The Bivalve Class
This class (formerly, Pelecypoda or Lamellibranchia) is the second largest among the molluscs. It has about 10,000 living species, 2,000 of which may be freshwater. All have a laterally compressed body with two shell halves ("valves") that are hinged dorsally and can completely enclose the body. The shells can be closed by one or two large
and kept ajar by a chitinous, elastic ligament located at the
The hinge may have a variable number of interlocking "teeth," like notches on a door hinge. Their shape and number serve, in part, to characterize individual species. Because of their utility in fossil identification and dating, such details of shell structure probably have received much more attention than would be warranted by consideration of body plan, alone.
The bivalve body plan is quite simple. It has lost a defined head end with pharyngeal glands and radula. Tactile, chemosensory, and visual receptors where present, have migrated to the mantle at the open edges of the shell.
With the exception of the
and several other superfamilies in the Order, Anomalodesmata, almost all other bivalves are sedentary filter feeders, that live on minute particles of algae and detritus. The partcles are swept by a powerful ciliary current, aided by muscle contractions, then filtered out on pairs of highly enlarged,
states that a typical bivalve can move from 30 to 60 times as much water per hour as its body volume. Cilia further assist the transfer of food particles from the gills to the lip-like "palps," which funnel food particles to the mouth.