Cockles Bivalves are kinds of molluscs with flattened bodies contained in a shell that has two hinged parts. They include clams, cockles, oysters, scallops, mussels, razor shells and many others. Some, for example the cockles, are rounded, while razors have elongated shells and shipworms are long and thin. Most bivalves are filter feeders: they use their special gills for both feeding and breathing. To protect themselves from predators, they either bury themselves in the sand or mud on the seabed, or attach themselves to rocks. Some, such as shipworms, bore into wood. Others, including scallops, swim away from danger.
Oysters and marine mussels remain permanently attached to rocky ground. Bivalves that are fixed in one place (sessile) and become exposed at low tide, such as marine mussels, keep their gills wet by ensuring their shells remain tightly shut.
Clams burrow into sand or mud using their foot. Scallops can swim at speed by suddenly slamming their shells together and ejecting water from inside, effectively using jet propulsion to move through the water.Scallops move by forcing water from their shells to push themselves forward.
There are about 8000 species of marine bivalve.
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