, shallow grooves answering to folds of the cuticle, which run from the intervals between the articular cavities for the limbs towards the tergal end of the inner wall of the branchial chamber, mark off the epimeral portions of as many somites as there are sterna, from one another. A short distance above the articular cavities a trans- verse groove separates a nearly square area of the lower part of the epimeron from the rest. Towards the anterior and upper angle of this area, in the two somites THE CEPHALOTHORAX. 1538 which lie immediately in front of the hindermost, there is a small round aperture for the attachment of the Fie. 39.—Astacus fluviatilis.—The cephalothoracic sterna and the endo- phragmal system (x 2). A, from beneath; B, from above. a, a’, arthrophragms or partitions between the articular cavities for the limbs ; ¢c.ap, cephalic apodeme ; of, cervical fold ; epn. 1, epimeron of the antennulary somite; %, anterior, and h’, posterior horizontal process of endopleurite; Jb, labrum ; m, mesophragm; mt, meta- stoma ; p,paraphragm ; J—XJV, cephalothoracic sterna; 1—14, articular cavities of the cephalothoracic appendages. (The anterior cephalic sterna are bent downwards in A so as to bring them into the same plane with the remaining cephalothoracic sterna; in B these sterna are not shown.) 8 154 THE MORPHOLOGY OF THE COMMON CRAYFISH. rudimentary branchia. These aree of the epimera, in fact, correspond with the shield-shaped plate of. the hindermost somite. In the next most anterior somite (that which bears the first pair of ambulatory legs) there is only a small elevation in the place of the rudimentary branchia; and in the anterior four thoracic somites.no- thing of the kind is visible. On the sternal aspect of the thorax (figs. 3 and 39, A) a triangular space is interposed between the basal joints or coxopodites of the penultimate and the ante-penultimate pairs of ambulatory legs, while the coxopodites of the more anterior limbs are closely approximated. The triangular area in question is occupied by two sterna (fig. 39, A, XIZ, XIII), the lateral margins of which are raised into flange-like ridges. The next two sterna (X, XT) are longer, especially that which lies between the forceps (X), but they are very narrow; while the lateral processes are reduced to mere tubercles at the posterior ends of the sterna. Between the three pairs of maxil- lipedes, the sterna (VII, VIII, IX) are yet narrower, and become gradually shorter; but traces of the tubercles at their posterior ends are still discernible. The most anterior of these sternal rods passes into a transversely elongated plate, shaped like a broad arrow (V, VJ), which is constituted by the conjoined sterna of the two, posterior somites of the head. Anteriorly to this, and between it and the posterior end of the elongated oral aperture, the sternal region is THE CEPHALIC SOMITES. 155 occupied only by soft or imperfectly calcified cuticle, which, on each side of the hinder part of the mouth, passes into one of the lobes of the metastoma (mt). At the base of each of these lobes there is a calcified plate, united by an oblique suture with another, which occupies the-whole length of the lobe and gives it firmness. The soft narrow lip which constitutes the lateral boundary of the oral aperture, and lies between it and the man- dible, passes, in front, into the posterior face of the labrum (J). In front of the mouth, the sternal region which apper- tains, in part, to the antenne, and, in part, to the man- dibles, is obvious as a broad plate (III), termed the epistoma. The middle third of the posterior edge of the epistoma gives rise to a thickened transverse ridge, with rounded ends, slightly excavated behind, and is then continued into the labrum (lb), which is strengthened by three pairs of calcifications, arranged in a longitudinal series. The sides of the front edge of the epistoma are excavated, and bound the articular cavities for the basal joints of the antenne (8); but, in the middle line, the epistoma is continued forwards into a spear-head shaped process (figs. 39 and 40, II), to which the posterior end of the antennulary sternum contributes. The antennulary sternum is very narrow, and its anterior or upper end runs into a small but distinct conical median spine (fig. 40, ¢.). Upon this follows an uncalcified plate, bent into the form ot a half cylinder (J), which lies between the inner ends of 156 THE MORPHOLOGY OF THE COMMON CRAYFISH. the eye-stalks and is united with adjacent parts only by flexible cuticle, so that it is freely movable. This represents the whole of the sternal region, and probably more, of the ophthalmic somite. The sterna of fourteen somites are thus identifiable in the cephalothorax. The corresponding epimera are Fig. 40.—Astacus fluviatilis —The ophthalmic and antennulary somites (x 8). J, ophthalmic, and ZZ, antennulary sternum; 1, articular surface for eyestalk; 2, for antennule; cpm, epimeral plate; pep, procephalic process ; 7, base of rostrum ; ¢, tubercle. represented, in the thorax, by the thin inner walls of the branchial chamber; the pleura, by the branchiostegites ; and the terga, by so much of the median region of the carapace as lies behind the cervical groove. That part of the carapace which is situated in front of this groove occu- pies the place of the terga of the head ;-while the low ridge, skirting the oral and pre-oral region, in which it terminates laterally, represents the pleura of the cephalic somites. The epimera of the head are, for the most part, very narrow; but those of the antennulary somite are broad plates (fig. 40, epm.), which constitute the posterior THE ENDOPHRAGMAL SYSTEM. 157 wall of the orbits. I am inclined to think that a trans- verse ridge, which unites these under the base of the rostrum, represents the tergum of the antennulary somite, and that the rostrum itself belongs to the next or antennary somite.