Valve is an anatomical term applied to the
shell of those
molluscs that have a shell. Although in theory any mollusc shell can be termed a "valve", the word is now most commonly applied to members of two
classes of molluscs: the
Bivalvia (clams) and the
Polyplacophora (chitons), in other words, to those molluscs whose shells are normally composed of more than one articulating part, each one of which is known as a valve (and also, in the case of chitons, as a "plate".)