gore Sete centre iveoeee or pont Zee ee Cornell Mniversity Library BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henry W. Sage 1891 2114196 Cornell University Library or 805.L26 1891 Vv. Tuan 3 1924 024 558 284 olin 805 la & (S89 Yo TEXT-BOOK OF COMPARATIVE ANATOMY 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 PART I. London MACMILLAN AND CO., Lrp. NEW YORK: MACMILLAN & CO. 1896 A. % 2154 441 Bee 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 vi 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, e.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 .Vachwort 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 WirH 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. Iam 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 5 2 II. GasTRopopA (CEPHALOPHORA) 3 III. ScapHoropa . F : i) IV. LAMELLIBRANCHIA (PELECYPODA, Bivatva, ACEPHALA, AGLOSSA) 14 V. CEPHALOPODA 21 I. ORGANISATION OF THE PRIMITIVE MoLLusc 26 II. REVIEW oF THE OUTER ORGANISATION CHARACTERISING THE CHIEF GROUPS OF THE MOLLUSCA ‘ 28 A. PLACOPHORA OR PoLYPLACoPHORA (CHITONIDZ) . 29 B. APLACOPHORA, SOLENOGASTRES 29 C. GASTROPODA (CEPHALOPHORA) 380 D. ScapHOPoDA 34 E. LAMELLIBRANCHIA . 34 F, CEPHALOPODA ft 36 III. Tue INTEGUMENT, THE MANTLE, AND THE ViscERAL DoME ‘ 39 A. PLACOPHORA 39 B. SOLENOGASTRES 41 C. GASTROPODA 42 D. ScaPHoPoDA 49 E, LAMELLIBRANCHIA . 49 F. CrEPHALOPODA 53 IV. THE SHELL 55 A, AMPHINEURA 58 B. GASTROPODA 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 B. ScaApHoPoDA C. LAMELLIBRANCHIA. D. CEPHALOPODA VI. THe RESPIRATORY ORGANS . Tur True GILLS oR CTENIDIA . AMPHINEURA . GASTROPODA LAMELLIBRANCHIA . . CEPHALOPODA ADAPTIVE GILLS Lunes SOomtp VII. THE HypopraNcHIAL GLAND VIII. Tor Heap A. GASTROPODA B. ScaPpHopopAa C. CEPHALOPODA IX. Tue Ornau Loses or THE LAMELLIBRANCHIA X. THE Foor anpD THE PEDAL GLANDS . AMPHINEURA . GASTROPODA . SCAPHOPODA . LAMELLIBRANCHIA . CEPHALOPODA Ho Ob XI. SwELLING or THE Foor (Turgescence) XII. Muscutature anp ENDOSKELETON AMPHINEURA GASTROPODA ScaPHOPODA . LAMELLIBRANCHIA . CEPHALOPODA HPO Ome XIII. THe Nervous System A. AMPHINEURA B. GASTROPODA 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 ©. ScarPHOPoDA D. LAMELLIBRANCHIA . E. CEPHALOPODA XIV. AN ATTEMPT TO EXPLAIN THE ASYMMETRY OF THE GASTROPODA XV. THE SENsoRY ORGANS A. INTEGUMENTAL SENSORY ORGANS 1. TACTILE ORGANS 2. OLFACTORY ORGANS ‘ - THe ‘‘ LATERAL ORGANS” OF THE DIOTOCARDIA . GUSTATORY OnGaANs ‘ . SUBRADULAR SENSORY ORGAN OF CHITON . . THE SENSORY ORGANS ON THE SHELL OF CHITON B. AUDITORY ORGANS C. VisvuAL ORGANS 1. Optic Prrs Opric VESICLES OR VESICULAR EYEs . THE EYE OF THE DIBRANCHIATE CEPHALOPODA , . THE Dorsat Eyres OF ONCIDIUM AND THE EYES AT THE EpcGg oF THE MANTLE IN PECTEN . THE EYES ON THE SHELL OF CHITON 6. THe Compounp Eves oF ARCA AND PECTUNCULUS 7. DEGENERATION OF THE CEPHALIC EYES me Ww to aD Oe CO oO XVI. THE ALIMENTARY CANAL A. BuccaL Cavity, SnNout, PRrozoscis B. Toe PHARYNX AND Jaws, THE TONGUE AND SALIVARY GLANDS ‘ ForMATION OF THE RADULA C. THE GisopHacus D. Tue Mip-cur wiTH THE SromMAcH AND DIGESTIVE GLAND (LIVER) . . AMPHINEURA GASTROPODA ScaPHOPODA LAMELLIBRANCHIA . CEPHALOPODA E. Hinp-cur (REctvM) Seg Ge: XVII. THE CrrcuLaTory SysTEM A. GENERAL 137 142 143 145 149 162 162 162 162 165 166 166 166 167 169 169 170 170 173 175 175 176 176 178 180 188 187 190 191 192 193 194 194 195 198 198 xu COMPARATIVE ANATOMY B. SPECIAL . AMPHINEURA . GASTROPODA . SCAPHOPODA . LAMELLIBRANCHIA . CEPHALOPODA nr wl et XVIII. Tur Bopy Cayiry XIX. Tue NeEpuripia AMPHINEURA GASTROPODA ScAPHOPODA . LAMELLIBRANCHIA . . CEPHALOPODA AOOnF XX. GENITAL ORGANS A. GENERAL B. SpEcrAL XXI. Parasiric GASTROPODA XXII. ArracHED GASTROPODA XXIII. Ontrocreny A, AMPHINEURA B. GAsrRopopa XXIV. PHyLoceny Review of the most Importunt Literature APPENDAGE,—RHODOPE VERANIL CHAPTER VIII ECHINODERMATA Systematic Review Crass I, HoLoTHURIOIDEA II. EcurnomDEa III. AsTERoIDEA IV. OpHIUROIDEA V. PELMATOZOA 1. CRINOIDEA 2. CYSTIDEA 3. BLASTOIDEA J. GrNnERAL MorpPHoOLoGY OF THE ECHINODERM Bopy II. MorpHoLocy OF THE SKELETAL SYSTEM CONTENTS INTRODUCTION A. Tar ApicaL System (Calyx) 1. EcHINOIDEA 2, ASTEROIDEA 3. 4+. PELMATOZOA OPHIUROIDEA (a) CRINOIDEA (b) BLASTOIDEA . (c) CYSTIDEA B. THE ORAL SYSTEM OF ee . a THE PERISOMATIC SKELETON oo ~ oe . HoLoTHURIOIDEA . EcHINOIDEA p j ; ie ) THe NUMBER OF THE VERTICAL mans oF PLATES (6) Tat Pores oF THE AMBULACRAL SYSTEM (c) THE SYMMETRY OF THE EcHINOID SHELL (d) THE RELATION OF THE AMBULACRAL AND INTERAMBU- LACRAL PLATES TO THE PERISTOME (e) MANNER IN WHICH THE SKELETAL PLATES ARE Con- ¢ NECTED ; : (f) SPECIAL HeviierinreNs OF THE AMBULACRA (g) SpEcIAL MopIFICATIONS OF THE INTERRADII (2) ForM OF THE PERISTOME ({) ORNAMENTATION (k) MarcInaL INCISIONS OR PERFORATIONS (1) THE PERIGNATHIC APOPHYSIAL GIRDLE . ASTEROIDEA (a) THE AMBULACRAL Seaneat (b) THE INTERAMBULACRAL SKELETON (ec) THE Accessory SKELETAL SysTEM : (@) COMPARISON OF THE PERISOMATIC SKELETON OF ASTEROIDEA WITH THAT OF THE ECHINOIDEA OPHIUROIDEA (a) SKELETON OF THE ARMS (6) Tar OraL SKELETON CRINOIDEA (a) THE PERISOMATIC Bicwan OF THE CALYX a. THE APICAL CAPSULE oR Dorsal Cup b. THE TEGMEN CALYCIS . (6) Tot BracHIAL SKELETON (c) Tue Stem (CoLumNa) ( THE d) Tot MANNER OF CONNECTION BETWEEN THE SKELETAL PIECES 2 F : (e) THE NERVE Civiie OF THE ARMS AND OF THE APICAL CAPSULE xill PAGE 317 319 319 326 327 328 328 330 332 333 337 337 338 3389 340 340 344 346 348 376 377 x1V COMPARATIVE ANATOMY (f) THE WatTER PoRES 6. BLASTOIDEA (a) THE AMBULACRAL SKELETON (b) Tue STEM 7. CYSTIDEA D. Tue SPINES AND THEIR DERIVATIVES—THE SPHERIDIA AND THE P&DICELLARLE ‘ E. THe Masricatony APPARATUS OF THE ECHINOIDEA. (Aris- totle’s Lantern) F. Tue Catcarzeous Rinc oF THE HOLOTHURIOIDEA G. Furruern Deposirs or CALCAREOUS MATTER H. Concntupinc REMARKS ON THE SKELETON III. THE OvrER MorpHoLocy oF THE HoOLOTHURIOIDEA IV. Tue PosiTion AND ARRANGEMENT OF THE MOST IMPORTANT ORGANS IN THE RADII V. Tue INTEGUMENT VI. THE WatTER VASCULAR SYSTEM A. THE MADREPORITE AND STONE CANAL B. THE WaTeR VascuLtar RING : C. THe RapraL CANALS, THE CANALS OF THE TENTACLES AND TUBE-FEET, ETC. D. THe AMBULACRAL APPENDAGES VIL. Tur Caetom . THe Bopy Cavity THE BRACHIAL CAVITIES THE PERIESOPHAGEAL SINUS THE PERIANAL SINUS Tue AXIAL SINUS . THE AXIAL ORGAN THE CHAMBERED SINUS Qt hU¥O tb VIII. Tort PsEUDOHSMAL SYSTEM IX. Tur ErInruraL SysTemM X. Tue Bioop VascvuLarR oR LAcUNAR SysTEM XI. Tut Nervous SysTEM A. THE SUPERFICIAL ORAL SYSTEM . B. Tue Deerer ORAL NERVOUS SYSTEM C. THe APICAL OR ABORAL NERVOUS SysreM D. Tue Tuirp Nervous SYSTEM OF THE CRINOIDEA CONTENTS XII. THE Sensory ORGANS A. THE AMBULACRAL APPENDAGES AS SENSORY ORGANS B. Nerve ENpInes IN THE INTEGUMENT C. AUDITORY ORGANS, ORGANS OF ORIENTATION D. Eves XIII. Tar Bopy MuscunatTure HOoLOTHURIOIDEA ECHINOIDEA ASTEROIDEA . OPHIUROIDEA CRINOIDEA . HOOP XIV. Tat ALIMENTARY CANAL GENERAL REVIEW HoLoTHURIOIDEA EcHINOIDEA . CRINOIDEA . . ASTEROIDEA . OPHIUROIDEA Span > XV. RESPIRATORY ORGANS A. THE (INNER) REsprraToRY TREES OF THE HoOLOTHURIOIDEA B. REVIEW OF THE RESPIRATORY ORGANS OF THE ECHINODER- MATA XVI. THE CUVIERIAN ORGANS OF THE HOLOTHURIOIDEA XVII. Excrerion XVIII. Tue Saccuti oF THE CRINOIDEA XIX. GENITAL ORGANS . GENERAL MorPHoLocy . HoLOTHURIOIDEA ASTEROIDEA . OPHIUROIDEA 1. THE Bursa i 2. THE GENITAL APPARATUS . ECHINOIDEA CRINOIDEA . . ORIGIN OF THE SEXUAL PRODUCTS . HERMAPHRODITISM IN ECHINODERMS 1. CARE OF THE Broop AND SExuAL DIMORPHISM von mat XX. Capacity ror REGENERATION AND ASEXUAL REPRODUCTION XXI. ONTOGENY ‘ : ; i : A. Tue VArtous LArvAL Forms oF THE ECHINODERMATA Xvi COMPARATIVE ANATOMY B. ONTOGENY OF THE HOLOTHURIOIDEA C. ONTOGENY OF THE ECHINOIDEA D. ONTOGENY OF THE ASTEROIDEA E. ONroGENY OF THE OPHIUROIDEA . F. ONTOGENY OF THE CRINOIDEA XXII. PHYLoGENY I. Il Review of the most Important Literature CHAPTER IX ENTEROPNEUSTA . OUTER ORGANISATION . . THE Bopy ErrraeLium . THE Nervous SysTEM . THE SENSORY ORGANS . THE ALIMENTARY CANAL . THe CaeLomic Sacs AnD THE Bopy MuscuLaTURE. . THE ‘‘Hranr VESICLE” . Tae Limiting MeEMbRANEs, THE PROBOSCIDAL SKELETON, AND THE BRrANCHIAL SKELETON . Tur Bioop VascuLan SysTEM . THE GONADS . ONTOGENY . PHYLOGENY Literature APPENDAGE TO THE ENTEROPNEUSTA CEPHALODISCUS . RHABDOPLEURA Literature INDEX . 562 600 602 603 = CHAPTER VII SIXTH RACE OR PHYLUM OF THE ANIMAL KINGDOM MOLLUSCA. THE Mollusca 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 lies 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 cceelom (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, I . ot 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 Molluscs are divided into the five following classes :— I. Amphineura. II. Gastropoda. III. Scaphopoda. IV. Lamellibranchia. V. Cephalopoda. Systematie Review. CLASS I. Amphineura. Bilaterally-symmetrical Molluses. 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 del’ Astrolabe). provided with ganglion cells throughout their whole length ; these pass anteriorly into the cerebral ganglion. Special sensory organs are reduced. Marine. OrpDER 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 branchie 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), (L+1+1), (1+2), (1+3). Chiton (Fig. 1), Chitonedlus. OrpER 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; el, cloaca. 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 :— Fam. rnc o eaam Cheetodermatide. Neomeniide. a Proneomeniide. Neomentine Dondersiide. 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 (Yrochid, after Pelseneer). 1, Head; 2, anterior epipodial lobes ; 3, foot ; 4, pigmented prominence at the base of the epipodial tentacles (5); 6, visceral dome. 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- iorly to the ventricle. The Proso- branchia are chiefly marine, and are sexually separate. The foot is generally pro- vided with an opereulum for closing the aperture of the shell. A shell is wanting only in Titiscaniv, 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 frecly. 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. Geugobranchia (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. AHatliotidw, radula w1L(5.15)lo, Fissurellidu (Pissu- ing Fic. 4.—Patella vulgata (from eneath, after Lankester). «, Tentacle ; ad, etterent branchial vessel : v, free edge of the shell; ¢, free edge of the mantle; v-y, median line; g, afferent branchial vessels 3 J, branchial lainelle ; h, one of the afferent vessels ; i, spaces between the shell muscles 3b, foot. rella, vad, «©1,(4.1,4)1., with secondarily symmetrical shell. Emarginula, Seutum VII MOLLUSCA—SYSTEMATIC REVIEW 5 = Parmophorus), Pleurotomaride (Pleurotomaria, Scissurella, Polytremaria), Bellero- phontide (exclusively fossil). 6. Azygobranchia. — One gill, homologous with the left gill of the Zeugo- 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.)1.00, marine, Neritide, rad. 01.(3.1.3.)1.0 (marine, but along the shore able to live out of water), Neritin (marine and fresh-water). The Hydrocoenida, rad. e@l.(1.1.1.)l.0, and Helicinide, rad. 0 1.(4.1.4.)l.0, have no gills but a lung resembling that of the Pulmonata. The Helicinide are terrestrial. ce. 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). «, Proboscidal snout or rostrum; }, tentacle ; e, eye; d, foot; e, metapodium with operculum /. 1. Left true ctenidium present. Acmacide, rad. 1.2.(1.0.1.)2.1.; with numerous accessory gills in the mantle furrow: Scurria;—without such gills: demaea (Tectura). 2. True ctenidia altogether wanting, accessory gills very numerous in the mantle furrow.—Fam. Patellidee (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 Cypraca (and in other forms ?) a rudiment of the right auricle persists. Fam, Cypraeide, rad. 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), Littorinidw, Cyclostomide (terrestrial), Planaxide, Hydrobiide (fresh-water), Aciculide (terrestrial), Truncatellide (partly terrestrial), Hipponycide, Capulide, Calyptracide, Pseudomelanide, Melanide, Cerithiide, Fic. 6.—Rostellaria rectirostris (after Owen). «, Snout ; b, tentacle; c, stalked eye; d, foot; e, metapodium with operculum f; hk, beak (for the siphon). Vermetide, Turritellide, Nenophoride (Fig. 5), Struthiolaride, Chenopide, Strombide (Fig. 6). Proboscidifera holostomata. Fam. Scalaride, rad. «Ow i Solaride, rad. «Ow ; Pyraimidellide, rad. 0; Eulimide, rad. O. Proboscidifera siphonostomata. Fam. Colombellinide, Tritoniide, Cassidiide (Fig. 7), Doliide. Fic. 7.—Cassis suclosa (after Poli). u, Shell; b, beak; ¢, siphon; d, head; g, proboscis; e, eye; f, tentacle; h, foot; i, operculuin. Janthinide, rad.«Ox. Heteropoda (marine Taenioglossa, with foot transformed into a perpendicular rowing fin). Fam. Aélantide (Fig. 8), Pterotrachacide (Fig. 9). c. Stenoglossa. — Normal vad. 1.1.1. Rachiglossa. Fam. J'urbinellidie, Fuside, Mitride, Buceinide, Muricide, Purpuride, Hatiadea, Cancellariide, Polutide, Olivide, Marginellide, Harpide. Toxiglossa. Fam. Plewrotomide, Terebride, Conide. Fic. 8.—Atlanta Peronii (after Gegenbaur). «, Pharynx; b, buccal ganglion ; ¢, tentacle ; «, eye; é€, cerebral ganglion ; f, aorta cephalica ; g, pleuro-visceral connective ; h, columellar inuscle ; i, k, osphradium ; 1, vagina; m, ctenidium ; , anus; 0, uterus; p, nephridium ; 4, aorta cephalica ; r, auricle; s, ventricle; t, aorta visceralis; wu, digestive gland (liver); v, ovary; w, stomach; a, pedal ganglion ; y, operculum; z, metapodium ; 1, sucker of the fin-like foot (rudimentary sole); 2, toot; 8, auditory organ; 4, cesophagus ; 5, snout; 6, salivary gland. u Fic. 9.—Pterotrachea (Firola) coronata (after Leuckart). «, Pharynx; b, proboscidal snout ; «, eye; d, cerebral ganglion; e, pedal ganglion; f, pedal artery; g, intestinal canal; h, pleuro- visceral connective; i, parieto-visceral ganglion ; k, osphradium; 7, ventricle; i, auricle; n, anus; 0, ctenidium; p, metapodium; g, appendage ; r, aorta cephalica; s, nerve running to the metapodinm ; t, artery; u, foot; v, connmon pedal artery; w, cephalic artery; #, auditory organ ; y, buceal ganglion. 8 COMPARATIVE ANATOMY cHAr. OnveER 2. Pulmonata. The pleuro-visceral connectives are not crossed. The ctenidium has disappeared from the mantle complex and is replaced by a lung, or respiratory vascular network, on the inner surface of the mantle. The pallial organs lie primitively to the right, anteriorly on the visceral dome. The edge of the mantle, with the exception of a branchial aperture on the right, unites with the integument of the neck. In terres- trial Pulmonata the visceral dome is often flattened down and the shell becomes rudi- mentary (Slugs). The operculum is always wanting. The heart has one auricle, which almost always lies anteriorly to the ventricle. The Pulmonata are hermaphrodites with herma- phrodite glands or ovotestes, and complicated efferent ducts. They are either terrestrial or fresh-water. Fic, 10._Amphipeplea leuconensis (after Adams). «a, Lobe of the mantle Syb-Order 1. Basommatophora (fresh-water). bent back over the shell; }, portion of the shell uncovered ; ¢, foot. Eyes at the bases of the non-invaginable optic tentacles. Genital apertures separate, to the right anteriorly, the male in front of the female. Fam. Limneide, (Limnea, Amphipeplea [Fig. 10], Physa [Fig. 11], Planorbis, Aneylus), Auriculide. Fic. 11.—Physa fontinalis (after L. Reeve). uv, Mantle lobes folded back over the shell; b, evaginated penis. Eyes at the tips of the optic tentacles ; tentacles invaginable. Sub-Order 2. Stylommatophora. a. Monogonopora.— With a single genital aperture to the right. Fam. Helicide (Helix [Fig. 12, A], Arion [Fig. 12, D], Bulimus). Testacellide (Duudebardia {Fig. 12, B], Testacella [Fig. 12, C]. Limacide (Ariophanta, Limax, Vitrina, Zonites, Helicarion). Bulimulide (Fig. 13), Pupide (Buliminus, Pupa, Clausil ia), Succineide. b. Digonopora.—Shell-less snails with separate male and female genital apertures, the male anterior, the female at the posterior end of the body, both to the right. Pallial complex at the posterior end of the body, lung cavity reduced. Fam. Vagi- nulide (terrestrial), Oncidiide (marine or amphibious) ; respiration partly by means of dorsal branchial appendages. VIEW 4 MOLLUSCA—SYSTEMATIC RE VI ', Testacella halio- , Daudebardia (Helicophanta) brevipes ; ( in D shield (from Lankester). B Helix pomatia ; Fic. 12.—4, tidea; D, Arion ater ; s, Shell, > Fic. 13.—Peltella palliolum (Bulimu/id, after Ferussac). 10 COMPARATIVE ANATOMY CHAP. ORDER 3. Opisthobranchia. The pleuro-visceral connectives do not cross. * There is one auricle placed behind the ventricle. A shell is sometimes present, more frequently wanting. An operculum is rarely found. Respiration by means of true ctenidia, or of adaptive gills, or through the skin. The visceral dome is very often levelled down. Herma- phrodites with ovotestes. Marine. Sub-Order 1. Tectibranchia. The pallial complex is to the right of the body, and is more or less covered by the mantle fold belonging to that side. One true ctenidium (viz. that which was originally the right) is always retained in the mantle cavity, but is often very incompletely covered by the mantle. The visceral dome tends to disappear. A shell is always present, but tends to become rudimentary. Generally with para- podia, and mantle lobes covering the shell. A. Reptantia. a. Cephalaspide.—With frontal or cephalic disc. Fam. Acteonide (with operculum), Scaphandride, Bullide (Bulla, decre), Gastropteride (Fig. 14), Philinide, Doridiide. b, Anaspide.—Head without frontal dise ; four triangular or ear-like tentacles. Fam. Aplysiide (Aplysia, Dolabella, Notarchus). conf, Fic. 14.—Gastropteron Meckelii, Fic. 14.—Pleurobranchus aurantiacus, with internal with internal shell (after Vayssiére). shell (atter Leuckart’s Wendtafeln), seen from the right 1, Cephalic shield (frontal disc); 2, para- side. «, Rhinophores ; }, labial sail; c, genital aperture ; podium ; 3, ctenidium, left almost un- d, nephridial aperture (7); ¢, ctenidium ; f, anus. covered by the rudimentary mantle fold ; 4, flagellun=appendage of the mantle fold. c. Notaspide.—Head short, with or without tentacles. Large dorsal dise (noteeum) in or on which a shell may lie. Fam. Pleurobranchide (Plewrobranchus (Fig. 15], Plenrobranchwa, Oscainius), Umbrellide (Umbrella, Tylodina), Peltide. B. Natantia sive Pteropoda.” These formerly constituted a separate class of the Molluscs, but are now recog- nised to be Tectibranchia adapted to a free-swimming pelagic life. The parapodia of the Tectibranchia develop as fins or wing-like swimming organs. " Except in Actwon, which is streptoneurous, and thus forms a connecting link between the Opisthobranchia and Pulmonata on the one hand, and the remaining Gastropods on the other [Bouvier and Pelseneer], v. Nod. Sei., July 1893. ; 2 The classification of the Opisthobranchs, which places the Pteropoda thecosomata with the Cephalaspide, and the Pteropoda gymnosomata with the Anaspide, is accepted ou p. 110 and elsewhere. : VII MOLLUSCA—SYSTEMATIC REVIEW 11 a, Pteropoda thecosomata.——These are nearly related to the Cephulaspidea, and possess a mantle, mantle cavity, and shell. The head is not distinct, and has only one pair of tentacles. The fins, at their anterior edges, are fused over the mouth; the anus lies to the left. Fam. Limacinide. An external calcareous shell, with left-handed or sinistral twist, and a spiral operculum. Anus to the right (Lima- cinw (Fig. 16], Peraclis). Fam. Cavoliniide. External symmetrical shell (Clio, Cavolinia). Fam. Cymbuliide. Internal cartilaginous shell (Cymbulia, Cymbuli- opsis, Gleba). The Thecosomata feed chiefly on small Protozoa and Alge. Fic, 16.—Limacina Lesueuri (dorsal aspect, after Pelseneer). 1, Penis; 2, fin (parapo- dium); 8, seminal furrow; 4, mantle process (‘‘ balancer”); 5, visceral dome; 6, head with two tentacles and the seminal furrow 3. Fic. 17.—Pneumoderma (diagram from the right, after Pelseneer). 1, right evaginated process bearing hooks (hook sac); 2, proboscis ; 3, right buccal tentacle ; +, position of the right nuchal tentacle ; 5, right tin (parapodium); 6, seminal furrow ; 7, genital aperture ; 8, position of the jaw ; 9, ventral proboscidal papilla; 10, right buccal appendage provided with suckers ; 11, head ; 12, aperture for penis; 13, right an- terior pedal lobe ; 14, anus ; 15, posterior pedal lobe; 16, ctenidium; 17, posterior adaptive gill; d, v, a, p denote dorsal, ventral, anterior, and posterior. acy b. 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 hes to the right. Fam. Pneumodermatide. One ctenidium to the right (Dextobranchea, Spongiobrancheea, 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; b, oral tentacle; ce, 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. WO \ 1 . 1 ' i 2a & Fic. 19.— Phyllirhoé bucephalum (lateral aspect, after Souleyet, modified). 1, Tentacle - 2, cerebral ganglion; 3, 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- branchiatz. The branchial rosette round the dorsal anus can be with- drawn into a cavity. (Bathydoris, alrchiduris, Discodoris, Diaulula, Kent- rodoris, Platydoris, Chronodoris, ete.) Dorididz phanerobranchiate. Bran- chial rosette not retractile. (Gonio- doris, Polycera, Acanthodoris, Idalia, atneule, Euplocamus, Triopa, ete.) 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, Galvin, Coryptella, Rizzolia, Facellina, Flabel- lina, Fiona, Glaucus, Janus, Hero), Fam. Tethymelibide, without radula (Tethys, Mclibe). 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 (Z7ritonta, Murionia). 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 IV. 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. OrpeEr 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). «, Anterior adductor ; b, anterior retractor of the foot > ¢, elevator of the foot; d, genital mass ; é, hypobranchial gland ; f, posterior retractor of the foot 3 9, posterior adductor : h cteni- dium ; i, mantle cavity ; k, creeping sole of the foot (1); m, oral lobes (labial palps) with posterior appendages n and o. 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). OrvDER 2. Pilibranchia. ; 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 (¢f. Fig. 88 B). Fam. Anomiids: mantle open VII MOLLUSCA—SYSTEMATIC REVIEIF 15 without siphons ; Monomyarian. Foot small ; body and shell asymmetrical. Attached. Branchial filaments entirely free (Anomia, Placuna). Fam. Arcide: the branchial filaments of each row connected by ciliated discs ; Dimyarian. No siphons. Foot large (Arca, Pectunculus). Fam. Trigoniide: ctenidia like those of the Arcida ; Fic. 22.—Mytilus edulis (after Meyer and M6bius), 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 (Trigonia). 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). OrpvEr 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 (¢/ 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. Wendtafeln), Fic, 24.—Anatomy of the Oyster (Ostrea edulis), right aspect (after Mébius, Leuckart, and Nitsche, Zool. Wiwdtufeln). br, Gills ; Pu, posterior mantle nerve ;.