As a vertebrate species, fish differ from all the other numerous representatives in that they live predominantly in the aquatic environment, and for breathing a special organ is used – the gills. There is no other representative of the vertebrate class, which would have such a variety of forms of the body, limbs and organs as it exists in fish.
The shape of the fish body can be stretched like a snake or worm, be compressed laterally or have a ribbon look, be strongly flattened or look very massive. The outer covering of fish is also quite diverse: it can be plates, flakes and shields of various shapes, located both along and across their bodies. The most interesting is the coloring of fish, no animal species in the world can compete with them in terms of its brightness and diversity. In addition to the color of all precious stones and metals in this color, it can contain bizarre patterns and even change, depending on the circumstances.
In addition, that the fish are the most perfect swimmers, some of them can make short flights over the surface of the water, crawl along the silt, the ground and even along tree trunks. Few animals can compete with fish and endurance, in search of food or a place for spawning, they can overcome huge distances, almost without eating at the same time.
By the number of their species, their external features, their habits and behavior, fish are of great interest for observation and study, what has long been the concern of mankind.