Bivalve Seashells Also known as pelecypods, bivalve shells are mollusks with two valves joined by a hinge. Most of the 20,000 species are marine including clams, mussels, oysters and scallops. About one third of the species live in fresh water. They lack a head and radial teeth. They feed on microscopic plant life aided by their gills. The tow shells (valves) are kept closed by strong muscles. The sexes may be combined in one individual or may be separate. Most lay their eggs directly into the water. Oysters, clams, and scallops are a major source of food.
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