“, 71, apertures of the cavities between the fused plates of the two left gills; ./, large adductor muscle ; a, anus; Mi, posterior portion of the adductor muscle ; J’, mantle ; P, pericardium ; I’, heart ; go, gonad (hermaphrodite gland); d, intestinal canal; /, digestive gland (liver); 0, mouth ; os, 0s,, oral lobes (labial palps) of the left side ; ('y, cerebral ganglion ; 7, kidney ; bx, branchial nerve ; I’g, visceral ganglion ; Pj, abdo- yninal process ; P11, 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 REVIEIV 17 swimming. (Pecten [Fig. 23], Chicinys). Fam. Aviculide: Monomyarian or Hetero- myarian without siphons. Valves equal or unequal (dvicule [Alelengrina], Malleus, Vulsella, Perna, Inoccramus, Pinna, Meleagrine maryaritifera, 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]). OrpeEr 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 Lamed/i- 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. Tam. Carditide : with open mantle and large foot (Cardita, Venericardia). Fam. Lucinide : with simple, and as a rule single, siphonal aperture. Foot often vermiform. Fam. Fic. 25.—Anatomy of Unio (Margaritana) margaritiferus, left aspect (after Leuckart and Nitsche). v, Mouth; cg, cerebral ganglion; J), anterior adductor muscle; @, cesophagus ; 1, digestive gland (liver); no, nephridial aperture ; lo, aperture of the digestive gland in the stomach m ; Aa, anterior aorta ; x, nephridium, the outlines given in dotted lines ; V, heart; r, proctodeum ; Ap, posterior aorta; Mo, posterior adductor ; a, anus; Vg, visceral ganglion; Br, gill; Bh, 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. (Lrycina, Kellya, Lasea, 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 ; Muteda). VOL. I C 18 COMPARATIVE ANATOMY CHAP. Sub-Order 2. Tellinacee. Dimyarian with completely separate siphons. Foot large. Gills smooth. Fam. Tellinide (Tellina). Fam. Donacide (Donax), Mactride (Mactra). Sub-Order 3. Veneracea. Dimyarian with somewhat folded branchial lamelle. Siphons separate, and foot large. Fam. Veneride (Menus, AMeretria [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. Cardiidee : Dimyarian. Fic. 26.—Anatomy of Cardium tuberculatum, left aspect (after Grobben, Leuckart, and Nitsche, Zool. Wondtafeln). p, Foot; go, gonad; S, shell; Pa, mantle; os, labial palps ; 0, mouth; My, anterior adductor muscle; @, esophagus; m, stomach; J, digestive gland; d, intestinal canal; goo, genital aperture ; 20, pericardial aperture of the kidney ; V, ventricle; At, auricle ; P, pericardium ; no, aperture of the kidney in the mantle cavity ; », kidney; Mo, posterior adductor ; Bl, point of concrescence 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, ctenicdium. (Cardiwm (Fig. 26].) Fam. Chamide: Dimyarian. Valves of shell unequal. (Chama, Diceras, Requienia.) To these the fossil forms Jfonoplewride, Caprinide, Hip- puritide,*Radiolitide., Fam. Tridacnide: Monomyarian. (Zridacna, Hippopus.) Sub-Order 5. Myacea. Dimyarian with folded branchial lamellew. 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, Mesodesmatide, 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 M, Mo ie At oe Fic. 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; ol, anterior upper mantle lobe; Rpp, posterior retractor of the foot ; Ss, partition between the two siphons; M3, accessory adductor ; mb, intestinal cecum ; x, pericardial section of the kidney, which opens into the pericardium by means of the renal funnel at u. Fia. 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 Mébius). The centre is omitted, the calcareous tube is for the most part uninjured. Fic. 30.—Shell of Aspergillum (Bre- chites) vaginiferum, dorsal view. uw, 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. vit MOLLUSCA—SYSTEMATIC REVIEW 21 Xylophaga). Fam. Teredinide : boring mussels ( Zeredo [Fig. 29]). Fam. Clava- Sellidee (Claragell, Brechites [Aspergillum, Fig. 30)). Sub-Order 7, Anatinacea. Mantle to a great extent closed. With siphons and foot. Hermaphrodite. Fam. Pandoride, Lyonsiide, Anatinide (Anatina, Thiacia), OrvEr 5. Septibranchia. The ctenidium on each side is transformed into a muscular septum pierced by d 10 Fic. 31.—Soft body of Silenia Sarsii (Cuspidaria), after Pelseneer. 4, Left aspect after removal of the mantle ; LB, ventral aspect after removal of most of the mantle; a, p, anterior and posterior ; d, v, dorsal and ventral ; 7, 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 (lig. 81 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. Orper 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, Cephalie hood; b, tentacles; c, infundibulum (siphon); ¢, eye; ¢, projection caused by nidamental gland ; f, point of attachment of the adductor muscle; g, upper portion of the visceral dome; h, last (inhabited) chamber of the shell; i, anterior lobe of the mantle; /, last chamber but one; J, 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. 1 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. VII MOLLUSCA—SYSTEMATIC REVIEW 23 Fia. 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). Fic. 34.—Loligo vulgaris (after D’Orbigny). 4, Dorsal (physiologically ventral) view; 3B, 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 cHaY. OnvER 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, Owenin, Thysanoteuthis, Onychoteuthis, Ommatustrephes). Fam. Myopside (Rossia, Sepiola, Sepiadarium, Idiosepion, Loligo [Fig. 34], Sepivteuthis, Belosepia [fossil], Sepiv, 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. Fie, 35.—Female Argonauta, in the swimming position, right aspect (after Lacaze-Duthiers). 1, Uncovered part of the shell; 2, the right arin 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, secondarm. The second, third, and fourth arms are stretched backwards inside the shell. Oviducts paired. Fam. Cirrhoteuthide : with fins. Fam. Philonexide : Aryonauta (Figs. 35, 36, and 200, p, 243). Female with external unchambered shell. Philunewis, Tremoctopus. Fam. Octopodide (Octopus, radula 1.3.1. (Fig. 37], Eledoue). VIL MOLLUSCA—SYSTEMATIC REVIEW 25 Tic. 36.—Female of Argonauta Argo (after Vérany). Second, third, and fourth pairs of arms stretched downwards. «, Siphon; b, eye; ¢, 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. J. Organisation of the Primitive Molluse. The hypothetical primitive Mollusc, reconstructed from the results of morphological research, may he described as follows :— The body is bilaterally symmetrical and dorsally arched ; its Fic. 38. — Hypothetical Primitive Mollusc, diagrammatic, left aspect. 0, Mouth; k, head; sm, shell muscle ; oso, upper aperture of the shell; «, anus; x, 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 eavity, 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 Fic. 39.—Hypothetical Primitive Mollusc, from above. 0, Mouth; wle, ulpl, ulp, primitive left cerebral pleural and pedal ganglia; ulpa, urpa, primitive left and right parietal ganglia ; ula, primitive left auricle; wos, uros, primitive left and right osphradia (Spengel’s organ) ; ulect, urct, primitive left and right ctenidia (gills) ; mb, base of the mantle ; mr, edge of the mantle; m, mantle cavity ; v, visceral ganglion; vc, ventricle ; uw, 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 Mollusc 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 branchie on each side. Each gill is feather-like, 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 eomplex. The inner organisation may thus be briefly described. The intestinal canal. 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 esophagus, 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). Musculature.—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 (ecolumellar or shell muscles), and special muscles for the different organs. Nervous system.—Two well-developed eerebral 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 Amphineura and Diotocurdiu 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 viseeral ganglia lying posteriorly in the body cavity. Giving the name connectives to such nerves as unite the ganglia of one side of the body, ze. 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 ecelom 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 ducts or vesicles. The eireulatory 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. II. 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. vu MOLLUSCA—OUTER ORGANISATION 29 A. Placophora or Polyplacophora (Chitonide). 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 in a 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 le 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. Aplaeophora, 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. 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 Fissurellu, Oliva, Turritella, Cleodora, Pterotrachea, Phyllirhoc, Linux, 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. u, Anus; ve, ventricle ; ula, right auricle ; wret, left ctenidium ; The large visceral dome is 5 wros, left osphradium. 2 By : coiled spirally, generally to the right (dextrally), the shell naturally assuming the same form. The well- VII 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 SS ule o t fold hangs down from the anterior side of the f aN Var ch visceral dome, and covers IP the spacious branchial or ulpt wre mantle cavity, in which of le certain organs of special morphological importance. These, which may be called the mantle or pallial organs, are, in such forms as a ede ae tive, (1) the anus, which lies, not posteriorly, but on the anterior side of the visceral dome, shifted forwards to- 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, jag. 41 ee .—Diagram of a Prosobranchiate Monotocardian. The nephridial aperture, and outer form, shell, mantle, pallial complex, heart and pericardium, osphradium to the left nervous system and operculum, are depicted. Lettering mostly é asin Fig. 39. In addition: f, foot; si, siphon; sup, sub, supra- of the anus being T€- and sub- intestinal connectives ; op, operculum ; of, auditory tained, while the hind- organ; »p, penis; sr, seminal groove; mk, mantle cavity ; hy, hypobranchial gland; , male genital aperture; 7, rectum ; au, gut with the anus moves rrr ee tacle, 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. «a/, Respiratory aperture; rgn, vas- at the inner surface of the mantle cwar network on the inner surface of the mantle. fold, in which runs a fine network Se year oi aaa Further letter- 6f vessels lying in front of the 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. fore at 2 3. Opisthobranchia. The respiratory organs lie behind the heurt. (a) 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. (b) Nudibranehia..— 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 Fic. 43.—Diagram of a Tectibranchiate Opistho- pranchiate. 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. IL D 34 COMPARATIVE ANATOMY CHAP. D. Seaphopoda. The body is symmetrical and long, i.e. 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 euough 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 cirele 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 lies 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, p- 1138). E. Lamellibranchia. 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 foot being we cygnea (ordinary freshwater mussel) (after Howes). tracted), be viewed from the side, ig, Ligament ; ty, typhlosolis ; kb, pericardial gland the outline will be found to be (Keber’s organ); re, kidney (glandular portion); for dd ll sbe, chambers at the bases of the gills; gd, genital a ne ui EOD Sa Ys by the dorsal duets ; briy, brl, outer anil inner brauchiallameliew; Median line of the body; an- ibe, mantle cavity ; s, shell; s,, edge of the shell ; ] r tap) E Si, foot ; pm, pallial muscle ; i, intestine ; pi), right terior] ‘ Poster iorly, and ventrally mantle fold; ggl, gonad; r, rectum; cp, cerebro- by the free edge of the mantle pedal connective ; re], non-glandular vestibule of fold. The two mantle folds en- kidney ; reg, renal aperture ; pe, pericardium. ean » Pia Dencarcuun close a space whose transverse axis is always markedly shorter than either its dorso-ventral or its longitudinal axis, i.e. 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 lies 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 Fic. 46.—Anatomy of Unio (Margaritana) margaritiferus, left side (after Leuckart and Nitsche). o, Mouth; Cg, cerebral ganglion ; MZ), anterior adductor muscle ; a’, cesophagus ; 1, digestive gland (liver) ; no, nephridial aperture ; lo, apertures of the digestive gland in the stomach m; da, anterior aorta; n, nephridium, the outline given in dotted lines; I’, heart; r, hind-gut; Aj, posterior aorta; Mo, posterior adductor; «, anus; ly, 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, ate.). 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 Fic. 47.—Diagram of Sepia, median section from the left side. ¥v, Ventral (physiologically anterior); d, dorsal (physiologically posterior); an, anterior (physiologically upper); po, posterior (physiologically lower); 1, 2, 3, 4, 5, the tive arms of the left side; au, eye; co internal shell; go, gonad; d, pigment gland =ink-bag; m, stomach; x, kidney; ct. ctenidium ; @, anus; mh, mantle cavity ; in, siphon, The arrows indicate the direction of the respiratory current. distinct, and is surrounded by the foot, 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 certain well-known Cephalopods does not agree with it. A Sepia, for example, swims or lies at rest in such a way that the strongly pigmented anterior side of the visceral dome and of the “head” (Kopffuss) isuppermost, and the posterior side lowermost. The accompanying dia- 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 Tetrabranehia (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 contain gas.!_ The septa separating the consec- utive chambers are pierced in the middle to allow of the passage of a siphunele, which runs through all the compartments, and is attached to the visceral dome. That portion of the foot which sur- rounds the mouth is produced into numerous Fic. 48.—Diagram of Nautilus, left view. ve, Ventral; do, tentacles, which can be dorsal ; vo, anterior ; hi, posterior ; f, foot (tentacles and siphon) ; retracted into special sm, shell muscle ; ct, ctenidia ; mh, mantle cavity ; u, anus; s, shell; si, siphuncle ; au, eye; 0, mouth. sheaths. The anterior portion of the foot, which lies in front of and over the head, is widened out into a concave lobe, the hood ; this is applied to the outer surface of the occupied chamber of the shell anteriorly, and, when the tentacles are withdrawn, can close its aperture. The hood carries two tentacles, and on each side of the head there is an eye. 1 Or water ; v. Ford’s Introduction to Brit. Mus, Cat., Fossil Cephalopoda, 1889. 38 COMPARATIVE ANATOMY CHAP. Above the head, the mantle fold encircles the whole body. It is short at the sides, but anteriorly it forms a large lobe which is folded back over the shell in the way shown in Fig. 32, p. 22. Posteriorly, the mantle covers a very deep cavity which contains the whole posterior side of the visceral dome. The siphon consists of two entirely distinct lateral lobes (epipodial lobes), whose free edges overlap in such a manner as to form a tube, open above and below. As we shall see later, this siphon is a part of the foot. Deep down in the mantle cavity, two pairs of pinnate gills—a lower and an upper pair—spring from the visceral dome. Nine apertures of inner organs are also found in this cavity ; a single median anal aperture, and four paired apertures, viz. one pair of genital, two pairs of nephridial, and one pair of viscero- pericardial apertures. The position of these is depicted in Figs. 78 and 79, p. 82. The Dibranchia. With one exception, viz. the female ryonvutv, which has an external unchambered shell, the Dibranchia either have an internal shell lying on the anterior side of the visceral dome, covered by an integumental fold, or no shell at all. The visceral dome is sometimes compact and pouch-like (in reptant animals, Fig. 37), sometimes, in the good swimmers, much elongated dorso-ventrally, produced dorsally to a point, and flattened antero-posteriorly (Fig. 34). In the latter case, the body is further generally encircled by a fin-like integumental fold, which marks the limit between the anterior and posterior sides of the visceral dome. The “head” is usually distinct from the visceral dome, and carries to the right and left the well-developed eyes. The mouth is sur- rounded by eight or ten arms for seizing prey; these are provided with suckers on their lower adoral sides. The mantle fold covers nearly the whole posterior surface of the visceral dome, and thus encloses a very deep and spacious cavity. Laterally and anteriorly to the visceral dome, the mantle fold is continued as a narrow border which, immediately above the “head,” covers a shallow groove or furrow. The two lateral lobes which form the siphon of the Tetrabranchia have in the Dibranchia grown together at their free edges, and form a tube open at each end. There are only two gills in the mantle cavity, one right, and one left. Near the upper siphonal aperture in the mantle cavity he the anus, and the genital and nephridial apertures as well as that of the ink-bag. Details as to the arrangement and number of these apertures will be given further on. 7 vir MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 39 III. The Integument, the Mantle, and the Visceral Dome. The whole body is covered by a single layer of epithelium, which, in parts not protected by the shell, may be more or less ciliated. This layer is very rich in glands which are almost exclusively unicellular ; some of these lie in the epi- thelium itself, while some have sunk into the subjacent tissue, their ducts, however, passing between the epithelial cells. The layer immediately beneath this body epithelium is called the corium, and con- sists of connective tissue and muscle fibres. It is, how- ever, not distinctly marked off from the tissues beneath it. Fic. 49.—Section of the integument of Daudebardia é z rufa (after Plate). 1, Epithelium ; 2, 3, 9, various forms The pigment is almost of unicellular glands; 4, globular pigment cells; 5, 7, always found in the cells of wnpigmented cells of the connective tissue ; 6, muscle the subepithelial connective fibres 5 8, branched and anastomosing cells of the con- i nective tissue containing pigment. tissue. A. Placophora. (Cf. the sketch of the Outer Organisation, p. 29.) The Chiton is provided dorsally with eight consecutive shell-plates (Fig. 1, p. 2), which overlap in such a manner that the posterior edge of each plate covers the anterior edgeofthe next. These plates are bilaminar. The outerand upper layer which forms the dorsal surface is called the tegmentum, the lower hidden layer the articulamentum. As a rule, the tegmentum of the anterior plate only is as large as the articulamentum beneath it; in the other plates, the latter is the larger and projects beyond the former laterally and anteriorly. These projecting parts of the articulamentum, called apophyses, slide under the plate next in order anteriorly. Between these two layers, tissue is found, which is a continuation of the dorsal integument. The tegmentum is penetrated by canals of various sizes, which open at its surface through characteristically arranged pores.1 The tegmentum consists of a horny or chitinous substance, which may be considered as a cuticular formation, impregnated with calcareous salts. The articulamentum is compact and free from canals ; it contains little organic substance, and much calcareous salt. It alone answers to the shell of other Molluscs, while the tegmentum must be considered as a calcified cuticle covering the true shells (the articulamenta) as a continuation of the cuticle of the zone which encircles the eight shell-plates. This zone carries 1 On the relation of these canals and pores to peculiar sensory organs and eyes on the shell of the Chiton, cf. section on Sensory Organs, p. 166. 40 COMPARATIVE ANATOMY CHAP, chitinous or calcareous spines, sete, scales, granules, etc., varying in number and arrangement according to the genus and species. Each spine, as a rule, arises as a globular vesicle within an epithelial papilla and above a very large formative cell (Fig. 50). As it grows, it is pushed upwards by the newly -forming cuticular layers. The formative cell at its base persists, but remains connected with the epithelial papilla only by a protoplasmic process which continually lengthens, and may surround itself with a nucleated sheath. In fully-developed spines, the remains of this cell are still found as a small terminal swelling (Endkélbchen). There are, however, spines and specially flat scale- or plate-like calcareous formations in the integument which do not arise from single large formative cells, but are probably produced by several cells in the base of an epithelial papilla. é Just as we have recognised the tegmentum Fic, 50.—A, B, C, Three stages in de- . . velopment of a spine in the Chiton (after covering the articulamenta to be merely a Blumrich), diagrammatic. sf, Spine; bz, its Special portion of the general cuticle, so we formative cell; e, epithelium; c, thick may further recognise in the articulamenta cuticle secreted by the epithelium; ek, the homologues of the calcareous spines, ee scales, ete., which are developed in the integu- ment of the mantle. The articulamenta would thus be nothing more than very large and expanded calcareous scales. A sm J (Sa ea tas 8 10 Ta fem NS aa 16 @ 15 Fic. 51.—Transverse section through a Chiton near the nephridial apertures, highly diagrammatic (after Sedgwick), somewhat modified. 1, Pericardium ; 2, ventricle; 3, auricle; 4, branchial “vein”; 5, branchial groove (mantle cavity); 6, gill (etenidium); 7, foot; 8, pleuro- visceral connective; 9, branchial “artery”; 10, secondary ccelom; 11, intestine; 12, posterior portion of the gonad lying below the pericardium; 13, 14, the two posterior branches of the nephridium, one of which (13) opens into the branchial groove (at 16), the other (14) being connected in a way not here depicted with the pericardium ; 15, pedal nerves. This view, finally, leads to the conclusion that the shell (if it may here be so vir. MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 41 called) of the Molluscs originally consisted of isolated calcareous spicules or spines, which were enclosed in a thick cuticle, and projected from the same as in the Pronecomeniau, Neomenta, ete. (v. below). In Cryptochiton the shell is internal, i.c. it is entirely covered by a fold of the integument, which grows over it from all sides. It consists exclusively of the articulamentum, since the whole dorsal integument is covered by an even cuticle, which therefore forms no tegmentum. The only part of a Chitux which can be called the mantle fold is the marginal zone of the body, the ventral side of which encircles the head and foot and forms the lateral boundary of the branchial groove or furrow. Just as the dorsal side of this mantle, which is called the zone, carries large spines, sete, or scales, so may the under surface be covered with small closely- crowded spines. The rest of the integument is bare, - being merely covered with a simple epithelium. The genus Chitonellus is of great importance in comparing the outer or- ganisation of the Placo- phora with that of the Solenogastres. The body is not dorso-ventrally flat- tened, as in the Chiton, but nearly cylindrical ; the ventral surface, however, Fic. 52.