Bailey, Reeve M. Division of Fishes, Museum of Zoology, University of Michigan, Ann Arbor, Michigan.
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A term that embraces all fishes and fishlike vertebrates. In early zoological classifications, fishes (as well as mammals, birds, reptiles, and amphibians) were ranked as a class of the vertebrates. As the knowledge of fishes increased, it became apparent that not all fishes were closely related, despite their common possession of gills and fins and their dependence on an aquatic environment. At least five groups of fishes with modern descendants were already established before the tetrapods appeared. Not only are these groups older, but some are decidedly more divergent structurally than are the four classes of tetrapods. For these reasons, several classes of fishes are now recognized, although there often is variation. See also: Chordata; Tetrapoda; Vertebrata
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