Class Pelecypoda
Class pelecypoda, or bivalvia, houses such well-known mollusks as clams, oysters, scallops and mussels. The shell of a bivalve actually consists of two shells, or valves, connected by a thick, horny, elastic ligament that enclose the mollusk. These valves are brought together by either one or two strong adductor muscles and forced ajar by a rubberlike wedge, called risilium.
Pelecypods are typically flattened and bilaterally symmetrical lacking a head, eyes, tentacles, jaw and the characteristic molluscan radula. They have a well-developed foot for digging at one end and, in most species, a tubular siphon - used to carry in water and food particles and discharging waste products - at the other. The nervous system of the pelecypod consists of three ganglia - cerebral, visceral and pedal - connected by two pairs of long nerve cords. Sensory cells are capable of discriminating touch, light and chemical stimuli. A statocyst, comparable to the vertebrate inner ear, is used for balance control.
Bivalves are mostly sedentary creatures with the exception of some, such as the scallop, that can rapidly open and close their two valves. Most bivalves will inch along in the sand or mud with the help of their muscular foot while others stay attached to rocks by thick threads called a byssus. Pelecypods are filter feeders taking in water and food through the siphon. Most bivalves are divided into separate genders although some are hermaphroditic.
Although many mollusks lay their eggs in compact capsules of horny material most pelecypods shed their eggs into the open water where they are fertilized by free floating sperm. Some bivalves? eggs are internally fertilized and incubated within the gills.
The shells themselves are quite simple in structure, consisting of two most often arched valves, usually mirror images of each other. Bivlves may be smooth, or have concentric or radial sculpture, or both (latticed sculpture).