Tee rennet be ae ea ert Maen inw avons tee b enone ee eae eS NCTE TTR Eon A ice a ma Ot A en SPE IAG > Ape “s."% id ; TEXTBOOK OF OO EP aa | Vie AN AO MY TEXT-BOOK OF COMPARATIVE ANATOMY BY DR. ARNOLD LANG” PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF ZURICH FORMERLY RITTER PROFESSOR OF PHYLOGENY IN THE UNIVERSITY OF JENA TRANSLATED INTO ENGLISH BY HENRY M. BERNARD, M.A. Canras. AND MATILDA BERNARD PATS EL London MACMILLAN AND CO, Lrp. NEW YORK: MACMILLAN & CO. LIZ All rights reserved tion Ms - ae Me a) el ee A ls a TRANSLATORS’ PREFACE THE fact that this second volume of the translation appears four years after the first is due partly to the delay in the issue of the third and fourth German parts of which it is composed, and partly to the increased difficulty in the work of translation. A comparison of the two volumes will show at a glance that the work has developed under the hands of the author: the treatment has become more elaborate. The two “chapters” which practically fill this volume are in reality more like comprehensive treatises on the groups with which they deal, and as such could only be adequately translated from the German by some one with a very special knowledge of both groups. There are probably few zoologists who have attempted to make a special study of two such heterogeneous phyla as the Mollusca and the Echinodermata. In addition, therefore, to frequent references to the original literature and to constant applications to kind friends, the whole of the text relating to the two chief groups was submitted to specialists for revision. The translators beg to tender their warmest thanks to their friends who kindly undertook this laborious task. Mr. B. B. Woodward read the text of the chapter dealing with the Mollusca, revising the terminology, and suggesting slight alterations, which have been either adopted without comment in the text or else placed in short footnotes. Mr. W. Percy Sladen and Mr. F. A. Bather revised the text dealing with the Echinodermata, each with special reference to the group with which his name is most asso- ciated. Thanks are also due to Professor Jeffery Bell for his kind assistance in the solution of difficulties. We have no hesitation in saying that it is to the generous help of these gentlemen that vl COMPARATIVE ANATOMY our translation owes much of the value it may possess for the English student. In the use of certain technical terms we have given the English or the Latin form indifferently, ¢.g. pinnule or pinnula, auricle or auricula, with deliberate inconsistency. On the other hand, we have throughout used the terms madreporite, madreporitic, and Echinoder- mata, although some authorities are more in favour of madrepore, madreporic, and Echinoderma. We feel it our duty to call the atten- tion of students to these points. The following author’s preface is a free translation of the Nachwort which appeared at the end of the fourth German part. In it the author answers the only serious charge against the work as a text-book which has been brought to our notice. It finds its most appropriate place as a preface to the second volume of the translation. H. & M. BERNARD. AUTHOR’S PREFACE TO THE SECOND VOLUME WitH the publication of the last two chapters, dealing with the Echinodermata and the Enteropneusta—that is of the fourth German portion—I bring this text-book to a close for the time being, as a comparative anatomy of the Invertebrata. I feel that some excuse is necessary for the tardy appearance of the separate parts, especially of the third (Mollusca). This was mainly due to my call to the University of Zurich, where official duties left only the holidays and vacations for my own work. When I add that the greater number of the illustrations were drawn by my own hand, the reader will, I trust, pardon the lapse of time. Indeed, if he be a trained zoologist, he will be specially sympathetic and indulgent, and will be able to realise my feelings as I watched the fresh relays of books piling up before me at the commencement of each new chapter. Original sources alone have been relied upon for the subject matter of the work. In spite of the imperfections and deficiencies of which I am only too conscious, the book appears to have been found useful, judging from the favourable reception almost universally given to it, and from the circumstances that, even during its appearance, it was translated into foreign languages. I am fully aware that the matter is unequally worked up. The divisions treated in the first volume are too briefly dealt with, a defect which must be remedied in a new edition. Any criticisms or advice with which my colleagues may favour me will be gladly accepted in the spirit in which they are intended. I have been blamed by many for not mentioning the names of Vill COMPARATIVE ANATOMY authors in the text. From the very first this question caused me much perplexity, and I made repeated attempts to indite single chapters so as to bring in the historical development of the branch dealt with, together with the names of the most important authors. I then found that if this course were pursued the book would attain twice its present dimensions, that is, if strict impartiality were to be invariably observed. This latter I was resolved on no account to renounce, and I therefore determined to exclude from the text the names of all authors without distinction. Any one who is interested in knowing how a special question stands, can easily find his bearings by careful comparison of the text with the illustra- tions (the origin of which is everywhere given), and by consulting the literature. I have convinced myself of this among my own students. I must here express my thanks to my honoured and dear friend, Mr. Gustav Fischer, for the care and patience he has exercised in connection with this work. ARNOLD LANG. ZuricH, July 1894. CONTENTS CHAPTER VII MOLLUSCA PAGE Systematic Review : : : : ; : ; : 2 Class I. AMPHINEURA : : : : ; ; ‘ 2 II. GAsTROPODA (CEPHALOPHORA) . : : : : 3 III. ScapHorpopA ‘ : : ; ? ; : 13 IV. LAMELLIBRANCHIA (PELECYPODA, BIvAtva, ACEPHALA, AGLOSSA) : : ; : ' ‘ : 14 VY. CEPHALOPODA : ; : : : : 5 21 I. ORGANISATION OF THE PRIMITIVE MoLLusc : : : 26 II. REVIEW OF THE OUTER ORGANISATION CHARACTERISING THE CHIEF GROUPS OF THE MOLLUSCA 5 : : ; 28 A, PLACOPHORA OR POLYPLACOPHORA (CHITONIDZ). : ; 29 B. APLACOPHORA, SOLENOGASTRES . : : : : 29 C. GASTROPODA (CEPHALOPHORA) . - ; : : 30 D. ScaPHOPoDA : ‘ . : ‘ ; F 34 E. LAMELLIBRANCHIA . 5 ; ‘ . 2 . 34 F. CEPHALOPODA F : ‘ : ; ; , 36 III. Tur INTEGUMENT, THE MANTLE, AND THE VISCERAL DOME : 39 A. PLACOPHORA : 5 : ; - $ : 39 B. SOLENOGASTRES ‘ : ; : : ; : 41 C. GASTROPODA : : ; : ; ; : 42 D. ScaPHOPODA ‘ ; : : ; ‘ A 49 E. LAMELLIBRANCHIA . , z : ’ ; : 49 F. CEPHALOPODA : ‘ F : : F : 53 IV. THe SHELL . : é : : ; ; : 55 A, AMPHINEURA 58 B. GASTROPODA 4 F : : : ; : 58 COMPARATIVE ANATOMY C. LAMELLIBRANCHIA . D. CEPHALOPODA V. ARRANGEMENT OF THE ORGANS IN THE MANTLE CAVITY, AND OF THE OUTLETS OF INNER ORGANS IN THAT CAVITY A, GASTROPODA 5 B. ScAPHOPODA C. LAMELLIBRANCHIA . D. CrEPpHALOPODA VI. THE RESPIRATORY ORGANS . THE TrurE GILLS OR CTENIDIA AMPHINEURA GASTROPODA LAMELLIBRANCHIA . CEPHALOPODA ADAPTIVE GILLS LuNcs : eS) ered VII. Tort HyprosBrRANCHIAL GLAND VIII. THz Heap . A. GASTROPODA B. ScAPHOPODA C. CEPHALOPODA IX. THe Ora Lopes OF THE LAMELLIBRANCHIA X. THe Foor AND THE PEDAL GLANDS A. AMPHINEURA B. GASTROPODA C. ScAPHOPODA D. LAMELLIBRANCHIA . E. CEPHALOPODA XI. SwELLInG or THE Foor (Zurgescence) XII. MuscuLaturE AND ENDOSKELETON A. AMPHINEURA B. GASTROPODA C. SCAPHOPODA D, LAMELLIBRANCHTA E. CEPHALOPODA ‘ XIII. Tort Nervous SysteM A. AMPHINEURA B. GASTROPODA 101 CONTENTS 1. THE AREAS OF INNERVATION OF THE VARIOUS GANGLIA 2. ORIGIN OF THE CROSSING OF THE PLEUROVISCERAL Con- NECTIVE (CHIASTONEURY) . : ; : ; 3. SPECIAL REMARKS ON THE NERVOUS SYSTEM OF THE GAS- TROPODA . C. SCAPHOPODA D. LAMELLIBRANCHIA . E. CEPHALOPODA ; XIV. AN ATTEMPT TO EXPLAIN THE ASYMMETRY OF THE GASTROPODA XV. THE SENSORY ORGANS F ; A. INTEGUMENTAL SENSORY ORGANS 1. TACTILE ORGANS 2. OLFACTORY ORGANS 3. THE ‘‘ LATERAL ORGANS’ 4. GuSTATORY ORGANS . 2 5. SUBRADULAR SENSORY ORGAN OF CHITON . 6. THE SENSORY ORGANS ON THE SHELL OF CHITON B. AUDITORY ORGANS C. VISUAL ORGANS 1. Opric Pits 2. Opric VESICLES OR VESICULAR EYEs 3. THE Eyk oF THE DIBRANCHIATE CEPHALOPODA 4. THE DorsaAL EYES OF ONCIDIUM AND THE EYES AT THE EpGE OF THE MANTLE IN PECTEN 5. THE EYES ON THE SHELL OF CHITON 6. THE CompouND Eyes or ARCA AND PECTUNCULUS 7. DEGENERATION OF THE CEPHALIC EYES 3 OF THE DIOTOCARDIA XVI. THE ALIMENTARY CANAL A. BuccaL Cavity, SNout, PRogoscis : B. THE PHARYNX AND JAWS, THE TONGUE AND SALIVARY GLANDS FoRMATION OF THE RADULA C. THE CsopHacus : : : 5 ‘ D. THE Mip-gur with THE SToMACH AND DIGESTIVE GLAND (LIVER) . 1. AMPHINEURA 2. GASTROPODA ScAPHOPODA LAMELLIBRANCHIA 5. CEPHALOPODA E. Hinp-cur (Rectum) XVII. THE CircuLatory SysTEM A. GENERAL X1i COMPARATIVE ANATOMY B. SPEcIAL 1,: AMPHINEURA GASTROPODA . SCAPHOPODA . LAMELLIBRANCHIA . CEPHALOPODA . Oe ow bP XVIII. Tor Bopy Cavity XIX. THE NEPHRIDIA A. AMPHINEURA B. GASTROPODA C. SCAPHOPODA D. LAMELLIBRANCHIA . E. CrEpHALOPODA XX. GENITAL ORGANS A. GENERAL B. SPECIAL XXI. PARASITIC GASTROPODA XXII. ATTACHED GASTROPODA XXIII. OntrocrNy A. AMPHINEURA B. GASTROPODA XXIV. PHYLOGENY ‘ ; Review of the most Important Literature APPENDAGE,—RHODOPE VERANII CHAPTER VIII ECHINODERMATA Systematic Review CuAss I. HoLoTHuRIOIDEA II. EcHINOIDEA III. AsTEROIDEA IV. OPHIUROIDEA V. PELMATOZOA 1. CRINOIDEA 2. CYSTIDEA 3. BLASTOIDEA I. GENERAL MorpHoLoGy oF THE ECHINODERM Bopy Il. MorPHoLoGy OF THE SKELETAL SYSTEM CONTENTS INTRODUCTION 5 : : A. THE APICAL SysTEM (Calyx) 1. ECHINOIDEA . 