\\ \ VAY CAS ANN NAY AN ‘S'\ 8 \ St, a Library Bureau Cat. No. 1137 2787 Cornell University Library QL 363.M16 | joa book of invertebrate m i i} WATT Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924001027758 ZOOLOGY AND PHYSIOLOGY. BUMPUS’ INVERTEBRATE ZOOLOGY. A Lab- oratory Course. Revised Edition. 157 pp. 12mo. HOW BGLS DISSECTION OF THE DOG. 100 pp. vO. MARTIN’S THE HUMAN BODY. (American Science Series.) Advanced Course. xvi+621+34pp. 8vo. Briefer Course. xii+ 377 pp. 12mo. Elementary Course. vi+261 pp. 12mo. The Human Body and the Effects of Narcotics. viii + 399 pp. 12mo. McMURRICH’S IW BR LEeE ATS MORPHOL- OGY. vii+ 661 pp. 8vo. PACKARD’S ZOOLOGIES. (American Science Series.) Advanced Course. vii+722pp. 8vo. Briefer Course. viii+338 pp. 12mo. Elementary Course. viii+290pp. 12mo. PACKARD’S ENTOMOLOGY FOR BEGIN- NERS. xvi-+ 367 pp. 12mo. Fehon GUIDE TO THE STUDY OF SECTS. xii+715 pp. 8vo. soonnias BUTTERFLIES. x+322pp. 12mo. SCUDDER’S BRIEF GUIDE TO BUTTERFLIES. xi+ 206 pp. 12mo. SCUDDER’S THE LIFE OF A BUTTERFLY. (For General Readers.) 186 pp. 16mo. SEDGEWICK AND WILSON’S GENERAL BI- OLOGY. (American Science Series.) viii +193 pp. 8vo. The Publishers’ Educational Catalogue with descriptions and prices, free. - HENRY HOLT & CO., 29 WEsr 28D STREET, New York. A TEXT-BOOK OF INVERTEBRATE MORPHOLOG pe ay Je i € 7 \ . if aa RIOARDV LIBRARY, 7 ; A, aS as iT ia sad me cite a \ wt a ee ae BY J. PLAYFAIR McMURRICH, M.A., Px.D. Professor in the University of Michigan. NEW YORK HENRY HOLT AND COMPANY 1894 nn No. 24a Copyright, 1894, BY Henry Hort & Co. QL B63 [4 /é ROBERT DRUMMOND, ELECTROTYPER AND PRINTER, NEW YORK, PREFACE. THE Morphology of Invertebrate Animals may be treated either from the standpoint of Comparative Anatomy or from the zodlogical side, and either method of treatment has much to recommend it. In my experience, however, the zodlogical method has proved most satisfactory for the presentation of the subject to students, Inasmuch as it is necessarily the method employed in the laboratory, and accordingly in the present work that plan of presenting the facts of morphology has been followed. A bare statement of the structural peculiarities of the various groups, however, is simply collect- ing the bricks and stones without the mortar necessary to unite them together into a substantial edifice, and where the opportunity has presented itself attention has been directed to the comparative significance of various organs and to the affinities of the various groups. A word is perhaps necessary in regard to the classification adopted, which presents many radical changes from the schemes usually employed. For the larger groups, following the ex- ample of Claus, the term type has been employed, and no less than twelve of these types are adopted. This increased num- | ber has resulted from a division of two groups usually recog- nized, namely, the Vermes and the Arthropoda. As regards the former it has long been acknowledged to be a heterogeneous collection, and its retention is to be regarded as a survival. It is true that the forms assigned to it do present certain phylogenetic affinities; but if this is to be the reason for its retention, then the Mollusca and Prosopygia (Molluscoidea) should also be assigned to it. It has seemed more satisfac- tory to retain the Mollusca and Prosopygia as distinct groups, and to divide the Vermes into several types, such as the Platyhelminthes, Nemathelminthes, and Annelida, each of the same rank as the Mollusca, and presenting approximately similar degrees of affinity among themselves. iii iv PREFACE. As to the Arthropoda, its right to exist as a group coordi- nate with, for instance, the Mollusca has been questioned by several authors. Undoubtedly in this case also many similar structural features obtain among the various members of the group, but embryology has indicated a probability of a more or less independent origin of two Arthropodan groups usually regarded as closely related, namely, the Arachnida and the Tracheata proper. Apparently the former have originated . from Crustacean ancestors, while, if the supposed significance | of Peripatus be accepted, the Tracheates are to be traced back _ to Annelidan forebears, and for the purpose of calling the attention of the student to this probable phylogeny the Crustacea, Arachnida and Tracheata have been regarded as distinct types coordinate with the Annelida and Mollusca. A book of this kind must necessarily be highly tinged with the individual opinions of the writer, and for these indulgence must be craved. So far as the facts are concerned every care has been taken that they should be accurate and as far as possible up to date with the most recent investigations. Errors have no doubt crept in, a misfortune almost inevitable for the mass of material which must pass under consideration during the progress of the work, and for these again indul- gence must be asked. Refraining from further apologies, the more pleasant duty remains of thanking the many friends who have so kindly aided the work by suggestion or otherwise, and especially those who have permitted the use of figures taken from special papers. A large number of the figures employed are original and the great majority have been especially drawn for this work, the attempt being made to diagramma- tize them to a greater or less extent for the sake of clearness. In all cases where figures have been borrowed the original authorship has been duly acknowledged. Finally, I desire to make public recognition of my indebt- edness to my wife for the invaluable assistance she has ren- dered in many ways during the progress of the work. J. Puayrarr McoMurricu. UNIVERSITY oF MICHIGAN, September, 1894. TABLE OF CONTENTS. CHAPTER J. PROTOPLASM AND THE CELL........... Composition of Protoplasm, pp. 1-8. Structure of the Cell, pp. 4-8. Cell-division, pp. 9-12. Literature, p. 12. Cuapter II. Tur SuBKINGDOM PROTOZOA.......... The Class Rhizopoda, pp. 14-24. The Class Sporozoa, pp. 24-— 28. The Class Flagellata, pp. 28-33. The Class Infusoria, pp. 33-88. Synoptical Classification, pp. 38, 39. Literature, pp. 39, 40. Cuapter III. Tuer Supkrnapom METAZOA........ an Individuality of the Metazoa, pp. 41-42. Sexual Reproduction, pp. 42-51. The Segmentation and Early Development of the Ovum, pp. 51-58. Non-sexual Reproduction of the. Metazoa, pp. 58-60. Alternation of Generations, pp 6 62. Literature, p. 62. Cuarter IV. TRICHOPLAX, THE DICYEMIDH AND THE ORTHONECTIDA «sie 4 na hse Wo ee ewe weno we ee am Trichoplax, pp. 63, 64. The Dicyemide, pp. 64, 65. The Or- thonectida, pp. 65-67. Literature, p. 67. CHAPTER V. THE TYPE C@LENTERA............005 The Subtype Porifera, pp. 69-76. The Subtype Cnidaria, pp. 76-115. The Class Hydromeduse, pp. 78-97. The Class Scyphomeduse, pp. 97-104. The Class Anthozoa. pp. 104— 115. Synoptical Classification, pp. 115-117. Literature, pp. 118, 119. Cuaprer VI. THE OTENOPHORA.......ce cee ee ee cees Description of the Group, pp. 120-126. Synoptical Classifica- tion, p. 126. Literature, p. 126. PAGES 1-12 13-40 41-62 63-67 68-119 120-126 vl TABLE OF CONTENTS. CHapterR VII. THE Typz PLATYHELMINTHES........ The Class Turbellaria, pp. 180-148. The Class Trematoda, pp. 148-152. The Class Cestoda, pp. 152-161. The Class Nemertina, pp. 162-169. Synoptical Classification, pp. 169, 170. Literature, pp. 170, 171. Cuapter VIII. THE Typz NEMATHELMINTHES,...... The Class Nematoda, pp. 173-179. The Class Acanthocephala, pp. 179-182. Synoptical Classification, pp. 182, 188. Lit- erature, p. 183. Cuaprer 1X. THE OkDER ECHINODERA; THE CLASS CHATOGNATHA; THE OLass ROoOTIFERA; THE ORDER GASTROTRICHA; AND DINOPHILUS..... The Order Echinodera, pp. 184-186. The Class Cheetognatha, pp. 186-189. The Class Rotifera, pp. 189-195. The Order Gastrotricha, pp. 195-198. The Genus Dinophilus, pp. 198-200. Synoptical Classification, p. 200. Literature, p. 201. CHAPTER X. THE TYPE ANNELIDA............. ae The Class Chetopoda, pp. 204-227. The Class Hirudines, | pp. 228-237. The Class Gephyrea, pp. 237-2438. The Class Myzostomex, pp. 244-246. The Class Phoronide, pp. 247- 251. Synoptical Classification, pp. 251, 252. Literature, pp. 252, 253. CHaprTeR XI. Tur Typr RROSOPYGIA......... (tases The Class Polyzoa, pp. 255-268. The Class Brachiopoda, pp. 268-274. Synoptical Classification, p. 274. Literature, p. 275. CuHapter XII. THe Type Mobuusca................. The General Characteristics of the Type, pp. 276-284. The Class Amphineura, pp. 284-293. The Class Gasteropoda, pp. 298-822. The Class Scaphopoda, pp. 322-826. The Class Pelecypoda, pp. 326-340. The Class Cephalopoda, pp. 340-362. The Affinities of the Mollusca, pp. 362, 363. Synoptical Classification, pp. 363-365. Literature, pp. 365-367. CuapterR XIII. Tur Type CRUSTACEA.............. The General Characteristics of the Type, pp. 868-885. The Class Entomostraca, pp. 885-403. The Class Malacostraca, pp. 403-417. The Development and Affinities of the Crus- tacea, pp. 417-423. Synoptical Classification, pp. 428, 424. Literature, pp. 424-427. The Order Xiphosura, pp. 427- 484. Literature, p. 484. 172-183 184-201 « 202-253 254-275 276-367 368-434 TABLE OF CONTENTS. CHAPTER XIV. THE TYPE ARACHNIDA..........0006 The General Characteristics of the Type, pp. 485-441. Descrip- tion of the Various Orders, pp. 441-456. The Phylogeny of the Arachnida, pp. 456-458. Synoptical Classification, pp. 458, 459. Literature, pp. 459, 460. The Order Penta- stomide, pp. 461-463. The Order Pycnogonida, pp. 463- 466. The Order Tardigrada, pp. 466-468. Literature, p. 468. CHAPTER XV. THE TYPE TRACHEATA ...........05. The General Characteristics of the Type, pp. 469-474. The Class Protracheata, pp. 474-480. The Class Myriapoda, pp. 480-487. The Class Insecta, pp. 487-522. The Phy- logeny of the Tracheata, pp. 528-525. Synoptical Classifi- cation, pp. 525-528. Literature, pp. 528-530. CHAPTER XVI. THE TypE ECHINODERMA........... The General Characteristics of the Type, pp. 531-540. The Class Crinoidea, pp. 541-551. The Class Asteroidea, pp. 552-560. The Class Ophiuroidea, pp. 561-570. The Class Echinoidea, pp. 570-584. The Class Holothuroidea, pp. 584-590. The Phylogeny of the Echinoderma, pp. 590- 592. Synoptical Classification, pp. 592, 593. Literature, pp. 593-595. CuapTerR XVII. THE Type PRoTOCHORDATA......... The Class Hemichorda, pp. 596-608. The Class Cephalochorda, pp. 608-618. -The Class Urochorda, pp. 618-639. Synop- tical Classification, pp. 689, 640. Literature, pp. 640, 641. INDEX OF PROPER NAMES........ 000s eee cece eens INDEX OF SUBJECTS....... bi SRG elee hed ane hae ere ocats 469-530 531-595 596-641 643-653 . 654-661 INVERTEBRATE MORPHOLOGY. CHAPTER I. PROTOPLASM AND THE CELL. In the examination of organisms presenting the series of phenomena which we term life, the invariable presence of a peculiar semi-fluid transparent or hyaline substance becomes quickly apparent. Whether the organism be a plant or an animal, whether it be of the simplest or of the most complex organization, it is still composed of this substance, which is known as protoplasm, and it may be said that so far as our knowledge extends life never exists except in association with this material. Protoplasm is “the physical basis of Life,” and it becomes of great importance that its nature should be fully understood, in order that the results of its activities, Life, may become more intelligible. Much has yet to be accomplished, however, before an accu- rate knowledge of the structural and chemical characters of this substance is obtained, and indeed it is incorrect to regard it as a substance, since it is rather the aggregate of a large number of exceedingly complex chemical compounds, none of which are sufficiently known. From the very nature of things it is impossible at present to get acorrect idea of these substances and the relations which they bear to one another, since our present analytical methods are not capable of deter- mining and isolating them in living protoplasm and the mere act of subjecting protoplasm to analysis destroys those very relationships which are the cause of the vital manifestations, 2 INVERTEBRATE MORPHOLOGY. Dead protoplasm is something very different from living pro- toplasm, and our present knowledge only imperfectly extends to this much-altered material. Furthermore even in the dead material the chemist has to deal not only with the complex’ substances which constitute protoplasm proper, but also with numerous secondary prod- ucts either in the process of being built up into protoplasmic molecules or else resulting from the destruction of these molecules. For both these processes are continually going on, the living organism continually uniting simple chemical compounds to form new complex molecules, a process known as anabolism, and resulting in growth ; and just as continually it is resolving into simpler compounds the complex mole- cules already formed, a process known as catabolism, and resulting in the manifestation of energy in its various forms, such as heat, motion, electricity, and even light. Growth and the manifestation of energy are then two most important phenomena exhibited by living organisms, standing in oppo- sition to one another and determining the general condition of the organism. If anabolic changes are the more active, then the animal or plant grows, as we express it, adds new protoplasm and increases in size ; if the anabolic and catabolic changes are practically equal in amount, stability results ; while the preponderance of catabolism leads to a lessening of material, and finally to what we term death. These changes constitute a cycle occurring in the life-history of probably every organism and causing the periods which we denote as youth, maturity, and old age. Dead protoplasm then, together with the anabolic and catabolic constituents which are inextricably associated with it, will be found on analysis to consist to a large extent of the chemical elements Carbon, Hydrogen, Oxygen, and Nitrogen, together with Sulphur and Phosphorus, as well as a number of substances present in varying amounts, such as Chlorine, Potassium, Sodium, Iron, Calcium, and Magnesium. Exactly how these various elements are united together it is difficult to determine, but especial importance has been assigned to the C,H, O, N, and S compounds which occur and which form a group of chemical compounds known as Proteids. Of such PROTOPLASM AND THE CELL. 3 compounds several, such as Albumin, Globulin, Fibrin, Plastin, Nuclein, have been isolated from protoplasm, some being probably secondary products resulting from the alteration of the protoplasmic molecules proper, but others, such as Plastin and Nuclein, are especially constant, and seem to be important constituents of the protoplasmic complex. Plastin forms when isolated a sticky fibrous mass, insoluble in concen- trated alkaline solutions and unaffected by the peptic and tryptic ferments, and consists of C, H, O, N, 8, and P. Nuclein is more especially charac- teristic of a special portion or modification of protoplasm termed the nucleus, of which more will be said hereafter, and resembles plastin very closely, being, however, less insoluble than it, and consists of the same _ chemical elements. Analyses of these substances, however, differ greatly, | the nuclein from spermatozoa, for instance, containing no sulphur ; and it seems probable not only that they differ materially according to the source from which they are obtained, but also that they are not realy chemical compounds, but a mixture of several highly complex substances. With these proteids, then, there exist in protoplasm vari- ous salts, such as Potassium, Sodium, and Calcium phosphate, Potassium and Sodium chloride, Magnesium sulphate, and other such salts, the exact significance of which it is difficult to estimate. How living protoplasm differs chemically from dead has not up to the present been accurately determined. As regards its general structure protoplasm appears as a moderately consistent jelly-like substance, usually colorless and more or less granular in appearance. As a rule the’ peripheral portion of a mass of protoplasm is less granular than the central, appearing therefore clearer, and is espe- cially distinguished as the ectoplasm from the more opaque’ endoplasm. Imbedded in the endoplasm are to be found usually various bodies, the products of the activities of the protoplasm, such as large, clear spaces occupied by fluid and known as vacuoles, food-particles of various kinds in the simpler organisms, starch granules and crystals in plant-pro- toplasm, and depositions of pigment. One particular struc- ture, the nucleus, however, seems to be invariably present, occupying the central portion of the mass, and, as will be seen later, playing a very important role in the life of the protoplasm. It is indeed a specially modified portion of the protoplasm and cannot, therefore, be placed in the same 4 INVERTEBRATH MORPHOLOGY. category as the vacuoles and other accidental or secondary constituents which have been mentioned, and every mass of protoplasm may be considered as consisting of two essential parts, the protoplasm proper or cytoplasm and the special modification of it, the nucleus, which for convenience is termed the caryoplasm. Such a combination of cytoplasm and caryoplasm forms what is technically known as a cell, and all living organisms are composed of one or more such struc- tures, which are to be regarded therefore as morphological units. If the more intimate structure of the cytoplasm of such a unit or cell (Fig. 1) be examined, disregarding the various Fie. 1.—DIAGRAM SHOWING THE STRUCTURE OF AN ANIMAL CELL. c = centrosome. m = microsome. el = cytolymph. nt = nucleolus. cr = chromatin, nm = nuclear membrane. r = reticulum. secondary constituents it may enclose, it will be found to consist of a network of exceedingly fine fibrils, along which, and more especially at the points where two or more of them meet, are to be found minute granules which stain deeply with the ordinary microscopical staining reagents. The fibrils constitute the reticulum (Fig. 1, 7) of the cytoplasm, and the granules are termed the microsomes (m). The reticulum seems to be formed principally of’the proteid substance already mentioned as plastin, and its meshes are occupied by a more fluid substance which has been termed the cyto- lymph (cl). PROTOPLASM AND THE CELL. 5 Several opinions have been given in regard to the structure of the cyto- plasm, in addition to that here presented, according to which it may be — compared to a sponge the meshes of whose network are occupied by the | cytolymph. According to another view it is composed of a number of fibrils of varying lengths imbedded in a matrix, the fibrils corresponding to the reticulum of the reticular theory and the matrix to the cytolymph. According to still another theory which rests on the appearance produced in the cytoplasm by a special method of treatment, there is present a color- less matrix in which are imbedded numerous exceedingly small granules sometimes scattered and sometimes united together into chains. Indeed the upholder of this granular theory has carried his view to the extent of regarding the granules as structural units of which the cell is composed, its structure being comparable to that of a zooglea of micrococci. It seems probable, however, that the granules are to a large extent secondary products of the activities of the cytoplasm and have therefore but a sub- ordinate value in its composition. The reticular theory seems to stand more nearly in harmony with the majority of observations, though it must be admitted that some observers do not seem to have perceived the true reticulum, confining their attention to the coarser network produced in some cases by extensive vacuolization of the cell. An imitation of the cytoplasm has been recently obtained by the mix- ture of thickened oliye-oil with a solution of potassium carbonate or of [2/00 chloride of sodium, the watery solution taking the form of polyhedral globules each surrounded by a thin layer of oil which from its higher refractive index gives the appearance of the plastin reticulum surrounding the cytolymph. Solid particles finely divided and mixed with the oil tend to collect at the points where the oil-films of three of the globules come together, and resemble the microsomes, while it is further noticeable that under certain conditions the superficial globules of the emulsion take on a columnar form and may be compared with the ectoplasm of the cell. It is possible that the cytoplasm may have this structure, in which case the reticular theory would require to be modified, since there would no longer be a spongy structure, but rather an emulsion in which the cytolymph is divided into a number of globules each surrounded by a thin layer of plastin. At present, however, the reticular theory seems to correspond most accurately with the actual appearances, and therefore may be pro- visionally accepted. The caryoplasm or nucleus, as already stated, lies usually about the middle of the cytoplasm and to a certain extent re- sembles it, though it presents certain peculiar features. It is usually round or oval, though occasionally it may assume elongated, horseshoe-shaped, moniliform, or even branching forms, and is as arule clearly marked off from the cytoplasm by a membrane (Fig. 1, mm), which, however, at certain periods of oie a 6 INVERTEBRATE MORPHOLOGY. nuclear activity seems to disappear, a new one subsequently forming. Traversing the space enclosed by the membrane, so as to form a network, are fibres which do not stain very deeply with the usual staining fluids and which are composed of a substance termed linin, which does not, however, appear to differ essentially from the plastin of the cytoplasm. In- deed it is not improbable that the linin network is con- tinuous through the nuclear membrane with the plastin reticulum and that both are identical, as is also the caryo- _u a you? lymph contained in the meshes of the linin with the cyto- lymph. A more characteristic substance is the chromatin (Fig. 1, cr), so called from the strong affinities it shows for many staining fluids, such as carmine, hematoxylin solutions, and certain aniline stains. It seems to consist of the substance nuclein, already alluded to, and in the resting nucleus forms a reticulum intimately associated with the linin network, which it usually to a considerable extent obscures. Where the various strands of the network meet, thickenings of the chromatin sometimes occur, producing densely staining bodies (nl) to which the term nucleoli is given, though it is probable that bodies of a somewhat different composition are also included under this name; for there are usually to be found in the nucleus, imbedded in the substance of the network, one or more spherical bodies whose chemical re- actions differ noticeably from those of the chromatin nucleoli, the substance of which they are composed being termed paranuclein or pyrenin. There are then in the cell the following structural con- stituents : membrane (cell-wall), I. Cytoplasm: < reticulum (plastin), Pes ‘earyolymph. feu ve membrane, Micck.. . reticulum (linin), II. Caryoplasm :< caryolymph, (Catrcdem oti’ chromatin network (nuclein), ( nucleoli (nuclein and paranuclein). PROTOPLASM AND THE CELL. ° 7 In addition to these there is, however, still another body to be mentioned which is especially evident in cells which are undergoing multiplication, but which has also been found in various resting cells, especially in lymph-corpuscles, various kinds of endothelial cells, and in pigment-cells. This is the structure known as the centrosome (Fig. 1, c). It is usually an exceedingly small spherical body which does not readily stain with the reagents which place the chromatin in evi- deuce, but has a strong affinity for certain acid aniline stains, such as safranin, fuchsin, or orange. Usually but a single © centrosome is present in each cell, though occasionally two or | even more may occur, and it is situated in the cytoplasm in the neighborhood of the nucleus, sometimes resting in a slight concavity on the surface of that structure. Surround- ing the centrosome there is frequently to be seen, more especially in dividing cells, a radial arrangement of the cyto- plasmic reticulum, the centrosome being comparable to a . star from which rays pass out in all directions, whence the term aster which is applied to the combination of the cen- trosome and the cytoplasmic rays. The significance of the centrosome will be seen later when the phe- nomena, of cell-division are under consideration, but its origin may be inquired into at this place. Two views are current in regard to this matter, according to one of which the centrosome has its origin in the © nucleus and at a certain period of the cell’s existence is extruded from it. In favor of this view the intimate association of the centrosome and the nucleus are pointed out, an association which becomes especially pro- nounced during cell-division, the astral rays connected with the cen- trosome appearing to penetrate the nucleus and in fact to bring about its division into two parts. According to the other theory, however, the cen-: trosome is a constituent of the cytoplasm and in its origin has nothing to do with the nucleus. Quite recently an interesting amplification of this ess idea has been suggested to the effect that the centrosome is nothing more , or less than an aggregation of cytoplasmic microsomes. The astral rays are cytoplasmic fibres converging from all sides, and since microsomes occur along their course an aggregation of these bodies might be found where the fibres meet. This idea cannot be discussed in detail here, but it may be pointed out that the absence of a centrosome in cells which have lost their powers of reproduction is readily explicable on this idea, the ag- gregated microsomes having scattered in such cells, and furthermore that the theory explains certain peculiar arrangements replacing the typical aster during the division of some cells. 8 INVERTEBRATE MORPHOLOGY. Such a combination of cytoplasm and caryoplasm consti- tutes a morphological element capable of carrying on all the functions of life. It is not only a morphological but also a physiological element. It is capable of assimilating the necessary substances and building up protoplasm ; metabol- ism and the consequent evolution of energy goes on in it; it excretes waste products ; it is contractile and may therefore be capable of motion; it responds to stimuli of various kinds, or in other words it is irritable; and, finally, it is capable of reproduction. The question naturally arises, however, whether this combination of the two substances mentioned is essential—whether, that is to say, organisms without nuclei do not exist and manifest all the phenomena of life. At one time the existence of unicellular organisms destitute of a nucleus was recognized, the term cytode being applied to them to distinguish them from nucleated cells. Within re- cent years, however, a growing skepticism has come into existence ‘as to the non-nucleate character of these organ- isms, the recent improvements of the microscope and the application of modern staining reagents having revealed the existence of nuclei in many of the forms at one time regarded as typical cytodes. It would perhaps be going too far to state that cytodes do not exist, but the evidence at hand indi- cates that their existence is highly problematical. This conclusion is strengthened by the results which have been obtained from the observations of artificially produced cytodes. Some of the larger unicellular organisms have been cut into fragments some of which can be definitely shown to be destitute of nuclear or caryoplasmatic substance. In such cases it is found that the nucleated fragments if placed under proper conditions will regenerate and carry on their existence as before, while the cytode fragments, though manifesting signs of life for a considerable length of time, will not regenerate and do not possess the power of repro- _ duction. The nucleus seems to possess a marked regulating - or coérdinating action upon the cytoplasm, codrdinating the anabolic and catabolic activities upon which the continuance of life depends. It would be beyond the scope of the present work to enter PROTOPLASM AND THE CELL. 9 into a discussion of the various forms of physiological activity of the cell, but one of its physiological functions, reproduc- tion, must receive special attention in connection with the remarkable structural changes which accompany it. Since the disproval of the doctrine of spontaneous generation the epigrammatic statement Omnis cellula e celluld las been the watchword of modern histology and embryology, and to-day it finds its complement in a corresponding epigram, Omnis | nucleus e nucleo. Every cell at present in existence may be assumed to have descended from some previously existing | cell, and the nucleus it contains to be a portion of the nucleus of the ancestral cell. New cells arise by the division of previously existing cells, and each division of the cytoplasm is accompanied by a division of the nucleus. Not but that under certain conditions a division of the nucleus may occur without a corresponding division of the cytoplasm, multinucleated cells thus arising, and conversely a division of the cytoplasm may possibly in certain cases be inaugu- rated without entailing a division of the caryoplasm; but, as might be expected from the relation which exists between the nucleus and the cytoplasm, the division of the latter is usually preceded by a division of the caryoplasm. This latter process may take place in two ways. It may. begin as a simple constriction of the nucleus which, becoming deeper and deeper, finally separates off a portion of it, a divis- ion of the cytoplasm in a similar manner then occurring, so that each of the new cells thus formed contains a portion of the original nucleus. This method of nuclear division, which .. is rather rare, occurs for instance in the embryonic mem- branes of the Scorpion and is termed direct or amitotic divis- ion, to distinguish it from the more usual indirect or mitotic method which is accompanied by a series of complicated phenomena to which the general term hkaryokinesis or mitosis is applied. Starting with a typical cell, consisting of the various parts mentioned above, the karyokinetic phenomena may be re- garded as affecting two constituents, i.e. the centrosome and’ the nuclear chromatin. The centrosome which lies at one pole of the nucleus first divides, the two resulting portions 10 INVERTEBRATE MORPHOLOGY. gradually separating from one another (Fig. 2, A) until they lie at opposite poles of the nucleus, usually taking up a posi- tion ninety degrees distant from the point at which the origi- nal centrosome lay. During this process the radiating fila- ments which surround the centrosome become especially distinct and may be divided into two portions, those which ‘come in contact with the nucleus and which from their appearance in later stages are termed the spindle,ibres, and Fie. 2.—D1aGRAM SHOWING THE PHENOMENA OF CELL-DIVISION. A, separation of the centrosomes ; chromatin in skein-stage. B, fully formed spindle ; chromatin loops formed. @, longitudinal division of the chromatin loops. D, separation of chromatin loops and commencement of the division of the cytoplasm. those which radiate outwards and are lost in the cytoplasmic network and form the aster. In the meantime, however, im- portant changes have been taking place within the nucleus, “The chromatin substance, which originally was scattered in a reticulum, begins to arrange itself in a band (Fig. 2, 4) which with many turns traverses the nuclear substance, the nucleoli which were present at the same time gradually van- ishing. This stage of the process is termed the skein stage, PROTOPLASM AND THE OELL. 11 The spindle-fibres of the centrosome then appear to penetrate through the nuclear membrane, which sooner or later disap- pears, and by their growth push the chromatin skein towards | the equator of the nucleus, the skein at the same time break- ing into a number of fragments, termed chromosomes. The number of these chromosomes is practically constant for the | cells of any species of animal, and though there is consider- able variation in different species, yet in the majority of ob- served cases the number belongs either to the series 2, 4, 8, 16, 32, or to that of 6, 12, 24. They vary considerably in size in different forms, being in some cases V-shaped or in others dumbbell-shaped, and arrange themselves finally in a more or less definite ring surrounding the equator of the nucleus. At this stage, which is known as the equatorial-plate stage, the appearance presented in Figure 2, B,is found. At each pole of the nucleus is a centrosome surrounded by the astral rays and with the spindle-fibres extending towards and coming in contact with the chromosomes lying at the equator of the | nucleus, and to the entire complex the term amphiaster is sometimes applied. In the next stage the V-shaped chromosomes, to take this as a typical shape, which are arranged with the apex of the V towards the nuclear axis, divide longitudinally. Assuming that there were originally six chromosomes in the equatorial plate, as the result of the division there are now twelve ar- Gc letras ranged in pairs (Fig. 2, C). One of each pair, now proceeds to move'towards one Gf the ‘poles, of the nucleus and the other | to the other, so that eventually near each pole there is a group of six chromosomes, and between the two groups there may be seen stretched a number of connecting fibres identical in appearancé with the original spindle-fibres, while in some , cases at the equator of the egg there is to be seen on these fibres a number of darkly staining dots which may be termed — the intermediate bodies (Fig. 2, D). At about this stage the cytoplasm begins to divide, the plane of its division passing through the equator of the nucleus, and there are thus formed two cells, each containing a nucleus composed of six chromo- somes and a centrosome. The chromosomes now begin to become irregular in shape, they gradually fuse and are finally 12 INVERTEBRATE MORPHOLOGY. scattered in the form of a chromatic reticulum through the substance of the nucleus, which thus passes again into the resting stage, developing a new nuclear membrane. Our knowledge of many of the details of karyokinesis is yet imperfect, and especially is this the case with regard to the mode in which the cen- trosome exerts its influence. It has been regarded as a simple centre of attraction, similar to the pole of a magnet, but the spindle-fibres seem to be more than passive in the phenomena. A comparison of the centrosome with an aggregation of microsomes has already been referred to, and if this idea be extended some light may be thrown upon the spindle-fibres. They would then naturally be regarded as reticular fibres, i.e. fibres of plastin to which a certain amount of contractility and extensibility may be ascribed. During the earlier stages of karyokinesis their extensibility is more manifest, and extending into the nucleus they compress its chromatic substance, the contractility manifesting itself later and determining the migration of the chromatin loops or chromosomes towards the poles of the nucleus. Furthermore, since the linin reticulum of the nucleus is probably continuous with the plastin reticulum of the cytoplasm, it is conceivable that the activities of the centrosomes may call out in it changes of contrac- tion or extension which may suffice to bring about the characteristic skein formation of the chromatin and the subsequent fragmentation of the skein . into the chromosomes, as well as the formation of the connective fibres in later stages, the intermediate bodies upon these being regarded as micro- somes. These views, which have been but recently suggested, require con- firmation, however ; if true they afford a new basis from which to attack the problems involved in the phenomena of karyokinesis, and even at present throw no little light upon the structural details associated with the process. It must be mentioned, however, that certain recent observations have been held to prove that the centrosome has a nuclear origin, and for the present ' the important question of its significance must be considered as open. LITERATURE. 0. Hertwig. Dée Zelle und die Gewebe. Jena, 1892. W. Flemming. Zellsubstanz, Kern und Zelitheilung. Leipzig, 1882. 0. Biitschli. Untersuchungen tiber mikroskopische Schiume und das Proto- plasma. Leipzig, 1892. C. Rabl. Ueber Zelitheilung. Morpholog. Jahrbuch, x. 1884. G. Platner. Beitrige zur Kenntniss der Zelle und ihrer Theilung. Archiv fiir mikrosk. Anatomie, xxx11I. 1889. M. Heidenhain. Uber Kern und Protoplasma. Leipzig, 1892. H. P. Johnson. Amitosis in the Hmbryonal Envelopes of the Scorpion. Bulletin of the Museum of Comp. Zoology, xx11. 1892. SUBKINGDOM PROTOZOA. 18 CHAPTER II. SUBKINGDOM PROTOZOA. A stmpLe cell, as has already been stated, possesses the power of performing all the functions of life, and conse- quently the existence of unicellular organisms is possible. Such organisms, together with those which consist of a number of cells grouped together, each cell, however, retain- ing to a greater or less extent its own individuality, are grouped together in a subkingdom and are collectively termed Prorozoa. In its simplest form a Protozoon may show but little differentiation of its protoplasm, but in the majority of cases various portions of the cell-substance take upon themselves special functions, and in accordance with this physiological differentiation undergo various structural modifications. Locomotor and prehensile structures of vari- ous forms may be developed, excretory pulsating vacuoles, a permanent mouth and pharynx, special contractile bands, and even pigment spots presumably connected with light absorption may occur, and in addition the power of secreting horny, calcareous, or siliceous skeletons, serving either as protective or supportive structures, is frequently present. A high degree of complexity may therefore occur in a unicel- lular organism, a complexity produced by a differentiation of various portions of the protoplasm composing the individual. For the most part the organisms are simple, but occasionally they associate together to form colonies. The individuals of the colonies are as a rule all alike, each carrying on all the functions of existence for itself, and there is no division of labor among the various individuals. The complexity which exists is individual and not colonial. A few forms, however, such as Volvoxz, do present a certain amount of colonial differ- entiation ; all the cells composing the colony are not perfectly identical physiologically, some becoming, for instance, spe- 14 INVERTEBRATE MORPHOLOGY. cialized for reproductive purposes, while the rest take but little part in this process. Such a colony presents indica- tions of a passage towards a higher grade of individuality, some of the various cell-individuals merging to a certain ex- tent their individualities in that of the entire colony, and becoming somewhat dependent for existence on the coopera- tion of their fellows. This dependence, however, never reaches a high degree of development in the Protozoa and is for the most part entirely absent. It is in this respect that colonial Protozoa differ from the higher organisms, but the difference is one of degree, not of kind. Four well-marked classes may be distinguished among the Protozoa: I. Cl. Rhizopoda. IT. Cl. Sporozoa. III. Cl. Flagellata. IV. Cl. Infusoria. I. Crass RHIZOPODA. The simplest Rhizopods present an approach to the least complicated condition under which protoplasm is known to us. They are simply small masses of protoplasm, more or less granular towards the centre, clearér towards the periph- Fia. 3.—Ameba proters (after Grtper). cv = contractile vacuole. nm = nucleus. ps = pseudopodium. ery, and continually alter their shape by pushing out lobe. or thread-like processes known as pseudopodia (Fig. 3, Pps). SUBKINGDOM PROTOZOA. 15 By throwing out such a process and flowing after it, as it were, locomotion is performed, which from a well-known genus of the class is termed ameboid. Food is simply en- gulfed by the protoplasm flowing around it, after it has come in contact with a pseudopodium, and the digestion of the food-substance takes place within the protoplasm, being thus intracellular. Undigestible material is discarded at any part of the body; respiration and excretion are carried on by ‘the general surface; and reproduction is limited to the sim- ple process of division. It is rare, however, to find such a simple condition as this; even among the simpler forms a certain differentiation of the protoplasm exists, and it is doubtful if it is really absent in any of the forms known to us. The structural dif- ferentiations most usually occurring are the nucleus (Fig. 3, n) and the contractile vacuole (Fig. 3,cv). The former, as was noticed in the preceding chapter, is of great importance to the cell, and itis questionable whether it is really absent even in those Rhizopods in which it has not yet been dis- covered. It is presumable, of course, that it is a structure which has gradually become elaborated, that has evolved, and that in the simplest conceivable organism it may have been undifferentiated, but whether such an organism now exists is questionable. The contractile vacuole is excretory in its function, fluid containing products of metabolism in solution accumulating at one or more definite regions of the protoplasm to form it, and being by the sudden and rhythmi- cal contraction of the surrounding protoplasm periodically expelled from the body. Various degrees of complexity are, however, found among the Rhizopods, the higher forms presenting a considerable degree of differentiation both in structure and in the modes of reproduction, and three orders based upon structural charac- teristics may be distinguished. 1. Order Foraminifera. The Foraminifera contains the simpler members of the class. In the genus Amoeba (Fig. 3) are organisms presenting the simple characters above alluded to, being simple naked 16 INVERTEBRATE MORPHOLOG Y. masses of protoplasm containing a nucleus and a contractile vesicle and presenting a slight differentiation into a peripheral more transparent ectoplasm and a central more granular endoplasm in which the nucleus is imbedded. The pseudo- podia are as a rule blunt lobose processes, though in some species they are more or less fila- mentous and may even be some- what permanent. The majority of forms, however, secrete a protective shell of varying composition and complexity. In Arcella (Fig. 4) it is chitinous and smooth, and len- ticular in shape, completely sur- rounding the protoplasm, the pseu- Fie. 4.—Arcella mitrata dopodia projecting from the cir- ee cular opening on the flat surface ; in Euglypha it is similar in composition, but sculptured on the convex surface ; in Difflugia the shell is flask-shaped and composed of particles of sand and similar foreign bodies cemented together, while in a large number of forms, es- pecially those which are marine in habitat, the shell is calcareous in composition. It is in these forms with calcareous shells that the great- est complexity of structure occurs. In some, such as Gromia, the shell is simple and flask-shaped, the protoplasm pro- truding from the mouth of the shell and covering its entire surface as a delicate layer, from which the long, slender, and frequently anastomosing pseudopodia take their origin. Al- though the pseudopodia are practically permanent in form their protoplasm is continually changing, currents streaming from the body towards the tips of the pseudopodia and re- turning again to the central mass, a constant circulation being thus maintained, and food-particles caught by the delicate pseudopodia conveyed to the central mass, there to be di- gested. A simple shell is, however, comparatively rare among these calcareous forms; more frequently it consists of several chambers, as in Miliola, the chambers varying in size, the first-formed one being the smallest, and, in addition, in very many forms the shell is perforated by minute pores SUBKINGDOM PROTOZOA. 17 through which the pseudopodia are emitted. The successive chambers are arranged in various ways, sometimes end to end as in Nodosaria, sometimes alternately on opposite sides of an axis as in Textularia, sometimes as a spiral as in Globigerina, sometimes as a helix as in Rotalia (Fig. 5), and sometimes more or less irregularly as in Acervularia. \ Fig. 5.—Rotalia venata (after M. ScuutzE from HaTscHeER). Notwithstanding the complexity of the shell, however, the protoplasm retains throughout the order its simple structure, and though in the more complicated forms the single nucleus may be replaced by several, yet beyond this they present no more marked differentiation than is found in the simpler genera. 2. Order Heliozoa. In the second order, the Heliozoa, the pseudopodia are slender as in the calcareous Foraminifera and are permanent and somewhat rigid, the central protoplasm of each one being differentiated into an elastic axial support. The ani- mals are usually globular in shape, the slender pseudopodia radiating out from the central mass, an appearance being thus produced which is sufficient cause for the popular term “sun-animalcule’’ which is applied to several of the genera, such as Actinophrys and Actinospherium (Fig. 6). Currents 18 . INVERTEBRATE MORPHOLOGY. of protoplasm traverse the pseudopodia as in the Foraminifera and carry the food-particles to the body proper. This has a delicate ectoplasm and a central endoplasm which is fre- quently highly vacuolated and contains one or more nuclei Fic. 6.—Actinospherium Hichhornti (after Lery). cv and cv’ = contractile vacuoles, if = ingested food, of = egested food. ‘ps = pseudopodium. and contractile vacuoles. In some forms also a skeleton is developed; it reaches its most perfect form in the stalked Clathrulina, in which it consists of a delicate fenestrated siliceous sphere. 3. Order Radiolaria. The Radiolaria are exclusively marine and are the most complicated of all the Rhizopods. Their pseudopodia re- semble closely those of the Heliozoa, being slender and pos- sessing an axial support. The body varies in shape somewhat in accordance with the shape of the siliceous shell with which almost all the forms are provided. In those forms in which the shell is simplest, as in Thalassicolla (Fig. 7), where it is in reality absent, the body is spherical and is clearly differen. tiated into two regions, not, however, corresponding to the ectoplasm and endoplasm of other Rhizopods. The centre of SUBKINGDOM PROTOZOA. 19 the body is occupied by a spherical mass surrounded by a firm chitinous covering and forming the central capsule. This contains usually many uuclei as well as vacuoles, oil-globules, and in some cases crystals aud pigment-granules. The wall of the capsule is probably comparable to the shell of the Foraminifera, being perforated as in those forms by minute pores through which the intracapsular protoplasm becomes Fie. 7.—Thalassicolla pelagica (after Harcken from Hatscuex). continuous with the extracapsular. This latter portion on this supposition, notwithstanding its greater relative thick- ness, is equivalent to that portion of the protoplasm of the Foraminifera which is outside the shell and from which the pseudopodia arise. Itis usually richly vacuolated and pig- mented, but contains no nuclei; the axial supports of the pseudopodia traverse it and take their origin from the inner layers which immediately surround the central capsule and are more homogeneous than the outer portions. The shell is very various in form in the different genera, reaching a high degree of differentiation in some forms, such as Heliosphera (Fig. 8), where it consists of a fenestrated globe traversed by radiating spines. Its greatest simplicity is seen in Spherozoum, in which it is represented by scattered 20 INVERTEBRATE MORPHOLOGY. spicules, while in Thalassicolla, already alluded to (Fig. 7), it is entirely absent. As stated, itis usually siliceous in char- acter, though in Acanthometra it is composed of a peculiar horny material termed acanthin. Scattered through the protoplasm of the Radiolarians there are usually to be seen numbers of small yellowish bodies long known as the “yellow cells.” They are not con- stant, however, individuals of any species frequently being destitute of them, a peculiarity due to the “yellow cells” not being really constituent parts of the Radiolarian, but Fic. 8.