The structure of a shark. Whale shark
Currently, more than 450 species of sharks are known: from the deep-sea shallow Etmopterus perryi, only 17 cm long, to the whale shark, whose length reaches 12 meters.
Sharks are widespread in all seas and oceans, from the surface to depths of more than 2000 meters. Mainly live in sea water, but some species are also able to live in fresh water.
Most sharks are what are called true predators, but individual species, in particular whale, basking and largemouth sharks, are filter feeders; they feed on plankton, squid and small fish.
Skeleton
The skeleton of a shark is noticeably different from the skeleton of bony fish - it has no bones, and is formed entirely from cartilaginous tissue. Leather
Sharks are covered with placoid scales, the scales of which are rhombic plates ending in a spine protruding outward from the skin. In structure and strength, the scales are close to bones, which gives reason to call them dermal denticles. These teeth have a wide base, a flattened shape and a very relief-defined crown. In most cases, the crowns are very sharp and fit tightly together, so the skin may appear relatively smooth if you run your hand from head to tail, and vice versa - rough, like sandpaper, if you move in the opposite direction. Teeth and jaws
The teeth of most sharks are shaped like sharp dentin cones and sit on the cartilage of the upper and mandible. Teeth are regularly replaced as they fall out or wear out according to the conveyor belt principle - their replacement is constantly growing from the inside. In their structure and origin, these are modified placoid scales. Teeth and jaws vary greatly depending on diet and lifestyle. different types sharks Benthic sharks, whose food is usually protected by a hard shell, have hundreds of small, smooth teeth. Pelagic species are characterized by the presence of very sharp teeth, adapted for easy penetration into the flesh of prey. Sharks such as tiger sharks have knife-shaped teeth designed for tearing the flesh of large prey. Plankton-eating sharks have vestigial small teeth.
Buoyancy
Unlike bony fish, sharks do not have a swim bladder. Instead, a huge liver, cartilaginous skeleton and fins help them compensate for negative buoyancy. Most species of sharks need to constantly move in order to maintain their breathing, so they cannot sleep for long periods of time. However, some species, such as the whiskered nurse shark, are able to pump water through their gills, allowing them to rest on the bottom.
Digestive system
After a hearty meal, sharks are able to starve for a long time, slowly and economically using up accumulated resources, and in general their need for food is relatively small. For example, a three-meter Australian sand shark weighing 150 kg kept in captivity ate only 80-90 kg of fish per year. Sharks periodically perform stomach eversion - they turn it out through the mouth into aquatic environment for the purpose of cleansing. It is curious that they never damage the stomach with their numerous teeth.
Smell
Do sharks have a sense of smell? one of the main sensory systems. Experiments have shown the high sensitivity of sharks to odors. Are the olfactory organs represented by the nostrils? small bags on the muzzle that allow water to reach the olfactory receptors. The sense of smell is involved in searching for prey and breeding partners. The white shark uses 14% of its brain for smell. Do hammerhead sharks have a particularly well-developed sense of smell? The uniquely shaped nostrils, spaced at a decent distance from each other on the head, make it possible to more clearly determine the direction of the source of the smell. Research has shown that sharks respond better to the odors of wounded or alarmed prey.
Sharks can smell blood diluted 1:1,000,000, roughly equivalent to one teaspoon in a medium-sized swimming pool.
Vision
The structure of a shark's eye is for the most part the same as that of all vertebrates, but with some peculiarities. Does a shark's eye have a special reflective layer? tapetum? located behind the retina. The tapetum directs the light that passes through the retina back so that it once again affects the receptors, thereby increasing the sensitivity of the eye. This significantly improves visual acuity, especially in low light conditions. Another feature of some species is the presence of a blinking eyelid, which closes the eye directly during an attack on the victim, protecting it from damage. Sharks, which do not have a blinking eyelid, roll their eyes when attacking a victim.
Previously, it was believed that the shark's eye contains too few cones and is not able to distinguish colors and small details. However modern technologies made it possible to prove the opposite. The visual acuity of some shark species is up to 10 times sharper than humans.