2. ASTEROIDEA . 3. OPHIUROIDEA 4, PELMATOZOA (a) CRINOIDEA (6) BLASTOIDEA . (c) CYSTIDEA . : B. THE ORAL SYSTEM OF PLATES . C. THE PERISOMATIC SKELETON 1. HoLorHuRIOIDEA 2. ECHINOIDEA . ; P ‘ é (a) THE NUMBER OF THE VERTICAL Rows OF PLATES (b) THE PorES OF THE AMBULACRAL SYSTEM (c) THE SYMMETRY OF THE ECHINOID SHELL (dq) THE RELATION OF THE AMBULACRAL AND INTERAMBU- LACRAL PLATES TO THE PERISTOME (¢c) MANNER IN WHICH THE SKELETAL PLATES ARE CoN- NECTED é : z (7) SpectAL MopIFICATIONS OF THE AMBULACRA (g) SpEcIAL MopIFICATIONS OF THE INTERRADII (2) ForM OF THE PERISTOME . (it) ORNAMENTATION. : : (k) MARGINAL INCISIONS OR PERFORATIONS (1) THE PERIGNATHIC APOPHYSIAL GIRDLE 3. ASTEROIDEA . : : : (a) THE AMBULACRAL SKELETON : (6) THE INTERAMBULACRAL SKELETON : (ec) THE AccEsSSORY SKELETAL SYSTEM (d@) COMPARISON OF THE PERISOMATIC SKELETON OF ASTEROIDEA WITH THAT OF THE ECHINOIDEA 4, OPHIUROIDEA . : ‘ (a) SKELETON OF THE ARMS (6) THe OrAL SKELETON 5. CRINOIDEA ; (2) THE PERISOMATIC SKELETON OF THE CALYX a. THE APICAL CAPSULE OR DorsAL Cup b. THE TEGMEN CALYCIS . : (6) THr BRACHIAL SKELETON . (c) THE Stem (CoLuMNA) ? : ; THE (d@) THE MANNER OF CONNECTION BETWEEN THE SKELETAL PIECES i ‘ ‘ ‘ (e) THE NERVE CANALS OF THE ARMS AND OF THE APICAL CAPSULE xiil PAGE 317 319 319 326 327 328 328 330 332 333 337 337 338 339 340 340 B44 D. E. iH G. H Ill. Toe OvrER MorpHoLocy oF THE HOLOTHURIOIDEA IV. THE PoOsITION AND ARRANGEMENT OF THE MOST COMPARATIVE ANATOMY (f) THE WaTER PORES 6. BLASTOIDEA (a) THE AMBULACRAL SKELELON (b) THE STEM 7. CYSTIDEA THE SPINES AND THEIR DERIVATIVES THE PrDICELLARI® THE SPHHRIDIA AND Tue Masticarory APPARATUS OF THE ECHINOIDEA. (Aris- totle’s Lantern) THE CaLcargEous RING OF THE HOLOTHURIOIDEA FurTHER Deposits oF CALCAREOUS MATTER . CONCLUDING REMARKS ON THE SKELETON ORGANS IN THE RADII V. THe INTEGUMENT VI. THE WATER VASCULAR SYSTEM A. THrt MADREPORITE AND STONE CANAL B. THE WATER VASCULAR RING C. THe RapDIAL CANALS, THE CANALS OF THE TENTACLES AND D TUBE-FEET, ETC. . THE AMBULACRAL APPENDAGES VII. THE Ca@Lom UOw> Tue Bopy Cavity . THE BRACHIAL CAVITIES Tuer PERI@SOPHAGEAL SINUS THE PERIANAL SINUS THE AXIAL SINUS. THE AXIAL ORGAN THe CHAMBERED SINUS VIII. Tor PsrEUDOH#MAL SYSTEM IX. THe EpPINEURAL SYSTEM X. Tur Bioop VASCULAR OR LACUNAR SYSTEM XI. THe Nervous SYSTEM A. Tur SUPERFICIAL ORAL SYSTEM . B C. D . Toe DrerrER OrnAL NERVOUS SYSTEM Tue APICAL oR ABORAL NERVOUS SYSTEM . Toe Turrp Nervous SYSTEM OF THE CRINOIDEA IMPORTANT CONTENTS XII. THE Sensory ORGANS A. Bb. THE AMBULACRAL APPENDAGES AS SENSORY ORGANS NERVE ENDINGS IN THE INTEGUMENT C. AUDITORY ORGANS, ORGANS OF ORIENTATION 1D). EYEs XIII. THE Bopy MuscunarvureE A. B. HoLoOTHURIOIDEA ECHINOIDEA C. ASTEROIDEA D. E. OPHIUROIDEA CRINOIDEA . XIV. Tat ALIMENTARY CANAL Ano OoR Pe GENERAL REVIEW . HOLOTHURIOIDEA ECHINOIDEA CRINOIDEA . ASTEROIDEA OPHIUROIDEA XY. RESPIRATORY ORGANS A. THE (INNER) REsprraATORY TREES OF THE HOLOTHURIOIDEA REVIEW OF THE RESPIRATORY ORGANS OF THE ECHINODER- B. MATA XVI. THE CUVIERIAN ORGANS OF THE HOLOTHURIOIDEA XVII. Excretion XVIII. THE Saccunt or THE CRINOIDEA XIX. GENITAL ORGANS GOW GENERAL MORPHOLOGY HoLOTHURIOIDEA ASTEROIDEA OPHIUROIDEA 1. THe Burs 2. THE GENITAL APPARATUS EcCHINOIDEA CRINOIDEA . ORIGIN OF THE SEXUAL PRODUCTS . HERMAPHRODITISM IN ECHINODERMS 1. CARE OF THE BRoop AND SEXUAL DIMORPHISM XX. CApAciry FoR REGENERATION AND ASEXUAL REPRODUCTION XXI. ONTOGENY NG THE VARIous LARVAL FoRMS OF THE EKCHINODERMATA Xvl COMPARATIVE ANATOMY PAGE B. ONTOGENY OF THE HOLOTHURIOIDEA : ; 5 : 510 C. ONTOGENY OF THE ECHINOIDEA . ; 3 ’ p 519 D. ONTOGENY OF THE ASTEROIDEA 524 E. ONTOGENY OF THE OPHIUROIDEA . : : ‘ : 532 F. ONTOGENY OF THE CRINOIDEA . : 3 é : 533 XXII. ‘PHYLOGENY . ; : : : : ; ; 545 Review of the most Important Literature . c ‘ ‘ 5b1 CHAPTER IX ENTEROPNEUSTA I. OUTER ORGANISATION. , ; ; ; : , 562 Il. THt Bopy ErirHELIUM : f , : i : 563 III. Tot Nervous System ; 3 , } ‘ ; 564 IV. THE SENSORY ORGANS 3 s : 5 : ; 565 V. THE ALIMENTARY CANAL : 5 ; ; é : 565 VI. Tur Ca@tomic Sacs AND THE Bopy MuscuLATURE. : : 571 VII, Tur *‘ HEART VESICLE” ; 3 ; 4 : : 578 VIII. Tot Limiting MEMBRANES, THE PROBOSCIDAL SKELETON, AND THE BRANCHIAL SKELETON . : ; 5 E : 579 IX. THr BLoop VASCULAR SYSTEM ; : ‘ : ; 581 Xe Dr (GONADS eee : ; F k 5 : 585 XI. ONTOGENY : : : d ; : x : 586 XII. PHYLOGENY . : : ‘ : ; : é 591 Literature. : ; : , s ; : 595 APPENDAGE TO THE ENTEROPNEUSTA I. CEPHALODISCUS : : : P ; ; : 596 Il. RHABDOPLEURA : : , : 5 A f 600 Literature. ; : ‘ ; : , d 602 INDEX . : , . : ; r 2 . 3 603 -_~ CHAPTER. VIL SIXTH RACE OR PHYLUM OF THE ANIMAL KINGDOM MOLLUSCA. THE Mollusea are essentially bilaterally symmetrical animals with unsegmented bodies. The ventral wall is thick and muscular, and forms a foot which is used for locomotion, and assumes the most varied shapes. A fold of the body wall forms a circular mantle, which hangs down round the body, enclosing a space which is called the mantle or pallial cavity. This cavity is originally deepest and most spacious posteriorly, and contains, at the sides of the median anus, symmetrically grouped, the two gills and the renal and genital apertures. The dorsal portion of the animal is generally developed into a visceral dome or sac, and is protected down to the edge of the mantle by a shell. The mouth les at the anterior end of the body and leads into a pharynx, which is usually provided with jaws and a rasp-like organ called the radula. The mesenteron or mid-gut is supplied with a large digestive gland (liver). The secondary ccelom (enclosed by its own walls) is reduced, but always persists as a pericardium. The blood vascular system is open, and generally to a great extent lacunar. The heart is dorsal and arterial, and was primitively provided with two symmetrical auricles. The nephridia were originally paired, and in open communication with the pericardium. The central nervous system consists of paired cerebral, pleural, pedal, and visceral ganglia. The Mollusca are either sexually separate or hermaphrodite. The gonads are usually single, with paired or unpaired ducts. In the course of development a modified Trochophora arises from the gastrula; this is the Veliger larva, typical of the Mollusca. These general characteristics of the Molluscan body have to be modified for each class. In each class there are series of forms which deviate from the typical organisation in some one important point, or in several. The shell may disappear, and so may the mantle. Either one or both of the gills or ctenidia may be lost, VOL. II B 2 COMPARATIVE ANATOMY CHAP. and new, morphologically different respiratory organs may be substituted. The visceral dome may be flattened down, and the foot become rudimentary or disappear. Teeth of all kinds may be wanting. The complex of the sub-pallial organs may be so displaced as to lie anteriorly, thereby causing a very pronounced asymmetry of the whole organism. But the typical Molluscan characteristics are never so entirely obscured that the members of the race cannot be recognised, on the one hand by means of transition forms leading to well-known Molluscan types, and on the other by their developmental history. The Molluses are divided into the five following classes :— I. Amphineura. Il. Gastropoda. III. Scaphopoda. IV. Lamellibranchia. V. Cephalopoda. Systematie Review. CLASS I. Amphineura. Bilaterally-symmetrical Molluscs. The nervous system consists of two lateral and two ventral nerve trunks, bound together’ by numerous commissures, and Fic. 1.—Chiton, from life (after Prétre, in the Voyage de l’ Astrolabe). provided with ganglion cells throughout their whole length ; these pass anteriorly into the cerebral ganglion. Special sensory organs are reduced. Marine. OrpER 1. Placophora (Polyplacophora) sive Chitonide. On the dorsal side there are eight consecutive shelly plates overlapping like the tiles on a roof. There is a distinct snout. The branchiew are numerous, and are arranged in two longitudinal rows, one on each side in the groove between the foot and mantle. The foot (except in Chitonellus) is strongly developed, with a large flat VII MOLLUSCA—SYSTEMATIC REVIEW 3 sole for creeping or for attachment. The sexual ducts and the nephridia are paired. The sexes are separate. The heart is provided with two auricles. Radula (3 +1), (2+1), (1+1+1), (142), (1+8). Chéiton (Fig. 1), Chitonelius. fo) ORDER 2. Aplacophora sive Solenogastres.! The body is almost cylindrical, and generally vermiform. There is no shell. The much thickened cuticle contains calcareous spicules. The foot is rudimentary, a mere ridge being left, and the mantle cavity is reduced to a groove at the sides of this ridge, and a cavity (cloaca) at the posterior part of the body, into which the intestinal canal and nephridia open, and in which are found, when present, the rudimentary gills. The nephridia serve as ducts for the genital products. Family 1. Neomeniide. The foot is a longitudinal ridge, which rises from the base of a medio-ventral Fic. 2.