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Branchiura (phylum Arthropoda, subphylum Crustacea) Small class of crustaceans, all of which are parasites, and which differ from the Copepoda in having a pair of sessile compound eyes, and a large, shield-like carapace covering the head and thorax. Branchiurans are common, blood-sucking ectoparasites on the skin or in the gill cavities of marine and freshwater fish and a few amphibians. Attachment to the host is via a large, claw-like modification of the first antennae, and by a pair of large suckers derived from the bases of the first maxillae, the rest of the maxillae being vestigial. A sucking mouth cone is formed from the labrum and labium. (In Argulus a hollow spine in front of the mouth cone pierces the skin of the host and injects a supposedly poisonous secretion.) There are no maxillipeds, and the second maxillae are uniramous, well-developed, and have terminal claws. There are four large, biramous, thoracic appendages, armed with swimming setae used when swimming between hosts. There are 75 species.

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