External structure of osteochondral fish. Class cartilaginous fish

Cartilaginous fish- this is a class of vertebrates that, unlike bony fish, have a cartilaginous skeleton. The class includes about 630 species, distributed mainly in salt water bodies, some species enter rivers. The body shape of cartilaginous fish is streamlined, elongated (sharks) or flattened, wide (stinging rays).

Features of the external structure

Body parts: head, body, tail.

Chairman, on which are located eyes, doubles nostrils, mouth opening(on the underside of the body and looks like a transverse slit), 5-7 pairs gill slits on the sides of the body (there are no gill covers), a pair of holes - brizkalets. Brizcalca - the first pair of gill slits through which water enters the oropharyngeal cavity during respiration is underdeveloped.

Tulub has paired December And pelvic fins(play the role of rudders, and the abdominal one also performs the function of a paruvial organ) and odd ones - dorsal (front and back).

Tail ends caudal unequal fin(the upper blade is larger than the lower one), which is an organ of movement.

Coverings of the body - leather With big amount mucous glands and placoid scales. Placoid scales - scales, built from a special substance (dentin and enamel), which is part of the teeth of all vertebrates. The scales consist of a dentin plate located in the dermis and covered with a thin layer of tooth enamel that penetrates the epidermis. The skin of cartilaginous fish secretes mucus, which prevents bacteria from penetrating the skin and reduces friction when moving.

External structure of a shark: 1 - head; 2 - torso; 3 - tail; 4 - eyes; 5 - snout (rostrum) ; 6 - gill slits; 7 - anterior dorsal fin; V - posterior dorsal fin; 9 - pectoral fins (doubles) ; 10 - ventral fins (doubles) ; 11 - caudal fin (asymmetric) ; 12 - anal fin

Features of the internal structure and life processes

Musculoskeletal system

skeletal system consists of individual metameric segments, separated by layers of connective tissue; Only the muscles of swimmers and jaws are well differentiated.

Skeleton is cartilaginous , DOES NOT ossify during life and is divided into: a) head skeleton (scull), which unites the brain and visceral (jaws and gill arches) sections; b) ridge, cartilaginous vertebrae, between the bodies of which the remains of the chord are contained; V) fin skeleton

Digestive system consists of: a) oral cavity, in which there are several rows of teeth on the jaws; 6) throats, which turns into a short esophagus; V) stomach, G) small intestine with the ducts of the digestive glands - the liver and pancreas; d) large intestine, which has spiral valve(formation of the colon, slows down the passage of food And increases the absorption internal surface of the intestine) f) rectum, opening in cloaca(expansion of the hind intestine into which the ureters and genital ducts open) f) anus.

swim bladder absent, buoyancy is provided by a fat-like substance that accumulates in the liver.

Circulatory system closed, consists of a two-chambered heart (atrium and ventricle), venous sinus (sinus) and conus arteriosus; The blood in the heart is venous.

Respiratory system presented gills(5-7 gill slits), which open at one end into the oropharyngeal cavity, and at the other to the surface of the body. Gills have: gill arches, gill rakers(filter water and prevent food from entering the gill cavity), interbranchial membranes(separate the gill openings) and gill filaments(carry out gas exchange).

Excretory system consists of paired trunk kidneys and excretory channels ( ureters), opening directly into the cloaca.

Features of the nervous system: V the brain is better developed compared to bony fishes forebrain, and the middle and cerebellum are smaller in size.

Sense organs presented organs of vision (eyes, in which spherical lens and flat cornea, due to which these animals can only see at close range) hearing organs And equilibrium(in the inner ear) olfactory organs(paired olfactory capsules that open outward with the nostrils) side line.

Reproductive system dioecious with clear differences between males and females (sexual dimorphism): males have paruvial organ, testes And vas deferens; females have pairs ovaries And oviducts, opening into the cloaca.

Reproduction sexual, characterized by perfection; internal fertilization; using eggs which have a supply of nutrients and are surrounded by leathery membranes.

Development straight, oviparity and viviparity are typical. Egg production- a method of reproduction in which the egg develops in water, outside the female’s body. Live birth- a method of reproduction of offspring, in which the development of the egg and embryo occurs inside the mother’s body due to its nutrients.

