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Show 354 MOLLUSCA. . . 'bl 1 n find no other organ of respiration than its thin and lS VlSI e; ca vascular skin( 1 )· ORDER. VI. PECTINIBRANCIIIATA(2). This order forms beyond all comparison the most numerous division, inasmuch as it comprises the whole of the spiral uni· valves and several that are simply conical. Their branchi:e, comp:sed of numerous lamellre or strips laid parallel with each other like the teeth of a comb, are attached on one, two, or three' lines, according to the genus, to the ceiling of the pulmo· nary cavity, which occupies the last whorl of the shell, and which has a large opening between the edge of the mantle and the body. In two genera only, Cyclostoma and Helicina, do we find, instead of branchire, a vascular network, covering the ceiling of a cavity, that is otherwise similar; they are the only ones that respire the natueal air ; all the others respire water. All the Pectinibranchiata have two tentacula and two eyes, sometimes placed on particular pedicles, and a mouth resem· bling a more or less elongated proboscis; the sexes are sepa· rated. The penis of the male, attached to the right side ~f the neek, cannot usually be retracted within the body, butJs reflected into the cavity of the branchire; it is sometimes ver! stout, and the Paludina is the only one which can retractJt through an orifice perforated in its right tentaculum. !~~ rectum and oviduct of the female also run along the rJg ( 1) These observations are made from individuals presented to me by M. Quoyd M. de Blainville makes a family of Pltilliroe, which he names PsuLosoMA, an which is the third ofhis APononnANCIIIATA: the others are llyalre, &c. (2) M. de Blainville's sub-class PARACE.PIIA.LOl'HOilA Dxorc.A. GASTEROPODA PECTlNIBHANCHIAT A. 355 side of this cavity, and between them and the branchire is a peculiar organ composed of ce11s, from which exudes an extremely viscid fluid, that forms a common envelope which contains the ova, and which is deposited with them. The figure of this envelope is often very complex and singular( I). Their tongue is armed with little hooks, and by slow and repeated rubbing acts upon the hardest bodies. The greatest difference in these animals consists in the presence or absence of the little canal formed by a prolongation of the edge of the pulmonary cavity of the left side, and which passes through a similar canal or emargination in the sheiJ, to enable the animal to breathe without leaving its shelter. There is also this distinction between the genera-some of them have no operculum ; the species differ from each other by the filaments, fringes, and other ornaments of the head, foot, or mantle. These Mollusca are arranged in several families according to the forms of their shells, which appear to bear a constant relation to that of the animal. FAMILY I. TROCHOID A. This family is known by the shell, the aperture of which is entire, without an emargination or canal for a siphon of the mantle, as the animal has none, and is furnished with an operculum or some organ in place ofit(2). TRocnus, Lin. (3) The external margin of the angular aperture approaching more or less to a perfect quadrangular figure, and in an oblique plane, with respect to the axis of the shell, because the part of the margin next to the spire projects more than the rest. Most of these animal~ (1) Fot• Murcx, see Lister, 881, Baster, Op. Subs., I, vi, 1, 2; for Buccinum, llaster, lb. V, 2, 3. (2) They are the PA.RACEPHALOPHORA DroiCA AsiPHONOliRANCIIIA..TA of Blainville. (3) This great genus constitutes the family GoNIOSTOllr A, Ulainv. · |