* The sharp convex ventral edge of the rostrum (fig. 41) is produced into a single, or sometimes two divergent spines, which descend, in front of the ophthalmic somite, towards the conical tubercle mentioned above: it thus gives rise to an imperfect partition between the orbits. Fig. 41.—Astacus fluviatilis.—The rostrum, seen from the left side. The internal face of the sternal wall of the whole of the thorax and of the post-oral part of the head, presents a complicated arrangement of hard parts, which is known as the endophragmal system (figs. 39, B, 42, and 43), and which performs the office of an internal skeleton by afford- ing attachment to muscles, and serving to protect im- portant viscera, while at the same time it ties the somites together, and unites them into a solid whole. In reality, however, the curious pillars and bulkheads which enter into the composition of the endophragmal system are all * There are some singular marine crustacea, the Szuillide, in which both the ophthalmic and the antennary somites are free and movable, while the rostrum is articulated with the tergum of the antennary somite. 158 THE MORPHOLOGY OF THE COMMON CRAYFISH. mere infoldings of the cuticle, or apodemes; and, as such, they are shed along with the other cuticular structures during the process of ecdysis. Without entering into unnecessary details, the gene- ral principle of the construction of the endophragmal skeleton may be stated as follows. Four apodemes are developed between every two somites, and as every apodeme is a fold of the cuticle, it follows that the anterior wall of each belongs to the somite in front, and the posterior wall to the somite behind. All four apodemes lie in the ventral half of the somite and form a single transverse series ; consequently there are two nearer the middle line, which are termed the endosternites, and two further off, which are the endopleurites. The former lie at the inner, and the latter at the outer ends of the partitions or arthrophragms (fig. 89, A, a, a’, fig. 42, aph), between the articular cavities for the basal joints of the limbs, and they spring partly from the latter and partly from the sternum and the epimera respectively. The endosternite (fig. 42, ens.) ascends vertically, with a slight inclination forwards, and its summit narrows and assumes the form of a pillar, with a flat, transversely elongated capital. The inner prolongation of the capital is called the mesophiragm (mph.), the outer the paraphragm (pph.). The mesophragms of the two endosternites of a somite usually unite by a median suture, and thus form a complete arch over the sternal canal (s.c.), which lies between the endosternites. THE ENDOPHRAGMAL SYSTEM. 159 The endopleurites (en.pl.) are also vertical plates, but they are relatively shorter, and their inner angles give off two nearly horizontal processes, one of which passes obliquely forwards (fig. 39, B, h, fig. 42, h.p.) and unites with the paraphragm of the endosternite of the somite in front, while the other, passing obliquely backwards (fig. 89, h’), becomes similarly connected with the endo- sternite of the somite behind. Fic, 42.— Astacus fluviatilis.—A segment of the endophragmal system (x 8). aph, arthrophragm; arth, arthrodial or articular cavity ; cep, coxopodite of the ambulatory leg; enpi, endopleurite; ens, endosternite ; epm, epimeron ; hp, horizontal process of endo- pleurite ; mph, mesophragm ; pph, paraphragm ; s, sternum of somite ; sc, sternal canal. The endopleurites of the last thoracic somite are rudi- mentary, and its endosternites are small. On the other hand, the mesophragmal processes of the endosternites of the two posterior somites of the head (fig. 39, B, c.ap), by which the endophragmal system terminates in front, are particularly strong and closely united together. They thus, with their endopleurites, form a solid partition be- tween the stomach, which lies upon them, and the mass of 160 THE MORPHOLOGY OF THE COMMON CRAYFISH. coalesced anterior thoracic and posterior cephalic ganglia situated beneath them. Strong processes are given off from their anterior and outer angles, which curve round the tendons of the adductor muscles of the mandibles, and give attachment to the abductors. In front of the mouth there is no such endophragmal system as that which lies behind it. But the anterior gas- tric muscles are attached to two flat calcified plates, which appear to lie in the interior of the head (though they are really situated in its upper and front wall) on each side of the base of the rostrum, and are called the procephalic processes (figs. 40, 48, p.cp). Each of these plates con- stitutes the posterior wall of a narrow cavity which opens externally into the roof of the orbit, and has been regarded (though, as it appears to me, without sufficient reason) as an olfactory organ. I am disposed to think, though I have not been able to obtain complete evidence of the fact, that the procephalic processes are the representa- tives of the ‘ procephalic lobes”? which terminate the anterior end of the body in the embryo crayfish. At any rate, they occupy the same position relatively to the eyes and to the carapace; and the hidden position of these processes, in the adult, appears to arise from the extension of the carapace at the base of the rostrum over the fore part of the originally free sternal surface of the head. It has thus covered over ‘the procephalic processes, in which the sternal wall of the body termi- nated; and the cavities which lie in front of them are TIIE THEORY OF THE SKELETON. 