Quadravalvia
Quadravalvia ("four valve") is a class of haveing four-part shells, being symmetrical along the hinge lines. The class has 80,000 species. The class is exclusively aquatic; they include both marine and freshwater forms. They lack a radula and feed by siphoning and filtering large particles from water. Some are epifaunal, attaching themselves to surfaces in the water, by means of a byssus or organic cementation. Others are infaunal, burying themselves in sand or sediments. These forms typically have a strong digging foot.
Quadravalves have very simple nervous system, considting of two cerebropleural ganglia - the pedal ganglia, controlling the foot, are at its base, and the visceral ganglia under the posterior adductor muscle. Epifaunal species also have a siphonal ganglia.
The sensory organs are likewise poorly developed, and are largely a function of the posterior mantle margins. The organs are usually tentacles and most are typically mechanoreceptors and chemoreceptors. There are also light-sensitive cells in all speciece able to detect shadows falling on the animal. Additionally, statocysts within the organism help sense orientation, which then be corrected if need be.