—Proneomenia Sluiteri, two-thirds natural size. A, From the right side; B, from beneath ; 0, mouth; cl, cloaca. to} longitudinal furrow. This family is hermaphrodite. Proneomenia (Fig. 2), Neo- menia, Lepidomenia, Dondersia. Family 2. Chetodermide. The foot and the pedal furrow are quite degenerated. The sexes are separate. Cheetoderma. CLASS II. Gastropoda (Cephalophora). Snails. The body is asymmetrical. The head, which carries tentacles and eyes, is generally distinct from the body. The foot is well developed—usually with a flat sole for creeping. The large protruding visceral dome may be flattened down secondarily in all the groups. It is covered by a shell, consisting of a single piece, into which the animal can withdraw. In all divisions, however, though rarely among the Proso- 1 Simroth, in the new edition of Bronn’s Klassen und Ordnungen des Thierreiches, vol. iii., 1893, divides the Solenogastres as follows :— Sub-Order. Fam. Chetodermatina . 3 é : Chetodermatide. Y Neomeniide. \ Proneomeniide. Dondersiide. Neomeniina : : : Bebe Parameniide. 4 COMPARATIVE ANATOMY CHAP. branchia, this shell may become more or less rudimentary (generally in connection with the reduction of the visceral dome). The pallial complex becomes shifted forward on to the right (seldom the left) Fic. 3.—Margarita Groenlandica (Trochid, after 1, Head; 2, anterior epipodial lobes ; 3, foot ; 4, pigmented prominence at the base of the Pelseneer). epipodial tentacles (5) ; 6, visceral dome. branchia are chiefly marine, and are sexually separate. vided with an operculum for closing the aperture of the shell. A shell is wanting only in T%tiscania, a genus of the Neritacea. Sub-Order 1. Diotocardia. The heart has two auricles (except- ing in Docoglossa). There are two kidneys. Instead of the pedal ganglion of other Gastropoda, there are two longitudinal nerves in the foot, sup- plied with ganglia and connected with one another by numerous commissures. The gills are feathered on two sides, their points projecting freely. The epipodium is well developed, and there is a circle of more or less numerous tentacles around the base of the foot. Proboscis, penis, and siphon are all wanting. a. Zeugobranchia (Rhipidoglossa, Aspidobranchia).—Two gills; both auricles well developed. Heart. tra- versed by the rectum. Shell with marginal cleft, or with apical perfora- tion or with a row of perforations. Generally without operculum. Marine. Fam. Haliotide, radula w1.(5.1.5)lo, Fissurellide (Fissu- side, or along this side so as to lie quite anteriorly. The visceral dome and shell (with some exceptions) are spirally coiled. In all except the lowest Proso- branchia, the asymmetry is evidenced by the disappearance of one gill, of one kidney, and of one auricle. The radula is rarely wanting. ORDER 1. Prosobranchia. The pleuro-visceral connectives are crossed. The mantle complex is twisted round to the front side of the visceral dome. In most forms there is only one gill, placed anteriorly to the heart, and in the heart the auricle lies anter- lorly to the ventricle. The Proso- The foot is generally pro- Fic. 4.—Patella vulgata (from beneath, after Lankester). «a, Tentacle ; d, efferent branchial vessel ; c, free edge of the shell; e, free edge of the mantle ; v-Y;, median Jf, branchial lamelle ; h, one of the afferent vessels ; line; g, afferent branchial vessels ; i, spaces between the shell muscles ; b, foot. relia, rad. #1.(4.1.4)1.%©, with secondarily symmetrical shell. Emarginula, Scutwm VII MOLLUSCA—SYSTEMATIC REVIEW 5 =Parmophorus), Pleurotomaride (Pleurotomaria, Scissurella, Polytremaria), Bellero- phontide (exclusively fossil). b. Azygobranchia. — One gill, homologous with the left gill of the Zengo- branchia. Right auricle ending blindly. Heart perforated by the rectum. Fam. Turbonide, rad, ©0.(5.1.5.)0.0, Trochide (Fig. 3) Stomatiide, Neritopside, rad. o1.(2.0.2.)1.co, marine, Neritide, rad. 01.(3.1.3.)1.0 (marine, but along the shore able to live out of water), Neritine (marine and fresh-water). The Hydrocoenide, rad. oo1.(1.1.1.)1.00, and Helicinide, rad. 01.(4.1.4.)l.0, have no gills but a lung resembling that of the Pulmonata. The Helicinide are terrestrial. c. Docoglossa.—Heart with one auricle, and not perforated by the rectum. Left kidney shifted to the right side of the pericardium. Visceral dome and _ shell secondarily symmetrical, the latter usually cup-like. Operculum wanting. Marine. Fic. 5.—Phorus exutus (after Lankester). a, Proboscidal snout or rostrum; b, tentacle ; c, eye; d, foot; e, metapodium with operculum f. 1. Left true ctenidium present. Acmacide, rad. 1.2.(1.0.1.)2.1.; with numerous accessory gills in the mantle furrow: Scwrria;—without such gills: Acmaea (Tectura). 2. True ctenidia altogether wanting, accessory gills very numerous in the mantle furrow.—Fam. Patellide (Fig. 4), rad. 3.1.(2.0.2.)1.3. 3. Neither ctenidia nor accessory gills found (Lepetide), rad. 2.0.1.0.2. Sub-Order 2. Monotocardia (Pectinibranchia). Heart with one auricle. A single true ctenidium feathered on one side, the point not projecting freely (except in Valvata), Pedal nerve trunks a rare exception, pedal ganglia the rule. Only one kidney. Siphon and penis generally present. Epi- podium weakly developed or wanting. The Monotocardia are very numerous and are chiefly marine. a. Architaenioglossa.—Pedal nerve trunks. In Cypraea (and in other forms ?) a rudiment of the right auricle persists. Fam. Cypraeide, vad. 3.1.1.1.3, Paludinide (fresh-water), Cyclophoride (terrestrial, pulmonate). 6 COMPARATIVE ANATOMY CHAP. VII b. Taenioglossa. — Typical radula, 2.1.1.1.2. Semiproboscidifera. Fam. Naticide (Fig. 98, p. 107), Lamellaride. Rostrifera. Fam. Valvatide (fresh-water), Ampullaride (fresh-water), Littorinide, Cyclostomide (terrestrial), Planaxide, Hydrobiide (fresh-water), Aciculide (terrestrial), Truncatellide (partly terrestrial), Hipponycide, Capulide, Calyptraecide, Pseudomelanide, Melanide, Cerithiide, Fic. 6.—Rostellaria rectirostris (after Owen). «, Snout; b, tentacle; c, stalked eye ; d, foot ; e, metapodium with operculum f; h, beak (for the siphon). Vermetide, Turritellide, Xenophoride (Fig. 5), Struthiolaride, Chenopide, Strombide (Fig. 6). Proboscidifera holostomata. Fam. Scalaridw, rad. ~O~ ; Solaride, rad. xO” ; Pyramidellide, rad. O; Eulimide, rad. O. Proboscidifera siphonostomata. Fam. Colombellinide, Tritoniide, Cassidiide (Fig. 7), Doliide. Fic. 7.—Cassis suclosa (after Poli). «, Shell; b, beak; c, siphon; d, head; g, proboscis; e, eye; f, tentacle ; h, foot; 7, operculum. . Janthinide, rad. ~»O». Heteropoda (marine Taenioglossa, with foot transformed into a perpendicular rowing fin). Fam. . Pteropoda gymnosomata.— These are nearly related to the Anaspidae. They have no mantle, mantle cavity, nor shell. The head is distinct, and carries two pairs of tentacles. The fins are separate; the anus lies to the right. Fam. Pneumodermatide. One ctenidium to the right (Dexiobranchea, Spongiobranchea, Pneumoderma [Fig. 17]). In the last two genera there is an adaptive posterior gill as well. Fam. Clionopside and Notobrancheide. No ctenidium, but a posterior adaptive gill. Fam. Clionide. Neither ctenidium nor adaptive gill. All Gymnoso- mata are carnivorous, feeding principally on Thecosomata. Sub-Order 2. Ascoglossa. This sub-order is characterised by the fact that the worn-out teeth of the long narrow radula, which consists of a single row of dental plates, are preserved in a sac 12 COMPARATIVE ANATOMY CHAP. at its anterior end. No jaws. The anus almost always dorsal. Except in the Steganobranchia, the disappearance of the mantle and its cavity is accompanied by the disappearance of the single ctenidium of the Tectibranchia. Section 1. Steganobranchia.—With mantle, cavity, and ctenidium to the right ; with a shell and parapodia. Fam. Oxynoidea (Oxynoe, Lobiger). Section 2. Cirrobranchia.—Leaf- or club-shaped processes found laterally on the back. Fam. Hermeide, Phyllobranchide. Section 3. Pterobranchia.—The sides of the body produced into lobes, in which the branches of the glands of the mid-gut spread out. Fam. Elysiade, Placobranchide. Section 4. Abranchia.—Neither ctenidium, nor dorsal appendages, nor leaf-like lateral expansions of the body. Respiration through the skin. The body is almost like that of a Planarian. Fam. Limapontiide. Sub-Order 3. Nudibranchia. Without mantle fold, shell, or ctenidium. Jaws almost always found. Radula Fic. 18.—Aeolis rufibranchialis (right aspect, after Alder and Hancock). «, Eye; 3, oral tentacle; c, cephalic tentacle; d, anus; e, genital aperture; f, dorsal respiratory appendages (cerata). generally well developed, with teeth which fall off and are lost. Adaptive gills very variously developed, but occasionally wanting. Fic. 19.— Phyllirhoé bucephalum (lateral aspect, after Souleyet, modified). 1, Tentacle ; 2, cerebral ganglion; 38, stomach; 4 and 12, intestinal ceca (forming the digestive gland) ; 5 ventricle ; 6, auricle; 7, pericardial aperture of the kidney; 8 kidney; 9, external aperture of the same (on the right side); 10, anus (on the right side); 11, hermaphrodite glands, the ducts not drawn ; 13, genital apertures ; 14, buccal ganglion ; 15, salivary glands. Section 1. Holohepatica.