161 simply the interspaces left between the inferior or posterior wall of the prolongation of the carapace and the originally exposed external faces of these regions of the cephalic integument. Fourteen somites having thus been distinguished in the cephalothorax, and six being obvious in the abdomen, it is clear that there is a somite for every pair of append- ages. And, if we suppose the carapace divided into segments answering to these sterna, the whole body will be made up of twenty somites, each having a pair of appendages. As the carapace, however, is not actually divided into terga in correspondence with the sterna which it covers, all we can safely conclude from the anatomical facts is that it represents the tergal region of the somites, not that it is formed by the coalescence of primarily distinct terga. In the head, and in the greater part of the thorax, the somites are, as it were, run together, but the last thoracic somite is partly free and to a slight extent moveable, while the abdominal somites are all free, and moveably articulated together. At the anterior end of the body, and, apparently, from the an- tennary somite, the tergal region gives rise to the rostrum, which projects between and beyond the eyes. At the opposite extremity, the telson is a corresponding median outgrowth of the last somite, which has become moveably articulated therewith. The narrowing of the sternal moieties of the anterior thoracic somites, to- 162 THE MORPHOLOGY OF THE COMMON CRAYFISH. gether with the sudden widening of the same parts in the posterior cephalic somites, gives rise to the lateral depression (fig. 89, ef) in which the scaphognathite lies. The limit thus indicated corresponds with that marked by the cervical groove upon the surface of the carapace, and separates the head from the thorax. The three pair of maxillipedes (7, 8, 9), the forceps (10), the ambulatory Pep: “ey, Fic. 43.—Astacus fluviatilis Longitudinal section of the anterior part of the cephalothorax(x 8). J—ZLX, sterna of first nine cephalo- thoracic somites; 1, eyestalk ; 2,-basal joint of antennule ; 3, basal joint of antenna ; 4, mandible; a, inner division of the masticatory surface of the mandible ; a’, apophysis of the mandible for muscular attachment ; cp, free edge of carapace ; ¢, endosternite ; enpl, endo- pleurite ; epm, epimeral plate; J, labrum ; m, muscular fibres con- necting epimera with interior of carapace ; mt, metastoma 5 pep. precephalic process. ; THE THEORY OF THE SKELETON. 163 limbs (11—I4), and the eight somites of which they are the appendages (VII—XIV), lie behind this boundary and belong to the thorax. The two pairs of maxille (6, 6) the mandibles (4), the antenne (3), the antennules (4), the eyestalks (J), and the six somites to which they are attached (I—VJ), lie in front of the boundary and com- pose the head. Another important point to be noticed is that, in frout of the mouth, the sternum of the antennary somite (fig, 43, ITD) is inclined at an angle of 60° or 70° to the direc- tion of the sterna behind the mouth. The sternum of the antennulary somite (IZ) is at right angles to the latter ; and that of the eyes (J) looks upwards as well as forwards. Hence, the front of the head beneath the rostrum, though it looks forwards, or even upwards, is homologous with the sternal aspect of the other somites. It is for this reason that the feelers and the eyestalks take a direction so dif- ferent from that of the other appendages. The change of aspect of the sternal surface in front of the mouth, thus effected, is what is termed the cephalic flexure. Since the skeleton which invests the trunk of the cray- fish is made up of a twenty-fold repetition of somites, homologous with those of the abdomen, we may expect to find that the appendages of the thorax and of the head, however unlike they may seem to be to those of the ab- domen, are nevertheless reducible to the same funda- mental plan. 164 THE MORPHOLOGY OF THE COMMON CRAYFISH. The third maxillipede is one of the most complete of these appendages, and may be advantageously made the starting point of the study of the whole series. Fig. 44.—Astacus fluviatilis.—The third or external maxillipede of the left side (x 3). e¢, lamina, and-br, branchial filaments of the podobranchia ; cap, coxopodite; cas, coxopoditic sete ; bp, basi- podite ; ex, exopodite; ip, ischiopodite ; mp, meropodite; cp, carpopodite ; pp, propcdite ; dp, dactylopodite. Neglecting details for the moment, it may be said that the appendage consists of a basal portion (fig. 44, exp, bp), THE MAXILLIPEDES, 165 with two terminal divisions (ip to dp, and ex), which are directed forwards, below the mouth, and a third, lateral appendage (e, br), which runs up, beneath the carapace, into the branchial chamber. The latter is the gill, or podo- branchia, attached to this limb, and it is something not represented in the abdominal limbs. But, with regard to the rest of the maxillipede, it is obvious that the basal portion (cxp, bp) represents the protopodite, and the two terminal divisions the endopodite and the exo- podite respectively. It has been observed that, in the abdominal appendages, the extent to which segmentation occurs in homologous parts varies indefinitely; an endo- podite, for example, may be a continuous plate, or may be subdivided into many joints. In the maxillipede, the basal portion is divided into two joints; and, as in the abdominal limb, the first, or that which articulates with the thorax, is termed the coxopodite (exp), while the second is the basipodite (bp). The stout, leg-like endopodite appears to be the direct continuation of the basipodite ; while the much more narrow and slender exopodite arti- culates with its outer side. The exopodite (ex) is by no means unlike one of the exopodites of the abdominal limbs, consisting as it does of an undivided base and a many-jointed terminal filament. The endopodite, on the contrary, is strong and massive, and is divided into five joints, named, from that nearest to the base onwards, ischiopodite (ip), meropodite (mp), carpopodite (cp), propo- dite (pp), and dactylopodite (dp). 