Shell Anatomy, Structure and Composition
The bivalve shell is composed of two calcareous valves. The mantle, a thin membrane surrounding the body, secretes the shell valves, ligament and hinge teeth. The mantle lobes secrete the valves, and the mantle crest creates the other parts.
The mantle itself is attached to the shell by numerous small mantle retractor muscles, which are arranged in a narrow line along the length of the interior of the shell. The position of this line is often quite clearly visible on the inside of each valve of a bivalve shell, as a shiny line, the pallial line, which runs along a small distance in from the outer edge of each valve, usually joining the anterior adductor muscle scar to the posterior adductor muscle scar. The two adductor muscles are what allow the bivalve to close the shell tightly.
In some bivalves the mantle edges fuse to form siphons, which take in and expel water during suspension feeding. Species which live buried in sediment usually have long siphons, and when the bivalve needs to close its shell, these siphons retract into a pocket-like space in the mantle. This feature of the internal anatomy of a bivalve is clearly indicated on the interior of the shell surface as a pallial sinus, an indentation in the pallial line.
The valves of the shell are made of either calcite (as with, e.g. oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer, as is the case with the Pterioida which have this layer in the form of nacre or mother of pearl. The outermost layer of the shell is known as the periostracum, which is composed of a horny organic substance. This forms a yellowish or brownish "skin" on the outside of the shell. The periostracum may start to peel off of a shell when it is allowed to dry out for long periods.
The shell is added to, and increases in size, in two ways - by increments added to the open edge of the shell, and by a gradual thickening throughout the animal's life.
The two shell valves are held together at the animal's dorsum by the ligament, which is composed of the tensilium and resilium. In life the ligament opens the shell, and the adductor muscle or muscles close the shell.
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