There are over 30,000 species of bivalves, including the fossil species. There are about 9,200 living species in 1,260 genera and 106 families. All of them live in the water, most of them in the sea or in brackish water. Some live in fresh water. All are filter feeders: they lost their radula in the course of evolution. A few are carnivorous, eating much larger prey than the tiny microalgae eaten by other bivalves.
Bivalves have two shells or valves connected by a hinge with hinge teeth. They are made of a calcareousmineral, calcite or
aragonite. The valves are covered by a
periostracum, which is an organichorny substance. This forms the familiar coloured layer on the shell.
The shells are usually held shut by strong adductor muscles. Scallops can use their muscles to flap the valves and swim.