A term that embraces all fishes and fishlike vertebrates. In early zoological classifications fishes, like mammals, birds, reptiles, and amphibians, were ranked as a class of the vertebrates. As knowledge of fishes increased, it became apparent that, despite their common possession of gills and fins and their dependence on an aquatic environment, not all fishes were closely related. 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. The number of classes varies; one reputable but extreme classification recognizes 11 classes of fishes.