Hearing
Sharks have a hearing organ? This is the inner ear, enclosed in a cartilaginous capsule. Sharks perceive predominantly low sounds of 100–2500 Hz. Most sharks are able to detect infrasound with frequencies below 20 Hz. The inner ear is also an organ of balance. Electro- and magnetoreception
Are the electroreceptive apparatus of sharks represented by ampullae of Lorenzini? These are small connective tissue capsules immersed in the skin with tubes emanating from them that open to the surface of the skin. Sharks respond to electric fields as low as 0.01 µV/cm. Therefore, they are able to detect prey by electrical fields created by the work of the respiratory muscles and heart.
Lifespan
Each species has a specific lifespan, and it is not easy to estimate it for all sharks. In general, sharks grow relatively slowly, and in general it can be said that most species live 20–30 years. However, the spotted spiny shark has a record life expectancy, living for more than 100 years. Whale sharks of a similar age are also known.
Reproduction
Sharks have the internal fertilization characteristic of cartilaginous fish, a primitive uterus and a fairly perfect placental connection. The fetus develops in the uterus and is born well adapted to independent life. Newborn sharks have well-developed musculoskeletal systems, digestive system and sensory organs, which allows you to feed yourself and quickly gain weight. Do sharks produce different numbers of babies? some species up to 100, others only two or three. A white shark gives birth to approximately 3–14 pups at a time.
Unlike most bony fish, which produce millions of eggs, shark procreation focuses on quality rather than quantity.
The care of some species for their offspring (the baby shark is under the care of the mother for some time) allows sharks to have a high survival rate, and therefore lower fertility.
Lifestyle
In the traditional view, the shark looks like a lone hunter, roaming the ocean in search of prey. However, this description applies only to a few species. Many sharks lead sedentary, inactive lives. Contrary to the popular belief that the shark is just a “hunting machine” driven only by instincts, recent research has shown the ability of some species to solve problems, social behavior and curiosity. In 1987, off South Africa, a group of seven white sharks worked together to drag a half-stranded whale to deeper water for a meal.
The ratio of brain to body mass in sharks is roughly equivalent to that of birds and mammals.
Sharks generally move at a cruising speed of approximately 8 km/h, but when hunting or attacking, the average shark accelerates to 19 km/h. The mako shark can accelerate to speeds of 50 km/h. The white shark is also capable of similar jerks. Such exceptions are possible due to the warm-blooded nature of these species.
Nutrition
The food preferences of sharks are very diverse, and they depend on the characteristics of each species, as well as on their habitat. The main food for sharks are fish, mammals, plankton and crustaceans. For example, lamna, mako and blue sharks feed primarily sea fish pelagic species, and the shape of their thin, sharp teeth is adapted to grab prey in motion.
The white shark prefers seals and sea lions, but if possible, it also hunts whale mammals, since the features of its teeth allow it to snatch off large pieces of flesh.
The diet of benthic sharks consists mainly of crabs and other crustaceans, and their teeth are short and adapted to breaking shells.
Basking, largemouth and whale sharks feed on plankton and small marine organisms. Most species are carnivores.
Some species, such as the tiger shark, are almost omnivorous and swallow almost anything that comes their way.
After all, for the most part these are large and aggressive fish, which, while fishing with bait, are in a state of hunting for prey? that is, in increased excitement.
In addition, when removed from the water, some species may simply crush their internal organs its own weight, and this must be taken into account when moving a shark from the ocean to an artificial tank.
Further difficulties arise upon the arrival of sharks in the aquarium, which must have the necessary capacity for the normal life of these fish, and also take into account their increased sensitivity to electromagnetic waves.
Fishing and hunting
Along with other fish, sharks have been the subject of fishing for many years (more than 100 species). The fishing industry is interested in sharks:
Meat used as food by many cultures (although observations have shown that sharks are predisposed to accumulate mercury, the content of which in meat has increased significantly due to environmental pollution).
Fins, which in Asia are the main ingredient for a delicious soup, are also used in oriental medicine.
