They also have no radula. A foot is present but laterally compressed. Bivalves generally have a large mantle cavity with gills. The posterior edges of the mantle may be fused to form inhalant and exhalant siphons that circulate water to and from the gill chamber. They are dioecious and fertilization is external.
Embryos first develop into a trochophore larva, then a veliger, and finally a spat.
Freshwater clams differ in that their fertilization is internal, with brooding taking place in the gill chamber. The fertilized eggs develop into bivalved glochidium larvae, which are discharged through the exhalant siphon. These larvae attach to the gills of fish, where they remain as parasites until they drop to the bottom to become sedentary adults. Most adult bivalves are sedentary filter feeders, using currents set up by cilia on their gills to bring in food particles. Particles are trapped in mucus secreted by the gills and labial palps, and moved to the mouth by ciliary action.