Cartilaginous fish class skeleton representatives structure difference swim bladder systems signs organs gill covers characteristics
Latin name Condrichtityes
general characteristics
A relatively small modern (about 600 species) group of fish, the organization of which combines primitive features (mainly) with progressive features.
Skeleton Cartilaginous fish remains cartilaginous for life. The shoulder girdle of the limbs is represented by a solid cartilaginous arch, covering the body from the sides and below.
Cartilaginous fish skin covered with the most primitive type of scales - placoid (rarely bare skin). There are relatively many gill slits (5-7), and each slit opens outwards with an independent slit-like opening (the exception is the frilled shark and chimaeras). The paired fins are arranged horizontally. There is no swim bladder.
Along with the above, cartilaginous fish are characterized by such progressive features as the presence of nervous substance in the roof of the forebrain, internal insemination, and in many species, viviparity.
The body sizes of cartilaginous fish are very different: from 20 cm to 15 and even 20 m. They are distributed in all seas (except the Caspian) and oceans, mainly in tropical latitudes. Some species enter rivers. In some places they are important in commercial fishing.
Cartilaginous fish are divided into two subclasses:
- Elasmobranchii
- Whole-headed, or chimera (Holocephali).
Subclass Elasmobranchii
Cartilaginous fishes such as sharks and rays belong to this subclass. They are characterized by the presence of placoid scales. Each external gill opening opens independently on the surface of the body. There is a cesspool. Due to the presence in most species of a growth at the anterior end of the head, the so-called rostrum, the oral opening is located on the underside of the head in the form of a transverse slit. The skull is often hyostylic, less often amphistylous.
To understand in more detail the features of the organization of elasmobranchs, let us consider the structure of a shark.
Appearance.
The general body shape of most sharks is elongated and fusiform. The head bears a rostrum in front. On the sides of the head gill slits are visible, the number of which is usually 5 on each side; only a few modern sharks have up to 6-7 of them (Chlamydoselachidae and Hexanchidae). Behind the eyes are two openings leading into the pharynx. These are the so-called squirters, representing the rudiments of gill slits located between the maxillary and hyoid arches. The cloaca is located on the lower surface of the body at the root of the tail. The caudal fin is unequally lobed. The axis of the skeleton extends into the upper, large lobe of the fin. This type of caudal fin is called heterocercal.
Paired limbs are represented by pectoral and pelvic fins, which are located horizontally. In males, the inner parts of the ventral fins form finger-like projections that serve as copulatory organs.
Skin
The epidermis is multilayered with numerous glandular cells that secrete their secretions onto the surface of the skin. Corium is dense and fibrous. The skin is covered with placoid scales. The latter in the diagram represent a plate lying in the fibrous layer of the skin and a tooth sitting on this plate; the top of the tooth is directed backwards. The scales develop in the corium and are composed of the bone substance osteodentin, similar to the dentin of the teeth of other vertebrates.
The outside of the scale tooth is covered with a thin sheath of enamel, which is a derivative of the gland of the same name, which is formed during the development of scales in the inner layers of the epidermis. Scales cover the entire body of the fish and extend along the edges of the mouth opening on the jaw. Here it is larger than on other parts of the body and serves as teeth. This circumstance, as well as the similarity in the development of teeth and placoid scales, emphasize their homology.
Cartilaginous fish Skeleton
The skeleton remains cartilaginous for life, although deposits of calcareous salts are observed in some parts of it.
The axial skeleton of cartilaginous fish consists of the vertebral column and the brain part of the skull. The spine is divided into only two sections: the trunk and the caudal. Each vertebra consists of a vertebral body, from which extend the upper arches, which have the appearance of an arch, limiting the spinal canal; the superior spinous processes extend from above these arches; The lower arches extend from the inferolateral part of the vertebral bodies, which form the hemal canal in the caudal spine, and in the trunk they are represented by short transverse processes extending to the sides.
The vertebral bodies are concave anteriorly and posteriorly (amphicoelous vertebrae). The notochord is preserved in the cavities formed between the bodies of adjacent vertebrae. In addition, the notochord is also preserved within the body of each vertebra, where it lies in a narrow foramen located in the center of the vertebral body.
The ribs attached to the spine limit the body cavity only from above and only slightly from the sides.
The cranium consists of the braincase, sensory organ capsules and rostrum. All sensory organ capsules are paired; they are tightly fused with the main plate of the skull. A cartilaginous roof of the braincase is formed, which, however, is not complete, since in its anterior part there remains a large area covered by a membrane (anterior fontanelle). By growing into the skull of the first vertebra, the occipital region of the skull is formed.
The rostrum of cartilaginous fishes consists of three rod-shaped cartilages extending from the front of the skull
The visceral skeleton consists of the branchial arches, the hyoid arch and the maxillary arch.
