AY Honors/Shells - Advanced/Answer Key

European Squid (Loligo vulgaris). The mantle is all that is visible behind the head.

The mantle is an important part of the anatomy of molluscs. It is the dorsal body wall which covers the visceral mass.

In many, but by no means all, species of molluscs, the epidermis of the mantle secretes calcium carbonate and creates a shell.

The word "mantle" (a 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

The mantle cavity is a central feature of molluscan biology. This cavity is formed by the mantle skirt, a double fold of mantle which encloses a water space. This space containing the mollusc's gills, anus, osphradium, nephridiopores, and gonopores.

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.

Shell formation

In shelled molluscs, the mantle is what forms the shell, and what adds to the shell to increase its size and strength as the animal grows. Shell material is secreted by the ectodermic cells of the mantle tissue.

Mollusc blood is rich in a soluable form of calcium, and this calcium is concentrated and crystallized as calcium carbonate (CaCO₃).

The shell of a mollusc is formed of 2 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.

In some mollusc shells the inner layer is especially strong, and is iridescent. This layer is known as nacre or mother of pearl.

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.

See also

• Aperture
• Columella
• Operculum