The name "bivalve" refers to the two-part shell that characterizes these mollusk species. The two halves of the shell are joined by a ligamentous hinge and held shut by a pair of strong adductor muscles. The shell is made of calcium carbonate and is secreted by the mantle (soft body wall). Shells grow with the organisms, extending out from the hinge area. Most bivalve species go through a free-swimming larval stage before taking on their characteristic adult form and lifestyle.
Most species of bivalves are filter feeders. Currents of water are drawn into the body and through the gills, where tiny food particles are caught in the gill mucus. This flow of water also functions in respiration, allowing organisms to obtain fresh oxygen. A few bivalve species are predatory, including some deep-sea scallops. Bivalves make use of a variety of lifestyles. Sedentary species (e.g., mussels and oysters) spend their lives attached to a substrate , whereas others burrow underground (e.g., clams) or live on the water bottom and swim (e.g., scallops). Bivalves have highly reduced heads and simple nervous and sensory systems. Most species have some chemosensory cells at the edge of the mantle that are used to detect chemical signals in the water. Some also have simple eyes. Bivalves have an open circulatory system. A number of bivalve species have some commercial value, including representatives of all the groups discussed here. Bivalves are valued as food sources, and throughout history their shells have been used for many purposes. Most buttons were made from shells before the advent of plastic. In addition, shell fragments have been used for everything from roads to fertilizer. Finally, the beautiful pearls found in oysters have always been treasured. Read more: http://www.waterencyclopedia.com/Bi-Ca/Bivalves.html#ixzz3Yirmv5jM |