FISH

Of the more than 25,000 known species, all fish are aquatic, generally cold-blooded and gill-breathing vertebrates, that swim with the aid of fins. Unlike mammals, birds, reptiles and amphibians, which all represent single classes of vertebrate animals within the animal kingdom, fish have three classes that are some what different from one another. These are the jawless, the cartilaginous, and the bony fish.

Jawless fish are the most primitive of all, the last survivors of the world's first vertebrates. They lack both scales and jaws. Dating from over 500 million years ago, only one order now remains.

Cartilaginous fish evolved about 100 million years later. They are ancestors of the oceans' most feared predator today, the shark. As the name suggests, the skeleton of these fish is made of cartilage, a substance similar to gristle, which is not as hard as bone. Possessing a jaw, they also have teeth which are generally hard and sharp. Their bodies are covered with hard scales. However, they lack a feature of most bony fish, a swim bladder, which is an organ that helps to give a fish bouyancy in the water. Pectoral fins, a tail and a characteristic flattened head help to streamline this group.

Bony fish, fish with bony skeletons, appeared at the same time as cartilaginous fish. They are the most numerous group, with about 20,000 species.

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