So, cartilaginous fish, unlike bony fish, have the following features: 1 ) cartilaginous skeleton; 2) absence of gill covers and swim bladder; 3) multi-lobed caudal fin; 4) the presence of a spiral valve and cloaca in the digestive system; 5) internal fertilization; 6) direct development and etc.

Variety of cartilaginous fish

Characteristics and diversity of cartilaginous fish

Sharks (about 250 species)	1 Length from 20 cm to 20 m. Streamlined body. Paired fins move only in the vertical plane. Caudal fin of rhiznolopateus. Adapted to fast swimming. Huts. Fecundity is low (from 1 to 500 eggs). Most breed live births	Huge, tiger, whale, dwarf, fox, white, hammerfish, carcharodon, katran, mako
Stingrays (about 350 species)	Length from 3.5 cm to 6-7 m. Body flattened. The paired fins are wide. The dorsal and caudal fins are very small. Sedentary bottom lifestyle. Benthophages. Reproduce by ovovival birth	Manta ray, electric ray, sawfish, sea fox (spiny slope) , sea 1 cat (tailtail)
Chimeras (about 30 species)	Length up to 2 m. Body elongated, fusiform. Almost no scales. Brizcalce is missing. There are leathery gill covers, which is why the gill membranes are reduced	chimera European

The importance of cartilaginous fish in nature and human life

■ Object of fishing (for example, a giant shark, from which oil is extracted, meat from polar sharks and blue sharks - fruit).

■ Used as raw material for industry (shark skin called “fish shagreen” is used to make bags, boots, fish oil, which contains a lot of vitamin D, is extracted from the liver).

■ Some species are dangerous to humans (white sharks, tail sharks, etc.).

■ May cause harm to fisheries (eg herring shark).

Currently, of the most ancient vertebrates, namely fish, more than twenty thousand species live on Earth. The so-called superclass, fish, contains two classes: Bony and Cartilaginous fish. It is these two subclasses (both cartilaginous and vertebrates) that are the oldest vertebrates on Earth. Cartilaginous fish are an earlier “product” of evolution compared to bony fish, and today there are about 730 species of them. Their typical representatives are sharks (more than 200 species), in addition to sharks, the most famous cartilaginous rays are stingrays, catraniforms (representatives are found in the Black Sea), and chimaeras.

The structure of cartilaginous fish allows them to live mainly in salt water bodies, but some species can not only enter fresh water bodies, but also constantly stay in them.

Cartilaginous fish and their features in the choice of food

Representatives of this species are inherently predatory animals, they feed mainly on their smaller “relatives”, as well as bottom-dwelling “inhabitants” of oceans and seas (mollusks, crayfish, crabs), and can, on occasion, also eat jellyfish.

As for sharks, they are unique creatures, voracious predatory fish that do not limit themselves in any way in their choice of food. Cases of shark attacks on humans are often recorded; white and tiger sharks primarily pose a danger to people.

Fortunately for residents and vacationers of the Mediterranean, as well as the Black Sea basin, these waters are cold for white and tiger sharks. And such representatives of sharks as the catanoids that live in these seas do not pose a threat to humans.

Evolution

The peculiarity of cartilaginous fish is that they are the first animals that evolved from “jawless”. Fast movement in water (due to the structure and shape of the body), jaws with sharp teeth, all this allowed them to dominate until the Mesozoic era. But with the development of evolution, they began to be gradually replaced by more adapted higher bony fish.

It is important to note that the class of cartilaginous fish cannot be considered as ancient animals that simply do not have a formed bone skeleton. It has been proven that their ancestors had a skeleton made of bone tissue, but evolution led to the fact that the skeleton was transformed into a cartilaginous one.

The structural features of these fish are that the skeleton is completely cartilaginous, its strength is achieved by the fact that it is saturated with calcium salts, there are no gill covers (each gill slit is an independent opening), the scales are placoid, they completely lack a swim bladder, and fertilization in this fish internal type

Also, interesting feature structure is their buoyancy, it is practically zero, so they must either lie on the bottom (rays) or be constantly in motion, even in sleep (sharks). Well, other external signs practically do not distinguish cartilaginous fish from bony fish, inhabitants of the World Ocean.

