In the early larval stage of gastropods, the visceral mass and mantle cavity rotate up to 180 degrees counterclockwise, in a process called torsion. This brings these organs from a posterior position to an anterior position behind the head. The animal in most cases is able to retract into the shell. Gastropods are one of the few groups of organisms well-represented in marine, freshwater, and terrestrial habitats. There are about 60,000 species.
Bivalves have two valves connected by a flexible ligament.
The mantle cavity is enlarged, enclosing the visceral mass and other internal organs. There is no differentiated head or cephalic region and the radula is absent. Most bivalves are filter feeders, with the gill acting as a food collecting and sorting organ, in addition to filling its respiratory function. The mouth usually has a pair of labial palps on either side, which handle food collected by the gill. There are about 10,000 living species of bivalves in marine and freshwater habitats worldwide.