166 THE MORPHOLOGY OF THE COMMON CRAYFISH. The second maxillipede (fig. 45, B) has essentially the same composition as the first, but the exopodite (ez) is relatively larger, the endopodite (ip—dp) smaller and softer; and, while the ischiopodite (ip) is the longest joint in the third maxillipede, it is the meropodite (mp) which is longest in the second. In the first maxillipede Fig. 45.—Astacus fluviatilis.—A, the first ; B, the second maxillipede of the left side (x 3). exp, coxopodite ; bp, basipodite ; e, br. po- dobranchia ; ep, epipodite; en, endopodite; ea, exopodite ; ip, is- chiopodite ; mp, meropodite ; cp, carpopodite ; pp, propodite ; dp, dactylopodite. (fig. 45, A) a great modification has taken place. The coxopodite (cp) and the basipodite (bp) are broad thin plates with setose cutting edges, while the endopodite (en) is short and only two-jointed, and the undivided portion of the exopodite (er) is very long. The place of PODOBRANCHIA AND EPIPODITES. 167 the podobranchia is taken by a broad soft membranous plate entirely devoid of branchial filaments (ep). Thus, in the series of the thoracic limbs, on passing forwards from the third maxillipede, we find that though the plan of the appendages remains the same; (1) the protopodite increases in relative size; (2) the endopodite diminishes ; (3) the exopodite increases; (4) the podobranchia finally takes the form of a broad membranous plate and loses its branchial filaments. Writers on descriptive Zoology usually refer to the parts ofthe maxillipedes under different names from those which are employed here. The protopodite and the endo- podite taken together are commonly called the stem of the maxillipede, while the exopodite is the palp, and the metamorphosed podobranchia, the real nature of which is not recognised, is termed the flagellum. When the comparison of the maxillipedes with the abdominal members, however, had shown the funda- mental uniformity of composition of the two, it became desirable to invent a nomenclature of the homologous parts which should be capable of a general application. The names of protopodite, endopodite, exopodite, which I have adopted as the equivalents of the ‘“‘stem” and the “‘palp,” were proposed by Milne-Edwards, who at the same time suggested epipodite for the “‘ flagellum.” And the lamellar process of the first maxillipede is now very generally termed an epipodite ; while the podobranchie, which have exactly the same relations to the following 168 THE MORPHOLOGY OF THE COMMON CRAYFISH. limbs, are spoken of as if they were totally different structures, under the name of branchie or gills. The flagellum or epipodite of the first maxillipede, however, is nothing but the slightly modified stem of a podobranchia, which has lost its branchial filaments; but the term ‘‘epipodite’’ may be conveniently used for podobranchie thus modified. Unfortunately, the same term is applied to certain lamelliform portions of the branchiz of other crustacea, which answer to the laminz of the crayfishes’ branchie ; and this ambiguity must be borne in mind, though it is of no great moment. On examining an appendage from that part of the thorax which lies behind the third maxillipede, say, for example, the sixth thoracic limb (the second walking leg) (fig. 46), the two joints of the protopodite and the five joints of the endopodite are at once identifiable, and so is the podobranchia; but the exopodite has vanished altogether. In the eighth, or last, thoracic limb, the podobranchia has also disappeared. The fifth and sixth limbs also differ from the seventh and eighth, in being chelate; that is to say, one angle of the distal end of the propodite is prolonged and forms the fixed leg of the pincer. The produced angle is that which is turned downwards when the limb is fully extended (fig. 46). In the forceps, the great chela is formed in just the same way; the only important difference lies in the fact that, as in the external maxillipede, the basipo- dite and the ischiopodite are immoveably united. Thus, Fig. 46.— Astacus fluviatilis—The second ambulatory leg of the left side (x 3). cxp, coxopodite ; bp, basipodite ; br, gill; ews, coxo- poditic sete ; ¢, lamina of gill or epipodite ; ip, ischiopodite ; mp, meropodite ; ep, carpopodite ; pp, propodite ; dp, dactylopodite. 170 THE MORPHOLOGY OF THE COMMON CRAYFISH. the limbs of the thorax are all reducible to the same type as those of the abdomen, if we suppose that, in the posterior five pair, the exopodites are suppressed; and that, in all but the last, podobranchie are superadded. Turning to the appendages of the head, the second maxilla (fig. 47, C) presents a further modification of the disposition of the parts seen in the first maxillipede. The coxopodite (exp) and the basipodite (bp) are still thinner and more lamellar, and are subdivided by deep fissures which extend from their inner edges. The endopodite (en) is very small and undivided. In the place of the exopodite and the epipodite there is only one great plate, the scaphognathite (sg) which either is such an epipodite as that of the first maxillipede with its anterior basal process much enlarged, or repre- sents both the exopodite and the epipodite. In the first maxilla (B), the exopodite and the epipodite have dis- appeared, and the endopodite (en) is insignificant and unjointed. In the mandibles (A), the representative of the protopodite is strong and transversely elongated. Its broad inner or oral end presents a semicircular mastica- tory surface divided by a deep longitudinal groove into two toothed ridges. The one of these follows the con- vex anterior or inferior contour of the masticatory surface, projects far beyond the other, and is provided with a sharp serrated edge; the other (fig. 43, a) gives rise to the straight posterior or superior contour of the masticatory surface, and is more obtusely tuberculated. In front, the inner THE MANDIBLES AND MAXILLA. 