—One large unbranched hepatic gland (liver). Fam. VII MOLLUSCA—SYSTEMATIC REVIEW 13 Phyllidiide. Numerous branchial lamelle lie in a groove which encircles the body. No jaws and no radula. Pharynx transformed for sucking. Fam. Doridopside. Without jaws or radula; pharynx adapted for sucking. Branchial rosette round the dorsal anus. Doridide crypto- branchiate. The branchial rosette round the dorsal anus can be with- drawn into a cavity. (Bathydoris, Archidoris, Discodoris, Diaulula, Kent- rodoris, Platydoris, Chronodoris, ete.) Doridide phanerobranchiate. Bran- chial rosette not retractile. (Gonio- doris, Polycera, Acanthodoris, Idalia, Ancula, Euplocamus, Triopa, etc.) Section 2. Cladohepatica.—Diges- tive glands more or less broken up into separate branched canals spreading widely in the body. Variously formed dorsal appendages chiefly connected with respiration. Anus usually to the right. Fam. Aeolidiade (Acolidiu [Fig. 18], Berghia, Tergipes, Galvina, Coryptella, Rizzolia, Facellina, Flabel- lina, Fiona, Glawcus, Janus, Hero). Fam. Tethymelibide, without radula (Tethys, Melibe). Fams. Lomanotide, Dotonide, Dendronotide, Bornellide, Scyllaeide, Phyllirhoide (Fig. 19; marine free-swimming animals with narrow laterally - compressed body, without foot or respiratory append- Fic. 20.—Pleurophyllidia lineata (from below, ages). Fam. Pleurophyllidiide. Nu- after Souleyet). 1, Genital apertures; 2, branchial merous branchial lamelle arranged in leaflets; 3, anus; 4, pedal gland; 5, mouth; 6, a single row on each side along a_ tentacle shield; 7, foot. furrow between the dorsal shield and the foot (Fig. 20). Fam. Pleuroleuride, Tritoniade (7Z7ritonia, Marionia). CLASS III. Scaphopoda. The body is symmetrical, and elongated dorso-ventrally. The mantle is a tubular sac with a narrow dorsal and a wider ventral aperture. Posteriorly, the mantle cavity reaches to the apical (dorsal) aperture. The shell forms a high tubular cone, and, like the mantle, has a small apical and a larger ventral aperture. Ctenidia are wanting ; the kidneys are paired. The vascular part of the circulatory system is reduced to a ventricle ; without auricles. The sexes are separate. There are no special ducts for the sexual products, which are ejected through the right kidney. The mouth lies at the end of a barrel-shaped snout, and is surrounded by a circle of leaf-like appendages. At the base of this snout there are numerous filamentous appendages, which can be protruded through the lower aperture of the shell and mantle. The foot is ventrally elongated. A radula is found. Limicolous. Marine. Fam. Dentalium (Fig. 101, p.113). The foot is relatively short ; it is shaped somewhat like an acorn, with a conical central portion and two lateral lobes. Siphonodentalium. The foot is long and worm-like, but broadens out at the end into a dise edged with papille. 14 COMPARATIVE ANATOMY CHAP. CLASS LY. Lamellibranchia (Pelecypoda, Bivalva, Acephala, Aglossa). Mussels. The body is symmetrical and more or less transversely flattened ; it has two large lateral leaf-like mantle lobes, enclosing a spacious mantle cavity large enough to contain the foot, which is usually hatchet- or wedge-shaped. The shell consists of two lateral valves connected together only at the dorsal hinge. It is closed by means of two adductor muscles passing transversely from one valve to the other (Dimyaria) ; occasionally the anterior adductor degenerates and only one remains (Monomyaria). On each side in the mantle cavity there is a ctenidium. There are no jaws, no pharynx, no radula, no tentacles, and no distinct head. The kidneys and genital organs are paired, and the latter either have separate ducts or eject their products through the nephridia. The heart has two auricles. At each side of the mouth there are two oral lobes. Either sexually separate or hermaphrodite. They live in salt or fresh water, and are either limicolous or attached. OrpDER 1. Protobranchia. The gills with two rows of leaflets, in the posterior part of the mantle cavity ; they correspond in all respects with the ctenidia of the Zeugobranchia, their ends Fic. 21.—Nucula nucleus, left aspect after removal of the left valve and mantle (after Pelseneer). «a, Anterior adductor ; b, anterior retractor of the foot ;c, elevator of the foot; d, genital mass ; e¢, hypobranchial gland ; f, posterior retractor of the foot ; g, posterior adductor ; h, cteni- dium ; 7, mantle cavity ; k, creeping sole of the foot (J); m, oral lobes (labial palps) with posterior appendages n and 0. project freely backward into the cavity. The foot has a sole for creeping. The pleural ganglion can be distinguished from the cerebral. Fam. Nuculide (Nucula [Fig. 21], Leda, Yoldia, Solenomyide). OrpDmER 2. Filibranchia. The branchial leaflets of the ctenidium have become lengthened out into long filaments hanging far down into the mantle cavity. Each is in two parts, the proxi- mal descending and the distal ascending (cf. Fig. 88 B). Fam, Anomiide: mantle open vil MOLLUSCA—SYSTEMATIC REVIEW 15 without siphons; Monomyarian. Foot small; bodyand shellasymmetrical. Attached. Branchial filaments entirely free (Anomia, Placuna). Fam. Arcide: the branchial filaments of each row connected by ciliated dises ; Dimyarian. No siphons. Foot large (Arca, Pectunculus). Fam. Trigoniide: ctenidia like those of the Arcida; Fic. 22.—Mytilus edulis (after Meyer and Mobius), left aspect, with extended foot attaching a byssus thread ; d, byssus threads ; a, exhalent aperture (anal siphon); b, fringed edge of the in- halent mantle aperture ; c, object to which the animal is attached. Dimyarian. No siphons (Zrigonia). Fam. Mytilide (excluding Aviculide) : cten- idia connected by means of non-vascularised trabecule. The anterior adductor is smaller than the posterior (Heteromyarian). With siphons. Foot long. (Mytilus [Fig. 22], Modiola, Lithodomus [boring mussel], Modiolaria). ORDER 3. Pseudolamellibranchia. The consecutive ctenidial filaments of each row are connected by means either of ciliated discs or of vascularised trabecule ; and the ascending and descending 16 COMPARATIVE ANATOMY CHAP. portions of each filament are similarly united (cf Fig. 88. p. 92). Fam. Pectinide: Fic. 23.—Pecten Jacobus, ventral aspect, shell opened. The mantle cleft is seen between the fringes of the mantle, which are beset with numerous tentacles and eyes (after Leuckart and Nitsche, Zool. Wandtafeln). Fic. 24.—Anatomy of the Oyster (Ostrea edulis), right aspect (after Mobius, Leuckart, and Nitsche, Zool. Wandtafeln). br, Gills; Pn, posterior mantle nerve ; 7, 7, apertures of the cavities between the fused plates of the two left gills; 1, large adductor muscle ; a, anus; MJ/m, posterior portion of the adductor muscle ; Pa, mantle ; P, pericardium ; V, heart; go, gonad (hermaphrodite elaud); d, intestinal canal; 1, digestive gland (liver); 0, mouth ; os, 0s], oral lobes (labial palps) of the left side ; Cg, cerebral ganglion ; 7, kidney ; bn, branchial nerve ; Vg, visceral ganglion ; Py, abdo- minal process ; Pry, nerve of the pallial edge ; m, stomach, with the apertures of the digestive gland. Monomyarian with mantle entirely open, and eyes at its edge. Without siphons. Foot small and linguiform. Valves of the shell equal or unequal. Capable of VII MOLLUSCA—SYSTEMATIC REVIEW 17 swimming. (Pecten [Fig. 23], Chlamys). Fam. Aviculide: Monomyarian or Hetero- myarian without siphons. Valves equal or unequal (Avicula [Meleagrina], Malleus, Vulsella, Perna, Inoceramus, Pinna, Meleagrina margaritifera, pearl mussel). Fam. Ostreide : Monomyarian without foot, with completely open mantle, without siphons. Valves unequal, the left valve attached to the substratum. (Ostrea : oyster [Fig. 24]). OrpeErR 4. Eulamellibranchia. The gills no longer consist of distinct filaments. On the contrary, the filaments in each row and the two parts of each filament are so connected by means of vascularised trabecule or sutures as to form together a lamella or trellis- work. There are, on either side, two such branchial lamelle (hence the name of Lamelli- branchia), which in fact correspond with the two rows of leaflets of the typical ctenidium. ‘This order includes the majority of the Lamellibranchia. Sub-Order 1. Submytilacea. Branchial lamelle smooth. The mantle edges usually grown together only between the inhalent and the exhalent apertures. Dimyarian. Fam. Carditide : with open mantle and large foot (Cardita, Venericardia). Fam. Lucinide : with simple, and asa rule single, siphonal aperture. Foot often vermiform. Fam. ae Ze ao a Fic. 25.—Anatomy of Unio (Margaritana) margaritiferus, left aspect (after Leuckart and Nitsche). o, Mouth; cg, cerebral ganglion; Jj, anterior adductor muscle; @, cesophagus; J, digestive gland (liver); no, nephridial aperture ; Jo, aperture of the digestive gland in the stomach m ; Ad, anterior aorta ; 7, nephridium, the outlines given in dotted lines ; V, heart ; 7, proctodeeum ; Ap, posterior aorta; Mo, posterior adductor ; a, anus; Vg, visceral ganglion; Br, gill ; Bk, mantle cavity ; go, gonad and ducts go;; Pg, pedal ganglion; p, foot. The arrows mark the direction of the inhalent and exhalent streams of water. Erycinide : mantle closed except at the two siphonal and the pedal apertures. Foot long. (rycina, Kellya, Lascea, Lepton, Galeomma.) Fam, Crassatellide : mantle open without siphons. Foot moderately developed. Fam. Cyrenide : mantle open, two siphons. Foot large. In fresh or brackish water. (Cyrena, Corbicula, Spherium, Pisidium, Galatea.) Fam. Dreissensiide (fluvial). Fam. Unionide: fresh- water ; foot large, hatchet- or wedge-shaped, two simple siphonal apertures or clefts, mantle open (Unio [Fig. 25], Painter’s Mussel ; Anodonta, pond Mussel ; Mutela). VOL. II C 18 COMPARATIVE ANATOMY CHAP. Sub-Order 2. Tellinacez. Dimyarian with completely separate siphons. Foot large. Gills smooth. Fam. Tellinide (7Zellinz). Fam. Donacide (Donax), Mactride (Muctra). Sub-Order 3. Veneracea. Dimyarian with somewhat folded branchial lamellie. Siphons separate, and foot large. Fam. Veneridee (Venus, Meretrix [Cytherea], Tapes). Fam. Petricolide : boring muscles. Sub-Order 4. Cardiacea. Dimyarian or Monomyarian. Branchial lamelle much folded. Mantle closed except at the two siphonal and one pedal apertures. Fam. Cardiide: Dimyarian. Fic. 26.