Page:Natural History, Mollusca.djvu/321

 (Arm-footed Mollusks.)

animals bearing the above title are inhabitants of bivalve shells, one valve of which is frequently perforated, to give exit to a fleshy peduncle or stem, by which the animal is affixed to the rocks. When we open the two valves of the shell, we find each of them lined with a broad membrane, very thin, delicate, and semi-transparent, which together constitute the mantle. The edges of these membranes are thickened, and fringed with organs, already several times mentioned in these pages, and which we shall see assuming great importance as we investigate the economy of the lower tribes of animals. These organs are called cilia and consist of very subtile and microscopically minute hairs, arranged in close series, and capable of moving in unison, with vibratile waves, and of thus producing rapid currents in the water.

At the bottom of the cleft formed by the two leaves of the mantle, the mouth is placed, on each side of which is a long fleshy process, fringed all along one side with delicate hairs. In some species, these arms (which give name to the Class) are enormously developed; they are free for their whole length, and are capable, at the will of the animal, of being coiled up in many spiral folds, or of being protruded from the shell to a distance equal to thrice its length. The mechanism by which they