Appearance and its features in cartilaginous fish

The appearance of cartilaginous animals is very diverse. Thus, sharks have a mostly spindle-shaped (elongated) shape; on the sides of the head you can see from 5 to 7 gill slits (the operculum is absent). Sharks' mouths, or rather jaws, are located on the ventral side. There are two nostrils in front of the jaws; sharks are believed to have a very keen sense of smell. Well, the teeth located in the jaws are not only unusually sharp, but also covered with enamel.

Rough scales (dantin) are very similar in structure to the teeth of vertebrates (higher). The respiratory system has, as mentioned above, five to seven gill slits. They are separated by septa (interbranchial), on which the so-called “gill filaments” are located; they are penetrated by a network of blood vessels.

To increase the absorption surface, a special spiral-shaped valve runs along the entire length of the intestine.

A high concentration of urea is almost always observed in the blood of cartilaginous fish. In sharks, the kidneys practically work to produce urea (excreted from urine), while it remains in the blood. Compared to mammals, the content of urea in the blood is hundreds of times higher, but for sharks it is necessary, because it is a component of all body fluids, which is necessary for the full functioning of internal organs. And the two-chambered heart, when contracting independently, gives the blood additional impulses.

The organs of touch are well developed (there is a lateral line), as well as the organs of smell. With all their structural features, cartilaginous fish are perhaps the only representatives of the aquatic world that can drown (lack of a respiratory bladder) - this imposes on them the need to constantly be in motion.

Movement is carried out using paired fins, which are located horizontally. In this case, the caudal fin is unequally bladed (the spine extends into the upper blade).

A feature of the digestion of cartilaginous fish is that the digestive system consists of a “mouth” (teeth, jaws), a pharynx in which the gill slits are located, the esophagus and the stomach. There is also an intestine and anus. The digestion process takes place in such a way that both the pancreas and the liver and gall bladder take part in it.

The base of the tail of almost all cartilaginous animals has an extension that serves to remove food debris from the body that, for some reason, has not been poisoned by the body, as well as urine (the so-called cloaca).

The sense organs of cartilaginous fish, as mentioned above, are adapted to the characteristics of their environment (naturally, over tens of thousands of years). Their eyes are capable of determining both color and shape at short distances. The head contains both the organs of hearing and the organs of balance. Well, the oral cavity, in addition to the “terrible” teeth, has sensitive cells whose task is to determine the taste of food.

Well, if we talk about reproduction, the peculiarity of cartilaginous fish is that it occurs in two ways: ovoviviparous and viviparous.

With the so-called ovoviviparous method, fertilized eggs are attached to the posterior section of the oviduct and further development of the fry occurs there. And when hatching occurs, the eggs break and the fry are born.

In some species of cartilaginous fish, such as the stingray, the role of the uterus is played by the posterior section of the oviduct and on its walls there are special outgrowths, thanks to which the stingrays can feed their young with the necessary nutritional fluid.

And with the viviparous method, the female develops specific structures in the posterior section of the oviduct that resemble the placenta of mammals, thanks to which the female can feed her embryo nutrients. This method of reproduction is characteristic of some species of sharks that live at great depths.

It is worth noting that the main advantage of the above methods of reproduction in cartilaginous fish is that, thanks to intrauterine development, the greatest survival rate of the young is achieved.

And if you want to learn more about cartilaginous fish, check out these articles:

About 600 species of cartilaginous fish are known - exclusively marine aquatic organisms. Their skeleton remains throughout life cartilaginous, no swim bladder. The length of these fish is from 20 cm to 15 - 20 m. Cartilaginous fish are common in all oceans and seas except the Caspian. Many species serve as a commercial item, since their meat is edible, and technical fish oil is obtained from the liver.

The class of cartilaginous fish includes sharks And stingrays. The body of sharks is elongated - torpedo-shaped.

The front part of the head forms a growth - rostrum. The sides of the heads open gill slits. Most often there are 5 pairs, and only a few species have 6-7 pairs. Behind the eyes are two openings leading into the pharynx. These are the so-called sprinklers

The body has paired and unpaired fins. Doubles chest And abdominal fins are located horizontally. In males, the internal parts of the pelvic fins form finger-like outgrowths that serve as a copulatory organ. Unpaired fins - dorsal, caudal and anal. Caudal fin unequally bladed. The axis of the skeleton extends into its upper large lobe.

