ABSTRACT
Bivalves, comprising the oysters, mussels, and clams, are the second largest group of molluscs. They are headless, their shell consists of a pair of laterally compressed, hinged valves connected by a ligament, and the mantle cavity typically surrounds the entire body. Several detailed treatments of bivalve evolution, morphology, and systematics are available, with a number of books being devoted to this class. Bivalves are divided into two main groups, the Protobranchia and the more typical bivalves with sheet-like gills, the Autobranchia. A major modification of bivalve classification was that of Newell in the Treatise on Invertebrate Paleontology. Purchon classified bivalves based on morphological criteria, particularly stomach anatomy. He divided them into two subclasses, Oligosyringia, including two orders, Protobranchia and Septibranchia, and Polysyringia, with the latter ‘group’ including all the ‘lamellibranch’ bivalves. Protobranch bivalves have a simple gill and, except for the solemyoideans, deposit feed using their elongate labial palp proboscides.
