Bivalve Mollusk

Mantle and shell
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Bivalve shell In bivalves the mantle, a thin membrane surrounding the body, secretes the main shell valves, ligament and hinge teeth, the mantle lobes secreting the valves and the mantle crest the other parts. The mantle is attached to the shell by the mantle retractor muscles at the pallial line. In some bivalves the mantle edges fuse to form siphons, which take in and expel water for suspension feeding purposes.

The shell is composed of two calcareous valves, which are made of either calcite (as with, e.g. oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer, as with the pterioida. The outermost layer is the periostracum, composed of a horny organic substance. This forms the familiar coloured layer on the shell.[5] The shell is added to in two ways - at the open edge, and by a gradual thickening throughout the animal's life.

The shell halves are held together at the animal's dorsum by the ligament, which is composed of the tensilium and resilium. The ligament opens the shells.

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