The jaw arch is made up of two paired cartilages. The upper pair of cartilages, called palatoquadratum cartilages, acts as the upper jaw. The lower pair of cartilages, which acts as the lower jaw, are called Meckel's cartilages (cart, meckeli). In most sharks, the palatoquadrate cartilage is attached to the brain skull only in its anterior part. The posterior section of this cartilage is not directly connected to the skull, but is attached to it through the upper element of the hyoid arch - the hyomandibular cartilage (hyostylium). Only in a few species the posterior part of the palatoquadrate cartilage is directly attached to the skull. However, in this case, the hyomandibular cartilage articulates with the palatoquadrate cartilage (amphistyly).
Anterior to the jaw arch are two pairs of small cartilages called labial cartilages, which represent the remains of the first and second visceral arches of ancient, primitive vertebrates. Consequently, the jaw arch of modern sharks is not the first, but the third visceral arch.
The hyoid, or hyoid, arch consists of paired hyomandibular cartilage (hyomandibulare), paired hyoid cartilage (hyoideum) and unpaired cartilage - copula. The hyomandibular cartilage articulates with the skull and hyoid, as well as with the jaw arch. The copula connects the hyoids of the left and right sides.
The branchial arches, of which there are usually five, consist of four paired cartilages and one unpaired one, lying on the ventral side of the arch and connecting its left and right parts. Along the posterior edge of the gill arches there are cartilaginous rays that serve as a support for the interbranchial septa.
Limb skeleton
The skeleton of the paired limbs of cartilaginous fish is divided into the limb girdle, which lies in the body body and serves as a support for the limbs, and the skeleton of the free limb.
The girdle of the forelimbs (pectoral fins) is represented by a cartilaginous arch lying freely in the thickness of the muscles and its apex facing the ventral surface. In the middle of each (left and right) half of the arch there are protrusions to which the skeleton of the free limb is attached. The sections of the belt lying upward from the indicated protrusion are called scapular, and those lying downward are called coracoid.
The skeleton of the forelimb itself consists of three sections. The fin is based on three cartilages - basalia, which are attached to the belt. Rod-shaped cartilages, radials, arranged in several rows, are attached to the basalia on one side. Such a fin is called a uniserial fin (in contrast to a biserial fin, in which the radials are located on both sides of the basalia. Finally, long and thin elastoidin filaments of cutaneous origin are attached to the radials.
The girdle of the hind limbs (pelvic fins) consists of unpaired cartilage located across the body in front of the cloaca. The skeleton of the ventral fin itself contains only one or two basalia. The radials are attached to their outer edge.
The skeleton of unpaired fins consists of radials and elastoidin filaments.
Digestive organs
The jaws that limit the mouth opening usually bear rather large teeth. The oral cavity continues into the pharynx, perforated by gill slits. The previously mentioned squirts, representing rudimentary gill slits, also open into the pharynx. The short esophagus opens into an arched stomach, from which a short small intestine also extends. The pancreas lies in its mesentery. The large intestine has a significant diameter and is equipped with a spiral valve. The latter has the same meaning in sharks as in cyclostomes. But in its structure, attention is drawn to the steeper convolutions of the valve fold, due to which they form a greater number of whorls than in cyclostomes. The bilobed liver is equipped with a gallbladder; The bile duct flows into the initial part of the small intestine.
As can be seen, the general morphological feature of the digestive tract of cartilaginous fish, which characterizes its complexity, is, firstly, its greater division into sections than in cyclostomes and, secondly, the general elongation of the entire tract, which is associated with the formation of bends in the digestive tube. Let us remember that cyclostomes have a straight digestive tube.
In the body cavity, near the convex side of the stomach, lies the spleen.
Respiratory system
Each gill slits opens at one end into the pharynx, and at the other - independently on the surface of the body. The gill openings are delimited from each other by wide interbranchial septa, in the thickness of which lie cartilaginous gill arches. The gill filaments sit on the anterior and posterior walls of the gill slits, where they form the semi-gills.
Unlike cyclostomes, the gill filaments of fish are of ectodermal origin.
Circulatory system
The heart of cartilaginous fish is two-chambered, consisting of an atrium and a ventricle. Adjacent to the atrium is a wide, thin-walled venous sinus into which venous blood flows. Adjacent to the final (along the blood flow) part of the ventricle is the arterial cone, which is essentially part of the ventricle, although outwardly it looks like the beginning of the abdominal aorta. The belonging of the conus arteriosus to the heart is proven by the presence in it (as well as in other parts of the heart) of striated muscles.
The abdominal aorta originates from the conus arteriosus, giving off five pairs of branchial, arterial arches to the gills. Parts of the arches up to the gill filaments are called afferent gill arteries, while their parts coming from the gills and carrying already oxidized blood are called efferent gill arteries. The latter flow into paired longitudinal vessels - the roots of the aorta, which, merging, form the main arterial trunk - the dorsal aorta, which lies under the spine and supplies blood to the internal organs. The carotid arteries, carrying blood to the head, extend forward from the roots of the aorta (or from the first pair of efferent branchial arteries).
