Page:Natural History, Mollusca.djvu/175

 (Comb-gilled Mollusks.)

have now arrived at the most numerous division of the, which comprehends nearly the whole of the spiral univalves, and many with simply conical shells. Their distinctive character is the possession of gills composed of numerous leaflets, or fringes, ranged in parallel order, like the teeth of a comb, and attached, in one, two, or three lines, (according to the genus) to the ceiling of the breathing-chamber, a cavity opening by a wide orifice between the edge of the mantle and the body. All the members of the Order respire water, and nearly all are marine.

A pair of tentacles are always present, accompanied by a pair of eyes, often highly organized, carried, sometimes, on spinal footstalks, and sometimes seated, as it were, on the side, or at the base of the tentacles. The mouth takes the form of a proboscis more or less lengthened, and conceals a tongue armed with small recurved hooks, which wear down the hardest bodies by slow and repeated friction. The sexes are always separate.

The shell is in general turbinated, or twisted spirally into a cone more or less regular; the aperture of which is sometimes entire, sometimes notched, sometimes drawn out into a canal. The orifice is in general capable of being closed by an operculum, a horny or shelly disk, attached to the