Fish belongs to the vertebrate group, consisting of 5 types of vertebrates, such as birds, mammals, rodents, marsupials, primates, fish, amphibians, reptiles.
Fish were among the first developed vertebrates, the earliest fossils of fish being discovered were dated as being 400 millions years old.
The skeleton of fishes is made of bone or cartilage. The cartilage skeleton increase the weight of the fish, producing sinking, hence, the fish must swim to be able to float. Fishes having cartilage skeleton are sharks and rays. Unlike cartilaginous fishes, bone fishes may float, even if they are not swimming, since they possess a gas-filled bladder that can be deflated or inflated, at need.
Fishes breathe through gills, which allows them to obtain the oxygen and exchange gases, in water, while primate have developed lungs that allow them to breathe.
Fish locomotion is allowed by means of tail fin, whose muscles push the water back and the fish forward. Fish locomotion by means of fin is adapted to water habitat, as the presence of wings in birds allows their locomotion through the air, by flying. While fishes are cool-blooded and scaly, animals are warm blooded and hairy.