Cartilage, around which there is still controversy about its medicinal properties against cancer tumors.
Liver contains fat rich in vitamin A and B vitamins, and is used as a raw material for the manufacture of medicines.
Leather that is used in haberdashery and as an abrasive material.
The main fishery is carried out in the Atlantic Ocean, where 26 species are commercial, about a third of sharks are caught in Indian Ocean, and another one and a half times fewer sharks are caught in the Pacific. Every year, approximately 100 million sharks are caught worldwide.
Shark fishing can be divided into three areas:
Fishing for the purpose of using their meat, liver, cartilage, skin and fins? that is, the full use of fish.
So-called bycatch? when the shark is incidental prey when catching other fish.
Fishing for the purpose of obtaining fins only. This is the most irrational (the weight of fins is up to 4% of the entire body) and inhumane method of catching sharks, which has received English language name Finnish? when the only target is the fins, and the rest of the carcass is thrown out to rot on the shore or back into the sea.
In addition to fishing for industrial purposes, there are also reasons for hunting sharks in the world such as ensuring the safety of beaches, reducing the natural threat of industrial fish species, and simply extreme hunting and fishing.
Common Misconceptions About Sharks
A shark must constantly swim to stay alive. In fact, many species are able to rest by lying on the bottom and pumping water through their gills. Most sharks attack and kill humans. Only a few species of sharks regularly commit unprovoked attacks on humans, and this is mostly due to misidentification of prey.
Sharks swim at high speed. In fact, sharks' cruising speed is quite slow, as they need to conserve energy. However, this does not prevent them from developing a high, so-called “throwing” speed immediately before attacking the victim.
Sharks love human blood. Sharks do not have a preference for any blood. On the contrary, having grabbed a piece of flesh from a person, they usually spit it back, because this meat is not the high-fat food that they need to replenish their energy reserves.
Sharks are omnivores. Most species prefer to wait until they can get their regular food instead of eating everything.
Sharks are not susceptible to cancer. This belief, which existed for a long time, caused the death of a huge number of sharks caught by humans for the sake of “anti-cancer” cartilage. However, observations of sharks in captivity, as well as in their natural habitat, have shown the presence of individuals with organs affected by cancerous tumors. The number of cancer cases turned out to be higher where the water is more polluted (including from human activity).
Fish.
Type Chordata
n./type Vertebrates
n./class of fish
class Cartilaginous fish
n./class Cartilaginous or elasmobranchial
squad Sharks and rays
The body is clearly divided into head, trunk and tail. At the front end of the head there is a growth - rostrum or snout. The eyes are located on the sides of the head, and the squirts are located somewhat behind and above them. The mouth opening in the form of a transverse slit is located on the underside of the head, paired nostrils are located in front of the mouth, and behind there are five pairs of vertical gill slits.
The trunk section includes the part of the body from the last gill slit to the cloaca opening, its opening is located on the lower surface of the body near the tail; then the caudal section of the body continues.
The limbs of fish are represented by fins - skin outgrowths penetrated by cartilaginous or bone rays - elastotrichy, which can be paired or unpaired.
The pectoral and pelvic fins are paired, both the dorsal and caudal fins are unpaired.
The caudal fin has unequal blades - the upper one is much larger and part of the vertebral column extends into it. This type is called heterocoelous.
The main organ of movement is the caudal fin; paired fins allow steering, and all unpaired fins provide balance to the body.
Covers of the body.
Multilayered epidermis with numerous glandular cells; cutis or dermis. Scales develop in the dermis, covering the skin and performing a protective function. Cartilaginous fish have scales placoid, each scale is a rounded plate of osteodentine, on which a tooth rises, directed backwards. The outside of the tooth is covered with a thin layer of enamel. On sharks' jaws, such scales turn into teeth. Because scales are constantly being formed, broken teeth are repeatedly replaced by new ones.
Skeleton.
