Bivalves

form a cla of animals whose shell consists of two limy valves, lying one on each side of the body mu els, oysters, and cockles arc familiar examples,; The inactive lives they lead result in (1) an undeveloped structure of head, the absence of head eves, the absence of biting organs, the marked development (3) of plate-like gills producing (4) nutritive as well as respiratory currents, the presence of thick, intricately-formed shells.