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; phylum Arthropoda, subphylum Crustacea)

Class of widely distributed marine and freshwater crustaceans in which the entire body is enclosed in a rounded or elliptical, bivalved carapace, the outer layer of which is impregnated with calcium carbonate. A cluster of transverse adductor muscle fibres closes the valves, the muscles being inserted near the centre of each valve. The trunk is greatly reduced, so that the head accounts for much of the body. Trunk appendages are usually reduced to two pairs and there is no external segmentation of the trunk. By contrast, the head appendages, especially the antennules and antennae, are well developed and are the means of locomotion, even in the remarkable terrestrial species of Mesocypris, which plough through damp humus in the forests of southern Africa and New Zealand. Ostracods show a wide range of feeding types, and the group includes filter feeders, carnivores, herbivores, and scavengers. The sexes are separate, and parthenogenesis is common in freshwater forms. There are more than 2000 living species and the group is known from more than 10 000 fossil species, ranging from the early Cambrian to the present.

Subjects: Earth Sciences and Geography — Zoology and Animal Sciences.

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