—Heliosphera actinota (after HaeckeL from HatscHEr). foreign bodies, in fact unicellular plants, for which the term Zooxanthelle has been proposed. They cannot be consid- ered parasites, since they do not appear to exist at the ex- pense of the host, but, on the contrary, their presence seems actually to be beneficial. Mutual benefits are conferred by the plant and the Radiolarian, the coexistence constituting an example of the phenomenon known as Symbiosis. Reproduction in the Rhizopods.—Throughout all the groups the simplest form of reproduction, fission, is probably preva- lent (Fig. 9), though it is not yet definitely known to occur in SUBKINGDOM PROTOZOA. 21 the marine Foraminifera nor among the Radiolaria. In the fresh-water Foraminifera and Heliozoa it is, however, the usual method in genera both with and without shells. Where the shell is thin it may be divided during the process, but where it is thicker the protoplasm divides within it, one of the new individuals retaining the old shell, while the other wanders forth and constructs a new house for itself. This is the case, for instance, in Arcella, in which the wandering indi- Fie. 9.—Division of Amba (after ScuuuzE). vidual protrudes from the mouth of the parent shell until it forms its new shell, only separating when this is accom- plished. Colonies, produced by repeated divisions and the imper- fect separation of the forms so produced, are occasionally formed, but they are simply aggregations of similar individ- uals, no differentiation or individualization of the colony as a whole occurring. Among the fresh-water Rhizopods this is the case with Microgromia, a shelled form, numerous individ- 22 INVERTEBRATE MORPHOLOGY. uals of which may remain in connection with one another by means of their profusely-branching pseudopodia. Colonies of Actinophrys are also formed in asimilar manner, and among the Radiolaria the forms with rudimentary shells—such as Spherozoum, produce, apparently by the division of the cen- tral capsule, numerous individuals which remain in contact. A modification of fission known as budding or gemmation also occurs in some forms. It differs from fission only in that the products of the division differ in size, so that it is possible to regard the larger individual as the parent and the one or more smaller ones formed from it by budding as the progeny. The process is, however, fundamentally the same as fission and is a derivative of that process. In Arcella bud-like processes arise from the periphery of the parent protoplasm, separate, and assume amceboid movement leav- ing the shell in an Ameba-like condition, and it seems prob- able that the marine Foraminifera and certain Heliozoa re- produce in a similar manner. Spore-formation also occurs, the parent protoplasm break- ing up more or less completely into a number of small por- tions termed spores, which later increase in size and assume the characters of the parent. This process is sometimes pre- ceded by encystment, a phenomenon not, however, in its origin connected with reproduction. It is more prevalent among fresh-water than among marine forms, and seems to have been originally developed as a protection from injurious ex- ternal conditions, such as the drying up of the pools in which the organisms live. When about to encyst an dmeba, for instance, withdraws its pseudopodia and assumes a spherical shape, and then secretes a more or less dense chitinous case or cyst which completely encloses it. In virtue of the resist. ent and non-conductive nature of the cyst the organism may, while in this state, suffer uninjured prolonged exposure to conditions which would quickly entail the death of the non- encysted individual, and on the return of favorable condi- tions may leave the cyst and reassume its active life. Occa. sionally, too, encystment may occur as the result of good nutrition, an individual which has engulfed a number of diatoms, for instance, secreting a cyst around itself within SUBKINGDOM PROTOZOA. 23 which it remains until the food-matter has been thoroughly digested, when the cyst is thrown off together with the empty diatom shells and the animal again becomes active. Plentiful nutrition and reproduction by division (including under this term the various modifications of fission) are related to a certain extent, and it is easy to understand why the two processes of encystment and spore-formation should be associated together. The Heliozoan Vampyrella (Fig. 10, A) feeds in its active condition on diatoms, and especially on a stalked form, Gomphonema. After having digested the contents of the diatom frustules which it engulfs it pushes Fie. 10.— Vampyrella (from Harcxen after BUrscHLt). A. Vampyrelia feeding upon the stalked diatom Gomphonema. B. Vampyrelia encysted upon the stalk of the diatom. them aside and encysts itself upon the stalk previously occu- pied by them. Within the cyst the animal divides into four spores (Fig. 10, B), each of which escaping from the cyst becomes a new Vampyrella. Among the Radiolaria spore-formation seems to be the most usual method of reproduction, and a complication occurs among them in that spores of two kinds may be formed. In some cases the spores, which are formed from the intracap- sular protoplasm, are all equal in size (isospores), while in others some of the spores may be large (macrospores) and others small (microspores). Both macrospores and micro- spores may be formed in the same individual, or each indi- vidual may produce only one of the two forms. In such cases itis easy to determine whether one has to do with macro- spores or isospores, which closely resemble each other in size, 24 INVERTEBRATE MORPHOLOGY. from the fact that the isospores are spherical in shape and each possesses a peculiar whetstoue-like crystal, wanting in the macrospores. All the spores are provided with single whip-like processes, flagella, by which they are propelled through the water when set free from the parent. The various processes so far mentioned concern a single individual only and are therefore non-sexual. Whether sex- ual reproduction, the union of two individuals (conjugation), occurs among the Rhizopods is uncertain, although the fusion of two individuals preceding spore-formation has been ob- served in several instances, That the fusion, however, is the predisposing cause of the spore-formation seems probable, but cannot be positively asserted until the behavior of the nuclei of the two fused individuals is ascertained. It seems exceedingly probable, also, that the macrospores and micro- spores of the Radiolaria are sexual cells, their further de- velopment depending on the conjugation of a micro- with a macrospore, but the fate of these spores has not as yet been ascertained, and their conjugation can only be imagined from analogy with other forms. II. Cuass SPOROZOA. The Sporozoa, which constitute the second class of Proto- zoa, are all parasitic, living in the cavities, cells, or tissues of other animals and deriving their nutrition from their hosts. At present much is lacking to an adequate knowledge of the various members of the group, but at least three orders are to be recognized. — 1. Order Gregarinida, The Gregarinida include some of the largest Sporozoa, and are parasitic either in the body-cavity, intestine, or organs of various Invertebrata (especially in Annelids and Tracheata), or in the cells especially of Vertebrated Animals, these intracellular parasites being .usually known as the Coccidia in contradistinction to the former, the Gregarinida proper. The members of both groups show a marked diffey- entiation of their protoplasm into ectoplasm and endoplasim, SUBKINGDOM PROTOZOA. 25 a relatively large nucleus lying in the latter, and none are known to possess pseudopodia. Indeed in many Gregarinida a well-marked cuticle covers the exterior of the body (Fig. 11), sometimes distinctly striated or occasionally tuberculated. The Coccidia and many Gre- garinida show little differentiation beyond what has been mentioned, but the Gregari- nida which inhabit Tracheate hosts usually present the appearance of being composed of two cells, owing to the anterior portion of the body being separated by a partition of ectoplasm from the posterior part, and in addition to this the anterior moiety in some cases is furnished with hooks, bristles, or finger-like processes (Fig. 11) of use in fixing F6- 1!.—Hoplorhyn- a ay ‘ chus oligacanthus the animal to the walls of the cavity in which Gfter gcunsiver). it lives. Even in these cases, however, but a single nucleus is present and the organism is unicellu- lar. Reproduction is carried on by spore-formation, preceded in some cases by conjugation (Fig. 12), but simple division or gemmation is not known to occur, apparent instances of division being more probably cases of conjugation. In spore- formation, preceded or not by conjugation, the animal as- sumes a spherical shape and forms a cyst about itself, the greater portion of the protoplasm splitting up into usually a number of nucleated spores, a small portion of it, how- ever, remaining undivided (residual body) (Fig. 12). When inature the spores are usually spindle- or boat-shaped and have received the name of pseudonavicelle. They do not, however, develop directly into Gregarines, but their proto- plasmic contents break up into 2, 8, or more crescentic spores (Fig. 12), a residual body being again formed as in the formation of pseudonavicelle. The further history of these crescentic spores is not thoroughly known, but in some cases (Porospora from the intestine of the lobster) each seems to become converted into an amceboid structure which later elongates to an actively moving thread-like organism, 26 INVERTEBRATE MORPHOLOGY. the pseudofilaria, and this, gradually losing its motility, de- velops into the adult form. Fig. 12.—REPRODUCTION OF GREGARINE (from HERTWIG). 1. Clepsidrina blattarum in conjugation; ck = ectosarc, en = endosarc,cu = cuticula, pm = anterior portion, dm = posterior portion, m = nucleus. 2. Cysts in transformation into. pseudonavicelle; pn — pseudonavicelle; rk = residual protoplasm. 3. A, a pseudonavicella strougly magnified; B, the same divided into spores, sk; n = nucleus, rk = residual protoplasm. ; 2. Order Myxosporidia. The Myxosporidia are found almost exclusively parasitic in Fishes, affecting principally the skin, but also occurring in the internal organs, such as kidneys, spleen, and urinary bladder. They consist of irregularly-shaped masses of pro- toplasm, sometimes reaching a length 0.1 mm., but usually falling considerably short of this size. Frequently they are enclosed in cysts developed from the tissues of the host, but SUBKINGDOM PROTOZOA. 27 when not so enclosed seem to possess the power of slow anceboid movement. The endoplasm is usually well differ- entiated from the ectoplasm and contains in the adult condi- tion a large number of minute nuclei. Reproduction by division is not known to occur, spore- formation being the only method as yet observed. In the Myxosporidium occurring in the urinary bladder of the Pike the protoplasm breaks up into a number of spherical masses each containing a number of nuclei. The fate of all of these masses is not known, but some, containing only six nuclei, form a wall about themselves and divide into two portions each of which contains three nuclei. These trinucleated bodies elongate, develop a wall, and become pseudonavicella-like spores, one of the three nuclei per- sisting as the spore-nucleus, while the other two, situated at the extremities of the spore, seem to give rise to a sac-like structure containing within its interior a spirally rolled fila- ment which is emitted when the spore is subjected to press- ure and probably serves for the fixation of the spore to the body of a host. The further history of the spores is not thoroughly known, but it seems probable that the contents escape as amceboid masses which develop into adult Myxo- sporidia. In many respects the Myxosporidia resemble closely the Gregarinida, but the possibility of their being in reality not of an animal but of a plant nature must not be overlooked. By some authors their nearest re- lations have been found in the Myxomycetous and Chytridiaceous fungi, a view which certainly has not a little to recommend it. 3. Order Sarcosporidia. The Sarcosporidia are, with a single exception. parasites in the muscle-tissue of warm-blooded animals, especially of Mammalia, being found in the interior of the primitive fibrils of the striated muscles, whose contents they more or less destroy. They form somewhat elongated sacs 1-2 mm. in length, the wall of the sac being formed of a distinct membrane which has the appearance of being covered with fine bristles. The contents of the sac consist of a protoplasmic ground- 28 INVERTEBRATE MORPHOLOGY. substance in which a large number of nuclei are imbedded, sometimes aggregated into masses each of which is sur- rounded by a delicate membrane. It seems probable that these masses represent a process of spore-formation, but as yet nothing is known regarding the further development of the spores. III. Crass FLAGELLATA. The Flagellates are characterized by the possession of one or more long filamentous processes of protoplasm, known as flagella, which, by whip-like movements, propel the organ- isms through the water in which they live, and at the same time by the production of currents in the water bring food- particles within their reach. Some forms possess pseudo- podia in addition to the flagella, which are indeed simply at- tenuated and mobile pseudopodia, but the majority have a more or less permanent body-form. This in many species is accompanied by the formation at the exterior of the body of a skin or cuticle which in some cases, as in the Dino- flagellata, may assume a sufficient density and thickness to entitle it to be termed a shell. - 1. Order Autoflagellata. In the Autoflagellata the body is usually more or less oval, and while in many forms it is naked and capable of changing form (Fig. 13, A), yet in others special cuticular in- vestments may be present, taking the form in some cases of a simple cuticular covering, as in Huglena (Fig. 13, B), in others forming a stalk by which the organism is attached to a foreign body ; in some forms, as in Codosiga (Fig. 13, C), a cuticular collar surrounding the base of the flagellum is present, while in others, such as Dinobryon, a cup is formed, within which the organism lives. Usually but one or two whip-like flagella are present, though oceasionally a larger number (6 or 8) may occur, and in some instances one or more may assume a firmer character and serve for fixation of the organism. All forms possess a single nucleus and a contractile vacuole. In the simpler SUBKINGDOM PROTOZOA. 29 forms, such as Monas,in which no cuticle is developed, no special mouth-orifice is present, though the in- , gestion of food takes place at a more or less definitely localized region at the base of the flagellum, the food-particles drawn to the | organism by the currents established by the flagellum usually impinging at this point; | where, however, a definite cuticle or shell is | developed a definite mouth occurs, and in some cases, as Luglena (Fig. 13, B), this leads into a distinct tubular pharynx projecting some distance into the interior. No hollow digestive tract is, however, present, but the food-parti- cles, after traversing the gullet, are received directly into the protoplasm of the body, and are digested there as in Ame@ba. A localized 4 oy nonas egestive region, situated usually towards the @fter pérscata), posterior end of the body, has been ascer- B fugienaacus. tained to occur in some species, but in no G, Codosiga instance is it a permanent orifice, as is the ‘fter Burscau). case with the mouth. In addition to the nucleus, contractile vacuole, and food-particles, other definitely organized particles, such as starch-like granules and pigment-granules, may be imbedded in the protoplasm. In Luglena the pigment is green and resembles plant-chlorophyll, probably too possess- ing a similar function. A red pigment-spot (stigma) is also present in this and other genera at the base of the flagellum and is supposed to be concerned in light-perception. The typical Flagellate is a free-swimming single organism, but many forms are fixed, developing a stalk by which they are fastened to foreign bodies ; the stalk may be very much branched, each terminal branch supporting an individual, the whole thus forming a colony, without, however, any differ- entiation among the individuals. Free-swimming colonies also exist, such for example as Volvox, in which a large number of individuals are grouped together to form a spheri- cal hollow colony. Each individual contains chlorophyll- granules and a red stigma, and is provided with two fla- gella by the action of which the entire colony is propelled Fie. 18. 30 INVERTEBRATE MORPHOLOG Y. through the water with a rotatory motion. The rotation is around a definite axis, one portion of the spherical colony always being in front in progression, and it is noticeable that the stigmata of the individuals of this anterior hemisphere are slightly larger than those of the cells of the posterior hemisphere, a slight differentiation of the individuals being thus present. 2. Order Dinoflagellata. : The Dinoflagellata are distinguished from the members of the preceding order by the almost general occurrence of a rather dense shell composed of plates of a substance resem- bling closely vegetable cellulose. Some of the forms, such as Ceratium (Fig. 14), present a rather bizarre shape on account of the shell being prolonged into horns, and in the majority the shell-plates are delicately sculptured, while around the equator of the shell runs a furrow, and from an opening in the line of the furrow two flagella protrude, one of which possesses the ordinary whip-like char- acter, while the other lies in the fur- row and in some cases has the form Fie. 14.—Ceratium tripos of a delicate undulating band. Chlo- oe of two figures yonhyll-like pigment is almost invari- ably present, as is also the red stigma. Peculiar cysts are also present in the protoplasm of many forms, consisting of a hollow capsule having rolled up within it a hollow thread, which on occasion may be rapidly evagi- nated and no doubt has a protective function, resembling very closely in its structure the nematocysts of the Coelenterates. 3. Order Cystoflagellata. The order of the Cystoflagellata includes only two genera, Noctiluca and Leptodiscus. The latter is a somewhat disk-like structure nearly 2 mm. in diameter, while Voctiluca (Fig. 15) is almost globular with a slight depression at one point where SUBKINGDOM PROTOZOA. 31 the flagella are situated, and at the botton of which is situated the mouth-opening. Noctiluca has the form of a cyst, pos- sessing an external thin membrane- like outer wall, to which branching strands of protoplasm extend from the central mass containing the nucleus and lying slightly below the depression which contains the fla- gella. These are two in number, one being short and whip-like, while the other, usually known as the 1 ore “tentacle” (Fig. 15, t), is a highly Le contractile, somewhat flattened, and, F!* al mations relatively to the flagellum, thick pro- ,_ ta a ae cess of the internal protoplasm. This structure is unrepresented in Leptodiscus, which other- wise closely resembles Noctiluca. Noctiluca is of considerable physiological interest, since it is one of the forms to which the phosphorescence of the ocean is due. The cause of the light and its character are, however, as yet unknown. Reproduction in the Flagellata.—The most frequent method of reproduction in all the orders of the Flagellates is simple division, either transverse or longitudinal. Encystment, fol- lowed or not as the case may be by spore-formation, is also common, and when accompanied by spore-formation may be preceded by the conjugation and fusion of two individuals. In Cercomonas the spores are exceedingly abundant and small, presenting the appearance of minute granules even under the highest powers of the microscope, but in other forms, as Chlamydomonas, the spores are larger and much fewer in num- ber, being only 4 or 8 in this particular case. An interesting modification occurs in closely-related species (Fig. 16), some individuals of which divide into a number of small spores (microspores), while others undergo a more restricted division and give rise to a few large spores (macrospores). The latter develop directly into the adult forms, but the microspores show a tendency to conjugate in pairs before undergoing further development. This differentiation of two kinds of spores is carried still farther in other forms where neither 32 INVERTEBRATE MORPHOLOGY. macro- nor microspores develop directly but further develop- ment is contingent upon the conjugation of a micro- with a macrospore. In this respect considerable interest attaches to Volvox ; certain cells, usually those situated in the posterior hemi- sphere, enlarge and project into the interior cavity, dividing RC ooR eT Fie. 16.—1. Phacotus lenticularis; 2. MacrRosPpoRES AND MICROSPORES OF THE SAME SPECIES (after BurscH.t). sch = shell. n = nucleus. when they have reached their full growth into a number of cells which arrange themselves in a hollow sphere forming daughter colonies in the interior of the parent. In addition to this a sexual process occurs ushered in by certain indi- viduals gradually enlarging, and leaving their position at the surface of the colony. In the interior some of them continue to enlarge, forming ova (macrospores), while others divide frequently, forming packets of elongated cells furnished with flagella; these may be termed spermatozoa (microspores). The ova develop into colonies similar to the parent after conjuga- tion with spermatozoa. Since many of the cells of the parent colony do not participate in this reproductive act, but disin- tegrate and die on the development of the daughter colonies, it is clear that we have in this form a rather marked differ- entiation of the individuals of the colony, the individualities of the constituent cells being to a slight extent merged in the individuality of the colony. In Noctiluca in addition to simple division a process of reproduction occurs which partakes of the character of budding. It is apparently pre. SUBKINGDOM PROTOZOA. 33 ceded by the conjugation of two individuals, the combined central proto- plasms coming to the surface of the cyst where they form a protuberance. Repeated division of the nucleus into 2, 4, 8, etc., up to 256 or more now takes place accompanied by only a partial division of the protoplasm, so that the surface of the protuberance is covered by a large number of bud- like structures. Eventually these separate, develop a flagellum, and take on the character of motile spores. Their further development into the adult Woctiluca has, however, not yet been followed. IV. Crass INFUSORIA. The Infusoria are the most highly specialized of all the Protozoa, showing a differentiation of the protoplasm unat- tained by other members of the group. They are character- ized by the possession during the whole or part of their lives of numerous delicate short motile hair-like processes termed cilia by means of which locomotion is performed and food procured. In one of the orders into which the class may be divided, the Ciuiata, these structures are present during the adult life of the organisms, while in the other, the Sucrortia, though present in the young stages they are replaced later by immovable processes of the body, which extract the nourish- ment from the food-particles which come into contact with them. 1. Order Ciliata. The Ciliata are for the most part free-swimming organ- isms, though some, e.g. Vortivella (Fig. 17, C), adhere to foreign bodies by means of a stalk, similar to that found in Flagel- lates, and colonial stalked forms also occur as in that class. In these stalked forms the body is enveloped in a chitinous case, of which the stalk is a prolongation, the surface oppo- site the stalk being, however, left naked and being surrounded by cilia which are absent on the portions of the body pro- tected by the chitin (Peritrichous forms, Fig. 17, C). In the free-swimming forms, however, the cilia are more universally distributed, covering either the entire surface (Holotrichous forms, Fig. 17, A) or else one surface of the flattened body, some of them in this case being modified into stout movable 34 INVERTEBRATE MORPHOLOGY. bristles upon which the animal creeps (Hypotrichous forms, Fig. 17, D). A definitely localized mouth-opening is always present, situated frequently at the extremity of a peristomial groove and leading into a gullet of variable extent, usually lined by cilia, though sometimes furnished with a chitinous support Fig. 17.—A, Paramecium ; B, Stentor ; C, Vorticella ; D, Euplotes. cov = contractile vacuole. n = nucleus. m = mouth. n' = micronucleus. my = myophane. tr = trichocyst. (Chilodon). There is, however, no special digestive tract, the food-particles after traversing the gullet being received into the body-protoplasm, where they are digested. Usually there is a localized cgestive region, and in a few cases there is a definite anal opening. The food is procured as in the Flag- ellates by the currents set up in the water by the cilia carry- SUBKINGDOM PROTOZOA. 35 ing minute organisms to the neighborhood of the mouth, the cilia surrounding this opening directing them to the gullet. The body-protoplasm is usually very granular in its cen- tral part, and filled with food-vacuoles and products of diges- tion. Pigment-granules are sometimes present and may con- sist of Chlorophyll, as in Stentor, and one or more excretory contractile vacuoles are always present. The nucleus is usu- ally single, though occasionally two are present, and in the genus Opalina, which occurs in the intestine of the Frog, they are numerous in the adult condition. When single the nu- cleus may be very large and either spherical, elongated, horseshoe-shaped as in Vorticella (Fig. 17, c), moniliform as in Stentor (Fig. 17, B), or otherwise shaped. In addition to the nucleus there are one or two minute structures usually to be found in its vicinity which play an important part in repro- duction and are known as micronuclei (Fig. 17, A, 2‘). Other differentiations of the protoplasm are also found in certain forms, as, for instance, special bands differentiated so as to be specially contractile and therefore corresponding in function to the muscles of the higher animals, and hence termed myo- phanes. In Vorticella a more striking differentiation of spe- cially contractile protoplasm occurs (Fig. 17, C, my); running in an open spiral through the centre of the supporting stalk of this organism is a strong myophane terminating above in the protoplasm of the animal. Whien the latter is stimulated the myophane contracts, coiling the stalk into a close spiral and withdrawing the animal from the source of irritation. In some of the Holotricha, such as Paramecium, numerous mi- nute rod-like structures occur imbedded in the protoplasm near the surface of the body (Fig. 17, A, tr). They are appar- ently defensive in function, since when stimulated they sud- denly, as if by an explosive action, become transformed into long threads or needle-like structures projecting beyond the cilia. These trichocysts also occur in some Flagellates. 2. Order Suctoria. The Suctoria lack the active movements of the Ciliata, being destitute in the adult stage of cilia, and many of the 36 INVERTEBRATE MORPHOLOGY. forms, e.g. Acineta (Fig. 18), are attached to foreign bodies by a stalk. They do not possess any mouth, but a number of simple or branched stiff processes project from the body which serve for the prehension and digestion of the organisms upon which they feed. A contractile vacuole, nucleus, and micronucleus are always present, the nucleus having sometimes a very complicated shape. It seems pretty clear that they have been de- rived from the Ciliata, since in their young stages they are free-swimming ciliated struc- tures; the tentacular processes have been compared to the pseudopodia of the Rhizo- pods, but good reasons for such an homology do not exist, and it is more probable that they are structures peculiar to the group. The Reproduction of the Infusoria.—In the Fie. 18.—Acineta Trtysoria the reproductive processes reach a grandis (after é ‘ : SaviteKenr), much higher grade of complication than occurs in other Protozoa, though the simple pro- cesses of fission and spore-formation likewise occur. The former occurs in the majority of forms, and may be the only mode of reproduction occurring throughout a number of gen- erations. Long-continued fission seems, however, to lead in many cases to structural and physiological derangements, unless the process of conjugation be interposed. Encystment is also of frequent occurrence and may occur under various conditions. In Colpoda, in which the process has been most thoroughly studied, encystment may or may not be followed by reproduction. In the latter case the cyst, a resting cyst, is perfectly closed, and the walls are thick and resistent so as to withstand unfavorable conditions, such as insufficient aeration or dryness. When reproduction is asso- ciated with encystment it may be either fission or spore-forma- tion. The division cyst is thin-walled and is not completely closed, and within it the animal undergoes division into two or four parts. In spore-formation a thin cyst is first formed, within which the animal slowly rotates, at the same time gradually growing smaller by the expulsion of fluid. Finally SUBKINGDOM PROTOZOA. 37 it contracts to a round mass and surrounds itself with a sec- ond cyst within the first. At the surface of the encysted animal from eight to thirty minute spherical and highly re- fractive bodies appear which are the spores, and by the bursting of the cyst they, with the remains of the protoplasm in which they arose, escape to the exterior and soon begin to develop. Losing its spherical shape each spore becomes amoeboid; then, drawing in all the pseudopodia but one, which elongates and becomes a flagellum, it passes from the Rhizopod to the Flagellate stage; and finally the flagellum is withdrawn, cilia appear, and the animal gradually assumes _ the adult form. Spore-development somewhat similar to this has been observed also in Vorticella, and special interest at- taches to it as probably indicating the line of descent of the Infusoria. Conjugation is a frequent process among the Infusoria, where it seems to have a rejuvenating rather than a strictly reproductive function. If prevented, and fission goes on through a number of generations, marked degeneration en- sues; while if it be allowed, the same number of generations may be produced without any signs of degeneration. The process consists of a renewal of the nuclei and micronuclei of the conjugating forms, and the process as it occurs in Colpid- ium colpoda may be described thus. Two individuals come Into contact by the anterior portions of their body, actual fusion of the two protoplasms taking place at the point of contact. The micronucleus in each individual then enlarges and divides, the two thus formed subsequently dividing again, so that each of the conjugating individuals contains four micronuclei and one nucleus. One of the four micronuclei in each individual now divides, and one of the two thus tormed (the male pronucleus) crosses over to the other indi- vidual and unites with the other product of the division, the female pronucleus, there being thus a mutual interchange of micronuclei. The individuals now separate and resume their independent existences, and a rearrangement of the nuclear structures accompanied by fission takes place. The three micronuclei which did not take part in the formation of the pronuclei of conjugation degenerate, as does also the original 38 INVERTEBRATE MORPHOLOGY. nucleus. The conjugation micronucleus, formed by the fusion of the male and female pronuclei, divides twice, forming four micronuclei, and this is followed by a fission of the entire Infusorian, each of the daughter forms so produced possessing two micronuclei. One of these, enlarging, becomes the new nucleus, while the other remains as the micronucleus. This complicated process may perhaps be better followed in the accompanying diagram (Fig. 19). O O Fig. 19.—Dragram To ILLUSTRATE THE BEHAVIOR OF THE NUCLEI AND MICRONUCLEI DURING CONJUGATION IN INFUSORIA (after Maupas). In the majority of forms the conjugation is a temporary process, the two individuals separating after the exchange of pronuclei. In Vorticella, however, a permanent fusion occurs. By repeated longitudinal fission a Vorticella becomes divided: into a number of small individuals which leave their stalks and swim about freely in the water. Should one of them come into contact with a large individual a complete and permanent fusion of the small with the large one occurs. SUBKINGDOM PROTOZOA. I. Class Ru1zopopa.—Protozoa with lobe-like or filamentous pseudopodia. 1. Order Foraminifera.—Pseudopodia without axial support; shell when present horny or calcareous. (a) Shell absent. Amba. (6) Shell horny. Arcella, Euglypha. (c) Shell of foreign particles cemented together. Difflugia. (d) Shell calcareous, imperforate. Groméia. (e) Shell calcareous, perforate. Jfiliola, Nodosaria, Textu- laria, Globigerina, Rotula, Acervularia. SUBKINGDOM PROTOZOA. 39 2. Order Heliozoa.—Pseudopodia slender, with axial support; shell if present siliceous ; no central capsule. (a) Shell wanting. Actinophrys, Actinospherium, Vampy- rella, Microgromia. (6) Shell present. Clathrulina. 8. Order Radiolaria.—Pseudopodia slender with axial support; shell usually present and siliceous (rarely horny); central capsule present. (a) Shell wanting. Thalassicolia, Spherozoon. (6) Shell siliceous. Actinomma, Heliosphera. (c) Shell horny. Acanthometra. II. Class SPorozoa.—Parasitic ; without pseudopodia, flagella or cilia. 1. Order Gregurinida.—Parasitic in cavities of the body especially of Invertebrates or in the cells especially of Vertebrates. 2. Order Myxosporidia.—Parasitic usually in the skin, sometimes in internal organs of fishes. 8. Order Sarcosporidia.—Parasitic in the muscle-fibres of Mammalia. TII. Class FLAGELLATA. Provided with one or more flagella. 1. Order Autoflagellata.—Without shell, protoplasm not especially vacuolated. (a) Without collar.—Monas, Cercomonas, Chlamydomonas, Huglena, Volvoz. (0) With collar.—Codosiga, Dinobryon. 2. Order Dinoftagellata.—With shell composed of cellulose. Cera- tium. 3. Order Cystoflagellata.— Without shell, protoplasm highly vacuo- lated, marine. Noctiluca, Leptodiscus. TV. Class Inrusor1a.—Provided with cilia or immovable processes. 1. Order Ciliata.—Provided with cilia in adult stage. (a) Cilia of nearly uniform length all over the body (Holo- tricha). Paramecium, Colpoda, Colpidium, Chilodon, Opalina. (6) Cilia around anterior end of body longer than the rest (Heterotricha). Stentor. (©) Cilia limited to anterior end of body (Peritricha). Vorti- cella. (d) Cilia or sete only on ventral surface of the body (Hypo- tricha). Stylonychia. ; 2. Order Suctoria.—With cilia only in the young stages, in the adult with immovable processes. Podophrya, Acineta. LITERATURE. 0. Biitschli. Protozoa. Bronn’s Klassen u. Ordnungen des Thierreichs. Leip- ; zig u. Heidelberg, 1883-87. W.S. Kent. A Manual of the Infusoria. London, 1880-82. 40 INVERTEBRATE MORPHOLOGY. bm Leidy. Fresh-water Rhizopods of North America. U. 8. Geological Sur- vey of the Territories, x11. 1879. . Hertwig. Bemerkungen zur Organisation und systematischen Stellung der Foraminiferen. Jenaische Zeitschr., x. 1876. . Haeckel. Die Radiolurien. Hine Monographie. Berlin, 1862-88. . Brandt. Die Holonie-bildenden Radiolarien (Spharozoen) des Golfes von Neapel. Fauna u. Flora des Golfes von Neapel. Monographie, XIII. 1885. . Stein. Der Organismus der Infusionsthiere. Leipzig, 1859-79. . Maupas. La rajeunissement karyogamique chez les Ciliés. Archives de Zool. expérimentale, 2™¢ Sér. vir. 1889. . Rhumbler. Die verschiedenen Cystendildungen und die Hntwicklungsge- schichte der holotrichen Infusoriengattung Colpoda. Zeitschr. fiir wis- sensch, Zoologie, XLVI. 1888. . Hertwig. Ueber Podophrya gemmipara, etc. Morpholog. Jahrbuch, 1. 1876. SUBKINGDOM MET'AZOA. 41 CHAPTER III. SUBKINGDOM METAZOA. Tue Metazoa are equivalent to colonies of Protozoa, the individual cells of which have differentiated in various direc- tions, some being more especially contractile, others nutritive, others irritable, others reproductive, ete., instead of each one for itself performing equally all the functions necessary for existence. A physiological division of labor of a more or less perfect kind is introduced among the individuals composing the colony, and the welfare of each individual becomes de- pendent upon the proper performance by its colleagues of their special functions; in short, the individualities of the component cells are merged in the higher individuality of the whole organism. Physiologically a Metazoon is equivalent to a Protozoon, but morphologically it is the equivalent of a large number of them. Each is physiologically an individual, but morpholog- ically the Metazoon is a colony of Protozoan individuals. To harmonize the physiological and morphological conceptions of an individual it is necessary to recognize several grades of morphological individuality of which the cell may be as- sumed to be the lowest. In the Metazoa the physiological differentiations of the .cell-individuals are accompanied by structural differentiations, so that it is possible, as a rule, to determine from its structure what the function of a cell may be ; aggregates of similar cells are termed tissues or tissue-indi- viduals, and as the simplest Metazoa are complexes of various tissues, such a complex forms the third grade of individu- ality and may be termed an Organ-individual. A complex of organ-individuals united to form a physiological unit consti- tutes an individual of the third grade, the Metamere-individual, while the fourth grade, the Cormus, is formed by a similar union of a number of metameres, as, for instance, in the Earthworm, each joint or segment of which is a metamere. 42 INVERTEBRATH MORPHOLOOY. It has been pointed out that the Flagellate Volvox presents a tendency towards a higher individuality, being somewhat higher than a mere colony of cell-individuals and yet not ~ quite reaching the dignity of an organ-individual; similarly intermediate conditions between the other grades may occur. In certain worms, for instance, considerable independence of the constituent metameres exists, any one of them, when de- tached, being capable of carrying on an independent exist- ence, and of developing into an organism similar to that of which it was originally a part. In the Earthworm the depend- ence of the various segments or metameres upon one another is greater than this, but in it, too, a certain amount of inde- pendence is shown by the power it possesses of regenerating lost metameres. In other cormi, as, for instance, in the Lobster, the interdependence of the component metameres proceeds still farther, and a differentiation of the various meta- meres occurs, a process carried to its greatest extent in the higher Vertebrates. A physiologica] division of labor among the metameres develops, some of them losing, for instance, their excretory organs, while in others these organs lose their excretory functions and serve as ducts by which the repro- ductive elements may pass to the exterior. The subordina- tion of the metameres proceeds most rapidly and is most complete at the anterior extremity of the organism, leading to the formation of a head bearing highly developed sense- organs and containing a complex nervous system, which rep- resents originally distinct metamere nervous systems, now fused and destitute of all independence. Sexual Reproduction in the Metazoa.—In cell-individuals it has been seen that fission is the most frequent and simplest mode of reproduction; in the Metazoa this method and its modification, budding, also occurs, but, as a rule, only in forms of a low grade of individuality or in a transition stage between a lower and a higher grade. In organ-individuals it is of frequent occurrence, the imperfect separation of the individuals so produced leading, in many cases, to the forma-— tion of colonies, and in cormi in which the integration of the constituent metameres is but slight it also occurs. In the Protozoa cell-division naturally entails reproduc- SUBKINGDOM METAZOA. 43 tion, but in organ-individuals reproduction of the constitu- ent cell-individuals is not necessarily connected with the reproduction of the entire individual, but may simply increase the number of lower-grade individuals of which it is com- posed. Similarly multiplication of the organ-individuals of a metamere, or of the metamere-individuals of a cormus may occur without producing reproduction of the whole; it is simply growth. From growth to reproduction by budding the path is short, and various intermediate stages connecting the two processes can be found. Hence reproduction has been aptly defined as “discontinuous growth,” though perhaps it would be even more apt to define growth as reproduction with- out discontinuity, growth in a Metazoon depending on the reproduction of the lower-grade individuals of which it is composed. It is possible to carry this idea still farther back and refer the growth of a cell to the reproduction of the constituent elements, plasomes, of which, it may be imagined, it is composed. In the simplest cells the various forms of plasomes are distributed throughout the cell, but in the higher Protozoa, for instance, an aggregation of similar plasomes occurs, giving rise to such structures as the myophanes. In a similar manner in the lower Metazoa, although a division of labor and structural differentiation has taken place among the constituent cells, yet the cells possessing similar functions, as, for instance, the nerve-cells, are more or less irregularly scattered throughout the body, only becoming aggregated. in the higher forms into distinct tissues, and giving rise to the most perfect type of an organ-individual. Likewise in a metamere-individual a multiplication of the organs leads to a transition form with discretely arranged parts, the definite aggregation of which produces a cormus, composed in the simpler forms of distinct metameres, which become more and more integrated and subordinated to the individuality of the cormus in higher types of that grade of individual. According to this view the segmentation or metamerism of the higher Metazoa is the result of the multiplication and subsequent integration of the organ-individuals of an ancestral metamere-individual, and explains the occurrence of imperfect metamerism in certain forms of that grade of individuality (Turbellaria). Some authors have considered metamerism to have arisen by the reproduction by budding of an ancestral metamere, an idea which fails to explain satisfactorily the condition just referred to. The view presented here considers metamerism to be the result of growth. It has not arisen by the reproduction of the metamere, but by that of its organs, just as a typical organ-individual has arisen by the reproduction and integration of its constituent cell-individuals. 44 INVERTEBRATE MORPHOLOGY. As a mode of reproduction in the Metazoa division plays but a secondary part, the sexual process being the character- istic method. Attention has already been called to the par- tial specialization in Volvox of reproductive cells which serve to perpetuate the species, the remaining cells of the colony perishing. This condition is a premonition of the more per- fect specialization found in the Metazoa of reproductive or germ cells and non-reproductive or somatic cells, the latter serving for the nutrition and protection of the germ-cells, to which the perpetuation of the species is entrusted. Comparatively early in the development of an individual certain cells differ- entiate from the others, not undergoing like them a physi ological and structural specialization, but retaining a general- ized character. These are the germ-cells usually grouped together to form the reproductive organs. In describing the methods of reproduction occurring in the Flagellata, the manner of the development of sexual repro- duction was indicated. It appears to have been originally a more or less accidental fusion of two similar cells or spores, and from being accidental this fusion gradually became the rule on account of the greater vitality which the conjugate in- dividual possessed over cells which did not conjugate. The next step was the differentiation of microspores and macro- spores, which reaches a high development in Volvox, where it is associated also with a differentiation into somatic and germ cells. In the Metazoa both these differentiations are carried to a higher degree, the macrospores being known as ova and the microspores as spermatozoa, while the agegre- gates of these cells are termed respectively ovaries and testes. In a young embryo a mass of germ-cells which is to give rise to spermatozoa cannot be distinguished from one which is destined to be converted into ova. Fundamentally both are the same, and occasionally a portion of a mass of germ- cells may be differentiated into ova, while the rest of it devel- ops into spermatozoa. This has not unfrequently been seen in fishes in which there is normally a separation of the sexual elements in distinct individuals, and throws considerable light upon the occurrence of forms which normally possess both elements. This condition of hermaphroditism, which oc- SUBKINGDOM METAZOA. 45 curs in many parasitic forms and in certain sponges, Flat- worms, Mollusks, and Crustacea, seems to have been second- arily acquired. It is probable that the ancestral Metazoa were unisexual, possessing reproductive elements of only one kind, a supposition borne out by the frequent association of hermaphroditism with a parasitic or sessile mode of life, such conditions being what may be termed abnormal, and usually accompanied by marked structural characters which are to be regarded as secondary modifications. On the other haud, it is noticeable that the lowest free Metazoa (such as the free- swimming Cnidaria) are unisexual. An ovum is a single cell, and in its typical form consists of a mass of protoplasm containing a nucleus, and may or may not be surrounded by a membrane. Seldom, however, does such a simple ovum occur; usually more or less yolk, consisting of fatty and albuminous globules, is distrib- uted throughout the protoplasm, and fre- quently the amount of yolk far overbalances the amount of protoplasm. Other structures, such as albumen and one or more enveloping membranes, may be added, the ova of different species differing greatly in this respect. Among the lower forms the ova are usually extruded freely from the body of the parent, but in many of the higher Metazoa they are enclosed within protective cases (cocoons), as in the Earthworm, or imbedded in jelly-like masses, as in the common Pond-snails. In the ovary of a young individual all the Fie.20.—Ovarra. germ-cells are alike, and all are potentially TUBE or 4 Brx- reproductive cells; very frequently, however, ne wae many of the primitive germ-cells relinquish 9 — germinal re- their reproductive function and serve as pur- o=ova. [gion. veyors of nutrition to certain of their com- 9 =mature ovum. rades which enlarge and become mature ova. % =yolk-eells. ae ee : a SF = follicle-cells. This is well seen in insects, in which each ovary (Fig. 20) consists of a number of tubes tapering to a point at one end, while at the other they open into a common duct, 46 INVERTEBRATE MORPHOLOGY. the oviduct, leading to the exterior. At the tip of each tube the primitive germ-cells (Fig. 20, g) are located, and lower down ova (0) in various stages of development towards matu- rity are to be found, each surrounded by a number of small undeveloped germ-cells, known as follicle-cells (f), whose func- tion it is to transfer food-yolk (y) to the growing ovum. As the latter approaches maturity the follicle-cells secrete around it a thick, sometimes highly sculptured shell and finally degenerate. As arule, conjugation with a spermatozoon, ie. fertiliza- tion, is necessary as an antecedent to further development. Before this takes place, however, certain modifications of the ovum are necessary, the phenomena which accompany them being known as the maturation of the ovum. In this process » Fig. 21.