1. Axial skeleton.
A) Spinal column, formed by interconnected vertebrae. Each vertebra consists of a body and two pairs of arches: the upper arches, connecting, limit the opening of the spinal canal, and the lower arches. The vertebral body has a hole in the center through which the notochord passes - its remains are preserved throughout life. Vertebrae – amphicoelous– biconcave.
The spinal column has two sections - the trunk and the caudal. In the trunk region, the ribs are attached to the lower arches. In the vertebrae of the caudal region, the lower arches grow together and form hemal channel, in which large blood vessels pass.
b) Scull divided into cerebral and visceral. The skull is immovably connected to the spine.
The cranium includes the brain case, sensory organ capsules, and the rostrum skeleton. The braincase includes several sections: occipital, auditory, olfactory, orbital, floor and roof.
The visceral skull consists of the jaw apparatus, the hyoid arch and the branchial arches. The hyoid arch movably connects.
visceral skull with brain. The number of gill arches corresponds to the number of gills
2. The accessory skeleton is the skeleton of the limbs. Paired fins have additional support, which becomes the girdles of the limbs.
A) forelimb belt(thoracic or humeral) is formed by a solid arcuate cartilage, covering the body from below and from the sides. This cartilage is not connected to the axial skeleton and lies freely in the muscles of the body. Three basal cartilages, or basalia, to which they are attached radials, forming several consecutive rows. Thin ones are attached to the distal part of the radials. elastin threads, forming a direct support for the paired pectoral fins - the free forelimbs.
b) hind limb belt(abdominal or pelvic) is the internal support of the free hind limbs, which are the paired pelvic fins. It is a rod-shaped cartilage lying across the body in front of the cloaca. One is attached to the cartilage on each side basalia, to the outer edge of which they are attached radials free hind limb. The internal support of the fin itself is also elastin threads. In male sharks, the elongated basalia of the ventral fins form a copulatory organ.
c) the internal support for unpaired fins is a series of rod-shaped cartilages - radial, located in the muscles of the body. In the thickness of the fin itself there are numerous elastin threads, which are of cutaneous origin.
Muscular system.
Represented by striated somatic skeletal muscles and smooth muscles of internal organs and blood vessels. Skeletal muscles are represented by myomeres, separated by thin layers - septa - of connective tissue.
Digestive system.
The mouth opening is located on the underside of the head and is limited by jaws covered with numerous teeth. They are arranged in several rows, have a conical shape and are facing backwards; pharynx pierced with gill slits; short esophagus; stomach, in the walls there are numerous glands that secrete components of gastric juice; the small intestine, into which the ducts of the liver and pancreas flow; The large intestine has a spiral valve, which is a helical fold. This significantly increases the functional surface of the intestine. Absorption and digestion processes end in the large intestine; The rectum opens into the cloaca, into which the reproductive ducts and ducts of the urinary system flow.
Cartilaginous fish do not have salivary glands or a real tongue. The pancreas is represented by small, loosely located lobules in the mesentery of the small intestine.
The liver forms two or three lobes. Its size makes up 25% of the total body weight. From the liver, bile enters the gallbladder. The liver not only actively participates in digestion, but also disinfects toxic substances that are contained in the portal vein flowing from the digestive organs, and also normalizes the concentration of monosaccharides in this blood. Reserve substances are deposited in the liver, which significantly increases the buoyancy of these fish, since they do not have a swim bladder.
Respiratory system.
Gills. On both sides of the interbranchial septum there are numerous outgrowths - gill filaments. Only plankton feeders have gill rakers.
Gas exchange occurs when water enters the pharynx through the mouth and exits through the gill slits into the pharynx. external environment, while washing the gill filaments. In sharks, gas exchange occurs passively during movement.
Circulatory system.
Represented by the heart and blood vessels. Closed. One circle of blood circulation. The heart consists of an atrium and a ventricle. It contains only venous blood.
Ventricle; abdominal aorta; five pairs of afferent gill vessels: microvasculature: gills; gas exchange; efferent branchial arteries; dorsal aorta; the carotid arteries and dorsal aorta are directed to the caudal region giving off small vessels; venous blood from the anterior part of the body collects into the anterior cardiac veins, and from the posterior part into the posterior cardiac veins; venous sinus; heart.