171 ridge is continued into a process by which the mandible articulates with the epistoma (fig. 47, A, ar). The endo- Fig. 47.—Astacus fluviatilis—A, mandible ; B, first maxilla ; C, second maxilla of the left side (x 8). a, internal, and a”, external articular process of the mandible ; bp, basipodite ; cap, coxopodite ; en, endopodite ; p, palp of the mandible; sg, scaphognathite ; z, internal process of the first maxilla. podite is represented by the three-jointed palp (p), the terminal joint of which is oval and beset with numerous strong sete, which are especially abundant along its anterior edge. 172 THE MORPHOLOGY OF THE COMMON CRAYFISH. In the antenna (fig. 48, C) the protopodite is two- jointed. The basal segment is small, and its ventral face presents the conical prominence on the posterior aspect of which is the aperture of the duct of the renal gland (gg). The terminal segment is larger and is subdi- vided by deep longitudinal folds, one upon the dorsal and Fig. 48.—Astacus fluviatilis.—A, eye-stalk ; B, antennule; C, antenna of the left side (x 3). a, spine of the basal joint of the antennule ; e, corneal surface of the eye; exp, exopodite or squame of the antenna ; gg, aperture of the duct of the green gland. one upon the ventral face, into two moieties which are more or less moveable upon one another. In front and ‘externally it bears the broad flat squame (exp) of the an- tenna, as an exopodite. Internally, the long annulated “‘feeler ” which represents the endopodite, is connected with it by two stout basal segments. THE ANTENNULES AND THE EYESTALKS. 173 The antennule (fig. 48, B) has a three-jointed stem and two terminal annulated filaments, the outer of which is thicker and longer than the inner, and lies rather above as well as external to the latter. The peculiar form of the basal segment of the stem of the antennule has already been adverted to (p. 116). It is longer than the other two segments put together, and near the anterior end its sternal edge is produced into a single strong spine (a). The stem of the antennule answers to the protopodite of the other limbs, though its division into three joints is unusual; the two terminal annulated filaments represent the endopodite and the exopodite. Finally, the eyestalk (A) has just the same structure as the protopodite of an abdominal limb, having a short basal and a long cylindrical terminal joint. From this brief statement of the characters of the appen- dages, it is clear that, in whatever sense it is allowable to say that the appendages of the abdomen are constructed upon one plan, which is modified in execution by the excess of development of one part over another, or by the suppression of parts, or by the coalescence of one part with another, it is allowable to say that all the appen- dages are constructed on the same plan, and are modified on similar principles. Given a general type of appendage consisting of a protopodite, bearing a podobranchia, an endopodite and an exopodite, all the actual appendages are readily derivable from that type. 174 THE MORPHOLOGY OF THE COMMON CRAYFISH. In addition, therefore, to their adaptation to the pur- poses which they subserve, the parts of the skeleton of the crayfish show a unity in diversity, such as, if the animal were a piece of human workmanship, would lead us to suppose that the artificer was under an obliga- tion not merely to make a machine capable of doing cer- tain kinds of work, but to subordinate the nature and arrangement of the mechanism to certain fixed architec- tural conditions. The lesson thus taught by the skeletal organs is re- iterated and enforced by the study of the nervous and the muscular systems. As the skeleton of the whole body is capable of resolution into the skeletons of twenty separate metameres, variously modified and combined; so is the entire ganglionic chain resolvable into twenty pairs of ganglia various in size, distant in this region and approximated in that; and so is the muscular system of the trunk conceivable as the sum of twenty myotomes or segments of the muscular system appro- priate to a metamere, variously modified according to the degree of mobility of the different regions of the organism. The building up of the body by the repetition and the modification of a few similar parts, which is so ob- vious from the study of the general form of the somites and of their appendages, is still more remarkably illus- trated, if we pursue our investigations further, and trace HISTOLOGY. TISSUES. 175 out the more intimate structure of these parts. The tough, outer coat, which has been termed the cuticula, except so far as it presents different degrees of hardness, from the presence or absence of calcareous salts, is obviously everywhere of the same nature; and, by macerating a crayfish in caustic alkali, which destroys all its other components of the body, it will be readily enough seen that a continuation of the cuticular layer passes in at the mouth and the vent, and lines the alimentary canal;. furthermore, that processes of the euticle covering various parts of the trunk and limbs extend inwards, and afford surfaces of attachment to the muscles, as the apodemata and tendons. In technical language, the cuticular substance which thus enters so largely into the composition of the bodily fabric of the crayfish is called a tissue. The flesh, or muscle, is another kind of tissue, which is readily enough distinguished from cuticular tissue by the naked eye; but, for a complete discrimination of all the different tissues, recourse must be had to the microscope, the application of which to the study of the ultimate optical characters of the morphological constituents of the body has given rise to that branch of morphology which is known as Histology. If we count every formed element of the body, which is separable from the rest by definite characters, as a tissue, there are no more than eight kinds of such tissues in the crayfish; that is to say, every. solid constituent 176 THE MORPHOLOGY OF THE COMMON CRAYFISIL. of the poay consists of one or more of the following eight histological groups :— 1. Blood corpuscles; 2. Epithelium; 3. Connective tissue; 4. Muscle; 5. Nerve; 6. Ova; 7. Spermatozoa; 8. Cuticle. 