—DIaGRAMS8 ILLUSTRATING THE MATURATION OF THE Ovum. A = formation of the first polar globule (pg). B = formation of the second polar globule and entrance of the sperm-nucleus (sp). (Hig. 21, A) the nucleus approaches the surface of the ovum and there undergoes a karyokinetic division which is peculiar in that in the equatorial-plate stage twice as many chromo- somes are formed as are typical for the species. These do not undergo longitudinal division, and by the karyokinesis their number is reduced to the typical number, a small cell, the polar globule (pg), being separated from the ovum with half the chromosomes, while the others are retained within the ovum. The nucleus of the ovum, instead of now returning to the resting stage, divides again (Fig. 21, B), a second polar globule being formed and receiving half the chromosomes SUBKINGDOM METAZOA. 47 which remain, so that the nucleus of the ovum now possesses only half the number of chromosomes which are character- istic for the species. At the time of the formation of the second polar globule the first frequently divides without its nucleus passing into a resting stage, so that as the result of this maturation process four cells have been formed, three of which are small, while the third is relatively very large and will alone undergo further development. When these divi- sions have been completed and the chromosomes have been reduced to one-half their proper number the nucleus of the ovum passes into the resting stage, migrates back towards the centre of the ovum, and is ready for senjuganlgn with the nucleus of a spermatozoon. The spermatozoa are always much smaller than the ova, and are, as a rule, capable of active motion, though in certain Crustacea, for instance, they lack this power. The ova and spermatozoa have specialized in opposite directions in this respect. The ova of the Metazoa are specialized as the nutritive cells of conjugation, possessing abundant protoplasm and usually a considerable amount of yolk for the nutrition of the young embryo. They consequently have lost their motility, and in order that conjugation may be made prob- able the spermatozoa lack all unnecessary material which would interfere with their motility, no yolk being stored up and the protoplasm even being reduced to the smallest amount consistent with the development of a locomotor organ. The nuclei, as will be seen later, are essential ele- ments in conjugation, and the spermatozoa are to all intents locomotor nuclei, the ova supplying the protoplasmic nidus necessary for the growth and division of the nucleus formed — by conjugation. In their typical form spermatozoa are composed of a globular or pyriform head consisting of a nucleus surrounded by a small amount of protoplasm, and a long filamentous tail continuous with the protoplasm and frequently provided with a delicate fringe-like membrane (Fig. 22, F’). By the rapid whipping movements of the tail the organism is propelled through the water, or other fluid in which it may find itself, and so may come into contact with an ovum. 48 INVERTEBRATE MORPHOLOGY. The transformation of the germ-cells present in an em bryo into spermatozoa is usually a somewhat complicated process. In the Round-worm Ascaris, in which it retains somewhat primitive characters, the process closely resem- bles what takes place during the maturation of the ovum. Fig. 22.—DIAGRAMS TO ILLUSTRATE THE MATURATION OF THE SPERM-CELL- A = division of the spermogone. B = division of the two spermocytes. C = the four spermatids. D, # = conversion of a spermatid into a spermatozoon. F = fully developed spermatozoon. The embryonic germ-cells (spermatogones, Fig. 22, A) undergo karyokinetic division, the number of chromosomes being, as in the ovum in the division which results in the formation of the first polar globule, twice that which is characteristic for the species. They do not undergo longitudinal division, and one half of them passes into one of the daughter cells (sper- matocytes) and the other half into the other, so that these two cells possess the number of chromosomes characteristic for the species. - A division of these daughter cells (Fig. 22, B) immediately takes place without a return to the resting stage, and unaccompanied by a longitudinal division of the chromo- somes, so that four cells (spermatids, Fig. 22, C) are formed, each of which contains only half the typical number of chro. mosomes, and each one of these cells becomes a spermato- zoon. This process is comparable step by step with the SUBKINGDOM METAZOA. 49 maturation of the ovum and seems to indicate that the polar globules are to be regarded as abortive ova. The conversion of the spermatids into spermatozoa is simply a differentiation of structures already present. In the air-breathing Mollusca, for instance, the spermatids consist of a mass of cytoplasm containing a nucleus, in close proxim- ity to which may be found the centrosome, while an irregular mass of filaments represents the remains of the spindle-fila- ments. In the differentiation which follows (Fig. 22, D, E, and #’) the nucleus elongates and its chromatin-filaments fuse to form a homogeneous mass; the cytoplasm likewise elongates, and in it appears an axial filament which later will form the tail-filament. The origin of this filament is doubt- ful, some authors maintaining that it is a differentiation of the cytoplasm, while others believe it to be a prolongation of the nuclear substance; but, however that may be, the spiral fringe which surrounds the axial filament is certainly the remains of the cytoplasm of the spermatid. The remains of the spindle-filaments disappear, while the centrosome prob- ably persists as a structure lying behind the head and termed the “ Mittelstiick.” In some cases, as the insect Pyrrhocoris and the crustacean Diaptomus, the doubling of the chromosomes previous to division into spermatocytes does not take place. In Pyrrhocoris twenty-four chromosomes are typi- cally present and twelve of these pass into each of the spermatocytes, and in the division of these to form the spermatids each of the twelve chromo- somes divides so that each spermatid possesses half the typical number. In Diaptomus the same result is brought about somewhat differently. The spermatogones possess eight chromosomes which assume a dumbbell shape and divide transversely, so that each spermatocyte has the typical number of chromosomes; the spermatocytes divide without passing through a resting stage, and each spermatid thus contains four chromo- somes, i.e. half the typical number. Fertilization of the Ovum.—So soon as the formation of the polar globules has been completed, the nucleus of the ovum migrates towards the centre of the protoplasm and is the female pronucleus (Fig. 23, fp) of conjugation. The penetra- tion of the spermatozoon may occur at any portion of the sur- face of the ovum and may take place before, during (Fig. 21, B, sp), or after the formation of the polar globules, a single 50 INVERTEBRATE MORPHOLOGY. spermatozoon, as a rule, in healthy ova, penetrating and tak- ing part in the conjugation, though apparently in some cases polyspermy, or the penetration of several spermatozoa, may occur. The head of the spermatozoon comes into contact with the protoplasm of the ovum, which in some cases rises up to meet it, and is rapidly engulfed. The tail likewise of Fic. 23.—Dr1aGRAMs TO ILLUSTRATE THE PHENOMENA OF FERTILIZATION. A, the approximation of the male and female nuclei. B, division of the centrosomes. C, rotation of the centrosomes. D, fusion of the centrosomes and nuclei, and formation of the segmentation spindle. ce = compound centrosome. oc = ovum centrosome. Jp = female nucleus. s¢ = sperm centrosome. mp = male nucleus. sn = segmentation nucleus. the spermatozoon is taken into the ovum and seems to be com- pletely absorbed, the head alone being visible in later stages ; it constitutes the male pronucleus (Fig. 23, mp) and moves towards the centre of the egg until it comes into contact with the female pronucleus, without, however, fusing with it. A spindle now makes its appearance, and the two pronuclei pass through the various karyokinetic stages, forming equatorial plates each with half the typical number of chromosomes, SUBKINGDOM METAZOA. ~ 51 which divide longitudinally in the usual manner, one half the chromosomes of each nucleus passing towards one of the cen- trosomes. The ovum then divides into two cells and the compound nucleus of each passes into the resting stage, the chromosomes now uniting to form a single chromatic net- work, It will be seen from this that the conjugation or fertiliza- tion process consists of the union of two distinct nuclei, whose complete fusion does not necessarily occur until after the first division or segmentation of the ovum. A conjugation of centrosomes to form those of the first segmentation-spindle also occurs. A centrosome accompanies each of the conjugating nuclei (Fig. 23, A), and before the formation of the spindle each divides into two (Fig. 23, B, oc and sc), which conjugate in pairs (Fig. 23, Cand D), forming the centrosomes of the spindle, each of which thus contains elements of both the original centrosomes. Furthermore, in some cases at least, it is possible to dis- tinguish the nuclear elements derived from the male and female pronuclei respectively in stages later than the first segmentation, owing to a slightly different behavior to certain staining reagents which characterizes them. The pronuclei undergo a morphological fusion during the first cleavage of the ovum, but a physiological differentiation persists. Segmentation and Eurly Development of the Ovum.—The development of the ovum into the embryo consists in its divi- gion into a number of cells, which gradually undergo a phy- siological and morphological differentiation resulting in the formation of tissues, organs, ete. These divisions constitute the segmentation of the ovum. The first division has already been described ; it bears a definite relation to the formation of the polar globules, the plane of the division passing through the point at which they were separated from the ovum. Considering this point to represent one pole of the ovum, the first division is meridio- nal, and the second division likewise, though its plane is at right angles to that of the first division (Fig. 24, A). The third division is, on the other hand, equatorial, its plane cutting the planes of previous divisions at right angles (Fig. 24, B). 52 INVERTEBRATE MORPHOLOGY. Eight segmentation-cells are thus formed which remain in contact with each other and enclose a small cavity, the seg- mentation-cavity or blastocel. The further division of the cells (Fig. 24, C) results in the formation of an oval or spheri- cal organism (Fig. 24, Y”) which may be compared to Volvoz, consisting of a single layer of cells enclosing a more or less voluminous blastocel. ‘lhis embryonic stage is known as the blastula. In its simplest form it shows no special differ- entiation into tissues, its cells being uniformly ciliated, and Fig. 24 —DIAGRAMS ILLUSTRATING THE SEGMENTATION OF THE OVUM. A, four-celled stage. B, eight-celled stage of a telolecithal ovum. C, sixteen-celled stage. D, biastula. The arrows indicate the mode of division. the organism free-swimming, moving through the water with a rotatory movement about a definite axis, one and the same end of whichis always anterior. In many blastulas, however, especially in those which for one reason or another are not free-swimming, an early differentiation of the cells takes place, especially at the extremity which is posterior in the free- swimming forms or which corresponds to that pole in the non-motile embryos. These posterior cells are usually some- what larger than those at the anterior pole, and if much food- yolk is present in the embryo it is especially concentrated in SUBKINGDOM METAZOA, — 5B these cells, which in the later development will assume the vegetative functions of the organism. In many ova the processes just described are modified to a greater or less extent, but from the frequency of their occurrence they must be re- garded as fundamental and the modifications as secondary. Ova which contain but little yolk usually follow more or less closely the typical processes, but where the yolk is abundant, being an inert sub- stance, it acts as a drag upon the protoplasmic activity and produces modi- fication of the segmentation-processes. Two methods of arrangement of the yolk may be recognized: (a) it may be aggregated more or less com- pletely at one pole of the ovum, such ova being termed ¢elolecithal, or (b) it may be distributed in the meshes of a protoplasmic network, a small quantity of yolkless protoplasm being concentrated around the nucleus of the ovum, while another portion of it forms a thin peripheral layer sur- rounding the yolk, this arrangement being termed centrolecithal. In telolecithal ova the third segmentation-division results in the forma- tion of four cells containing very little yolk at one pole of the ovum, while nearly all the yolk is concentrated in the four cells at the other pole (Fig. 24, B). This arrangement, which occurs in many Mollusca, constitutes what is termed a total irregular segmentation, in which, owing to the large size of the yolk-containing vegetative cells, the blastoccel is usually com- paratively small. Inthe Squids the amount of yolk present at the vegetative pole is very great and the protoplasm of the ovum collects upon its surface, there undergoing division and producing a plate of cells, the blastoderm, which by further division gradually extends and finally encloses the inert yolk. This partial segmentation is the result of the presence of a very large quantity of yolk and its telolecithal arrangement, and necessarily obscures greatly the blastula stage. In centrolecithal ova which occur in Crustacea and Insects, the division of the nucleus is accompanied by a division of the central yolkless proto- plasm only, the yolk-containing reticulum and the peripheral layer not tak- ing part in the process. As the divisions continue the nuclei gradually approach the surface and finally come to lie in the peripheral protoplasm, which then takes part in the division, a greater or less portion of the inert undivided yolk occupying the blastoccel of the resulting blastula. Many intermediate gradations occur between such a typical centrolecithal and a total regular segmentation, from which both the centrolecithal and telo- lecithal methods are to be derived. The blastula is a single layer of cells surrounding a large blastoccel in typical cases, and is a stage quickly passed over in the Metazoa. Itis succeeded by a stage in which the em- bryo consists of a double-walled sac open at one end, the gas- trula (Fig. 25). This is most frequently produced from the 54 INVERTEBRATE MORPHOLOGY. blastula by the pushing in or invagination of the cells of one pole (the posterior in free-swimming blastulas) into the blas- toccel, which thus becomes more or less perfectly obliterated. The cavity lined by the invagi- nated cells is the primitive di- gestive tract or archenteron, its opening to the exterior being the gastrula-mouth or blastopore. The gastrula is a two-layered organism or is diploblastic, and the cell-layers of which it is composed are the primitive germ- layers. The outer layer in the higher Metazoa gives rise to the integument, nervous system, and sense-organs of the adult and is known as the ectoderm, while the inner one, from which the digestive tract and its glands, such as the liver, will develop, is termed the endoderm. Just as the presence of yolk in the ovum may modify the segmentation, so too it may produce decided modifications in the formation of the gastrula. The method just described, which occurs in embryos containing little food-yolk, is distin- guished as embolic from the epibolic method occurring in telo- lecithal ova which undergo a markedly irregular segmentation. In such ova, as has been stated, one pole is occupied by inert yolk-laden spherules, while at the other are almost yolkless active cells. These latter divide rapidly and extend as a cap over the yolk-laden cells and finally completely enclose them. The result is practically the same as in the embolic method, the yolk-laden endoderm cells being enclosed within the yolk- less ectoderm. Among the lower Metazoa especially, another method oc- curs by which the diploblastic embryo is formed. Instead of certain cells invaginating, each cell of the blastula divides in a plane parallel to the surface of the organism, one of the two cells thus produced becoming ectoderm, while the other is a portion of the endoderm. A diploblastic closed sac thus re- sults, the blastopore appearing later and placing the archen- Fig. 25.—DIAGRAM OF A GASTRULA. SUBKINGDOM METAZOA. ° 55 teron, which in this case is identical with the blastoccel, in communication with the exterior. This process is known as delamination (Fig. 26, ). A third method also exists, occurring like delamination in its most typical form among the lower Metazoa. This is the immigration method (Fig. 26, B), certain cells of the blastula leaving their position at the surface and passing into the blastoccel. Here they undergo division, and, by the addition of other cells by immigration, the blastoccel gradually be- comes filled up and a solid organism, consisting of an exter- nal layer of cells surrounding a central more or less solid Fig. 26.—DiaGram ILLUSTRATING THE FORMATION OF THE DIPLOBLAS8TIC SvraGcEe (A) BY DELAMINATION, (B) BY IMMIGRATION. mass, results. This is known as the parenchymella or sterrula. Later a cavity appears in the centre of the solid mass, whose cells gradually are pushed towards the periphery, where they form eventually a single layer, the endoderm. Finally a blas- topore is formed and the embryo becomes a gastrula. It does not seem easy to bring the delamination and invagination methods of gastrulation into direct relation with each other, or to derive one from the other, but it is probable that both must be referred back to the immigration method. In typical cases of immigration the cells which migrate are situated irregularly at any part of the blastula, but frequently, especially in free-swimming blastulas, the migrating cells are all located at. the posterior extremity. If in such cases of polar immigration the migrat- ing cells were to pass into the blastoccel en masse instead of individually, invagination would result. On the other hand, if a considerable amount of yolk were present in all the cells of a blastula, it might happen that, in- stead of migrating, the cell might undergo division, cutting off the yolk- containing protoplasm from the yolkless, delamination thus taking place. 56 INVERTEBRATE MORPHOLOGY. The fact that in some cases both immigration and delamination may occur simultaneously, leading to the formation of a sterrula, bears out the idea that the latter process has arisen from the former. Furthermore, it may be pointed out that the occurrence of immigration in such colonial Flagellates as Volvoaw indicates the primitive character of immigration in the Metazoan blastulas, as well as the manner in which diploblastic organisms have arisen from the more primitive single-layered organisms. It is only in the lowest Metazoa, however, that the adult organism is diploblastic. In all others a triploblastice (Fig. Fie. 27.—DIAGRAMMATIC TRANSVERSE SECTION OF AN EARTHWORM TO SHOW THE TRIPLOBLASTIC CONDITION. bm = basement membrane. en = endoderm. C = celom. sm = somatic mesoderm. dm =: dorsal mesentery. spm = splanchnic mesoderm. ec = ectoderm. vm = ventral mesentery. 27) condition supervenes during embryonic life, by the devel- opment of a third layer, primitively separated from the endo- derm, and occupying the space which may remain between the two primitive layers. This is the secondary germ-layer or mesoderm. From it there arise the muscular, excretory, circulatory, and reproductive systems in the triploblastic ani- mals, the first and last of these being derived in diploblastic forms from either one or both of the primary layers, while the excretory and circulatory systems are not differentiated. The manner of formation of the mesoderm in the embryo varies greatly. In some cases it arises as bilateral pouch-like outgrowths of the archenteron, which later form closed sacks completely surrounding the digestive tract, the sack of either SUBKINGDOM METAZOA. 57 side coming into contact above and below, the united walls forming the dorsal and ventral mesenteries which suspend the intestine (Fig. 27, am and bm). That wall of each sack which surrounds the digestive tract is termed the splanchnic layer of the mesoderm (Fig. 27, spm), while that lying immediately below the ectoderm is the somatic layer (sm), and the enclosed cavity is the celom (C) or body-cavity. In other cases the protoplasm destined to give rise to the mesoderm segregates into a small number of cells, or sometimes even into a single cell, at an early period of the development, frequently while the embryo is still in what may be considered the blastula stage. These cells, known as mesoblasts, give rise by repeated division in one direction, and by the subsequent division of the daughter cells so formed, to bands of mesodermic tissue extending along the ventral surface of the embryo (see Fig. 105), and later growing dorsally so as to enclose the diges- tive tract. The coelom forms by the hollowing out of the mesodermic bands, and when fully developed presents the game appearance as in the former case. In many animals, such as some Turbellarian worms, a well-developed ccelom is not present, the only traces of it being minute scattered cavities in a mass of mesodermic tissue which fills up the space between the endoderm and ectoderm. A strict demarcation of this form of ccoelom (schizocel) from the other variety (enterocel) does not, however, exist, grada- tions occurring in various groups of animals and both varie- ties sometimes being coexistent in the same form, as for instance in bivalve Mollusca, where the pericardial cavity is to be regarded as an enteroccel, while the spaces existing else- where in the mesoderm are schizoceels. If the conditions which exist in the lowest triploblastic animals known to us, the Turbellarian worms, throw any light upon the origin of the meso- derm, it would seem that primitively it was a solid tissue, not completely marked off from the endoderm, and that any ccelom that it contained was of the nature of aschizocel. From this condition it became more and more differentiated from the endoderm proper, and either tended to appear as a separate germ-layer at an early stage of development in the form of the mesoblasts, or was delayed in its development until after the formation of the primitive digestive tract, from which it then separated in the meso- dermic pouches. According to this view the mesoderm is a secondary 58 INVERTEBRATE MORPHOLOGY. derivative of the endoderm, and the endoderm of the diploblastic organ- isms is equivalent to the endoderm plus mesoderm of the triploblastic forms. The apparent derivation of the mesoderm from the ectoderm in some of the latter (e.g. Annelida) is to be regarded as resulting from the precocious segregation of the mesoderm at an early period of development and is not to be regarded as indicating its original derivation. Non-sexual Reproduction in the Metazoa.—Reproduction by division and by budding, though playing by no means so im- portant a part as in the Protozoa, is neverthe- less of frequent occurrence in the Metazoa, especially in certain groups. In certain Tur- bellarian worms (Microstoma) division is the usual mode of reproduction, replacing almost completely the sexual method, and, the indi- viduals so produced remaining in connection with one another, longitudinal chains are produced, consisting of individuals in various degrees of separation (Fig. 28). In certain Annelids also (Naidide) division frequently takes place, occasionally each metamere being capable of developing into a new animal, as in Ctenodrilus. Budding, however, is a rather more fre- quent method and is characteristic of certain groups, such as the Hydroids, Anthozoa, and Fie. 28.—Dra- Bryozoa. In some cases, as in Hydra and saa aurrin ey some meduse, the buds separate from the pRopuction or parent and lead an independent existence ; aTursetLartran but frequently the separation is not complete, Worm Microsto- resulting in the formation of colonies the a as sai individual components of which are in or- ganic connection with each other. In such colonies a physiological division of labor among the con- stituent individuals may take place, as in the Hydroid HHydractinia (see p. 87) where some of the individuals devote themselves to the nutrition of the colony, others to its reproduction, and others again to the protection of their weaker companions. The assemblages produced by bud- ding may assume very complicated shapes, though ocea- SUBKINGDOM METAZOA. 59 sionally linear colonies are formed which are with difficulty to be distinguished from those formed by division. Indeed a definite distinction between budding and division is not possible, though where an alternation of older and younger individuals occurs in a linear colony division is indicated, while in one produced by budding there is a regular succes- sion of gradually older individuals from before backwards. Closely related to budding is the power of regeneration. of parts. The higher Crustacea possess an extraordinary power of regenerating lost limbs, and provision is present in crabs and the lobster for the self-amputation of a limb when such a mutilation seems,to be demanded by the exigencies of the situation. In the lower forms, however, the extent to which such regeneration may be carried is much greater, ex- tending even to the reproduction of the whole by a compara- tively small part. A Starfish is not only able to regenerate an arm which has been accidentally lost, but from an arm and a portion of the disk all the missing parts may be developed ; and Hydra or a Sponge may be divided into a large number of pieces each of which is capable of developing into an entire animal. Such phenomena, as well as budding and division, depend either upon a low degree of differentiation of the tissues, as in such a form as a Sponge or in Hydra, or else to the persistence of a certain amount of tissue in an embryonic or undifferentiated condition. In a Bryozoan bud, for in- stance, as its tissues gradually differentiate into the adult condition, a number of cells lag behind and do not take part in the differentiation, and later give rise to a new bud; and similarly in the Annelid worms the tissues of a regenerating part show an appearance and mode of differentiation similar to what they present in the development from the ovum. Conversely, the greater the degree of differentiation and in- tegration of the tissues and organs of an animal the less is the power of regenerating lost parts or of reproducing by budding. As a general rule ova are incapable of developing into the adult form unless fertilized by a spermatozoon. In a number of forms, however, a development of unfertilized ova occurs constituting a mode of reproduction known as_ partheno- 60 INVERTEBRATH MORPHOLOGY. genesis. Examples of this phenomenon are to be met with in Insects, a familiar one being the common Hive Bee, the queens of which species deposit large numbers of eggs, those last deposited, which give rise to drones, being unfer- tilized and developing parthenogenetically. In certain flies (Cecidomyia) this parthenogenetic development of the ova may occur while the insect is still in the larval or maggot stage, a phenomenon which is known as peedogenesis (Fig. 29). Alternation of Generations.—The majority of forms which possess the power of non-sexual reproduction also repro- duce by the sexual method, no definite relation existing, however, between the two processes. In some cases, however, a definite relation is established, the one method succeeding the other with rhythmic regularity, the individuals also which reproduce sexually differing materi- ally in form and organization from those which gave rise to them by a non-sexual method; such a condition of affairs is termed Alter- nation of Generations, a generation of in- dividuals reproducing only by a non-sexual method alternating with a second generation reproducing exclusively or almost so in the sexual manner. Typical examples of this process are afforded by the Discomeduse, in many of which the individual produced by the development of the ovum is a fixed, Fig. 29. — Pmpo. cylindrical organism of simple structure, known GENETIC Cecido- 88 a polyp, possessing the power of non-sexual myia LARVA (after reproduction (see Fig. 55). By a series of apeetslias rom transverse divisions it gives rise to a linear colony of individuals which in the course of development assume a form very different from that of the parent polyp, becoming more complicated in structure, more highly organized, and free-swimming. These organisins, known as Meduse, are the sexual generation, producing sper- matozoa and ova, the latter after fertilization developing a non-sexual generation, a polyp, with which the cycle begins again. SUBKINGDOM METAZOA. 61 Schematically such an arrangement may be represented thus, A repre- senting the non-sexual and B the sexual generation : B—A, ete. A // B—A, etc. \ B—A, ete. B—A, ete. Among the Hydromeduse, in which group alternation of generations likewise occurs, the process is usually complicated by a number of non- sexual generations succeeding one another before the intervention of the Medusa, thus : oe, Ad. SA B=A, ete Se Al = 8 = A, ate Ca" And in some cases the succession is still further complicated by non-sexual reproduction on the part of the medusa, thus : A, ete ACh 2B = A, et & = A, ete. 1» _ p/B! =A’ ete Nadas = ri = A’ ete A, ete. But such complications do not interfere with the general alternation which invariably occurs in such forms before the completion of the repro- ductive cycle. Such a phenomenon as this where a true non-sexual gen- eration alternates with a sexual one presenting a different structure is usually distinguished as metagenesis from another form of alternation of generations known as heterogony, in which the first generation reproduces parthenogenetically, giving rise to a second generation differing in form from the first and reproducing by the sexual method. Typical exam- ples of this process are to be found among the Trematode worms (q.v.), where the sexual worm gives rise to a sporocyst in the interior of which ova, developing parthenogenetically, give rise to a larva which later on transforms to the adult worm. In a less perfect form heterogony occurs in many lower Crustacea (Daphnia), which throughout the warmer portion of the year produce “summer eggs”’ which develop parthenogenetically, male animals appearing only for a short period in the autumn, as a rule, when the females produce 62 INVERTEBRATE MORPHOLOG Y. “winter eggs” which develop after fertilization. Here no difference of form exists between the two generations, but such cases, as well as those in which two sexual generations unlike in form and habitat alternate with each other, are usually associated with the more typical examples as in- stances of heterogony. LITERATURE. 0. Hertwig. Beitrige zur Kenntniss der Bildung, Befruchtung und Thetlung des thierischen Hies. Morpholog. Jahrbuch, 1., 111. and rv. 1875-78. E. van Beneden and A. Neyt. Nouvelles recherches sur la fécondation et la division mitosique chez V Ascaride mégalocéphale. Bulletin de la Société royale Belgique, xrv. 1887. Th. Boveri. Zellenstudien. Jenaische Zeitschrift, xx11. 1888 and xxtv. 1890. H. Fol. Le quadrille des centres. Archives des Sciences phys. et naturelles _ Genéve, xxv. 1891. 0. Hertwig. Vergleich der Hi- und Samenbildung bei Nematoden. Archiv fiir mikrosk. Anatomie, xxxvr. 1890. F. M. Balfour. A Treatise on Comparative Embryology. London, 1880. E. Metschnikoff Hmbryologische Studien an Medusen. Vienna, 1886. E. Haeckel. Die Gastrula und die Hifurchung der Thiere. Jenaische Zeit- schr., 1x. 1875. TRICHOPLAX, THE DICYEMIDH AND ORTHONECTIDA. 63 CHAPTER IV. fRICHOPLAX, THE DICYEMIDA AND ORTHONECTIDA, BEFORE passing on to a description of the first type of Metazoa, it will be necessary to consider a few forms which can hardly be assigned to it and yet present too great a dif- ferentiation of their component cells to warrant their reference to the Protozoa. A third subkingdom, the Mesozou, has been proposed for them, but until more is known of the relatious of some of them at least to other forms the establishment of such a subkingdom seems inadvisable. Trichoplax adhcerens. Fie. 80.—A, SuRFAcE VIEW AND B TRANSVERSE SECTION THROUGH Tricho- plax (after ScHULZE). b = botryoidal structure. 7” = refractive bodies. In the marine aquaria at Gratz, Vienna, and Berlin there has been found a small organism (Fig. 30, 4) measuring from 64 INVERTEBRATE MORPHOLOGY. 1.5 to4mm., but capable of great alteration of form. It is’ flattened, and creeps about upon the walls of the aquaria in an amoeboid manner. It consists, however, of numerous cells (Fig. 30, 2), the upper surface being covered by a flattened ciliated epithelium, and the lower formed by a layer of columnar cells also ciliated, while the space between the two _ surfaces is occupied by a network of branching cells, the branches appearing to unite with those of adjacent cells and with prolongations from both the upper and the lower epithe. | lium. The arrangement suggests the three germ-layers ecto- derm, endoderm, and mesoderm, but until more is known con- cerning the reproductive processes such an homology is unwarranted. At present the organism is only known to re- produce by division, and no structures have been discovered which may be identified as ova or spermatozoa. Beneath the upper epithelium, imbedded in the cells of the middle tissue, large refractive spheres (Fig. 30, B, r) and yellowish-green botryoidal masses (>) occur, but they have apparently no con- nection with reproduction. THe DIcyEMIDz. The Dicyemide are elongated vermiform organisms which are parasitic in the renal organs of the Cephalopods. The various species of Dicyema (Fig. 31) vary in length from 0.5-7 mm. and are all very simple in structure, consisting of a single elongated central cell (Fig. 31, C’) extending from one end of the body to the other and covered by a number of ciliated cells arranged in a single layer. Some of these, situ- ated at one end of the body, are smaller than the others and mark off the anterior extremity ; there is no mouth or diges- tive tract and no sense-organs. Reproduction is carried on by the development of germ- cells (g) produced by the division of the nucleus of the central cell and the concentration around the nuclei so produced of a portion of its protoplasm. The development of these germ- cells is apparently parthenogenetic and no male Dicyema is as yet known. In young individuals the germ-cells segment in the interior of the central cell and give rise to “vermiform ” TRICHOPLAX, THE DICYEMIDH AND ORTHONECTID4. 65 embryos (Fig. 31, V) similar to and developing directly into the adult form. Another form of embryo is, however, produced by older individuals, its formation being accompanied by a peculiar behavior of the germ-cells. The nucleus of each one first divides into two unequal parts, the smaller part separating as a paranucleus and under- going no further development. The germ- cell now segments, and an embryo (Fig. 31, e) consisting of a single large cell partially surrounded by smaller cells re- sults. The smaller cells are now thrown off and separate somewhat from each other, and the larger cell repeats the segmentation- process, the smaller cells being again thrown off; and this may happen three or four times, the result being the production of three or four concentric layers of small cells surrounding a single larger one, all lying in the central cell of the parent. The large cell undergoes no further develop- ment, but the smaller ones, except those _ of the last generation, develop into “infu- soriform” embryos of a peculiar and com- plicated structure. The cells of the last ea generation develop into “vermiform” em- yg, 31.—Dicyema tie bryos similar to those found in young pus (combined from Dicyemids. several figures by WHITMAN). The fate of the “infusoriform ” embryos ¢ = central cell. has not been determined. Since they are ¢ = embryos. ciliated it seems not improbable that they 7 = ser™-cells. é j 7 : 1 = nucleus of central serve for the dissemination of the species walt: and its transference from one Cephalopod p= vermiform em- host to another. It has, however, been bryo. suggested that they may develop into males. THE ORTHONECTIDA. The Orthonectids are parasitic on Echinoderms and Nemertean worms and resemble in structure the Dicyemids, 66 INVERTEBRATE MORPHOLOGY. the ectoderm consisting of a number of ciliated cells arranged in a single layer and enclosing a mass of germ-cells which correspond to the central cell of Dicyema. Between the germ- cells and the ectoderm fine nucleated fibres occur which are presumably muscular. Three forms of individual are known to occur in the genus Rhopalura, one being a male, and the other two females. B Fig. 82.—Rhopalura Giardti (after Juutn). A, male; B, round female; Q, flat female. The male (Fig. 32, A) is about half the size of the females, which measure about 0.25 mm. in length, and presents a met- americ arrangement of the ectoderm which does not extend to the internal cells. The cells of the anterior segment have their cilia directed anteriorly, and are succeeded by a segment consisting of several rows of small non-ciliated cells each containing a refractive body, and behind this there follow three or four segments formed of cells provided with cilia directed backwards. One of the female forms (Fig. 32, B) is elon. gated, and is segmented like the male except that the segments are more numerous and the second non-ciliated segment con- sists of a single row of cells destitute of refractive bodies, TRIGHOPLAX, THE DICYEMID# AND ORTHONECTIDH. 67 The other female (Fig. 32, C) is, on the contrary, ovoid, flat- tened, and unsegmented, being ciliated all over; it differs furthermore from the elongated female in possessing on one side near the anterior extremity a granular mass containing a large nucleus whose significance is entirely problematical. Associated with the difference of form of the two females there is a difference of function. In the elongated form when the ova are mature the anterior two segments split off asa cap and allow the ova to escape, and, on fertilization, these give rise to males. In the ovoid form, however, the ova are imbedded in a gelatinous mass, and are liberated by the breaking up of the parent into a number of fragments; from the ova females of both forms develop. The systematic position and affinities of the Dicyemide and Orthonec- tide is a matter of uncertainty. They have been held by some authors to possess affinities with the Gregarinida and by others to be degenerate flat worms, while others have sought to trace resemblances to the Rotifers. The granular mass with the large nucleus which occurs in the ovoid Rhopalura has been supposed to represent a rudiment of a digestive tract, while the superficial metamerism of the male and elongate female of the Orthonectida may possibly point to a derivation from more highly organ- ized ancestral forms. There can be but little doubt that the Dicyemide and Orthonectida are closely related, but at present sufficient evidence is want- ing to warrant any definite conclusions as to their relationships to other forms. LITERATURE. TRICHOPLAX ADHHRENS. F. E. Schulze Uber Trichoplaw adherens, Abhandl. Akad. Wiss. Berlin, 1891 (see also Zoolog. Anzeiger, vi. 1883). DICYEMID&, C. 0. Whitman. P bunches of ciliated filaments. egies, The ectoderm rests upon a layer of muscular tissue in which Fig. 12%—Diacram or Ar- both circular and longitudinal RANGEMENT OF OrGaNns AT Jayers can be distinguished, and Hino Exp or Chetoderma yomerous bands of transverse (after Huprecut from LANKESTER). ‘ ae fe TS fibres, in some cases arranged to nm = nephridium. form septa placed at regular o = ovary. intervals, traverse the body- B= perlenrdiute: cavity. A fairly-capacious peri- ecm cardium is present, lying dorsally to the posterior portion of the digestive tract and into its upper portion the heart projects slightly, not being, however, completely enclosed by the pericardium. No auricles seem to be developed, nor are any definite blood-vessels present, the circulation being throughout lacunar. Il Ul This condition of the heart in relation to the pericardium is interesting as showing its original independence of that portion of the body-cavity. Its enclosure in the pericardium in the higher Mollusca is a secondary con- dition, the heart and its cavity belonging to the schizoceelic structures rather than to the so-called enteroceelic pericardium. This agrees perfectly with the relationships of the blood vascular system of the Nemerteans and Annelids. (See pp. 165 and 231.) The mouth is a longitudinal slit upon the ventral surface of the body and opens into a pharynx provided usually with a radula and with salivary glands, though both these structures are absent in Neomenia. The intestine pursues a straight course towards the anal opening, being, however, in some forms pouched, owing to its constriction at more or less regu- lar and close intervals by muscular transverse septa. The walls of the pouches thus formed are glandular and represent the digestive gland of other Mollusca, though in Cheetoderma there is a simple outgrowth of the digestive tract which rep- resents it more perfectly. TYPE MOLLUSCA. 287 The nervous system varies in the details of its arrange- ment in the different species, but is characterized in general by a tendency to form ganglia, although nerve-cells are scat- tered along the nerve-cords throughout their entire length. In Proneomenia there is present a well-developed and closely- approximated pair of cerebral ganglia from which arise the pleuro-visceral cords which extend backward along the sides of the body and possess a number of ganglionic swellings near their posterior extremity. ‘Two nerve-rings surround the cesophagus: (1) the cerebro-pedal connectives, which end below in the pedal ganglia, from which two pedal cords extend backward along the foot, in some forms (Dondersia) connected at regular intervals by transverse commissures in an almost metameric manner, ganglionic enlargements of the cords being developed in connection with the commissures; and (2) the cerebro-buccal connectives, which pass to two buccal ganglia lying below the pharynx. Special sense-organs have not yet been discovered in the Solenogastres. The nephridia consist of a pair of tubes which communi- cate internally with the pericardial cavity and, bending around Fia@. 128.—D1aAGRAMMATIC LONGITUDINAL SECTION oF Chiton (after Haier). c = perivisceral ccelom. n = nerve. h = heart. = pericardium. m = mouth. ro = reproductive organ. 1-8 = shell-plates. the posterior part of the digestive tract, unite to open into the cloaca ventral to the anus by a common orifice. The walls of the tubes are glandular and probably, therefore, excretory in function, but the nephridia also serve as the ducts for the reproductive elements. With the exception of Cheetoderma the Solenogastres are hermaphrodite, the single reproductive 288 INVERTEBRATE MORPHOLOGY. gland producing both ova and testes. This hermaphrodite gland is a hollow sac divided into two principal compart- ments by a longitudinal partition and lies above the digestive tract. It is a hollow structure (Fig. 128, ro), the reproductive elements developing from the cells lining its walls and pass- ing from its cavity into that of the pericardium (p), with which the reproductive sacs communicate. They are in fact simply prolongations of the pericardial body-cavity, and the epithelium lining them is continuous with that of the pericar- dium. From the pericardial cavity the ova and spermatozoa pass to the exterior by the nephridia. The Solenogastres are especially interesting on account of the many structural peculiarities of a primitive character which they present and in consequence of which they have been regarded as representatives of ances- tral Molluscan forms. By others, however, this important position is denied them on the ground that many of their peculiarities are due to degeneration produced in accordance with their life in the mud at the bot- tom of the ocean. The absence of a shell, the reduction of the mantle- lobes, foot, and radula may with plausibility be accounted for in this manner, but there are other peculiarities that are certainly primitive which are not thus explicable. The relation of the heart to the pericardium is one of these, and others are the communication of the hermaphrodite gland with the -pericardium, and the functioning of the nephridia as ducts for the reproductive organs. The Solenogastres are unquestionably primi- tive Mollusca; the only question which is yet to be settled is to what extent, if any, degeneration is responsible for their external peculiarities, such as the absence of a shell, the reduction of the mantle-lobes and of the foot. It must be noted in this connection that one form belonging to the genus Dondersia has been described as passing through in its development a stage in which indications of a shell consisting of several plates and simi- lar to that of the Polyplacophora was present, a condition which would seem to indicate the derivation of the members of this group from forms provided with a distinct shell. 2. Order Polyplacophora. The Polyplacophora, like the preceding: order, contains only marine forms. For the most part they are somewhat flat- tened animals with a rather broad foot occupying the ventral surface, while from the sides of the body a slight fold, the mantle-fold, projects. In one genus, Chitonellus, the form of the body is more cylindrical and the foot is rather narrow TYPE MOLLUSCA. 289 and situated, as in the Solenogastres, at the bottom of a median ventral furrow, the lips of which correspond to the more dorsally situated mantle-folds of such forms as Chiton, Trachydermon (Fig. 129), etc. In all cases, in the groove be- tween the mantle-folds and the foot a number of gills, pinnate processes of the body-wall, are to be found, in some cases occurring at definite intervals along the entire side of the body, in others (Chitonellus) limited to the posterior a only. One of the most characteristic features of the = Polyplacophora is, however, the shell, which consists of eight calcareous plates arranged in a longitudinal series along the dorsal surface of the body so that the posterior border of one overlaps the anterior border of the other. The series covers only the median portion of the surface, the more peripheral portions and the outer surface of the mantle-lobes possess- ing a large number of scattered spicules, plates or granules imbedded in their wall. The body-wall has not so definite an arrangement of the muscle-fibres lying below the ectoderm as is the case in the Solenogastres, but, on the other hand, the body-cavity is well developed. Indeed the schizoccelic lacune play a rather sub- ordinate part in the Chitonide, as the order is sometimes termed, the enteroccelic cavity (Fig. 128) being very large and divisible into three usually separated parts united by bands, which indicate the original continuity. One of the parts (c) surrounds the intestine and the digestive gland ; another, lying rather towards the anterior end of the dorsal portion of the body, contains the reproductive cells (ro); while the third part (p), lying dorsally and posteriorly, is the so-called pericar- ‘dium. The two auricles of the heart are elongated tubes which open about the middle of their length into the single ventricle and also unite together posteriorly, the ventricle, also an elongated tube, again communicating with this united portion. Anteriorly the ventricle is continued into a short aorta from which the blood passes to the lacunar spaces of the schizoceel. Two vessels with distinct walls run longitudinally in the foot, Fie. 129.