The cartilaginous skin is represented epidermis And dermis (cutis). The epidermis contains numerous glandular cells, secreting secretions onto the surface of the skin. Skin covered placoid scales. Each scale consists of plastic sitting in the skin and a tooth extending from it. The top of the tooth is directed backwards.

The skeleton remains cartilaginous throughout the life of the individual. The spine is divided into 2 sections - trunk and caudal. The vertebrae are cartilaginous. The vertebral bodies are concave anteriorly and posteriorly. The notochord is preserved in the cavities formed between the bodies of adjacent vertebrae. The ribs are attached to the spine. They limit the body cavity from above and slightly from the sides. Well developed specialized muscles.

The alimentary canal of cartilaginous fish begins oral cavity. The jaws limiting the mouth opening bear large teeth. Side walls throats permeate gill slits, they open into it sprinklers. Short esophagus opens at stomach. There is a short line coming from the stomach small intestine, which goes into fat. Hanging inside from the back side fold. It increases the absorption surface of the intestine. Cartilaginous fish have developed pancreas And liver.

The respiratory organs are plate gills. Gill filaments sit on the anterior and posterior walls of the gill slits.

The circulatory system of cartilaginous fish is presented two-chambered heart, arterial and venous vessels. The heart is made up of atria And ventricle. The hematopoietic organ is spleen.

The nervous system of cartilaginous fish consists of head And spinal cord with nerves extending from them.

Of the sensory organs, cartilaginous fish have developed organs smell, sight, hearing. Organ side line It is a channel lying in the skin and communicating with the external environment through many holes. With the help of this organ, fish perceive water vibrations.

Cartilaginous fish dioecious. Males have pairs testes, and in females - paired ovaries. Fertilization internal. Some of them are viviparous, while others lay large eggs.

Shark Squad

The Order Sharks contains about 250 species of sharks. The largest of them is the whale shark, 15-20 m long. It feeds on plankton and small fish, and is completely safe for humans. The reproduction of this shark has been little studied. Another large species is the basking shark, 12 m long (can be up to 15 m). Feeds on plankton. Swims at a speed of 3 km/h.

Representatives:

Gray sharks can be up to 5-9 m long. They live in tropical ocean waters. They reproduce by viviparity. Prolific (30-50 cubs 0.5 m long). They feed on shellfish, fish, and turtles. They often attack people. A typical representative is the tiger shark.

The giant hammerhead shark can be up to 6 m long. It is found in coastal areas. Viviparous (up to 40 cubs). It feeds on mollusks, crustaceans, and fish. Attacks people.

Spiny, or dogfish, sharks range in length from 20 cm to 1 m. They have spiny spines in front of their dorsal fins. Safe for humans. In some countries they are subject to regular fishing.

The order of sharks also includes sea angels up to 2.5 m long, similar to stingrays: the body is flattened, the pectoral fins are large, wide gill slits are located on the sides of the body. Stay in shallow water. Ovoviviparous.

Squad of stingrays

Stingrays are cartilaginous fish, the body of which is flattened in the dorso-ventral direction. Their pectoral fins are large. Due to the flattening of the body, the gill openings are located on the ventral side. The squirts are better developed than those of sharks. Through them, the stingrays lying on the bottom collect water for breathing. These are benthic sedentary animals. The caudal fin is poorly developed. They feed on bottom animals. Their teeth are blunt, adapted for grinding the shells and chitinous cover of arthropods.

Representatives:

Common stingrays have a long tail with a barely visible fin. Stay at a depth of up to 850 m. They are mined to produce fat.

The stingray is characterized by rounded fins that grow together in front of the snout. At the base of the tail on the back is a jagged spine with which it can stab.

Electric stingrays have a round body shape. There is no rostrum, the tail is short, the skin is smooth. The electrical organs are located between the pectoral fins and the head. They can produce voltages up to 70 W.

Sea devils are large stingrays weighing up to 500 kg, up to 6 m wide. Frightened and caught, they can sometimes attack people.

Class bony fish

The class of bony fishes unites the vast majority of modern species. All representatives of the class are characterized by common features: the skeleton is more or less bony; there is an operculum covering the outside of the gill apparatus; most have a swim bladder , which helps fish move up and down; in most bony fish fertilization is external, eggs are small.

From the class of bony fishes, 3 subclasses will be considered: Cartilaginous, ray-finned and lobe-finned.

