The 25,000 known species of fish are aquatic, generally cold-blooded and gill-breathing vertebrates that swim with the aid of fins. There are three classes of fish; jawless, cartilaginous, and bony fish.
Jawless fish are the most primitive of all. They lack both scales and jaws. The jawless fish are the last survivors of the world's first vertebrates.
Cartilaginous fish's skeleton is made up of cartilage, a substance like gristle, which is not as hard as bone. They have a jaw and teeth, which the teeth is generally hard and sharp. Their bodies are covered with scales. The cartilaginous fish lack a feature of most bony fish, which is a swim bladder. The swim bladder is an organ that helps give a fish buoyancy in the water.
Bony fish have bony skeletons and appeared at the same time as the cartilaginous fish.
Fish Fact For Today