—Cheto- pleura apiculate. 290 INVERTEBRATE MORPHOLOGY. and presumably receive the blood which they contain more or less directly from the aorta and distribute it to the lacunar spaces of the foot. The mouth lies on the ventral surface, in front of the anterior end of the foot, and leads into a pharynx provided with a well-developed radula characterized by a somewhat ‘complex arrangement of the teeth. Into the esophagus a pair of glands opens in Chiton whose secretion contains an amylolytic ferment, and in addition a pair of small glands open into the mouth-cavity. The esophagus communicates with a sac-like stomach, into which open the ducts of the paired digestive gland, and the intestine, being considerably longer than the body, is thrown into numerous coils, and terminates by a short rectum which opens at the posterior extremity of the body. The nervous system is characterized by the diffuse arrangement of the nerve-cells, no well-defined ganglia oc- curring on the principal nerve-cords. These consist of a strong circumcesophageal ring (Fig. 130), the upper part of which gives off numerous nerves and: evidently corresponds to the cerebral ganglia of other Mollusca, while the lower part, corresponding to ‘the pedal ganglia, gives rise to two nerve-cords (pe), the pedal nerves, which pursue a parallel course throughout the foot, giving off a number of nerves laterally and being connected by a number of somewhat irreg- ularly arranged transverse commissures, which almost suggest a metameric arrangement. From the sides of the circum- cesophageal ring two other strong nerves, the pleuro-visceral nerve-cords, arise and pass backwards along the sides of the body, uniting with each other posteriorly above the terminal portion of the digestive tract. These cords (pl), like the cir- cumcesophageal ring, present no distinct ganglionic enlarge- ments, but contain the elements of the pleural, visceral, and parietal ganglia, sending off numerous nerves to the branchia, the mantle, and probably alsc to the heart and nephridia. In addition to these principal nerve-cords others of smaller size also arise from the circumcsophageal ring. One pair of these pass to a pair of ganglia, the buccal ganglia, lying beneath the buccal mass and send nerves to the cesophagus TYPE MOLLUSCA. 291 while another pair pass to a pair of ganglia lying below the radula and in intimate connection with a peculiar subradular organ, probably sensory, lying in this region. VOUrITI® Fig. 180.—D1aGRraM or NERVOUS AND ExcrRETORY SysTEMs oF Ohiton siculus (combination of two figures by Hauer). an = anus. no = nephridial orifice. Br = ctenidia. oe = esophagus. go = genital orifice. ‘pe = pedal nerve cord. n = nephridium. pl = pleural nerve-cord. As regards sense-organs, in addition to this subradular organ whose function is entirely problematical, ridges of sensory epithelium exist along the sides of the body in the mantle-cavity. One such ridge runs along the inner wall of the mantle-fold, while the other is found at the bottom of the mantle-cavity passing over the bases of the branchial plumes and sending a short prolongation outwards upon each of these structures and seeming thus to correspond with the osphradia of other Mollusca. . 292 INVERTEBRATE MORPHOLOGY. A much more peculiar series of organs, found, however, in their perfect form only in certain species, is developed in connection with the shell of the Chitonide. They consist of club-shaped structures contained in pores which traverse the shell-plates and possess a definite arrangement, being ar- ranged in groups of larger and smaller organs (megalcesthetes and micresthetes). Each group is in connection with a num- ber of large glandlike cells, which terminate in the megal- esthete, covered externally by a cup-shaped layer of. chitin, and from this cell-mass more or less numerous branches arise, the micresthetes, which terminate in club-shaped swellings likewise covered by a chitinous layer. Below the group of cells is in connection with fibrils which unite to form a nerve probably passing to the pleuro-visceral nerve- cords, and it thus seems tolerably certain that these struc- tures are sensory and perhaps tactile in function. In some species the megalesthetes become modified into eyes consist- ing of an external convex chitinous cap, the cornea, below which is a lens and below this a layer of retina-cells con- nected with nerve-fibrils and surrounded by a cup of pig- ment-cells. No eyes other than these occur in the Polypla- cophora, nor are tactile tentacles or otocysts, of such fre- quent occurrence in other Mollusca, found. The nephridia (Fig. 130, ») are paired, one lying on each side of the body and consisting of a long tube giving rise to numerous dendritic branches. Posteriorly the tube branches, one of the branches opening into the mantle-cavity in. its posterior part, while the other communicates with the peri- cardial portion of the enteroccel. In function these organs of the Chitonide differs from the corresponding ones of the Solenogastres in being excretory only and in not serving as ducts for the reproductive elements. These are developed in a portion of the enteroccel which lies anteriorly to the pericardium and make their way to the mantle-cavity and so to the exterior by special ducts arising one on each side from near the posterior part of the reproductive enteroccel and ending (go) on the sides of the body slightly in front of the openings of the nephridia (no), The Polyplacophora are with- out exception bisexual. TYPE MOLLUSCA. 293 The structural peculiarities of the Polyplacophora point strongly to their primitive character, though in many respects they are less primitive than the Solenogastres. Thus they possess special reproductive ducts, in all probability a secondary acquisition, and furthermore the reproductive and . pericardial moieties of the enteroccel no longer communicate freely. If the Solenogastres have been derived from forms with Chiton-like shells (see p. 288), then it must be supposed that the two groups represent di- verging lines of development from a common ancestor whose character- istics have been partly retained in the one group and partly in the other. II. CLass GASTEROPODA. The Gasteropods form a very complex group, the various members differing so much in the details of their organiza- tion that it is difficult to give a general description which will apply to all the forms. Certain features may, how- ever, be considered typical of the class, and these may be mentioned here, reserving notice of the more important varia- tions until the various subdivisions are being considered. One of the most characteristic features is the occurrence of what may be termed the “ visceral hump” whose presence is responsible for many of the peculiarities of Gasteropod struc- ture. It consists of an elevation into a dome-like structure of the dorsal region of the body, the digestive tract and gland being contained within the elevation. The mantle arises as a circular fold surrounding the hump, but usually is more highly developed, and therefore encloses a deeper cavity, upon the right side or anterior surface of the hump, and in the cavity so arranged lie the structures which usually are associated with the mantle-cavity, namely, the branchiz and the openings of the digestive tube and of the nephridia. There is thus a very decided asymmetry in most Gasteropods, usually emphasized by the visceral hump being coiled into a spiral, a coiling which is shared by the shell, usually present and consisting of a single tubular structure surrounding the visceral hump, but usually sufficiently ample to permit of the retraction within it of the rest of the body. In a number of forms the visceral hump may be very much reduced, and with this reduction there is generally con- comitant a reduction of the shell, but such conditions are 294 INVERTEBRATE MORPHOLOGY. plainly secondary inasmuch as the primitive asymmetry is indicated in certain of the organs in all cases. In order to understand the exact nature of this asymmetry it will be nec- essary to consider what may have been the original form of the Gasteropoda. Judging from what is known of the Amphi- neura, it may be supposed that in the primitive Gasteropod (Fig. 131, A) the anus (a) was terminal and opened into a mantle-cavity, the mantle being, except posteriorly, only a slight fold. In this mantle-cavity there was present also a single pair of branchial plumes (ct), and into it the two nephridia opened (n), passing from the posteriorly-situated pericardium which contained the heart provided with two auricles. It may be imagined now that in such a form the visceral hump enclosed by a dome-like shell became elevated to such an extent that it could no longer be retained in an erect posi- tion, but fell over to one side—it may be supposed the left side. The result of this would be an interference with the development of the mantle-cavity towards the left side, and a prevention of the perfect growth of the left branchia and of the proper functioning of the left nephridium. There would be a tendency then for the mantle-cavity, and with it the anus and indeed the entire posterior region of the body with the heart and nephridia, to be pushed over towards the right side (Fig. 131, 2), and this process might in some cases be con- tinued until the mantle-cavity and the organs associated with it had been pushed round through 180° (Fig. 131, C, D) and had come to lie apparently in front of the visceral hump (D). The anus in such a case would open into the mantle-cavity in the mid line, dorsal to the mouth, and what was originally the right branchia would lie upon the left side of the body; the digestive tube, which may originally have been practically a straight tube, would now be bent upon itself, and furthermore the original right parietal nerve-ganglion would have passed over to the left side of the body and the original left ganglion to the right side, a crossing of the pleuro-parietal connectives (ve) being thus brought about. The original pressure of the shell upon the left half of the mantle-cavity would, however, as pointed out, have TYPE MOLLUSCA. 295 tended to produce a retardation in the growth or even the complete abortion of the organs lying in that region. Accord- ingly the original left nephridium is in many Gasteropods com- pletely suppressed as well as the original left branchia, and in accordance with the disappearance of this latter structure Fig. 131.—D1aAGRAM8 TO ILLUSTRATE THE ROTATION OF THE MANTLE-CAVITY AND ITS ORGANS IN A GASTEROPOD (after figures by BUTSCHLI and Lane from KorscHe.t and HEIDER). @ = anus. m = mouth. ao = aorta. n = nephridial pore. ct = ctenidium. peg = pedal ganglion. cg = cerebral ganglion. plg = pleural ganglion. ve = visceral connective. there is a disappearance also of the left auricle of the heart which receives blood from it. The visceral hump does not, however, retain its original conical form, but, owing perhaps to unequal pressure, grows more rapidly upon one surface, the anterior, and so becomes coiled into a right-handed spiral, the shell covering the hump naturally assuming a similar form. In the majority of 296 INVERTEBRATE MORPHOLOGY. Gasteropods consequently a shell coiled in a right-handed spiral occurs, but this rule has not a few exceptions. Where the shell forms a left-handed spiral it is to be explained by supposing that in such cases the visceral hump tended towards the right side of the body rather than the left, and this is confirmed by the fact that in most left-handed forms it is the left branchia and nephridium that have persisted. It must be pointed out, however, that the extent to which the rotation of the mantle-cavity, the abortion of the organs of either the left or right side of the body, and the crossing of the pleuro-parietal nerve-cords has been carried varies in different forms. In some the rotation has been carried so far that the original right branchia, etc., has passed the median line in front so as to lie on the left side of the body, and in such cases the crossing of the nerve-cords (chiastoneurism) is completed. Many forms, however, stop short of this, and numerous gradations are to be found. The rotation, however, is present in all forms to some extent and forms a character- istic feature of Gasteropod morphology. The anterior portion of the body (Fig. 132) is usually well marked off by a more or less distinct constriction or neck, and consequently it is possible in the Gasteropods to speak of a head in contradistinction to the trunk region of the body ; in- deed so prevalent is this character that the term Cephalo- phora has been applied to the group. Tentacles, either one or two pairs, are borne by the head, and furthermore eyes are usually present upon it either at the bases of one of the pairs of tentacles or else borne at the tips of these structures. The foot is generally well developed and usually has a flat creeping sole. It undergoes many modifications, however, sometimes becoming more keel-like, or becoming differentiated into three regions differing in form, the propodium, mesopo- dium, and metapodium, the last-named portion frequently secreting a chitinous plate, the operculum (Fig. 132, op), which serves to close the mouth of the shell when the animal is withdrawn within it. In addition to these portions an epipo- dium is frequently highly developed, consisting in its primi- tive form of a fold arising from the sides of the foot where it passes into the body-wall. In many cases, however, it loses. TYPE MOLLUSCA. 297 this simple form, its margin becoming fringed or tentaculate, or else it may be reduced to one or more separate lobes or tentacular processes on either side of the body. Opening upon the surface of the foot is frequently to be found a so- called ‘“foot-gland ” which secretes a sticky mucous fluid and is comparable to the byssus-gland of the Pelecypoda (q. v.). Fie. 182.—Buccinum undatum. _ op = operculum. si = sipho. The respiratory organs (Fig. 133, ct) consist in typical cases of a single pair of pinnate branchial plumes lying in the mantle-cavity, but, as has already been mentioned in connec- tion with the rotation of that cavity, one of these structures is very frequently aborted. Other changes, however, also occur, such, for example, as the fusion of the central axis of the branchial plume throughout its entire length to the inner surface of the mantle (Haliotis), or the disappearance of the pinne from one side of the plume in connection with such a fusion (Sycotypus, Fig. 133). In some forms accessory bran- chise may be produced as folds of the mantle, richly supplied ’ with blood, and their development may be carried to such an extent that they may entirely supplant the branchiw proper (Patella). From such a condition as this a passage is not dif- ficult to such a condition as is found in the air-breathing Gas- teropods (Pulmonata) in which the entire inner surface of the mantle serves a respiratory purpose, an interchange of gases taking place between the air contained in the mantle-cavity and the blood which is richly supplied to the mantle. The musculature of the body-wall does not as a rule pre- sent the Annelidan arrangement in layers, as in some Amphi- neura, but usually are irregularly arranged as dorso-ventral and oblique bands traversing the schizoccel. Special muscles, 298 INVERTEBRATE MORPHOLOGY. however, are developed in many forms, the most important being those connected with the foot and serving for locomotion, retractor muscles in connection with the head, proboscis, and tentacles, and the spindie-muscle, which has a general vertical direction running along the right side of the visceral hump from its insertion into the shell to the foot in whose wall its fibres spread out, interlacing as it were with the horizontal and transverse muscles there developed ; it serves to retract the entire animal within the shell, and its development is naturally in proportion to that of the shell, those forms in which the shell is rudimentary or absent frequently lacking it. The enteroceelic portion of the celom is much reduced in comparison with what occurs in the Amphineura, being dis- tinctly represented only by a comparatively small pericar- dium surrounding the heart, the auricle in some cases not being enclosed by it. From analogy with the Amphineura, however, the reproductive organs must be regarded as repre- senting a portion of the enteroccel whose connection with the pericardium has been completely severed. A glandular struc- ture, the pericardial gland, is in some Gasteropods developed by the folding of the pericardial walls, and has apparently an excretory function acting as an accessory nephridium ; it is not, however, as highly developed as in some of the other Molluscan groups. The circulatory organ possesses in some forms the charac- teristic Molluscan structure, consisting of an unpaired ven- tricle lying in the pericardium and receiving the blood from two lateral wing-like auricles. In many cases, however, as al- ready pointed out, the asymmetry produced by the develop- ment of the visceral hump affects the heart, resulting in the suppression of one of the auricles, that of the left (or right) side (Fig. 183). In such cases the persisting auricle may secondarily assume a terminal position with regard to the ventricle, and the latter, instead of being continued into an ar- tery at either extremity, gives off a single artery at the end op- posite to that at which the blood enters from the auricle, this artery dividing into two main trunks which distribute the blood to the various regions of the body. These arteries may be continued as distinct tubes with definite walls for some TYPE MOLLUSCA. 299 distance from the heart, but sooner or later the blood passes into the system of lacunar spaces constituting the schizoccel, whence it is again returned to the auricle through a series of veins. The position of the single auricle with reference to the body axis differs in different orders of Gasteropods, in accord- Fig. 183.—STructuRE oF Sycotypus canaliculatus. The mantle is divided in the middle line and turned aside, exposing the mantle-cavity. an = anus. os = osphradium. et = ctenidium. p = pericardial cavity. dg = digestive gland. pe = penis. ¢ = intestine. pr = proboscis. nm = nephridium. st = sipho/~ 1 no = nephridial opening. t = tentacle. o = eye. te = testis. op = operculum. » = ventricle. vd = vas deferens. The arrows show the openings of nephridium to the mantle-chamber and to the pericardium. ance with the varying position of the branchia. In those forms in which the branchia lies in front of the heart the auricle lies at the-anterior end of the ventricle, while when the branchia is posteriorly situated the auricle lies behind the ventricle. The mouth lies in all Gasteropods at the anterior ex- tremity of the body, towards the ventral surface of the head, 300 INVERTEBRATE MORPHOLOGY. and opens into a mouth-cavity frequently provided with two or more chitinous teeth. The pharynx usually receives the ducts of a pair of salivary glands, contains a well-developed radular organ in practically all cases, and communicates posteriorly with a tubular cesophagus. In many cases the anterior portion of the digestive tract is capable of being protruded as a proboscis (Fig. 133, pr), which lies when re- tracted within a proboscis-sheath, formed by a circular infold- ing of the body-wall around the mouth. The intestine (7) is usually more or less coiled, extending into the visceral hump, | and presents a stomach-like enlargement which receives the ducts of the digestive gland (dq), a structure usually well developed and forming the greater portion of the visceral hump. The intestine terminates in a straight portion, the rectum (7), which passes forward to the anus (an), which, as has already been indicated, lies in the mantle-cavity, slightly to the right, but occasionally to the left, of the middle line, its position depending upon the amount of rotation which the mantle-cavity and the associated organs have undergone. It should be mentioned that in one suborder of Gasteropods the pericardium and ventricle have wrapped themselves around the rectum in such a way that the diges- tive tube seems to have penetrated the ventricle, a feature which will later be seen to be characteristic of one of the other groups of Mollusca. The nervous system has the arrangement which has been described as characteristic of the Mollusca (Fig. 124), the peculiar feature being the crossing of the pleuro-parietal con- nectives which is found in many forms. Numerous modifica- tions of the typical condition are to be found, consisting principally in (1) the concentration of the ganglia, more especially the cerebral, pleural, and pedal, or the pedal, pleural, parietal, and visceral (Fig. 137), to form a single mass; (2) in the suppression in some cases of one of the parietal ganglia; and (3) in the occurrence of several visceral ganglia. In accordance with the flat elongated form of the foot in many species, the nerve-cords passing backward from the pedal ganglia may be of considerable size, and further- more may be connected by regularly-arranged transverse TYPE MOLLUSCA. 301 commissures, recalling the condition seen in the Chitons, as well as the ladder-like arrangement of the ventral nerve-cords of the Annelida, though there cannot in the Gasteropods be any question of metamerism in this connection. Special sense-organs are very generally well developed in the Gasteropods. The tentacles so usually found upon the head have probably a tactile function as well as the tentacular or winglike processes sometimes found in connection with the anterior extremity of the foot, and the epipodial ten- tacles which occur in some forms (/aliotis). On the ventral side of the bases of the epipodial tentacles of some forms special sensory thickenings have been found which have suggested a comparison with the sense-organs of the lateral line of the Annelida, a comparison which, however, at present seems rather strained; it seems probable, notwithstanding their innervation from the pedal ganglia, that these sensory patches are to be placed in the same category as the osphra- dia and the sensory ridges of the mantle-cavity of the Chi- tons. The osphradia (Fig. 133, os) in all Gasteropods which are provided with branchix are associated with these organs; and even where one or both branchis have been suppressed the osphradia may still persist. Eyes (Fig. 134) are very generally present in the Gasteropods, being situated at the base of the ten- tacles, or at their summit in some forms. They present a very uni- c form structure throughout the 2 nena group and arise as a depression Fig. 184.—Eve or Hiliotis (atter if i PATTEN). of the integument, the lips of eres icachon the cavity fusing and giving rise i = lens. to a globular sac lying beneath rt = relina, the epidermis, which remains thin and transparent, forming an outer cornea (co). The cells of the outer wall of the sac likewise remain clear, forming the inner cornea, while over the remainder of the wall of the sac they are sensory in function, pigmented cells being scattered among them, the two together forming the retina (ret). The nerve-fibres pass- 302 INVERTEBRATE MORPHOLOGY. ing to the eye from the cerebral ganglia pass through an optic ganglion lying beneath the optic sac and are distributed to the sensory cells, and the centre of the sac is filled up by a cuticular mass which serves as a refractive lens (J). In some forms (Patella, etc.) the development of the eye ceases while it is still in the cup form, there being then no formation of corneal layers and no central lens, though the retina is usu- ally covered by a thin cuticular layer. In some species of a peculiar genus of the air-breathing Gasteropods, Onchidium, eyes are developed upon the dorsal surface of the body, the shell being lacking and the visceral hump undeveloped. In structure these eyes differ very materially from those usually occurring in the Gasteropoda and will be described later (p. 318). Otocysts are usually imbedded in the tissues of the foot close to the pedal ganglia, though in all cases they receive their innervation from the cerebral ganglia; they have the usual sac-like form and are lined with sensory hair-bearing cells and contain otoliths. The nephridia of the Gasteropods are in nearly all cases modified from the original typical condition in accordance with the asymmetry of the body (Fig. 133, re). In only a few forms, so far as known (Fissurellu, Patella), are two functional nephridia, opening on the one hand into the pericardial cavity and on the other to the exterior through the mantle- cavity, perfectly developed. In other forms, such as Hali- otis, Turbo, etc., both nephridia are present and are struct- urally perfect, though the left* one has lost its secretory function, but in the majority of cases the left (or, in forms with a left-handed coiling of the visceral hump, the right) nephridium is completely aborted. : The Gasteropods are in some cases bisexual, in others hermaphrodite. The reproductive sac (Fig. 133, t) is quite unconnected with the pericardial enterocceel and is an un- paired structure lying in the visceral hump. The ova and spermatozoa in most cases reach the exterior by a special * The terms left and right refer to the position of the nephridia as they are supposed to have been arranged in the primitive symmetrical Gasteropod. pp 8 y Pp TYPE MOLLUSCA. 803 duct (vd), having apparently no relation to the nephridia and Opening into the mantle-cavity to the right side of the anus. In the more primitive Gasteropods, however, such as Haliotis, Fissurella, and Patella, the nephridia, as in the Solenogastres, serve as reproductive ducts; and it has been suggested that the special reproductive duct of the remaining Gasteropods may represent the left nephridium, which is usually described as having disappeared. The reproductive duct, especially in hermaphrodite forms, has developed in connection with it accessory glandular structures as well as external copulatory organs, the whole reproductive system becoming highly com- plicated. An account of the more important arrangements will be more satisfactorily given in connection with the various orders. 1. Order Prosobranchia. The Prosobranchia are, with very few exceptions, marine Gasteropods, provided with well-developed shells, which are usually spirally coiled, the height of the spiral varying, however, in different forms. In some, such as Patella and Fissurella, the shell has a simple conical form, without any indication of a spiral; and since these forms in Meee many respects show primitive charac- | ters, it might be supposed that this type of shell was also primitive. These very forms, however, show also that asymmetry of parts, which is character- Fia.135,—Suexis or Proso- istic for the Gasteropods, and which Branca GasreRorops. accompanies the rotation of the mantle- A, Acmaa ; testudinalis , 5 < (after Goutp); B, Haliotis cavity, and furthermore, in Fissurella (eer teonis): C, Turritella at least, a distinct indication of a after Levys). spiral coiling, is present in the shells of young animals. It seems more probable, accordingly, that these conical shells are to be regarded as secondary modifi- cations of an originally spirally-coiled shell. The mantle-cavity is situated in front of the well-devel- oped visceral hump, and is usually somewhat capacious, com- 304 INVERTEBRATE MORPHOLOGY. municating with the exterior freely. In some forms the mantle is slit from its margin upwards and backwards, a corresponding slit occurring in the shell (marginula). In Haliotis and Pleurotomaria the slit in the shell becomes closed at regular intervals, producing a row of round perfora- tions, beneath which lies the mantle-slit, and through which _ water finds a ready exit from the mantle-cavity, and, in Pis- surella, in which at an early stage the margin of the shell possesses a slit, by the subsequent growth and obliteration of the spiral coiling the slit becomes converted into an aperture which lies almost at the apex of the conical shell and leads into the mantle-cavity, functioning as a means of exit of the water and excrementa from that cavity. In the greater num- ber of forms, however, such slits or apertures do not exist; but one finds frequently the margin of the mantle produced at one point on the left side into a projecting narrow lobe whose edges may be brought into opposition, thus producing a tube or siphon through which water may pass into the mantle-cavity. Where this siphon is well developed a dis- tinct notch is found in the margin of the shell, through which it may be protruded, or else the lips of the notch are pro- longed so as to form a grooved process, the siphonal canal, in which the siphon lies, being by these arrangements able to function even when the mouth of the shell is closed by the operculum. In many forms the mantle-folds are sufficiently large to allow of their being reflected over the outer surface of the shell when the body is fully protruded. The foot is as a rule adapted for creeping, but in many cases is differentiated into pro-, meso- and metapodium, the last usually bearing a chitinous or more or less calcified operculum. In certain forms belonging to a group of pelagic forms, however, which were formerly associated together as a distinct order, the Heteropoda (Fig. 138), the pro- and meso- podium are modified into a kecl-like structure and bear a peculiar sucker. The epipodium is frequently developed in the Prosobranchia, especially in the more primitive species— most frequently, however, being reduced to tentacle- or lobe- like processes arising from the sides of the foot. In the majority (Fig. 133) of forms there is but a single TYPE MOLLUSCA. 805 branchia which lies in front of the heart, whence the name of the order, but in a few genera the original left gill also per- sists. In many forms a gland is developed in the floor of the mantle-cavity close to the rectum—hence called the adrectal gland—which in some forms, e.g. J/urex secretes a purple pigment. The rotation of the mantle-cavity and the associated organs has called forth a crossing of the pleuro-parietal nerve- cords, a feature which is lacking in the other orders and therefore forms a characteristic of the Prosobranchs. In all but a few cases the members of the order are bisex- ual, the unpaired reproductive gland lying in the visceral hump. The oviduct has in connection with it one or more receptacula seminis and dilates into a glandular uterus in which the eggs are supplied with the albumen in which they are usually imbedded and also surrounded by a shell. In the males, except in the more primitive forms, there is present a well-developed intromittent organ or penis (Fig. 133, pe), situated upon the right side of the head or neck and there- fore removed at some distance from the opening of the vas deferens into the mantle-cavity. A groove or tube extends, however, from the reproductive orifice to the grooved or tubu- lar penis, and along this groove or tube, by the ciliary action of the cells lining it, the seminal fluid is carried. 1. Suborder Diotocardia. This suborder includes the more primitive Prosobranchs, in which, although a considerable rotation has occurred, yet nevertheless the abortion of the organs of the original left side of the body has not been carried very far. Thus, except in Patella and some allied forms, there are two auricles to the heart, although in Turbo, Trochus, Neritina, and allied genera that of the right side (ie., the original left one) does not com- municate with the ventricle. Attention may again be called to the fact that in those forms which possess two functional auricles the ventricle and pericardium have wrapped them- selves round the rectum which seems to perforate the ventri- cle. Such forms as Haliotis, Fissurella, and Plewrotomaria pos- sess two branchiez, but in the majority of the members of the 306 INVERTEBRATE MORPHOLOGY. group only one is present, while in Patella both have disap- peared, their place having been taken by respiratory folds of the mantle. Both kidneys are invariably present. The primitive character of the suborder is further shown in the absence of certain structures found in more specialized forms. Thus the foot is flat and undifferentiated into pro-, meso-, and metapodium ; the anterior part of the digestive tract is not evertible as a proboscis; there is no siphonal prolonga- tion of the mantle, and no notch or siphonal groove on the margin of theshell; and thereis no penis. On the other hand the epipodium is usually well developed, as are also the pedal — nerve-cords, which are connected by numerous cross-commis- sures. A further distinguishing feature of the suborder is the arrangement of the teeth of the radula. Each transverse row - of teeth presents an indefinite number of marginal teeth, usu- ally a single lateral, a single median, and a varying number of admedian teeth, an arrangement known as rhipidoglossate. Thus in Haliotis the arrangement is indicated by the formula Fig. 1386.—DENTITION OF Trochus (after LANKESTER). #,1,5,1,5,1, ©;in Fissurella by x, 1, 4,1, 4,1, 2; and in Zro- chus (Fig. 186) and Turbo by 2, 0, 5, 1, 5, 0, x, the single lateral tooth being absent in these forms. In Patella, however, an- other arrangement is found characterized by the occurrence of only a small number of marginal teeth and by the absence of the median, the formula being 3, 1, 2, 0, 2,1, 3; this ar- rangement is termed docoglossate. 2. Suborder Monotocardia. In this suborder the effect of the pressure of the visceral hump on the organs of the left side of the mantle-cavity is more pronounced than in the Diotocardia. The heart possesses a TYPE MOLLUSCA. 307 single auricle only, except in Cyprea, where the rudiment of a second occurs, and throughout the group but a single nephrid- ium is present. There is never more than a single gill, which is usually more or less united to the mantle-wall and bears lateral branches only upon one side. The foot is in some cases flat and broad, as in the Dioto- cardia, and in such cases may possess the parallel pedal nerve- cords with transverse commissures asin Cypreea and Paludina, but usually it becomes more or less differentiated, a propodi- um being in many cases well defined (Strombus, Natica), while a chitinous or calcareous operculum is usually carried by the metapodium, and the pedal nerve-cords are very much re- duced or wanting, the pedal ganglia being on the other hand more highly developed than in the Diotocardia. The epipo- dium is usually entirely wanting, and when present is but slightly developed, reaching its fullest development as a con- tinuous fold upon the sides of the foot only in Janthina. In Paludina itis represented by two anteriorly-situated tentacle- like lobes, and in Calyptreea by a semicircular fold on each side of the neck region. The Monotocardia are further distinguished by the fre- quent occurrence of a well-developed siphon and a more or less developed siphon groove at the margin of the shell, and furthermore a well-developed penis is usually present. The anterior portion of the digestive tract isin many forms capable of being protruded as a proboscis. The arrangement of the teeth of the radula varies considerably in different forms, but the rhipidoglossate arrangementis not represented. In one group, including the genera Cyprea, Natica (Fig. 187, A), Littorina (the periwinkles), Calyptrea, Strombus, etc., the tenioglossate arrangement is found, represented by the for- mula 2 or 8,1, 1,1, 2 or 3, the admedian teeth, however, being very similar to the lateral. In other cases buta single median tooth or the median with a single admedian on each side is found, as in Fusus, Buccinum (the whelks), Nassa (Fig. 137, B), Murex, Purpura, Oliva, Marginella, etc., forming the rachi- glossate arrangement represented by the formulas —, 1, —, or 1,1,1. In Zerebra, Conus, Pleurotoma (Fig. 137, C), and allied genera the median tooth is absent, and the single admedian 308 INVERTEBRATE MORPHOLOGY. tooth on either side peculiarly long, forming the toxiglossate arrangement with a formula 1, 0,1; and finally certain forms, such as lanthina, Scalaria (Fig. 137, D), Solarium, ete., have a ptenoglossate arrangement in which the median is wanting but in which there are a large number of admedians, 2, 0, x. The suborder is relatively very rich in species, and conse- quently considerable variety of form is found. The majority are marine, but a few are fresh-water or even terrestrial in habitat. In these latter adaptations to their mode of life are found in modifications of the respiratory processes. In Am- ADE feo Wwe AY Fie. 137.—A, Dentition or Natica ; B, or Nassa; C, oF Pleurotoma, D, oF Scalaria (from Bronn). pullaria the single branchia persists, but in addition a com- paratively capacious “lung-cavity ” is formed by a fold of the mantle, its walls being richly supplied with blood-vessels and its cavity being in communication with the exterior, so that air can be taken into and expelled from it. The species of this genus live partly in fresh water and partly are terrestrial, but in other forms, such as Cyclostoma, which are purely ter- restrial, the branchia has entirely aborted, respiration being aerial and performed by the highly vascular wall of the mantle-cavity. The majority of the marine Monotocardia have a creep- ing habit, but a number are pelagic and form a group pre- senting many adaptive peculiarities which obtained for it the dignity of an order in older classifications. The members of this group, HETEROPODA, are more or less transparent animals, some of which, with this exception, present few differences TYPE MOLLUSCA. 309 from the other Monotocardia, while others are extensively modified. The genus Atalanta possesses a large transparent shell within which the animal can be completely retracted. The foot is no longer adapted for creeping, but is differenti- ated into a laterally flattened keel-like pro- and mesopodium which bears a sucker on its posterior surface, and a metapo- dium provided with an operculum. In Carinaria (Fig. 138) Fig. 188.—StRuctTuRE oF Carinaria mediterranea (after Owxn). ao = aorta. o = eye. 6 = buccal mass. p = penis. cg = cerebral ganglion. peg = pedal ganglion. et = ctenidium. $ = salivary gland. Ah = heart. su = sucker. 7 = intestine. te = testis. 2 = liver. od = vas deferens. mp = mesopodium. og = visceral ganglion. vs = seminal vesicle. the visceral hump is reduced to a comparatively small mass upon the dorsal surface of the elongated body and is enclosed in a transparent shell shaped like a liberty-cap. The pro- and mesopodium have the form of a plate hanging down from about the middle of the under-surface of the body, and the metapodium is directed backwards, forming in reality the posterior portion of the body. The same relationships of the foot are found in Pterotrachea, which presents the extreme of modification found in this group; in this form the visceral hump is still more reduced than in Carinaria, forming only a small oval mass imbedded in the dorsal surface of the body and being destitute of any shell. Considering these two forms, 310 INVERTEBRATE MORPHOLOGY. Pierotrachea and Carinaria, by themselves, the formation of a separate order for their reception would perhaps be justi- fiable, but Atalanta shows their close relationships with the Prosobranchia and indicates their true position as Monoto- cardia. 2. Order Opisthobranchia. The Opisthobranchs are exclusively marine forms, present- ing numerous modifications of shape and structure, but all agreeing in certain important particulars. The rotation of the mantle-cavity has not proceeded quite so far as in the Prosobranchs, the cavity and its organs lying upon the right side of the body, but at the same time the abortion of the organs of the primitively left side of the body has occurred. Thus in those forms which possess respiratory organs homol- ogous with the branchiz of the Prosobranchs, but one (that of the right side) is present, and with this character is associ- ated the occurrence in the heart of but a single auricle, which lies behind the ventricle. Only one nephridium occurs, and a distinction from the Monotocardiate Prosobranchs is found in the fact that the branchia when present lies as in- dicated by the position of the auricle, behind the heart—the name bestowed upon the order being suggested by this pecul- iarity. A more important distinguishing character perhaps is, however, to be found in the arrangement of the nerve-cords. The rotation of the mantle-cavity and its associated parts has not been carried to such an extent as to produce a crossing of the pleuro-visceral connectives, which run more or less par- allel with one another and present what is termed an ortho- neurous arrangement in contradistinction to the chiastoneu- rism of the Prosobranchs. In addition to this character a tendency towards an aggregation of the various ganglia to a complex mass lying behind the pharynx may also be con- sidered a characteristic of the Opisthobranchs. One or both parietal ganglia may disappear, and in some cases where there is a marked concentration of the ganglia the visceral ganglion may also be unrepresented, though usually from one TYPE MOLLUSCA. 311 to three such ganglia may be distinguished. In the figure of the nervous system of Fiona (Fig. 1389) the concentration of the ganglia is well marked, but a decided asymmetry is made evi- dent in some forms by the exist- ence of a single parietal ganglion and of three visceral ganglia. In Fiona, however, the ganglionic concentration has been carried still farther, and at the same time by the suppression ut the parietal Fia. 189.—Nervous System or ganglion as a distinct mass of Fiona atlantica (after Bercx from cells an apparent symmetry has @ecznzasvn). resulted. A = cerebro-pleuro-visceral gan- With regard to many other pA ieee ee Sra Eerlon: features of their anatomy con- 7 — gastro-cesophageal ganglion. siderable variations are to be e= pedal commissure. found. Thus in some forms a @ = Visceral commissure. well-developed spirally-coiled visceral hump is developed, - ‘while in others it loses its spiral arrangement, and in others again is elongated in the direction of the foot and can hardly be said to exist. So, too, with the occurrence of the shell, mantle, and branchie ; all are well developed in some forms, but entirely absent in others. These peculiarities will be more conveniently referred to in connection with the various groups, and it is only necessary here to refer to another feature in addition to those already given, which is common to all the members of the order—ie., the hermaphroditic character of the reproductive gland. This forms part of the visceral mass and is usually com- posed of numerous lobes, these again being divided into secondary lobes or acini, the lining epithelium of which give rise to both ova and spermatozoa. In some forms, such as Bulla and Aplysia, both elements are formed in ail the acini; but in others, such as Doris, Janus, Pteropoda, etc., the epithelium of the terminal acini gives rise to ova only ; the epithelium of the lobes, ie., the central portions of the gland, producing spermatozoa. Whether or not, however, there be such a separation of the epithelium into male and female 312 INVERTEBRATE MORPHOLOGY. areas, the reproductive elements make their way into a com. mon hermaphrodite duct, which presents variations of structure in different forms and receives the secretion of certain acces- sory glands. In its simplest form, as seen for instance in Aplysia, the duct runs forward, pursuing a somewhat tortuous course and becomes surrounded by an albuminiparous gland, from which it receives a viscid secretion, within which the ova become imbedded just in front of the point where the gland opens into the duct. The latter has attached to it a pouch-like structure, the vesicula seminalis, and is continued on as a somewhat wider tube to open to the exterior at the genital pore situated on the right side of the body, shortly before reaching the pore, however, receiving a duct from a globular sac, the spermatheca. From the anterior edge of the pore a groove, the seminal groove, extends along the right side of the body to the neck region, where it ends in a mus- cular evertible penis, situated near the anterior right tentacle. It seems probable that the spermatozoa mature before the ova, and passing to the vesicula are stored up there. During copulation the seminal fluid is transferred through the penis to the spermatheca of another individual (perhaps the trans- ference is a mutual interchange), and when later the ova pass along the duct they are impregnated by the spermatozoa so stored away, a cross-fertilization being thus . brought about. This arrangement of the reproductive duct is found in the more primitive Opisthobranchs, i.e., in those in which the mantle-lobe still persists, and in the group Pteropoda; in the more highly-modified forms, such as Doris, #olis, etc., and, among the more simple forms, in Pleurobranched the hermaphrodite duct divides into an oviduct and a vas deferens. The former after receiving the spermathecal duct opens into a genital atrium, with which communicate also the albuminipa- rous gland and a nidamental gland, which manufactures the outer shell-like investment of the ova. The vas deferens, after a more or less tortuous course, enters the muscular saclike penis-sheath which communicates with the genital atrium; the enlarged termination of the vas, the penis, being thus capable of eversion through the pore by which the atrium communicates TYPE MOLLUSCA., : 313 with the exterior. This condition seems to be a secondary modification of one in which the oviduct and vas deferens open independently at widely separated points—a condition which is represented by a few Opisthobranchs. Suborder Tectibranchia. The Tectibranchiates are those Opisthobranchs which present the smallest amount of modification from what has been considered the typical Gasteropod structure. A more _ or less developed mantle-fold is usually present, sometimes sufficiently voluminous to cover in the single branchia which persists (Bulla), but frequently represented only by a slight fold, which leaves the branchia exposed (Aplysia, Casterop- teron). A shell is very generally present, sometimes well de- veloped (Bulla), but in other cases reduced to a plate-like structure enclosed within the mantle which has been reflected over it and the lips of the reflected portion meeting and fusing (Aplysia, Pleurobranchus). The visceral hump, however, is as a rule low and elongated in the direction of the long axis of the body instead of at right angles to it, as in the majority of Prosobranchs. In many members of the group the foot pos- sesses a broad creeping surface, but its margins are prolonged into broad thin wings, the parapodia, which may be bent up- wards, as in Aplysia, so as almost to enclose the body. The Tectibranchiates are divisible into two groups accord- ing to their habits, in accordance with which the form of the foot and especially of the parapodia is modified. Those forms which possess a broad flat sole to the foot have a creeping habit; but there are many forms which are pelagic in habit and present many modifications of structure in adaptation to ~ this mode of life, and were consequently classified at one time as a distinct order, the PTEropopa, and consequently call for special mention. One of the most characteristic features of this group is the foot, which is limited to the anterior portion of the body and consists of a small median portion and two lateral wing-like parapodia arising from the sides of the median portion, and by the rapid flapping of which the 314 INVERTEBRATE MORPHOLOGY. animals are propelled through the water. In their general form much diversity is observable. In accordance with their pelagic habits the majority are more or less transparent; and some, the Gymnosomata, e.g. Pneuwmoderma, Clione, etc., are entirely destitute of a shell, mantle, and, except in Pneumo- . derma and its allies, of a branchia. Others, the Thecosomata, possess these structures, however—the shell in Limacina being spirally coiled, the mantle-cavity situated in front of the Fie. 140.