Blood consists of plasma and formed elements. The main organs for the formation of formed elements are the spleen and kidneys. Red blood cells, even in adulthood, contain a nucleus. Fish have a primitive lymphatic system.
Excretory system.
Pair of trunk buds - mesanephros- in the form of elongated bodies along the spine. The ureter is the Wulffian canal, they connect and empty into the cloaca. In cartilaginous fish, the final product of nitrogen metabolism is urea.
Nervous system.
CNS – spinal cord and brain.
The spinal cord looks like a white cord and is located in the spinal canal. Spinal nerves arise from it.
The brain includes 5 sections. The cerebellum is well developed. The midbrain is the largest and most developed section. 10 pairs of cranial nerves.
Sense organs
Organ of vision– eye: flat cornea and spherical lens. Sharks have a nictitating membrane that covers the eye like an eyelid. Hearing organ- inner ear. Olfactory organ- opens with nostrils. Very thin . Lateral line organs located on the head and sides of the body. Located at the bottom of pits or in grooves; perceive water fluctuations and pressure differences. Tactile cells scattered throughout the body.
Reproductive system.
Paired testes; Wolffian duct; connecting they flow into the urogenital sinus, which opens in the urogenital papilla.
Paired ovaries; Müllerian ducts; cloaca.
Fertilization is internal. Shark eggs are large, have a dense shell and often have a complex shape. Development is direct.
semi-class Whole-headed.
Chimeras.
n/class lungfish.
Australian scalefly (horntooth), American scalefly (lepidosirenus), protonterus. These animals have the ability to breathe not only O 2 dissolved in water, but also atmospheric air with the help of the lungs. The lungs communicate with the abdominal side of the esophagus and have a cellular structure. They don't have a swim bladder .
Front view of a whale shark
Wobbegong shark, related to the whale shark
Appearance and structural features
The whale shark is difficult to confuse with other fish - in addition to its huge size, it is distinguished by its characteristic appearance. The whale shark has a powerful and thick body, and a relatively small head. The shape of the head is very peculiar - it is strongly flattened, and becomes increasingly flatter towards the end of the snout. Gill slits 5; they are extremely wide and long. The mouth is at the end of the snout, and not under it, like most other sharks. The mouth is very wide, reaching one and a half meters in width. It can open quite strongly and, when fully extended, takes on the appearance of a wide oval. At the corners of the mouth there are leathery outgrowths, like small antennae.
The eyes are very small and deep-set, located close to the end of the snout almost at the edges of the mouth. They are located on the line separating the dark color of the back and sides from the white belly. The largest sharks have eyes barely the size of a golf ball. The whale shark does not have a nictitating membrane, but the eye can be closed by a thick fold of skin that moves forward. If some fairly large object is too close to the eye, the shark draws the eye into its orbit and closes it with this fold. This is a unique trait among sharks. Almost immediately behind the eyes there are round splashes.
Whale shark eyes
The body of the whale shark behind the head becomes thick, the back rises in the form of a gentle hump. The body is at its greatest thickness just behind the head, and then begins to become thinner. There are two dorsal fins, both of them are shifted far back. The first fin is high and wide, in the shape of an almost equilateral triangle. The caudal fin, like that of all sharks, is sharply asymmetrical; its upper lobe is approximately one and a half times longer than the lower one. At the same time, there is no notch on the upper blade, characteristic of the tail fins of most sharks. The 12-meter fish had a caudal fin width of 4.8 m, a pectoral fin length of 2.4 m. On the back of the body there are several longitudinal folds of skin in the form of long ridges on the sides and on the back, reaching all the way to the tail.