—Transverse section of Chitonellus, diagrainmatic, b A adapted from figures by Pelseneer and Blumrich. 4g, Shell (articu- is flattened (Fig. 52), and lamentum); go, gonad; i, intestine ; ab, vb, branchial arteries and has a median longitudinal veins; pr, pleuro-visceral nerves; 2, latero-ventral thickening of groove. The foot is not the cuticle ; p, foot; ct, ctenidium ; pn, pedal nerve; h, digestive gland (liver) ; ¢, secondary ccelom ; ao, aorta. externally visible, but can be discovered, much reduced, in the base of the median groove, itself possessing a ventral median groove representing a narrow contracted sole. The flat ventral surface is therefore the mantle. In the narrow cleft on each side, between mantle and foot, in the posterior half of the body, lie the gills. The lateral margin of the body in Chiton is represented in Chitunc//us by a mere blunted ridge, which is almost exclusively caused, as may be seen in transverse sections, by a great thickening of the cuticle. B. Solenogastres. In the Solenogastres (Aplacophora), whose outer organisation has already been sufficiently described (p. 29), the shell is altogether wanting, but the cuticle secreted by the epithelium over the whole body is usually exceedingly thick (Fig. 53). It contains calcareous spicules, which sometimes project above the surface. These, like the spines of the Polyplacophora, rise from cellular cups, which are connected with the basal epithelium of the cuticle by nucleated stalks. There can be no doubt that the spicules are formed by these cups and nourished by them during growth. The foot, as we have seen, is reduced to a narrow ciliated longitudinal ridge, which rises from the base of the medio-ventral groove. The term mantle is here inappli- cable, except perhaps to the integument which forms the lateral boundary of this groove. 42 COMPARATIVE ANATOMY CHAP. In Chaetoderma the foot finally atrophies, and the medio-ventral groove also disappears. The‘long series of undoubtedly primitive characteristics in these two groups—the Placophoraand Solenogastres—obliges us to place them, as we shall have repeatedly to point out, near the root of the Molluscan phylum. In some points the Solenogastres are perhaps more primitive than the Polyplaco- phora, and the vermiform body, the slight development of the mantle, the foot and the gills have been thought to be primitive characteristics. More recently, however, it has been main- tained, as the present writer thinks, with justice, that these conditions are rather the result of secondary adapta- tion to a limicolous habit of life (most Solenogastres inhabiting mud). The shell, mantle, gills, and foot are such essential characteristics of the Mol- lusca that we must assume their Fic. 53.— Transverse section of Proneomenia 5 ‘ Sluiteri in the region of the mid-gut. 1, Mid-gut; eXistence in the racial form. 2, rudimentary foot; 3, sepia projecting into the mid- The series Chiton, Chitonellus, gut; 4, testicular portion of the gonad; 5, ovarial Noypenin, Chetoderma does not, there- ortion of the saine; 6, thick enticle secreted by the oe . . - P fore, illustrate for us the rise and epithelium. ; ‘ development of typical Molluscan characteristics, but rather their progressive obscuration and disappearance. C. Gastropoda. (Cf Sketch of Outer Organisation, pp. 30-33.) Integument. The free edge of the mantle, which takes the chief part in the formation and growth of the shell, is particularly rich in mucous, pig- ment, and ealeareous glands. The epithelium is ciliated over areas of varying extent, especially in aquatic Gastropods. In many of the shell-less Opisthobranchia the whole surface of the body is ciliated. The remarkable marking and colouring of the integument especi- ally seen in the Nudihranchia are caused by pigment cells, which are more often found in the cutis than in the epithelium. Where there is no firm shell, calcareous granules or spicules may be found scattered throughout the cutis, In several Nudibranchia stinging cells have been discovered in the integument. Mantle, Visceral Dome. The mantle fold is, as a rule, well developed in Gastropods, and covers a spacious pallial cavity. Whenever the fold is small or alto- gether wanting, the condition is secondary rather than primitive. vir MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 43 1. Prosobranchia. In the Prosobranchia, the mantle fold develops on the anterior side of the visceral dome, and there covers a spacious cavity. It further usually extends like a narrow collar right round the base of the visceral dome. In the symmetrical Fissure?/ide, the mantle cavity is short, and opens out- wardly by means of a dorsal aperture through the mantle fold, which corresponds with the perforation at the apex of the shell. A circular fold, provided with a highly sensitive fringe, is formed by the mantle around the aperture, and projects for a short distance beyond the perforation in the shell. The water needed for respira- tion passes into the pallial cavity through the slit-like aperture at the free edge of the mantle fold, over the nuchal region, and flows out through the apical aperture just described. This aperture also serves for the ejection of excretory matter from the rectum, which lies immediately behind it. In Rimuda, the apical apertures in shell and mantle have moved somewhat forward, and lie anteriorly between the apex and edge of the shell. In Emarginula, the mantle has an anterior cleft, the edges of which, in the living animal, are folded in such a way as to form a tubular siphon, which can be protruded through the marginal cleft of the shell. In Par- mophorus there is no second opening into the mantle cavity, but the lateral edges of the mantle are very much widened, and bent back dorsally over the outer surface of the shell in such a way as to cover the greater part of it. In Hadliotis, the enormous development of the columellar muscle on the right side confines the mantle cavity to the left. The mantle fold has a long slit reach- ing from its edge to the base of the pallial cavity. This slit lies under a row of perforations in the shell which are characteristic of Haliotis, and through these the respiratory water is expelled. In the spaces between the consecutive perforations, the edges of the mantle cleft are apposed, merely separating beneath each aperture to allow of free communication between the cavity and the exterior. The edges carry three tentacular processes, which can be thrust outward through the perforations. The anus is always found under the posterior perforation. The edge of the mantle surrounding the body splits into two narrow lamelle, which bend round to form a groove for the reception of the edge of the shell. The Trochide, Turbinidw, Neritide, and nearly all Monotocardia have no second aperture and no mantle cleft. In Docoglossa (Patella, etc.) the mantle forms a circular fold round the visceral dome, which is in the form of a blunt cone. It covers the edge of the almost circular broad-soled foot. The mantle is broadest anteriorly, where it covers the head and neck, 7.e. the pallial cavity or groove is here deepest. The visceral dome, in the Monotocardiu, is almost always distinctly constricted at the base, and spirally coiled. The pallial cavity occupies its typical position. In many Monotocurdia, the free edge of the mantle fold is prolonged on the left side, projecting forward, sometimes to a great extent ; the lower edges of this projecting fold bend round towards each other to form a tube or semi-cylindrical channel, which is called the siphon. Through this siphon, the water needed for respiration flows into the mantle cavity. It can generally be told, by the shape of the shell, if there is a siphon or not, since most MJonotocurdia which possess one have either a notch in the edge of the shell at the columella, or a process called the canal or heak, at this same point, which encloses the siphon. The length of this latter canal need not, however, correspond with that of the siphon. 44 COMPARATIVE ANATOMY CHAP. The Monotocardia have even been grouped, according to the presence or absence of a siphon, into the Siphoniata or Siphvnostomata, and the Asiphoniata or Holo- stomata ; but this classification is artificial, since siphons are sometimes present and sometimes absent in forms which are un- doubtedly nearly related. In most Aonotocardia, the shell is not outwardly covered by the mantle, but in some groups, the edges of the mantle bend back over the shell, and finally grow over it to such an extent as to unite above it. The external shell in such cases becomes an internal shell. In the Harpide among the Rhachiglossa, the mantle bends back over the columellar lip of the shell. In the AMerginellide, it covers a large part of the outer surface, and the same is the case in Pyrula among the Tuentogloss, in most Cypreidw and in the Lamellaride. In Lamellaria, the shell is completely grown over by the mantle. In Stiliter among the ELudimidce also, the shell is more or less covered by the mantle. The edge of the mantle may be fringed or notched, or (Cypraeide) provided with wart-like, tentacular, or branched ap- pendages. A B 2. Pulmonata. In the Pulmonata, the arrange- ments of the mantle fold and visceral dome and of the shell, which is in- timately connected with them, are of great interest. We have, on the one hand, forms such as Hrliv, with large protruding spirally-coiled visceral Fic. 54.—Testacella haliotidea (after Lacaze-Duthiers). A, right view; b, enormous pharynx evaginateil through the buccal cavity, carrying on its surface the radula (a); c, open- ing of the pharynx into the cesophagus; d, position of the genital aperture ; e, latero-dorsal groove along the body; /, latero-ventral groove ; g, mantle, rudiment of the visceral dome. B, dorsal view: a, b, the two pairs of tentacles ; c, the latero-ventral groove; d, the latero- dorsal groove ; e, shell. even within some of the natural divisions of the Pulmonata. dome and large mantle fold enclosing a spacious cavity; on the other, forms such as Qneidium, without distinct visceral dome or mantle fold and without shell. Between these two extremes there are numerous transition forms; indeed, complete series of such forms may be found The following are a few characteristic types. Helix (Figs. 12 A, p. 9; 72, p. 75).—The visceral dome is large and spirally coiled, and is covered by a spiral shell sufficiently large to shelter with ease the whole body. The mantle fold covers a cavity lying anteriorly to the visceral dome (pul- monary cavity). Its free thickened glandular edge unites with the nuchal integument vi MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 45 near it in a way characteristic of the Pulmonata, leaving only one aperture, the respiratory aperture—on the right. (In Pulmonata whose shells have the sinistral twist, the respiratory aperture lies to the left.) The apertures of the hind-gut and excretory organ are close to the respiratory aperture, through which their excreta have to pass out. In many species of the genus Vitrina, the shell cannot contain the whole animal, The mantle fold projects in front of the shell, and has a process which is bent back over the shell, and is used for cleansing it. In Daudebardia (Helicophanta) (Fig. 12 B, p. 9) the visceral dome and shell are, in comparison with the rest of the body, much smaller than in Vitrina. The animal cannot be sheltered by the shell. The visceral dome begins to be levelled down to a certain extent, disappearing into the dorsal surface of the foot. It lies far back on the body, the respiratory aperture being on its right side. A somewhat similar arrangement is found in the genus Homalonyx, in which the low visceral dome lies on the centre of the back. The respiratory aperture lies to the right at the edge of the mantle. The edge of the flat ear-shaped shell is fixed into the mantle fold. Davdebardia and Homalonyx begin to look like slugs. In Testacella (Figs. 54 and 55) a visceral dome hardly exists. The only re- mains of it is a small mantle at the dorso-posterior end — j,.,. 55,_Testacella haliotidea, posterior portion of the body of the body, which is from the right (after Lacaze-Duthiers), The shell is removed to covered by an ear-shaped show the rudimentary visceral dome, «, latero-dorsal groove ; b, shell. Beneath the mantle latero-ventral groove ; c, end of the muscle attached to the shell ; ; e, mantle edge of the visceral dome ; g, respiratory aperture. lies a reduced respiratory cavity. The respiratory aperture lies to the right posteriorly, beneath the edge of the shell. The viscera lie dorsally on the foot. The common terrestrial snails Limaz and Arion (Fig. 12 D, p. 9) resemble Testaeclla in the reduction of the visceral dome, but in them the mantle or so-called shield which takes its place lies anteriorly behind the head. At its right edge lies the respiratory aperture. In Zimx there is a small round rudimentary shell which is internal, i.e. it is entirely enveloped in or overgrown by the mantle jfold. In rion this shell is represented by isolated calcareous granules. In Onehidiwn and Tuginulus there is no trace of a visceral dome, nor, in the adult, of a shell. The visceral dome has to a certain extent spread out over the whole dorsal surface of the foot, and has disappeared. There is, further, no outwardly recognisable mantle fold distinct from the rest of the dorsal integument. A longitudinal furrow still divides the dorsal part of the body from the foot. The respiratory aperture with the anus lie posteriorly in the median line. In the genus Physa (Fig. 11, p. 8), the edge of the mantle takes the form of lobe-like or finger-shaped processes, which bend back over the shell, and can be applied to its outer surface. In Amphipeplea (Fig. 10, p. 8) the mantle is much widened and, when bent back over the shell, covers all but an oval spot on the dorsal side of the last coil. The dorsal integument of the Onchidia has wart-like protuberances or (in Peroni«) 46 COMPARATIVE ANATOMY CHAP. branched appendages. These are richly supplied with blood-vessels, and serve for respiration. In Peronia there are besides these also dorsal prominences which carry eyes. The dorsal integument projects all round the body above the foot, and thus forms, as in Chiton, a peripheral zone, which is ventrally separated from the foot by a groove. In Oneidivlla the edge of this zone, i.e. the lateral edge of the body, is dentate or fringed. 3. Opisthobranchia. The typical outer organisation of the Gastropoda here suffers even more varied and thorough modification than in the Pulmonatu. We have, on the one hand, forms with head, foot, visceral dome, shell, mantle and gill; on the other, forms which possess none of these organs and nevertheless are both Gastropods and Opisthobranchia. In one principal division of this order, the Pilliata or Tvetibranchia, the mantle fold is retained on the right side of the body, and partially covers a typical Molluscan ctenidium ; in other divisions both mantle and ctenidia are wanting. We do not here apply the term mantle to the fold or edge of the dorsal integument which surrounds the body at the part where the head and foot take their rise ; such an edge is more or less developed in most Opisthobranchia and distinctly marks off the foot and head from the rest of the body or back. The mantle here means only the broader fold which covers the mantle cavity, in which les a typical molluscan gill. The edge of the mantle never forms a distinct siphon in the Opisthobranchia, though there is an approach to such a structure in the Linyiculide. (a) Tectibranchia. (a) Reptantia.—In this division we have, on the one hand, forms which still have a distinctly projecting visceral dome, whose integument secretes a coiled shell, into which the whole body can be withdrawn. On the other hand, forms occur in which the flattened visceral dome has spread out over the whole dorsal surface of the foot, the shell being rudimentary and internal. Examples of the former are found in the Cephalaspide, e.g. the Actaconidw, Tornatinide, and some Scaphandr ide (dtys, Cylichnu, Amphisphyru), a few Bullite (Bulla), and the Ringiculide. In Seaphonder among the Secphandride, and Accra among the Buillide, the body cannot be completely withdrawn into the shell. In the Cephalaspide, to which so far reference has been made, the shell is external. In Gustropteron the mantle is rudimentary, and is provided posteriorly with a filiform appendage. It covers a delicate membranous internal shell, into which the body cannot be withdrawn. The same is the case in Philiae and Doridlivm, where there is also a delicate internal shell covering only a small portion of the viscera ; this shell, in Dori?ium, is produced in the form of two lobes, the one to the left ending in a filiform process. The visceral dome in the Awrspidee is small as compared with the size of the animal, but rises distinctly above the rest of the body, and is covered by a thin inconspicuous shell. The mantle and shell often only partially cover the gill. In Aplysia, the shell is internal, é.¢. it is entirely overgrown by the mantle ; in Dolu- bella, this enveloping overgrowth is not quite complete, as a circular median dorsal vir MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 47 aperture is left, through which the dorsal surface of the shell is visible. The mantle in Dolabella forms a small anal siphon posteriorly. Notarehus has a microscopically minute shell. the integument forms protuberances or delicately branched appendages. In the Oxynotden, the shell is only partially covered by the mantle, and is, further, much too small to shelter the body. Among the Notaspide, the Uinbrellide have a small flattened cap-like visceral dome lying upon the massive foot. The visceral dome is surrounded by a mantle fold which, on the right side, covers the gill. The integument of the dome and mantle is covered by a flattened disc-shaped shell. In Pleurobrunchia, the visceral dome is relatively large. The right and left margins project as short mantle folds, but there are no such folds to the front and back, so that at these latter parts the flattened visceral dome is not distinct from the rest of the body. In Pleurobranchus, the integument of the flattened visceral dome broadens out into a large fleshy disc which projects on all sides beyond the large, broad- soled foot ; its margin (mantle fold) is separated from the foot by a deep continuous groove running right round the body ; in this groove, to the right, lies the large gill, while in Plewrobranchus a small flat internal shell, thin and membranous, is still found ; in related forms this may be wanting. The dorsal integu- ment is often strengthened by a layer of calcareous granules. (8) Natantia. Pteropoda Thecosomata.—The Limacitnidie have a well-developed visceral dome and corresponding shell, with sinistral twist ; the shell can be closed by means of a typical operculum. The mantle fold covers a cavity which lies anteriorly to the visceral dome. The anus is to the right. The animal can with- draw into its shell. In the Cavolintide the dome and shell are bilaterally symmetrical, not twisted, and the body can be entirely hidden within the shell. The mantle cavity here lies posteriorly to the visceral dome, on what is usually called its lower side. In certain species of this” genus, Fic, 56.—Diagrammatic trans- verse sections of Gastropods, to illustrate the arrangement of the shell (black, 1), visceral dome and mantle (dotted, 2), and foot (streak - ed, 3). A, Prosobranchiate with outer shell and epipodium (4). B, Tectibranchiate with lobes (6) of the mantle turned back over the outer surface of the shell. Dorsally the shell is still uncovered ; 5, para- podia; 7, ctenidium. ©, Tecti- branchiate with internal shell, i.c. completely overgrown by the lobes of the mantle. The symmetrical shell of the Cymbuliidee does not correspond with the shell of other Thecosomata ; it is a cartilaginous ‘ the mantle cavity also lies posteriorly. position of this cavity among the Thecosomata. pseudoconch” covered with body epithelium. We shall return later -to the varying In the Cymbuliida The mantle, in the genus Cavolinia, shows peculiarities which can best be described in connection with the shell. In the latter, two surfaces are distinguished, a slightly arched anterior surface (usually described as the upper), and an arched posterior surface. The anterior surface projects forwards and downwards beyond the posterior for a third of its length. The shell has three slit-like apertures, one 48 COMPARATIVE ANATOMY CHAP. anterior and ventral, through which the fin-like processes of the foot can be protruded, and two lateral apertures stretching far up, so that the shell appears almost bivalve. At these lateral slits, which admit water to the mantle cavity, the mantle bends round on to the outer surface of the shell, covering the greater part of it; and, at the upper angles of the slits, has two freely projecting processes. Pteropoda Gymnosomata.—In these, the long outwardly symmetrical body is naked and without a mantle, and the foot, which is much reduced, is found on the ventral side of the most anterior part of the body. (b) Ascoglossa and Nudibranchia. In mature Ascoglossa and Nudibranchia, with the single exception of the Stegano- vranchia, a shell is always wanting, as also a distinctly demarcated visceral dome. The latter, indeed, spreads out over the whole dorsal surface. The dorsal integu- ment, nevertheless, forms a circular fold (mantle fold) separated from the foot by a groove sometimes deep, sometimes shallow ; but, except in the Phyllidiide, no gills lie in this groove. Where this groove has nearly disappeared, the animals strongly resemble Planaria. Phyllidiide.—In these, the mantle fold is distinct, and carries on its lower surface, to the right and left, a row of branchial leaves, herein recalling Patella and Chiton. The genus Dermatobranchus, which, judged by its organisation, belongs here, has, however, no gills. Dorididz.