Definitions

Mollusc

mollusk

[mol-uhsk]
or mollusc

Representative mollusks. Bivalves have a shell with two halves. Filter feeders, they take in food elipsis

Any of some 75,000 species of soft-bodied invertebrate animals (phylum Mollusca), many of which are wholly or partly enclosed in a calcium carbonate shell secreted by the mantle, a soft covering formed from the body wall. Between the mantle and the body is the mantle cavity. Mollusks occur in most habitats from the deep sea to high mountains. Living mollusks are usually grouped into eight classes: Gastropoda (see gastropod), Bivalvia or Pelecypoda (see bivalve), Cephalopoda (see cephalopod), Scaphopoda (tusk shells), Aplacophora (Solenogasters), Caudofoveata (sometimes included in the Aplacophora order), Polyplacophora (chitons), and Monoplacophora. Mollusks are economically important as food, and their shells are widely used in jewelry and decorative items.

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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" (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

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 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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