This is the second largest group of mollusks.
All species are aquatic, most are marine.
About 264 species occur in the freshwater of North America. Recognition Laterally compressed mollusks with a bivalved shell with a dorsal hinge. The head is poorly developed. The mantle cavity is capacious, the gills are large and serve also in feeding. Life Cycle Marine forms normally have external fertilization and free-swimming trochophore and veliger larvae , freshwater forms usually shed the sperm into the water, but the eggs are fertilized and brooded within the gill cavity.