—Anatomy of Cardium tuberculatum, left aspect (after Grobben, Leuckart, and Nitsche, Zool. Wandtafeln). p, Foot; go, gonad; S, shell; Pa, mantle; os, labial palps ; 0, mouth; My, anterior adductor muscle; @, cesophagus; m, stomach; J, digestive gland; d, intestinal canal; goo, genital aperture ; 701, pericardial aperture of the kidney ; V, ventricle; At, auricle ; P, pericardium ; no, aperture of the kidney in the mantle cavity ; n, kidney; Mo, posterior adductor ; Bl, point of conerescence of the right and left ctenidia behind the foot ; a, anus ; Ak, anal chamber of the mantle cavity) with anal siphon As; Bk, branchial chamber of the same cavity with branchial siphon Bs; Br, ctenidium. (Cardium [Fig. 26].) Fam. Chamide: Dimyarian. Valves of shell unequal. (Chama, Diceras, Requienia.) To these the fossil forms Monopleuride, Caprinide, Hip- puritide,® Radiolitide. Yam. Tridacnide: Monomyarian. (7’ridacna, Hippopus.) Sub-Order 5. Myacea. Dimyarian with folded branchial lamelle. Tendency towards concrescence of the edges of the mantle folds. Siphons very long and foot large. Fam. Psammobiide : pedal cleft of the mantle still very large (Psammobia). Fam. Mesodesmatida, VII MOLLUSCA—SYSTEMATIC REVIEW 19 Lutrariide, Myide (Mya, Corbula), Fam. Glycymeride (Glycymeris, Saxicava [boring mussels]). Solenide: shell with anterior and posterior cleft ; foot very large (Solenocurtus, Cultellus, Ensis, Solen). Sub-Order 6. Pholadacea. Dimyarian with closed mantle and well-developed siphons. Foot varies, and is Fia. 27.—Anatomy of Pholadidea, left aspect (after Egger). Lettering as before. In addition, Npa, Npp, anterior and posterior nerves of the mantle edge ; mo, anterior aperture of mantle; Ks, sac of the crystalline stylet; Kv, branchial vein; o/, anterior upper mantle lobe ; Rpp, posterior retractor of the foot ; Ss, partition between the two siphons; Ms, accessory adductor; mb, intestinal eexeum ; 2, pericardial section of the kidney, which opens into the pericardium by means of the renal funnel at wv. Fria. 28.—Anatomy of Jouannetia Cumingii, left aspect (after Egger). Lettering as in last figure. sometimes rudimentary. Shell open, often having accessory pieces added to it. Fam. Pholadide: boring mussels (Pholas, Pholadidea |Fig. 27], Jouannetia [Fig. 28], 20 COMPARATIVE ANATOMY CHAP. _ Fic. 29.—Teredo Navalis in its boring, ventral aspect (after Meyer and Mobius). The centre is omitted, the calcareous tube is for the most part uninjured. Fic. 30.—Shell of Aspergillum (Bre- chites) vaginiferum, dorsal view. a, An- terior; p, posterior; d, right; s, left; 1, siphonal aperture of the pseudoconch; 2, pseudoconch (calcareous tube) ; 3, true shell embedded in the pseudoconch ; 4, anterior aperture of the pseudoconch. VIL MOLLUSCA—SYSTEMATIC REVIEW 21 Xylophaga). Fam. Teredinide: boring mussels (7eredo [Fig. 29]). Fam. Clava- gellide (OClavagella, Brechites |Aspergillum, Fig. 30]). Sub-Order 7. Anatinacea. Mantle to a great extent closed. With siphons and foot. Hermaphrodite. Fam. Pandoride, Lyonsiide, Anatinide (dnatina, Thracia). OrvbER 5. Septibranchia. The ctenidium on each side is transformed into a muscular septum pierced by Fic. 31.—Soft body of Silenia Sarsii (Cuspidaria), after Pelseneer. A, Left aspect after removal of the mantle ; B, ventral aspect after removal of most of the mantle ; a, p, anterior and posterior ; d, v, dorsal and ventral; d, s, right and left ; 1, anterior adductor ; 2, mouth; 3, anterior group of branchial slits; 4, hepatic mass ; 5, branchial septum ; 6, posterior group of branchial slits ; 7, posterior adductor; 8, anal siphon; 9, siphonal tentacles ; 10, valve of the branchial or inhalent aperture ; 11, point where the free mantle edges limiting the pedal aperture fuse ; 12, median group of branchial slits ; 13, free edges of mantle ; 14, foot ; 15, posterior labial palps; 16, anterior labial palp. slits, which divides the mantle cavity into two chambers, one lying above the other. Hermaphrodite. Fam. Poromyide, Cuspidaride (Fig. 31 A and B). CLASS V.—Cephalopoda (Cuttlefish). Body symmetrical with high visceral dome. The mouth is surrounded by tentacles or prehensile arms, which may be considered as portions of the foot developed round the mouth. Another portion of the foot forms the siphon. In 22 COMPARATIVE ANATOMY CHAP. the posterior mantle cavity there are two or four ctenidia. The heart has two or four auricles, and there are two or four kidneys. Gonad unpaired, with single or paired ducts. The sensory organs are highly developed, especially the eyes, which lie anteriorly and laterally on the ‘‘head” (Kopffuss). The jaws and radula are powerful. There is sometimes a shell, either external or internal. An ink-bag is generally present. The Cephalopoda are large, highly-developed marine carnivora. Dicecious. ~OrvER 1. Tetrabranchia. An external chambered shell, the animal inhabiting the last (and largest) chamber. It is symmetrical, and exogastrically coiled. The mouth is surrounded by numerous tentacles without suckers, which rise from large lobes and can be Fic. 32.—Nautilus Pompilius, after Owen. Median section of shell. «a, Cephalic hood; b, tentacles; c, infundibulum (siphon); d, eye ; e, projection caused by nidamental gland; /, point of attachment of the adductor muscle; g, upper portion of the visceral dome; h, last (inhabited) chamber of the shell; 7, anterior lobe of the mantle; k, last chamber but one; /, siphuncle. retracted into special sheaths. There are four ctenidia, four auricles, and four kidneys. The siphon consists of two lateral lobes distinct from one another, which by the apposition of their free edges form a tube. Without ink-bag. The eyes are simple pits. The only living form is the Nautilus, radula 2.2.1.2.2 (Fig. 32). The two large divisions of this order, Nautiloidea and Ammonitidea,' occur as fossils. ! The Ammonitidea, owing to the uncertainty concerning their anatomy, are by many authorities arranged in a separate order, ‘‘Ammonea,” and placed between the other two. NAME MOLLUSCA—SYSTEMATIC REVIEW Fic, 33.—Spirula prototypos, right aspect (after Chun and Owen), from Leuckart and Nitsche, Zool. Wandtafeln. Both portions of the shell are visible, the inner portion seen through the mantle. The eye should be placed more anteriorly on the ‘‘ head” (Kopffuss). a: i St eet to Fig. 34.—Loligo vulgaris (after D’Orbigny). 4, Dorsal (physiologically ventral) view; B, anterior (physiologically dorsal) view. Of the five pairs of arms, the fourth are seen to be developed as long prehensile tentacles ; the eyes, the edge of the mantle, the fins, and the chromatophores in the skin are depicted. 24 COMPARATIVE ANATOMY cHar. OrprER 2. Dibranchia. The shell is either internal, rudimentary, or altogether wanting. When present it is endogastrically coiled. There are two ctenidia, two auricles, and two kidneys. The mouth is surrounded by eight or ten sucker-bearing prehensile arms. The free edges of the two lobes which form the siphon have grown together. The eyes are vesicular. An ink-bag is present. Sub-Order 1. Decapoda. Shell internal and often rudimentary. There are ten arms, the fourth pair being developed into long prehensile tentacles, which can be withdrawn into special cephalic cavities. The Decapoda are good swimmers; their bodies are elongated dorso- ventrally, and provided with lateral fins. The oviduct is unpaired. Fam. Spirulide : internal shell spirally (endogastrically) coiled. Spirula (Fig. 33). Fam. Belem- nitide : fossil forms with internal chambered shell, usually long and _ straight (Belemnites, Spirulirostra, Belemnoteuthis). Fam. Oigopside (Ommastrephes, radula 3.1.3, Loligopsis, Cranchia, Chiroteuthis, Owenia, Thysanoteuthis, Onychoteuthis, Ommatostrephes). Fam. Myopside (Rossia, Sepiola, Sepiadarium, Idiosepion, Loligo [Fig. 34], Sepioteuthis, Belosepia [fossil], Sepia, radula 3.1.3). Sub-Order 2. Octopoda. Without shell or ‘‘ guard” (rostrum) ;. eight arms; without specialised prehensile tentacles. Body thick, generally without fins, and little adapted for swimming. Fic. 35.—Female Argonauta, in the swimming position, right aspect (after Lacaze-Duthiers). 1, Uncovered part of the shell; 2, the right arm of the first (anterior) pair, with its lobe-like expan- sion (sail) 3, covering a large part of the shell; 4, fourth arm; 5, third arm; 6, siphon; 7, eye; 8, jaw; 9, second arm. The second, third, and fourth arms are stretched backwards inside the shell. Oviducts paired. Fam. Cirrhoteuthide : with fins. Fam. Philonexide : Argonauta (Figs. 35, 36, and 200, p. 243). Female with external unchambered shell. Philonexis, Tremoctopus. Fam. Octopodide (Octopus, radula 1.3.1. [Fig. 37], Eledone). VII MOLLUSCA—SYSTEMATIC REVIEW 25 Fic. 36.