The shell halves are hinged at the mid-dorsal line and are drawn together by two adductor muscles to protect the animal. Bivalves may extend the foot for motility or anchorage when the shell is open. Most are suspension feeders that trap small food particles in the mucus coating of the gills and then use cilia to move the particles to the mouth. Water enters the mantle cavity through an incurrent siphon, passes over the gills, and then exits through an excurrent siphon. No radula or distinct head is present.
Bivalves lead sedentary lives.
They use the foot as an anchor in sand or mud. Sessile mussels secrete threads that anchor them to rocks, docks or other hard surfaces. Scallops can propel themselves along the sea floor by flapping their shells. The Class Cephalopoda contains the squids and octopuses. Cephalopods are agile carnivores.