Class Bivalvia
Body in a bilohed mantle enclosed in a two-valved shell; head reduced; mouth with labial palps but no radula; foot wedge-shaped; plate-like gills; sexes separate; trochophore and veliger larvae. Class Bivalvia is as well known as Pelecypoda or Lamellibranchia. It includes clams, oysters and muscles. The body of these molluscs is laterally compressed and the shell is made up of two valves hinged dorsally. The shell covers the body completely. The foot of the animal is also laterally compressed. Bivalves have mainly spacious mantle cavity among the molluscs. The large gills that are enclosed in the mantle cavity function like organs of gas exchange and food gathering apparatus producing ciliary currents assisting in filter feeding. Bivalves do not have a distinct head or radula. Most of the bivalves burrow in the bottom of fresh water bodies or sea and the body is properly modified for such a habitat. But there are other forms which have .taken to other modes of life also.