—The dorsal integument (noteum), which here covers the body like a shield, being generally distinctly demarcated from the foot and the head, contains numerous caloan eous particles, which give it a firmer consistency. Anteriorly, there are two feeler-like processes, the aonephenes which can generally be withdrawn into special sheaths or pits; these are not to be confounded with the tentacles. The anus lies in the median line, generally behind the middle of the body, and ds surrounded by an ornamental circlet of pinnate gills. The noteum is often covered with prominences, and in some genera the margin carries variously shaped processes. Cladohepatica.—Here there are no anal gills. The dorsal integument has variously formed and variously arranged appendages ; these may be conical, club- or finger-shaped, lobate or branched ; they are, for the most part, very striking in colour and appearance. Sacs of nematocysts are generally found at their tips, and weca of the intestinal canal (branches of the digestive gland) penetrate them. These dorsal appendages, which, like the rest of the body, are ciliated, have, at least partly, a respiratory function. In many forms they easily fall off, and are later regenerated (Fig. 18, p. 12). Many Cladohepatica have a certain external likeness to Planaria with dorsal papille (Thysanozoon), but this likeness is still more marked in the following family :— . Ascoglossa.—Anal gills and also, as a rule, dorsal appendages are here wanting. The whole body is naked and ciliated. The lack is indistinctly demarcated from the head. Phyllirhoé.—This Nudibranchiate genus, of all Opisthobranchia, shows least of the typical external organisation of the Mollusca. The body here is naked and laterally compressed, with sharp dorsal and ventral edges. It has neither foot nor gills (Fig. 19, p. 12). vil MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 49 D. Seaphopoda. ((f. Review of Outer Organisation, p. 34.) E. Lamellibranchia. From each side of the body there typically hangs a large leaf-like mantle fold of the same shape as the shell-valye formed by it. These mantle folds project beyond the body in front, below, and behind, and enclose a mantle cavity which everywhere, except dorsally, opens outward by means of the slit left between the edges of the folds. This large single cleft serves for the admission of nourishment and water into the mantle cavity, and for the expulsion of the excreta, genital products, and respired water; through it also the foot is protruded. Such a primitive mantle is thus completely open, its simple edges (i.e. without folds, papille, tentacles, or eyes) are quite free, coalescing nowhere. The above serves for a description of the mantle of Nucwla— one of the Protolranchia—and must be considered as the primitive arrangement. In most Lamellibranchia, however, special differentiations of the margin of the mantle occur; these take the form of folds, thickenings, protuberances, papille, tentacles, glands, eyes, etc., and this is the case both in forms which have an open mantle and in those in which the mantle is partially closed. The partial closing of the mantle is brought about by the con- crescence at one or more points of the free edges of the mantle folds. A. A completely open mantle, i.c. one single large cleft entirely separating the edges of the mantle, is found: (a) Among the Protobranchia in Nucula. (b) Among the Filibranchia in the Anomitde, Areide, Trigontide, and a few WMiytilide (Pinna). (e) In all Pseudolamellibranchia except Mrlengrina. (d) Among the Eulamellibranchia, only in a few species of Crassatella, B. The mantle folds of the two sides grow together at one point.—In this case the point of concrescence almost always lies high up posteriorly ; and marks off a small aperture from the originally simple cleft. This aperture, occurring on a level with the anus, forms the exhalent or anal aperture of the mantle. Its edge may be more or less prolonged posteriorly to form an anal siphon, which can be protruded beyond the valves of the shell. At a point a little below this exhalent aperture, the mantle edges usually become applied to one another, although no concrescence takes place. Above this point, between it and the anal siphon, they separate to form an inhalent or branchial aperture. The edges of this aperture also may be produced posteriorly into a branchial siphon, which, however, in this case, has a cleft extending along the whole of its lower side, which is a continuation of the large cleft of the mantle. A branchial siphon formed in this way, by mere apposition of the mantle edges, is found in the genus MeMZctiv among the Protobranchiu. VOL. II E 50 COMPARATIVE ANATOMY CHAP. An anal aperture, separated by a point of concrescence from the large mantle cleft, is found in the following Lamellibranchia : (a) Among the Protobranchia in Malletia. (b) Among the Filibranchia in most Mytitide. (c) Among the Pseudolamellibranchia in the Aviculide (genus Meleagrina). (2) Among the Eulamellibranchia, in the Carditide (Ve enericardia, Cardita Milneria), the Astartide, and most Crassatellide ; among the Cyrenida, in the genus Pisidium ; among the Unionide in the Unionine (Unio, Anodonta) ; and among the Lucinacea, in Cryptodon Mosele yt. In Solenomya, among the Protobranchia, the two mantle edges grow together only at one point, but to such an extent as to close the whole posterior half of the we > Fic. 57.—Diagrams to illustrate the various ways in which concrescence of the mantle and formation of siphons take place in the Lamellibranchia. The foot (7) protruded forward through the mantle cleft; A, mantle completely open; B, mantle open, but with its edges applied to one another at two points, thus giving rise to incompletely separated anal and respiratory cavities ; C, edges of the mantle grown together at one point (1), the anal or exhalent aperture of the mantle (4) is separated ; D, edges grown together at two points (1, 2), the branchial or inhalent aperture (5) is also separated, the mantle has three apertures; E, mantle closed by the extension of the place of concrescence (2), three limited apertures remain, viz. the anal, branchial, and pedal apertures—the first two are produced into siphons; F, a third concrescence (3) takes place. Mantle with four apertures (4, 5, 6a, 6b), the most anterior (6b) for the protrusion of the foot. The siphons have united. ventral mantle opening. In this way the mantle cleft is divided into two ; the anterior aperture serves for the protrusion of the foot, while the posterior serves at the same time as inhalent (branchial) and exhalent (anal) aperture. Solenomya is the only bivalve in which this arrangement is found. C. The mantle folds grow together at two points, thus forming three apertures.—This condition arises in consequence of the complete separation (through concrescence) of the branchial aperture from the rest of the large anterior mantle cleft. The anal and branchial apertures may remain as slits, or may be produced into longer or shorter anal and branchial siphons. The large anterior and ventral mantle cleft serves for the protrusion of the foot, and is called the pedal cleft. These two points of concrescence are found: (a) Among the Protobranchia, in Yoldia and Lede. vit MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 51 (0) In most Hulamellibranchia, viz. in most Lucinide, most Cyrenide ; among the Untonide, in the Mutelinee, in the Donacide, Psammobiide, Tellinide, Scrobi- eulariide ; among the Veneracea, in the Veneride, in the Cardiide, the Mactride, Mesodesmatidw, and the Solenide (excepting Solen and Lutraria). (c) In all Septibranchia (Poromyia, Cuspidaria). In the above forms the mantle is still wide open, i.e. the points of concrescence are small and local. But these points may become lines of concrescence of con- siderable length. In the Chamacea, for example, and especially in the Tridacnide among the Lulamellibranchia, the three apertures of the mantle are found at con- siderable distances from one another, being divided by long intervals where the edges have grown together. In some groups of Lamellibranchia, the concrescence between the anal and branchial apertures or siphons remains short, é.c. the one aperture lies directly below the other, but in such cases the edges anterior to the branchial aperture grow together to a greater extent, so that the pedal aperture becomes reduced to a small anterior fissure. In this condition the mantle is closed. Such a mantle is found : Among the HLulamellibranchia in the Modiolarca, Dreissensia, Petricola, all Pholadide (Pholas, Pholadidea, Jouannetia, in which the pedal aperture is said to close entirely in old animals, Xylophaga, Martesia); in the Teredinide, and among the Pandoride, Pandora, the Verticordiide and Lyonstide (Anatinacec). D. There are some Lamellibranchia with closed mantle, in which a fourth aperture is added to the three found in the above groups, the mantle thus having three points of concrescence. The fourth aperture is always small, and is found between the pedal and branchial apertures ; it probably corresponds with a rudi- mentary fissure for the byssus. This arrangement is found in the Eulamellibranchia ; among the Solenide, in Solen and Lutraria ; among the Pandoride, in Myochamea ; in Glycymeris ; among the Anatinacea, in the genus Thracia ; in the Pholadomyide and the Clavagellide (Clavagella and Brechites [Aspergillwm)) ; and, finally, in Lyonsta norvegica, The anal aperture is often and the branchial aperture nearly always fringed, or in various ways edged with protuberances, papilla, or ten- tacles, and this is the case whether these apertures are found on the edge of the mantle or at the ends of (longer or shorter) siphons. The siphons can be contracted and extended, and either wholly or partly withdrawn into the shell, by means of special muscles. These muscles are attached on the inner surface of the shell-valves to the right and left posteriorly, and their line of attachment forms the pallial sinus, which will be described later on. The length of the siphons varies greatly. Specially long siphons are found in the Mactride, Donacide, Psammobtide, Tellinide, Scrobiculartide, many Veneracea and Curdiide, the Mesodesmatide, Lutraria, the Pholadide, Teredinide, Anatinide, and Clavagellide. The siphons may be separated throughout their whole length, and often diverge one from the other (¢.g. Galatea among the Cyrenide, the Donacide, Psammobtide, Tellinide, Scrobicularide (Fig. 58), Mesodesmatide, Pharus, etc.). In other forms they coalesce along their entire length ; they may even look like a single tube, which is, however, always internally divided into an upper (anal) and a lower (branchial) channel. This common siphon is sometimes protected by a special sheath of epidermis, particularly in those forms in which it cannot be 52 COMPARATIVE ANATOMY CHAP. withdrawn into the shell. Siphons united throughout their whole length are found in the Muetridw, a few Tenerucea, Lutrarin, Solenocurtus, Solen, the Phaladide, many Anatinide, and the Claviyellide. In some cases, siphons which are united for some distance at the base, separate near their ends and even diverge, ¢.g. in Petriculy among the I’eucrucea, Teredo, ete. The two siphons are often of unequal length. In the Modinluria (Mytilidu) only one, the anal, is developed, while the branchial aperture remains unseparated from the large mantle cleft. The reverse is the case in Prvissensie and Scrobicularia, where the branchial siphon is much longer than the anal. The siphons are sometimes provided with valves ; these occur more often in the anal than in the branchial siphon. , Significance of the development of the Anal and Branchial Apertures and Siphons. Most Lamellibranchia inhabit mud or sand, into which they sink the anterior part of the body, burrowing by means of the protrusible foot. The water necessary for bathing the gills and for respiration can only be received into and expelled from the mantle cavity through the cleft at the posterior end of the body which projects above the mud. The fe:cal masses from the anus near this point must also here be ejected from the cavity. The development of localised inhalent and exhalent apertures is explained by the fact that a constant regulated stream of water into and out of the mantle cavity is necessary both for respiration and for conducting particles of food Fic, 58.—Serobicularia piperata buried in mud. ‘The inhalent siphon takes im imud as nourishment ; the anal siphon stands erect (after Meyer and Mobius). to the mouth. The most advantageous point for the exhalent aperture is obviously directly behind the anus. Siphons attain development in consequence of the habit of life of many bivalves, which bury themselves deep in mud, sand, wood, and even rock. By means of their siphons they can still remain connected with the water which bathes the surface of their place of concealment, and, as long as the animals remain undisturbed, a con- stant current enters the mantle cavity through the branchial and leaves it through the anal siphon. Where the mantle folds have grown together to a large extent (closed mantle) the siphons are always well developed. Such closing of the mantle is found principally among bivalves which bore into wood, clay, rock, etc., and in which the foot of the adult is weakly developed, or altogether rudimentary. The degeneration of the foot leads to the shortening of the pedal aperture which originally served for its pro- trusion. The mantle is found completely open with only slightly developed anal and branchial apertures or none at all, in bivalves which do not burrow, but live surrounded by water, either attached to the bottom or lying freely on it. vi MOLLUSCA—INTEGUMENT, MANTLE, VISCERAL DOME 53 In such animals the swrounding water can circulate through the usually open mantle cleft and the mantle cavity. We here find protuberances, papille, tentacles, etc., carrying sensory organs, all along the free edges of the mantle, whereas, in bivalves which inhabit mud or bore into wood, rock, ete., such organs are mostly found massed together round the edges of the branchial and anal apertures. The Edge of the Mantle. The edge of the mantle often forms a number of diverging folds, which in trans- verse section look like finger-shaped processes. The outermost fold is always applied to the shell. The edge of the mantle may also be beset with one or more rows of pro- tuberances, papille, or tentacles, and often contains unicellular or multicellular glands, mucous glands, and others which have been considered to be poison glands for protective purposes. Tactile sensory cells are very common. Eyes are rarely developed on the edge (¢7. section on the Sensory Organs). In the Peetinide, Spundylide, and Limide, the inner fold of the mantle has a somewhat broad border, which, when the shell is open, projects from the mantle towards the median line of the body (Fig. 23, p. 16). The free opposite edges of these folds (flaps, or curtains), springing from right and left, may meet in such a way as to shut off the central part of the mantle cavity even while the shell is open, apertures only remaining anteriorly and posteriorly. F. Cephalopoda. Integument. The integument of the Cephalopoda consists of an outer cylindrical epithelium, and a subjacent cutis in the form of thick connective tissue. In this cutis, not far removed from the epidermis, and above a layer of connective tissue plates (which are refractive and often shimmer like silver), there are Jarge pigment cells or chromatophores which, by their alternate contraction and expansion, bring about the well-known changes of colour in these animals. These chromatophores are single cells containing yellow, brown, black, violet or carmine pigment, either as fluid or in small granules. _The layers containing them are either single or double ; in the latter case, the colour of the pigment in the one layer of chromatophores differs from that of the chromatophores in the other. Radial fibres, arising from the surrounding connective tissue, are attached to each chromatophore, round that equator which lies parallel to the integument. The chromatophores are enveloped in a special, possibly elastic, membrane, and when contracted are almost globular ; the pigment corpuscles are then crowded together. The chromatophores expand equatorially, diminishing the distance between their poles, i.e. they become much flattened. In this condition, they may, further, throw out fine branches, the pigment granules being thus spread out over a large surface. It was formerly believed that the expansion of the chromatophores was caused by the contraction of the radial fibres, which were thought to be muscular, but more recent investigations have shown the fibres to be of the nature of connective tissue. The changes of colour, which are of great physiological and biological interest, and which are partially under the control of the animal, are brought about by the alternate contraction and expansion of these variously coloured chromatophores 54 COMPARATIVE ANATOMY CHAP. Mantle, Visceral Dome. Some of the most important points connected with the mantle and visceral dome have already been mentioned (pp. 36-38). In Nuutilus, the body is attached right and left to the inner surface of the shell of the last or inhabited chamber by powerful muscles, which may make a slight impression on the shell. Between the points of attachment of these lateral muscles, the integument of the visceral dome coalesces with the inner surface of the shell of the inhabited chamber in a narrow circular zone, so that the gas? enclosed in the upper chambers of the shell cannot escape. While the integu- ment and mantle beneath this zone of concrescence (i.. towards the free aperture of the last chamber) are rough, fleshy, and muscular, the integument of that portion of the visceral dome which lies above the zone and is applied to the last septum is delicate and soft. The siphuncle, which arises at the dorsal end of the visceral dome and passes through all the septa, is membraneous and hollow and filled with blood. It is said to communicate with the pericardium, In the female Naw- tilus, the nidamental gland (see Genital Organs, p. 241) lies in the free mantle fold, near the point at which it separates from the visceral dome. We thus have parts which usually lie in the visceral dome wandering into the mantle fold. Among the Dibranchia, which are good swimmers, fins are found. In the Octopoda, which are distinguished by the round, compact form of the visceral dome, these are wanting, except in the remarkable genus Cirrhotcuthis. Fins are universal among the Decapoda, and vary much in form, size, and arrangement. In Sepia (Fig. 80, p. 83) and Sepiofeuthis, the fins are inserted on the lateral edges of the body, along the whole height (length) of the visceral dome, forming the boundary between the anterior and posterior (physiologically the dorsal and ventral) surfaces of the latter. In Lossia, Sepiola, and Sepivloidea they are almost semicircular, and are like distinct appendages situated on the anterior surface of the dome, about half-way up it. This is also the case in Cirrhotvuthis, where the more or less circular fin-lobes rise from the body on stalk-like bases. The triangular or semicircular fins of Cranchia, Histioteuthis, Onychoteuthis, Loligo (Fig. 34, p. 23), Loligopsis, Omumastrephes, etc., are found at the dorsal end of the visceral dome, on its anterior side. In many Dibranchiu, there is a coucrescence of the free edge of the mantle fold with the integument of the ‘‘ head” (Kopffuss), which lies below it. This connec- tion is effected by means of a muscular band, which passes over the neck (nuchal band). In most Decapoda, this connection is wanting, and the edge of the mantle is free all round the body ; the exceptions are the genera Sepiola, Cranchia, and Loligopsis, which have a narrow connection of this sort. All Ortupuda have this concrescence, commencing with the Argonauta ; it lengthens in Philoneais and Octopus, till in Cirrholcuthis it spreads to the posterior surface (physiologically the ventral surface), so that the edge of the mantle remains free only at the aperture through which the funnel or siphon is protruded. Arrangements for fastening the mantle fold to the adjacent body wall are very common. Such attachment is either temporary or permanent. In the former case, there are prominences with cor- responding depressions for locking the mantle (appareil de résistance) ; in the latter case, dermal or muscular fusions take place between the mantle and body wall. 1 Of. note, p. 37. VIL MOLLUSCA—THE SHELL 55 1. Apparatus for locking the mantle.—These are paired or unpaired. The former are to be found on the posterior side of the body, in the mantle cavity, near its lower end; they lie to the right and left at the base of the funnel, and on the corresponding points of the inner surface of the mantle fold. The unpaired, on the contrary, are found on the anterior surface of the neck. Since all the arrange- ments serve the purpose of cutting off the mantle cavity from the external medium, it is easy to see that their development is in inverse ratio to the extent of the con- crescence of the edge of the mantle round the neck before mentioned. Where no concrescence is found, as in Sepia, the arrangements for locking the mantle are highly developed ; while, where the line of concrescence is very long, as in Octopus, the locking apparatus may be altogether wanting. The locking apparatus consists, in general, of cartilaginous prominences (often accompanied by depressions) on the inner surface of the mantle fold, ¢.c. the surface turned towards the mantle cavity, which exactly fit corresponding cartilaginous depressions accompanied, as the case may be, by prominences, on the opposite body wall (cf. Fig. 80). The special forms of the mantle and nuchal locking cartilages are of importance in classification. The cartilaginous arrangements for locking, which are almost always found in the Decapoda (they are wanting only in Owenia and Cranchia), are still retained in a few Octopoda in the form of more or less modified fleshy processes (Argonauta, Tremoctopus). The nuchal locking apparatus is the first to disappear on the rise of the pallio-nuchal concrescence. It has disappeared among the Decapoda in the genus Sepiola, where the mantle is firmly attached to the neck. 2. Permanent connections between the mantle fold and the adjacent body wall traversing the mantle cavity are found only in those Cephalopods which have no locking apparatus. Thus in Octopws and Eledone the mantle is attached to the body wall by means of a median muscle above the funnel. This muscle consists of two closely-applied lamelle, having the anus between them. In Cranchia the free dorsal edge of the funnel (at its so-called base) has become united by an integu- mental band on the right and left with the mantle fold, and « similar arrangement is found in Loligopsis. Water pores.