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. Squids swim backwards in open water by drawing water into the mantle cavity, and then firing a jetstream of water through the excurrent siphon which points anteriorly. Directional changes can be made by pointing the siphon in different directions.