—Hyalea complanata (after Gzcensaur, from HERTWIG). @ = anus. m = mantle. br = branchie. oe = esophagus. c = heart. re = nephridia. G = reproductive organs. » = stomach. h = digestive gland. IIT = pedal ganglion. visceral hump being without a branchia; in Styliola the shell is not coiled, but is cone-shaped and bilaterally symmetrical, the mantle-cavity containing a gill; while in Cymbuliopsis the original shell is replaced by a cartilaginous case formed by the subepidermal tissues of the mantle, and the voluminous mantle-cavity contains no gill. The head of the Gymnoso- mata carries a non-retractile proboscis, at the extremity of which is situated the mouth, and it may furthermore bear in addition to the tentacles usually present peculiar tentacle- like processes, sometimes provided with suckers and perhaps TYPE MOLLUSCA. 815 modifications of portions of the foot, being innervated from the pedal ganglia. In these forms also fringed or simple pro- cesses of the posterior portion of the body occur which serve as respiratory organs, though they are not homologous with the true branchia which in Pneumoderma coexist with them. Suborder Nudibranchia. In the Nudibranchs the visceral hump has undergone elongation parallel with the long axis of the foot, from which it is not distinctly marked off, and an apparent bilateral sym- metry is manifested by the body. This condition, however, is evidently entirely secondary, as is shown by the structure of the heart and nephridium, in which the usual asymmetry is’ well marked. There is no shell, mantle, or ctenidia. Adaptive branchie are, however, frequently developed, as in Pleurophyl- lidia, where they form a series of folds which lie in a groove at the side of the body and recall somewhat the arrange- ment inthe Chitonida, or in Doris, where they surround the anus, which has a dorsal position, and form a circle of pin- nate processes. In the pelagic Phyllirhoé and in the creeping Limapontia, however, there is no trace of respiratory organs. Many forms (Fig. 141), such as vlis, Facellina, and their allies, bear upon the dorsal surface of the body numbers of finger-like processes usually arranged in bunches, and frequently brightly colored. These cerata frequently enclose branches from the intestine which correspond to the digestive gland of other forms, and p, 44), Nuprpran- bear at their extremities a sacin which oyarg OpisrHo- are developed nematocysts. Theseorgans Brancn (olid). are usually richly provided with blood- vessels, and are probably respiratory in function, though the presence of nematocysts renders it probable that they are also protective—an idea which is confirmed by their 316 INVERTEBRATE MORPHOLOGY. usually brilliant coloration. The foot in the pelagic Phyl- lirhoé has entirely disappeared, but is usually elongated and provided with a broad flat surface, in accordance with the creeping habits of the Nudibranchs. Parapodial folds, such as occur in the Tectibranchs, are never developed. Order Pulmonata. The Pulmonates differ from all the other groups of Gas- ‘teropods in that they are, with the exception of a single genus, Onchidium, either terrestrial or aquatic; and in adaptation to this assumed habit certain well-defined changes have occurred. In some genera, more especially the aquatic forms, such as Limnea, Physa, and Planorbis, the visceral hump has its typi- cal Gasteropod development, and is spirally coiled; but in many terrestrial forms, such as Limawx (Fig. 142, A), Arion, and Vaginula, it is low and elon- gated parallel to the long axis of the foot with which it is fused. The mantle is in all forms well developed, but pre- sents the peculiarity that it is fused by its edges to the body- wall except at one point upon the right side, where an open- ing is left by which the other- wise completely-closed mantle- cavity communicates with the exterior and through which air may be taken into the cavity. The position of the mantle- cavity, when not interfered with by secondary changes, is upon the right side of the body and somewhat in front of the visceral hump when this is present. A spirally-coiled shell is present in all forms in which the visceral hump is well developed, as in Limnwa, Physa, Helix (Fig. 142, B), and Planorbis, but in the elongated terrestrial forms a rudimentation of the shell accompanies the diminution of the visceral hump. Thus in Daudebardia, in which only a slight trace of the hump persists, the shell has become Fig. 142.—A, Limax maximus; B, Helix (after Howss). TYPE MOLLUSCA. 317 quite small, though still showing plainly a spiral form ; but in Limacx it is represented only by a partially calcified plate, im- bedded in the roof of the mantle-cavity by the closure over it of a fold of the mantle. In Arion only a few isolated parti- cles of carbonate of lime persist, while in Vaginula and Onchi- dium all trace of it has disappeared. A marked characteristic of the Pulmonata is found in the character of their respiratory organ. A ctenidium is entirely wanting, the only trace of its existence being the occurrence in some of the aquatic forms (Limnea, Physa, etc.), of an os- phradium near the mantle-pore. Its place is taken by the roof of the mantle-cavity, which receives a rich vascular net- work and functions as a lung, the mantle-cavity containing air which can be renewed through the mantle-pore. The heart is situated far back in the mantle-cavity, its auricle lying in front of the ventricle and receiving the blood from the more anteriorly-situated lung, so that the relation of the respiratory organ to the heart is the same as obtains in the Opisthobranchs. Inthe immediate neighborhood of the heart lies the single nephridium, opening into the mantle- cavity or else into the terminal portion of the rectum (Helix), this structure opening on the right side of the body in close proximity to the mantle-pore. As in the Opisthobranchs, the rotation of the mantle-cavity and its organs, as indicated by its position on the side of the body, has not extended as far as in the Prosobranchs, and consequently there is no crossing of the pleuro-visceral con- nectives. The Pulmonates are orthoneurous. The ganglia are present in typical number, and are massed together, as in some Opisthobranchs and Prosobranchs, behind the buccal mass. Special visual organs are invariably present with the struct- ure which has already been described. In some forms they are situated, as in the Prosobranchs, at the bases of the ten- tacles; while in others they are found at the tips of these structures—the Pulmonates being divisible, according to the situation of the eyes, into the Basommatophora, including such forms as Limnea, Physa, Planorbis, and in general the aqua- 318 INVERTEBRATE MORPHOLOGY. tic forms, and the Stylommatophora, which includes the ter- restrial forms, and Onchidium. This last genus in addition to the usual eyes borne upon the tentacles is in some species further provided with a number of eyes situated upon the back and differing from the typical eye in the arrangement of the retinal cells. As has been seen, the optic nerve in typical eyes on entering the eye spreads out in a layer to form the retina, the terminal optic cells being situated on that surface of the retina which is turned towards the light. The dorsal eyes of Onchidium, however, present a somewhat different arrangement, the cells in which the nerve-fibres terminate having their distal ends turned away from the light, which to affect them must pass through the layer of nerve-fibres formed by the spreading out of the optic nerve. Compared with the retine of typical eyes, those of the dorsal eyes of Onchidium are inverted and have assumed an arrangement exceedingly rare in Invertebrates, but typical for the lateral eyes of the Vertebrata. Otocysts are always present, and the tentacles borne by the head are probably tactile in function. In the Stylommatoph- orous Pulmonates there are in some cases (Helix) two pairs of such tentacles, the eyes being situated upon the posterior pair, both pairs furthermore being capable of being invagi- nated for protection into the body-cavity, a peculiarity not presented by the tentacles of the Basommatophora. As stated above, the osphradium is represented in certain aquatic forms, but in the Stylommatophora it has disappeared with the suppression of the ctenidium. The Pulmonata are hermaphrodite, the epithelium of the reproductive gland (Fig. 143, hg) differentiating into both spermatozoa and ova, there being no localization of the for- mation of either one or the other in a special portion of the gland, as happens in some Opisthobranchs. In the Basom- matophora and certain terrestrial Pulmonates, such as Vagi- nula and Onchidium, the common duct (hd) for the spermatozoa and ova divides and passes to the exterior by two distinct and separate apertures. Thus in Limnea the hermaphro- dite duct shortly after leaving the gland divides, and into one of the branches immediately after the division there opens a well-developed albuminiparous gland (al), and it then becomes somewhat folded, forming what is termed the uterus (ut). Beyond this structure the duct, now known as the oviduct (od), receives the duct of a.nidamental gland and dilates into a TYPE MOLLUSCA. 319 large pyriform structure, which tapers somewhat to form a vagina opening to the exterior and receives a duct from the receptaculum seminis. The vas deferens (vd) shortly after its separation from the hermaphrodite duct dilates into a glandular structure, the prostate gland, from which the nar- row duct passes onward to terminate in an enlarged penis- sheath (pe) which contains the muscular protrusible penis and opens to the exterior quite independent of the opening of the vagina. In the majority of the Stylommatophora (Fig. 143), however, the two ducts open into a common atrium so that only one genital orifice occurs, as in some of the Opistho- branchs (see p. 312). Other- wise the arrangement is simi- Jar to what has been de- scribed for the Basommato- phora, except that in some forms, as Helix, one or two Fic. 148.—REPRODUCTIVE ORGANS OF additional accessory struc. Limax maximus (after SimrorH). al = albuminiparous gland. tures are added. Thus the ha i= hemmaalpodiin duct, atrium has communicating hg = hermaphrodite gland. with it a sac which contains ii = ligament. a sharp calcareous rod, the fa = Oxmiuce, pe = penis-sheath. “dart,” which serves as a he stimulus during copulation, ut being plunged into the body od of the other party to the act; % and again just at the point where the vas deferens opens into the penis it has communicating with it an elongated tubular structure, the “flagellum,” which perhaps furnishes the material of which the capsule of the spermatophores is composed. Development and Affinities of the Gasteropods.—The devel- opment of the Gasteropods is made interesting on account of receptaculum seminis. uterus. vas deferens. vesicula seminalis. eu wd de th ton 320 | INVERTEBRATE MORPHOLOGY. the occurrence in the majority of forms of a larva known as the Veliger (Fig. 144) which presents many interesting aftin- ities to the Annelid Trochophore. In the early stages of de- velopment the embryo is strictly bilateral, with the mouth and anus at the extremities of the longitudinal axis. Upon the dor- sal surface posteriorly is a de- pression lined with columuar cells which secrete the larval shell (Sh), and in front of this is an area enclosed by two rows of cells “S bearing stout cilia and forming Fig. 144.—Veicer Larva. the velum (V). This band of F = foot. cilia is preoral (Pro) in position, M = mouth. and in addition to it a second OS er Gye: band of smaller cilia is to be Poo = postoral band of cilia. Pro = preoral band of cilia. found which passes ventrally to Sh = shell. the mouth and constitutes a post- T = tentacle, oral band (Poo), the groove be- V = velum. : tween it and the preoral band being occupied by the adoral cilia. On the ventral surface is found a prominence which represents the foot. In later stages the lateral edges of the velum are drawn out so as to form a broad lobe, sometimes divided into two arms, projecting on each side of the head; the preoral and postoral bands of cilia extending round the margin of the fold, not, however, completely enclosing the velar area, but re- maining open on the dorsal surface. The shell area increases markedly in size, the shell becoming spirally coiled, the vis- ceral hump which develops in the shell area likewise assum- ing the coiled form. At the margins of the shell area a fold appears, the rudiment of the mantle, which gradually increases in size as the shell area extends, and at the same time the anus becomes rotated forwards from its original terminal posi- tion along the right side of the body to a greater or less ex- tent. As these changes progress, the embryo gradually ap- proaches more and more to the adult form, differing from it mainly in the existence of the velum, by means of which it TYPE MOLLUSCA. 321 leads a free-swimming pelagic existence, assuming the adult habit only after a further growth which is accompanied by a reduction of the velum. Such a Veliger larva occurs in the life-history of the majority of the Gasteropoda, though, as might be expected, it undergoes certain modifica- tions more especially in terrestrial forms, though even in these there are ample indications of its existence. Indeed the Veliger is so frequent in its occurrence that the conclusion is almost unavoidable that it has an ances- tral significance and represents in a more or less modified condition a primitive form from which the Mollusca have descended. A comparison of the Veliger with the Annelid Trochophore brings out, as already men- tioned, numerous similarities. These are especially noticeable in the ar- rangement of the ciliary bands, which resemble those of the Trochophore part for part, even to the dorsal break in their continuity. It is difficult to believe that such marked similarities should have been acquired inde- pendently in the larve of two different groups of animals and have become so characteristic, a difficulty rendered all the greater by the occurrence of other points of similarity, such as the development of the mesoderm, in some forms at least, from a pair of mesoblasts situated at the posterior extremity of the blastoccel ; the existence of a thickening of the ectoderm in the centre of the velar area in some forms, corresponding to the apical plate of the Trocophore ; and the occurrence of a larval excretory organ or nephridium in some Veligers which may be compared to the larval ne- _ phridium or head-kidney of the Trochophore. The probable significance of this larval form will be more suitably discussed at the conclusion of this chapter ; it remains to be said here regarding it that the occurrence among the Pteropods of larve with several bands of cilia surrounding the visceral hump is probably to be explained as a secondary adaptation, just as the mesotrochal Annelid larve are probably secondary modifications of a Tro- chophore. As regards the relationships of the various groups of Gasteropods among themselves, there is little doubt but that the Diotocardiate Proso- branchs are, on the whole, the most primitive of all the groups and stand nearest to the Amphineura, and from them the Monotocardia have devel- oped. The Opisthobranchs and Pulmonates are apparently closely re- lated, the latter group having been derived from Tectibranchiate ancestors somewhat more generalized probably than any Opisthobranch now living. The orthoneurous character of the nervous system and the structure of the reproductive system in the two groups indicates their affinity, and it seems probable that the Pulmonates are to be regarded as Opisthobranchs which have accommodated themselves at first to an amphibious life, somewhat similar to that now led by Onchidiwm, and later to one purely terrestrial, at the same time differentiating an organ for aerial respiration. Such an origin would imply that the aquatic species have secondarily taken to fresh water as a habitat, having originally been terrestrial, an idea which on 322 INVERTEBRATE MORPHOLOGY. a priori grounds seems improbable ; but there seems to be no good reason, if the aquatic forms are derived directly from marine ancestors, why their ctenidia should have become replaced by a lung, since in the aquatic Pro- sobranch Paludina the ctenidium is still retained. On the other hand, it may be again mentioned that the terrestrial Prosobranchs such as Cycio- stoma, Acicula, etc., have lost their ctenidium and resemble a Pulmonate in their mode of respiration. III. Crass SCAPHOPODA. The class Scaphopoda contains a small number of closely- related genera of marine Mollusca, Dentalium, Siphonodenta- lium, Cadulus, etc., living imbedded in the sand in depths of from 10 to 100 fathoms and possessing but slight powers of locomotion. They resemble the Gasteropoda in possessing a visceral hump which is relatively enormously elongated but does not undergo a spiral twisting, nor has it fallen over to the right or left side of the body. Consequently the Scaph- opods are bilaterally symmetrical and stand in marked con- trast in this respect to the Gasteropods. The mantle-folds are two in number, arising from the ‘anterior surface of the visceral hump and extending around the body so as to completely enclose it, meeting posteriorly and fusing together, except for a short extent, dorsally and ventrally, and forming thus a tube to the anterior wall of which the body is as it were attached. This tube is open at either end, the ventral opening being somewhat larger than the dorsal one, and the whole is enclosed within a tubular shell (Fig. 145, sh) whose shape corresponds essentially to that of the mantle. From the ventral opening the foot (/) pro- jects to a greater or less extent, being in Dentalium a cylin- drical structure, terminating in a conical process provided with two lateral lobes. The mouth (m) is situated at the extremity of a cylindrical proboscis (not to be confounded with the protrusible proboscis of a Gasteropod) and is surrounded by a number of leaflike tentacles, while at the base of the snout there is upon each side a bunch of long filamentous tentacles (¢) capable of being protruded from the mouth of the shell and of being withdrawn within it. Each tentacle terminates in a spoon-shaped struct- TYPE MOLLUSCA. 323 ure whose concave surface is furnished with ciliated cells and also towards the margin with unicellular glands. These structures have been supposed to represent the ctenidia of the other Mollusca, but this view cannot, in the present condition of our information concerning their structure and devel- opment, be accepted without reser- vation. The mouth opens into a short cesophagus provided with a single chitinous jaw-tooth apparently formed by a fusion of two chitinous masses, and behind this there is a pharynx provided with a radula and opening posteriorly into the some- what U-shaped more or less convolu- ted intestine (¢) which terminates in the anus (a) lying in the mid-ventral line behind the foot. Into the intes- tine at the turn of the U there open the ducts of the digestive gland (), and into the posterior portion of the intestine, the rectum, there open in Dentalium several ducts from a rectal gland which surrounds this portion of the digestive tract and whose sig- nificance is quite obscure. The nervous system presents the majority of the ganglia characteristic of the Gasteropoda, and the pleuro- . visceral connectives do not cross one another. The cerebral ganglia (ce) lie at the base of the proboscis an- terior to the csophagus and have closely associated with them the Fie. 145. — StructurE oF Dentalium (after LeuckaRt). @ = anus. ce = cerebral ganglion. f = foot. z = intestine. ¢ = liver. m = mouth. me = mantle-cavity. pe = pedal ganglion. r” = reproductive organ. rm = right nephridium. sh = shell. t = tentacle. vt = visceral ganglion. pleural ganglia, the cerebro-pedal and pleuro-pedal connectives fusing with one another to pass downwards and forwards to the pedal ganglion (pe) situated in the foot. Posteriorly in the vicinity of the rectum lie the two visceral ganglia (vi) from which long 324 INVERTEBRATE MORPHOLOGY. nerves pass dorsally, but no special parietal ganglia occur. Two pairs of buccal ganglia are also present. Otocysts are present imbedded in the foot in the neighborhood of the pedal ganglia, but no other special organs of sense, unless the bunches of tentacles be considered such, occur. No special respiratory organs are developed, the mantle probably subserving the respiratory function. The heart, a simple invagination of the wall of the pericardial cavity, lies in the posterior region of the body, on the dorsal surface of the intestine. It possesses no auricle, but receives the blood through small slits in its walls. There are no special blood- vessels, but the blood circulates through a series of sinuses traversing the body in various directions. A pair of nephridia occurs in the posterior region of the body, opening to the exterior by a pore on either side of the anus, but a communication with the pericardial cavity is said to be wanting. However this may be, the right nephridium (rn) serves for the exit of the reproductive elements, though the exact method by which these latter make their way into the duct is unknown. Between each nephridial pore and the anus there is a pore which seems to be the opening of a short tube which communicates directly with the schizoccelic sinus surrounding the terminal portion of the intestine and places it in communication with the surrounding water, a peculiar arrangement which recalls the dorsal pores of the oligo- chetous Annelids. The Scaphopods are bisexual, and the reproductive organs, ovaries or testes, are single, consisting of long completely closed sacs with lateral diverticula, lying along the posterior wall of the visceral hump. As already stated, the reproductive elements after the rupture of the wall of the reproductive gland make their way to the exterior through the right nephridium. Development and Affinities of the Scaphopoda.—The larva of Denta- lium, though presenting considerable resemblance to the Trochophore, differs from it nevertheless in several points of detail. It possesses a dis- tinct apical tuft of cilia and the prototroch is present, though represented by three or more circles of cilia-bearing cells. The mantle-folds develop at a relatively early stage as two lateral folds, quite separate along the ventral line, the fusion characteristic of the adult only appearing later. It is this early development of the mantle-lobes and the multiplication of the proto- TYPE MOLLUSCA. 325 troch bands which obscure the Trochophore characters, a still earlier larva presenting greater similarities to the annelid larva. By the earlier writers the Scaphophods were considered more élosely related to the Pelecypoda than to the other Molluscan groups, this relation- ship being indicated more especially by the symmetrical form, the apparent lateral arrangement of the mantle-folds and the absence of eyes. On the other hand, there are a large number of differences between the members of the two groups, as for instance the univalve character of the shell, and especially the occurrence of a radula andjaw. This latter feature suggests Fic. 146.—D1aGRAMs TO SHOW THE ORIGIN OF THE SCAPHOPODS FROM A F%s- surella-like ANCESTOR (after PLATE). ct = ctenidium. go = reproductive organ. Sf = foot. m = mouth. sh = shell. the Gasteropods, and it seems most probable that it is to this group that the Scaphopods should be considered as related. They must, however, be referred to the more primitive Gasteropods, those in which the rotation of the mantle-chamber had not occurred. An elongation of the dorsal hump of a Fissurella-like ancestor unaccompanied by a twisting to one side, as represented in Fig. 146, would bring about a condition from which it does not seem a great step to reach the Scaphopods. Among recent Gasteropods it is with the Diotocardiates that the Scaphopods seem to be most affiliated, and, as we shall later see, itis from the primitive members of this order that the Pelecypods have probably been derived, and thus any similarities which may exist between the Scaphopods and Pelecypods is readily explicable on the basis of a similar ancestry, both groups being derived from Prosobranch-like forms. The absence of a larva corresponding closely to the Gasteropod Veliger would seem to oppose such a view, but it must be remembered that the Veliger is characteristic only of the more highly-differentiated Prosobranchs—such 326 INVERTEBRATE MORPHOLOGY. forms as Patella, for instance, having a larva destitute of some of the more characteristic Veliger features and more closely resembling the Annelid Trochophore and the Scaphopod larva. IV. CLass PELECYPODA. The class Pelecypoda, also known as the Zamellibranchia, contains a number of fresh-water genera, though the majority are marine, and all its members retain the primitive bilateral symmetry of form, no visceral hump being developed. The body is more or less laterally compressed and two large mantle-folds (Fig. 149, m) are developed, arising one on each side a short distance ventrad of the dorsal mid-line and extend- ing downward so as to meet below. They thus enclose a wide space, the mantle-cavity, between their inner surfaces and the body-wall, within which lie the ctenidia (Fig. 148, ct) and the foot (p). Upon the mantle-edge in many forms tentacles, papille, glands, and eyes are developed, and in many cases the edges of the two lobes may fuse more or less completely, openings being, however, left for the entrance and exit of water into the mantle-cavity, and also for the protrusion of the foot. All gradations of fusion are represented: thus in Nucula, Ostrea, etc., there is no fusion whatever; in Unio (Fig. 149) and other forms the posterior edges of the mantle- folds are modified, so that while the edges of the folds are in contact throughout the greater portion of their extent two openings are left, through the uppermost of which, the exha- lent opening (eo), water carrying with it the excreta and the reproductive elements finds an exit, while through the lower one, the inhalent opening (io), fresh water passes in; in the next gradation the point of separation between these two openings, which in Unio was simply formed by the contact of the mantle-edges, becomes permanent by the fusion of these latter parts, and a further stage, seen in Venus for example, is formed by the fusion of the mantle-edges ventral to the branchial opening, a fusion which may extend forward a con- siderable distance. In this last condition there are three openings which place the mantle-cavity in communication with the exterior, one anterior, through which the foot is pro- TYPE MOLLUSCA. 327 truded, and two posterior, the branchial and anal openings. The mantle around these latter frequently becomes prolonged so that two tubes, or siphons as they are termed, are formed, sometimes in contact with one another (Pholas), sometimes quite separate (Venus), sometimes capable of retraction within the shell, sometimes so large as to be incapable of retraction (Mya). Fie. 147.—A, Mya arenaria with the siphons slightly expanded; B, inner sur- face of the right valve of the shell of Mya. aa = impression of anterior ad- pa = impression of posterior ad- ductor muscle. ductor muscle. ¢ = ligament. pl = pallial line. m = mantle edge. 8 = siphon. p = foot. st = siphonal impression. In conformity with the form of the mantle-lobes the shell consists of two similar portions or valves, lying on the sides of the body and united along the dorsal mid-line by a hinge. The hinge is formed by a ligament, as it is termed, which is really a portion of the shell substance, and consists of an external portion continuous with the epidermis of the shell and an internal elastic portion, frequently calcified to a cer- tain extent, and continuous with the middle layer (prismatic layer) of the shell. When at rest the two valves of the shell are kept apart along the ventral line by the elasticity of the hinge-ligament, and it is only by the application of force that 328 INVERTEBRATE MORPHOLOGY. the two valves can be brought together, the ligament being then compressed. The hinge is frequently complicated by the development of tooth-like processes and corresponding sockets so that the two valves may be firmly locked together. Upon the inner surface of the valves are certain impressions produced by the softer parts and of considerable value in systematic conchology. A short distance from the margin of each valve and parallel to it is a distinct line, the pallial impression (Fig. 147, pl), produced by the attachment of the muscle-fibres which bind the mantle-lobes to the shell. In some forms, such as Anodon, this pallial line follows the shell margin throughout its entire course, but in those genera which possess well-developed and retractile siphons it is deeply incurved in the posterior portion of its course. Other markings of the shells are produced by the insertion into them of a number of muscles. -The largest and most import- ant of these are the adductor muscles of the shell (aa), large muscles passing from one valve to the other, by their contrac- tion overcoming the elasticity of the hinge-ligament and clos- ing the shell. In the majority of forms there are two such muscles, situated towards the anterior and posterior portion of the body, but not unfrequently, as in Ostrea and Pecten, but one, corresponding to the posterior adductor of other forms, is present. In the immediate vicinity of the adductor- impressions other smaller muscle-impressions are usually observable, produced by the protractor and retractor muscles of the foot and siphons. Although in the Pelecypod shell the two valves are typically similar and symmetrical, yet in a number of cases a marked dissimilarity is found in their shape. Thus in Ostvea the valve upon which the animal rests, usually the left valve, is large and concave, while the other is smaller and flattened, and a similar relation is found in other forms which be- come temporarily fastened to rocks, ete. Occasionally additional cal- careous plates are added to the usual shell, as in the boring mollusk Pholas, in which three accessory calcareous plates are developed on the dorsal sur- face of the body. In the Ship-worm, or Teredo, which bores extensively into timber and is in some cases exceedingly destructive, the true shell- valves are very small and situated at the anterior end of the body, and the mantle projects backwards far beyond them and secretes a thin calcareous tube which lines the interior of the passages excavated by the animal. A TYPE MOLLUSCA. . 329 similar peculiarity is found in the Aspergillum. Here, too, the true shell- valves are exceedingly small and are united together by and imbedded in a calcareous tube secreted by the mantle, which projects far beyond the shell proper and is fused throughout the greater portion of its extent. The calcareous tube is open behind for the passage of the two siphons, but anteriorly is closed by a perforated plate, the margins of the perfora- tions being sometimes prolonged into tubes which may branch dichoto- mously. The animal lives imbedded in the sand, the posterior ex- tremity of the shell being directed upwards, and seems to have been derived from forms originally possessing a boring habit, such as is seen in Teredo. The foot of the Pelecypoda is as arule very simple. In the most primitive members of the group, such as Nucula (Fig. 151), it is a flat disk-like structure, recalling somewhat the foot of the Gasteropoda, but more usually itis a keel- shaped structure (Fig. 149, p). The modifications in shape which it undergoes are, however, numerous and it may even in some cases be almost absent, as in the Oyster (Ostrea), but special developments, such as epipodia, are never found in connection with it. A “byssus-gland” is a characteristic development of the Pelecypod foot, consisting of a cavity with usually greatly folded walls lying in the tissues of the foot and connected with the exterior by a canal opening on the sole of the foot. By the cells lining the cavity threads of a horny consistency are secreted by means of which the animal is enabled to fasten itself to stones, etc., or even in some cases, as Mytilus, to move about in the absence of a well-developed foot, throwing out byssus filaments, attaching them, and then drawing itself forward towards them. The respiratory organs (Fig. 149, br) of the Pelecypoda consist of a pair of platelike structures situated on each side of the body, and being attached along their dorsal margins hang down between the mantle and the body-wall. Notwithstand- ing their platelike form they are modifications of the plumose ctenidium of the Gasteropods. If the typical bipinnate ctenidium be imagined to be directed parallel to the long axis of the body and the median axis to have fused with the body-wall, so that the two rows of pinne are bent down so as to lie parallel to one another, the simplest form of the Pelecypod ctenidium, such as occurs in Nucula (Fig. 151), will 330 ’ INVERTEBRATE MORPHOLOGY. be obtained. In the majority of forms, however, the arrange- ment is much more complicated than this. Thus in Dytilus it will be found that the various pinne composing each plate are held together by a series of patches of strong cilia 86 poreeteliiga,, GR DdNyy, Cet 1000000009 ppp, Vegaysanvo LTA D Fie. 148.—A, diagrammatic section through Pecten, and B, through Anodon; @, section through gill-lamella of Pecten, and D, of Anodon. au = auricle. ol = outer lamella of outer gill. SF = foot. pe = pericardial cavity. @f = gill-filament. p = pore. wt = inner Jamella of inner gill. 8 = blood-sinus, al = interlamellar junction. sbr = suprabranchial chamber. ne = nephridium. sh = shell. which interlock forming the “ciliated junctions,” and further- more the pinne are at their free ends bent abruptly upon themselves, those of the outer row outwards and those of the inner row inwards, so that each gill-plate is composed of two lamellw (Fig. 148, .1). This condition may be regarded as the next step in the modification, which is continued even further by the permanent union of the outer and inner limbg TYPE MOLLUSCA. 331 of the pinna, or gill-filaments as they may be called, by hollow processes, the “interlamellar junctions” (Fig. 148, C, ‘l). A still greater departure from the primitive condition is found, however, in the greater number of existing Pelecy- pods, consisting of a fusion of all the filaments of each lamella into a plate (Fig. 148, D), small openings (p) only being left here and there between adjacent filaments ; furthermore the inter- lamellar junctions become very well developed, so that the two lamellz of each gill become firmly united together to form a plate, containing in the interior a cavity, the interlamellar space. In addition to these various modifications which lead to the formation of a true lamellate gill, the edge of the external lamella of the outer plate fuses with the inner surface of the mantle, and the internal lamella of the inner plate fuses sim- ilarly with the side of the foot (Fig. 148, 2), and the mantle- cavity thus becomes divided into two chambers. Into the ven- tral chamber the inhalent siphon opens, and the water which enters by it passes through the openings left between the filaments and so reaches the interlamellar spaces which com- municate above with the dorsal or suprabranchial chamber (sbr), whence it passes to the exterior through the exhalent siphon. In the region of the foot the suprabranchial cham- ber is of course divided into two portions, one of which lies on each side of the base of the foot, and each of these is again divided longitudinally into an inner and an outer portion by the line attachment of the gills to what may be considered the roof of the mantle-cavity. Behind the foot the inner cavities of the two sides unite and in some forms open ventrally into the mantle-cavity proper ; in others, however, the inner lamella of the inner gill-plates fuse with one another along the middle line so that a distinct partition, formed by the gills, sepa- rates the suprabranchial chamber from the ventral mantle- chamber throughout its entire length. In a few forms, such as Cuspidaria, the gills become reduced to simple muscular partitions perforated by pores and separating the two cham- bers, practically all indication of the original ctenidium characters having disappeared. The muscular system of the Pelecypoda reaches a some- 332 INVERTEBRATE MORPHOLOGY. what extensive development in connection with the presence of the bivalved shell. The mantle-folds are as a rule some- what richly provided with muscle-fibres especially near the margin; and where siphons are developed some of the fibres are specialized into retractors for these organs. For the closure of the shell-valves, however, more extensive muscular bands are present which seem, like the siphonal retractors, to be special- ized portions of the mantle musculature. Of these shell- adductors there may be one, as in Ostrea and Pecten, or two, as in Anodon (Fig. 149, aa and pa), which pass transversely across the body from one shell-valve to the other, in the form of stout compact muscular bands. In connection with the foot special bands are also developed which function as protrac- tors ( pp), retractors (rp), and elevators arranged in pairs and extending from the inner surfaces of the shell-valves to spread out below in the foot. These various bundles seem to cor- respond to the spindle-muscle of the Gasteropods. The celom presents an arrangement similar to that of other Mollusca, both schizoccelic and enteroccelic portions being distinguishable. To the former portion belong the numerous lacunar spaces which traverse the body and mantle- folds, and to the latter the pericardial cavity (Fig. 149, p) and the cavity of the reproductive glands. The blood-vascular system consists of a heart provided with two lateral auricles and lying in the pericardium. In the majority of forms the ven- tricle (v) seems to be traversed by the terminal portion of the digestive tract, a condition produced by its having folded itself longitudinally around the rectum, and which recalls what occurs in certain Diotocardiate Gasteropods (see p. 305). This arrangement does not, however, obtain in all forms, some of the more primitive (Nucwla, Arca) having the ventricle entirely dorsal to the intestine, as it is in the Amphineura, for example, while in a few others (Ostrea) it has assumed a secondary position ventral to the intestine. From both the anterior (ao) and posterior extremities of the ventricle arteries arise which, after branching a number of times, pour the blood into the schizoccelic lacunar system. Traversing this the venous blood is returned to a longitudinal sinus lying in the middle line of the body just below the pericardium (Fig. TYPE MOLLUSCA. 333 148, B, s), whence the greater portion passes into the compli- cated network of the nephridia and thence to a blood-vessel, the branchial artery, running along the base of the gill of each side. After traversing the gill-filaments it becomes arterial and is returned to the branchial veins which run parallel to the branchial arteries and thence is returned to the auricles of the heart. The digestive tract has a much simpler structure than in the majority of the Mollusca, lacking all trace of a radula and muscular pharynx. On each side of the mouth are two usually triangular plates, the so-called labial palps, the upper- most of which meet above the mouth forming a sort of upper lip, while the lower ones similarly form a lower lip. At the bottom of the space separating the two palps of each side is a groove which, starting at the sides of the mouth, runs back- wards along the sides of the body to the gills. This groove serves for the conduction to the mouth of the particles of food brought into the mantle-cavity by the action of the cilia of the gills, the food of the Pelecypods consisting of diatoms and other minute organisms capable of being captured in this manner. The esophagus opens into a stomach (Fig. 149, s) which receives by numerous openings the secretion of the usually voluminous digestive gland (J), the so-called liver, and passes posteriorly into the intestine (7), which, usually in sev- eral convolutions, lies imbedded in the tissues of the base of the foot. In the wall of the anterior portion of the intestine is a groove, frequently converted into a canal, which may open into the stomach by an independent opening; the epi- thelium of this groove or canal secretes a substance which forms a transparent glass-like rod lying in the canal and pyro- jecting into the lumen of the intestine. The function of this crystalline style, as it is termed, has been the subject of much speculation, the most plausible theory being that the secre- tion serves to surround sharp-edged particles of sand or simi- lar substances, taken into the intestine with food, with a jelly- like coating which will prevent them from injuring the delicate walls of the intestine. Towards its posterior end the intes- tine bends upwards, i.e. dorsally, to a point in front of the heart and then passes directly backwards to terminate in the anus 334 INVERTEBRATE MORPHOLOGY. (a) which opens into the suprabranchial chamber (sbr) in the vicinity of the exhalent siphon. The relations of this rectum to the heart have already been noted (p. 332). The nervous system of the Pelecypoda differs somewhat apparently from that of the Gasteropods, a smaller number LY) i. Uy ; ep) ip SCE Fie. 149.—StRucturgE oF Anodon. @ = anus. np’ = nephridial opening into supra- aa = anterior adductor. branchial chamber. ao = aorta. p = pericardial cavity. br = gill. pa = posterior adductor. cg = cerebral ganglion. pg = pedal ganglion. eo = exhalent orifice of siphon. pp = protractor pedis. Sf = foot. r = reproductive organ. go = genital orifice. rp = retractor pedis. ¢ = intestine. s = stomach. zo = inhalent orifice of siphon. sbr = suprabranchial chamber. t = liver. sh = shell. m = mantle. » = ventricle. ne = nephridium. vt = visceral ganglion. np'= nephridial opening into pericar- dial cavity. of ganglia being discernible. Above the cesophagus a short distance behind the mouth is on either side a well-marked ganglion (Fig. 149, cg) connected with its fellow of the oppo- site side by a transverse commissure. In the more primitive forms (Nucula) two ganglia are found on either side, of which one evidently corresponds to the cerebral and the other to the pleural ganglion of the Gasteropods. Where, therefore, asin the majority of the Pelecypods, buta single ganglion occurs on TYPE MOLLUSCA, 335 each side, it is to be regarded as a cerebro-pleural ganglion. From each of these a pedal connective passes downwards into the-foot to terminate in a paired pedal ganglion (pg), and a second strong connective passes backwards on each side of the base of the foot to terminate in a large ganglion (vi), sit uated below the rectal portion of the intestine and frequently in close proximity to the posterior adductor muscle, and which from its relations is evidently to be regarded as representing both the parietal and the visceral ganglia of the Gasteropods and hence may be termed the viscero-parietal ganglion. The sense-organs are of essentially the same nature as in the Gasteropods. Tactile cells exist scattered over the sur- face of the body, and are especially numerous in certain lo- calities, as upon the siphons when these are present. A pair of osphradia are also present situated above the viscero-parie- tal ganglion close to the insertion of the bases of the gill- plates into the side of the body ; and imbedded in the tissues of the foot, usually in close proximity to the pedal ganglia, though innervated by the cerebro-pleural, are a pair of oto- cysts having the usual structure (see p. 283). In a number of forms paired elevations, evidently of a sensory nature, have been found in the neighborhood of the inner ends of the siphons, or on the sides of the body a little in front of the anus; the function of these is doubtful, though it has been suggested that they are olfactory. Eyes are present in a number of forms and present various degrees of complexity. In some cases a perception of sudden variations in the intensity of light is present, as in the siphons of some forms, without any distinct optic sense-organs being developed. Sensory and pigment cells are present, however, and may be regarded as forming a diffuse optic organ. No eyes occur upon the head, nor are tentacles developed in any of the Pelecypods, but large numbers of eyes are developed upon the edge of the mantle of many forms, such as Pecten and Spondylus. These eyes may be simple depressions of the mantle-margin, the bottom of the depression being lined with pigmented and sensory cells, a cuticle of varying thickness cov- ering this retinal surface. Another form of eye (Fig. 150) also occurs upon tentacular processes which presents an arrange- 336 INVERTEBRATE MORPHOLOGY. ment unusual for Invertebrates. The extremity of the pro- cess 1s occupied by a number of clear transparent cells which serve as a cornea (co) and which are continuous with a zone of pigmented cells (pg) analogous to an iris, and which pass grad- ually over into ordinary ectodermal cells. Upon the inner surface of the cornea is a mass of transparent cells constitut- ing a lens (J), and below this lies the sensory portion of the eye. The optic nerve as it comes towards the eye branches; Vee Y Vy ~ Tie a Ha RE NWS a “GZ Fig. 150.—Eyr or Pecten (modified slightly from Parte). co = cornea. op, op’ = optic nerve. ¢ = lens. pg = pigment-cells. ta = blood-lacuna. rt = retina. tw = tapetum lucidum. one branch (op’), passing to one side of the eye, bends inwards towards the axis of the eye between the retina-cells (rt) and the lens. The sensory portion of the eye consequently is in- verted, the retina-cells being turned away from the light which must pass through the fibres of the optic nerve to reach them. Below the retina and separated from it by a space is a layer of tissue, the tapetum lucidum (tl), which serves as a reflector and gives the metallic lustre which is characteristic of the TYPE MOLLUSCA. 337 eye of Pecten, and below this again comes a pigment-layer (pg). In a small number of forms, e.g. Arca, peculiar compound eyes are also found on the edge of the mantle. They form slight rounded elevations and consist of a number of conical retinal cells, each surrounded by a sheath of six cylindrical pigment-cells. Each of these groups of retinal and pigment cells is known as an ommatidium and is separated from the © adjoining ones by slender intermediate cells, so that on sur- face view the composite character of the eye is very distinct. The nephridia (Fig. 149, ne) of the Pelecypoda are always paired, and each consists of a tube bent upon itself lying im- mediately beneath the pericardial cavity into which one of the limbs opens (np'), while the other communicates with the suprabranchial chamber (np’), and so with the exterior. In the simplest forms the entire extent of both limbs is glandu- lar, but in the majority the limb which opens to the exterior loses its glandular character and surrounds to a certain ex- tent the glandular or proximal limb. In addition to these nephridia, frequently known as the organs of Bojanus, peri- cardial glands are of common occurrence in all but the simplest Pelecypods, and apparently assist the nephridia in their excretory function. They are known also as Keber’s organs and consist either of outpouchings of the anterior portion of the pericardial wall into the space between the two walls of the mantle (Unio, Venus) or of similar evagina- tions of the walls of the auricles into the pericardial cavity (Mytilus), both methods of formation usually being associated. The reproductive organs (Fig. 149, r) are paired, lying usu- ally in the tissue forming the base of the foot, though extend- ing in some cases into the lacunar spaces between the walls of the mantle (Mytilus). They are very richly branched and usually contain in any one individual only ova or sperma- tozoa, as the case may be, though a number of forms are hermaphrodite—such, for example, as the members of the genus Cyclas and some species of the genera Ostrea and Pecten. The ducts which convey the reproductive elements to the exterior open into the nephridia near their proximal ends in Nucula and a few other primitive genera, but more 338 INVERTEBRATE MORPHOLOGY. usually open directly into the suprabranchial chamber quite near the openings of the nephridia (Fig. 149, go) ; conditions connecting these two extremes are to be found, as in Pecten, where the reproductive ducts communicate with the nephridia near their distal ends, and in Cyclas and Ostrea, where both nephridial and reproductive openings are contained in a common groove. No complex accessory structures are de- veloped in connection with the reproductive organs, as in some of the Gasteropods, nor is there an intromittent organ in the male, the ova and spermatozoa being usually extruded to the exterior, where fertilization takes place, or else the ova pass from the suprabranchial chamber into the interlamellar spaces of the gill-plates and are fertilized there. The structure of the gills forms a suitable character for a classification of the Pelecypoda. 