—Near the mouth, or at the bases of the arms, or laterally on the head, in many Cephalopods, there are apertures leading to integumental pouches of varying size. The function of these organs is unknown. IV. The Shell. General. The Shape of the Shell, and its Relation to the Soft Body. All the various forms of shell found in the Mollusca are deducible from a cup- or plate-like shell covering the dorsal region. Such a shell affords sufficient protection for animals such as Iissurella, Patella, etc., which can firmly and almost immovably attach themselves to a hard surface by the sucker-like foot. The soft body is in this case protected on one side by the shell, and on the other by the surface of attachment. Free-moving Mollusca, however, show a tendency to protect the whole body exclusively by means of their shells, and this object is attained in various ways. In the Chitonide, for instance, the shell is made up of consecutive 56 COMPARATIVE ANATOMY CHAP. joints, overlapping in such a way as to be movable one upon the other. This segmented shell can protect the whole body, since it allows the Chiton to roll up like an Armadillo or a Wood-louse. In the Lumellibranchia, the protection of the whole of the soft body is provided for by the development of a bivalve shell, from which the foot can be protruded, and which, by the closing of its two valves, completely envelops the soft body as well as the retracted foot. In the Gastropoda, Scaphopoda, and Cephalopoda, the most complete protection on all sides of the body by means of the shell is attained on another plan. The shell becomes much elongated and _ turret- like, and is thus so capacious that not only the visceral dome but the head and foot also can find place in it. Even the only remaining unprotected aperture, the one weak point of this fortification, can very often be completely closed by a hard operculum. A long, turret-like shell is an inconvenient burden for a freely moving animal, being, in consequence of its large surface, a hindrance to locomotion. A reduction of the surface is brought about in the Gastropoda and Cephalopoda by the coiling of the shell, either in one plane or in a conical spiral. In the latter case the spiral twist is almost always right-handed or dextral. In order to decide the direction of the twist, the shell should be held in such a manner that the point of the spiral is uppermost, while the aperture is directed downwards and towards the observer (Fig. 60, p. 60). If, in this position, the aperture lies on the right of the axis, the shell has a dextral twist if to the left its twist is left-handed or sinistral. We have a striking and in most cases unexplained phenomenon in the reduction and even complete disappearance of the shell, which takes place not only in nearly all the classes, but even within some of the smaller groups of Mollases, e.g. the Solenoyasires among the Amphineura, a few Heteropoda and Titiscania among the Prosobranchia, many Pul- mowitu, very many Opisthobranchia, and most extant Cephalopoda. In almost all cases the forms in which the shell is rudimentary or wanting can be shown to be derived from forms in which it is well developed. All shell-less snails (slugs) have shells in the early stages of their development. The process of the gradual reduction of the shell to a rudiment, which will be more fully described later on, is often as follows: (1) the shell becomes internal; (2) it decreases in size, so that it no longer can cover the body ; (3) the visceral dome disappears ; (4) the shell is only to be found in the form of isolated calcareous particles in the dorsal integument ; (5) even these vanish, and the shell is only to be found in the embryo. Only in a few cases is it possible distinctly to recognise the reason or the advantage of this reduction of a protective covering so useful to and exercising so profound an influence on the organisation of the VIL MOLLUSCA—THE SHELL 57 whole race. The following are a few cases in which the utility of the reduction of the shell in adaptation to special conditions is to some extent evident: (1) In free-swimming marine forms, where the shell is too heavy and increases friction; (2) in Testacellu and allied forms, which prey upon earthworms, where a large shell would prevent them from following their prey into narrow holes and passages ; (3) in Gustropods, which browse among thick tangles of Corals, Bryozoa, Hydroida, or Alge (e.g. many Nudibrunchia). The loss of the shell is generally followed by compensatory adaptations for protection, such as great capacity for regeneration, especially of the easily detachable appendages, voluntary amputation of portions of the body, stinging cells, and colouring which may be protective in various ways. The carnivorous ('ephulopods are protected (1) by their extraordinary swimming powers, which are in keeping with their highly developed organisation ; (2) by their well-developed sight ; (3) by great muscular strength ; (4) by strong jaws; (5) by the discharge of the secretion of the ink-bag; (6) by their partly mimetic changes of colour, ete. Certain peculiarities of organisation, which can only be under- stood as remains of a shelled condition (¢.7. the lateral position of the genital and renal apertures and also to some extent of the anus in the Nudibranchia), always persist after the shell has disappeared. Chemical Composition of the Shell. The shell of the Mollusca consists principally of carbonate of lime, with traces of phosphate of lime and of an organic substance related to chitin,—concliyolin. Besides these, various colouring matters may occur. Structure of the Shell. The shell of the Lamellibranchia consists of three layers, the innermost layer being applied to the surface of the mantle. The shell is to be regarded as a cuticular structure. The outer layer (shell-integument, epidermis, cuticle, periostracum) is, so far as its physical constitution is concerned, horny and wanting in lime. It generally disappears off the older portions of the shell. The middle layer (columnar, prismatic, or porcelanous layer) consists of slender prisms of carbonate of lime, usually perpendicular to the surface of the shell and closely crowded together. The inner (nacreous) layer has a finely lamellated structure. The very delicate transparent laminze of which it is composed are thrown into slight waves ; these cause the wavy lines on that surface of the shell which lies on the mantle, which, by interference, produce the characteristic nacreous lustre. The pearls of the pearl oyster are formed of the same substance as this layer. ‘The constitution of these three layers varies greatly in details both in the Zamel/i- branchia and in other Mollusca. The outer and middle layers are formed at the free margin of the mantle, the inner layer is yielded by the epithelium of its whole outer surface. The shell in the Gastropoda and Cephalopoda consists principally of the middle 58 COMPARATIVE ANATOMY CHAP. or porcelain layer, which, however, has a structure very different from that of the same layer in the Lamellibranchia. This layer is generally, if not always (at least in the young), covered by a periostracum. The inner (nacreous) layer is very often wanting. Growth of the Shell. In the Arthropoda, the chitinous exoskeleton, which we may compare with the Molluscan shell, develops at the surface of the whole body and its appendages. This skeleton, when once formed and hardened, encases the body on all sides within fixed boundaries, and is incapable of growth. Hence the moults of the Arthropoda, by which alone growth becomes possible. The Molluscan shell, on the contrary, is open. In the Gastropoda and Cephalopoda, it assumes the shape of a conical mantle, coiled round a single axis and open at the base of the cone. By continual additions at the edge of its aperture, it grows with the growth of the animal, without materially altering its form. The lines on the surface of the shell of the adult snail register its phases of growth. During growth, the oldest, uppermost coils or whorls of the shell either continue to be filled by the apex of the visceral dome (in many Guastropods), or are deserted by the animal which, as the shell grows, withdraws farther and farther from its tip. These whorls may remain empty, or may be partially or completely filled with shell substance. In the latter case, they may be successively thrown off. The Nautilus and allied forms, during growth, periodically form transverse septa, so that the forsaken parts of the shell become chambered and filled with gas,! the animal occupying the largest and last-formed chamber, which opens externally. In the Lamellibranchia, the growth of the shell keeps pace with the growth of the body in exactly the same manner, the free edge of the shell valve continually receiving additions of shell substance from the edge of the mantle to form the periostracum and the prismatic layers, while the whole external surface of the mantle yields an additional nacreous layer. The consecutive phases of growth are here also registered by the concentric markings on the surface of the shell. Special. A. Amphineura. (Cf. pp. 39-42.) B. Gastropoda. A few details concerning the shell of the Gastropods must here be added. As a rule, the shell is coiled spirally round an axis. This spiral is, in rare instances, so flattened that the coils come to lie almost in one plane, giving rise to a nearly symmetrical shell (e.g. Planorbis). There are, however, among the Gastropoda, uncoiled shells which are symmetrical, and these require special attention. The most important are the cup-shaped or more or less bluntly conical shells of the Patedlide and Fissurcllide. Since (1) we derive 1 Cf. note, p. 37. VIL MOLLUSCA—THE SHELL 59 the Gastropoda from bilaterally-symmetrical ancestors with symmetrical shells ; and since (2) the Fisswreldide undoubtedly possess the most primitive organisation of all the Gastropoda, and thus stand nearest to the racial form, and are moreover (3) strikingly symmetrical in their organisation, it seems, at first sight, natural to consider this syinmetry a primitive feature. Certain peculiarities of the nervous system, however, especially the crossing of the pleuro-visceral connectives, taken in connection with other conditions explained more fully elsewhere, make it certain that the cup-shaped shell of Fissurella is only secondarily symmetrical, ¢.e. that Fissurella is descended from forms which possessed a spirally coiled shell. The same is the case with the Patellide. The following important facts are in harmony with this conclusion: (1) the young shell of Fisswrella is asymmetrical and coiled, and it only gradually assumes Fic. 59.—Shells of—A, Pleurotomaria; B, Polytremaria; C and E, Emarginula; D, Haliotis; F, Fissurella; G and H, stages in the development of the shell of Fissurella; I, shell of the twisted racial form of the Gastropoda with marginal cleft; K, the same with apical aperture ; L, shell of Lamellibranch ; M, shell of Dentalium, seen from the apical aperture. The holes and clefts of the shells are black ; 0, mouth; a, anus; cf, ctenidium. the symmetrical form (Fig. 59, G, H); (2) the apparently symmetrical shape of some forms nearly related to Fissurella and Patella prove on closer inspection to be somewhat asymmetrical, the apex especially being more or less excentric; (3) other forms nearly allied to Fissurella, such as Haliotis, Scissurella, and Pleuro- tomaria, have spirally coiled shells (Fig. 59, A, B, C, D). In the Fissurellide, many Pleurotomariide, and the Haliotide, i.e. in the most primitive Gastropods, peculiar and noteworthy perforations of the shell occur, such as are occasionally found in other divisions. These perforations lie above the slit in the mantle which is characteristic of this order (¢f p. 43), and they everywhere establish communication between the mantle cavity and the exterior, especially needed when the mouth or edge of the shell is closely applied to the object on which the creature crawls. 69 COMPARATIVE ANATOMY CHAP. In Seissurrlla, Pleurotomaria, and Emarginula, there is a median indentation in the anterior edge of the shell, which corresponds with an incision in the mantle edge. This is the case in the young Fissuwrel/a, but, during further development, the cde of the shell grows across the incision, so that in the adult animal the aperture lies near the apex. Beneath it is the anus, placed high up in the mantle cavity. If such a cleft were to arise at both the anterior and posterior edges, and to become very deep. a double shell would result comparable with the bivalve shell of the Lamvllibranchia, It is in fact probable that this notching of the shell edge is of great phylogenetic significance. In Huliotis we have a row of perforations of the shell, the process of formation of the perforation in Fisswre2lu being often repeated ; the older apertures are always, however, closed by shell substance, and the younger only remain open as long as they lie immediately over the respiratory cavity. In very many Prosobranchia (the Siphaniate of earlier writers), there is, at the columnar edge or lip of the shell, a notch which gives passage to a channel-like fold of the mantle margin. This channel keeps up communication between the mantle cavity and exterior, even when the shell is closed hy the operculum. Instead of a Fic. 60.—A, Dextrally twisted ; B, sinistrally twisted shell of Helix pomatia. notch, a more or less long process or beak may enclose a corresponding process of the mantle, the siphon. The latter may become a tube by the apposition of its edges. It has already been mentioned that the shells of most Gastropods are dextrally twisted. There are, however, a few families, genera, or species in which the shell has a sinistral twist ; and in some species where the twist is dextral, a few individuals with sinistral twist may occur, and vice versd. It is a curious fact that some species, in which the shell has a sinistral twist, show the asymmetry of the dextral twist in the soft body, whereas, in others, the asymmetry of the soft body corresponds with the twist of the shell. We shall return to this point. For details as to the growth of the shell, and the capacity of the animal to dissolve the shell already formed, both of which are points full of interest, we must refer to special works on Conchology, as also for detailed descriptions of forms of the shell and opercula, and differences due to age. Progressive reduction of the shell occurs in each of the three divisions of the Gastropoda. In the Prosobranchiu, this has only been observed in marine, free-swimming Heteropods and in Titiseania ; in the Pulmonata, it is much more cominon ; and in the Upisthobranchia, so frequent that nearly all the members of this division have more or less rudimentary shells. Many adult Opisthobranchia have even lost every trace of a shell (Pleropoda yymnosomata, Nudibranchia, and most Ascoglusse), although, in their earliest stages at least, they possessed a coiled VIL MOLLUSCA—THE SHELL 61 shell, for the closing of which there may even be an operculun, secreted by the foot, as in the Prosobrunchiu. The following are some of the principal stages and concomitant phenomena of the reduction of the shell: (@) The well-developed shell ceases to be large enough to shelter the whole body. () The shell, which becomes thinner and smaller, is dorsally overgrown, partially or altogether, by extensions of the mantle. (c) As the shell (which is then either cup- shield- or ear-shaped) becomes continually smaller, the visceral dome begins to be levelled down, till it no longer rises above the rest of the body, its contents spreading out to a certain extent over the dorsal surface of the foot. (@) The external asymmetry of the body passes by degrees into symmetry, whereas the internal asymmetry never entirely disappears. (¢) The shell is reduced to a number of isolated calcareous particles in the integument of the flattened visceral dome. (jf) There is at last no trace of a distinct visceral dome ; calcareous particles are to be found in the dorsal integument of the long and now naked Gastropod. (g) Even these particles finally disappear. In connection with the reduction of the shell in Opisthobranchia and Pulmonata, compare the section on the mantle, pp. 43-48. The Heteropoda present the following interesting series :— Atlanta. The shell is very light and thin, but large and spirally coiled (with an incision at its aperture); the animal can entirely withdraw into it, and close it by means of an operculum developed ou the distinct metapodium. Carinaria. The shell is thin, light, and delicate ; it is cup-shaped, and covers the large stalked visceral dome, but is incapable of sheltering the long and thick cylindrical body and foot. There is no operculum. Pterotrachea. The visceral dome is small, and there is no shell and no operculum. C. Lamellibranchia. The two lateral valves of the Lamellibranch shell are connected, at their dorsal edges, by means of a hinge and a ligament. The ligament counteracts the muscles of the shell, which will be described later on, and which, by their contraction, close the shell. It is usually composed of two layers, the inner layer being elastic, while the outer is not. The outer non-elastic layer passes into the epidermis or periostracum of the shell. The inner layer of the ligament is elastic and calcareous, and is often called cartilage, but this is histologically incorrect. The ligament lies either externally, distinctly seen dorsally between the prominences of the hinge edges of the valves, or internally, stretched between the apposed edges of the hinge, which are ftunished with depressions for its reception. These depressions can easily be distinguished from those belonging to the hinge itself, since the former are alike on the two valves, whereas the furrows and other depressions belonging to the one face of the hinge correspond with teeth, ridges, etc., on the opposite face. When the elastic ‘‘cartilage” of the ligament is at rest, as in a dead bivalve, or when the adductor muscles of the living animal are relaxed, the valves open. When the adductors contract, the ‘‘cartilage” is—apparently in all cases—compressed. On the other hand, when the adductors are relaxed, the elasticity of the ‘‘ cartilage” forces the shell open again (Fig. 61). The continuity established between the two valves, by means of this dorsal ligament, causes the Lamellibranch shell to appear to consist, strictly speaking, of one dorsal piece, developed to the right and left ventrally into two valves. The constitution of the ligament and hinge are of importance in classification. We must refer the reader to systematic zoological works for the special forms taken by the shell, and content ourselves with the following remarks :— 62 COMPARATIVE ANATOMY CHAP. The Lamellibranch shell is originally symmetrical, that is to say, the two valves, apart from the almost invariable asymmetry of the hinge, are exactly alike (equivalve). This is the case in most of the Lamellibranchia. The two valves may, however, become unlike, i.e. the shell (and to a mich lesser extent, and only in unimportant details, the soft body also) may become asymmetrical. As far as we can at present judge, this asymmetry is caused by adaptation to an attached manner of life. The left valve of the Oyster is firmly cemented to the surface on which it rests. This valve is thicker, more convex and spacious, and forms a sort of basin in which the soft body lies, while the right valve acts rather as a lid, and is thinner and flatter. We have thus an ‘upper” (the right) and a ‘‘lower” (the left) valve, but it is hardly necessary to point out that this use of the terms upper and lower has as little morphological significance as in the Pleuronectide among the fishes. The attached valve is sometimes the right, sometimes the left, and this A B Nad Fia. 61.—Diagram in illustration of the mechanism for opening and closing the Lamelli- branch shell. 1, 2, 3, The three layers of the shell—1, prismatic layer ; 2, cuticle or periostracum; 3, nacreous Jayer. A, Shell closed by the contraction of the adductor muscle (6), by means of which the elastic inner portion of the ligament (5) is compressed. B, Shell opened by the elastic pressure of the inner portion of the ligament during the relaxation of the adductor muscle. 4, Non-elastic outer portion of the ligament, which passes into the periostracuin. variation may occur within one and the same genus (Chama), or even species (Aetheri«a). Besides the above-named, the following bivalves are also attached, and have dissimilar valves: Spondylus, Gryphiw p. p., Evogyra p. p., and especially the fossil Hippuritcs (Rudistes), in which the right valve assumes the form of a high cone attached by its point, while the left looks like a lid. The conical valve has, however, no corresponding internal cavity, but is almost entirely filled up with shell substance, so that, in spite of the form of the shell, the space occupied by the animal between the two valves is very limited. This same condition is found in certain fossil Chaimacea. In Requienia, the left valve is produced spirally and is attached by its point, while the spirally-coiled flattened right valve covers it like a lid, so that the whole shell closely resembles a Gastropod shell closed by its operculum. There are also free, unattached bivalves with unequal valves, c.g. many Pectinide. In these animals, however, many peculiarities of organisation, such as VI MOLLUSCA—THE SHELL 63 the rudimentary foot, the constitution of the mantle edge, and the absence of siphons, indicate descent from sedentary forms. In the case of other forms with unequal valves, however, no such descent can be established. In Anomia we have an example of an inequivalve bivalve, in which the valve turned to the surface it rests on is flat and the upper arched. The lower valve is here the right one, and takes the exact imprint of the surface on which it rests, so that, for example, the mark- ings of the shell of the Pecten or the Oyster, to which noma frequently attaches itself, are exactly reproduced. In this right attached valve there is a perforation into which a shelly plug, the calcified byssus, fits ; by means of this, the animal fixes itself to its substratum. The ex- planation of this perforation is seen in the course of development. Itcommencesasasimple notch Fic. 62.—Three stages in the develop- at the edge of the shell, as found also in other ment of the shell valves of Anomia. bivalves, for the passage of the byssus. By the 4» Very young shell ; B, older shell with further growth of the shell, this notch to a mpbehy for the: Dyssus su) eult older shells 7, the byssus notch surrounded by the shell certain extent is grown round, and thus ap- anq persisting as a hole (after Morse). parently travels away from the edge of the shell, with which, however, it is still really connected (Fig. 62). In related forms (Carolia) this aperture becomes quite filled up by a homogeneous calcareous mass. Impressions on the inner surfaces of the shell.