The first vertebrates
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.
To bone the fish is about 24 thousand species — more than half of all modern vertebrate diversity. This group has perfectly mastered, and salt and fresh water. The structure is almost the same as the cartilaginous fishes, but with some important differences. The skeleton, as the name implies, at least in part of bone, including fin rays. The caudal fin is usually, but not always, externally symmetrical. Most bony fish have a swim bladder, regulating the volume of which can be supported at different depths of the so-called neutral buoyancy, that is, not to float and not sink. This ability still hang in place – an important advantage in comparison with those of cartilaginous fishes, which, ceasing to move, go to the bottom. The throat on each side of the body in bony fish usually penetrate 4 Gill slits — they open in a common branchial cavity, covered by a bony Gill cover. The cartilaginous fishes Gill cover has only a few chimeras. The body of most bony fish are covered with overlapping each other like roof tiles, thin flattened scales, strongly different from the odontoid (placoid) scales of sharks.
Bony fish have mastered underwater all ecological niches. Some species feed on plants, other plankton, otziva it using a comb-like Gill structures (arches, covered with stamens). Many — quick hunters with sharp teeth, and some, like flatfish, waiting for its prey in ambush, blending into the background. Some even manage to catch outside the water insects make the jump or hit the victim with plants, with the power of firing into it a stream of water from his mouth, as, for example, bryzguny. Other feed from the bottom sludge.
In many species, the female spawns just spawn in the water, where it seeds the male. Take care of offspring is rare, so from a huge number of eggs get a little Mature individuals. However, the males of some fish (particularly cartilaginous all) of the pelvic fins is formed of a pair of copulatory organs, internal fertilization is accomplished. The development of the eggs in this case can occur inside the female, culminating in asciiportal or a real live birth. Most bony fish belongs to the subclass of ray-finned, and 4 species to the subclass of lobe-finned. The latter is represented by latimeria marine and 3 freshwater species of lungfish, which breathe and gills, and a kind of light (i.e., atmospheric air). Light role does a cellular swimming bladder connected to the esophagus. Two species of lungfish live in temporary ponds and their drying fall into a long period of “hibernation”, buried in bottom mud.