Most bivalved mollusks have laterally compressed bodies and a shell consisting of two calcareous valves hinged dorsally by interlocking teeth and an elastic ligament. The shell valves are usually similar in size, sculpture, and color, and retain a permanent record of shell growth in their concentric layers. Growth can be traced from the first stage (prodissoconch) at the umbo (rounded or pointed extremity) to the latest stage at the ventral margin. The shells of many bivalve species are rather bland, although some groups (e.g., Pectinidae, Spondyl-idae) show characteristic colors or color patterns. Lining each valve is mantle tissue that secretes the aragonitic or calcitic shell, including its organic outer layer, or periostracum, and the interior aragonitic nacre (mother-of-pearl) present in many species. Between the two shell-mantle layers, an internal mantle (or pallial) cavity contains the ctenidia and visceral mass (containing digestive and reproductive organs), the latter ending in a muscular extensible foot. The foot is usually equipped with a gland that secretes the byssus for attachment to hard substrates.
The posterior mantle edge of many bivalves is fused into incurrent and excurrent siphons that direct water in and out of the mantle cavity for respiration, feeding, and discharge of waste and reproductive products. In most bivalves, water flows in and out of the posterior end; some, however, are secondarily modified for an anterior-posterior flow. Paired adductor muscles connect the inner valve surfaces, enabling the bivalve to close; relaxation of these muscles allows the ligament to open the valves. The head of bivalves is reduced, so that the cephalic eyes, tentacles, and radular teeth typical of most mollusks are absent. The stomach is one of the most complex organs of bivalves, comprising various ciliated sorting areas and ridges as well as the crystalline style, which is an enzymatic rod that rotates against a hardened gastric shield lining the dorsal stomach surface to facilitate extracellular digestion. The bivalve nervous system consists simply of three pairs of ganglia connected by nerve connectives. The circulatory system is equipped with a three-chambered heart, simple vessels, and open hemocoels. The hemostatic pressure of the fluid in these blood cavities is responsible for the expansion and retraction of many bivalve structures, particularly the foot, siphons, and tentacles.
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