—Various organs of the Mollusc, attached to or adjacent to the inner surface of the shell, leave more or less distinct impressions on this surface, which are visible when it is empty. These impressions are of great importance, especially to the paleontologist, for by their means fairly Cc Fic. 63.—Dimyaria, inner surface of the left shell valve. A, Cytherea chione (Sinupollinta); B, Lucina Pennsylvanica (Jntegripalliata); 1, impression of the anterior; 2, impression of the posterior, adductor ; 3, sinus of the pallial line (4); 5, ligament. safe conclusions may be arrived at as to certain points in the organisation of the soft body which has disappeared. 1. The most distinct impressions are those caused by the adductor muscles. Where there are two powerful adductor muscles, one anterior and the other posterior (Dimyaria), there are two impressions in the corresponding parts of the inner surface of the shell (Fig. 63). In cases where the anterior muscle is rudimentary, while the posterior is unusually powerful, and has moved anteriorly towards the middle of the shell (Monomyaria), there is only oue large impression (Fig. 64). The anus 64 COMPARATIVE ANATOMY CHAP. is always to be found close to the posterior (which in the Monomyaria is the only) adductor. 2. Parallel to the edge of the shell, and more or less removed from it, we tind on the inner surface of the shell the so-called pallial line, caused by the inuscle fibres which attach the edge of the mantle to the valves. The course taken by this line undergoes charac- teristic modification in such Lamellibranchs as have siphons ; at the posterior part of the shell it suddenly bends forward and upward, and then again passes backward and upward towards the lower edge of the posterior adductor. The pallial line, in this case, forms an indentation, leaving a sinus or bay opening posteriorly, the pallial sinus, which has been utilised for systematic purposes (Strupalliati, Integripal- lint, Fig. 63). The sinus marks the line of attach- Fic, 64.—Monomyarian, internal ment of the sipho-retractor muscles ; the stronger surface of a shell valve of Perna these retractors and the hetter developed the siphons Ephippium. 1, Hinge edge; 2,im- the larger and clearer is the sinus. pression of adductor. a é : "i : 3. The foregoing impressions are the most dis- tinct and constant, but others may oceur as well, caused by the protractors and retractors of the foot, by the muscles or ligaments which attach the visceral dome to the shell, ete. : but these cannot be further described. In most Lamellibranchia, when the shell is closed, the edges of the two valves meet exactly, so that the soft body can be entirely enclosed and cut off from the exterior (closed) shell. There are, however, shells in which, in the closed condition, the valves gape posteriorly, or, more frequently, both posteriorly and anteriorly (yp, Myider, CTyeymeride, Solenidw). This is accounted for by the great develop- ment of the siphons and of the foot, which can only partially (Zyide, Solenocurtus) or with difficulty be withdrawn into the shell. Such gaping shells are found in most boring bivalves, whose shell formation is specially interesting owing to the develop- ment of accessory valves or calcareous tubes. In this respect Pholas, Pholadiden, and Joucnnetia represent the most important stages in a remarkable series. The shell of Pho/us is elongated longitudinally, aud gapes anteriorly and ventrally for the passage of the short club-shaped foot, and posteriorly for that of the strongly developed siphons, As many as three accessory valves are developed dorsally (prosoplax, mesoplax, metaplax), The shell of Pholadider somewhat resembles that of Pholas. In the young animal it gapes anteriorly, as in Pholus, for the passage of the foot. Posteriorly, each valve is produced into a horny process, Which is succeeded by an accessory piece (siphonoplax), hollowed out like a trough. The siphonoplax of the one valve often fuses with that of the other to form a single tube for the reception of the siphons. There are two pieces of prosoplax, while the meso- and metaplax are rudimentary. In the adult the boring activity is suspended, and the anterior opening becomes entirely closed by the secretion of an accessory piece, the callum (hypoplax). The functionless foot atrophies, and the animal can move no farther in the substance into which it has bored. The shell of the adult Jowenartin is much shortened longitudinally, and is globular, and the animal cannot move in the round hole it has bored for itself in a block of coral. Any alteration in its position in the hole, which might he fatal to the animal, is avoided hy means of a posterior tongue-like process of the shell, which, however, only belongs to the right valve. The shell is completely closed anteriorly, and a foot is wanting (cf. also Figs. 27, 28, p. 19, and 66, p. 67) VII MULLUSCA—THE SHELL 65 The adult condition of Jowannetia is explained by its developmental history. The shell of the young animal is like the segment of a sphere, whose greatest height is hardly half of the radius. It covers the dorsal upper portion of the body, its free edges thus bounding a very wide aperture, which corresponds with the anterior pedal gape of Pholas, In this Pholas-stage, in fact, Jouannetia really possesses u foot. Twisting the body about and rasping the stone with the anterior edge of the shell, the animal excavates a hole, which is spherical in consequence of the shape of its shell. When this hole is made, new accessory shell material is secreted at the free edge of the shell ; this forms the ‘‘callum,” and as the edge of the mantle follows the lines of excavation, the form of the accessory shell is here (as in 7vrdo) determined by the form of the hole, and the sphere of which the original shell was but a segment is completed. Setting aside a few related forms (J/artesia, Teredinu, Nylophaga, Gastrochaena, and Fistulana), in which the conditions are somewhat similar, we come to the ship- worm Teredo (Fig. 29, p. 20). This animal has a long tubular mantle which is produced posteriorly in two long siphons. The body lies at the anterior end of the mantle. Teredo bores cylindrical passages in wood. The valves of the shell are very small in comparison with the body ; they take the form of tri-lobate pieces, which encircle the anterior end of the mantle. This rudimentary shell gapes anteriorly for the passage of the pestle-shaped foot, and very widely posteriorly. The mantle further secretes over its whole surface a calcareous tube which lines its burrow, but which does not fuse with the shell valves. Two small accessory shell- pieces, the so-called ‘‘ palettes,” lie at the place where the siphons separate. If the anterior portion of the animal reaches (z.c. if it Lores through to) the water, the calcareous tube is rounded off and closed. Aspergillwm (Brechites, Fig. 30, p. 20, and Fig. 65) and Clavayclla show similar con- ditions. In the club-shaped shell, which inserts its anterior thicker end into rock, shell, coral, or sand, we can distinguish a true and a false shell. The false shell forms by far the larger portion of the tube, and corresponds with the secreted tube of eredo, and with a callum like that of Pholas. The true shell is very small and lies anteriorly. The two valves of this true but rudimentary shell are, in Aspergillum, placed saddle-like over the anterior end of the tube, with which they are firmly fused (Fig. 30, p. 20). Were they isolated, their gape would be unusually wide, not only anteriorly and posteriorly, but ventrally. The shell-tube is open posteriorly, over the apertures of the siphons ; anteriorly, however, it is closed (in the adult) by means of a disc perforated like the rose of a watering-can, which corresponds in position with the callum of the Pholadide. The perforations at the edge of the disc, or even over its whole surface, are sometimes produced into cal- careous, and at times dichotomously branched tubules. In the middle of the disc there is sometimes found a narrow slit-like aperture corresponding with the pedal aperture in the mantle beneath, but this is often wanting. Less frequently, we find another aperture in the ventral middle line, corresponding with the fourth mantle aperture above described (p. 51). Aspergillum buries its anterior end in mud or sand, but its whole organisation, and especially its shell arrangement, point to a former boring mode of life. Clavagella, which is nearly related to Aspergillum, bores into rock or the cal- careous shells of various other animals, The arrangement of its shell differs from that of Aspergillum chiefly in the somewhat greater size of its true valves, and in the fusion of only the left valve with the calcareous tube, the right lying free within that tube. In the Pholadidw, the ligament, which is still found at the hinge, no longer acts for opening the shell. In consequence of a peculiar arrangement of the anterior VOL. II F CHAP. COMPARATIVE ANATOMY 66 “uoydis Terpouviq ayy Jo aanqrede ayno ‘A { AqLAvO aTZUBUT ay Jo TaquIBYD [eIYoURIG ‘mM f LaquILY [wUB ‘A | aALeU TeLYOWEIG w fampade apueU yIMoJ {7 faAreM [eITLed ‘s { eIBDSTA aT} Surmpequoco Apoq ‘4 !4ooy ATepUUIpUs 9yy JO aseq att} 7B uolsues peped ‘A ‘jays pur atjuRut usaaqeq AyAvo ‘x f satnqny oJUL psonpoad [ays oy4 Jo edad Sursoyo ayy UL suorw1ojzad ayy ‘d ‘o :(sosn[[oOW 10yjO UL aanjqiedy peped att Yara snoSopouoy) adnqlade [jeys coweque ‘w famyrede ayurm rojeqyuy fw {uorpsuvs [Biqeteo ‘) ! peuos & ‘y fqaveq % {unrptuaya yysi ‘yf uorpsues yexeasta ‘6 Ssnuer yy yns-pury Y fuMIpraezo yer ‘a ‘ uoydis Teryouesq ‘p ‘ uoydis [wus ‘9 :suoYydis ey} SuIsvToua eqn} snoetvo[vo oy} ‘q Suoydis [eue oy] Jo ornqiedy ‘M “(SIOTUING-ezvoe'T 10}J¥) PSAOUlal apls JST aif} Jo apJUVUM pue [[ayS “WNMoPOYOTIp uni s1edsy jo Amoyeuy—'s9 “91g VII MOLLUSCA—THE SHELL 67 adductor, the opening of the shell, such as it is, is brought about by the muscles. The anterior and upper edges of the valves are bent outward, and to these edges the anterior muscle is attached. We thus have external instead of internal points a Fic. 66.—Pholas dactylus, right valve, internal aspect (after Egger). 1-2, Axis round which the valves move upon one another ; 3-4, longitudinal axis of the shell; 5-8, line connecting the shell lnuscles ; 6, anterior muscle ; 7, posterior muscle ; 9, rotating point of the valves ; 10, anterior and upper edge of the shell, which is bent outwards, and to which the muscle 6 is attached ; 6-9, shorter anterior ; 9-7, longer posterior arm of the lever. of attachment, and the whole shell may be compared to a double-armed lever acting along the longitudinal axis of the body, its fulcrum being at the point where, in other bivalves, the hinge is found. When the anterior muscle contracts, the shell opens posteriorly and ventrally ; when the posterior adductor contracts, the shell closes (Fig. 66). D. Cephalopoda. The Cephalopoda are all to be derived from an ancient fossil form which possessed a chambered shell, in the last and largest portion of which the animal lived, leaving the rest of the shell empty, or rather filled with gas (or water) and traversed by the siphon or siphuncle. Such a shell is now found only in the sole living repre- sentative of the Tetrabranchia, the Nautilus, an animal of great importance to the comparative anatomist. Many fossil forms allied to the Nautilus, and grouped in the order Nawtiloidea, possessed such a shell, as did also the Ammonoidea, with their enormous wealth of forms which, rightly or not, are generally considered to be nearly related to the Nautiloidca, i.e. to belong to the Tetrabranchia. In nearly all these animals the shell, when coiled at all, is, unlike the Gastropod shell, coiled anteriorly or exogastrically. One group of the Nawtiloidea, the Endoceratide, which includes only very old forms (Cambrian and Lower Silurian), is distinguished by the fact that the chambers of its straight shell, which were filled with gas (or water), lay at the side of and not behind the inhabited chamber. There was no real siphuncle, but the upper end of the visceral dome, much narrowed by the air chambers, stretched as far as to the apex of the shell. In other Nautiloides, the air chambers always lie, as in Navwtilus, ‘above the occupied chamber, and are traversed by a thin membraneous siphuncle, which, how- ever, in old forms, is much thicker, and represented the narrow prolonged portion of the visceral dome (Fig. 32, p. 22). Some forms of Nautiloidea have shells coiled endogastrically ; this is never the case, however, when the shell forms a complete spiral. The sutures, which corre- spond with the lines of insertion of the septa, are simple in the Nautiloidea, as 68 COMPARATIVE ANATOMY CHAP. compared with those in the Aa nonvidea, in which they are folded in a complicated manner. Nautiloidea.—In the following table we have the chief forms of the shell among the NeutiJoidea :4— (a) Orthuceras group.—Shell straight or slightly bent. Silurian—Trias. (b) Cyrtuceras group.—Shell curved like a horn, but not regularly spirally coiled. Cambrian—Permian. (c) (yroceras growp. —Shell regularly spirally coiled, the coils, however, not touching each other. Silurian—Permian. (d) Nautilus growp.—Shell regularly spirally coiled, the coils touching, or the outer clasping the inner. Silurian—recent. (ce) Lituites. —Shell at first regularly spirally coiled, straightening later. Silurian. The siphuncle runs either through the centre of the septa, or through their anterior or posterior sides. Ammonoidea.—The shells of the (fossil) Ammonoidea are distinguished by very complicated sutures, their zigzag lines are like the outlines of sharply-indented leaves or richly-branched mosses, they are due to the extraordinary folding of the edges of the septa, which are attached to the inner surface of the shell. The siphuncle is always very thin in the Ammonoidea, and almost always pierces the septa on the posterior side. The following quotation summarises the chief peculiarities in the form of the Ammonite shell :—* ‘‘The shell, as a rule, forms a closed symmetrical spiral, the coils touching or clasping one another. Some of the oldest forms are straight, or in youth incom- pletely coiled. In certain groups of the mmenoide we find a tendency repeated at different times (Trias, Jurassic, Chalk) to depart from the close symmetrical spiral, and to adopt what are called accessory forms, The first step in this process of change is in most cases the detachment of the occupied chamber from the next inner whorl ; then, little by little, the inner whorls also separate, though they still remain in the same plane—the Criveerus stage. Sometimes the shell grows straight for a time, then becomes hooked—the dAneyloeervs and Hamites stages, and, if only the occupied chamber separates from the coiled part—the Scaphites stage. Finally, entirely straight shells arise in the Baculites stage. Rarely, the coils leave the symmetrical plane and assume the shape of a snail’s shell; in this case, the shells may be either closely or loosely coiled,—the Turrilites stage.” Dibranchia.—The shells of all known Dibranchia, extinct or recent, are more or less rudimentary, since they are never capable of sheltering more than a small portion of the animal. Further, they are always internal, on the anterior side of the visceral dome, and are overgrown by a fold of the integument. In Spirula (Fig. 33, p. 23) alone, the shell is not completely overgrown, a portion at the apex of the visceral dome remaining uncovered. The shell of the (fossil) Belemnites (Fig. 67 C) is straight, conical, and chambered; the septa are near one another, and are pierced on the posterior or ventral side by the thread-like siphuncle, which is enclosed in short, calcareous sheaths. The apex of the shell (phragmocone) is protected by a conical calcareous sheath (rostrum or guard), the only part usually preserved. The anterior wall of the last chamber is produced downwards into a broad thin process, the pro-ostracum. In Spirudivostra (Fig. 67 D), the phragmocone begins to bend posteriorly (endo- gastrically), The rostrum is triangular and pointed at the top. 1 Steinmann-Doderlein, Elemente der Paléontologie, 1890. 2 Dbid. o---- VII MOLLUSCA—THE SHELL 69 In Spirulu (E), the shell is coiled spirally and endogastrically. The siphuncle is thick, and is surrounded along its whole length by septal envelopes. The rostrum is rudimentary, and there is no pro-ostracum, Starting again from the Belemmnites, the modification of the shell may take another direction. The phragmocone may become smaller and shorter in comparison with the continually lengthening pro-ostracum (e.g. Ostracoteuthis, F). The rostrum also may become thinner and smaller. Finally, the shell may be reduced to a very A B c D E F G H pt Fic. 67.—A-H., Diagrammatic median sections through the shells of eight extant or fossli Dibranchia, from the left side. The point of the visceral dome is turned downwards, the posterior side of the shell is to the left and the anterior to the right (cf. the position of the Cephalopod body, p. 36). A, Sepia; B, Belosepia (fossil); (, Belemnite (fossil); D, Spirulirostra (fossil); 1, Spirula; F, Ostracoteuthis (fossil); G, Ommastrephes; H, Loligopsis; ph, chambered shell= phragmocone; pr, pro-ostracuin ; 7, rostrum (guard); s, siphuncular canal, or space which con- tains the siphuncle ; 1, 2, 3, last three septa (the most recent); «, anterior wall of the siphuncle ; p, posterior ; 2, anterior edge of the first septal or siphuncular envelope =anterior or posterior edge of the siphuncular canal. small hollow cone at the end of a long narrow horny lamella which corresponds with the pro-ostracum, and is called, in the extant Decapoda, the gladius or calamus (or pen) (Loligo, Ommastrephes (G), Onychoteuthis). In Dosidicus, this terminal cone is almost solid, and in Loligopsis (H) it is nothing more than a thickening at the upper end of the gladius ; in other Decapoda, there is no trace of it on the gladius. In the Octopoda, the shell has completely disappeared. Again starting from the Belemnite, the shell may develop in a third direction to form the Sepia shell. The transition form is found in Belosepia (B) (Eocene), pe 70 COMPARATIVE ANATOMY CHAP. that is, if this interpretation is correct. This shell is somewhat bent, the septa are crowded together and slope downwards anteriorly. They are penetrated posteriorly by an extremely thick siphon, which is enclosed throughout its whole length in an envelope with a very thick anterior B wall. The completely enclosed siphuncular ! space is thus a wide funnel running through the chambers of the shell on its posterior side (Fig. 67 B). The phragmocone is enclosed in a thick, strongly - developed rostrum, and its anterior and lateral walls are produced downwards into a broad, posteriorly concave shell (pro- ostracum ?). These arrangements seem to have culminated in the extant Sepia (Figs. 67 A and 68). The siphuncular space fits over the visceral dome like a mould. The anterior portions of the septa slope downward much more obliquely from behind anteriorly, so that, in a back view of the shell, the whole area of the last septum is visible at the surface (Fig. 68,1). The septa are thin calcareous lamelle, closely superim- posed one upon the other, with very narrow air chambers between them; and these latter are traversed by perpendicular trabecule. The shell is thus very light, its specific gravity is less than that of water. Behind the siphuncle, on the posterior very much shortened side of the shell, the short septa are closely contiguous, without any intervening air spaces. The dorsal end of the shell is enclosed in a small pointed rostrum. The whole anterior sur- face is covered by a thin lamella of conchyolin, which projects laterally beyond the edge of the shell, and is itself covered by a calcareous layer which is an anterior and ventral extension of the rostrum. Fic. 68,—Shell of Sepia aculeata. The female Argonaut is the single exception Posterior (physiologically ventral) aspect. to the rule stated above, that in the Octopoda Lettering as in Fig. 67. The last septum the shell has entirely disappeared. This animal 7 is seen in its whole extent; s, the mouth hag a licht, thin external shell coiled anteriorly of the broad, slipper-shaped siphuncular 3 : : s cavity ; J, lateral wall of the cavity ; «8, O° exogastrically, which is not firmly attached line of the section which in Fig. 67 A is to the body at any point, and serves more for diagrammatised. The two figures should receiving the eggs (Figs. 35, 36, pp. 24, 25) than he compared (principal details after for protecting the body. This shell is sur- PORE): rounded and secured by lobate processes of the anterior pair of arms. It has no nacreous layer, but is porcelanous, and is apparently produced from the integument of the visceral dome and the mantle. The dorsal pair of arms is said only to deposit the so-called black layer on its surface. It is usually considered that this Argonaut shell is not the homologue of the shell of other Cephalopods, but is a formation peculiar to the Argonaut female. An opposite view has, however, recently been very ably advanced—that the Argonaut shell is an Ammvnite shell which has lost its septa and siphuncle and also its VII MOLLUSCA—THE PALLIAL COMPLEX 71 nacreous layer.! | Should this view prove correct, the Cephalopods would have to be differently classified. The division into Tetrabranchia and Dibranchia would have to disappear, as we cannot tell whether the fossil Ammonoidea were Tetra- branchia, and are also ignorant as to when the Dibranchia developed from the Tetva- branchia. The Cephalopods would then have to be divided into (1) Nautiloidea with the extant genus Nautilus ; (2) Ammonoidca with the still living Octopoda ; and (3) Belemnoidew with the extant Decapoda. Bivalve shelly plates called aptychi have been found sometimes in the last chamber of the mmonoidca, sometimes isolated. These have been proved to belong to the bodies of certain species of Ammonoidea, and have been considered by some to be protectives for the nidamental gland, by others as opercula, and by others again as the analogues or homologues of the infundibular cartilage of the Decapoda. No one of these three views has as yet been generally accepted. VY. Arrangement of the Organs in the Mantle Cavity and of the Outlets of Inner Organs in that Cavity. A discussion of this subject at this stage will help to explain the asymmetry of the Gastropoda and to simplify the discussion in later chapters. There are, in the mantle cavity, many important organs crowded,together in a comparatively small space, and into it also open all the apertures of the inner organs except the oral aperture of the alimentary canal. The term ‘‘ circum-anal complex,” though especially applicable to the arrangement in the Gastropoda, is not so suitable as ‘‘ pallial complex,” which applies to nearly all Mollusca, and comprises not only the pallial organs themselves, but the apertures of inner organs that lie in the mantle cavity. The most important constituents of the pallial complex are the ctenidium, the osphradium (Spengel’s organ, olfactory organ, or accessory gill), the hypobranchial gland, the anus, and frequently the rectum as well, the nephridial apertures and often the renal organ also, the genital apertures, and frequently the pericardium, with the enclosed heart. Starting with the Chitonide, which, as has already been described (p. 42), must be considered as the most primitive of all living Molluscs, we have :— The median anus, lying at the posterior end of the body in the mantle groove ; on each side of it anteriorly the nephridial apertures, and again on each side, in front of these, the genital apertures. Assuming this to be the primitive arrangement, we have the following important variations. A. Gastropoda. 