Fish are divided into two classes – Cartilaginous fish (sharks, rays) and Bony fish (sturgeon, salmon, herring, crucian carp, perch, pike, etc.). The main criterion for such a division is the substance that makes up the internal skeleton of fish (cartilage or bones).
Class Cartilaginous fish
The main characteristic features of cartilaginous fish are:
* cartilaginous skeleton, preserved throughout life;
* lack of a swim bladder;
* no gill covers, gills open outward by 5–7 * gill slits;
* placoid scales;
* internal fertilization;
* most species live in salty waters, although there are * freshwater ones.
Class Bony fish
Signs of bony fish:
* there are bony gill covers covering the gill slits;
* internal skeleton, at least partially, bony;
* fins are supported by bony rays (except for lungfish);
* scales ganoid or bony (but not placoid);
* there is either a swim bladder (which may be underdeveloped for the second time), or (in rare cases) a lung.
* external fertilization, caviar is small.
Bony fish are subdivided into bone-cartilaginous, lung-breathing, cross-finned and bony fish.
Most species of fish that live in almost all water bodies of the Earth belong to bony fish. This group includes fish of the orders of herring (herring, sardines, anchovies, two species of which are called anchovies), salmon (noble salmon, or salmon, chum, pink salmon, chinook salmon, sockeye salmon, whitefish, grayling, smelt), carps (chub, roach, bream, ide, dace, asp, carp, crucian carp), catfish (catfish), cod-like (cod, navaga, haddock, blue whiting, pollock, burbot), flatfish (flounder, halibut).
Bone-cartilaginous, or sturgeon
Bone-cartilaginous, or sturgeon, fish have a bone-cartilaginous skeleton with a well-developed chord, there are gill covers, a swim bladder. Along the body of sturgeon there are 5 rows of bone plates, between which there are small bone plates.
Sturgeon are anadromous fish of the northern hemisphere. They live up to 50-100 years or more. These fish are widely known for their particularly tasty meat and black caviar.
Examples: beluga, sterlet, Russian sturgeon.
Lungs are an ancient group of fish. There are only 6 species of them, for example, the Australian horntooth, African and South American flake.
In lungs, the notochord is preserved throughout life, the bodies of the vertebrae do not develop, which indicates their antiquity. Along with the gills, these fish have lungs that have developed from the swim bladder. The structure of the heart has also changed: the atrium is divided by an incomplete septum into the left and right halves. The right half receives blood from the gills, and the left half receives blood from the lungs.
The Australian horntooth lives in rivers heavily overgrown with plants. In the summer, when the reservoirs become shallow, he completely switches to breathing atmospheric air.
Other representatives of liverworms – African flake (up to 2 m long) and South American flake (up to 1 m), when water bodies dry up, burrow into silt and hibernate.
Cyst-finned fish are an ancient group of fish close to lungfish. The swim bladder has become a paired lung, the nostrils communicate with the oropharynx. Currently, one modern representative is known – coelacanth.
Latimeria is a large fish (up to 180 cm long). Its body is covered with massive scales, and fins (especially paired ones) look like fleshy lobes. Coelacanths live at the bottom, at a depth of up to 400 m (possibly deeper), in the southwestern part of the Indian Ocean.