Cartilaginous fish(lat. Chondrichthyes) - one of two classes of currently existing fish. The most famous representatives are sharks ( Selachii) and stingrays ( Batoidea).

In cartilaginous fish, the skeleton consists of cartilage, which, however, due to the deposition of minerals, can become quite hard. Cartilaginous fish are not, as previously thought, a group of prehistoric animals that did not develop a bony skeleton.

A number of cartilaginous fish are characterized by viviparity and even the formation of a vitelline placenta, which has a number of functions similar to the functions of the real placenta in placentals.

Features of anatomy [edit]

Cartilaginous fish, unlike bony fish, do not have a swim bladder. In this regard, in order not to sink to the bottom, cartilaginous fish must be in motion. And in cartilaginous fish, unlike bony fish, the gills open outward with gill slits; There are no gill covers.

Visual inspection

The body of the katran has fusiform (torpedo-shaped) shape and is divided, like all fish, into the head, body and tail (Fig. 6). The head is not separated from the body; the boundary between the head and the body can be considered to be the last gill slit, and between the body and the tail - cloaca.

The head ends in an elongated snout – rostrum, on the underside of the head there is a large arched mouth located transversely. At the outer corners of the mouth, the labial cartilages, the rudiments of the anterior visceral arches, can be felt under the skin. In front of the mouth, on the right and left, nostrils are visible, leading to the olfactory organ, located in the cartilaginous capsule of the skull of the same name.

Big eyes lie on the sides of the head. Behind them are round holes leading into the pharynx. This sprinklers, or vestigial gill slits. Five vertical gill openings open from the back on the sides of the head (Fig. 6).

The unpaired fins collectively represent an intermittent fold of skin on the back, then bordering the tail and passing to the ventral side in the form of a small anal fin (Fig. 6). Heterocercal the caudal fin consists of a larger upper lobe, into which the end of the spine passes, and a smaller lower lobe (Fig. 6). In a shark, we can distinguish a pair of pectoral and a pair of pelvic fins, which occupy a horizontal position in relation to the body. In males, the internal parts of the ventral fins are somewhat isolated and transformed into peculiar copulatory organs.

The shark's tail is the main organ of forward movement; the pectoral fins move backward and, together with the ventral fins, serve as rudders for depth and rotation.

The entire surface of the shark's body is covered with small, hard placoid scales, they are well felt by hand. In some parts of the body (the base of the fins), the scales can transform into sharp spines. The placoid scales found on the jaws are real. teeth.

On the sides of the shark’s body and tail, a lateral line is clearly visible, which is a series of small external holes: a special channel buried deep in the skin. It contains the skin's sensory organs. The canal of the lateral line organ continues in the form of branches and further onto the head.

Autopsy and general location of internal organs

To examine the internal organs, you need to open the shark. To do this, taking the fish in your left hand, use scissors to make a cut along the mid-ventral line from the cloaca (the base of the ventral fins) towards the head to the base of the pectoral fins. Then, from the extreme points of this opening line, we make deep transverse incisions so that it is easy to turn the side flaps of the body wall to the sides. Without cutting, both elongated rectangular muscle flaps thus obtained are folded to the sides and pinned to the bottom of the dissection bath (Fig. 7).

In the area of ​​the pectoral fins on the ventral side, it is necessary to dissect the shoulder girdle with scissors, and in order to examine its structure in more detail, in the area of ​​the gill apparatus it is useful to prepare the skin with a scalpel.

The dissected shark (Fig. 7) clearly shows the internal organs lying in the abdominal and pericardial cavities. In the abdominal cavity, a mesentery is developed, on which the digestive organs are suspended. The large two-lobed liver partially covers the large curved stomach, near which a dark red liver is suspended on the mesentery. spleen. Differentiated into sections departs from the stomach intestines ending in a cloaca. Near the cloaca, an outgrowth of the intestine is noticeable - rectal gland. In the depths of the abdominal cavity on both sides of the spine there are oblong kidneys.

The abdominal cavity at the upper end is separated by a septum from the pericardial cavity. In the latter, one can distinguish the heart and the internal gill openings located in front of it, leading to the pharynx.

