Some bivalves do not attach themselves to anything but remain free.

Scallop shells, Pecten maximus .

Scallops have a row of bright blue eyes rimming the mantle and can detect approaching predators. When a starfish (its main predator) approaches, scallops are able to snap their valves shut using their large adductor muscle. This muscle acts against a rubber-like material (‘pectin’) in the hinge that opens the shell again. By rapidly contracting the adductor muscle, and then allowing the shell to spring open again, the scallop can lift itself from the sea bed and ‘swim’ out of trouble by jet propulsion.