The word "mantle" (an old word, meaning cloak) is used for this anatomical structure because in many kinds of molluscs, the edge of the mantle extends beyond the main part of the body, forming flaps or double-folded structures.
The mantle cavity functions as a respiratory chamber in all molluscs. In bivalves it is usually part of the feeding structure. In some mollusks the mantle cavity is a brood chamber, and in cephalopods and some bivlaves such as the scallops, it is a locomotory organ.
The mantle is highly muscular. In cephalopods the contraction of the mantle is used to force water through a tubular siphon, the hyponome, and this propels the animal very rapidly through the water. In other mollusks, it is used as a kind of "foot" for locomotion.
The shell of a mollusc is formed of two or three layers. The outermost shell layer in many molluscs is composed solely of organic material, and is known as the periostracum. The inner layers of the shell are formed of calcium carbonate crystalized into an organic matrix. The individual crystals of each shell layer differ in shape and orientation, such that one layer is calcite and another aragonite. Thin new layers of shell are continually deposited onto the inner surface of the animal's shell.
The calcium carbonate layers in a shell are generally of two types: an outer, chalk-like prismatic layer and an inner pearly, lamellar or nacreous layer. The layers usually incorporate a substance called conchiolin, often in order to help bind the calcium carbonate crystals together. Conchiolin is composed largely of quinone-tanned proteins.
Some shells contain pigments which are incorporated into the structure. This is what accounts for the striking colors and patterns that can be seen in some species of seashells, and the shells of some tropical land snails. These shell pigments sometimes include compounds such as pyrroles and porphyrins.