Osteichthyes is a class of vertebrates. Size from 0.7 cm to 6-7 m, mass up to 1.5 tons. The body is covered with bony (cycloid or ctenoid), ganoid or cosmoid scales. There is a bony gill cover, covering the gill cavity. The skeleton is bony or cartilaginous. There are skin bones on the head and shoulder girdle. There is a swim bladder. Fertilization in most species is external, there are also viviparous forms. They live in bodies of water with varying salinity. Russ and Linberg distinguish two subclasses: Sarcopterygii and Actinopterygii.

A group of fishes that includes all bony vertebrates except quadrupeds. Bony fishes have paired fins. The mouth of these fishes is formed by grasping jaws with teeth, gills are located on the gill arches with internal skeletal support, nostrils paired. Traditionally considered as an independent class or supraclass Osteichthyes, but due to the fact that such a group is paraphyletic in relation to quadrupeds, in accordance with the principles of cladistics, the name Osteichthyes is often extended to all bony vertebrates, and bony fishes are considered a non-taxonomic group. In J. S. Nelson's classification, a new taxon Euteleostomi was proposed to designate the monophyletic group of bony vertebrates.

The oldest known fossils of bony fishes date back to the Ludlowian epoch of the Silurian period (about 425 million years ago). It is now disputed from whom they, together with the cartilaginous ones, originated: from placoderms or acanthods. After the Devonian extinction, when placoderms became extinct, bony and cartilaginous fishes began to dominate the seas with no competitors.

Possess scales of ganoid, cycloid or ctenoid type. The scales are covered with mucus.

Consisting of a bony skeleton and muscles, it allows the fish to move in the water.

From the mouth, food passes into the pharynx, from it into the esophagus, and then into the voluminous stomach or directly into the intestine (crucian carp). In the stomach there is a partial digestion of food under the influence of gastric juice. The final digestion of food occurs in the small intestine. In the initial section of the small intestine enters the duct of the gallbladder, liver and pancreatic ducts. In the small intestine, nutrients are absorbed into the blood and undigested food remains are removed through the anus.

Most Osteichthyes have a swim bladder under the spine, filled with a mixture of gases that are released from blood vessels. When the volume of the bladder increases, the average body density of the fish decreases and it floats easily, and when it decreases, it sinks.

Breathing is gill breathing. From the oral cavity, water passes through the gill slits, washes over the gills and exits from under the gill covers. Gills consist of gill arches, which in turn consist of gill lobes and gill stamens. In some species, cutaneous respiration is essential or there are adaptations for air respiration.

The circulatory system of fish is closed, the heart consists of two chambers: atrium and ventricle. From the ventricle to the gills comes a large blood vessel - aorta, branching into smaller - arteries. In the gills arteries form a dense network of tiny vessels - capillaries. After enriching the blood with oxygen (oxygen-enriched blood is called arterial), the vessels are again collected in the artery, which branches into smaller arteries and capillaries. In the organs of the body, oxygen and nutrients enter the tissues through the capillary walls, and carbon dioxide and other waste products from the tissues enter the blood.

Unnecessary substances for the body are excreted from the blood when they pass through the excretory organs, the kidneys. The kidneys have two ureters that carry urine into the bladder and then out through the opening behind the anal opening.

In most species, fertilization is external. In species with internal fertilization, the copulatory organ of males is formed by a modified part of the anal fin. In males in the paired sex glands, testes, spermatozoa are formed, they are called mammary glands. In females, eggs are formed in the ovaries, they are called eggs. Fertilization, as a rule, external, occurs in the water, where the sex cells are removed through special spills. All the processes associated with the laying of eggs, milk and fertilization are called spawning. Spawning is characterized by special mating behavior.

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