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Bivalves are most easily recognized by their two shells (hence "bi-valv-ia"). Their
shells are comprised of a pair of laterally- compressed hinged valves and
the pallial cavity surrounds the whole body.
The bivalve shell consists of two valves that are hinged dorsally, usually with shelly interlocking teeth (the hinge), and always with a horny ligament that connects the two valves along their dorsal surfaces and acts to force the valves apart. The interior of the valves contains scars of the various muscles attached to it, in particular the (usually two, sometimes one) adductor muscles that, on contraction, close the valves. The shell can also be internal, reduced or even absent such as in "shipworms" (Teredo). Bivalves typically display bilateral symmetry both in shell and anatomy, but there are significant departures from this theme in taxa such as scallops and oysters. Bivalves lack a head, radula and jaws. Although the plesiomorphic feeding state for bivalves is probably deposit feeding utilizing long labial palps, the ctenidia provide an effective filter feeding mechanism in most taxa with numerous levels or grades of organization. In most bivalves, the mantle pallial cavity contains a pair of very large gills that are used to capture food particles suspended in the inhalant water current |
The visceral mass is primarily situated above the pallial cavity and continues ventrally into
the foot. The intestine is irregularly looped and opens dorsally into the exhalant area. Also
opening into this region are the paired kidneys and, when separate from the kidneys, the gonopores
of the paired gonads. The heart typically lies below the center of the valves and consists of two
auricles and a single ventricle that supplies both anterior and posterior aorta. The nervous system
is made up of three pairs of ganglia. These innervate the musculature, mantle, viscera, ctenidia,
and siphons.
The bivalve foot is modified as a powerful digging tool in many groups while in those that live a permanently attached life (e.g. oysters), it is very reduced. In most bivalves the two halves of the mantle are fused around the edges, with openings anteriorly for the foot and posteriorly for the exhalant opening through which the water is expelled from the pallial cavity. This exhalant also carries waste products and gametes. The inhalant opening, through which water is carried into the pallial cavity, is also posteriorly located in most bivalves, lying just below the exhalant opening. The mantle edge is also where contact is made with the external world and is, consequently where any sense organs are located. These are usually simple sensory cells but in some there are pallial eyes and/or sensory tentacles. |
