 A few bivalves , such as the granular poromya ( Poromya granulata ) , are carnivorous , eating much larger prey than the tiny microalgae consumed by other bivalves . In these animals , the gills are relatively small , and form a perforated barrier separating the main mantle cavity from a smaller chamber through which the water is exhaled . Muscles draw water in through the inhalant siphon which is modified into a cowl @-@ shaped organ , sucking in small crustaceans and worms at the same time . The siphon can be retracted quickly and inverted , bringing the prey within reach of the mouth . The gut is modified so that large food particles can be digested . 
