They are defined by the presence of two laterally-compressed shells, hinged together by an elastic ligament and shell teeth. The shells are closed by well-developed adductor muscles. Bivalves lack defined heads, although eyes may be present elsewhere on the body. 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.