1. Order Protobranchia. The gill is a true ctenidium attached by its axis to the roof of the mantle-cavity in its posterior part. In addition to Fie. 151.—Nuecula nucleus For tHE Lert SIDE AFTER THE REMOVAL OF THE Lerr SHELL AND LEFT MANTLE-LOBE (after PELSENEER). aa = anterior adductor. SF = foot. ar = anterior retractor pedis. g = reproductive organ. e = ctenidium. p = labial palp. ep = levator pedis. pa = posterior adductor. pr = posterior retractor. this primitive feature the foot has a creeping surface, the pleural ganglia are not completely united with the cerebral, TYPE MOLLUSCA. 339 and the reproductive ducts communicate with the proximal portion of the nephridia. To this order, which represents the most primitive Pelecypods, belong the genera Nucula (Fig. 151), Yoldia, and others. 2. Order Filibranchia. In this group the gill-filaments have elongated consider- ably and commenced to bend upwards at their ends to form the outer and inner lamellae (Anomia; Mytilus, Modiolaria, the mussels ; Arca). 3. Order Pseudolamellibranchia. In this the gill-filaments show a tendency to become united together and the inner and outer lamelle are united (Pecten, the Scallop; Ostrea, the Oyster). 4. Order Eulamellibranchia, In which the gill-filaments are united to form continuous lamelle. To this order belong the majority of forms, such as the fresh-water mussels Unio and Anodon, the small fresh- water Cyclas, the hard-shell clam or Quahog Venus, the soft- shell clam J/ya, the razor-shell Lnsatella, the boring-shell Pholas, the ship-worm Teredo, and a very large number of other genera. 5. Order Septibranchia. A small group in which the gills are reduced to a muscu- lar perforated septum dividing the suprabranchial chamber from the more ventral mantle-chamber (Cuspidaria), Development and Affinities of the Pelecypoda.—The larva which is characteristic of the Pelecypods resembles a Trochophore very closely in- deed and may be described as a Trochophore provided with a bivalved shell. In certain forms the characteristic ciliary bands may, however, be very much reduced, and in the fresh-water mussels (Unio, Anodon) a remarkable secondary larva known as the Glochidiwm is developed. The ova undergo their development in the interlamellar spaces of the gill plates, and the shell-valves assume a somewhat triangular shape, the apex usually constituting a somewhat curved tooth, while smaller teeth may also be present on the edges. Each mantle-lobe is provided with four tactile papillae on each side, the slightest stimulation of which causes the / 240 INVERTEBRATE MORPHOLOGY. undoubtedly homologous with the nephridia of the Chetop- oda, possessing the same relations. In a few forms (Bonellia, Phascolion) a single nephridium only is present. In addition to these in Achiurus, Thalassema, and allied genera there is a usually much-branched organ on either side lying in the body- cavity and opening into the terminal portion of the intestine. Numerous ciliated funnels occur upon the branches placing the organ in communication with the body-cavity. This so- called “ respiratory tree” (so named from a supposed homol- ogy with the similarly named organs of the Holothuria (q. v.) are probably nephridia, though whether or not they per- form excretory functions is not quite clear. In Priapulus these organs are represented by branched tubes, the branches of which terminate blindly in flame-cells, resembling thus the excretory organs of the Platyhelminths, and in Sipunculus rudiments of these organs have been described as short tubes. The Gephyrea are bisexual, the reproductive organs (ov) forming small digitate, elongate, or ovoid processes arising from the peritoneal lining of the body-cavity; but in some forms (Sipunculus) their products early escape into the ca- lomic cavity, in which they float. The exact manner in which the ova and spermatozoa escape to the exterior has not been definitely ascertained for the majority of forms, but it seems probable that the nephridia serve as the generative ducts. In Priapulus the “respiratory trees” are said to give rise tu the reproductive organs, and also to serve as the reproductive ducts—a behavior which would render exceedingly probable the supposition that they are modified nephridia. Two orders are recognizable in the Gephyrea. 1. Order Echiuree. The Echiurex, sometimes known as the Gephyrea armata, are characterized by the presence on the ventral surface of the body, in front of the openings of the nephridia, of a pair of setee—the genus Echiurus possessing, in addition to these, two circles of sete at the posterior extremity of the body. The anus is terminal in all the known species, and the ter- minal portion of the intestine has opening into it the branched respiratory trees. The anterior end of the body is IYPH ANNELIDA. 241 prolonged into a prostomium of considerable size overlying the mouth; it may be short and broad as in Hchiurus, more elongated and slender as in Thalassema, or deeply bifurcated at the extremity as in Bonellia. Fig. 110.—Bonellia viridis A, ADULT FEMALE OPENED 80 AS TO SHOW THE PRINCIPAL OrG@ANS; B, male much enlarged in proportion to the female (trom Heztwie). ¢ = cloaca. m = muscles. d = rudimentary intestine. 8 = proboscis. g = respiratory trees. s (in Fig. B) = spermatozoa. t = intestine. od = vas deferens. uw = single nephridium which serves also as the oviduct. The last-named genus is interesting as affording an exam- ple of sexual dimorphism, the males being small Turbellarian- like organisms which live parasitically in the anterior portion of the digestive tract of the female, only coming to the exterior for the purpose of copulation. 2. Order Sipunculacea, The Sipunculacea, to which the term Gephyrea inermes is also applied, is an order including forms which lack all traces 342 INVERTEBRATE MORPHOLOGY. organs of prehension. A second portion of the foot lies in the neck region on the ventral surface and has the form of two folds (st), whose edges may be approximated or even fused to form a tube, through which the water contained in the mantle-cavity may be violently expelled, the animal being thereby propelled through the water in a direction of their long dorso-ventral axis. This portion of the foot is termed the funnel and is perhaps equivalent to the epipodium of the Gasteropods. In the majority of forms there projects into the lumen of the funnel a fold (v) arising from the body-wall and termed the valve of the funnel. It is probably homolo- gous with the posterior portion of the foot, the metapodium, of the Gasteropoda, so that all portions of the Gasteropod foot are represented in the Cephalopods, the propodium and mesopodium by the arm-bearing portion, the metapodium by the valve just mentioned, and the epipodium by the funnel. In many forms two depressions are to be found on the outer surface of the funnel, which receive two corresponding eleva- tions on the inner surface of the mantle, which thus becomes locked as it were to the funnel during the expulsion of water from the mantle-cavity. The mantle (m) forms a circular fold surrounding the vis- ceral hump, but upon the anterior surface it has usually only a very slight development, while posteriorly there is a wide space, the mantle-cavity (mc), between it and the body-wall. Within this space lie the ctenidia (ct), and into it the nephri- dia (ne) and the digestive tract open, the excreta being ex- pelled from it during the expulsion of water from the funnel. The mantle-fold is rather thick as a rule, owing to the pres- ence in it of abundant muscle-fibres, by the contraction of which the mantle-cavity may be considerably reduced in size, and frequently there is a special muscular thickening around the edge of the mantle whereby the mouth of the cavity, widely open during the intaking of water, may be firmly ap- pressed upon the funnel during the expulsive act. In the majority of Cephalopods the integument covering the outer surface of the mantle and of the visceral hump is provided with abundant pigment-cells or chromatophores each of which is provided with a muscular arrangement by which its TYPE MOLLUSCA. 343. size may be rapidly diminished, remarkable flushes of color passing over the surface of the living animal. In the Nautilus (Fig. 160) a chambered calcareous shell is present having a rather complicated structure which will be described later, and in one or two other living forms, such as Argonauta and Spirula, an external shell also exists, but in the majority of forms the edges of the mantle close over the shell, which thus becomes internal and takes the form of a plate lying along the anterior surface of the body, being some- times calcareous as in the common Cuttlefish bone of com- merce obtained from the Sepia (Fig. 152, sh), or else chitinous as in the common Squid, ZLoligo. In connection with the mantle there are also frequently developed finlike expansions with a cartilaginous support and provided with muscles, sometimes running along the sides of the visceral hump or in other cases situated near its dorsal extremity. The respiratory organs or ctenidia (Fig. 154, ct) are present as either one or two (Nautilus) pairs of pinnate structures lying in the mantle-cavity. Hach consists of a central axis attached throughout its entire length to the body-wall, forming a rather high ridge upon it and containing near its outer edge two blood-vessels running throughout its entire length. The vessel nearer the summit of the ridge is the branchial vein carrying the aérated blood back to the body, and between it and the branchial artery is a cavity or canal which communicates with the mantle-cavity between each pair of branchial pinne. These structures arise from near the free edge of the axial ridge, but each is bound to the ridge by a thin triangular membrane so that they possess the form of lamelle rather than of pinne. Near the line of attachment of the axial ridge to the body-wall is a cord of cellular tissue richly supplied with blood coming from the branchial artery, forming what is termed a blood-gland, from which the blood is collected into two longitudinal canals which conduct it back to the heart. The ccelom of the Cephalopods is characterized by the great development of the pericardial cavity (Fig. 153, pe), which recalls the condition found in the Amphineura, and may perhaps be better termed the viscero-pericardial cavity. 344 INVERTEBRATE MORPHOLOGY. In the majority of forms it is a large sac occupying a con- siderable portion of the apex of the visceral hump and ex- Fig. 153.—Diacram oF Bopy cavity oF Sepia (after GROBBEN). bh = branchial heart. od = oviduct. f = funnel. Ov = ovary. go = reproductive opening into celom. p = pancreas. HT = heart. pa = partition partially dividing cc- z = intestine. lom. Ib = ink-bag. pe = celom. i = liver. 8 = stomach. id = liver-duct. Sh = shell. me = mantle cavity. u = external opening of nephridium. NV = nephridium. U' = opening of nephridium into cos. lom. tending ventrally a considerable distance, the more ventral portion being incompletely separated from the more capacioug TYPE MOLLUSCA. 345 dorsal portion by a transverse fold or partial partition (pa). In Nautilus itis placed in direct communication with the man- tle-cavity by two minute pores, but in other forms such direct communications do not occur. With a ventral prolongation of the ventral cavity the nephridia (V) communicate, and the walls of the cavity fold themselves around the heart (#) in the usual manner, and in addition also enclose the branchial hearts (6h), becoming thickened and considerably folded in this region so as to form the so-called appendages of the branchial hearts, which are homologous with the pericardial . glands of the Lamellibranchs. The wall of the dorsal cavity is in a similar manner folded over the viscera present in that region, and more or less completely encloses the reproductive organs (ov) so as to form around them a capsule, sometimes with muscular walls, into the cavity of which the reproduc- tive elements are shed when mature. In one group of Ce- phalopods, however, the Octopoda, the arrangement departs slightly from this owing to the reduction of the viscero-peri- cardial cavity to a number of comparatively small canals which constitute the so-called water vascular canals of the older authors. Three of these canals are found on either side of the body, meeting together in a common centre, the ne- phridia communicating with one, another passing to the branchial heart of its side to form the pericardial gland, while the third extends dorsally to dilate with its fellow of the opposite side into the capsule surrounding the reproductive organs (Fig. 158, we). The general relationships of these canals are evidently comparable with those of the viscero- pericardial cavity of the majority of the Cephalopods, but they differ in one very marked peculiarity, ie., the heart is not enclosed within their lumen. The tubelike condition of the cavity is evidently a secondary condition, and the exclu- sion of the heart can be understood as a result of the diminu- tion of the extent of the cavity, when the manner in which it is enclosed, as exemplified by the Solenogastres, is considered. The schizoccelic portion of the ccelom takes the form partly | of lacunar spaces, but partly of blood-vessels with definite walls. To a certain extent the blood system is completely closed, an unusual condition among Mollusca; well-defined 346 INVERTEBRATH MORPHOLOGY. veins return the blood carried by the arteries to various por- tions of the body, definite capillaries connecting the two sets of vessels. A lacunar system also exists, however, so that, while showing a much greater differentiation than the other Mollusca, the Cephalopods yet retain indications of the more primitive arrangement. The heart consists of a tubular ventricle (Fig. 154, »), eS 3 Ei 2 | Ef EE =a e eet ao = anterior aorta. to = lateral vein. ao’ = abdominal aorta. ne = excretory appendage. au = auricle. pg = pericardial gland. bh = branchial heart. © = ventricle. ct = ctenidium. va = abdominal vein. ve = cephalic vein. though in the Octopoda it is transverse, and has opening into it at each side one or two (Nautilus) auricles (au) which re- ceive the blood from the branchiz (ct). Two principal arte- ries arise from the ventricle, a larger one running ventrally (ao), and a smaller one which runs towards the tip of Hye visceral hump and supplies the viscera of that region (ao’). As already stated, these arteries pass into a fine capillary net. work from which the veins arise, sinuses, however, interyey- ing in some cases in the course of the latter, and Possibly some arterial branches may terminate in such sinuses. The TYPE MOLLUSCA. 347 principal venous trunk is the cephalic vein (vc), which lies on the posterior side of the cesophagus, and passing dorsally divides into two branches, the vene cave, with each of which an abdominal vein (va) unites, the conjoined trunk on each side passing into a contractile dilatation, the branchial heart (bh), at the base of the ctenidium of thatside. The ven cave and the abdominal veins are covered by .a much-folded mass of tissue, the venous appendages (ne), which are portions of the nephridia and will be considered in the description of those organs. Mention may also be made here of the peri- cardial glands (pg) attached to the branchial hearts, which have already been described in connection with the viscero- pericardial cavity. Slight variations from the arrangement here described may be found in various forms, of which the most important is that found in Nautilus, in which, in accordance with the presence of two pairs of ctenidia, each vena cava divides into two branches, one passing to each ctenidium. No branchial hearts occur, and, as has been already mentioned, the ventri- cle has opening into it two pairs of auricles instead of the single pair usually present. In the mesodermal tissue of the Cephalopods in various portions of the body there are developed plates and nodules of a consistency resembling cartilage and like it consisting of a hyaline or partly fibrous matrix through which numerous cells with branching processes are scattered. These cartilagi- nous structures resemble the tissue which is developed in the pharynx of the Gasteropods below the radula, but reach a much more extensive development in the Cephalopods, serving as a protection for some of the more important organs, and also as a point d’apput for the various muscles, and therefore constituting a true endoskeleton. In the Nautilus there is but a single cartilage which lies on the posterior surface of the cesophagus, being deeply grooved for the reception of the brain and optic ganglia. In other forms, however, the carti- lages are more numerous. There is a well-developed cephalic cartilage forming a deeply-concave disk perforated by the oesophagus, and partially enclosing the brain, being also ex- panded at the sides and hollowed out so as to form a support 348 , INVERTEBRATE MORPHOLOGY. for the eyes, which are further covered by a pair of plates which project anteriorly and laterally from the anterior margin of the disk. At the base of the arms a brachial cartilage, sometimes united with the cephalic mass, is found which serves for the origin of the brachial musculature, and further- more a nuchal plate is present lying below the anterior sur- face of the body just behind the head. In connection with the infundibulum plates and nodules are developed, the most important of which is the infundibular cartilage on the pos- terior (strictly speaking the ventral) surface of the body in the floor of infundibulum, nodules being found below the de- pressions on the side of the infundibulum and the corre- sponding elevations of the mantle which have already been described as interlocking during the expulsion of water through the funnel. Finally, it may be mentioned that the centre of each fin is occupied by a cartilaginous plate which serves for the origin of the muscles which move the fin. In harmony with the peculiar modification of the foot there is a considerable amount of differentiation of special muscles in the Cephalopods, which pass from cartilage to cartilage or from the shell to the various cartilages. Leaving aside the general musculature of the mantle and of the arms, mention may be made of the three or four strong retractor muscles, which pass from the shell to the cephalic cartilage and are sometimes fused together to form a single strong muscle which serves to retract the head; the collaris, which runs on either side of the neck from the infundibular cartilage to be inserted into the sides of the nuchal cartilage ; and finally the adductors and depressor of the funnel, which pass respectively from the cephalic cartilage and the shell to be inserted into the infundibular cartilage. Considerable varia- tion is to be found in the arrangement of muscles in various forms, but the typical arrangement may be regarded as being somewhat as described. Like the other organs the digestive system presents a con- siderable amount of differentiation. The mouth opens in the centre of the disk which bears the arms or tentaculiferous ‘lobes, and is guarded by two strong chitinous or partly calea- reous (Nautilus) jaws resembling in form the beak of a parrot. TYPE MOLLUSCA. 849 It leads into a muscular pharynx (Fig. 152, b), upon the floor of which lies the characteristic molluscan radula, while into its cavity the ducts of one or two pairs of salivary glands open. Succeeding the pharynx is a tubular cesophagus (@) which in some forms is provided with a lateral diverticulum, the crop, and which terminates below in the large pyriform stomach (s). The intestine leaves the stomach close to the entrance of the cesophagus, and a pouchlike structure, in some forms prolonged into a spiral czcum (ce), is to be found - either communicating with the stomach close to this point or else opening into the proximal portion of the intestine (Nauti- lus). Into this cecum the two ducts from the large digestive glands, or so-called liver, open, their walls being in the ma- jority of cases provided with sacculations arranged in bunches and constituting the pancreas, a structure which in oligo (Fig. 153, p) is imbedded in the thickened walls of the ducts or else, as in Octopus, attached to the digestive gland in the region where its ducts arise. From its origin in the stomach the intestine passes ventrally, the entire tract having thus a V-shaped arrangement, and opens into the mautle-cavity on the summit of a papilla situated a short distance from the dorsal end of the infundibulum. From each side of the anal papilla a fleshy appendage arises, the anal valve, which in some forms may be drawn down so as to completely close the anal opening. In connection with the posterior portion of the digestive tract there is found in all Cephalopods except Nautilus a sac- like gland (Fig. 152, 7) which secretes a dark pigment and is known as the ink-bag, the animal discharging the ink into the surrounding water to conceal its retreat when alarmed. It arises as a saclike diverticulum of the rectum close to its termination and, elongating, becomes differentiated into a duct of considerable length opening into the terminal portion of the rectum and closed by a circular band of muscle-fibres which surround it near its opening. The more or less globu- lar extremity of the diverticulum becomes differentiated into (1) a cavity which serves as a reservoir for the inky secretion manufactured in (2) a special glandular region, traversed by a series of trabecule lined by the secreting cells. 350 INVERTEBRATE MORPHOLOGY. The nervous system of the Cephalopods shows a high degree of concentration, the various ganglia being more or less fused with one another to form a mass surrounding the ceso- phagus just behind the pharyngeal mass. In JVautilus this mass takes the form of two rings surrounding the cesophagus, united in front but widely divergent behind, and in which the various ganglia are but indistinctly indicated, the condition which occurs in Chiton in this respect being recalled. That portion of the ring which lies in front of the esophagus rep- resents the cerebral ganglia; the lateral portions of the more ventral of the two rings found on the pos- terior surface are the pedal ganglia, giving rise to the nerves to the pedal lobes and the infundibulum ; while the more dorsal posterior ring rep- resents the combined visceral, pa- rietal, and pleural ganglia. In other forms the ganglia become more Hie. 168 —Nwadous Canenia perfectly marked off and at the same or (A) Loligo and (B) Octopus time more concentrated. A cerebral (after PetsenesR). ganglion (Fig. 155, c) is always dis- oS eee ee tinguishable, and with it are con- ¢ = cerebral ganglion. Z _ p = pedal ganglion. nected pleuro-parieto-visceral (pl p' = brachial ganglion. and v) and pedal (p and p’) ganglia; op = optic ganglion. the latter, however, are usually divi- 0 = pleuro- parieto - visceral oq into two portions—a more ven- - oe tral mass (p’) which sends branches to the armlike prolongations of the pro- and mesopodium and whichis hence termed the brachial ganglion, and a more dorsal one (p) which supplies the infundibulum and is known as the pedal ganglion proper. A study of a number of different forms shows clearly that the brachial ganglion is merely a separated portion of the pedal, and that the arms are to be con- sidered portions of the foot and are not cephalic appendages. At the sides of the cerebral ganglia there are to be found a pair of large ganglia (op) which stand in relation to the eye and are termed the optic ganglia; they are undoubtedly spe- cializations of the cerebral ganglia, owing their separate exist. LX . eS v Pp’ B TYPE MOLLUSCA. 351 ence to the remarkable development and differentiation of the eye which is found in the majority of the Cephalopods. A sympathetic system of nerves is well developed and con- sists of one or two pairs of buccal ganglia (0) innervating the large pharyngeal mass and united to the cerebral ganglia by connectives and giving rise to a strong nerve which runs dorsally along the cesophagus to end in a large gastric ganglion from which nerves pass to the viscera. Mention should also be made of two other ganglia, the ganglia stellata, which belong to the central system and are situated in the lateral portions of the mantle, being united with the pleuro- | visceral ganglia by strong nerves; they correspond probably with the parietal ganglia of the Gasteropods, sending branches to the tissues of the mantle. The special sense-organs are exceedingly well developed, and especially is this the case with the eyes. In Nautilus, however, the eye (Fig. 156, 4) stands on a much lower grade ee ee Fig. 156.—A, Eye of Nautilus (modified from Hansen); and B, of Loligo. ¢ = cartilage. 2 = lens. co = cornea. nm = nerve-layer. g = layer of ganglion-cells. op = optic nerve and retinal ganglion. iy = iris, pg = pigment-layer. r = layer of rods. of organization than that of the other Cephalopods, con- sisting of a cup lined by a retina composed of several layers and richly supplied with nerves. The outermost layer consists of rodlike bodies (r) below which is a layer of pigment (pg), below which again lies a layer of ganglion-cells (g). No refractive structures are, however, present, the cavity of the cup communicating freely with the 352 INVERTEBRATE MORPHOLOGY. external water through a small circular opening in the front flattened wall of the cup. The eye is a camera constructed on the “ pin-hole” type, the image being defined by the exclusion of all the more divergent rays of light which pass in from the object towards the eye. In the remaining forms the eyes (Fig. 156, B) are large globes imbedded in an orbit formed by the lateral portions of the cephalic cartilage and its processes. The retinal por- tion of the eye closely resembles that of Vautilus, consisting of an external layer of rods (7) bounded beneath by a pigment- layer (pg) beneath which is a nerve-layer (n) enclosed within a connective tissue-sheath in which cartilage (c) is developed. The optic nerve dilates into a retinal ganglion before being distributed to the retina, the rods of which, it will be noted, are turned towards the source of light. The eye-cup differs, however, from that of Nautilus in being completely closed, and the cells which form the outer and inner layers of the outer wall of the cup secrete chitinous material which acts as a lens (J), forming a powerful biconvex condenser. In ad- dition to this the eye is further complicated by the develop- ment of a series of folds from the skin in its neighborhood. One such fold is developed from the front wall of the optic sac, surrounding the region occupied by the lens and form- ing an iris (ir), the circular opening in its centre correspond- ing to the pupil of the Vertebrate eye. A second likewise forms nearer the base of the optic sac and, growing forward, may enclose a space bounded behind by the iris and lens, resembling the anterior chamber of the Vertebrate eye, the portion of the fold immediately in front of the lens becoming transparent and forming a cornea (co). The anterior chamber is not, however, closed in all forms, but remains in communi- cation with the exterior by an aperture produced by a failure of the edges of the fold to unite completely. Finally, in some forms other folds, which from analogy have been termed eye- lids, develop. The resemblance of such an eye to that found in the Vertebrates is ex- ceedingly striking, but a detailed study of the structure and mode of origin of the various parts demonstrates conclusively that the similarities are ana- logical only and not homological. One of the most important of the differ- TYPE MOLLUSCA. 853 ences is found in the arrangement of the layers of the retina, the rods being turned towards the light as is usual in Invertebrate eyes, while in the Verte- brates they are reversed, the nerve-fibre layer lying above them, the light of necessity penetrating it before reaching the rods. The structure of the lens is again very different, being cellular and formed as an invagination of the ectoderm in the Vertebrates, while in the Cephalopods it is a cuticular structure. These are fundamental differences and may suffice to show what is meant, but many other dissimilarities may readily be found. Otocysts also occur imbedded in a capsule forming part of the cephalic cartilage. They have the characteristic Mol- luscan form and receive a large nerve arising from the cere- bral ganglion. Osphradia occur only in Nawzlus, where they form a pair of sensory papille one of which lies at the base of each of the more ventral ctenidia. Other Cephalopods, though lacking these structures, are yet provided with special olfactory organs in the form of a pair of fosse or grooves lined by ciliated and sensory cells and situated above the eye in the position occupied by the eye-tentacles of Nautilus (see p. 358), from which they may possibly have been derived. The excretory organs consist of two comparatively large sac-like nephridia except in Navtilus, in which, in harmony with the number of ctenidia and auricles, there are four. In Octopus and the other members of the group Octopoda the two nephridia are quite separate from one another, but in the group Decapoda, to which Loligo and Sepia belong, they are placed in communication with one another by transverse canals one of which may be produced dorsally into a large sac occupying a great portion of the anterior region of the body. The vene cave and branchial veins lie between the walls of this anterior sac and the paired posterior nephridia, and along the course of the veins the walls of the excretory sacs are richly folded (Fig. 154, ne), constituting the venous appendages, for a long time considered to be the excretory organs in their entirety. The posterior paired nephridia present the same relations to the exterior and to the entero- coel which exist in other Mollusca, opening by two distinct apertures into the mantle-cavity on the one hand, and on the other communicating with the large enteroccel which has been shown to be the equivalent of the pericardial cavity of the Gasteropods and Pelecypods. 854 INVERTEBRATE MORPHOLOGY. The reproductive organs are situated near the dorsal ex- tremity of the visceral hump. The sexes are always sepa- rated in different individuals, there being occasionally well- marked differences between the two sexes of the same species, as in Argonauta, the female of which possesses a well-devel- oped shell which the male lacks. The ovary (Fig. 157, ov) is single and is enclosed in a capsule (c) formed by the walls of the enteroccel or viscero-pericardial cavity, into which the organ seems to project, though morphologically it is entirely outside it. The germ-producing region is nearly always the anterior surface of the organ, the stalked ova surrounded by their follicle-cells projecting forward into the capsule, into the cavity of which, ie. into the viscero-pericardial cavity, they burst when mature. In some forms the germ-producing sur- CEN ov i Fig. 157.—FEMALE REPRODUCTIVE ORGANS OF T'remoctopus violaceus (after Brock). ¢ = capsule. ov = ovary. od = oviduct. rs = seminal receptacle. og = oviducal gland. we = coelomic canal. face becomes more highly folded and more or less dendritic in form, the area over which the ova are formed becoming thus much greater. The ova reach the exterior after they have passed into the cavity of the capsule by means of one or two complicated ducts (od) opening into the mantle-cavity. In Nautilus two ducts are present, that of the left side, however, being non-functional, and in the Octopoda and some Decapods, such as Ommastrephes, both ducts are present. In other forms but a single duct persists, which, contrary to what occurs in TYPE. MOLLUSCA. 355 Nautilus, is that of the left side. The oviduct opens into the mantle-cavity at the extremity of a well-marked papilla, its terminal portion being richly supplied with glands, and in addition in some forms two small pear-shaped glands are attached to it in this region. In connection with the female duets:there should be mentioned a pair of glands which take part in the formation of the investments of the ova, but which open quite separate from the oviduct into the mantle-cavity. These are the nidamental glands which are present in the majority of forms, excluding the Octopoda, and consist of a pair of large pyriform structures lying on the posterior sur- face of the visceral mass; in connection with them in some forms are developed accessory nidamental glands consisting of a central and two lateral portions whose ducts open into the mantle-cavity in close proximity to those of the nidamen- tal glands proper. As stated, the gelatinous mass within which the ova are imbedded is probably manufactured by these glands. The testis in its general relations resembles the ovary, being single and enclosed in a capsule which isa portion of the viscero-pericardial cavity. The organ is attached to the wall of the capsule by a thin band of tissue and is in most cases almost completely surrounded by the capsule, into the cavity of which the spermatozoa are shed when mature. From the wall of the capsule the vas deferens arises and is usually a single tube opening upon the left side of the body into the mantle-cavity. In Nautilus there are, as in the female, two ducts, the right, however, being functionless, but in other forms a paired arrangement is very rare. The proximal por- tion of the duct is a coiled vas deferens, which opens into a thick-walled glandular seminal vesicle which on its part by means of a narrow duct passes into a saclike structure known as Needham’s pouch which finally passes into the muscular penis. In most forms the duct connecting the seminal vesicle with Needham’s pouch receives the secretion of a special gland known as the prostate. ; The majority of the accessory structures connected with the male ducts are concerned in the formation of cases or spermatophores in which a number of spermatozoa are en- 356 INVERTEBRATE MORPHOLOG ¥. closed. Such cases are cylindrical structures with a double wall, and are provided at one extremity with a somewhat complicated apparatus for the ejection of the spermatozoa. The exact method of their formation is not understood, but apparently the seminal vesicles and the prostate play an im- portant part in the process, the Needham’s pouches being a reservoir in which they may be stored up until required for fertilization. Since the genital capsule is a portion of the viscero-pericardial cavity, and the reproductive ducts are continuations of its walls, these structures must also be regarded as prolongations of the enteroccel; and indeed second- ary communications may exist between them and the viscero-pericardial cavity proper. The genital capsule is not completely separated off from the rest of the enteroccel, so that it might be possible for the reproductive elements to pass from its cavity into the viscero-pericardial cavity proper, and so to the exterior through the nephridia, though this method of exit does not seem to be made use of. A remarkable modification of one of the armlike processes of the foot occurs in the males of certain species in connection with reproduction. The arm—in Tremoctopus and Philonewxis the third arm of the right side of the body counting from the anterior mid line, in Argonauta (Fig. 158) Fra. 158.—MAae or Argonauta with HecrocotytizeD ARM (after H. Mutier from Hatscuex). A = arm still enclosed within a membranous sac. B = arm freed from the sac. the third of the left side—is at first enclosed within a sac, by the bursting of which it becomes free, the walls of the sac being reflected back so as to form a pouch which in some unexplained manner receives a TYPE MOLLUSCA. 357 spermatophore. The terminal portion of the arm, which is traversed throughout its entire length by a canal, is developed into a long terminal filament through which the spermatozoa may pass. During copulation the arm is probably thrown off and passes into the mantle-cavity of the female, the manner in which the spermatozoa reach the ova being, however, not yet understood. When first discovered in the mantle-cavity of a female the arm was regarded as a parasitic worm, and the name Hectocotylus was applied to it—a term which is still retained on account of its convenience. In other genera of Cephalopods one arm is generally peculiarly modified in the male—in the Decapoda usually the fourth of the left side and in the Octopoda usually the third of the right side, though frequent exceptions are found. This arm is termed the hectocotylized arm, though it is doubtful whether it takes any part in copulation. As will be seen from the above description the genus Nautilus differs in many important particulars from the re- maining genera of Cephalopods, and the class is therefore divided into two orders. 1. Order Tetrabranchia. This order, of which the genus Nautilus (Fig. 159) is the sole living representative, was in former periods of the earth’s history the dominant group of the Cephalopods—the Ortho- cerites of the Paleozoic and the Ammonites of the Mesozoic being extinct members of it. It is characterized by its mem- bers possessing four ctenidia, four auricles to the heart, and four nephridia; and in addition there may be mentioned, as further peculiarities, the presence of paired reproductive ducts, of which the right one alone is functional, and also of direct communication of the viscero-pericardial cavity with the ex- terior by two pores, and by the occurrence of a single pair of osphradia. For a more detailed account of the peculiarities of Nautilus the preceding general description may be con- sulted. It remains to discuss here the shell and the structure of the foot-lobes—structures which, with the other characters mentioned, serve to distinguish Nautilus from all its living congeners. The shell is voluminous, coiled, and calcareous, its cavity being divided by a series of transverse partitions into a num- ber of chambers, in the last—that is to say, the youngest—of which the animal lives, while the remaining ones are filled with 358 INVERTEBRATE MORPHOLOGY. gas. The centre of each partition is perforated, and: through the opening there extends to the tip of the shell a prolongation of the body of the animal, termed the sipuncle. The foot of Nautilus, or at least that portion of it which fuses with the head, has already been described as forming a number of tentaculiferous lobes. These lobes are arranged in the female in two series—one ventral, consisting of three Fria. 159.—Nautilus pompilius, —FEMALE. WITH THE SHELL SECTIONED LonGI- TUDINALLY TO SHOW ITS INTERNAL STRUCTURE (after Leunis from HERTWIG). 1 = mantle. 7 = nidamental gland. 2 = dorsal lobe of mantle. 8 = shell-muscle. 3 = tentacles. 9 = terminal chamber of shell. 4 = head-cap. 10 = partitions between the various 5 = eye. chambers. 6 = funnel. 11 = sipuncle. lobes which immediately abut upon the mouth, and a more dorsal ringlike lobe the anterior portion of which is de- veloped into a hood (4) which arches over and protects the re- tracted tentacles. Around the margins of both the ventral and dorsal lobes are arranged the tentacles, each of which is filiform and capable of being withdrawn into the basal por- tion, which thus serves as a sheath. In addition to these tentacles two other tentacles are found in close proximity to the eye, one being on its ventral side and the other on its dorsal. In the male the arrangement is very similar, except that the median lobe of the ventral series is transformed into a lamellated structure and does not bear tentacles, while a portion of each of the lateral lobes of the inner series is sepa. TYPE MOLLUSOA. 359 rated from the rest of the lobe—that of the left side becoming modified into a conical structure, lamellated at the extremity and destitute of tentacles, forming what is termed the spadix, probably homologous with the hectocotylized arm of the male Octopods and Decapods. 2. Order Dibranchia. The members of this order, which includes the majority . of living Cephalopods, possess but a single pair of ctenidia, nephridia, and auricles, and lack ; the direct communication of the viscero-pericardial cavity with the exterior as well as the osphradia which occur in Nautilus. The portion of the foot which is fused with the head is drawn out into a number of arms provided with suckers, which seem to represent the tentacles and their sheaths found in Nautilus. The suckers are very numerous and may bef arranged in from one to four rows Re arms, the margin of each sucker ee being in some forms strengthened | by a horny ring, which may be toothed. The number of the arms varies, being either eight or ten; and, since, other struc- tural differences are associated with this difference, the order may be divided into two suborders Fre. 160.—Lolégo pallida, Dorsat —the Octopeda with eight arms, ‘VIEW (after Emerton from VerRiLt). including the genera Octopus, Tremoctopus, and > i Gn \ iN Fig. 194.—MerGanopa-staGE oF Cancer irroratus (after EMERTON from VERRILL). i i ) HW iN | | lipeds are either entirely wanting or very rudimentary. Dur- ing successive ecdyses the missing appendages are gradually developed, though the actual transformation of the Phyllosoma into the youngest Scyllarus or Palinurus stage (which is de- cidedly smaller than the oldest Phyllosoma) has not yet been observed. The change from the Mysis stage to the adult is usually gradual, and no specially definite larval forms are to be found as a rule among the Macrura. In the Brachyura, however, 422 INVERTEBRATE MORPHOLOGY. the Metazoéa becomes transformed into a well-marked form, the Megalopa (Fig. 194), so called trom the usually large size of the cephalothorax. It resembles closely a Macruran, differing only in the abdomen being relatively small, and becomes converted into the adult form by the doubling of the abdomen beneath the thorax. A Megalopa stage occurs also in the Hermit-crabs, but is not so well marked off from the young fully-formed animals as in the Brachyura. Affinities of the Crustacea.—The relationships of the higher groups of the Malacostraca to one another are clearly shown by their larval forms, the Megalopa showing the origin of the Brachyura from Macruran forms, and the Mysis stage that of the latter from Schizopod ancestors. When attempts are made to go still further difficulties stand in the way. As regards the Stomatopoda it is to be noted that they pass through a stage, the Hrichthus, in which the thoracic appendages which are present are biramous, and it seems probable that both they and the Cumacea are re- ferable back to Schizopod ancestors. The Arthrostraca, on the other hand, are probably traceable to Cumacealike ancestors, while the Leptostraca represent more nearly the Entomostracan ancestors than any other group, though widely differentiated from them in certain particulars. It is even still more difficult to trace out relationships of the various Entomostracan orders, but it seems fairly clear that Phyllopodan forms such as Apus are to be considered as representing more nearly than any others the primitive Crustacea. As regards the affinities with other groups very interesting questions arise, two possibilities seeming to be open. According to one the Crustacea have been derived directly from segmented Annelids, through forms repre- sented in a modified condition to-day by Apus. The lobed appendage of Apus is a moditied parapodium, and the segmentation of the body has been inherited. What then as to the Nauplius? According to this view it has practically no ancestral significance, or at best can be considered only as representing a Trochophore larva highly modified and with many adult characters thrown back upon it. This latter idea does not seem, however, to agree with the facts, since the Trochophore is an unsegmented structure and can be comparable only to the prostomial and first appendage-bearing segments of the Nauplius. In other words, the Nauplius is comparable, if comparable at all, to a Trochophore p/is certain additional segments, It has recently been suggested that possibly the Nauplius may represent not the Trochophore but the larval Annelid with three parapodia, which, as indicated (p. 215), is a well-marked stage in the development of many Polychaeta. The number of segments is apparently similar in the two forms, and the idea is plausible. If, however, in all Crustacea a ganglion, representing a segment, intervenes between the archicerebral ganglia and the antennulary (see p. 378), then the Nanplius has potentially one seg- TYPE CRUSTACEA. 423 ment more than the Annelid larva and the comparison will not hold. If the direct Annelid origin is to be accepted, it seems most satisfactory at present to regard the Nauplius as a secondarily acquired larval stage without any ancestral significance. Another suggestion has, however, been made which gives the Nauplius a significance and traces the Crustacea back to unsegmented ancestors. It is to the effect that the Nauplius can be referred to Rotiferlike ancestors, the remarkable Hexarthra with its six processes being supposed to indi- cate the line of descent. It is exceedingly doubtful, however, whether this similarity can be regarded as anything more than a superficial one TYPE CRUSTACEA. I. Class ENToMostraca.—Number of segments varies ; abdomen without appendages ; larva a Nauplius. 1. Order Phyllopoda.Number of segments variable; appendages with branchiz. 1. Suborder Branchiopoda.—Body plainly segmented and seg- ments of thorax more numerous than six. Apus, Branchipus, Estheria, Limnadia, Limnetis. 2. Suborder Cladocera.—Body indistinctly segmented ; with bi- valved shell; four to six thoracic appendages. Daphnia, Moina, Sida, Evadne, Polyphemus. 2. Order Ostracoda.—With bivalved shell; body indistinctly seg- mented ; two thoracic appendages. Cypris, Cythere, Cypri- dina, Halocypris. 3. Order Copepoda.—Without shell; five pairs of thoracic limbs; many forms parasitic and degenerate. 1. Suborder Hucopepoda,—First thoracic segment only fused with head; abdomen cylindrical and segmented except in highly degenerated forms. Cyclops, Canthocamptus, Harpacticus, Calanus, Cetochilus (free-swimming) ; Notodelphys (commen- salistic) ; Coryceus, Sapphirina, Ergasilus, Caligus, Panda- rus (partly parasitic) ; Philichthys, Penelia, Lernea, Chon- dracanthus, Achtheres, Anchorella (parasitic). 2. Suborder Branchiura.—All thoracic segments fused with head ; abdomen small and lamellar, partly parasitic. Argulus. 4. Order Cirrhipedia.—Sessile or parasitic; segmentation indis- tinct ; six pairs of thoracic appendages ; pass through Cypris stage. Lepas, Scalpellum, Ibla, Balanus (sessile) ; Alcippe, Cryptophialus (boring); Proteolepas, Sacculina, Laura, Dendrogaster (parasitic). II. Class MaLacostraca.