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Bivalve shell
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<p>A bivalve shell is the enveloping <a href="/facts/Exoskeleton/Wlmru4GW">exoskeleton</a> or <a href="/facts/Mollusc_shell/v7rRM9nw">shell</a> of a <a href="/facts/Bivalve/IQDxHVc8">bivalve</a> <a href="/facts/Mollusc/1elgknsQ">mollusc</a>, composed of two hinged halves or <i><a href="/facts/Valve_(mollusc)/hL4IuSUN">valves</a></i>. The two half-shells, called the "right valve" and "left valve", are joined by a <a href="/facts/Ligament/xAf6lB1z">ligament</a> and usually articulate with one another using structures known as "teeth" which are situated along the <a href="/facts/Hinge_line/qTU2DrcR">hinge line</a>. In many bivalve shells, the two valves are symmetrical along the hinge line — when truly symmetrical, such an animal is said to be <i>equivalved</i>; if the valves vary from each other in size or shape, <i>inequivalved</i>. If symmetrical front-to-back, the valves are said to be <i>equilateral</i>, and are otherwise considered <i>inequilateral</i>. </p><p>The bivalve shell not only serves as protection from predators and physical damage, but also for <a href="/facts/Adductor_muscles_(bivalve)/Bf7KxQPU">adductor muscle</a> attachment, which can allow the mollusc to "swim" short distances by flapping the valves. The shell is secreted by a soft part of the molluscan body known as the <a href="/facts/Mantle_(mollusc)/GqSY3XB6">mantle</a> and has several layers, typically made of <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a> precipitated out into an organic matrix. </p><p>Bivalves are very common in essentially all aquatic locales, including <a href="/facts/Saltwater/Tgw5AgL8">saltwater</a>, <a href="/facts/Brackish_water/f6z3UXCa">brackish water</a> and <a href="/facts/Fresh_water/xMP2TxwI">fresh water</a>. The shells of dead bivalves commonly wash up on <a href="/facts/Beach/P8dhamMd">beaches</a> (often as separate valves) and along the edges of lakes, rivers and streams. They are collected by professional and amateur <a href="/facts/Conchology/QrDIVx60">conchologists</a> and are sometimes harvested for commercial sale in the international shell trade or for use in glue, chalk, or varnish, occasionally to the detriment of the local ecology. </p>