 The bivalves are a highly successful class of invertebrates found in aquatic habitats throughout the world . Most are infaunal and live buried in sediment on the seabed , or in the sediment in freshwater habitats . A large number of bivalve species are found in the intertidal and sublittoral zones of the oceans . A sandy sea beach may superficially appear to be devoid of life , but often a very large number of bivalves and other invertebrates are living beneath the surface of the sand . On a large beach in South Wales , careful sampling produced an estimate of 1 @.@ 44 million cockles ( Cerastoderma edule ) per acre of beach . 
