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Ostracod
Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typically around in size, but varying from in the case of ''Gigantocypris''. Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell". The hinge of the two valves is in the upper (dorsal) region of the body. Ostracods are grouped together based on gross morphology, but the group may not be monophyletic; their molecular phylogeny remains ambiguous.
Ecologically, marine ostracods can be part of the zooplankton or (most commonly) are part of the benthos, living on or inside the upper layer of the sea floor. Many ostracods, especially the Podocopida, are also found in fresh water, and terrestrial species of ''Mesocypris'' are known from humid forest soils of South Africa, Australia, New Zealand, and Tasmania. They have a wide range of diets, and the group includes carnivores, herbivores, scavengers and filter feeders.
As of 2008, around 2000 species and 200 genera of nonmarine ostracods are found. However, a large portion of diversity is still undescribed, indicated by undocumented diversity hotspots of temporary habitats in Africa and Australia. Of the known specific and generic diversity of nonmarine ostracods, half (1000 species, 100 genera) belongs to one family (of 13 families), Cyprididae.〔 Many Cyprididae occur in temporary water bodies and have drought-resistant eggs, mixed/parthenogenetic reproduction, and the ability to swim. These biological attributes preadapt them to form successful radiations in these habitats.
==Etymology==
''Ostracod'' comes from the Greek ''óstrakon'' meaning shell or tile. The word "ostracize" comes from the same root due to the practice of voting with shells or potsherds.〔Definition of (ostracod ) at dictionary.com.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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