—Number of segments constant ; thoracic seg- ments eight, abdominal seven or eight. 1. Subclass Leptostraca.—With bivalved shell; abdomen with eight segments. NVebalia. 424 INVERTEBRATE MORPHOLOGY. 2, Subclass Thoracostraca.—With carapace covering the whole or a part of the thorax ; abdominal segments seven. 1. Order Schizopoda.—Thorax completely covered ; thoracic append- ages biramous. Mysis, Euphausia, Siriella. 2. Order Cumacea.—Last four or five thoracic segments not covered by the carapace; eyes sessile or rudimentary. Diastylis, Campylaspis. 3. Order Stomatopoda.—Last three or four thoracic segments not covered by the carapace; eyes stalked; five maxillipeds. Squilla, Lystosquilla, Gonodactylus. 4, Order Decapoda. — Thorax completely covered; five posterior appendages uniramous and three maxillipeds; otocysts in antennules. 1. Suborder Macruwra—Abdomen usually well developed. Ser- gestes, Lucifer, Peneus, Palemonetes, Alpheus, Cambarus, Homarus, Eupagurus, Birgus, Hippa. 2. Suborder Brachyura.—Abdomen small and concealed beneath cephalothorax more or less perfectly. Porcellana, Libinia, Callinectes, Platyonychus, Cancer, Pinnotheres, Ocypoda, Gelasimus, Gecarcinus. 8. Subclass Arthrostraca.—No shell or carapace as a rule; with seven (or six) walking-limbs ; eyes sessile. 1. Order Anisopoda,—Carapace slightly developed ; first two tho- racic segments fused with head ; branchie on anterior maxille. A seudes, Tanais, Anthura. 2. Order Zsopoda.— No carapace ; first thoracic segment fused with head ; body flattened dorso-ventrally ; branchise on abdomi- nal appendages. Asellus, Oniscus, Porcellio, Armadillid- tum, Idotea, Spheroma (free); Cymothoa, Aga, Bopyrus, Entoniscus (parasitic). 8. Order Amphipoda.—No carapace, first thoracic segment fused with head; body flattened laterally ; branchiw on thoracic appendages. Gammarus, Orchestia, Corophium, Cyamus, Caprella. LITERATURE. GENERAL. H. Milne-Edwards. Histoire Naturelle des Crustacés. Paris, 1834-1840. F. Miller. Fir Darwin. Leipzig, 1864. H. Gerstaecker. Arthropoda. Bronn’s Klassen und Ordnungen des Thier- reichs, Bd. v. Abth. 1. 1866- (not yet completed). C. Claus. Untersuchungen zur Hiforschung der genealogischen Grundlage des Crustaceen-Systems. Wien, 1876. C.Grobben. Die Antennendriise der Crustaceen. Arbeiten a. d. Zoolog. Inst. Wien, 111, 1880. TYPE CRUSTACEA. 425 J.E. V. Boas. Studien iiber die Verwandtschaftsbezichungen der Malakostraken. Morpholog. Jahrbuch, vii, 1883. J. Carriére. Die Sehorgane der Thiere, vergleichend-anatomisch dargestelit. Munich and Leipzig, 1885. : C. Claus. Neue Beitrige zur Morphologie der Crustaceen. Arbeiten a. d. Zoolog. Inst. Wien, vi, 1886. S. Watase. On the Morphology of the Compound Hyes of Arthropoda. Studies from the Biolog. Laboratory, Johns Hopkins Univ., rv, 1890. G. H. Parker. The Compound Hyes in Crustaceans. Bulletin Museum of Comp. Zoology, Xx, 1891. C. Grobben. Zur Kenntniss des Stambaumes und des Systems der Crustaceen. Sitzungsber. Akad. wissensch. Wien, c1, 1892. PHYLLOPODA. A. S. Packard. A Monograph of North American Phyllopod Crustacea. Twelfth Annual Report U. 8. Geolog. Survey. Washington, 1883. CG. L. Herrick. A Final Report on the Crustacea of Minnesota. Twelfth An- nual Report of the Geolog. and Natural History Survey of Minnesota. Minneapolis, 1884. C. Claus. Zur Kenntniss des Baues und der Hntwickiung von Branchipus stag- nalis und Apus cancriformis. Abhandl. k. Akad. wissensch. Gottingen, xvin, 1873. A. Weismann. Ueber Bau und Lebenserscheinungen von Leptodora hyalina. Zeitschr. fiir wissensch. Zoologie, xxrv, 1874. Beitrége zur Naturgeschichte der Daphnoiden. Zeitschr. fiir wissensch. Zoologie, XXvI-xxxul, 1876-1879. E. Ray Lankester. Observations and Reflections on the Appendages and on the Nervous System of Apus cancriformis. Quarterly Journ. Microscop. Science, xx1, 1881. CG. Claus. Untersuchungen tiber die Organisation und Entwicklung von Branchipus und Artemia. Arbeiten a. d. Zoolog. Inst. Wien, v1, 1886, P. Pelseneer. Observations on the Nervous System of Apus. Quarterly Journ. of Microscop. Science, xxv, 1885. P. Samassa. Untersuchungen tiber das centrale Nervensystem der Crustaceen. Archiv fir mikr. Anat., xxxvuur, 1891. G. M. Bernard. The Apodide, a Morphological Study. London, 1892. C.Grobben. Die Entwickiungsgeschichte der Moina rectirostris, ete. Arbeiten a. d. Zool. Inst. Wien, 11, 1879. P, Samassa. Die Keimblitterbildung bet den Cladoceren. I. Moina rectirostris Brady. Archiv. fiir mikrosk. Anat., x1, 1893. OSTRACODA. G. E. Brady. A Monograph of the Recenc British Ostracoda. Transactions Linnean Soc. London, xxv. C. Claus. Ueber die Organisation der Cypridinen. Zeitschr. fiir wissensch. Zoologie, xv, 1865. Beitrdge zur Kenntniss der siisswasser Ostracoden. Arbeiten a. d. zoolog. Inst. Wien, xu, 1892. 426 INVERTEBRATE MORPHOLOGY. COPEPODA. CG. L. Herrick, A Final Report on the Crustacea of Minnesota, Twelfth An- nual Report of the Geol. and Nat. Hist. Survey of Minnesota. Min- neapolis, 1884. W. Giesbrecht. Pelagische Copepoden. Fauna und Flora des Golfes von Neapel. Monogr., xx, 1892. C. Claus. Ueber die Entwicklung, Organisation und systematische Stellung der Arguliden. Zeitschr. fir wissensch. Zoologie, xxv, 1875. C. Heider. Die Gattung Lernanthropus. Arbeiten a. d. zoolog. Inst. Wien, 1, 1879. ; M. Hartog. Zhe Morphology of Cyclops and the Relations of the Copepoda. Trans. Linnzan Soc. London, 2d Series, v, 1888. ; F. Leydig. Ueber Arguilus foliuceus. Archiv. fir mikr. Anat., xxxu11, 1889. C. Grobben. Die Entwicklungsgeschichte von Cetochilus septentrionalis, Goodsir. Arbeiten a. d. zoolog Inst. Wien, mr, 1881. CIRRHIPEDIA. C. Darwin. A Monograph of the Subclass Cirrhipedia. London, 1851-1854. H. de Lucaze-Duthiers. Histoire de la Laura Gerardie. Archives de Zool. expér. et. gen., vu, 1880. P. P. C. Hoek. Jteport on the Cirrhipedia. Scientific Results of Voyage of H.M.S. Challenger. Zool., vir, 1883; x, 1884. Yves Delage. Evolution de la Sacculine. Archives de Zool. expér. et. gén., 2me sér., 11, 1884. LEPTOSTRACA. C. Claus. Ueber den Organismus der Nebaliiden, und die systerratischen Stellung der Leptostraken. Arbeiten a. d. Zool. Inst. Wien, vin, 1888. SCHIZOPODA. G. 0. Sars. Report on the Schizopoda. Scientific Results of the Voyage of H.M.S. Challenger. x11, 1885. CUMACEA. A. Dohrn. Ueber Baw und Entwicklung der Cumaceen. Jenaische Zeitschr. fir Naturwiss., v, 1870. G. 0. Sars. Report on the Cumuacea. Scientific Results of the Voyage of H.M.S. Challenger. x1x, 1887. STOMATOPODA. C. Claus. Die Kreislaufsorgane und Blutbewegung der Stomatopoden. Arbeiten a. d. Zoolog. Inst. Wien, v, 1883. W. K. Brooks. Report on the Stomatopoda. Scientific Results of the Voyage of H.M.S. Challenger. xvi, 1886. DECAPODA. 8. I. Smith. Various Papers in Trans. Connecticut Academy and in Reports. of the U §. Commissioner of Fish and Fisheries. TYPE CRUSTACEA. AQT V. Hensen. Studien tiber das Gehororgan der Decapoden. Zeitschr. fir wissensch. Zool., x11, 1868. C. Grobben. Bettrdge zur Kenntniss der mdnnlichen Geschlechtsorgane der Decapoden. Arbeiten a. d. Zoolog. Inst. Wien, 1, 1878. T. H. Huxley. The Crayfish. London and New York, 1881. W. K. Brooks. Lucifer: a Study in Morphology. Philosoph. Trans. Royal Soc. London, cLxx1t, 1882. H. Reichenbach. Studien zur Hntwicklungsgeschichte des Flusskrebses. Ab- handl. Senckenburg. Gesellsch. Frankfurt, xrv., 1886. W. F. BR. Weldon. Celom and Nephridia of Palemon serratus. Journal Marine Biolog. Assoc., 1, 1889. G. H. Parker. The Histology and Development of the Hye in the Lobster. Bulletin Museum Comp. Zoolog., xx, 1890. P. Marchal. Recherches anatomiques et physiologiques sur Vappareil excréteur des Crustacés décapodes. Archives de Zool. expér. et gén., 2me sér., x, 1892. W. XK. Brooks and F. H. Herrick. The Embryology and Metamorphosis of the Macroura. Proc. U. 8. National Acad., v, 1892. E. J. Allen. Nephridia and Body-cavity of some Decapod Crustacea. Quarterly Journ. Microscop. Science, xxxtv, 1893. ARTHROSTRACA. 0. Harger. Report on the Marine Isopoda of New England and Adjacent Waters. Report of the U. 8. Commissioner of Fish and Fisheries for 1878. A. Delle Valle. Gammarini del Golfo di Napoli. Fauna und Flora des Golfes von Neapel. Monogr., xx, 1893. P. Mayer. Caprelliden. Fauna und Flora des Golfes von Neapel. Monogr., vi and xvi, 1882, 1890. 0. Nebeski. Bedtiriige zur Kenntniss der Amphipoden der Adria. Arbeiten a. d. Zoolog. Inst. Wien, 111, 1890. R. Kossmann. Studien tiber Bopyriden. Zeitschr. fiir wissensch. Zoologie, xxxv, 1881; Mitth. a. d. Zoolog. Station zu Neapel, 1, 1882. Yves Delage. Contribution a Vétude de Vappareil circulatoire des Crustacés édriophthalmes marins. Archives de zool. expér. et gén., 1x, 1881. APPENDIX TO THE TYPE CRUSTACEA. Order Xiphosura, The Xiphosura is a group which possesses many Crus- tacean peculiarities, and also many foreign to that group and more especially characteristic of the Arachnida; consequently it is advisable to consider it as an order by itself, intermediate between the two types. A single genus, Limulus (Fig. 195), with few species consti- tutes the order, the members of which are popularly known 428 INVERTEBRATE MORPHOLOGY. as King-crabs or Horseshoe-crabs. They are large forms measuring a foot or so in diameter, and the body is composed of three portions. The anterior is a broad semicircular ceph- ep | Fie. 195.—Limulus polyphemus, FEMALE, FROM THE VENTRAL SURFACE. ab = abdomen ep = cephalothorax, an = anus. ol = olfactory organ. ch = chelicera. op = operculum, cht = chilarium. sp = spine. alothorax (cp), prolonged backwards into sharp points at its posterior angles and bearing upon its dorsal surface a pair of compound eyes towards the sides and near the median line two simple eyes. The middle region is the abdomen (ad), TYPE CRUSTACEA. 429 showing but faint indication of segmentation, and bearing on its terminal segment the anus, behind which is a long mova- ble spine (sp), the post-abdomen, forming the third region and to be regarded probably as a movable prolongation of the dorsum of the last abdominal segment. The cephalothorax bears seven pairs of appendages. The first pair, the chelicerce (Fig. 195, ch), which lie in front of the mouth, are small and, like the following four pairs, are chelate. These together with the sixth are much longer and surround the mouth, their basal joints being provided with strong bris- tles and serving as Jaws. The sixth pair of appendages differ from their predecessors in not being chelate and in possessing upon their basal joints a peculiar process which has been termed the flabellum and by some is regarded as representing an exopodite' The seventh pair of appendages is very differ- ent from the others, forming a broad flat plate, the two appendages of the opposite sides meeting in the middle line. ‘This plate covers in the abdominal appendages to a certain extent and hence is termed the operculum (op). The abdom- inal appendages, of which there are five pairs, resemble the operculum in form, and like it allow an external larger exopo- dite and an inner smaller endopodite to be distinguished. They carry upon their posterior surfaces series of large leaf- like, thin-walled folds which function as branchie. The heart (Fig. 196, ht) is an elongated tubular organ lying in the posterior part of the cephalothorax and the anterior part of the abdomen, and possesses eight ostia. Arteries arise from it which. carry the blood to various parts of the body, eventually, however, opening into the general lacunar system. The blood has a distinct bluish color which deepens on exposure to the air and is due to a copper-containing respiratory pigment, hemocyanin. The body is enclosed in a hard chitinous cuticle, and in addition a peculiar fibro-cartilaginous plate, the endosternite, is found in the cephalothorax between the intestine and the nervous system. It is formed by the fusion of a number of tendons and may be regarded as an endoskeleton. The mouth is an elongated opening lying between the bases of the anterior cephalothoracic appendages and is 430 INVERTEBRATE MORPHOLOGY. bounded behind by a pair of processes which represent a lower lip and are known as the chilaria (Fig. 195, chi). The cesophagus passes upwards and forwards and dilates into a large proventriculus (Fig. 196, pr) in the front part of the cephalothoracic shield, and this, bending upon itself and constricting again, opens into the stomach (s), from which the intestine (i) passes straight back to open on the ventral sur- face of the body at the base of the terminal spine. The inner wall of the hind-gut, cesophagus, and proventriculus is lined by chitin, which in the last-named structure is thrown into Fie. 196.—LoneirupinaL SEcTION THROUGH A younG Limulus polyphemus, DIAGRAMMATIC (after PacKaRD). ce = cerebral ganglion. ¢ = liver. At = heart. pr = proventriculus. Z = intestine. s = stomach. on = ventral nerve-cord. folds and recalls the masticatory apparatus in the stomach of the Decapodous Crustacea. Into the stomach there open the ducts of two pairs of voluminous digestive glands (I) which occupy the greater portion of the cephalothorax and are much branched greenish structures. The nervous system consists of a syncerebrum (ce) com- posed apparently of three pairs of ganglia. It lies in front of the oesophagus, sending branches to the compound and simple eyes. Behind the csophagus and united with the syncere- brum by circumossophageal connectives comes a series of seven pairs of ganglia closely approximated, the first pair innervating the chelicerw and the remaining six the other thoracic limbs in succession. A chain of six pairs of ganglia lying in the abdomen is connected with the cephalothoracic series and innervate the abdominal appendages. TYPE CRUSTACEA. 431 As already noticed, a pair of simple eyes are borne upon the dorsal surface of the carapace, one on each side of the median line, while a pair of larger compound eyes are situated lat erally. The structure of these compound eyes is peculiar (Fig. 197). Over their surface the cuticle is considerably thickened and shows upon the outer surface no indication of corneal facets, but its inner surface is prolonged into a num- ber of papillee (7) each one of which projects into a depression of the ectoderm. At the bottom of each depression is a bulb- Fig. 197.—Compounp Eve or Limulus polyphemus, Two OMMATIDIA (after WatTAsE). e = central cell. ms = mesoderm, 2 = lens. opn = optic nerve. rt = retinula. like structure composed of a number of cells arranged in a circle and constituting a retinula (rt), the lower ends of the cells being continued inwards to form part of the optic nerves (opn). Upon the face which is turned towards its fellow each retinular cell secretes a layer of chitin, and these various chit- inous rods being in contact there is formed a structure com parable to the rhabdom of the Crustacean eye. In the centre of the retinular cells and below the rhabdom is a single clear cell (c) whose lower end is also prolonged into a nerve-fibre. Each depression with its retinula and the chitinous papilla which fits into it and represents its cornea is an ommatidium, and the development shows that the ommatidia arise as 432 INVERTEBRATE MORPHOLOGY. number of separate invaginations of the ectoderm, the sides of the retinular cells which secrete the rhabdom being in reality those sides which before invagination were at the surface of the body, and the rhabdom may therefore be regarded as composed of portions of the general cuticle which have been separated by the invagination. On the under surface of the carapace in the median line in front of the chelicere is a small tubercle (Fig. 195, ol) which contains an organ supposed to be olfactory in function, and probably some of the sete upon the basal joints of the limbs may also possess a similar function. Nephridia are represented by a single pair of large reddish bodies lying at the sides of the cephalothorax. They have no communication with the exterior in the adult, but in the early stages of development open upon the basal joint of the fifth appendage, and are at first tubular organs and nephridialike, later becoming much contorted and complex. What their function in the adult may be is uncertain, and to avoid possible misconceptions it seems preferable to speak of them as coxal glands, a term indicating their original point of opening on the basal joints (cox) of one of the pairs of limbs. The Xiphosura are bisexual, the genital ducts opening on both males and females on the posterior surface of the oper- culum near its base. The ovaries are much branched paired structures, the various branches frequently anastomosing even across the median line. The testes are numerous spherical bodies scattered through the body and situated on branching and anastomosing vasa deferentia. Development and Affinities of the Xiphosura.—When the young Limulus leaves the egg it presents a remarkable resem- blance to a Trilobite and suggests a possible aftinity with these forms which are known to occur only in the Paleozoic rocks. In these same rocks there occur also the remains of forms known as the Hurypteride which seem to have been even more nearly related to Zimulus than were the Trilobites. In them the cephalothorax bore apparently only six pairs of appendages which resembled more or less closely those of Limulus, except that the sixth pair was broad and oarlike, TYPE CRUSTACEA. 433 probably serving for swimming. The abdomen was com- posed of twelve segments, the anterior six of which were much more massive than the others and bore five pairs of platelike appendages on whose posterior surface were the branchie. The terminal segment bore a spine or finlike structure. Such a form as this, represented by the genus Eurypterus (Fig. 198), presents strong similarities to Limulus and also to the Scorpions, bearing out the numerous similarities of structure occurring between Limulus and those forms. This side of the affinity may be postponed, however, until the next chapter, and the comparison of Limulus with the Crustacea discussed here. Its chitinous cuticle, its jointed and biramous appendages, and its branchial respiration show similari- ties to the Crustacea, as do also the form of the heart and the compound eyes. Whether or not the coxal gland is comparable to the shell- gland is at present uncertain, but the other similarities are sufficient to justify the recognition of a Crusta- cean origin for Zimulus. It forms Fre. 198 —Hurypterus remipes indeed a connecting link between the Pen ene Crustacea and the Arachnida, presenting probably on the whole more affinities with this latter group than with the former. Since, however, a Crustacean ancestry is probable, a com- parision of the appendages of Limulus with those of a repre- sentative of the ancestral group ought to be possible. It has already been noticed that the brain of Zimulus is a syncere- brum composed of three segments; it represents, therefore, two segments of which the appendages and other parts have disappeared. Furthermore, recalling that, in the higher Crustacea at least, a ganglion occurring between the cerebral antennary ganglia in the embryo indicates a lost pair of 434 INVERTEBRATE MORPHOLOGY. appendages in these forms, the following table may represent the homologies of the appendages of the two groups. _ Crustacean, Limulus, 1 segment.......... no appendage no appendage 2, ce imaee eee 6c ii iT7 66 3 eS ....., antennules s fs 4 SS cis areas antennee chelicerze 5 E> Bigeoteaesds. mandibles 1st pair of legs 6 BOD basate Sedan 1st maxillee 9 « « «& 7 ames ad. * Sd. om Bm. owuaweta .. 1st thoracic appendages 4th “ “« « 9 eo ee 2d “ a Dthy ie ae = £8 10 6 Mie sie Od « fe operculum LITERATURE. A. Gerstaecker. Orustaceen. Bronn’s Klassen und Orduoungen des Thierreichs, Bd. V. 1. Abth., 1866-79. ; A. 8, Packard. The Anatomy, Histology, and Embryology of Limulus poly- phemus. Memoirs Boston Soc. Nat. History, 1880. E. R. Lankester. Limulus an Arachnid. Quarterly Journ. Microscopical Science, xx1, 1881. S. Watase. On the Morphology of the Compound Eyes of Arthropods. Studies from the Bio]. Lab. Johns Hopkins Univ., rv, 1890. W. Patten. On the Origin of Vertebrates from Arachnids.. Quarterly Journ. Microscop. Science, xxx1, 1890. J. 8. Kingsley. The Embryology of Limulus. Journ. of Morphology, vu, 1892 ; vir, 1893. W. Patten. On the Morphology and Physiology of the Brain and Sense-organs of Limulus, Quarterly Journ. Microscop. Science, xxxv, 1893. TYPE ARACHNIDA., 435 CHAPTER XIV. TYPE ARACHNIDA. Tue Arachnida are essentially terrestrial forms, for though a few species lead an aquatic or marine life, they are evi- dently descendants of forms which led a terrestrial existence and have only secondarily acquired the power of living under water. In all members of the group the body is covered by a more or less thick chitinous cuticle and the appendages are as a rule jointed. A characteristic feature of the group is the fusion of the head and thorax to form an unsegmented cephalothorax bear- ing usually six pairs of limbs. The first pair of these are the chelicere (Fig. 201, ch), composed of one to three joints and terminated either by a claw or a chela; they lie in front of the mouth, which is bounded at the sides by the basal joints of the second pair of appendages, the pedipalps ( pe), which may be long and limblike, or chelate, or in some cases clawlike, their basal joints serving in all cases as mandibles, Behind these follow four pairs of legs composed of six or seven joints, the basal joint being termed the coxa, the next, usually short, the trochanter, the third the femur, the next two to- gether form the tibia, then follows in some forms a metatarsus, while the terminal one, provided with two claws, termed ungues, and in some mites also with a suctorial disk, consti- tutes the tarsus. Variations from this structure of course occur, the chelicere, for example, in some mites being re- duced to short stylets, and in others the two posterior pairs of legs may be quite rudimentary (Phytoptus). The most impor- tant variation is, however, that found in the members of the order Solifuge, in which a head is distinctly marked off from a thorax composed of three segments. The abdomen in some forms is segmented, in others all trace of the segmentation is lost, and, finally, in the Mites it 436 - INVERTEBRATE MORPHOLOGY. may be united with the cephalothorax. In the Scorpions it is divisible into an anterior portion, the preabdomen, much broader and stouter than the posterior postabdomen, an ar- rangement also indicated in certain other forms. In the adults the abdomen is usually destitute of appendages, though they may be present in the embryos; the Scorpions, however, possess two highly-modified pairs, and it seems probable that the four or six papille upon which the ducts of the spinning- glands open in the Spiders represent also modified append- ages. A special respiratory system is entirely wanting in a few forms. In the majority there occur on the sides of the body from one to four pairs of pores termed stigmata (Fig. 201, st). In the Scorpions and some other forms each stigma opens into a cavity lined with chitin continuous with that which covers the general surface of the body, and into this cavity there project a number of lamelle arranged like the leaves of a book (Fig. = 199), whence the term lung-books frequently applied tothem. Each lamella is hollow, trabecule ex- tending across the cavity from one wall to the other, and the cavities communicate with the coelomic lacune, so that blood can ; readily flow into them and so Fie. 199.—TRANSVERSE SECTION change its gases through the thin rHRoveH tHe Lune-Boox or a Walls of the lamelle. In other SPIDER (after McLzop). cases there occurs in connection ch = sel tissue. with the lung-book apparatus, or = body-wall. * ce * ad : else entirely replacing it, a tra- lp = pulmonary lamella. poe st = stigma. cheal system consisting of a num- t= last compartment of lung-ber of tubes ramifying through book, trachealike in char-the body. In some cases a strong acter. . tube or trachea arises at each stigma and traverses the body, giving off branches to all parts as it goes; in others there is in connection with each stigma TYPH ARACHNIDA. 437 a bunch of unbranched trachex, and all gradations between these two conditions occur. The trachee are lined with chitin, which is sometimes thickened to form rings or spiral bands which serve to keep the lumen of the tubes open and thus permit a free passage of air into them. The ccelom is filled for the most part with the various organs and is reduced to a series of lacunar spaces containing blood, sometimes rich in hemocyanin and assuming a blue color when oxygenated. A heart is wanting in some Mites, but is present in the majority of forms, varying from a saclike organ with a single pair of ostia guarded by valves to an elongated cylinder with as many as eight pairs of ostia (Scorpi- ons). Itis for the most part situated in the abdominal region, and in the Spiders is enclosed within a space with definite walls which is termed the pericardium, though it cannot be considered homologous with the pericardium of the Mollusca, since it contains blood; muscle-bands extend from it to the walls of the body and by their contraction cause its expan- sion, fibres in its wall diminishing its cavity and forcing the blood through the ostia into the heart. Arteries in many forms arise from the heart, but after usually a short course open into the lacunar ccelom. The digestive tract pursues a more or less straight course through the body, but shows a tendency to develop cecal out- growths which sometimes reach a considerable size. _ The anterior and posterior portions of the tract are ectodermal, while the middle region or mid-gut is endodermal and is the portion with which the cceca are connected. In the Scorpi- ons the ducts of a digestive gland open into the mid-gut, and in many forms there is connected with the posterior portion of this same region a pair of tubular Malpighian vessels which are presumably excretory in function and recall the similar structures of the Amphipoda. The end-gut is fre- quently dilated into a large bladderlike structure, the rectal bladder. The nervous system consists of a supracesophageal syn- cerebrum composed of three pairs of ganglia fused together, and in some forms even four pairs may be included, since the chelicerss may be innervated from the mass, their ganglia in 488 INVERTEBRATH MORPHOLOGY. embryonic life being, however, distinct and postoral in posi- tion, only later moving forward. The succeeding ganglia are generally more or less fused, and indeed in some forms all the ganglia of the limb-bearing segments of the cephalothorax may be united with those of the abdominal region to form a single ganglionic mass. In some forms a single ganglion occurs behind this mass at the junction of the cephalothorax and the abdomen, and in the Scorpions there is posteriorly a ventral nerve-cord with seven pairs of ganglia, the anterior pair corresponding with the fifth abdominal segment. A sympathetic nervous system occurs in the Scorpions, Spiders, and Harvest-spiders, consisting of a nerve arising from the syncerebrum and passing to the digestive tract. Hairs situated upon the body and appendages serve as sense-organs of touch and apparently also of audition, since Spiders are sensitive to air-vibrations and possess no definite auditory organs. Eyes are very generally present and vary considerably in number, there being in the Spiders three or four pairs; in the Scorpions, in which there are from two to six pairs, one pair become closely approximated on each side of the mid-line and recall the median eyes of Limulus, while the remaining pairs are situated more to the sides of the cephalothorax. In structure the median eyes differ from the lateral ones; the chitinous cuticle is thickened over them to form a simple unfaceted lens below which lies a layer of transparent cells continuous with the general ectoderm (hypo- dermis) of the body and which may be termed the corneal hypodermis, though more usually they are known as the vitreous cells. Below them comes the retina, consisting of a single layer of elongated cells with their nuclei situated toward their inner ends, with which the fibres of the optic nerve come into connection. The various retinal cells are arranged in groups of five (retinule), which secrete a thin chitinous rod upon their contiguous faces, producing thus a rhabdom composed of five parts. Behind the retina is a thin layer of cells, the postretinular layer, and numerous pigment- cells occur between the various retinule. The lateral eyes are constructed upon a very different plan, lacking a corneal hypodermis between the retina and the cornea. They are TYPE ARACHNIDA. 439 cup-shaped structures, the cavity of the cup being filled by the cuticular cornea and its wall transformed into the retina, which is continuous at the margins of the cup with the gen- eral hypodermis. The retinal cells are of two kinds, viz., large sensory cells and smaller interstitial cells. Each sen- sory cell is surrounded by pigment and bears upon its lateral walls a chitinous secretion which, with the corresponding secretion of contiguous cells, forms a rhabdom. The nuclei of the cells are situated nearer their inner than their outer ends, and behind them in Huscorpius highly refractive spheres occur imbedded in the cells, constituting what have been termed the phaospheres. Upon its inner wall the retina is lined by a basement-membrane continuous with that lying below the general hypodermis and perforated by the fibres of the optic nerves which come into contact with the inner ends of the sensory cells. In the Spiders, in which there are six or eight eyes arranged upon the dorsum and sides of the cephalothorax, the ante- rior dorsal pair differs in structure from the remaining ones. In both forms of eyes (Fig. 200) the cuticular cornea (c) rests Fig. 200.—Eyrs or Sprper. A, ANTERIOR, AND B, PosTERIOR Eve (after BgertTKau from KorscHeExLT and HEIvER). b = rods. ” = retina. 2 = lens. t = tapetum lucidum. »® = Vitreous layer. upon a corneal hypodermis, (the vitreous cells, v), but the arrangement of the retina differs greatly. In the anterior dorsal pair (A) it is composed of a layer of elongated cells (r) whose nuclei are situated towards their inner ends, while near 440 INVERTEBRATE MORPHOLOGY. the outer ends are situated a number of rodlike bodies (rhab- doms, 6), whence these eyes have been termed prebacillar ; the nerve-fibres are continuous with the inner ends of the cells. In the posterior dorsal and lateral eyes (£2) an inversion of the retina (r) has taken place, so that the rods (0) are situated at the apparently inner ends of the cells and the nuclei at their outer ends, whence the term postbacillar applied to these eyes. The optic nerve-fibres enter at the sides of the eye and are distributed to the nuclear ends of the retinal cells, recalling the arrangement occurring in Pecten among the Mollusea. The innermost layer of the eye upon which the ends of the rods rest is cellular, numerous minute crystals being deposited in the cells, whence it has the function of a reflector and is termed the tapetum (t). It is quite wanting in the prebacillar eyes. The significance of the structure of the Arachnid eye may be under- stood by supposing it to have been derived from a compound eye similar to that of Limulus (see p. 431), the individualities of the various ommatidia being more or less subordinated. The cuticular cornea in Limulus is smooth upon its outer surface, the inner surface being produced into papille, one of which corresponds to each ommatidium. In the Arachnids even these papillae are wanting, the cornea showing no evidence of the presence of ommatidia. The lateral eyes of the Scorpions approach more nearly in their general structure the eyes of Limulus, though the conden- sation of the ommatidia has been carried further than in the median eyes” of that form, or in the posterior dorsal and lateral eyes of the Spiders. But in these eyes the condensation is associated with an invagination of the en- tire eye, a process which, it may be remarked, is indicated in the median eyes of Limulus. This invagination has been regarded as a pushing in, under and parallel to the hypodermis, of a pouch of that layer, a process which gives in cross-section the appearance of an S-shaped fold. The outermost layer of the fold forms the vitreous cells or corneal hypodermis, ‘the middle layer the retina, the inversion of which is plainly seen in the posterior dorsal and lateral eyes of the Spiders, while the innermost layer forms the postretinal layer in the Scorpions and the tapetum of the Spiders. The ommatidial retinule are more or less retained in these eyes, as is shown by the structure of the rhabdom, which in the Scorpions is composed of five parts, in the Spiders of two, and in the Harvest-spiders of three. The anterior dorsal eyes of the Spiders do not seem to have undergone an invagination, hence the absence of a tapetum and the praeba- cillar structure of the retina; a corneal hypodermis is, however, present, and would seem to indicate an invagination, but its mode of origin seems at present but imperfectly understood. If a generalization is to be made, it TYPE ARACHNIDA. 441 will be to the effect that the eyes of the Arachnids have been derived from compound eyes similar to those of Limulus, and that in the median eyes of the Scorpions, and the posterior dorsal and lateral eyes of the Spiders the entire optic area has been invaginated, making them comparable to the median eyes of Limulus, while the lateral eyes of the Scorpions and the anterior dorsal eyes of the Spiders have not undergone invagination and hence are comparable to the lateral eyes of Limulus. Whether the com- parability indicates also the homology from a phylogenetic standpoint of eye to eye must remain at present uncertain (see p. 457). In addition to the Malphighian tubules already mentioned as excretory organs occurring in connection with the digestive tract of the Spiders, there exist in many forms additional glands which probably are also excretory in function or sig- nificance. These are the coxal glands, so called on account of their openings when present being on the basal joints (coxe) of one of the pairs of legs. In the Scorpions and Spiders the ducts of the glands open on the third pair of legs (i.e., the fifth pair of appendages) in the embryo, but are usually wanting in the adults. In the Solifuge and Harvest-spiders coxal glands also occur in connection with the fourth pair of legs, and similar glands have also been observed in several genera of Mites, opening, however, at varying points. Glands are also of frequent occurrence in connection with the pedipalps, having apparently varying functions in differ- ent genera. They do not, however, seem to belong to the same category as the coxal glands and are in no case excretory. The Arachnida are bisexual throughout. The ovaries not infrequently fuse to form a single mass or a circular band, and in connection with the oviducts, which are in direct com- munication with them, there is usually developed a receptac- ulum seminis, and in the Harvest-spiders, an elongated ovi- positor. The testes are also frequently fused, and the vasa deferentia are provided with vesicule seminales and usually terminate in a copulatory organ. The majority of forms are oviparous, exceptions to the rule being found, however, as in the genus Phrynus and in the Scorpions, which are viviparous. 1. Order Scorpionida. In the Scorpions (fig. 201) the body is composed of an unsegmented cephalothorax and an elongated segmented 442 INVERTEBRATE MORPHOLOGY. abdomen. The seven anterior segments (the preabdomen) of the abdomen are broader and thicker than the remaining five segments (the postabdomen), the last one of which ter- minates in a curved stout spine which bears at its extremity the openings of two ducts leading from a pair of glands lying in the twelfth abdominal segment and secreting a poisonous fluid. The chelicerz (ch) are small chelate appendages situated in front of the mouth, while the pedipalps (pe) are long and provided with strong chele, their basal joints and those of the two succeeding appendages surround- ing the mouth and serving as jaws. The four pairs of appendages be- hind the pedipalps are all similar in form, being six-jointed walking- limbs. Upon the abdomen modi- fied appendages are also found, the second abdominal segment bearing a pair, each member of which consists of a single joint whose posterior edge is beset with a number of processes which give it the appearance of a comb, whence the name pectines (pt) applied to these appendages. In front of the pectines lies the geni- tal opening, protected by a small genital operculum (op) which may Fig. 201.—Scorpion (after OwEy). ch = chelicere. op = gevital operculum. possibly represent another pair of pe = pedipalp. appendages belonging to the first pt = pecten. abdominal segment. Upon the ventral surfaces of third, fourth, fifth, and sixth abdominal segments elongated pores are to be found which are stigmata (st'“) leading into the respiratory cavities containing the lung-books, of which there are in all four pairs in this group. No trachex occur. The intestine is quite straight in the Scorpions and lacks cecal outgrowths excepting the two Malpighian tubules sit st-4 = stigmata. TYPE ARACHNIDA. 443 uated at the posterior end of the mid-gut. The digestive gland is a large five-lobed structure which empties through several ducts into the mid-gut. The nervous system consists of a syncerebrum lying above the cesophagus and giving rise to nerves for the eyes and for the chelicere. It is connected with a subcesophageal mass from which the pedipalps and the three anterior legs are innervated, the fourth pair of legs receiving its nerves from a pair of distinct ganglia separated only by a short distance from the subcesophageal mass. Behind this in the abdomen is a chain consisting of seven pairs of ganglia united by long connectives. The eyes vary in number from two to six pairs, one pair being situated on or near the median line, while the others are lateral. Coxal glands occur in connection with the third pair of legs, and the heart is an elongated structure lying in the an- terior portion of the abdomen and possessing eight pairs of ostia. The Scorpions are viviparous. The ovaries are situated in the anterior abdominal region and are elongated, that of one side of the body being united with the other by several transverse connections. The oviducts, which are short, serve as uteri, and open to the exterior by a single median opening situated on the ventral surface of the first abdominal segment. The testes consist of four tubes, those of the same side being connected by transverse anastomoses, and unite together to open into a protrusible penis, accessory glands, vesiculz sem- inales, occurring in connection with each vas deferens. The single genital orifice occupies the same position as in the female. The Scorpions are confined to the warmer regions of the globe, but few genera being known. Of these the genera Hw- scorpius and Buthus are perhaps the commonest. 2. Order Pseudoscorpionida. This order includes a number of small forms which are found under the bark of trees or among dead leaves or moss, one genus, Chelifer (Fig. 202), occurring occasionally between 444 INVERTEBRATE MORPHOLOGY. the pages of books, and hence being known popularly as the Book-scorpion. The cephalothorax is unsegmented, and is followed by a broad flattened abdomen composed of eleven segments. A preabdomen and a postabdomen, such as can be distinguished in the Scorpionida, does not occur, nor is there a terminal poison-spine nor a poison-gland. The chelicere and pedipalps resemble those of the Scor- pions, being chelate, and the four succeeding appendages are : walking-legs, while the abdomen pos- &% sesses no appendages in the adult. Both 7% the second and third abdominal seg- ments bear upon their ventral surfaces a pair of stigmata which are the openings of tubular trachee which extend through the body sending off branches, except in Chernes, in which bunches of un- branched trachez arise from each stig- ¥ ma. A heart is present, but consists of Fie. 202.—Chelifer carci- a simple tube with either a single pair noides (from voter). of ogtia near its posterior extremity (Obisium) or with four ostia (Chernes). The endodermal portion of the digestive tract gives rise to a pair of lateral ceecal diverticula branched at the apex and to one unpaired ventral one. Two eyes are present in Chelifer and four in Obisium, while they are entirely wanting in Chernes. ‘The reproductive organs open upon the ventral sur- face of the second abdominal segment, and the opening is surrounded with glands which secrete a fluid which quickly hardens to silky filaments and serves to fasten the eggs to the abdomen of the parent. These glands are hypodermal in origin and correspond to the spinning-glands of the Spiders. 3. Order Solifugee. The members of this order are characterized by the head- region being separated from a thorax consisting of three seg- ments and bearing the three posterior pairs of legs. The abdomen is also segmented, its ten segments showing no dif- ferentiation into preabdomen and postabdomen, nor does it TYPE ARACHNIDA. 445 possess any sting or poison-gland. The chelicerz are chelate, but the pedipalps are long and leglike and possess glands which in Galeodes have been supposed to be poisonous, The anterior pair of legs lacks the terminal ungues found on the others, and functions as a second F pedipalp rather than a walking-leg. J ‘ No appendages occur on the abdo- 1 x. men. L< | Three pairs of stigmata occur on the ventral surface of the body, the most anterior pair being situ- ated on the first thoracic segment, . while the other two are on the »*” second and third abdominal seg- ts he ments. The anterior position of the 4 a: first pair is probably to be regarded 4 f as secondary, and produced by a forward migration of the pair which : i should occur upon the first abdomi- F nal segment. The stigmata lead ™'* Cig ee es into tubular trachezw which branch , extensively. A comparatively simple heart is situated in the abdomen. The mid-gut possesses numerous branched diverticula as well as Malpighian tubules. The nervous system consists of a syncerebrum connected with a subcesophageal mass which represents all the thoracic and abdominal ganglia fused to- gether. Two eyes are present, situated on a common eleva- tion at the front edge of the head. The reproductive organs resemble those of the Scorpions except that transverse anastomoses do not occur, and the gen- ital opening is situated upon the ventral surface of the first abdominal segment. The Solifugee is a small order living more especially in warm sandy regions. They are usually, on rather insufficient grounds, supposed to be capable of inflicting a poisoned wound. Only two genera, Solpuga and (aleodes, belong to the order. 446 INVERTEBRATH MORPHOLOGY. 4. Order Pedipalpi. The order Pedipalpi includes two genera, Phrynus and Thelyphonus, both of which are inhabitants of the warmer. regions of the earth. ‘The cephalothorax is unsegmented ; the abdomen in Phrynus is elongated and oval, and composed of eleven segments showing little differentiation of form, while in Thelyphonus there are twelve segments, the last three of which are much smaller than the others and bear a long, many- jointed terminal filament. The cheli- , cere are not chelate, but the terminal joint may be flexed upon the basal one and contains the duct of a poison- gland which opens at its extremity. The pedipalps in Phrynus are long and leglike, though richly provided with spines, and terminate with un- gues, but in Thelyphonus they are rel- atively short and stout with a flexible terminal joint as in the chelicere ; } in both genera the basal joints of the Fra. 204,—Phelyphonuscau- + nenipalps are fused. The first datus (from Cuvier). P ; leg is long and slender and termi- nates in a filament-like structure, the other three pairs being typical walking-legs. Four stigmata occur, one pair situated in the second and another in the third abdominal segment, and they open into cavities containing lung-books. The’ digestive tract is com- paratively simple, but the nervous system shows a concentra- tion of the postcesophageal ganglia similar to that described for the Solifuge, except that a single pair of ganglia occurs in the abominal region united by long connectives with the cephalothoracic mass. Hight eyes are present, two of which are larger than the others and situated at the anterior edge of the dorsal surface of the cephalothorax, while the other three pairs are situated laterally. The reproductive organs are paired and open by a median TYPE ARACHNIDA. 447 orifice situated on the ventral surface of the first abdominal segment. Phrynus is viviparous. fh, Order Phalangida. The Phalangida (Fig. 205), popularly known as the Harvest spiders, possess an unsegmented cephalothorax (ct) and have from six to nine segments composing the abdomen (ab). The cheliceree are chelate, while the pedipalps (pe) are long and leglike, with terminal ungues. The eight walking-legs are usually exceedingly long, though in the genera Cyphophthalmus and Gibbocellum they are shorter. A single pair of stigmata are usually all that occur; they are situated upon the first Fig. 205.—Letobunum. ab = abdomen. et = cephalothorax. pe = pedipalps. abdominal segment and open into branching trachee. In Gibbocellum, however, two pairs occur, situated upon the second and third abdominal segments, the anterior pair open- ing into branched trachew, while a bunch of simple unbranched trachew arises from each of the posterior ones. The heart is somewhat elongated and possesses three pairs of ostia; ar- teries are entirely wanting, the blood passing from the heart directly into the lacunar spaces. The digestive tract dilates into a sac-like stomach from which numerous much-branched cecal diverticula pass off. Malpighian vessels, two in number, are found in Cyphophthal- mus and Gibbocellum, and have been described as occurring 448 INVERTEBRATE MORPHOLOGY. in other forms also, though it is probable that two glandular tubes which open to the exterior on the sides of the cephalo- thorax have in some forms been mistaken for these organs. Odoriferous glands are also found in the abdomen of some forms, and so-called salivary glands occur in connection with the pedipalps. The nervous system shows a marked concentration of the postoral ganglia, a single pair only remaining separate from the fused mass formed of the remainder. The majority of forms possess but a single pair of eyes on the dorsum of the cephalothorax, but in (bbocellum two lateral pairs are found. Coxal glands have been described in connection with the coxal joints of the third pair of legs and have been observed to communicate with the exterior, differing therefore from those of other Arachnoids in being functional in the adult. The reproductive organs are unpaired, a condition which results from the fusion of originally paired structures, and the genital pore lies in both sexes at the junction of the cephalothorax and abdomen or on the first abdominal seg- ment. The vasa deferentia and oviducts are paired, each of the former communicating with a protrusible penis, while similarly each oviduct unites with a long protrusible ovipositor. Certain genera such as Letobunum (Fig. 205), Phalangium, and Opilio, are exceedingly common, and to them the terms Harvest-men, Harvest-spiders, or Daddy Longlegs are popu- larly applied. Other forms, such as Gonyleptus, with spinose pedipalps, are tropical in habitat, while Cyphophthalmus and Gibbocellum have a limited distribution, and on account of the many differences of structure which they present when com- pared with other forms are sometimes grouped together to form a separate order. It is to be noted especially that these two forms possess upon the second abdominal segment a pair of wartlike elevations at the summit of which the ducts of numerous spinning-glands open. 6. Order Aranee. The order Aranew includes a large number of forms possessing very definite characteristics. The cephalothorax TYPE ARACHNIDA. 