—Female of Argonauta Argo (after Vérany). Second, third, and fourth pairs of arms stretched downwards. «a, Siphon; }, eye; c, first pair of arms, covering with its sail d nearly the whole shell e. Fic. 37.—Octopus vulgaris, after Merculiano (in ‘‘ Aquarium Neapolitanum”). Above, in swimming position; below, quiescent, watching for prey. 26 COMPARATIVE ANATOMY CHAP. I. Organisation of the Primitive Molluse. The hypothetical primitive Mollusc, reconstructed from the results of morphological research, may be described as follows :— The body is bilaterally symmetrieal and dorsally arched ; its Fig. 38. —Hypothetical Primitive Mollusc, diagrammatic, left aspect. 0, Mouth; ck, head; sm, shell musele; oso, upper aperture of the shell; a, anus; 7, renal aperture; mh, mantle cavity; ct, ctenidium; f, foot. edge of the mantle and the foot. and of the mantle secretes a anterior end carries the mouth, eyes, and tentacles, forming a distinct head. The ventral side forms a powerful muscular foot, distinct from the rest of the body, with a flat sole for creeping. The soft integument of the arched dorsal side forms a fold, which hangs down all round the body, and is called the mantle or pallium. The mantle encloses a circular cavity, the mantle- or pallial cavity, which surrounds the body, and communicates freely with the surrounding medium between the free The dorsal integument of the body closely-applied shell, which consists of a chitinous matrix (conchyolin) in- ter-stratified with deposits of car- bonate of lime. This shell repeats the form of the dorsal surface, and is thus bilaterally symmetrical and arched. Such a shell detached and turned over would resemble a cup or plate. Since the dorsal shell covers the whole, or at any rate the greater part of the body, it forms a protection for it and at the same time plays the part of a skeleton, to which the muscles run- ning more or less dorso-ventrally into the foot and head, can be firmly attached. The mantle is of special im- portance as a protective structure. Apart from the fact that its edge secretes the greater part of the shell substance, and in this way adds to the shell as the animal grows, it covers the delicate gills, which thus also share the protection afforded by the shell. Analogous Were Fic. 39.—Hypothetical Primitive Mollusc, from above. 0, Mouth; ule, ulpl, ulp, primitive left cerebral pleural and pedal ganglia; wlpa, urpa, primitive left and right parietal ganglia ; ula, primitive left auricle; wos, wros, primitive left and right osphradia (Spengel’s organ) ; ulct, uret, primitive left and right ctenidia (gills) ; mb, base of the mantle ; m7, edge of the mantle; m, mantle cavity ; v, visceral ganglion ; ve, ventricle ; @, anus. arrangements are to be found in other divisions of the animal kingdom, VII THE HYPOTHETICAL PRIMITIVE MOLLUSC 27 e.g. the dorsal fold or carapace which, in the higher Crustacea, covers the branchial cavity, and the operculum of Fishes. The relations existing between the branchiew, the mantle, and the shell in the Mollusca are of the highest importance ; these organs should always be regarded as essentially interdependent structures. The branchiz lying in the mantle cavity are paired and symme- trical. It may be left an open question whether the primitive Molluse possessed more than one pair of gills. If only one, we must suppose that one gill lay on each side of the mantle cavity posteriorly ; if more than one, that there was a row of branchiz on each side. Each gill is feather-lke, with a shaft and two rows of very numerous leaflets. The shaft stands out freely from the body in the mantle cavity. Close to the base of each gill, a sensory organ, con- sidered to be olfactory, and called the osphradium, is found. Such a gill with an osphradium at its base has a very definite morphological value; in order to distinguish it from analogous though not homologous respiratory organs found in certain Mollusca, it has been named a etenidium. The head is provided with one pair of tentacles and one pair of eyes. The mouth lies anteriorly and ventrally. The remaining open- ings of the inner organs lie posteriorly above the foot ; the anus in the middle line, and on each side, between it and the ctenidium (supposing that there is only one pair of ctenidia), an aperture for the sexual organs, and another for the kidney (nephridium). These five apertures are covered by the mantle, and thus lie in the mantle cavity. We have thus, to recapitulate, in the posterior part of the mantle cavity two ctenidia, two osphradia, and five apertures, the median anus, and the paired symmetrical sexual and renal apertures. These, taken together, form what is known as the pallial complex. The inner organisation may thus be briefly described. The intestinal eanal. The mouth leads to a muscular pharynx, with horny jaws. At its base lies a chitinous rasp-like ribbon called the tongue or radula, which carries numerous consecutive transverse rows of sharp chitinous teeth. Paired salivary glands enter the pharynx, which passes into an cesophagus, which latter leads into the mid- gut. This, which we will suppose to be more or less coiled, runs right through the body, passing posteriorly into a very short hind- gut, which opens outward through the median anus. The mid-gut has large paired glandular diverticula (mesenteric gland, diges- tive gland, hepatopancreas, liver). Museulature.—The muscles of the foot are powerful, and are adapted for the creeping movement. There are, in addition, muscles running from the inner surface of the shell into the foot and head (columellar or shell muscles), and special muscles for the different organs. Nervous system.—Two well-developed cerebral ganglia lie dorsally in the head, and are connected by means of a short cerebral 28 COMPARATIVE ANATOMY CHAP. commissure, which runs over the cesophagus. Each cerebral ganglion gives rise to two powerful nerve trunks which are provided along their whole length with ganglion cells; there are thus two pairs of nerve trunks running right through the body longitudinally. One pair, the pedal cords, run right and left in the foot ; the other pair, the visceral cords, which lie more dorsally and are more deeply embedded in the body, run through the body cavity. The two visceral nerves are connected posteriorly. If we leave the Amphineuwra and Diotocardia out of the question, the following modified sketch of the Molluscan nervous system holds good. Two cerebral ganglia, two pedal ganglia, two pleural ganglia lying at the sides of the pharynx, two visceral ganglia lying posteriorly in the body cavity. Giving the name connectives to such nerves as unite the ganglia of one side of the body, i. dis- similar ganglia, and that of eommissures to the nerves that unite the similar ganglia of the two sides of the body, we have the following system: Commissures are found—(1) between the two cerebral ganglia (over the fore-gut); (2) between the two pedal ganglia (under the fore-gut); (3) between the two visceral ganglia (under the hind-gut). The connectives on each side are: (1) the cerebro- pedal connective ; (2) the cerebropleural connective ; (3) the pleuro- pedal connective ; (4) the pleurovisceral connective. There is a secondary eceelom or body eavity lined with endo- thelium, which has at least two divisions. In the anterior division, the genital chamber, the sexual products arise from the endothelium ; this chamber is connected by means of two canals, the genital ducts, with the mantle cavity. In the posterior chamber, or pericardium, lies at least one organ, the heart ; this chamber is connected with the mantle cavity by means of two nephridial duets or vesicles. The ecireulatory system is partly vascular and partly lacunar. The arterial heart lies in the pericardium above the hind-gut. It consists of one ventricle and two lateral auricles. Il. Review of the Outer Organisation characterising the Chief Groups of the Mollusea. Having given above a general plan of the morphology of the Mollusca, let us now see how far the various groups of Molluses agree with this description in their outer organisation. We shall at first only mention in connection with each group those special features which are now considered to be typical or characteristic of that group. In other words, we shall again give a general scheme of the outer organisation of each class of the Mollusca, in order that these more specialised schemes may be compared with that of the hypothetical primitive Mollusc above described. Later sections will deal with the changes which the separate organs undergo, not only in the different classes, but within one and the same class, so far, that is, as these modifications bear on external morphology. VII MOLLUSCA—OUTER ORGANISATION 29 A. Plaeophora or Polyplacophora (Chitonidze). The body of the Placophora is bilaterally symmetrical, and dorso- ventrally flattened ; viewed from the dorsal or ventral surface its shape is that of a long oval. On the ventral side there is a large muscular foot with a flat sole, the outline of which runs very nearly parallel with that of the body. In front of the foot, and also on the ventral side, there is a distinct snout which carries the mouth in the middle of its ventral surface. There