148 



MOLLUSCA 



veloped only in those few forms which are inequivalve. 

 The typical PERICARDIUM is well developed. It appears, 

 as in other Mollusca, not to be a blood-space although 

 developed from the cceloin, and it communicates with the 

 exterior by the pair of nephridia. As in Cephalopoda (and 

 possibly other Mollusca) water can be introduced through 

 the nephridia into this space. The ALIMENTARY CANAL 

 keeps very nearly to the median vertical plane whilst ex- 

 hibiting a number of flexures and loopings in this plane. 

 A pair of large glandular outgrowths, the so-called "liver" 

 or great digestive gland, exists as in other Molluscs. A 

 pair of pedal OTOCYSTS, and a pair of OSPHEADIA at the 

 base of the gills, appear to be always present. A typical 

 NERVOUS SYSTEM is present (fig. 144), consisting of a 

 cerebro-pleuro-visceral ganglion-pair, united by connectives 

 to a pedal ganglion-pair and an osphradial ganglion-pair 

 (parieto-splanchnic). 



A special cascum connected with the pharynx is some- 

 times found, containing a tough flexible cylinder of trans- 

 parent cartilaginous appearance and unknown significance, 

 called the "crystalline style" (Mactra), which possibly 

 represents the radular sac of Glossophora. In many Lamelli- 

 branchs a gland is found on the hinder surface of the foot 

 in the mid line, which secretes a substance which sets into 

 the form of threads the so-called " byssus " by means of 

 which the animal can fix itself. Sometimes this gland is 

 found in the young and not in the adult (Anodon, Unio, 

 Cyclas). In some Lamellibranchs (Pecteu, Spondylus, 

 Pholas, Mactra, TeUina, Pectunculus, Galeomma, &c.), 

 although cephalic eyes are always absent, special eyes 

 are developed on the free margin of the mantle-skirt, 

 apparently by the modification of tentacles which are 

 commonly found there (fig. 145). The existence of pores 

 in the foot and elsewhere in Lamellibranchia by which liquid 

 can pass into and out of the vascular system, although 

 asserted as in the case of other Mollusca, appears to be 

 improbable. It has yet to be shown by satisfactory micro- 

 scopic sections that the supposed pores are anything but 

 epidermal glands. 



The Lamellibranchia live chiefly in the sea, some in 

 fresh waters. A very few have the power of swimming by 

 opening and shutting the valves of the shell (Pecten, Lima) ; 

 most can slowly crawl or rapidly burrow ; others are, when 

 adult, permanently fixed to stones or rocks either by the 

 shell or the byssus. In development some Lamellibranchia 

 pass through a free-swimming trochosphere stage with pros- 

 oral ciliated band ; other fresh-water forms which carry the 

 young in brood-pouches formed by the ctenidia have sup- 

 pressed this larval phase. 



The following classification and enumeration of genera 

 are based primarily upon the characters of the adductor 

 muscles. The Heteromya and Monomya must be conceived 

 of as derived from forms resembling such Gastropodous 

 Isomya as Nucula and Trigonia, which undoubtedly are 

 the nearest living representatives of the ancestral Lipo- 

 cephala, and bring us nearest to the other branch of the 

 Mollusca, the Glossophora. 



Order 1. Isomya. 



Character. Anterior and posterior adductor muscles of approxi- 

 mately eijual size. 



Sub-order 1. Integripallia. 



Characters. Marginal attachment of the mantle to the shell not 

 inflected to form a sinus ; siphons not developed in some, present 

 in most. 

 Family 1. Arcacea. 



Genera: Area, L. (fig. 132) ; Cuctillxa, Lam. ; Pcctunculus, Lam. ; 

 Limopsis, Sassi ; Nueufa, Lam. (fig. '134) ; Isoarca, Minister : 

 Lcda., Scliu. ; 1'ulilin, Mull. ; Sulcnella, Sowerby, &c. 

 Family 2. Trii/on iacea. 

 Genera : Trigonia, Brug. ; Axinus, Sow. ; Lyrodcsma, Conrad. 



Family 3. Unionacea. 

 Genera : Unio, Retz. ; Castalia, Lam. ; Anodon, Cuv. (figs. 124, 



&c. ); Iridina, Lain. ; Mycctopus, d'Orb., &c. 

 Family 4. Lucinaeea. 



Genera : Lucina, Brug. ; Corlris, Cuv. ; Diplodonta, Brown ; 

 K 'Mia, Turtou ; Montacuta, Turtou ; Lepton, Turton ; Gale- 

 omma, Turton ; Astartc, Sow. ; C'mssatclla, Lam. ; Cardinia, 

 Ag. ; Cardita, Brug., &c. 

 Family 5. Cyprinacca. 



