All of us have seen fish. They are very elegant in their colouration and many people like to keep them as pets. A fish may be identified as an aquatic vertebrate animal whose body may or may not be covered by scales and bears two sets of paired fins and many unpaired fins. fish are cold blooded or ectothermic animals which mean that their body temperature keeps on changing with the temperature of water. fish are distributed in all aquatic habitats ranging from the mountain streams to the deepest oceans. About 32,500 species of fish are known and they together form the largest diversity of the organisms in comparison to other classes of vertebrates. fish are also caught by the fish keepers and are kept in aquaria as a source of decoration. They are also associated with movies, cultures and religions.
The term fish applies to an animal which suggests any non-tetrapod craniate that bears gills throughout its life and has fins instead of limbs. Like other vertebrates fish are of different types and are classified according to their characters. The main types of fish found in the modern world are the hagfish, sharks, rays, lampreys, ray-finned fish, coelacanths and lungfish. A fish possesses streamlined body to swim rapidly in the aquatic environment, has gills or accessory respiratory organs for getting oxygen and have paired as well as unpaired fins. The paired fins include the pectoral and the pelvic fins. The unpaired fins are the dorsal, anal and the caudal fins. Generally the body of the fish is covered with scales but there are some fish also which lack scales and are scale less. The jaws are very well developed in the fish. They are oviparous.