are no eyes or tentacles on the head. Between the mantle, which forms the outer edge of the body, and the body and head it covers, there is a deep groove, in the base of which lie numerous lancet-shaped gills, arranged ina single row on each side. These two rows of gills sometimes approach each other so nearly both anteriorly and posteriorly that there is an almost complete circle of gills around the foot, or else they are more or less shortened, and are in some forms so reduced as only to occupy the posterior third of the branchial furrow. The anus lies posteriorly in the median line, ventrally, immediately behind the foot. The two apertures of the nephridial ducts lie in the branchial furrow on each side, and slightly in front of the anus. The two genital apertures lie imme- diately in front of the nephridial apertures, also in the branchial furrow. The median dorsal region is covered by eight consecutive imbri- cating calcareous plates. The peripheral dorsal region, between the edge of the body and these shell plates, carries calcareous spicules, granules, etc. The corresponding peripheral region on the ventral side forms one of the boundaries of the branchial groove, and may be considered as the mantle. B. Aplacophora, Solenogastres. The body is here bilaterally symmetrical and vermiform ; in section it is round, and is sometimes long and thin, at others short and thick. The large oral aperture lies in the form of a longitudinal slit on the ventral surface of the anterior end of the body. The cloacal aperture —or common opening for the intestinal canal and the urogenital organs—lies ventrally at the posterior end of the body. A narrow median ventral groove runs forward from the cloacal aperture and terminates anteriorly near the mouth. In the base of this pedal groove rises a ciliated ridge or fold which runs along its whole length; this ridge, in cross section, is triangular, and represents the reduced foot. In the Chetoderma both foot and pedal groove are wanting. The Solenogastres have no distinct compact shell ; its place is taken by calcareous spicules embedded in the integu- ment. 4 30 COMPARATIVE ANATOMY CHAP. C. Gastropoda (Cephalophora). Although there can be no doubt as to the relationship to one another of the Mollusca grouped together in this class, it is almost impossible to give a general scheme of the outer form of the whole class. The greatest variation occurs, the body being sometimes out- wardly bilaterally symmetrical, sometimes in a high degree asym- metrical. Further, forms such as Fissurella, Oliva, Turritella, Cleodora, Pterotrachea, Phyllirhoé, Limax, Pleurobranchus, Thetys, differ so greatly in outward appearance that, at the first glance, it is almost impossible to believe that they are related. A shell may be present, and may show the most marvellous variation in form; or it may be rudimentary or even (in adult forms) altogether wanting. The foot also may assume the most varied forms, or may be entirely wanting. The same may be said of the mantle fold, the gills, ete. Setting aside those forms which are quite one-sidedly differenti- ated, it may be said in general—(1) that, in the Gastropods, the protective shell consists of one piece, and follows in a remarkable way the forms assumed by the body; (2) that the dorsal portion of the body, which contains the viscera, becomes constricted almost hernia- like from the head and foot, making a sac-like protuberance (visceral dome) ; (3) that, for the diminution of its surface, this dome or hump becomes coiled spirally, the shell repeating its shape ; (4) that the head and foot, which project through the aperture of this shell for purposes of locomotion, can be withdrawn into it. The large, long foot generally has a flat sole for creep-. ing. The head is distinct, and provided with tentacles and eyes. At some part of the body, the in- tegument of the visceral dome forms a mantle fold which hangs downwards, covering and protect- ing the respiratory organs. The outer surface of this mantle takes part with the rest of the integu- ment of the visceral dome in the formation of the shell. The follow- ing are more special descriptions of the outer organisation of the chief Gastropodan groups. Fic. 40.—Diagram of the Organisation of a 1. Prosobranchia. Zeugobranchiate Diotocardian. wu, Anus; ve, ventricle ; ula, right auricle ; wret, left etenidium ; The large visceral dome is wros, left osphradium. coiled spirally, generally to the right (dextrally), the shell naturally assuming the same form. The well- VIL MOLLUSCA—OUTER ORGANISATION 31 developed foot has a flat creeping sole. On the dorsal side of the posterior portion of the foot, the metapodium, there is a calcareous plate, the opereulum, which, when the animal withdraws its head and foot, closes the aperture of the shell. The mantle ule o 7 fold hangs down from the anterior side of the f \/ ae visceral dome, and covers \ Me! P the spacious branchial or utpf eps, mantle cavity, in which \l lie certain organs of special morphological a x\ importance. These, ou A) t which may be called the mantle or pallial organs, — wie are, in such forms as may be considered primi- tive, (1) the anus, which “+p lies, not posteriorly, but AN one the’ anterior: side ‘> of the visceral dome, shifted forwards _ to- oe wards the mouth; (2) the two apertures of the paired nephridia, one on each side of the anus; (3) the two gills, one to the left and one to the right ; (4) the two osphradia near the bases of the gills. In most Prosobranchia, however, the organs just mentioned as paired are unpaired ; only the gill, Fia. 41.— Diagram of a Prosobranchiate Monotocardian. | Tlie nephridial aperture, and outer form, shell, mantle, pallial complex, heart and pericardium, osphradium to the left metvous system and operculum, are depicted. Lettering mostly as in Fig. 39. In addition: f, foot; si, siphon; sup, sub, supra- of the anus being Ye€- and sub-intestinal connectives ; op, operculum; of, auditory tained, while the hind- organ ; p, penis; sv, seminal groove; mh, mantle cavity ; hy, cut with the anus moves pre mae ; 6, male genital aperture ; 7, rectum ; aw, to the right side of the mantle cavity. The single genital aperture lies on the right side, in the head, or on the floor of the mantle cavity. (In the Prosobranchia the sexes are separate.) The abortion of one of each of these originally paired organs, gills, nephridia, and osphradia, produces a very striking asymmetry of the whole body. The name Prosobranchia indicates the fact that the gills lie in front of the heart. 32 COMPARATIVE ANATOMY CHAP. 2. Pulmonata. Type: Helix pomatia.—The visceral dome is well developed, and protrudes hernia-like from the rest of the body ; it is dextrally coiled, and has a corresponding shell. The foot is large and long, and has a flat creeping sole. The head has two pairs of feelers, one of which carries the eyes. The mantle fold hangs down from the anterior side of the visceral dome, and covers a spacious mantle cavity (respiratory or pulmonary cavity). The free edge of the mantle fold unites with the integument of the neck near it, only leaving an aperture to the right, the respiratory aperture. This aperture serves for the inhala- tion and exhalation of the air. The anus and the unpaired nephri- dial aperture lie close to the re- spiratory aperture, and are thus on the right side. There are no gills in the mantle cavity, which con- ; tains air. Respiration takes place Fic. 42.—Diagram of a Basommatophoran : Pulmonate. al, Respiratory aperture: rgn,vas. 2b the inner surface of the mantle cular network on the inner surface of the mantle. fold, in which runs a fine network The icidney is inconrectly dra wa: Further letter- of vessels lying in front of the ing as in Figs. 39 and 41. : 7 heart. The foot, unlike that of the Prosobranchia, has no operculum. There is a common genital aperture on the neck, to the right, in front of the respiratory cavity (the Pulmonata being hermaphrodite). Many Pulmonata, however, differ greatly in their outer organisation from the Helix type. 3. Opisthobranchia. The respiratory organs le behind the heart. («) Teetibranchia.—The visceral dome is usually not large. It may be either spirally coiled or symmetrical, and is covered by a variously shaped shell. The foot is large, and usually has a flat sole for creeping. The head is variously shaped, and often carries tentacles or rhinophores, and unstalked eyes. The small mantle fold hangs down from the right side of the visceral dome, and often does not quite cover the single gill lying beneath it. The anus lies behind the gill, more or less removed from it. The Tecti- branchia are, like all Opisthobranchia, hermaphrodite; the genital Vil MOLLUSCA—OUTER ORGANISATION 33 and nephridial apertures lie on the right side of the body in front of the anus. (5) Nudibranchia.— The body is outwardly symmetrical, the visceral dome does not protrude from it, but is closely applied to the whole length of the foot, from which it is often not distinctly Wl Hid SS 3 Sor, BE) Fic. 43.—Diagram of a Tectibranchiate Opistho- branchiate. Lettering as before. In addition: gg, genital ganglion; s, shell; 9, female genital aperture; lpp, rpp, left and right parapodial lobes, that on the right laid back. Fic. 44.