1. Prosobranchia. u. Diotocardia.—In Fissuredla, the pallial complex is still quite symmetrical, but instead of lying posteriorly, as in Chiton, it, together with the mantle and the pallial cavity, lies on the front of the visceral dome. We have to imagine that the whole complex has shifted forward along the right side of the body, so that the gill originally on the left has come to lie on the right anteriorly, and that originally on the right now lies anteriorly on the left, and the same applies to the other organs belonging to the complex. 1 Steinmann, Bericht Freiburg Gesellsch., iv. pp. 113-129. 72 COMPARATIVE ANATOMY CHAP. In order to prevent confusion, the hypothetical original position of each organ will be denoted by #7 (=originally right) and wv (=originally left) in brackets. In the upper part of the mantle cavity in Fissurella, beneath the median aperture in the mantle and shell, lies the anus, and immediately to its right, the right (v2) nephridial aperture, immediately to its left the left (wr) nephridial aperture ; the right (72) and left (w7) ctenidia, again, lie symmetrically to the right and left. There are no distinct osphradia, and the genital apertures are wanting as the genital gland opens into the right nephridium. Haliotis.—The mantle cavity has here shifted to the left, and the rectum, attached to the mantle fold, runs forward some way through it, so that the anus is at a considerable distance from the posterior apex of the cavity. On the right of the rectum lies the right (v7), and to its left the left (17) ctenidium, both fastened to the mantle, and stretching far forward. The right and left nephridial apertures lie near the bases of the ctenidia, in the upper and posterior part of the mantle Fic. 69.—Anterior portion of Patella, from above, after removal of the mantle fold (after Ray Lankester). a, Tentacle; b, foot; c, pedal muscles (shell muscles); d, osphradia ; e, mantle fold ; f, aperture of the right neplii- dium; g, anal papilla and anus; h, papilla and aperture of the left nephridium ; 7, left nephri- dium; hk, right nephridium; J, pericardimn ; n, digestive gland (liver); m, cut edge of the mantle ; p, snout. Fic. 70.—The same specimen from the left side. Lettering as before ; 0, mouth. cavity. Between the rectum and the left ctenidium, also on the mantle, is found the long, well-developed hypobranchial gland (mucous gland), which stretches as far forward as the gill. Only a small portion of the gland lies to the right between the rectum, as far as it runs, and the right ctenidium. There are two osphradia which run as bands along the axes of the ctenidia facing the mantle cavity. Turbinide and Trochide.— Only the left (wr) ctenidium of Haliotis is here retained ; it lies far to the left on the roof of the mantle cavity, i.e. on the mantle. The rectum runs far forward along this roof. Two nephridial apertures lie on papille in the base of the cavity, at the sides of the rectum. The hypobranchial gland is found in various stages of development, the highest being attained in the Turbinide. Tt is largest between the rectum and ctenidium, ¢.r. between the right side of the latter and the left side of the former. In the Turbinida, however, a portion of it lies to the right of the rectum. There is a diffuse osphradium on the axis of the gill. Neritina.—There is here only one gill (the left (v7) in HuZiotis) shifted somewhat far to the right. The rectum lies asymmetrically to the right in the respiratory cavity, VIE MOLLUSCA—THE PALLIAL COMPLEX 73 reaching so far forward that the anus is found near the right edge of the mantle cleft. There is only one nephridial aperture to the left of the base of the ctenidium, far up in the mantle cavity. The inner surface of the mantle, between the rectum on the right 11 Fic. 71.—Pyrula tuba, male, taken out of the shell (after Souleyet). The mantle is cut open along its base and right side, and laid back to the left; the position of the pallial organs is thus reversed. 1, Proboscis; 2, snout; 3, foot ; 4, penis; 5, seminal duct, which is continued at 15; 6, floor of the pallial cavity=nuchal integument; 7, columellar muscle; 8, intestine ; 9, heartlin the opened pericardium ; 10, digestive gland (liver); 11, testes ; 12 and 13, renal organs ; 14, renal, aperture ; 15, seminal duct; 16, rectum ; 17, hypobranchial gland ; 18, anus; 19, ctenidium (gill) ; 20, mantle ; 21, osphradiun ; 22, respiratory siphon. and the gill on the left, is glandular, and represents the slightly differentiated hypo- branchial gland. The genital aperture lies close to the anus. Docoglossa.—In the Pateldidle (Figs. 69, 70) a short conical portion of the 74 COMPARATIVE ANATOMY CHAP. rectum projects into the small mantle cavity. This anal cone is not median, but is distinctly shifted to the right. To its right and left lie the nephridial apertures, raised on short conical papille. There is no separate genital aperture. In some forms (Zretura, Scurria, Acmeca) one ctenidium is found attached to the mantle, on the left side of the pallial cavity. Further details as to the gills in the Patedlide will be given later on. We further find, on the floor of the cavity, on each side, traces of an osphradium in the shape of a small patch of sensory epithelium, which may be raised on a prominence. It is doubtful if the prominence found in Patella close to each osphradium, containing a blood sinus divided up by septa, can be considered as a rudimentary gill. These prominences rise from the floor of the mantle cavity, whereas in Tecturv, for example, in which a true gill still occurs on the left, it lies far removed from the left osphradium, in the usual position on the roof of the cavity, i.e. on the inner surface of the mantle. b. Monotocardia.—In this division, the numerous forms of which show little variety of organisation, the arrangement of the pallial complex is very uniform. The single genital aperture is always distinct from the single nephridial aperture. The position of the organs in the spacious pallial cavity (Fig. 71), from right to left, is as follows :— 1. To the extreme right, lies the afferent duct of the genital organs (ovary or seminal duct), which runs more or less far forward, in the mantle cavity. 2. In contact with this, but quite on the roof of the cavity, is the rectum. 3. To the left of the rectum, far back in the base of the mantle cavity, lies the slit-like nephridial aperture, which pierces the wall separating the cavity from the renal organ behind and above it. Exceptions occur in Paludina and Valvata, in which this aperture is shifted forward to the end of a urinary duct which runs on the mantle. 4. On the roof of the mantle cavity are found the hypobranchial glands (mucous and purple glands), which are developed in varying degrees. 5. Quite to the left, and also on the roof of the cavity, the ctenidium, feathered on one side (the left (vr) of Hultotis and Fissurella), at whose base, deep back in the cavity, the pericardium is visible with the ventricle and auricle seen through it. 6. Finally, to the extreme left, lies the osphradium, which is always well developed and sharply circumscribed, and is either filamentous or feathered on two sides, and attached to the roof of the pallial cavity. The position of the organs in the pallial complex of the Heteropoda, certain forms of which, such as Atlanta, are closely related to the other Monotocardia, requires to be re-investigated. The osphradium lies at the base of the gill. 2. Pulmonata. In the Pulmonata, the single or double (2 and ¢) genital aperture (Fig. 72) no longer belongs to the pallial complex, but lies outside the mantle cavity laterally on the head or neck. In Onecidiwm the male aperture lies anteriorly under the right tentacle, the female posteriorly, near the anus. Bearing in mind that the mantle or pulmonary cavity communicates with the exterior only by means of the respiratory aperture lying on the right, we have the following arrangement of the pallial complex as typical (excluding such aberrant forms as Daudebardia, Testacella, and Oncidium). 1. On the extreme right of the pulmonary cavity lies the rectum, the anus opening in the respiratory aperture. 2. On the roof at the back of the cavity lies the nephridium (kidney). 3. To the left, near the kidney, also far up in the cavity, and on its roof, lies Vil MOLLUSCA—THE PALLIAL COMPLEX 75 the pericardium, containing the ventricle and auricle, the latter lying in front of a pl, Fic, 72.—Helix aspersa, fully extended from the right (after Howes). «, Anus appearing in the respiratory aperture, ply; s, shell; p, edge of shell aperture ; ga, genital aperture ; t), optic tentacle ; t, anterior tentacle ; 72, upper lip. the former. from the auricle rises the pulmonary vein, which runs forward along the roof of the pulmonary cavity. 4. The respiratory vascular network spreads over the whole remaining surface of the roof of the pulmonary cavity, and is thus in front of the kidney and peri- cardium. 5. An osphradium has till now only been found in the Basommatophora (Plan- orbis, Physa, Limnaeus), near the respira- tory aperture, and among the Stylommato- phora in Testacella on the floor of the pulmonary cavity at its extreme posterior angle. The floor of the pulmonary cavity (the dorsal nuchal integument) is smooth and devoid of organs. The arrangement of the efferent ducts of the renal organ varies and deserves special description (Fig. 73). 1. The anterior side of the renal sac opens on a simple papilla in the mantle cavity (Bulimus oblongus,and some species of Planorbis) (Fig. 73 A). 2. The papilla lengthens and runs for- ward as a straight ureter (primary ureter). This occurs in most Basommatophora, and some species of Bulimus, Cionella, Pupa, Helix (B). 3. The ureter runs backward along the From the ventricle the aortic trunk runs upward and backward, and Fic. 73.—Six diagrams illustrating the variations in the renal ducts in the Pul- monata. The organs are supposed to be seen through the mantle above them. 1, Free edge of mantle; 2, respiratory aperture; 3, rectum ; 4, kidney; 5, pericardium ; 6, auricle; 7, ven- tricle; 8, primary urinary duct; 9, secondary urinary dact, which, in D, isa groove. Further explanations found in the text. kidney, and opens at the base of the respiratory cavity. Testaccdla, and some forms of Helix (C). 4. A secondary urinary duct is added, becoming constricted from the wall of 76 COMPARATIVE ANATOMY CHAP. the pulmonary cavity, and at first forming a more or less closed channel along which the urinary discharge can be forwarded from the base of the cavity to the respiratory aperture. Some species of Budimus and Helix (D). 5. The secondary urinary duct becomes closed, and opens either alone or with the anus into the pulmonary cavity. Some species of Bulimus, Helix, Daudebardia, Vitrina, Hyalinia, Zonites, Arion, ete. (E). 6. The end of the secondary urinary duct and the end of the rectum together form a cloaca which is distinct from the pulmonary cavity,’ and opens close to the respiratory aperture. Limaz, Amalia, and some species of Daudebardia (F). When the primary urinary duct runs back along the kidney it is externally in- distinguishable from the substance of the latter, and it thus often appears as if the duct rose from the posterior end of the renal organ. The variations which occur in the position of the organs of the pallial complex in the carnivorous Pulmonata are specially interesting. In a series of car- nivorous forms, commencing probably with Hyalinia among the Stylommatophora, and proceeding through Deudebardiu to the extraordinary genus Testacella, we find progressive diminution of the visceral dome and its displacement to the posterior end of the body, simplification and diminution of the shell, and further, a shifting back of the liver and genital organs from the visceral dome into the nuchal portion of the ccelom, which now is found along the whole length of the dorsal surface of the foot. Finally, in Zestacel/a and certain Daudebardia, the visceral dome completely disappears, and the pulmonary cavity covered by the shell is alone left, the cavity reaching up to the apex of the shell. The floor of this cavity, and indeed the whole cavity, with the mantle and the shell, sink down into the body. In this way Testucella, which follows its prey, the earthworm, into its underground passages, is admirably adapted to its manner of life; its body is slender, and the somewhat flat shell at its posterior end, which does not stand out above the surrounding sur- face of the body, in no way hinders its movements. These alterations, however, especially the displacement of the visceral dome to the posterior end of the body, are accompanied by important alterations of position in the pallial organs, which finally lead to the condition called opisthopneumonic. It is important to note that concrescence of the mantle and the subjacent dorsal integument is complete except at the respiratory aperture on the right, and that the latter shifts farther and farther back, in its relation to the pulmonary cavity, till, in Testacella, its position is almost terminal. The first important step in the displacement of the pallial organs is seen in Daudebardia rufa. The pericardium, instead of lying far back at the base of the pulmonary cavity, here lies far forward on its roof, so that by far the greater portion of the vascularised pulmonary tissue lies on the roof behind the pericardium (Fig. 74 A). Daudebardia rufa is thus actually opisthopneumonic. But in this case the relative position of the ventricle and auricle is still unaltered. The auricle is, as before, placed in front of the ventricle ; the pulmonary vein from the auricle is thus obliged to bend round in order to run backward, while the aorta, which becomes almost exclusively the anterior or cephalic artery, supplying that portion of the body which lies in front of the visceral dome (by far the greatest part), must bend forward from the ventricle. In another Daudebardia, D. saulcyi, the case is somewhat similar, but the kidney and pericardium together form a sort of sac which hangs down into the pul- monary cavity from its roof. In this sac, the ureter lies dorsally and the peri- cardium ventrally to the kidney. The floor of the cavity sinks right and left deep into the subjacent region of the body, If we imagine that the pulmonary vein which runs back from the anteriorly VII MOLLUSCA—THE PALLIAL COMPLEX 77 placed auricle, and the aorta which runs forward from the chamber lying behind the auricle have pulled these chambers round in such a way that the flow of blood can have a straight course (cf. diagram, Fig. 74), the ventricle will then come to lie in front of the auricle. Indeed, the pericardium (with the ventricle and auricle) has actually twisted round 180°. In this twisting it has been followed by the kidney, which is connected with it by the reno-pericardial aperture, so that the latter organ no longer lies to the right but to the left of the pericardium, the aper- ture of the urinary duct remaining at its former place. The whole reno-pericardial complex, as compared with its typical position in the Pulmonata, is quite reversed. This reversal is characteristic of Testaer//u. It is, further, noteworthy that, in Zestaced/a, the floor of the pulmonary cavity becomes invaginated anteriorly into the body below it to form a large air sac. The walls of this sac are not supplied with blood vessels, and it seems to serve merely as a reservoir of air. In many Zestace/lidw the reno-pericardial complex hangs down in the shape of a sac into this air sac from the roof of the pulmonary cavity. In the Vaginulide and the Oucidi« the arrangement of the organs, originally belonging to the pallial complex, deviates still further from the type. A shell is Fic. 74.—Diagrams to illustrate the changes of position in the pallial organs of Daude- bardia and Testacella (adapted from figures by Plate). Mantle organs drawn as in Fig, 73. A, Daudebardia rufa; B, Hypothetical stage, the pallial complex of A twisted round 90°; C, Testacella. 1, Respiratory aperture ; 2, kidney; 3, ureter or urinary duct; 4, reno-pericardial aperture (renal funnel); 5, ventricle; 6, auricle; 7, aorta; 8, pulmonary vein ; 9, pulmonary vascular network. wanting in the adult and a mantle also; and the mantle- or pulmonary cavity seems in consequence to have atrophied. The pericardium lies posteriorly to the right, sunk into the integument, the ventricle lying, as in Zesface//a, in front of the auricle. Respiration takes place principally through the skin ; in the amphib- ious Oncidia it is assisted by dorsal papille. In Vuginulus, the urinary duct joins the proctodeum to form a cloaca which somewhat widens at the point of junction, and opens externally at the posterior part of the body. The same is the case in most Oncidia, but in Oneidium celticum, the urinary duct and the rectum emerge separately, but one close to the other, at the posterior end of the body. Close to these apertures lies, in all cases, the female genital aperture ; the male aperture, however, lies anteriorly to the right below the tentacle. The cloaca just mentioned, which is filled with air, has given rise to interesting discussions. From its wall there rise into the lumen closely packed folds, which may also be continued along the posterior portion of the urinary duct. The cloaca has therefore been considered by some to be a rudimentary pulmonary cavity, into which the urinary duct and the rectum open. The present writer holds the opinion, 78 COMPARATIVE ANATOMY CHAP. provisionally, that this cloaca has arisen by the junction of the terminal portions of the secondary ureter with the rectum, as in other Pulmonata, but that here the pul- monary cavity having atrophied, it opens outward direct, i.e. no longer through a respiratory aperture. Others, again, have thought the arrangement in Oncidiwm and Vaginulus to be primitive, the pulmonary cavity appearing here first as an insignificant widening of the terminal portion of the primary ureter. If this were the case, then the condition described above (p. 75, 1) for Budimus oblongus, where the kidney opens on a papilla direct into the base of the pulmonary cavity, would be thus explained: the pulmonary cavity would have to be considered as a much widened primary urinary duct. Then, in this primary ureter (pulmonary cavity) would follow the successive stages of the development of the secondary ureter, at first an open and later a partially closed channel, and finally a closed tube, so that at last, as in Helix pomatia, the primary ureter is divided into two distinct portions, viz. the much widened pulmonary cavity and the secondary ureter. But in the Limnewide, for example, the pulmonary cavity admittedly corresponds with the mantle cavity of other Gastropods. The Pulmonata would thus fall into two groups, the Nephropneusta (Stylommatophora), in which the pulmonary cavity=the widened primary ureter, and the Branchiopneusta (Basom- matophora, p. parte), in which the pulmonary cavity=the mantle cavity of other Gastropods. We consider this view incorrect because of the uniformity of the whole organisa- tion in the Pudmonata, and especially because of the occurrence of an osphradium in the pulmonary cavity of a Stylommatophore (Nephropneusta), viz. in the genus Testacella. For the osphradium invariably belongs to the mantle cavity, being primitively connected with the ctenidium, it never lies in the urinary duct. 3. Gastropoda Opisthobranchiata. We can here speak of a pallial complex only in connection with the Tectibranchia, since in them alone isa distinct mantle fold developed on the right side of the Fic. 75.— Aplysia, right aspect, the right parapodium (15) turned downwards ; the pallial complex is seen under the mantle fold 7 (after Lankester). 1, Anterior tentacle; 2, eyes; 3, posterior tentacle (rhinophore); 4, left parapodium ; 5, seminal furrow; 6, genital aperture; 7, mantle fold ; 8, gland ; 9, osphradium ; 10, outline of some inner organ seen through the integument ; 11, nephridial aperture; 12, ctenidium; 13, anus; 15, right parapodium ; 16, anterior portion of the foot. (There should be no connecting line between 6 and 9.) body. The general order of the organs in the pallial cavity (Fig. 75) is as follows :— 1. Far back, and often hardly or not at all covered by the mantle, sometimes at the summit of a conical prominence, lies the anus, and near it occasionally an anal gland. VII MOLLUSCA—THE PALLIAL COMPLEX 79 2. In front of the anus, between it and the ctenidium, is the nephridial aperture. Following these there may be— 3. A hypobranchial gland. 4, The ctenidium. 5. At the base of the ctenidium or on its axis, the osphradium. Were this complex of organs to be shifted along the edge of the body, we should have the arrange- ment found in the Monotocardia among the Prosu- branchia. The correspondence is, however, appar- ently marred by the position of— 6. The genital aperture, which in the Opistho- branchia lies farthest forward of all the pallial organs. In all other Opisthobranchia (after excluding the Tectibranchia) the pallial complex is broken up when the mantle and the true ctenidium disappear. The only exception to this is found in the Phyl- lidiide, where, apart from the gills, a similar arrangement to that in the Zectébimnchia occurs. The single or paired genital aperture always lies asymmetrically on the right side in front of the anus, which is sometimes found asymmetrically on the right side, and sometimes has a median dorsal position between the middle and the posterior end of the body. The renal aperture lies between the anus and the genital aperture, sometimes close to the latter. In the Pterupodu gymnosomata (Fig. 76) the shell and mantle are wanting. The ctenidium, when retained, asin the Dextobranchia and Preuno- dermat, lies somewhat far back on the right side of the body, far behind the anus. On the disappear- ance of the mantle, it evidently shifted back from its original position between the anus and the genital aperture, while the osphradium, which is generally found close to the ctenidium, has, as far as has yet been observed, retained its original position. The anus lies anteriorly behind the right fin ; the nephridial aperture lies close by, either distinct or united with the anus at the base of a common cloacal depression. Immediately in front of this lies the osphradium, then follows, considerably farther forward on the neck, to the right behind the base of the right fin, the genital aperture, from which, as in many Tectibranchia, a ciliated furrow runs forward along the surface of the body to the Fic. 76.