Fish for Life

Fish cold-blooded, limbless, completely aquatic vertebrates, having gills, commonly fins, and typically elongated torpedo-shaped body mostly covered with scales. Fish are usually grouped into four classes, three living (Agnatha, Chondrichthyes, and Osteichthyes) and one extinct (Placodermi): 1. Agnatha, the primitive jawless fishes, including the cyclostomes (lampreys and hagfishes) and extinct armoured fishes known as ostracoderms, none of which has gill arches (bony frames for the gills); 2. Placodermi, extinct armoured fishes with jaws; 3. Chondrichthyes, the cartilaginous fishes, including sharks, rays and their allies, all of which have a predominantly cartilaginous skeleton; and 4. Osteichthyes, the bony fishes, including the great majority of food and game fishes.
A number of aquatic invertebrate animals and groups have common names compounded with the term fish, which are unrelated to true fish. For example cuttlefish (cephalopod mollusks, relatives of the octopus), jellyfish (coelenterates), starfish (echinoderms), and shellfish (mollusks such as the oyster and clam), and the crustacean arthropods such as the crayfish.
The bony fishes are divided into two groups: the fleshy-finned fish (Crossopterygii) and the ray-finned fish (Actinopterygii). The latter group includes over 95% of all living fish species. The earliest bony fishes were fleshy-finned. They evolved during a period of widespread drought and stagnation and gave rise to the amphibians (the first terrestrial vertebrates) on the one hand, and to the ray-finned fish on the other. The only surviving fleshy-finned fishes are the lungfishes and one species of Coelacanth. These fishes retain some of the traits of ancestral bony fishes: fleshy fins with supporting bones (precursors of the limbs of land vertebrates), internal nostrils, and lungs. Ray-finned fishes are now predominant in both fresh and marine waters.
There are about 22,000 species of fishes worldwide, in about