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Mantle (mollusc) |
The mantle (also known by the Latin word pallium, adjective pallial) is a significant 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 conchiolin, and creates a shell.
The words mantle and pallium both originally meant cloak or cape, see mantle (vesture). This anatomical structure in molluscs often resembles a cloak because in many groups the edges of the mantle extend far beyond the main part of the body, forming flaps, double-layered structures which have been adapted for many different uses, including for example, the siphon.
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 bivalves such as 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.
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 soluble form of calcium, and this calcium is concentrated and crystallized as calcium carbonate (CaCO3).
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.
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.
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