Aquatic vertebrates which belong to one of the four classes Agnatha, Placodermi, Chondrichthyes and Osteichthyes. They are cold blooded strictly aquatic cranite vertebrates that have typically an elongated somewhat spindle-shaped body terminating in abroad caudal fin, limbs in the form of fins and a two chambered heart by which blood is sent through thoracic gills to be oxygenated.

Alternative form for fish : pisces.