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. Use beak-like jaws to crush prey. The mouth is at the center of several long tentacles. The chambered nautilus is the only shelled cephalopod alive today.