1. A drop of freshly-drawn blood of the crayfish con- tains multitudes of small particles, the blood corpuscles, Fig. 49.—Astacus fluviatilis.—The corpuscles of the blood, highly magnified. 1—8, show the changes undergone by a single cor- puscle during a quarter of an hour; , the nucleus; 9 and 10 are corpuscles killed by magenta, and having the nucleus deeply stained by the colouring matter, which rarely exceed 1-700th, and usually are about 1-1000th, of an inch in diameter (fig. 49). They are sometimes pale and delicate, but generally more or less dark, from containing a number of minute strongly refracting granules, and they are ordinarily exceedingly irregular in form. If one of them is watched continu- EPITHELIUM. 177 ously for two or three minutes, its shape will be seen to undergo the constant but slow changes to which passing reference has already been made (p. 69). One or other of the irregular prolongations will be drawn in, and another thrown out elsewhere. The corpuscle, in fact, has an inherent contractility, like one of those low organisms, known as an Amoba, whence its motions are frequently called amebiform. In its interior, .an ill-marked oval contour may be seen, indicating the presence of a sphe- roidal body, about 1-2000th of an inch in diameter, which is the nucleus of the corpuscle (n). The addition of some re-agents, such as dilute acetic acid, causes the corpuscles at once to assume a spherical shape, and renders the nuc- leus very conspicuous (fig. 49,9 and 10). The blood corpuscle is, in fact, a simple nucleated cell, composed of a contractile protoplasmic mass, investing a nucleus ; it is suspended freely in the blood; and, though as much a part of the crayfish organism as any other of its histological elements, leads a quasi-independent ex- istence in that fluid. 2. Under the general name of epithelium, may be in- cluded a form of tissue, which everywhere underlies the exoskeleton (where it corresponds with the epidermis of the higher animals), and the cuticular lining of the alimen- tary canal, extending thence into the hepatic ceca. It is further met with in the generative organs, and in the green gland. ‘Where it forms the subcuticular layer of the integument and of the alimentary canal, it is found to 9 178 THE MORPHOLOGY OF THE COMMON CRAYFISH. consist of a protoplasmic substance (fig. 50), in which close set nuclei (n) areimbedded. If anumber of blood corpus- cles could be supposed to be closely aggregated together into a continuous sheet, they would give rise to such a structure as this; and there can be no doubt that it really is an aggregate of nucleated cells, though the limits between the individual cells are rarely visible in the fresh state. In the liver, however, the cells grow, and become detached from one another in the wider and lower Fig. 50.—Astacus fluviatilis.—Epithelium, from the epidermic layer subjacent to the cuticle, highly magnified. A, in vertical section ; B, from the surface. , nuclei. parts of the ceca, and their essential nature is thus obvious. 3. Immediately beneath the epithelial layer follows a tissue, disposed in bands or sheets, which extend to the subjacent parts, invest them, and connect one with another. Hence this is called connective tissue. The connective tissue presents itself under three forms. In the first there is a transparent homogeneous-looking matrix, or ground substance, through which are scattered many nuclei. In fact, this form of connectiye tissue CONNECTIVE TISSUE, 179 very closely resembles the epithelial tissue, except that the intervals between the nuclei are wider, and that the substance in which they are imbedded cannot be broken up into a separate cell-body for each nucleus. In the second form (fig. 51, A) the matrix exhibits fine wavy parallel lines, as if it were marked out into imperfect Fig. 51.—Astacus fluviatilis.—Connective tissue; A, second form ; B, third form. a, cavities; n, nuclei. H'ghly magnified. fibres. In this form, as in the next to be described, more or less spherical cavities, which contain a clear fluid, are excavated in the matrix; and the number of 180 THE MORPHOLOGY OF THE COMMON CRAYFISH. these is sometimes so great, that the matrix is propor- tionally very much reduced, and the structure acquires a close superficial similarity to that of the parenchyma of plants. This is still more the case with a third form, in which the matrix itself is marked off into elongated or rounded masses, each of which has a nucleus in its interior (fig. 51, B). Under one form or another, the connective tissue extends throughout the body, ensheath- ing the various organs, and forming the walls of the blood sinuses. : The third form is particularly abundant in the outer investment of the heart, the arteries, the alimentary canal, and the nervous centres. About the cerebral and anterior thoracic ganglia, and on the exterior of the heart, it usually contains more or less fatty matter. In these regions, many of the nuclei, in fact, are hidden by the accumulation round them of granules of various sizes, some of which are composed of fat, while others consist of a proteinaceous material. These aggregates of granules are usually spheroidal; and, with the matrix in which they are imbedded and the nucleus which they sur- round, they are often readily detached when a portion of the connective tissue is teased out, and are then known as fat cells. From what has been said respecting the dis- tribution of the connective tissue, it is obvious that if all the other tissues could be removed, this tissue would form a continuous whole, and represent a sort of model, or cast, of the whole body of the crayfish. MUSCULAR TISSUE. 