Vertebrates

general characteristics jawless vertebrates

Jawless(Agnatha) - the lowest group of vertebrates. They differ from other gnathostomes by the absence of true jaws and paired limbs (fins). Jawless animals are also often called sacbranchs, since their gills are sac-shaped. The gill skeleton is located outside the bags and has the appearance of a solid lattice, in contrast to the dissected skeleton of fish. Jawless animals are the ancestors of all other vertebrates. Fossil representatives of this superclass appeared in the Ordovician and were widespread in the Silurian and Devonian.

Jawless animals are divided into two classes: cyclostomes and scutes (Ostracodermi). The latter looked like fish and lived in the bottom silt. The front part of their body was protected by a shell of bone scutes. Shield fishes flourished in the rivers and shallow seas of the Devonian, and at the end of this period they died out, giving way to their descendants - more advanced armored fish.

Extinct shield fishes

Hagfish and lampreys

Modern agnathans are represented by the class cyclostomes (Cyclostomata). Their ancestors are probably anaspids, extinct scutes. Cyclostomes have an eel-like body covered on the outside with dense skin that secretes a mucous membrane. There are no paired fins. The notochord persists throughout life; The internal skeleton is cartilaginous. The gills, like those of other jawless animals, are sac-shaped. The heart is two-chambered. Digestive system without stomach.

Cyclostomes have a funnel-shaped mouth, the shape of which is supported by annular cartilage. The mouth contains horny teeth, with which cyclostomes scrape off the soft tissues of their victims. On top of the head is an unpaired median nostril. The sense organs are represented by the organs of taste, smell and hearing; the organs of vision are poorly developed. There are organs of balance (semicircular canals).

All cyclostomes are aquatic animals. They are divided into two subclasses - hagfish And lampreys , which are now often treated as separate classes.

Hagfish(Myxini) are marine animals 45–70 cm long. Their body is so flexible that hagfish can tie themselves in a knot. The nostril is located at the end of the head and communicates with the pharynx. The mouth and nostril are framed by 6–8 antennae. Gill sacs communicate with external environment independently of each other or open with a common hole. The circulatory system is not closed; in addition to the main heart, there are three additional ones. The organs of vision on the head look like eyes covered with skin; light-sensitive cells are also present around the cloaca. The cerebellum is absent.

During the breeding season, hagfish lay dozens of large oval eggs up to 2 cm in size. Fertilization is external. Development is direct, there is no larval stage.

Internal structure of hagfish

The subclass includes one family with more than 40 species, distributed in temperate and subtropical latitudes of the World Ocean (especially off the coast North America). Hagfish are predators that prey on weakened fish. With the help of a powerful tongue and horny teeth, they bite into the victim, eating away internal organs and muscles. Hagfish cause some damage to fisheries by eating fish caught in nets.

Structural features of cartilaginous fish

In ordinary speech, we call all aquatic vertebrates that have gills fish. However, in this sense, the group of “fish” is very heterogeneous in structure and origin, so zoologists divide all fish into several classes. We have already become acquainted with jawless “fish” in the previous paragraphs; now it's time for cartilaginous fish.

Cartilaginous fish(Chondrichthyes) have a cartilaginous skeleton that retains strength due to the fact that it is impregnated with calcium salts. The notochord remains throughout life, but is partially reduced. The skull fuses with the jaws (in whole-headed animals) or forms 1–2 articulations with them (in elasmobranchs). There are caudal and paired ventral and pectoral fins. The mouth is located on the ventral side and is armed with jaws with teeth covered with enamel. There are two nostrils in front of the mouth.

On the outside, the body of these animals is covered with rough placoid scales formed by dentin. Each scale consists of a basal plate, a neck and a crown. In structure it is similar to the teeth of higher vertebrates; most likely, the teeth are derivatives of placoid scales. The respiratory system begins with 5–7 pairs of gill slits. In the intestine, a spiral valve stretches along its entire length, increasing the absorption surface. The swim bladder is absent; Cartilaginous fish are forced to constantly move to avoid drowning. There is a high concentration of nitrogen-containing substances (in particular, urea) in the blood. The arterial cone of a two-chambered heart is capable of independent contraction and provides an additional impulse of blood. The sense organs are represented by the organs of smell, vision, touch (lateral line); some species have electrical organs.

Fertilization in almost all cartilaginous fish is internal. Many of them have a cloaca; in males, several ventral fins form the male copulatory organ. Cartilaginous fish are viviparous or lay eggs.

