Any member of the mollusk class Bivalvia, or Pelecypoda, characterized by having a two-halved (valved) shell. Clams, cockles, mussels, oysters, scallops, and shipworms are bivalves. Most are completely enclosed by the shell, the two valves of which are joined by an elastic ligament, and by two sheets of tissue called the mantle. Bivalves have no head. They feed on phytoplankton by pumping water across the gills and trapping food particles that are then moved to the mouth. Bivalves are found in most parts of the ocean from the intertidal zone to abyssal depths.