A vivid description of the appearance of the whale shark was given by the famous Norwegian explorer Thor Heyerdahl, who observed this fish while sailing on the Kon-Tiki raft:
| « | The head belonged to a gigantic monster, and it was so huge, so terrible that the sea serpent itself, if it had appeared in front of us, would not have struck us so hard. Small eyes sat at the edges of the wide and flat muzzle, the toad's mouth with long fringes at the corners was at least one and a half meters wide. The powerful body ended in a long thin tail; a sharp vertical fin indicated that this was, in any case, not a whale. The body generally appeared brown in the water, but both it and the head were dotted with small white spots. The monster slowly, lazily swam after us, squinting like a bulldog and quietly working its tail... Now we could get a very close look at this giant... Even Walt Disney's rich imagination could not have created a more terrible monster. | » |
The number of teeth in a whale shark is extremely large and can reach several thousand - even up to 15 thousand. The shark, which had 3 thousand teeth in its mouth, had about 300 rows on each jaw. The teeth are small, even in the largest sharks not exceeding 6 mm in length. The whale shark's brain, relative to its body size, is significantly smaller than that of other sharks, such as the white shark. Its structure, studied using magnetic resonance imaging, showed noticeable differences from the brains of other sharks. The whale shark's cerebellum is more developed than that of other cartilaginous fish. Other features of her brain may be an adaptation to the herd lifestyle. The whale shark has a relatively much smaller liver than most other sharks. Therefore, the whale shark often swallows air to regulate the buoyancy of its body.
Whale shark compared to diver
Size
The whale shark is indisputably the largest living fish. The largest reliably recorded specimen was long thought to be 12.65 m long, with a size of 13.7 m also mentioned. For decades, reports of whale sharks up to 20 m long remained unverified. However, in the late 1990s, scientific information appeared about a whale shark 20 m long and weighing 34 tons. Therefore, in modern sources, the length of a whale shark of 20 m is indicated as already fully verified. A number of sources contain data on observations of whale sharks even 21.4 m long, but in general, specimens larger than 12 m are extremely rare. Like most sharks, female whale sharks are larger than males.
A young whale shark that fell into the hands of Indian ichthyologists near Tuticorin in 2003 was measured by them with high accuracy. With a length of 4.78 m, the fish weighed 1,700 kg. The width of her mouth was 77 cm, the size of her eye was 4 cm in length and 3.5 in width. The upper lobe of the caudal fin was 115 cm long, the lower - 74 cm. Based on the experience of studying whale sharks in captivity, it was concluded that they grow in captivity approximately 1.1-1.3 times faster than in wildlife. This may be due to the constant abundance of good food in aquariums. One of the measured sharks grew by 29.5 cm in a year, the other by 46 cm in 630 days. Apparently, at a young age, whale sharks grow relatively much faster than at an older age, which may be one of the adaptive features for protection from predators. One of the sharks, kept in an aquarium in Taiwan and brought there almost in a newborn state, grew by 1 cm per day - 143 cm in 143 days. The 60-centimeter calf kept at Oita lived in the aquarium for more than 3 years and grew to 3.7 m. In general, however, young whale sharks have been studied very poorly.
Skin and its color
The skin of a whale shark is very strong and thick - reaching a thickness of 10 cm and even 14 cm in large specimens. The skin, like that of other sharks, is covered with very small placoid scales, which look like sharp spines about 0.75 mm in height and 0.5 mm in width . Their structural features noticeably distinguish them from the scales of most sharks - the whale shark's scales have a very strongly developed and bent back point, while the lateral blades of the scales are poorly developed. Probably, scales of this structure improve the hydrodynamic properties of the fish’s body. On the belly, the skin is about a third thinner - apparently for this reason, when a diver approaches, a shark often instinctively turns its back to him, which is better protected than its belly.
The color of the whale shark is also very characteristic. The back and sides of this fish are dark, usually gray with a blue or brown tint. On a dark background in a pretty in the right order There are longitudinal and transverse narrow dirty white stripes, between which, also in a fairly regular order, there are rounded spots of the same color. On the head and pectoral fins the spots are smaller and more often and randomly located. The lower part of the body is off-white. The skin and fins usually have a large number of scratches, forming an individual pattern by which observers distinguish specific individuals. It is known that the pattern of spots on a shark's skin does not change with age, making it easier to track individuals. Interestingly, equipment designed for astronomical observations was successfully used to identify photographed whale sharks. Devices designed for image comparison starry sky and capable of detecting the slightest difference in the location of celestial bodies, they were just as effective in identifying differences in the spotted pattern on the skin of sharks.