—Dentalium, diagram- matic, leftaspect. g, Sexual glands ; kt, cephalic tentacles ; other letter- ing as before. differentiated. The foot has a flat creeping sole. There is no distinct mantle fold, no gill corresponding with that of the Tectibranchia, and no shell. The head carries tentacles or rhinophores, and sessile eyes. The anus lies either dorsally in the median line, or laterally to the right. The genital and renal apertures lie to the right in front of the anus. The gills, which vary much in form, number, and arrangement, are found dorsally or laterally, and are not homologous with the typical Molluscan ctenidia. VOL. II D 34 COMPARATIVE ANATOMY CHAP. D. Seaphopoda. The body is symmetrical and long, 7.¢. the visceral sac is elongated dorso-ventrally, and is completely enveloped in a tubular mantle. The mantle cavity lies posteriorly, and is prolonged ventrally far enough to allow the snout and retracted foot to be completely concealed in it. Besides the large ventral aperture, there is a smaller dorsal aperture further placing the mantle cavity in communication with the exterior. The shell, like the mantle, is tubular, or like a tapering cone, slightly curved anteriorly. It has two apertures corresponding with those in the mantle. The head is developed into a barrel-shaped snout, and has no eyes. The mouth, which lies at its ventral end, is surrounded by a circle of leaf-like tentacles. At the base of the snout there arise two tassels of long filamentous contractile tentacles, which hang down into the mantle cavity and can be projected far beyond the ventral aper- ture. Behind the snout, the cylindrical muscular foot rises from the body, and can be protruded downwards. ‘There are no gills. The median anus les posteriorly above the foot. The two nephridial apertures are at the sides of the anus. There are no special genital apertures (Figs. 44 and 101, paalilis): E. Lamellibranehia. The body is bilaterally sym- metrical ; somewhat elongated (from before backward). The integument forms leaf-like mantle folds to the right and to the left, which at their bases are attached to the trunk along its whole length, and grow down ventrally. If the body of a Lamellibranch, from which the shell has been Fic. 45.—Transverse section of Anodonta removed (the toot being es cygnea (ordinary freshwater mussel) (after Howes). tracted), be viewed from the side, lg, Ligament ; ty, typhlosolis ; kb, pericardialgland the outline will be found to be (Keber’s organ); ve, kidney (glandular portion) ; formed, dorsally, by the dorsal sbe, chambers at the bases of the gills; gd, genital 5 i duets ; bri), brly, outer and inner branchial lamelle ; median line of the body 5 als ’ ibe, mantle cavity; s, shell; s,, edge of the shell; teriorly, posteriorly, and ventrally fi, foot ; pm, pallial muscle ; 7, intestine ; ply, right lev th fr vey f tl antl mantle fold; ggl, gonad; 7, rectum; cp, cerebro- DY re Iree ec ge 0 he mantle pedal connective ; rez, non-glandular vestibule of fold. The two mantle folds en- kidney ; reg, renal aperture ; pe, pericardium. close a space whose transverse axis is always markedly shorter than either its dorso-ventral or its longitudinal axis, 7.c. the animal with its mantle is laterally compressed. VII MOLLUSCA—OUTER ORGANISATION 35 Projecting into the mantle cavity, there is a large muscular process of the body, the foot, which is directed downward and somewhat forward, and can be protruded between the free edges of the mantle. This foot is also laterally compressed. In certain cases which, though excep- tional, deserve special mention, its free end is flattened, and it thus has a flat sole. The outer surface of the trunk and mantle folds secretes a bivalve shell which covers the whole body. One valve les to the right, the other to the left of the median plane, and the two are exactly alike. Each valve repeats the outline of its own side of the trunk with its mantle fold. The two valves articulate dorsally, and are open anteriorly, ventrally, and posteriorly. Two strong muscles (adductors) run transversely il Da a Fic. 46.—Anatomy of Unio (Margaritana) margaritiferus, left side (after Leuckart and Nitsche). 0, Mouth; Cg, cerebral ganglion ; Mj, anterior adductor muscle ; w, cesophagus ; /, digestive gland (liver) ; no, nephridial aperture ; Jo, apertures of the digestive gland in the stomach m ; Aa, anterior aorta; m, nephridium, the outline given in dotted lines; V, heart; 7, hind-gut; Ap, posterior aorta; My, posterior adductor; a, anus; Vg, visceral ganglion; Br, gill; Bk, mantle cavity ; go, gonads with genital duct go,; Pg, pedal ganglion; p, foot. The arrows indicate the direction of the inhalent and exhalent streams of water. from one valve to the other. ‘Their contraction serves to shut the shell completely. One of these muscles lies anteriorly, the other posteriorly. Their points of attachment produce impressions on the inner surface of the shell, which are always distinctly visible when the shell is removed. The mouth lies below the anterior adductor, between it and the anterior base of the foot. The anus lies behind the posterior adductor. There is no distinct head. Near each side of the mouth, the body carries two leaf-like processes, the oral lobes or labial palps. At the line of insertion of the foot in the mantle cavity, a longitudinal ridge rises on each side in the middle and posterior regions of the body ; this carries two rows of long branchial leaflets. There is thus, 36 COMPARATIVE ANATOMY CHAP. on each side of the mantle cavity, one plumose gill, the shaft of which is attached lengthwise to the body (Figs. 45, 46, ete.). In various divisions of the Lamellibranchia, the outer organisation deviates very greatly from the above. F. Cephalopoda. The body is bilaterally symmetrical. The visceral dome is large and often much elongated dorso-ventrally. The head is more or less distinct, and is surrounded by the foot, ad which is transformed in a peculiar man+ ner. The foot has, in fact, grown round the head, and has developed numerous differently-shaped processes (arms and tentacles) arranged in a circle round the mouth; these serve principally for seizing and holding prey. In viewing the body of a Cephalopod, it must be remembered that the apex of the visceral dome (which a casual observer might take to be the posterior end of the body) is really the highest dorsal point, while the head and its arms lie lowest. We may thus dis- tinguish, both in the visceral dome and in the transformed foot which has been combined with the head, and drawn out into tentacles, an anterior and a posterior part (which to a casual observer would seem upper and lower parts), and a right and a left side. This at first sight seems a paradox to those not acquainted with the comparative anatomy of the Mollusca, since the normal position in the water of Fig. 47.—Diagram of Sepia, median section from the left side. v, Ventral f (physiologically anterior); d, dorsal certain well-known Cephalopods does not (physiologically posterior); an, anterior agree with it. IN Sepia, for example, (physiologically upper); po, posterior é 5 ‘ (physiologically ‘lower): 152, 8, 4, 6, the SWIMS Or lies at rest in such a way that five arms of the left side; au, eye; co, the strongly pigmented anterior side of internal shell; go, gonad; d, pigment gland o : i 73 ” =ink-bag; m, stomach; n, kidney; ct, the visceral dome and of the head ctenidium ; a, anus; mh, mantle cavity; (Kopftuss) isuppermost, and the posterior in, siphon. The arrows indicate the side lowermost. The accompanying dia- direction of the respiratory current. : : ; 5 gram illustrates the strict morphological position of the body, which alone concerns the comparative anatomist (Fig. 47). On the right and left of the “head” there is a highly-developed eye, and near it an olfactory pit. The mantle fold hangs down posteriorly from the visceral dome, covering a spacious mantle- or respiratory cavity, which communicates VIL MOLLUSCA—OUTER ORGANISATION 37 with the exterior at the free edge of the mantle fold, above the “head.” Within the mantle cavity there are two or four gills, arranged symmetrically, the median anus, and the apertures of the sexual and excretory organs. ‘Two symmetrical lobes are found on the posterior lower side of the visceral dome; the edges of these are apposed in such a way as to form a tube, the funnel or siphon, one aperture of which lies in the mantle cavity, while the other protrudes from the mantle cleft. The respiratory water enters the mantle cavity through the mantle cleft, and escapes through the siphon. The fecal masses, waste and sexual products, and the secretion of the ink-bag also leave the body through the siphon. Originally, no doubt, all Cephalopoda possessed a shell which covered the whole visceral dome as well as the mantle fold. In recent Cephalopods the shell is rarely developed in this way; it is often rudimentary, and may, indeed, be altogether wanting. Recent Cephalopods fall into two entirely distinct divisions, the Tetra- branchia and the Dibranchia. The Tetrabranchia (Nautilus, Fig. 48). These have a shell coiled anteriorly (exogastrically) in the plane of symmetry, and divided by septa into consecutive chambers. The animal occupies the last and largest chamber ; the others ee contain gas.!_ The septa Bi oe separating the consec- A utive chambers are ( pierced in the middle A\ to allow of the passage °° ~ iS A oN (7s of a siphunele, which (ess ca mt i i