Fins

Fins (allied to Lat. pinna or penna; see letter F), a term vaguely applied to outgrowths and limbs in aquatic animals. Thus, the paired fins of fishes are true limbs, the unpaired median fins are outgrowths. The term is similarly extended to cetaceans; while a still wider usage includes locomotor expansions in cuttle-fishes and some other molluscs. See FISHES.