Genera : Tridacna, Da C. ; Chama, L. ; Dimya, Ron. ; Diccras, 

 Lk. ; Isocardia, Lam. ; Eippopodium, Sow. ; Cardium, L. ; 

 Corbicula, Meg. ; Cyrena, Lk. ; Cydas, Brug. (fig. 146) ; Pisid- 

 ium, Pfr. (figs. 148-153) ; Cyprina, Lam., &c. 



Sub-order 2. Sinupallia. 



Cliaraetcrs. Marginal attachment of the mantle to the shell in- 

 flected so as to form a sinus into which the pallial siphons can be 

 withdrawn ; siphons always present, and large. 

 Family 6. Vencracca. 



Genera : Cypricardia, Lam. ; Tapes, Megl. ; Cyclina, Desh. ; 

 Cythcrca, Lam. (figs. 125, &c.) ; Chioiic, Megl. ; Venus, L. ; 

 Lucinopsis, F. H. ; Sanijuinolaria, Lam. ; Psammobia, Lani. 

 (fig. 130) ; Tcllina, L. ; Donax, L. ; Scrobiciilaria, Schu. ; 

 Cumingia, Sow. ; Raiirjia, Dsml. ; Mactra, L. (fig. 140) ; Trigo- 

 nella, Da C. ; Vaganclla, Gr. ; Lutraria, Lam. 

 Family 7. Myacca. 



Genera : Myochama, Stb. ; Chamostrca, Rois ; Pandora, Sol. ; 

 Thraeia, Leach ; Thetis, Sow. ; Pholadomya, Sow. ; Corlntla, 

 Brug. ; Mya, Lam. ; Saxicava, Fleur ; Panopsea, Ad. ; Glyci- 

 mcris, Lam. ; Siliqua, Mhlf., &c. ; Sokn, L. 

 Family 8. Pholadacca. 



Genera : Clavagella, Lam. ; Aspcrgillum, Lam. (figs. 128, 129) ; 

 Humphrcyia, Gr. ; Pholas, L. ; Pholadidea, Turt. ; Teredo, L. ; 

 Teredina, Lam. ; Furcella, Okeu, &c. 



Order 2. Heteromya. 



Characters. Anterior adductor (pallial adductor) much smaller 

 than the posterior adductor (pedal adductor) ; siphons rarely present. 

 Family 1. Mytilaeea. 



Genera : Mytilus, L. (fig. 133) ; Modiola, Lam. ; Crenella, Brown ; 

 Lithodomus, Cuv. ; Drcissena, Ben. (fig. 136) : Modiolarca, 

 Gr., &c. 



Family 2. Miitteriacea. 

 Genera : Acthcria,, Lam. ; Mullcria, Fer. 



Order 3. Monomya. 



Characters. Anterior adductor absent in the adult; siphons 

 never developed. 

 Family 1. Avieulacea. 



Genera : Cardiola, Brdp. ; Avicula, III. ; Malleus, Lam. ; Ino- 



ceramiis, Sow. ; Crcnatula, Lam. ; Pcrna, Brug., &c. 

 Family 2. Ostracea. 



Genera: Ostrca, L. (fig. 6); Anmnia, L. ; Spondylus, L. ; Plicatula, 

 Lam. ; Vulsella, Lam. ; Lima, Brug. ; Pecten, L. ; Hiunitcs, 

 Dfr., &c. 



Further Remarks on the Lamellibranchia. The Lamelli- 

 branchia are the only members of the Lipocephalous branch 

 of Mollusca existing at the present day; and we must 

 suppose that, whilst on the one hand the earliest Glosso- 

 phorous forms were developing from the archi-Mollusca by 

 the elaboration of the buccal apparatus, the bivalved sessile 

 Lamellibranchs were developing in another direction from 

 univalve cephalophoroua ancestors. The large bilobed 

 mantle-flap with its pair of shells covering in the whole 

 animal, the current-producing largely-expanded ctenidia, 

 and the reduced cephalic region are characters which go 

 hand in hand, and were simultaneously acquired, each being 

 related to the development of the others. Unless the 

 " crystalline style " of Lamellibranchs is to be considered 

 as the rudiment of the " radular sac " of Glossophora, as 

 suggested by Balfour, there is no indication whatever that 

 the ancestors of the Lamellibranchia had acquired a repre- 

 sentative of the buccal apparatus so highly developed in 

 Glossophora before diverging from the archi-Mollusca ; 

 that is to say, the common ancestors of the two great 

 branches of Mollusca presented the distinctive character 

 of neither branch they had not an aborted cephalic region, 

 and they had not a lingual ribbon. 



As an example of the organization of a Lamellibranch, 

 we shall review the structure of the Common Pond-Mussel 

 (Anodunta tygnea), comparing its structure with those of 



