Animal Kingdom - Pisces
PiscesClick thumbnail to view full-size
All species of fishes are included under Pisces. As of now, there are around 20,000 species of fishes. They are vertebrate animals and evolved some 500 million years ago. Fishes have boat shaped bodies. They swim with the help of their fins. Respiration in them takes place through gills. They are cold-blooded, that is, the temperature of their blood varies with the rise and fall in the temperature of environment. Most fishes lay eggs while others give birth to their young ones.
Fishes are divided into four categories- Cyclostomate, Placodermi, Chondrichthyes and Osteichthyes.
Cyclostomate fish have round mouth which works as a sucker. They have neither jaws in the mouth nor scales on their body. There is only one nostril on the head. Petromyzon, a cyclostomi fish, is found in the seas of North America and Europe.
Placodermi means plate-skin. Fishes of this category have a pair of jaws and sings. Some fish of this category resemble the shark.
The fishes in the Condrichthyes category have inner skeleton, made of cartilage. They have jaws in the mouth and a special kind of scales on their body. The external nostril and mouth are located on the lower part of the head. Scoliodon saw fish and torpedo belong to this category.
The Osteichthyes category of fishes has an inner skeleton made of bones. The opening of their mouth is located on the from part of the head, while the nostrils lie on their head’s back. This category is further sub-divided into: (a) The choaniachthyes; and (b) The Neopterygii. The choanichthyes have lungs to breathe and, therefore, are also called lungfish. The land vertebrates owe their evolution to the Choanichthyes. The Neoceratodus, a fish of this group, is found in Australia. The second sub category, neopterygii, includes developed fish. Their entire skeleton is made of bones. The gills on either side of their body are covered by a plate. Labes, seahorse and heteropinuestic belong to this sub category. Some fishes like eel generate electric current. Some fishes like lantern fish produce light.