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an order of higher crustaceans. The body, which is flattened in a dorsoventral direction, is from 0.1 to 27 cm long (1–2 cm usually). The eyes are sessile. One or, less frequently, two thoracic somites are fused with the head, and one or several abdominal somites are fused with the telson. The first pair of thoracic limbs have been transformed into maxillipeds. The remaining seven pairs are uniramose and are of approximately the same length and structure. The abdominal limbs are platelike and partially transformed into gills. The heart is in the abdominal section. Development is mostly direct. The female carries the embryos and young in a brood pouch formed by processes of the thoracic limbs.
There are approximately 4,500 species of isopods, distributed primarily in seas but also encountered in freshwaters and on dry land. Many species are eaten by fish. Mesidothea entomon damages fishnets and the fish caught in them; species of the genus Limnoria gnaw holes in wood, destroying wooden structures.