Marine bivalves are a class of 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. The majority are filter feeders. The gills have evolved into ctenidia, 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. Many are edible and are a commercially important food source. The largest of all living molluscs is the Giant clam, Tridagna Gigas measuring more than 1200mm and more than 200kgs.