181 4. The muscular tissue of the crayfish always has the form of bands or fibres, of very various thickness, marked, when viewed by transmitted light, by alternate darker and Fig. 52.—Astacus fluciatilis.—A, « single muscular fibre, transverse diameter ,1,th of an inch ; B,a portion of the same more highly magnified ; C,a smaller portion treated with alcohol and acetic acid still more highly magnified ; D and H, the splitting up of a part of a fibre, treated with picro-carmine, into fibrille; F, the connection of a nervous with a muscular fibre which has been treated with alcohol and acetic acid. a, darker, and 3, clearer portions of the fibrille ; , nuclei ; nv, nerve fibre ; s, sarcolemma ; ¢, tendon ; 1—5, successive dark granular striz answering to the granular portions, «, of each fibrilla. 182 THE MORPHOLOGY OF THE COMMON CRAYFISH. lighter striz, transversely to the axis of the fibres (fig. 52 A). The distance of the transverse strie from one another varies with the condition of the muscle, from 1-4,000th of an inch in the quiescent state to as little as 1-80,000th of an inch in that of extreme contraction. The more delicate muscular fibres, like those of the heart and those of the intestine, are imbedded in the connective tissue of the organ, but have no special sheaths. Fie. 538.— 44, 288, 298 distribution, chronologi- eal, 44 ecdysis, 32, 350 general characters, 6 growth, 31, 349 habits, 8 364 Astacus fluviatilis—continued histology, 174 mortality, 127 muscular system, 90 myths concerning, 44 name, origin of, 13 nervous system, 101 newly hatched young, cha- racters of, 219 nutrition, 48 occurrence, 5, 8 organs of alimentation, 51 circulation, 68 excretion, 82, 353 hearing, 116 reproduction, 128 respiration, 75, 353 sight, 118 smell, 114 taste, 115 touch, 113 prehension of food, 49 putrid, effect of smell of, 45 reproduction of lost limbs, 38 reproduction, sexual, 39, 128, 135, 350 sexual characters, 7, 20, 32, 145, 241 somites and appendages, 143 systematic description, 230 use as food, 10, 289 varieties, 289 Sontinalis, 290 japonicus, 304 klamathensis, 305 leniusculus, 305 INDEX. Astacus lcptodactylus, 299, 302 303, 310, 320 nigrescens, 244 nobilis, 290, 295, 296, 299, 310 oreganus, 305 pachypus, 302, 310 pallipes, 290 potitus, 344 saxatilis, 290 Schrenchii, 304, 310 torrentium, 290, 294, 298, 310, 311 tristis, 290 Lrowbridgit, 305 Atya, Atyida@, 331, 336 Auditory organ, 116 setee, 116 Australian Crayfishes, 306 province, 314 Austrocolumbian province, 314 Axius, 271 B. BALL, R., quoted, 36 Basipodite, 143 BELL, T., quoted, 37, 42 Bile-duct, 61, 66 Biological sciences, scope of, 4 Blastoderm, 207 Blastomere, 205 Blastopore, 209 Blood, 31, 68, 176 corpuscles, 69, 176 development of, 224 sinuses, 50, 69 BOBRETSKY, referred to, 356 Boutvak, Dr., 298 Branchiz, Astacoides, 266 Astacopsis, 264 INDEX. Branchize—continued Astacus, 25, 75, 265 development of, 224 Cancer, 276 Homarus, 257 Palemon, 270 Palinurus, 264 Peneus, 267 Branchial chamber, 25 formula, Astacoides, 266 Astacopsis, 264 Astacus, 266 Cancer, 277 hypothetically complete, 268 Palemon, 270 Palinurus, 265 Peneus, 267 Branchiobdella, 356 Branchiostegite, 25 development of, 217 BRAUN, quoted, 352 Brazilian Crayfishes, 306 C. Caecum, 61 Calcification of exoskeleton, 197 Californian Crayfisies, 243 Cambarus, 44, 247, 310, 312 Cancer, 272, 283 Carapace, 19 development of, 214 CARBONNIER, M., quoted, 297, 349, 350 Cardia, 52 Caridina, 330 Carpopodite, 165 Cell, 66, 199 Cell-aggregate, 190, 199 division, 200 " theory, 202, 204 Cephalic appendages, 170 development of, 217 flexure, 163 somites, 154 Cephalon, 19, 141 Cephalothorax, 19 Cervical groove, 19 spines, 234 CHANTRAN, M., quoted, 348, 350, 351 Chele, 22 Chilian Crayfishes, 308 Chitin, 50 composition of, 347 Chaeraps, 250 Chorology, 46 Circulation, 73 organs of, 68 Common knowledge and science, 3 Connective tissue, 178 development of, 224 CopE, Prof., quoted, 316 Cornea, 118 Coxopodite, 143 Coxopoditic setae, 78 Crab, see Cancer Crab’s-eye, see Gastrolith Crangon, 272 Crayfish, origin of name, 12 common, see Astacus fluviatilis Crayfishes, Amurland, 304 Asiatic, 304 Australian, 306 Brazilian, 306 Californian, 243 Chilian, 308 definition of, 254 Eastern North American, 247, 305 European, 288, 297 evolution of, 331 366 Crayfishes, Figian, 306, 313 Japanese, 304, 313 Mascarene, 308, 313 northern and southern, com- pared, 252 Novozelanian, 306, 313 southern, 249 Tasmanian, 306 Western North Amcrican, 305, 313 Crustacea, 271, 278 Crystalline cones, 121 Caticle, 33, 50, 175, 192 Cycelas, 356 Dz Dactylopodite, 165 Daphnia, asexual reproduction of 128 Darwiy, C., referred to, 4 DE HAAN, quoted, 313 Development, 205 abdomen, 213 abdominal appendages, 217 alimentary canal, 213, 222 antenne, 214, 218 antennules, 214, 218 blood and blood vessels, 224 anchiostegite, 217 carapace, 214 cephalic appendages, 217, 219 connective tissue, 224 ear, 225 eye, 225 eyestalk, 214, 218 gills, 224 heart, 224 kidney, 224 labrum, 218 mandibles, 214 INDEX. Development of muscles, 224 nervous system, 213, 224 reproductive organs, 225 rostrum, 217 thoracic appendages, 217, 219 Digestion, 63 Distribution, 46 chronological, of crayfishcs, 44, 316, 339 table of, 345 geographical, of crayfishes, 44, 288 causes of, 335 resuits of study of, 308, 314 DorMER, quoted, 356 DULKE, quoted, 349 E. Ear, 116 development of, 225 Ecdysis, 32, 350 Keorevisse 2 pieds blancs, 289, 297 a pieds rouges, 289, 247 Ectoderm, 141 Ectostracum, 194 Edelkrebs, 290 Endoderm, 141 Endophragmal system, 157 Endopleurite, 158 Endopodite, 145 Endoskeleton, 17 Endosternite, 158 Endostracum, 194 Enge@us, 250, 306 Enoplocytia, 342 Epiblast, 211 Epidermis, 140 Epimeron, 143 