LAMELLIBR ANCHIA . ] MOLLUSCA 685 vcloped 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 coelom, 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 OSPHRADIA 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 caecum 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 (Pecten, Spondylus, Pholas, Mactra, Tellina, 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 craAvl 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 pra> 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 equal size. Sub-order 1. Intcgripallia. 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. Arcacca. Genera: Area, L. (fig. 132) ; Cucullsea, Lam. ; Pectunculus, Lam. ; Limopsis, Sassi; Nucula, Lam. (fig. 134) ; Isoarca, Miinster ; Lcda, Scnu. ; Yoldia, Moll. ; Solcnclla, Sowerby, &c. Family 2. Trigoniacea. Genera : Trigonia, Brug. ; Axinus, Sow. ; Lyrodcsma, Conrad. Family 3. Unionacca. Genera : Unio, Retz. ; Castalia, Lam. ; Anodon, Cuv. (figs. 124, &c.) ; Iridina, Lam. ; Mycctopus, d Orb., &c. Family 4. Ludnacca. Genera : Lucina, Brug. ; Corbis, Cuv. ; Diplodonta, Brown ; Kellia, Turton ; Montacuta, Turton ; Lcpton, Turton ; Gale omma, Turton ; Astarte, Sow. ; Crassatclla, Lam. ; Cardinia, Ag. ; Cardita, Brug., &c. Family 5. Cyprinacca. Genera : Tridacna, Da C. ; Chama, L. ; Dimya, Ron. ; Diceras, Lk. ; Isocardia, Lam. ; Hippopodium, Sow. ; Cardium, L. ; Corbicula, Meg. ; Cyrcna, Lk. ; Cyclas, Brug. (iig. 146) ; Pisid- ium, Pfr. (figs. 148-153) ; Cyprina, Lam., &c. Sub-order 2. Sinupallia. Characters. 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. Vcneracca. Genera : Cypricardia, Lam. ; Tapes, Megl. ; Cyclina, Desh. ; Cytherea, Lam. (figs. 125, &c.) ; Chione, Megl. ; Venus, L. ; Lucinopsis, F. H. ; Sanyuinolaria, Lam. ; Psammobia, Lam. (fig. 130) ; Tellina, L. ; Donax, L. ; Scrobicularia, Schu. ; Cumingia, Sow. ; Rangia, DsmL ; Mactra, L. (fig. 140) ; Trigo- nella, Da C. ; Vaganella, Gr. ; Lutraria, Lam. Family 7. Myacca. Genera : Myochama, Stb. ; CJiamostrca, Rois ; Pandora, Sol. ; Thracia, Leach ; Thetis, Sow. ; Pholadomya, Sow. ; Corbula, Brug. ; Mi/a, Lam. ; Saxicava, Fleur ; Panop&a, Ad. ; Glyci- meris, Lam. ; Siligua, Mhlf., &c. ; Solen, L. Family 8. Pholadacca. Genera : Clavagclla, Lam. ; Aspergillum, Lam. (figs. 128, 129) ; Humphrcyia, Gr. ; Pholas, L. ; Pholadidea, Turt. ; Teredo, L. ; Tcredina, Lam. ; Furcella, Oken, &c. Order 2. Heteromya. Characters. Anterior adductor (pallial adductor) much smaller than the posterior adductor (pedal adductor) ; siphons rarely present. Family 1. Mytilacea. Genera : Mytilus, L. (fig. 133) ; Modiola, Lam. ; Crenella, Brown ; Lithodomus, Cuv. ; Dreissena, Ben. (fig. 136) ; Modiolarca, Gr., &c. Family 2. Mullcriacca. Genera : Aetheria, Lam. ; Mullcria, Fer. Order 3. Monomya. Characters. Anterior adductor absent in the adult ; siphons never developed. Family 1. Aviculacea. Genera : Cardiola, Brdp. ; Avicula, Kl. ; Malleus, Lam. ; Ino- ccramm, Sow. ; Crenatula, Lam. ; Perna, Brug., &c. Family 2. Ostracea. Genera : Ostrca, L. (fig. 6); Anomia, L. ; Spondylus, L. ; Plicatula, Lam. ; Vulsclla, Lam. ; Lima, Brug. ; Pecten, L. ; Niunitcs, 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 cephalophorous 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
(Anodonta cygnea), comparing its structure with those of