Venous blood from the head collects in the paired jugular veins (otherwise called the anterior cardinal veins). From the body, blood collects in paired cardinal veins, which at the level of the heart merge with the jugular veins of the corresponding side, resulting in the formation of paired ducts of Cuvier, which flow into the venous sinus. The cardinal veins form the portal circulatory system in the kidneys. From the intestines, blood flows through the intestinal vein, which forms the portal circulatory system in the liver. From the liver, blood flows through the hepatic vein (often a pair) into the venous sinus.
Nervous system
The brain is relatively large. Nervous substance on the forebrain hemispheres is present not only on its bottom and sides, but also on the roof of the brain. The midbrain is well developed, the cerebellum is large.
There are eleven pairs of head nerves. They have typical origins and areas of innervation.
Sense organs
The olfactory sacs are paired and end blindly. There is no parietal “eye”. The paired eyes have a structure typical of fish: their cornea is flat, the lens is spherical, there are no upper and lower eyelids. A few species have a nictitating membrane that can stretch the eyeball from its lower inner edge to the upper one. The organ of hearing is represented only by the inner ear - the membranous labyrinth. There are three semicircular canals. The lateral line is well defined. For the majority, it is a channel lying in the skin and communicating with the external environment through fairly frequently located openings. In primitive sharks (Chlamydoselachus), the lateral line represents a channel open at the top - a groove.
Urogenital organs.
The excretory organs are the primary kidneys - the mesonephros. The paired Wolffian canal, which flows into the cloaca, functions as their excretory ducts.
Most ovaries are paired. Paired oviducts (Müllerian canals) are not connected to the ovaries, but open into the body cavity in close proximity to them. The shell glands are located in the upper part of the oviducts. The expanded lower sections of the oviducts open into the cloaca. The reproductive and urinary tracts of females are separated along their entire length.
The paired testes are connected by a system of tubules to the anterior parts of the kidneys. These tubules pass through the substance of the kidneys and flow into the Wolffian canals. Thus, these latter serve not only as ureters in males, but also as vas deferens. The anterior parts of the kidneys do not have an excretory function and serve as appendages to the testes. Only the posterior parts of the kidneys function as excretory organs.
The vas deferens in its lower section are expanded and form thin-walled seminal vesicles. Both Wolffian canals empty into the urogenital sinus, which opens into the cloaca.
Classification General characteristics of units
Order Sharks (Selachoidei)
The body shape is usually fusiform and only in a few species is it somewhat flattened. Gill slits open on the sides of the front part of the body. The fins are developed proportionally. Numerous teeth with sharp tips. Body sizes are varied - from 20 cm to 15 m (sometimes more). About 10 families are known. Distributed in all oceans and seas (absent in the Caspian Sea). The total number of species is about 250.
Order Stingrays (Batoidei)
Cartilaginous fish with a dorsoventrally flattened body and highly developed pectoral fins. Due to the flattening of the body, the gill openings are located on the ventral side of the body. The squirts are better developed than those of sharks. The stingrays lying on the bottom collect water for breathing through splashers. Stingrays have adapted to a sedentary bottom lifestyle. The caudal fin, the most important organ of forward movement, is very poorly developed in them. Stingrays often feed on sedentary bottom animals - mollusks and crustaceans. Their teeth are usually blunt, adapted for grinding shells and chitinous covers of arthropods.
Fish appeared long before the emergence of humans as a species on our planet, survived the death of dinosaurs, the Ice Age, and now continue to thrive in the aquatic environment of the Earth. Depending on the composition of the skeleton, ichthyologists distinguish two classes - Cartilaginous and bony fish.
Superclass Pisces
1. Streamlined body shape due to the smooth transition of its sections - head, body and tail - into each other and flattened laterally. Sometimes - flattened in the dorso-abdominal direction.
2. The skin is rich in glands that secrete mucus abundantly and is covered with scales(or some species have no scales). Mucus reduces friction during fish movement and has a bactericidal effect.
3. The organs of movement and stabilization of the body position with the back up are paired and unpaired fins. Paired fins - pectoral and ventral - support the normal position of the body with the back up, serve as rudders, and in some fish (rays) - the main organs of movement. Buoyancy bony fishes are supported by a hydrostatic organ – swim bladder.
4. The skeleton is cartilaginous or bone. The skull is immovably connected to the spine. The spine has two sections: trunk and caudal. The limb girdles are not connected to the axial skeleton (i.e., the spine).
v Stingray and shark meat is used as animal feed or
bait in fishing.
v The skin of some sharks is used as sandpaper.
v Fossils are a source of knowledge about the evolution of life on the planet.
v Link in the power chain.
v Source of danger to human life and health (some types).
v Shark teeth have been used as amulets, decorations, and souvenirs since ancient times.
v Small, brightly colored species of mustelids, carpet sharks and catsharks,
some stingrays are sometimes kept in home aquariums.
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 cartilage.
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, an interesting feature of the structure is their buoyancy, it is practically equal to 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 with 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:
Vertebrates
General characteristics of 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. The gill sacs communicate with the external environment independently of each other or open through a common opening. 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 of 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 the 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.
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 a lot of 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).
Loading...