Epiostracum, 192 Epipodite, 167 INDEX. Epistoma, 155 Epithelium, 140, 177 Lyuus excelsus, occurring with fossil crayfishes, 316 Eryma, 341 Evolution of crayfishes, 331 Excretion, organs of, 82 Exopodite, 145 Exoskeleton, 17 chemical composition, 347 Eye, 118 compound, 122 development of, 225 Eye-stalk, 24, 173 development of, 214 F. Family, 252 Fat-cells, 180 Fibre, muscular, 185 Fibril, muscular, 185 Figian Crayfishes, 306 Filament, muscular, 185 Filter of stomach, 58 Flagellum, 167 Food-yelk, 206 Foot-jaws, see maxillipedes Forceps, 22 Foregut, 61 development of, 213, 222 Fossil crayfishes, 316 Foster, Dr. M., referred to, 110 France, consumption of crayfish in, 10 Function, 22 G. Galawida, 315 Gammarus, 323 Ganglion, 103, 105 Garglionic corpuscle, 87, 103 367 Gastric mill, 53 Gastrolith, 29, 347 chemical composition, 349 Gastrula, 211 GAY, quoted, 356 Genus, 249 Geographical distribution, see Dis- tribution GERBE, M., quoted, 350 Germinal disc, 209 layer, 206 spot, 133 vesicle, 133 GERSTFELDT, Dr., quoted, 290 Gills, see Branchiz GIRARD, quoted, 356 GorvuP-BESANEZ, quoted, 353 Green-gland, 83, 353 development of, 224 GROBBEN, Dr., quoted, 354 Growth of crayfish, 31, 349 Guanin, 82, 353 Gullet, see @sophagus GUNTHER, Dr., quoted, 315 H. HAGEN, Dr., quoted, 305, 312 Haplochitonida, 315 HARVEY, quoted, 5 Head, see Cephalon Hearing, organ of, 116 Heart, 27, 71 development of, 224 HELLER, Dr., quoted, 298, 330 Hepatic duct, see Bile duct Hind gut, 61 development of, 214, 223 Histology, 175 Histriohdella, 356 Homarida, 263 368 Homarina, 261 Homarus, 13, 42, 257, 332 Homology, homologous, homolo- gue, 148 Hoploparia, 342 Hypoblast, 211 I. Idothea, 323, 334 Impregnation, 135, 350 Integument, 50 Interseptal zone, 183 Intestine, 29, 61 Ischiopodite, 165 J. Japanese Crayfishes, 313, 314 Jaws, 23 JOHNSTON, J., quoted, 42 K, KESSLER, quoted, 298, 304 Kidney, see Green gland KLUNZINGER, referred to, 330 L. Labrum, 51 development of, 218 LAMARCK, referred to, 4 LEREBOULLET, quoted, 353 Legs, ambulatory, 168 LEMOINE, referred to, 353 LEyDIG, referred to, 115, 353 Liver, 30, 64 development of, 223 nature of secretion, 352 Lobster, common, see Homarus Norway, see Nephrops INDEX. Lobster, Rock, see Palinurus Loven, referred to, 327 M. Machine, living, 128 M‘Intosu, Dr. W. C., quoted, 288 Mandible, 23, 51, 170 ~ development of, 214 MARTENS, VON, 306 Mastodon mirificus, occurring with fossil crayfishes, 316 Maxille, 23, 170 Maxillipedes, 23, 164 Medullary groove, 213 Megalopa stage of development, 283 Meropodite, 165 Mesoblast, 212 Mesoderm, 141 Mesophragm, 158 Metamere, 143 Metastoma, 51 Metope, 278 Midgut, 61 development of, 211, 214, 223 MILNE-EDWARDS, quoted, 13, 289 Mollusca, 284 Morphology, 46, 138 comparative, 230 Mortality of crayfishes, 123 Morula, 206 Mosaic vision, 122, 354 Motor plates, 189 Mouth, 51 MULLER, JOHANNES, referred to, 122 Muscle, 57, 90, 175, 181 development of, 224 histology of, 90, 181 Muscles of abdomen, 99 INDEX. Muscles of chela, 93 of stomach, 57 Myosin, 186 Myotome, 174 Mysis, 281, 323 relicta, origin of, from AL oculata, 327 Mysis stage of development, 280 N. Natural History, 3 Philosophy, 3 Nauplius stage of development, 215, 280 Nearctic province, 314 Nephrops, 259, 332 Nerve, 101 auditory, 117 optic, 118 Nerve-cells, 103, 187 fibres, 101, 188 Nervous system, 105 development of, 213, 224 functions of, 354 Noble crayfish, see Astacus nobilis Nomenclature, binomial, 13, 15 Norway lobster, see Wephrops Novozelanian province, 314 Nucleated cell, 199 Nucleolus, 187 Nucleus, 177, 200 changes of, in cell-division, 200 oO. Gsophagus, 51 Olfactory organ, 114 Organ, 22 Origin of crayfish, evidence as to, 820, 331 17 369 Ovary, 31, 129 structure of, 131 Oviduct, 129 Oviposition, 351 Ovisac, 132 Ovum, 129 structure of, 133 P. Palearctic province, 314 Palemon, 268, 328 Palinuride 263 Palinurus, 261, 264 Palp, 171 Paranephrops, 250, 306, 313 Paraphragm, 158 Parasites of crayfish, 356 Parastacida, 252, 256, 306, 313 Parastacus, 250, 306 Pemphia, 341 Penaeus, 267, 280 Pericardium, 69 Perivisceral cavity, 50 Phyllobranchia, 271 Physiology, 46 Pleurobranchia, 79 Pleuron, 96, 143 Podobranchia, 75, 165 Podophthalmia, 279 Pore-canals, 195 Post-orbital ridge, 233 spine, 232 Potamobiida, 252, 256 Prawn, see Palemon Prehension of food, 49 Procephalic lobes, 160 development of, 213 Propodite, 165 Protopodite, 143 Prototroctes, 315 370 Protozoa, 285 Pseudastacus, 343 Pylorus, 52 R. Race, 292 RATHKE, quoted, 356 RLAUMODR, quoted, 33 Reflex action, 108 REICHENBACH, quoted, 356 Renal organ, see Green-gland Reproduction of lost limbs, 38 sexual, 39, 128, 135, 350 Reproductive organs, 128 development of, 225 Respiration, anal, 353 Respiratory organs, see Branchiz Retropinna, 315 RoBIN, quoted, 352 Rock lobster, see Palinurus ROESEL VON ROSENHOF, quoted, 41, 43 RONDOLETIUS, referred to, 4 Rostrum, 157 development of, 217 8. Salivary glands, 352 Salmonide, parallel between their distribution, and that of Asta- cid@, 315 Sarcolemma, 90, 182 Sars, G. O., referred to, 327 SARTORIUS VON WALTERHAUSEN, quoted, 322 Scaphognathite, 80, 170 Schizopod stage of development, 280 SCHLUTER, 317 INDEX. ScHMIDT, O., quoted, 354 SCHRANK, 290 Science, physical, 3 Science and common sense, 1 Segmentation, 174 Self-causation, 112 Sensory organs, 113 Septal line, 183 zone, 183 Sete, 197 Shrimp, see Crangon SIEBOLD, VON, referred to, 331 Sight, organ of, 118 Sinus, sternal, 69 Smell, organ of, 114 Somite, 143, 161, 355 abdominal, 142 cephalic, 154 thoracic, 150 SOUBEIRAN, M., quoted, 349 Southern Crayfishes, 249 Species, 243, 290 morphological, 291 physiological, 296 Spermatozoa, 129, 135, 354 Spontaneous action, 112 Squame of antenna, 172 Steinkrebs, see Astacus torrentium Sternum, 96, 143 Stomach, 29, 51 Stone-crayfish, see Astacus torren- tium Striated spindle, 121 Swimmeret, 20 T. 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