Chondrichthyans are a class of aquatic vertebrates. The skeleton is cartilaginous, calcified, without bone tissue. Fin rays in the form of numerous horny filaments. Scales, if present, are placoid. Gill slits 5-7 pairs. Gill cover absent. Swim bladder absent. Intestine with spiral valve. Heart with arterial cone. There is a cloaca. Fertilisation is internal. Live birth, oviparous, oviparous. Two subclasses, Elasmobranchii and Holocephali. 13 orders, 49 families, about 750 species. Reach up to 20 metres. Inhabit all seas and oceans from the Arctic to the Antarctic, some in fresh water.

In chondrichthyans, the skeleton is made up of cartilage, which can become quite hard due to mineral deposits. Chondrichthyans are not, as previously thought, a group of prehistoric animals that did not develop a bony skeleton.

Distinguishing features:

Placoid scales;

Cartilaginous skeleton;

Absence of swim bladder;

Absence of gill covers;

Fins are horizontal;

Mouth opening is ventral;

metabolic product is urea.

A number of chondrichthyans are characterised by live birth and even the formation of a yolk placenta, which has a number of functions similar to those of the true placenta in placental mammals.

Unlike bony fishes, chondrichthyans do not have a swim bladder. As a result, they must be in constant motion to avoid sinking to the bottom. In addition, unlike bony fishes, chondrichthyans have gills that open outward, rather than gill covers, except in chimaeras, where the gill slits are covered by a fold of skin. The number of gill slits and arches is usually 5, more rarely 6-7. The gills consist of a gill arch and gill plates. There are also splash gills, which are rudimentary gill slits. Unlike bony fishes, gills do not secrete products of nitrogen metabolism and salts. Sharks breathe through the mouth, and in rays, water enters the oropharyngeal cavity through the open valves of the sprays, and exits through the gills when they are closed.

Cartilaginous fishes have placoid scales, which are homologous to the teeth of all vertebrates; in the course of evolution, the placoid scales moved to the jaws and became the teeth of sharks and rays. Placoid scales are composed of dentin, which forms the base of the scales and is covered by enamel. The chemical composition of shark dentin and enamel is similar to that of human teeth. Lost placoid scales are not replaced, but their number increases as the fish grows. The fin spines of some chondrichthyans (e.g. Black Sea Catrana) are also transformed into placoid scales.

The skull is entirely cartilaginous. It is divided into the following sections: rostral, olfactory, ocular, occipital, auditory and occipital. The radial or pterygophores, which form the basis of the dorsal and anal fins, are cartilaginous; the fin rays are elastotrichia - skin elastin filaments. The caudal fin is heterocercal, represented by elastotrichia supported by the vertebrae. The shoulder girdle is represented by a cartilaginous arch with a scapular (dorsal) and coracoid (ventral) part. The limb skeleton is made up of three basal bones, the radial bones, which are made up of two or three cartilages, and the elastotrichia, which support the dermis. The pelvic girdle is represented by a cartilaginous plate to which the pelvic fin skeleton is attached. It consists of a long basal ray, radials and elastotrichia. In males, the posterior ends of the basal and radial rays form pterygopodia - an apparatus for copulation. From the trunk musculature of bony fishes, the muscles are characterised by weak differentiation. Muscles have a high urea content: up to 1.5-2.8% in marine species and up to 0.7% in freshwater species.

Insemination appears to be internal in almost all chondrichthyans. The males have a special copulatory organ - the pterygopodium - formed from the rays of the pelvic fins. Chondrichthyes are viviparous or oviparous. Most are predators. Most live in the sea, only a few plastinodzhaberny adapted to life in fresh water. Fossils of chondrichthyans have been known since the Devonian period.

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