Marine or freshwater mollusc whose body is enclosed between two shells hinged together by a ligament on the dorsal side of the body.

The shell is closed by strong 'adductor' muscles. Ventrally, a retractile 'foot' can be put out to assist movement in mud or sand. Two large platelike gills are used for breathing and also, with the cilia present on them, make a mechanism for collecting the small particles of food on which bivalves depend. The bivalves form one of the five classes of molluscs, the Lamellibranchiata, otherwise known as Bivalvia or Pelycypoda, containing about 8,000 species.