[L. mollis, soft.] The animals included in this group have soft, inarticulated bodies, usually inclosed in a shell, the body covered with a sensitive contractile skin, kept moistened by a viscid fluid which exudes from it. In one large division of molluscs (gastropods) the under surface of the body is covered by a broad fleshy disc or foot, on which the animal glides slowly along. Snails and slugs possess this foot. Of the shell-covered molluscs some live in univalve, others in bivalve, or multivalve shells. To the bivalves belong, oysters, mussels, and cockles; to the univalves snails, whelks, cowries, limpets, etc. The shells of the former class are joined by a hinge, and the inhabitanthas the power of opening and closing the valves at will. Many of the inhabitants of the univalve shells have a horny or shelly plate attached to their bodies, which serves the purpose of a door when the animal retires within its house. This is well illustrated in the common periwinkle.