Sharks, rays and chimeras Shark structure

The class of cartilaginous fishes is divided into two subclasses: whole-headed and elasmobranchs. About 850 living species are known.

Elasmobranchs or transverse - one of the subclasses of cartilaginous fish. The lamellar gills are washed by water through the gill slits, since these fish do not have gill covers. The intestine opens into the cloaca. Elasmobranchs appeared in the Devonian; To date, two superorders have been preserved: sharks (about 500 species) and rays descended from them (about 300 species). Elasmobranchs are found in many seas and oceans (off the coast of Russia - in the Barents, White, Black and Far Eastern seas).

Sharks have a spindle-shaped body, and in most rays it has acquired the shape of a disk, flattened in a horizontal plane and bordered by lateral fins.

The length of the giant shark exceeds 20 m, and its weight is 5 tons. Some fossil sharks (carcharadon) were even larger: several people could fit in their mouths. Many large sharks are predators that are deadly to humans; other sharks are subject to fishing. Dimensions stingrays more modest - up to 6 m; some of them are dangerous to humans due to their electric organs and poisonous tail spines. The ancient Greeks used the common electric stingray to treat gout.

U whole-headed or fused-skull, as already noted, the jaws are completely fused with the skull; in this they strongly resemble bony fish. The gill slits are covered by a fold of skin. There is no cloaca, the anal and urogenital openings are separated from each other. The naked body is up to 1.5 m long, gradually becoming thinner, turning into a long tail.

It is believed that chimeras descended from ancient sharks and are a side branch of evolution. Whole-headed animals have been known since the Upper Devonian; at present, only the order of chimeras exists. Of more than a dozen of his families, only 3 are living today; about 30 species living from the shelf to the great depths of the World Ocean. Chimeras feed on marine invertebrates and fish. They have practically no commercial significance.

Cartilaginous fish are related to ancient armored fish or plate-skinned fish (Placodermi). The primitive antiarchs had their entire body encased in a shell of skin plates; in carnivorous arthrodir the shell covered only the front part of the body. Armored fish are the first jawed fish; their jaws arose from gill arches that moved closer to the mouth, and consisted of pointed bony plates. Armored fish became extinct at the end of the Devonian.

Bony fish

External structure

Bony fish(Osteichthyes) - a class of aquatic vertebrates. All structural features of fish are determined by the environment in which they live. Long-term adaptation to life in water has not left a single unnecessary detail that interferes with movement.

Body sizes range from 1 cm (Philippine goby) to 17 m (herring king); blue marlin weighs up to 900 kg. The body shape is usually elongated and streamlined, although some bony fish are flattened dorsoventrally or laterally, or vice versa have a spherical shape. Translational movement in water is carried out due to wave-like movements of the body. Some fish “help” themselves with their caudal fin. Paired lateral, as well as dorsal and anal fins serve as stabilizer rudders. In some fish, individual fins have been transformed into suckers or copulatory organs.

Externally, the body of bony fishes is covered with scales: placoid (teeth laid “in parquet”), ganoid (rhombic plates with a spine), cycloid (thin plates with a smooth edge) or ctenoid (plates with spines), which are periodically replaced as the animal grows. The growth rings on it allow one to judge the age of the fish.

Different types of scales

Many fish have well-developed mucous glands on their skin; their secretions reduce the force of resistance to the oncoming flow of water. Some deep-sea fish develop luminous organs on their skin, which serve to identify their species, consolidate the school, lure prey, and scare away predators. The most complex of these organs are similar to a spotlight: they have luminous elements (such as phosphorescent bacteria), a mirror reflector, a diaphragm or lens, and an insulating black or red coating.

The coloring of fish is very diverse. Typically, fish have a bluish or greenish back (to match the color of the water) and silvery sides and belly (hardly visible against the background of the light “sky”). Many fish are covered with stripes and spots for camouflage. On the contrary, the inhabitants of coral reefs amaze with a riot of colors.

Internal structure

In accordance with the name, the skeleton of animals of this class is bony; only the most primitive forms have an osteochondral skeleton. The notochord is present only in embryonic stage. The number of vertebrae varies from 16 to 400. The skull consists of a large number of bones; as a rule, the upper jaw is freely articulated with the skull.