 Bivalvia , in previous centuries referred to as the <unk> and <unk> , is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts . Bivalves have no head , and they also lack a radula . They include clams , oysters , cockles , mussels , scallops , and numerous other families that live in saltwater , as well as a number of families that live in freshwater . The majority are filter feeders . The gills have evolved into <unk> , specialised organs for feeding and breathing . Most bivalves bury themselves in sediment , where they are relatively safe from predation . Others lie on the sea floor or attach themselves to rocks or other hard surfaces . A few bore into wood , clay , or stone and live inside these substances . Some bivalves , such as the scallops , can swim . 
