A fish is any gill-bearing aquatic vertebrate (or craniate) animal that lacks limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups.
Because the term is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification.
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