449 is unsegmented, as is also the abdomen, which is an oval, spherical, or sometimes irregularly-shaped region which narrows suddenly anteriorly so as to be much narrower than the cephalothorax. The chelicere project somewhat in front of the cephalothorax and each consists of a broad basal joint and a terminal strong claw which may be flexed upon the basal joint, and has opening at its tip the duct of a poison- gland (Fig. 206, pg) which lies in the cephalothorax. The § . Fic. 206.—DraAGRAM OF STRUCTURE OF A SPIDER (after Levcgart). ao = aorta. pe = pedipalp. ce = cerebral ganglion. pg = poison-gland. ch = chelicera. rb = rectul bladder. dg = digestive gland. rs = receptaculum seminis. gp = genital pore. s = stomach. At = heart. sd = stomach diverticulum. ib = lung-book. sp = spinueret. mt = Malpigbian tubule. spg = spiuning-glands. oc = eye. tg = thoracic ganglion. ov = ovary. tr = trachea. pedipalps of the females are leglike structures usually with a terminal unguis, but in the male are more or less swollen to serve as accessory organs in copulation. The four pairs of seven-jointed legs are all similar in structure and serve for walking, differing in relative length in different genera. In the embryo the abdomen is distinctly segmented and bears five or six pairs of rudimentary appendages, the more ante- rior of which later disappear, while the two or three posterior pairs persist as the spinnerets (sp), so called from the occur- rence on them of the openings of the ducts of the spinning- glands (spq). These are very numerous and open at the apices of the spinnerets, each gland producing a fluid secretion which quickly hardens on exposure to the air to form a silken 450 INVERTEBRATE MORPHOLOGY. thread. The thickness of the thread may be modified by uniting together the secretions of a greater or less number of the glands, which, moreover, differ among themselves, some producing, for instance, a sticky secretion with which certain of the threads may be covered. In some forms there is situ- ated upon the abdomen just in front of the swimmerets a chitinous plate, the cribellum, which is perforated, like the spinnerets, by the ducts of numerous spinning-glands. Its presence is associated with that of a calamistrum, a peculiar modification of the metatarsus of the last pair of legs, it being furnished with a double row of bristles which are rapidly waved over the cribellum and draw from its glands their secretion. The threads are used for several purposes, as, for example, to fasten the ova to the body of the parent or to form a cocoon for them, or else to form a snare by which insects may be caught to serve as food. These snares in some cases are composed of an irregular network of threads arranged without any definite pattern, as in Theridium, but some other forms show a certain amount of architectural skill, weaving a platform of felted threads which terminates in a tubelike place of concealment for the spider (e.g., Age- lena, Tegenaria) or webs composed of threads radiating from a central point and united by other threads arranged in a spiral or in concentric circles (e.g., Hpeira, the common garden- spider), or else using the threads to form a hinged trap-door covering in a burrow in the earth which serves as a domicile as in the Trap-door Spider. The digestive tract expands in the thoracic region into a saclike structure (s) from each side of which three or more usually five cecal diverticula (sd) arise, the anterior pair sometimes anastomosing so as to form a ring, while in some cases (petra) secondary diverticula extend from the more posterior ones into the coxal joints of the legs. In the abdo- men the intestine is more cylindrical, giving rise to much- branched lateral diverticula which together form the so-called liver (dg), and having connected with it, just as it joins the end-gut, two elongated Malpighian tubules (mt). The end- gut itself dilates into a large rectal bladder (7d) which a short TYPE ARACHNIDA. 451 rectum connects with the anus situated at the posterior ex- tremity of the body. In the genus Mygale and allied forms two pairs of stig- mata are found near the anterior portion of the abdomen, both of which lead into cavities containing lung-books. In the majority of forms, however, but one pair of lung-books (ib) occurs, the second pair of stigmata opening into a tracheal tube (tr) extending into the cephalothorax and terminating in a bunch of unbranched trachee, a similar bunch arising near its base and extending backwards into the abdomen (Seges- tria). In some forms the second or tracheal stigmata may be situated far back upon the abdomen, and may be united to a single median transversely-elongated cleft, from which a bunch of unbranched (Aétus) or branched trachee arises. The heart (Af), which lies in the abdomen, is enclosed within a so-called pericardium and possesses three pairs of ostia. It is continued anteriorly and posteriorly into aorte, and gives off also lateral arteries, all of which open after rel- atively short courses into the lacunar spaces. The blood is returned to the pericardial cavity, whence it passes into the heart, the greater portion on its way to the pericardium pass- ing through the lung-books. The nervous system consists of a syncerebrum (ce) and a large cephalothoracic ganglionic mass (fg). In addition to the nerves to the appendages, a posterior nerve arises from this mass and passes backwards towards the abdomen, in Mygale dilating at the junction of that region with the cephalo- thorax into a pair of small ganglia. A sympathetic or visceral system, consisting of a nerve arising by paired trunks from the brain, is distributed to the anterior portion of the diges- tive tract. The eyes are usually numerous, three or four pairs occurring on the anterior portion of the cephalothorax, their arrangement varying in different genera. Coxal glands have been found in several forms in connec- tion with the third pair of legs, but have not been found to open to the exterior in the adult. The reproductive organs open in both sexes by a single opening situated near the anterior end of the abdomen between the anterior stigmata. The ovaries (ov) are paired, or may unite to form a ring, and 452 INVERTEBRATE MORPHOLOGY. the two short oviducts unite to form a vagina with which may be associated receptacula seminis (rs), though more usu- ally these structures open independently in front of the genital orifice and may be single, or paired, or in some cases even three in number. The testes are cy- lindrical structures whose long, slender, and frequently-contorted vasa deferentia unite just before opening to the exterior, A remarkable copulatory organ is formed by the terminal joint of the pedipalp of the male (Fig. 207), which bears upon its inner surface a process containing a spirally- coiled tube. This tube opens at the ex- oa eae et tremity of the process, and is filled by the or Mate Spwer SPider with spermatozoa, and during copu- (after BerTKAU), lation is inserted into the receptacula semi- nis of the female. The males are usually smaller than the females, and their approaches are frequently resisted by the latter, who en- deavor to capture and destroy the persistent swains. In the Attide a process of courtship has been observed to occur, the male posturing before the female and displaying to their best advantage the highly-colored hairs with which the body is covered. The ova are in many forms (Lycosa) attached to the under surface of the abdomen, while in others they are enclosed in a silken cocoon which may either be carried about by the female or suspended in the webs or deposited in protected situations. Two suborders are recognized, according as there are two pairs of lung-books or only one. The Tetrapnewmones in- clude the forms with two pairs of lung-books, among which are the Trap-door Spiders, Cfeniza, already mentioned, and the Tarantula, Mygale, the largest of all the spiders and reputed to attack even small birds. The Dipnewmones have but a single pair of lung-books, the majority of living spiders be- longing to the suborder. Some, such as Hpeira, Agelena, Tege- naria, Theridium, and Segestria, spin webs, while others catch their prey by their rapid movements (Lycosa) or by suddenly springing upon it (Attus). TYPE ARACHNIDA., 453 %. Order Acarina. The Acarina are for the most part small forms, many being almost microscopic, while the largest, the Ticks (Jxodes), do not when at their greatest size exceed a centimeter in length, the males being much smaller. Some forms, such as Oribates and Nothrus, live among moss and in similar situa- tions, while others, such as Hydrachna and Ataz, are aquatic. Many forms are, however, parasitic either upon plants (Te- tranychus and Phytoptus) or on animals, the genus Sarcoptes being the cause of the disease termed the Itch in man, the symptoms being produced by the Mites burrowing beneath the skin. Other forms affect various animals and birds, the genera Dermaleichus, Analges, etc., feeding upon the feathers Fig. 208.—A, Sarcoptes scabiet; B, Demodex phylloides (after Csoxor from WRIGHT). of various birds, while others, such as Demodex (Fig. 208, B), live ia the hair-follicles or sebaceous glands of the skin, pro- ducing acnelike pustules. The larve of many forms which are non-parasitic in adult life have a parasitic habit, as for instance the larve of many of the Water-mites and of the Harvest-mites (Zrombidium), while other forms live upon organic matter of various kinds, as does the Cheese-mite, Tyroglyphus. A distinguishing characteristic of the Acarina is the ab- sence of any segmentation and the fusion of head-thorax and abdomen to a single mass (Fig. 208, A). The form of the appendages varies greatly in different genera according to the use to which they are put. The chelicere (Fig. 209, Md) are 454 INVERTEBRATE MORPHOLOGY. frequently chelate, but in parasitic forms are reduced to stylets enclosed by the fused basal joints of the pedipalps, a proboscis being thus produced which can pierce the integu- ment and thus render the juices of the host available as food. The pedipalps (Mxp) undergo various modifications, being. sometimes long and limblike, sometimes chelate, while their basal joints may or may not be fused. The four pairs of legs are generally adapted for walking, and terminate in ungues or bunches of hairs or, in some parasitic forms, in suctorial disks, while in the Water-mites they are provided with usually long bristles along the sides, serviceable swimming-organs being thus produced. In the genus Demodex the four legs are reduced to short unjointed structures each provided with four ungues, while in the Leaf-mites, Phytoptus, which pro- duce galls on the leaves of various plants, the two pairs of posterior limbs are reduced to wartlike elevations bearing bristles, the two anterior pairs being on the other hand five- jointed. , i The chitinous covering of the body is usually thick and delicately wrinkled. It usually bears numerous sete and occasionally also plates or lateral prolongations, as in Ori- bates and its allies. Dermal glands also frequently occur, producing oily fluids and sometimes odoriferous secretions. Spinning-glands opening on the pedipalps occur in Tetrany- chus, frequently parasitic on the leaves of the Rose, but as a rule they are not developed. A pair of stigmata (Fig. 209, sf) occurs in many forms, situated usually near the coxe of the last pair of legs, but not unfrequently they are much further forward, lying near the basal joints of the pedipalps or even of the chelicere. ‘They open into trachex which branch once, buuches of lateral traches being situated at intervals upon the two branches. Frequently, however, especially in parasitic and aquatic forms, both trachez and stigmata are wanting, as is usually also the heart. When present (Gamasus, [codes) this latter structure is small, with but a single pair of ostia, and is prolonged an- teriorly into a slender aorta. The digestive tract is frequently provided with glands opening into its anterior portion and supposed to be salivary. TYPE ARACHNIDA. 455 The mid-gut usually sends off a number of cwcal diverticula which may branch at the ends, and Malpighian vessels, sometimes one, sometimes a pair,and sometimes many are usually present, while in addition, in some forms, a rectal bladder, similar to that occurring in the Aranex, is found. Fra. 209.—Matz or Gamasus tergipes (after Winxuzr). an = anus. st = stigma. ce = cecal pouches of intestine. ste = stigma-canal. go = genital orifice. T = testis. Md = chelicera. t = tongue. mg = Malpighian tubules. Vd = vas deferens. Mzp = pedipalps. I-IV = limbs. The nervous system, as might be supposed from the con- centration of the body regions, is composed of a supracesoph- _./-geal syncerebrum and a larger subcesophageal ganglionic mass from which numerous nerves are given off. Eyes are usually wanting, or may be present in the form of one (Jxodes) or two pairs of small apparently simple ocelli. Coxal glands have been described as occurring at the bases of the second pair of legs (Oribatide). The repro- 456 INVERTEBRATE MORPHOLOGY. ductive organs show much variety in their arrangement, being sometimes paired and sometimes united to a single mass. The single genital orifice is situated far forward, in some cases even between the basal joints of the second pair of legs. Numerous accessory structures may be associated with the ducts, the receptacula seminis in some forms open- ing to the exterior quite independently of the oviduct, and protrusible organs serving for copulation in the male and for oviposition in the female may occur. The Acarina are as a rule oviparous, though a few forms are viviparous. Development of the Acarina.—Most of the Acarina whose development has been traced pass through a series of larval stages. While the young embryo is still within the egg and sometimes before the appendages have developed, a cuticular membrane is secreted around it lying between the embryo and the egg-shell. This is the dewtovwm, and within it further development proceeds. In those forms in which it does not appear until after the appendages are formed a degeneration of these structures takes place, and the egg-shell may also be thrown off leaving the embryo sur- rounded only by the deutovum (T7rombidiwm). New appendages now appear, and the larva hatches out from the deutovum as a six-legged form, sometimes showing traces of segmentation either in the thoracic region or in the abdomen. After a certain time a certain amount of de- generation of the tisues occurs (histolysis) and the appendages again dis- appear, a chitinous membrane forming around the now almost spherical body of the larva. A regeneration of the limbs and tissues takes place within this larval membrane, and the nymph is formed, resembling the adult in the number of appendages, but lacking fully-developed repro- ductive organs. A period of rest, and histolysis again occurs, accom- panied by the formation of a third cuticular membrane within which the nymph becomes transformed into the fully-developed and sexually-mature adult or imago, which finally issues from the membrane. This complicated process, it is needless to say, has no phylogenetic sig- nificance, the deutovum indeed being absent in certain forms (Tetrany- chus), nor does it seem likely that even the six-legged larva is anything but a secondary stage which has been developed within the group of the Acarina. There is no question but that the order represents the culmina- tion of a divergent line of evolution, perhaps from the Pseudoscorpionida, and since the separation many of the peculiarities characteristic of the group have been developed. Phylogeny of the Arachnida.—There seems little room for doubt but that the Scorpions among living forms represent most closely the ancestral Arachnoids, their segmentation being most perfect and their appendages more numerous than those of other forms. It is through the Scorpions TYPE ARACHNIDA. 457 accordingly, that relationships to other forms must be looked for, and a comparison of them with Limulus reveals similarities of structures so numerous and so detailed that the conclusion is unavoidable that both are to be traced back to a common ancestor. Thus the cephalothoracic ap- pendages in both are identical in number, and, so far as the first two pairs are concerned, in general structure also, while the genital opercula of the Scorpions are comparable in their relation to the genital orifices to the opercula of Limulus, and the pectines to the first pair of abdominal ap- pendages. The remaining abdominal appendages of Zimadlus, which are branchiate, seem at first sight to be unrepresented, but the embryo- logical investigation of the Scorpions appears to indicate that they are represented by the lung-books, which bear no little resemblance to the branchial lamelle of Zimulus, and the conversion of one set of organs into the other may be supposed to have been brought about by the formation behind each pair of abdominal appendages of an invagina- tion, which, deepening, has carried in with it the branchial lamellz, the original an- terior surface of the appendage forming the ventral wall of the body beneath the lung-sac, while the lamelle project into the sac for its ventral surface (Fig. 210). In the general form of the body Limulus corresponds fairly well with the Scorpions, y the cephalothoracic regions being strictly Fra. 210.—D1aGRaAM oF er comparable, as is also the terminal spine 9, [one-nooKs (after Kines- with the sting ; the abdomen, however, in rpy), the branchiate form has a smaller number i = indifferent stage. of segments which are all fused, a difference L = Limulus stage. readily explained by the probable derivation A = Arachnidan stage. of both forms from Hurypterws-like ancestors in which the abdomen possessed a relatively large number of distinct segments, and even showed indications of a differentiation into a przeabdomen and a postabdomen (see Fig. 198). In the internal structure quite as striking similarities are to be found in the presence of an endosternite in both groups and of coxal glands in connection with the fifth pair of appendages, in the tendency towards the concentration of the postoral ganglia, and in the invaginate origin of the median eyes, to mention but a few points. The Arachnida are accordingly to be traced back to Limulus or Huryp- terus-like ancestors, and through these finally to the Entomostraca, perhaps, a Crustacean ancestry being clearly indicated. As to the relationships of the various orders little that is definite can be said, differentiations having taken place along different lines in the various orders, so that while the 458 INVERTEBRATE MORPHOLOGY. Pedipalpi are more primitive as regards the number of abdominal seg- ments and their distinctness than the Aranez, yet the latter and especially the Tetrapneumones show a much more primitive condition of the respira- tory organs. With regard to these organs it may be stated that the con- dition in which they are represented by bunches of unbranched tracheze is more primitive than that in which they are branching tubes, the bunched condition being probably derived by a modification of original lung-books. TYPE ARACHNIDA. 1. Order Scorpionida.—Abdomen segmented and differentiated into pre- abdomen and postabdomen ; postabdomen terminating in poison- spine ; pedipalps chelate ; two pairs of abdominal appendages ; four of stigmata and lung-books. ZHuscorpius, Buthus. 2. Order Pseudoscorpionida.—Abdomen segmented but not differentiated ; no terminal spine; pedipalps chelate; no abdominal appen- dages ; two pairs of stigmata opening into trachee ; first pair of legs adapted for locomotion. Chelifer, Obisium, Chernes. 3. Order Solifuge.—Head separated from thorax with three segments; abdomen segmented but undifferentiated ; no terminal spine ; pedipalps palplike ; three pairs of stigmata leading into tracheex. Galeodes, Solpuga. ~ 4. Order Pedipalpi.No distinction of head and thorax ; abdomen seg- : mented, and either undifferentiated or with three small segments terminated by a multiarticulate flagellum ; pedipalps leglike or subchelate ; two pairs of stigmata and lung-books ; first pair of legs elongated and palplike. Phrynus, Thelyphonus. 5. Order Phalangida.—Abdomen segmented but undifferentiated and without appendages or terminal spine ; pedipalps leglike ; one pair of stigmata leading into traches; no spinning-glands. Leiobunum, Phalangium, Opilio, Gonyleptus, Cyphophthalmus, Gibbocellum. 6. Order Avanew.—Abdomen unsegmented and with two or three pairs of rudimentary papillalike appendages bearing the openings of ducts of numerous spinning-glands ; abdomen not fused with cephalothorax ; pedipalps long and palplike or leglike. 1. Suborder Tetrapneumones. — With four stigmata opening into sacs containing lung-books. Mygale, Cteniza. 2. Suborder Dipnewmones.—With four or three stigmata, the anterior pair opening into sacs with lung-books, the posterior one or two with trachew. Hpeira, Agelena, Tegenaria, Theridium, Segestria, Attus, Lycosa. 7, Order Acarina.— Abdomen unsegmented, without appendages, and fused with the cephalothorax ; pedipalps sometimes long and leglike, sometimes chelate; stigmata wanting or present as a TYPE ARACHNIDA. 459 single pair leading into tracheze; many forms parasitic ; fre- quently with complicated metamorphoses. Nonparasitic, or parasitic only in larval stage; terrestrial. Ort- bates, Nothrus, Trombidium. Aquatic. Hydrachna, Ataw. Living on organic matter. Tyroglyphus. Parasitic on animals. Demodex, Sarcoptes, Dermaleichus, Anal- ges, Gamasus, Ixodes. Parasitic on plants. Tetranychus, Phytoptus. LITERATURE. GENERAL. H. Grenacher. Untersuchungen tiber das Sehorgan der Arthropoden. Gottingen, 1879. E, R. Lankester. Limulus an Arachnoid. Quarterly Journ. Microscop. Sci- ence, xxi, 1881. J. Macleod. Recherches sur la structure et la signification de Vappareil respira- toire des Arachnides. Archives de Biologie, v, 1884. R. Sturany. Die Coxaldriisen der Arachnoiden. Arbeiten a. d. Zoolog. Inst. Wien, 1x, 1891. J. S. Kingsley. The Hmbryology of Limulus. Part II. Journ. of Mor- phology, virI, 1893. SCORPIONIDA. L. Dufour. Histoire anatomique et physiologique des Scorpions. Memoirs Acad. Sciences. Paris, xtv, 1856. E. R. Lankester. On the Coxal Glands of Scorpio, etc., and the Brick-red Glands of Limulus. Proceedings of the Royal Society, xxx1v, 1884. G. H. Parker. The Hyes in Scorpions. ‘Bulletin of the Museum of Compar- ative Zoology, x11, 1879. W. Patten. Zhe Origin of Vertebrates from Arachnoids. Quarterly Journal of Microscop. Science, xxx1, 1890. M. Laurie. The Embryology of a Scorpion (Huscorpius italicus). Quarterly Journ. Microscop. Science, xxx1, 1890. PSEUDOSCORPIONIDA. A. Croneberg. Beitrag zur Kenntniss des Bawes der Pseudoscorpione. Bulletin Soc. Imp. Naturalistes Moscou, m1, 1888. SOLIFUG. L. Dufour. Anatomie, physiologie, et histoire naturelle des Galéodes. Mé- moires Acad. Sciences, Paris, xvi1, 1858. PHALANGIDA. R. Rossler. Beitrdge zur Anatomie der Phalungiden. Zeitschrift fir wissensch. Zool., xxxvr, 1882. 460 INVERTEBRATE MORPHOLOGY. C. M. Weed. A Descriptive Catalogue of the Harvest-spiders (Phalangtide) of Ohio. Proceedings United States National Museum, xv1, 1893. ARANEA, E. Keyserling. Die Spinnen Amerikas. Nurnberg, 1880-91. G. W. and E. G. Peckham. North American Spiders of the Family Attide. Transactions of the Wisconsin Acad. Sciences, 1888. H. C. McCook. American Spiders and their Spinning Work. Philadelphia, 1889-90. J. H. Emerton. Papers in the Transactions of the Connecticut Academy, vu, 1889, and vit, 1891. G. Marx. Papers iv the Proceedings of the Entomological Society of Wash- ington, 1891, and in the Proceedings of the U. 8. National Museum, xu, 1890. W. Schimiewitsch. Etude sur Panatomie de l’Hpeire. Annales des Sciences Naturelles, 6me sér., xvi, 1884. Ph. Bertkau. Ueber den Verdauungsapparat bet Spinnen. Archiv. fir mi- kroskop. Anatomie, xxiv, 1885 Beitrige zur Kenntniss der Sinnesorgane bei Spinnen. I. Die Augen. Archiv fiir mikroskop. Anatomie, xxviI, 1886. E. L. Mark. Simple Hyes tn Arthropods. Bulletin Museum of Comparative Zoology, x1, 1887. A. T. Bruce. Observations on the Embryology of Insects and Arachnids. Baltimore, 1887. ACARINA. G@. Haller. Zur Kenntniss der Tyroglyphen und Verwandten. Zeitschr. fir wissensch. Zoologie, xxxtv, 1880. H. Henking. Beitrdge zur Anatomie, Entwicklungsgeschichte und Biologie von Trombidium fuliginosum. Zeitschr. fiir wissensch. Zoologie, xxxvm, 1882. A. D. Michael. British Oribatide. Loudon, 1884. A Nalepa. Anatomie der Phytopten. Sitzungsber. Akad. wissensch. Wien, xov, 1887. W. Winkler. Das Herz der Acarinen nebst vergleichenden Bemerkungen iiber das Herz der Phalangiden und Chernetiden. Arbeiten a. d. zool. Ins. Wien, vit, 1888. Anatomie der Gamasiden. Arbeiten a. d. zoolog. Inst. Wien, vu, 1888. APPENDIX TO THE ARACHNIDA. There are three orders which show a certain amount of affinity to the Arachnida, but which are not so closely related as to warrant the actual association of them with the orders which have been assigned to that type. They will be de- TYPH ARACHNIDA. 461 scribed here, and are the orders of the Pentastomide, the Pycnogonida, and the Tardigrada. Order Pentastomide. The Pentastomide are all parasitic, living in the adult stage in the lungs or nasal cavities of various animals, one species, Pentastomum teenioides, occurring in the nasal cavities or sinuses of dogs and wolves, while several species have been found in the lungs of different species of snakes (Fig. 211). They are all elongated wormlike forms, some- times slightly flattened and usually distinctly annulated, the annuli, however, not repre- senting a metamerism. The anterior end of the body is rounded and bears on the ventral surface the mouth, upon each side of which is situated a pair of strongly-recurved hooks (h) supplied with special muscles and serving for the attachment of the animal to the tissues of the host. With the exception of these hooks no appendages are present. The body is covered by a cuticle secreted by the ectodermal cells (hypodermis), be- neath which lies a layer of circular muscle- fibres, and beneath these again a layer of longitudinal muscles. The ccelom is ample and is traversed by dorso-ventral muscle- bands, which divide it into a central com- partment containing the various organs, sus- : F stomum teretiuscu- pended by mesenteries, and two lateral ones. ym Peyatm after There is no heart or circulatory apparatus, Spencer). and trachew or other respiratory organs are = books. : go = genital orifice. also wanting. The digestive tract is a straight tube extending through the body from the mouth to the terminal anus, giving off no lateral diverticula throughout its course. The nervous sys- tem (Fig. 212, ng) consists of a ganglionic mass lying below the cesophagus, a comparatively small commissural ring passing round that portion of the digestive tract, without, Fie. 211. — Penta- 462 INVERTEBRATE MORPHOLOGY. however, possessing any ganglionic enlargement which can be termed a cerebrum. Various nerves are given off from the mass, two of which extend backwards throughout nearly the entire length of the body. The only sense-organs pres- ent are a number of small papille on the anterior portion of the body, which are probably tactile in function. Glandular organs are highly developed. Scattered over the surface of the body are numerous flask-shaped glands, apparently ectodermal in origin, while lying in the ccelom on each side of the mid-gut and extending back almost to the posterior end of the body are two long cecal tubes, a glan- dular structure being also connected with them anteriorly. These glands open in the vicinity of the hooks and have hence been termed the hook-glands (Fig. 211, hg), and it has been suggested that they secrete a fluid which serves to keep the blood which the parasite ingests from coagulating, being thus similar to the glands in the pharynx of the Leeches od ov hg per Qierer eteromr - at i ‘ii a iii : LOR a= (a) S MSL GHHINAONS, ng Ts Fra. 212.—D1aGramM oF STRUCTURE OF FEMALE Pentastomum (after SpENECR). go = genital orifice. od = oviduct. hg = hook-gland. ov = ovary. ¢ = intestine. rs = seminal receptacle. ng = nerve-ganglion. ut = uterus. which serve the same purpose. Unless the ectodermal glands are excretory, no special organs for the carrying on of that function occur. The Pentastomide are bisexual, the male being smaller than the female, and recognizable by the situation of the geni- tal orifice (Fig. 211, go), which is near the anterior end of the body, while in the female it is near the posterior end. The ovary and testis.are both unpaired organs situated beneath the dorsal surface of the body and extending almost its entire length. Anteriorly a pair of oviducts (Fig. 212, od) arise from the extremity of the ovary (ov) and pass downwards and TYPH ARACHNIDA. 463 forwards towards the ventral surface, on nearing which they unite to form a long coiled tube, the uterus (ut), which passes backwards to the genital orifice, and just where the two ducts unite they have opening into them a pair of pyriform seminal receptacles (rs), The vasa deferentia are also paired, and arise at the anterior end of the testis, passing ventrally towards the genital pore, uniting before they reach it and dilating to form a complicated intromittent organ, from which two tubes with muscular walls and containing spermatozoa project backwards and serve as ejaculatory ducts for the ex- pulsion of the spermatozoa through the intromittent organ. The only genus belonging to the order is Pentastomum. Development of the Pentastomide.—During the life-history of a Pen- tastomum it passes through a marked metamorphosis associated with a change of hosts, recalling what occurs in the Cestoda. The ova are passed to the exterior with the excreta of the host, or, in the case of the dog, with the mucous discharge from the nasal passages, and the embryo which hatches out is a decidedly Mitelike form, possessing, however, only two pairs of legs terminating in ungues. No other appendages are present, but the embryo is provided anteriorly with a boring apparatus. If this larva of P. tenioides, the parasite of the dog, succeeds in gaining entrance to the digestive tract of a rabbit or cat, for instance, it bores through the wall of the intestine and, reaching the liver, encysts itself. Within the cyst it undergoes several moults, finally assuming a condition similar to the adult except that each annulus bears a circle of hooks. Leaving the cyst, then, it wanders through the tissues of the host, and if while it is in this condition the host is eaten by a dog, it adheres to the mucous mem- brane of the mouth of the latter, and makes its way into the nasal passages, there moulting again, losing the ring of hooks and assuming the adult form. The principal reason for supposing Pentastomum to be related to the Arachnids is the occurrence of the four-legged larva, which resembles, so far as its external form is concerned, a Mite. The internal structure is very different, however, although certain Arachnid features are indicated ; but it is evident that these forms must have undergone an enormous de- parture from the ancestral form during which the remarkable life-history and peculiar structure have been acquired. The parasitic habits of many Mites, and the general similarity of the body form of Demodew to that of Pentastomum, suggest the Mites as the ancestors of the latter, a theory which is as plausible as any other which can at present be suggested. Order Pyenogonida, The Pycnogonida are exclusively marine in habitat, and vary considerably in size, the smaller forms, such as Tanysty- 464 INVERTEBRATE MORPHOLOGY. lum, being only about a millimetre in breadth, while the purple Phoxichilidium measures over three millimetres from tip to tip of the legs, and the deep-sea form Collossendeis has a span of over sixty centimetres. The body proper is compar- atively small, the four pairs of long legs which arise from the thorax being exceedingly conspicuous, a feature which has i o% Abdomen Fig. 213.—Phozichilidium mazillare (after Morean). suggested the term Pantopoda sometimes applied to the group. Anteriorly there is a well-marked proboscis carrying the mouth at its anterior end, and at the base of this there arise the chelicere, which are rather short chelate limbs. The next segment of the body succeeding that which bears the chelicerze bears upon its dorsal surface the eyes, and may be regarded as a fusion of three segments since it bears three pairs of appendages. The most anterior of these are slender TYPE ARACHNIDA. 465 jointed palps; the second pair, wanting in the females of some species, but always present in the males, arise from the ventral surface of the segment, and are curved jointed struc- tures serving to carry the ova; while the third pair are ex- ceeding long jointed walking-legs. The next three segments also bear long walking-legs, the last one having attached to it the usually unsegmented rudimentary abdomen. The body and the appendages are encased in a well-de- veloped chitinous cuticle, and there are no indications of special respiratory organs. The heart lies immediately be- neath the dorsal integument and is a simple tubular organ with from two to three pairs of ostia. The portion of the digestive tract which lies within the proboscis is lined with chitin and opens behind into an elongated mid-gut, from which long diverticula extend out into the chelicerze and the proboscis and into the walking-legs, sometimes reaching even into the terminal joints of the latter. A short hind-gut leads to the anus at the tip of the abdomen. The nervous system consists of a supracesophageal gan- glionic mass, from which arise the optic nerves and those for the chelicerx, as well as certain nerves passing to the pro- boscis. Connected with this brain by circumoesophageal com- missures is a ventral chain consisting of five pairs of ganglia, the first pair of which is really formed by the fusion of two pairs, distinct in the embryo, and innervates the palps and the ovigerous legs, while the four pairs of walking-legs are sup- plied by the remaining four pairs. Finally one or two small ganglia also occur, innervating the abdomen. The eyes are four in number, situated at equal intervals upon a small domelike elevation on the dorsum of the first thoracic seg- ment, which, it is to be remembered, is compound. Each eye is covered by cuticle, sometimes thickened so as to forma lens, below which is a layer of cells forming the corneal or cuticular hypodermis. Below this comes a thick layer com- posed of retinal elements with nuclei in their outer portions and rodlike bodies towards the inner ends where they rest upon a layer of pigment. These eyes recall the postbacillar eyes of the Arachnids by their struct- ure, but show one remarkable peculiarity, i.e., a distinctly bilateral ar- 466 INVERTEBRATE MORPHOLOGY. rangement both of the corneal hypodermis and of the retinal elements, a distinct raphe being observable upon the inner surface of the eye, the reti- nal elements being arranged on either side of it. Such a condition as this cannot readily be explained by a simple unilateral inversion such as was described as probably occurring in Arachnidan eyes; it suggests rather an inversion of two sides of a primitive optic cup, the posterior wall at the same time forming the pigmented layer of the eye. Whether the Arachnid eye is not also traceable to such an arrangement, all traces of the original raphe being lost, is a question, though at present it seems more probable that it has been produced by a suppression of the inversion of one side of the cup. Glands, occurring in the palps and ovigerous legs, have been regarded as excretory in function, but no Malpighian tubules or coxal glands seem to exist, though an homology of the excretory glands just mentioned and of glands occurring in the walking-legs of the males with the latter is not impossible. The Pycnogonids are bisexual, the reproductive organs lying in the thorax and sending out branches into the walking-legs, on the fourth joints of one or more of which they open. As already stated, the male carries the eggs upon his ovigerous legs, fastening them as they are extruded by the female by means of the excretion of the glands occurring upon the walking-legs. Development and Affinities of the Pycnogonida.The young Pycno- gonid leaves the egg as a six-limbed embryo, which recalls, in a general way, the nauplius of the Crustacea, and indeed has suggested a derivation of the Pyenogonids from that group. The resemblance is, however, but superficial, important differences being found in the structure of the eyes and in the absence of an auus, to say nothing concerning the details of the early development. On the other hand these last, as well as the structure of the eyes, recall the Arachnids, and it seems most probable that the Pyecnogonids are to be regarded as having descended from ancestors which might have been included in the type Arachnida. Order Tardigrada. The Tardigrada are small forms not exceeding a milli- metre in length, with an unsegmented body provided with four pairs of short conical appendages tipped with claws, the last pair being situated at the posterior extremity of the body. The body is covered by a cuticle secreted by the subjacent hypodermis, below which and traversing the ccelom is a well- TYPH ARACHNIDA. developed system of muscle-bands. 467 There are no special organs either for respiration or circulation. The mouth, surrounded by papille (Fig. 214, p), lies at the anterior extremity of the body, and leads into a tubular mouth-cavity containing, imbedded in its walls, a pair of chitinous or partly calcareous teeth, and receiving the ducts of two glands (sy) which have been regarded as salivary or perhaps poisonous in function. Behind, this cavity opens into a muscular pharynx (ph) which is connected by a short esophagus with the mid-gut. At the junction of this with the rectum or hind-gut is a pair of cecal diverticula (lg), probably Malpighian tubules, and into the hind-gut there also open the ducts of the reproductive organs, the hind-gut thus serving asa cloaca. It opens on the ventral surface of the body a short distance from the pos- terior extremity and therefore in front of the last pair of appendages. The nervous system consists of a supracesophageal ganglion (ce) united with a chain of four pairs of ventral ganglia. No special sense-organs occur except two eyes situated at the sides of the head. The sexes are distinct, the reproductive organ being unpaired and opening into the cloaca, into which opens also in both sexes an unpaired accessory gland. - Fie. 214.—D1aGRAM oF Srructure or Macro- biotus Hufelandi (com- bination of figures by PLATE). an = anus. ce = cerebral ganglion. dg = dorsal gland. ig = lateral gland or Mal- pighian tubule. m = muscle. ov = ovary. p = papille. ph = pharynx. sg = salivary(?) glands. The Tardigrada occur in water usually, especially in such locations as the gutters on the roofs of houses, though some- times found also among moss. The group contains but a small number of genera, of which Macrobiotus is perhaps the most common. 468 INVERTEBRATH MORPHOLOGY. Affinities of the Tardigrada.—The embryological history of these forms has not yet been sufficiently studied to allow of any definite conclu- sions as to their affinities. ‘I'he presence of four pairs of limbs has usually been regarded as pointing to a relationship with the Acarina, but the ab- sence of all mouth-appendages, the structure of the legs, and the position of the last pair with regard to the anal opening, not to mention the peculi- arities of the internal organization, are opposed to any close relationship with the Arachnida. The Tardigrada must be considered as holding an independent position, without distinct indications of relationship with any of the types, until further information as to their developmental phenomena has been secured. | LITERATURE. PENTASTOMIDA. R. Leuckart. Bau und Entwickiungsgeschichte der Pentastomen. Leipzig und Heidelberg, 1860. C. W. Stiles. Buu und Entwicklungsgeschichte von Pentastomum proboscideum und P. subcylindricum. Zeitschr. fiir wissensch. Zoologie, L11, 1891. A. B. Spencer. The Anatomy of Pentastomum teretiusculum (Baird). Quar- terly Journal of Microscop. Science, xxxrv, 1892. PYCNOGONIDA. E. B. Wilson. The Pycnogonida of New England and Adjacent Waters. Re- port of the U. 8. Commissioner of Fish and Fisheries for 1878. Wash- ington, 1880. A. Dohrn. Die Pantopoden des Golfes von Neapel. Fauna und Flora des Golfes von Neapel. Monographie, 111, 1881. P.P.C. Hoek. Report on the Pycnogonida. Scientific Results of the Voyage of H.M.S. Challenger. Zoology, 11, 1881. T. H. Morgan. A Contribution to the Embryology and Phylogeny of the Pycno gonids. Studies for the Biol. Laboratory, Johns Hopkins University, v, 1891. : TARDIGRADA. L. Plate. Bettrige zur Nuturgeschichte der Tardigraden. Zoolog. Jahrbicher, Anatom. Abtheilung, 11, 1888. TYPE TRACHEATA, 469 CHAPTER XV. TYPE TRACHEATA. Tur Tracheata are, like the Arachnida, essentially terres- trial forms, for, though a few Insects have adapted themselves to an aquatic mode of life, they are nevertheless air-breathers, living either at the surface of the water or coming to the sur- face trom time to time to renew the air contained in the trachez which ramify through the body and serve as respira- tory organs. However, a few Insect-larve have acquired the power of extracting oxygen from the water by branchia-like processes of the body, but, even in these cases, trachee form the organs by which the respiration is carried-on, , the branchize being richly supplied with them. The body is distinctly segmented feecaph in Pertpatus,) and is covered by a chitinous cuticle secreted by the ecto- dermal cells, which constitute the so-called hypodermis. The appendages are usually uniramous, and with few exceptions (Peripatus) are jointed. The anterior pair in all cases are more or less elongated multiarticulate structures provided with sense-hairs, and are situated preorally, while of the remaining pairs, varying in number in different groups, the most anterior pair is specialized to serve as mandibles, while the succeeding one or two pairs usually form maxille. Numer- ous glands of varying function are developed in the hypo- dermis, the most interesting of which are the crural glands, well developed in Peripatus, and represented more or less perfectly in certain other forms. In addition to these, glands which secrete an acrid or offensive fluid (repugnatorial glands) are frequently present, as well as others which secrete waxy substances, or even in some cases silk. The ccelom except in Peripatus is lacunar throughout, pos- sessing no definite walls, and is traversed in various directions by muscles, serving to flex or extend the body and to move 470 INVERTEBRATE MORPHOLOGY. the appendages. A marked difference between the Tracheata and the Arachnida is the universal absence of an endo- sternite, a structure of considerable phylogenetic significance in the latter group. A heart is invariably present, lying above the intestine, and situated in a pericardial sinus incom- pletely partitioned off. In the majority of forms the parti- tion is composed of a varying number of triangular muscles, the alar muscles, which are attached by their bases to the walls of the heart, and by their apices to the body-wall. While at rest they are somewhat vaulted, the convexity being dorsal, and on contraction flatten down, thus enlarging the sinus and causing a flow of blood into it. The heart (Fig. 227, h) is elongated and imperfectly divided into a series of chambers, separated by pairs of valves which allow the blood to flow from behind forwards but not in the reverse direction, the heart being closed behind. Ostia are present in the lateral walls to allow of the entrance of blood into the heart- chambers, whence it is propelled through very short arteries which open widely into the lacunar spaces of the celom. In many forms a ventral sinus surrounds the ventral ganglionic nerve-chain, the blood flowing in it from before backwards, but with this exception definite vessels are wanting. This is compensated for by the rich branching of the tracheex, which, as stated, serve as respiratory organs and convey air to all parts of the body; the air is in fact brought directly to the tissues, instead of being carried to them by the blood from limited portions of the surface of the body. The blood is usually colorless, but in some cases is of a bright yellow or green color, owing to pigment contained in the plasma, and it contains in all cases colorless amoeboid corpuscles. The trachex (Fig. 215, tr) communicate with the exterior along the sides of the body by a varying number of pairs of stigmata (sé), and may either consist of bunches of unbranched tubes connected with each stigma, or of a number of richly- branching tubes, each one arising from a separate stigma and anastomosing in some cases through some of its branches with the tubes from other stigmata. Each stigma is usually provided with an apparatus by which it may be closed, and in the Insects the air is expired from the trachex by the con. TYPE TRACHEATA. 471 traction of certain dorso-ventral muscles of the abdomen, which cause a compression of the organs in that region of the body, inspiration following on their relaxation and the conse- quent re-expansion of the abdomen. In structure the trachez are simply to be regarded as invaginations of the body-wall, and consist of a single layer of cells continuous with the hypo- dermis of the body, lined within —that is to say, on the surface At with which the air is in contact —with chitin, which is thick- ened in such a way as to form a spiral band extending along \ the tube and serving as a spring ; to keep its walls apart. sate The digestive tract is in \ most groups a straight tube, but in Insects (Fig. 227) it may pa") P be coiledin a more or less com- af oo 3 plex manner and differentiated ie me into several parts. Glands of é = various kinds are usually asso- ciated with it, salivary glands @) (Fig. 227, sg) opening into the anterior portion and Malpighian gt i g tubules (mv), in connection with sd the posterior portion, being the most constant in occur- Z Fie. 215.— FIGURE SHOWING THE rence. It is to be noted that Disorrurion or TRACHER IN the fore-gut and hind-gut are Aphis pelargonti (after Wrrtaczit). ectodermal in origin, and that At = antenne. the Malpighian bodies arising g = gland-duct. as outgrowths from the hind- lance : : tr = trachea. gutarealso ectodermal, differing 4, 9 3 — thoracic appendages. thusin origin from the similarly- named organs of the Arachnida, which are apparently of endodermal origin, arising from the mid-gut. In function both organs are similar, the Malpighian bodies of Tracheates being excretory. The nervous system in the less-differentiated members of ESS ~Y tr 472 INVERTEBRATE MORPHOLOGY. the type consists of a supracesophageal ganglionic mass, con- nected by circumcoesophageal commissures with a chain of ventral ganglia, a pair of ganglia corresponding typically with each segment. In the Insecta (Fig. 228) more especially, however, considerable concentration occurs, a number of the postoral ganglia, or, in some cases, all of them, fusing to a single mass. A well-developed stomatogastric or sympathetic nervous system occurs in all forms, arising from the supra- cesophageal ganglionic mass by two trunks, which unite to form a single nerve, passing to the digestive tract, and in some cases provided with ganglionic enlargements both paired and unpaired. Sense-organs of various kinds are well developed in the Tracheata, with the exception of Peripatus, in which the only definite organs of special sense are the eyes. In other forms the antenne and other portions of the body are pro- vided with hairs connected with nerves and serving as tactile organs, and sete situated upon the mouth-parts and associated with peculiar nerve-endings have been supposed to represent organs of taste, and others again, on the antenne, olfactory organs. Jiyes are very generally present. In Peripatus and most Myriapoda simple eyes or ocelli are alone present; in Peripatus they resemble closely in structure the eyes of the Annelids or Mollusea (e.g. Haliotis, see Fig. 127), but in the Myriapods and Insects they are usually more complicated. Thus in a young larva of