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an order of invertebrates of the subclass of higher crustaceans. The body measures 1–18 mm long (only a few species reaching 35 mm). There are two divisions, the cephalothorax and the narrow abdomen. The cephalothoracic carapace covers the three front thoracic somites and is fused with them. There is a single eye. Of the three pairs of maxillipeds, the first bears complexly constructed gills. In the female, the second and third pairs of maxillipeds have plates that form the brood pouch. The five free thoracic somites each have a pair of two-jointed limbs. Abdominal appendages are found only on the male. There are about 600 species, living primarily in the seas. They serve as food for fish. Members of this order have been acclimatized to some reservoirs (for example, Pseudocuma cercaroides is found in the Dnieper Reservoir).