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Anomalosdesmata
Cryptodonta
Heterodonta
Paleoheterodonta
Palaeotaxodonta
Pteriomorphia
and see text Bivalves are exclusively aquatic; they include both marine and freshwater forms. However some, for instance the mussels, can survive out of water for short periods by closing their valves. Bivalves are unique among the molluscs for lacking a radula; they feed by siphoning and filtering large particles from water. Some bivalves are epifaunal that is, they attach themselves to surfaces in the water, by means of a byssus or organic cementation. Others are infaunal they bury themselves in sand or other sediments; these forms typically have a strong digging foot. Some bivalves, such as scallops, can swim. The systematic layout presented here is according to Newell's 1965 classification based on hinge teeth morphology. There exists no robust phylogeny, and due to the plethora of fossil lineages, DNA sequence data is only of limited use should the subclasses turn out to be paraphyletic. The monophyly of the Anomalosdesmata is especially disputed, but this is of less consequence as that group does not include higher-level prehistoric taxa.
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