—Pneumoderma, from the right side, diagrammatic (after Pel- seneer). 1, Right process bearing hooks (Hakensack) evaginated; 2, proboscis ; 3, right buccal tentacle ; 4, position of the right nuchal ten- tacle; 5, right fin (parapodium); 6, seminal furrow; 7, genital aperture ; 8, position of the jaw ; 9, ventral pro- boscidal papilla; 10, right buccal ap- pendage bearing suckers ; 11, head ; 12, aperture of penis; 13, right anterior pedal lobe; 14, anus; 15, posterior pedal lobe; 16, ctenidium; 17, pos- terior adaptive gill; d, v, a, p, dorsal, ventral, anterior, posterior. aperture of the penis, which lies to the right in front of the foot. All Thecosomata have « mantle and a mantle cavity, and often a shell as well ; in the Cymbuliide, the latter is replaced by a cartilaginous pseudoconch, a sub- cutaneous formation of the mantle. Among the Lhecosomata, the Linvacinide indicate the primitive arrangement ; they possess a dorsal or anterior mantle cavity, a coiled shell, and an operculum. 80 COMPARATIVE ANATOMY CHAP. The ctenidium, however, is wanting. In the base of the pallial cavity, to the left, lies the pericardium, and immediately in front of it the kidney, with a narrow aperture into the cavity ; then follow the osphradium (where this has been found), and, at the extreme right of the cavity, the anus with the anal gland. The mantle gland (hypobranchial gland, shield) is found on the roof of the pallial cavity. The genital aperture lies to the right anteriorly in the cephalic region ; from it a ciliated channel or furrow runs dorsally to the aperture of the penis, which lies anteriorly between the fins. As compared with the Limacinide, i.c. the Thecosomata with coiled shell, the A B Fic. 77.—A, B, C, Three diagrams to illustrate the relation of the Limacinide to the Cavoliniide (after Boas). A, Limacinide; B, hypothetical intermediate stage between the Limacinide and the Cavoliniide. The visceral doine twisted 90°. C, Cavoliniidas. All the diagrams from the ventral or posterior side. In A the visceral dome is drawn straight, whereas it is in reality coiled. 1, Right fin (parapodium); 2, foot bent forward ; 3, genital aperture; 4, ten- tacular appendage of the mantle edge ; 5, anus; 6, masticatory stomach ; 7, gonad. Cavoliniide and Cymbuliide, or Theeosomata with straight shell, show a very different arrangement of the pallial complex, which can only be explained by the supposition that the larger posterior portion of the body (the visceral dome) of the Linweiuide, with all the pallial organs belonging to it, has twisted round the longitudinal axis of the body 180°, in relation to the cephalic region with the genital apertures belonging to it. Such a twist gives the organs the position they actually occupy in the Cavoliniide and Cymbuliide ; the posterior (ventral) pallial cavity containing, on the left the anus, on the right the pericardium and kidney and the osphradium, the genital aperture occupying its original position to the right. The cause and significance of this twist are at present unknown. B. Scaphopoda. There is no gill in the posteriorly placed mantle cavity. The anus lies in the middle line above the foot, having a nephridial aperture on each side of it. There are no distinct genital apertures. VII MOLLUSCA—THE PALLIAL COMPLEX 81 C. Lamellibranchia. The general arrangement of the organs in the mantle cavity of the Lamelli- branchia has already been described. The strict symmetry of the body in this class must again be pointed out. All originally paired organs remain paired and symmetrical. The two nephridial apertures lie on the body above the base of the foot, or farther back near the posterior adductor muscle ; they usually lie beneath the point of attachment of the gill-axis, between it and the line of concrescence of the (inner) ascending lamella of the branchial leaf with the foot, where such concrescence takes place. In the Septibranchia, on the contrary, the apertures open into the upper pallial chamber. The outer genital apertures may be wanting, and in this case the genital products are ejected through the nephridial apertures, which is the primitive arrangement. When present, in diceceous bivalves, they are always found in one pair, and lie on each side just in front of the nephridial apertures, sometimes in the base of a common pit or furrow, less frequently at some distance from these aper- tures. There are no special copulatory organs. In hermaphrodite Lamellibranchia the arrangements may vary as follows :— 1. Both kinds of sexual products may be ejected on each side through a common aperture (Ostrwa, Pecten, Cyclas, Pisidium, etc.). 2. There may be, on each side, two distinct apertures, one male and the other female (Anatinacea). : 3. The seminal ducts and the oviducts may unite before opening to form a short, common, terminal piece (Septibranchia). The osphradium is paired in the Lamellibranchia, and always lies near the posterior adductor muscle over the visceral ganglion, at the point of insertion of the branchial axis on the body. A pair of sensory organs is found in many Lamelli- branchia, one on each side of the anus (abdominal sensory organs), or to the right and left on the mantle at the inner aperture of the siphons of the Siphoniata (pallial sensory organs). Hypobranchial glands have been found in the Protobranchia (Nuculide and Solenomyide). They are large and well developed, and belong to the mantle, lying in the posterior part of the body above the base of the gill on each side, to the right and left of the pericardium, and in front of the posterior adductor. The leaf-like oral lobes (labial palps), one occurring on each side of the mouth, between it and the anterior end of the base of the gill, will be described more in detail in another place. D. Cephalopoda. In the Cephalopoda the primitive symmetry of the pallial complex is on the whole retained. If we cut open the mantle of the Nautilus (Figs. 78 and 79), which covers the posteriorly placed pallial cavity, and lay it back on all sides, the following organs are revealed :— 1. On each side there are two gills, an upper and a lower. 2. The anus lies on the visceral dome, between the bases of the four gills. 3. Below the base of each gill is found a nephridial aperture—muaking four in all. 4, Close to the two upper nephridial apertures lie the two so-called viscero- pericardial apertures. 5. Between the bases of the lower gills there are in each sex, two genital VOL. II G CHAP. COMPARATIVE ANATOMY 82 Fic. 78.—Pallial complex and siphon of Nautilus pompilius ? (after Bourne and Lankester). v, Valve of the siphon ; ro, right genital aperture ; m, the mantle fold, with the nidamental gland, folded back ; an, anus ; cp, left aperture of the secondary celom ; lhn, left upper nephridial aper- ture; lo, aperture of the left rudimentary oviduct; lun, left lower nephridial aperture. The four ctenidia are not lettered, x anw deper Fic. 79.—Pallial complex of Nautilus pompilius g (after Bourne and Lankester). ec, Penis; a, muscle band of the siphon; Usp, aperture of the left rudimentary seminal duct; nepha, nephp, lower and upper nephridial aperture of the left side; olf, left osphradium ; viscper, left aperture of the secondary ccelom ; an, anus; , supra-anal papilla of unknown significance ; c, mantle cut off. VII MOLLUSCA--THE PALLIAL COMPLEX 83 Fic. 80.—Sepia Savignyana, from behind (after Savigny). The greater part of the mantle cut open and laid back on the right side (left in the figure). a, Prehensile tentacle; b, oral arm; ¢, mouth with jaws ; d, lower aperture of siphon; e, eye ; f, locking apparatus of the mantle g; h, right ctenidium ; i, siphon; &, locking apparatus of the mantle on the visceral dome; J, upper aperture of siphon; m, anus; n, depressor infundibuli; 0, penis ; p, right nephridial aperture ; g, posterior integument of the visceral dome; 7, fin. 84 COMPARATIVE ANATOMY CHAP. apertures, but only that on the right side is functional. In the male, the aperture is produced into a tubular penis. 6. Above the bases of the lower gills there is an osphradium on each side placed on a papilla. 7. Above the anus there is a large median papilla of unknown significance. 8. The nidamental gland lies dorsally in the mantle. If we compare with the above the pallial complex of a dibranchiate Cephalopod, such as Sepia (Fig. 80), we find the following arrangements :— 1. There is one gill on each side. 2, Along the median line of the visceral dome, the rectum and the duct of the ink-bag descend together, to open through a common aperture at the tip of a papilla at the base of the siphon. 3. On each side near the rectum, above the anus, a nephridial aperture occurs on the point of a papilla. 4. Of the two paired genital apertures only the left has been retained in Sepia and many other Cephalopods; this lies near the left nephridial aperture at the summit of a large papilla (penis). In the female Octopus, the genital apertures are paired and symmetrical, and lie to the right and left of the rectum. 5. The two nidamental glands (in Decapoda) lie in the visceral dome, sym- metrically with regard to the median line ; they open above the nephridial apertwres into the mantle cavity. VI. The Respiratory Organs. The True Gills or Ctenidia. The most important of the pallial organs in the Mollusca is the gill, for it is in order to protect it that the mantle, and with it the pallial cavity, develop. The gill found in the mantle cavity is throughout all the divisions of the Mollusca a homologous organ, to be derived from the gill of a common racial form. But since this gill is wanting in certain Mollusca (eg. many Opisthobranchia), and is functionally replaced by new organs which are morphologically altogether uncon- nected with it, it has been found useful to distinguish the primitive Molluscan gill by the name of etenidium. This word, therefore, has a special morphological significance. The etenidia of the Mollusca are originally paired and symmetri- eally arranged ciliated processes of the body wall, earrying two rows of branchial leaflets, andl projecting into the mantle cavity. Venous blood flows into the gills through afferent vessels (branchial arteries), and after becoming arterial by means of the respiration, flows through efferent vessels (branchial veins) back to the body, passing first through the heart. At or near the base of each ctenidium there always lies a sensory organ, which is considered as olfactory, the so-called osphradium or Spengel’s organ. Such primitive ctenidia are met with first in that group of the Mollusca which has undoubtedly retained more primitive characteristics than any other, viz. the Chifouide among the Amphinewra. They are, further, found in all other Mollusca which have retained the original VII MOLLUSCA—RESPIRATORY ORGANS 85 bilateral symmetry of the body, such as the Lamellibranchia, the Cephalopoda, and—a point of great importance—also in the primitive Fic. 81.—Ctenidia of various Molluscs (after Ray Lankester). A, Chiton; B, Sepia; ©, Fissurella; D, Nucula; E, Paludina. /t, Longitudinal branchial muscle ; abv, afferent branchial vessel ; ebv, efferent branchial vessel (branchial vein) ; gl, paired lamelle (leaflets) of the feathered gill; in D: d, position of the axis; a, inner; b and ¢, outer rows of branchial lamelle; in E: i, rectum ; br, branchial filaments ; a, anus. Gastropoda, the Zeugobranchia. In the latter, however, the left ctenidium was originally the right and vice versi, but this will be dealt with more in detail later. 86 COMPARATIVE ANATOMY CHAP. With regard to the number of gills originally present on each side of the body, opinions are divided. Those who hold that there were several seem justified by the arrangement in Chiton, where numerous consecutive ctenidia lie in a longitudinal row in the branchial furrow (mantle cavity) on each side, and also by that in the Nautilus, which is rightly considered the most primitive of extant Cephalopods, where four gills are found (Tetrabranchia). We shall, however, see later that the other view, viz. that the Mollusca originally possessed only one pair of ctenidia, has, to say the least, equal claim to be accepted. In all other Mollusca with paired ctenidia, including the Lamelli- branchia, there is only one pair at the posterior part of the body. Further, in the racial form of the Prosobranchia, a single pair of gills must be assumed to have occupied a posterior position in a mantle cavity which, with them, shifted forward later to the anterior position. The Zeugobranchia still retain this single pair of gills. In most Prosobranchia, the asymmetry of the body is also seen in the gills, only the left gill of the two in the Fisswrellide and Haliotide being retained, the right completely disappearing. In the forms which most resemble the Fissurellide and Haliotide, the single-gilled Dioto- cardia (Turbinide, Trochide, etc.), the gill is still feathered on both sides, but in all Monotocardia it has only a single row of leaflets. In one division of the Opisthobranchia, the Tectibranchia, one ctenidium is still retained, that on the right side. Other Opistho- branchia have lost the true ctenidium together with the mantle cavity ; it may besreplaced by analogous (but not homologous) respiratory organs, such as adaptive gills. The Pulmonata, in consequence of their adaptation to aerial respiration, have lost the ctenidia. The blood, which has become arterial in the ctenidia, reaches the heart through the auricle, and passes into the body through the arteries. It is therefore evident that a close relation must exist between the gills and auricles. This relation is briefly as follows: where the gills are paired, the auricles are paired, and unpaired gills are accompanied by a single auricle on that side of the body on which the gill is retained. Where gills are paired, there is almost always only one pair, and then there is one right and one left auricle. The Nautilus has four gills, and, to correspond, two right and two left auricles. The Chitonidw, on the other hand, in spite of their numerous pairs of gills, have only one right and one left auricle. The Scaphopoda possess neither true ctenidia nor any other localised gills. Respiration may take place at the various soft-skinned surfaces which come in contact with the water, such as the inner surface of the mantle, the tentacles, ete. A. Amphineura. Chitonide.—A single ctenidium of a Chiton (Fig. 82) may serve as a type of the Molluscan gill with its two rows of leaflets. The plumose ctenidium rises freely from VIL MOLLUSCA—RESPIRATORY ORGANS 87 the base of the branchial groove (mantle cavity). The axis here takes the shape of a thin septum. At each side, on the broader surface of the septum, extending fiom e | 3 SS = = eS f— == 53-6 S22 2S A ZS Zs Fic. §2.—Structure of the ctenidium of a Chiton (after B. Haller). A, Single ctenidium with its double row of branchial leaflets. B, Transverse section of the gill along the line a-b in Fig. A. 1, Narrow blood sinus in the branchial leaflet ; 2, septuni in its axis; 3, longitudinal muscle; 4, afferent branchial vessel; 5, efferent branchial vessel; 6, nerves; 7, long cilia on the branchial axis. C, 2 pairs of branchial leaflets cut through at right angles to their surfaces, along the line e-f in Fig. B. 1, Same as in Fig. B; 8, space between the consecutive branchial leaflets. D, Longi- tudinal section of the ctenidium somewhat laterally to the axis, and parallel to its septum, along the line c-d in Fig. A. This section is part of a transverse section of the body. Lettering as in Figs. Band C. In addition: 9, olfactory ridge of the branchial epithelium; 10, general afferent branchial vessel; 11, general efferent branchial vessel; 12, pleuro-visceral strand of the nervous system. The branchial epithelium is everywhere indicated by a thick black line. base to tip, there is one row of smooth, delicate branchial leaflets. In outline they are more or less semicircular, and stand crowded together in great numbers almost like the leaves of a book. The entire surface of the branchial epithelium is ciliated ; on the axial epithe- lium, the cilia are remarkably long. On that side of the axis which is turned towards the foot, a blood-vessel runs from base to tip, conducting venous blood to the gill (afferent branchial vessel). On the op- posite side, which faces the mantle, another vessel, the branchial vein, runs from the tip to the base of the gill, and carries the blood, which has become arterial by respiration, to the general branchial vein, Fi. $3,—Diagrams illustrating the arrangement of the and through it to the auricle. gills in the Chitonide. m, Mantle; 0, mouth; k, snout; J, These vessels have no special eee endothelial walls, but are surrounded by circular muscle fibres. The branchial vein is accompanied by a powerful longitudinal muscle. At the base of each branchial leaflet, the blood flows out of the branchial artery through an aperture into the narrow cavity of the leaflet, and passes through a similar aperture on the opposite side of the axis to enter the branchial vein. Nerves are supplied to the ctenidium from the pleuro-visceral nerve which runs close to its base. 88 COMPARATIVE ANATOMY CHAP. The number of ctenidia in each row varies very much in the different species of Chitonide ; it ranges from 14 to 75. The row extends along the whole length of the branchial furrow (Fig. 83 A), or else (in Chiton levis, C. Pallasii, and Chitonellus) is confined to its posterior half (B, C). Solenogastres. —(Proneomenia, Neomenia, Chetoderm«a). The mantle cavity, in these forms, is much reduced, consisting only of the groove on each side of the rudimentary foot; it opens into the cloacal cavity, or rather widens to form that cavity. The cloaca is thus the posterior portion of the mantle cavity. In Chetoderma (Fig. 84) the foot has disappeared, and the mantle cavity is reduced to the cloaca, in which one typical gill lies on ‘each side of the anus. These gills are regarded as __ the last ctenidia of the rows found in the Chitonidw, which in Fic. 84.—Posterior : i : 6 end of the body of Chitonellus and some species of Chiton are already confined to Chetoderma (dia. the posterior half of the body. In Neomenia, there is no longer gramafter Hubrecht). a pair of ctenidia, but a mere tuft of filaments rising from the 1, Gonad; 2, pericar- wall of the cloacal cavity, and in Proncomenia, there are only dium; 8, rectum; 4, =. < mageiiue andes irregular folds of the cloacal wall. ; ses : 6, ctenidium; 7, On the relation of the gills in the Chitonide to certain cloaca. patches of epithelium, which may perhaps be considered as osphradia, see the section on Olfactory Organs, p. 165. B. Gastropoda. The Fissurellide (Fig. 85, A and B) among the Prosobranchia stand nearest to the racial form of the Gastropoda. The mantle cavity is anteriorly placed ; into it from behind and above project two long gills feathered on each side; these lie symmetrically to the middle line, and to the right and left of the anus. The posterior portion of their axes is connected by a band with the floor of the respiratory cavity, while the anterior pointed portion projects freely. The fact that in the Fissurellide (and related forms) the gills are paired and symmetrical is very significant. It points to the primitive character of these forms, and enables us to compare their gills with those of the lower Lamellibranchia, i.e. the Protobranchia, and of the Cephalopoda. We must, however, again emphasise the generally-assumed fact that the left gill of Fissurella answers to the right gill of the Lamellibranchia and Cephalopoda, and the right gill of the former to the left of the latter, these latter having retained their primitive symmetry in this respect. This assumption becomes the more plausible when we consider that the mantle cavity with its organs originally lay posteriorly on the body, and shifted forward secondarily along its right side. The Haliotide are closely connected with the Fissurellida. Their spacious mantle cavity is, however, forced to the left side by the great development of the columellar muscle. There are two gills, feathered on both sides, of which the right is the smaller. The axis of each gill has united, for nearly its whole length, with the inner wall of the mantle, and only its anterior end is free ; its tip even projects a short distance beyond the respiratory cavity. Although the Fissurellidce and Haliotidee still possess two gills, other Diotocardia have retained only the left (ur) and larger gill of Haliotis. This gill is, however, still feathered on both sides, although this characteristic is obscured in a peculiar manner, The septum or axis of the gill, to the broader surfaces of which the branchial leaflets are attached, and one edge of which had, in Haliotis, already fused with the VII MOLLUSCA—RESPIRATORY ORGANS 89 inner wall of the mantle, becomes attached to the mantle by its other edge also (viz. that along which the branchial artery runs), somewhat to the right of the first line of concrescence. In this manner, which is illustrated by the accompanying diagrammatic sections (Fig. 86), the mantle cavity is divided by the branchial septum into two unequal parts, which open into one another anteriorly. Into the much smaller upper division the one row of smaller branchial leaflets projects, while the opposite row of larger leaflets hangs down into the lower and larger chamber. The anterior end of the gill, however, is still free, its point pro- jecting anteriorly (Trochide, Turbinide, Neritide). In the Docoglossa (Patellida) the ar- rangement of the gills is very varied. While the Lepetide have no gills whatever, we find in Pate//« a single row of numerous small branchial leaflets right round the body, on the inner or under side of the short encircling mantle fold, between it and the foot. This row is broken only in one place anteriorly on the left. It is, how- ever, evident that these gills, which some- what resemble those of the Chitonide, are no true ctenidia, from the fact that there are Docoglossa (e.g. some forms of Tectura and Scurria) which possess, in addition to this marginal row of leaflets, a typical ctenidium corresponding in every way with that of the Turbinide, Trochidce, ete. Other forms, such as deme, have only the true ctenidium and no marginal branchial leaflets. aha Fic. 85.—Subemarginula after removal of the In the large second division of the gyen (after Fischer). A, from above ; B, from Prosobranchia — the