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A fish is any member of a group of animals that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered obsolete or paraphyletic by excluding the tetrapods i.e., the amphibians, reptiles, birds and mammals which all descended from within the same ancestry. Because in this manner the term `fish` is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology.