This class, formerly known as Pelecypoda, contains the mollusks known as bivalves, including the mussels, oysters, scallops, and clams. All have shells composed of two pieces known as valves. In most, the valves are of similar size, but in some sedentary species, such as the oysters, the upper valve, which covers the left side of the body, is larger than the lower valve, which covers the right side and is attached to the substratum. Two large muscles, called adductors, hold the valves together at the top of the body. Bivalve shells vary greatly in size, color, and ornamentation. The freshwater seed shells are among the smallest known, being less than .1 in. (c.2 mm) in length, while the shell of the giant clam may exceed 4 ft (120.4 cm) in length.
The foot of bivalves is adapted for burrowing in all species except the sedentary ones, where it is reduced in size. Some species, e.g., the cockles, use the foot to hop about from place to place. Bivalves have a greatly reduced head and no radula. Most have a single pair of large gills used for respiration and for trapping minute food particles. Members of the order Protobranchia use another structure, the proboscis, to feed on bottom detritus. The order Septibranchia contains animals that have lost their gills; they are carnivores or scavengers. Bivalves have a relatively simple nervous system with three pairs of ganglia and two pairs of long nerve cords. An organ of equilibrium, called a statocyst, is present in most. Fertilization normally occurs in surrounding seawater, and most bivalves have separate sexes. All are aquatic, and they constitute an important food source for many animals, including humans.
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