Bivalves include clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, and well as a number of families that live in freshwater. 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.