Anyone who looks in Webster will find that a fish is a “cold-blooded, strictly aquatic, water-breathing vertebrate: having the: limbs (when present) developed as fins, and typically a long, scaly, somewhat tapering body ending in a broad vertical caudal fin.”
Although most fish do possess scales of one type or another, there are nevertheless a few like the catfish and the ocean sunfish which arc entirely scaleless. The limbs of fish, in the form of fins attached along the side or lower part of the body, arc so formed in some groups that the from pair allow the fish to crawl along the bottom of pond or ocean; or even in a few cases, to scramble about on land.
Breathing is accomplished by gills which arc placed just back of the head and usually covered by gill covers. However, many species have an organ called the air or swim bladder which, in some fish, is connected to the region just behind the mouth. For these forms, the swim bladder functions as a lung, and the fish can come: to the surface: of 1he water and take in air.
It should be noted that whales, porpoises, dolphins, manatees and similar animals are mammals, not fish, although they share the water habitat with fish and in general appearance are quite similar to fish.