Among vertebrates there are more than 25,000 species of fish. Depending on the environment they have many specialized forms, the variety of which at least not less than among all terrestrial tetrapods. The jawless fish are very similar.
Actually the Fish appeared slightly less than 400 million years ago. They are divided into two clearly obosobeni group. The first is a cartilaginous fish, i.e. sharks, rays, chimaeras. The second bone of the fish. Bone fish in turn are divided into ray-finned (most modern species) and lobe-finned or maistraatti that have paired fins similar to limbs of four-legged. In the latter group only 4 species: 3 types deacetylase fish and latimeria from the order Alcantaras.
To the cartilaginous fish is about 850 almost exclusively marine species, the length of which varies from 30 cm to over 15 m. Most — fast floating predators with several rows of sharp teeth in both jaws. Although their skeleton is cartilaginous, the General structure is the same as that of a typical bony fish. There is a real vertebrae and paired fins. The body is many streamlined with the maximum width closer to his chest, the muzzle sharp. Narrow, lobed tail bends from side to side, providing a translational motion. I bollenstreek fish the upper lobe of caudal fin longer than the lower: it goes far back to the spine. Front (breast) paired fins serve as rudders, and also with unpaired dorsal fins – stabilizers, helping the fish to maintain balance when moving. Gill slits can be from 5 to 7 pairs. Water flows freely through the throat and Gill slits extends outwardly, along the way giving oxygen to the blood, washing the delicate Gill filaments. Continue reading