Similar coloration with a dark back and light lower body, with big amount clear stripes and spots on a dark background are found in small wobbegong-shaped bottom sharks, which are related to the whale shark, and serve the purpose of camouflage. There is an opinion that this coloration of the whale shark is a feature left over from the ancestors of the whale shark along the evolutionary line. Another hypothesis states that the anti-shadow coloration may be due to the fact that whale sharks, which usually swim near the surface, are highly susceptible to ultraviolet irradiation from sunlight, the harmful effects of which are neutralized to a certain extent by the dark coloration.
The topic of our article is sharks, the internal structure of which we will consider in the following order:
- Skeleton and muscles;
- Nervous system: brain and sense organs;
- Circulatory system;
- Breath;
- Digestion;
- Genitourinary system.
Skeleton and muscles of a shark
Let's start studying internal structure sharks with the musculoskeletal system, which includes the skeleton and muscles. The skeleton of predatory fish consists of a skull, an axial skeleton, a skeleton of paired fins and their belts, and a skeleton of unpaired fins.
The skull is represented by the braincase and the visceral region, including the jaws and gills.
The braincase consists of cartilage tissue, protecting the brain from all sides. Only in the upper part there remains a hole (fontanelle), which is not overgrown with cartilage during the formation of the skull, but remains covered with a connective tissue film.
Behind the jaws there are paired cartilaginous arches of gills, connected by unpaired cartilages - copulas.
The skeleton of the shoulder girdle is represented by a semi-ring-shaped cartilage, on the sides of which there are processes for connection with the three basal cartilages of the pectoral fins. From the basal cartilages there are three rows of thinner radial cartilages and from the latter there are thin filaments of elastin.
The pelvic girdle is simpler in structure and has a cartilaginous plate lying in front of the cloaca slit, to which one row of radial fin cartilages is attached. Elastin threads also extend from the radial cartilages.
Watch the video: Anatomy of a shark - dissection and study of the internal structure
Unpaired fins (caudal, anal and dorsal) have a skeleton consisting only of radial cartilage and elastin filaments. The spine extends into the dorsal fin and into its upper section.
Spiny sharks do not have an anal fin, but have a dorsal fin, which gave the name to the family.
The muscular system of the shark’s internal structure is very developed and consists of myomeres (muscle segments) surrounded by a connective tissue membrane.
The muscular system is abundantly supplied with blood, since movement is life for a shark. After all, in order for the blood to return back to the heart, only the pressure created by the heart is not enough. And muscle contractions come to the rescue.
Watch video - Building muscular system white shark:
Shark nervous system and sensory organs
The nervous system is represented by the brain and spinal cord, from which nerves extend to organs and tissues.
It is still believed that vision in elasmobranchs is poorly developed, but is compensated by smell and sensitivity to electrical impulses.
At the bottom oral cavity there is a small fold of mucous membrane - a tongue that does not have muscles. Then the food enters the pharynx.
To prevent food from falling out of the gills, sharks have cartilaginous outgrowths on their gill arches - gill rakers.
The pharynx passes into the esophagus, through which food enters. The stomach of some sharks has the ability to “turn inside out,” freeing itself from undigested and inedible food debris.
The stomach is joined by the small intestine, which passes into the large intestine and then into the rectum. The large intestine has a spiral valve, which is an outgrowth of the mucosa that increases the absorption surface.
Read more in the article
A protrusion extends from the large intestine - the rectal gland, which secretes an odorous secretion to attract individuals of the opposite sex.
Sharks also have a very large liver (which serves part of the function), gall bladder and pancreas.
Remains of food enter the cloaca, where the ducts of the genitourinary system open.
Genitourinary system of sharks
From organs urinary system Sharks have kidneys, which in males act as an appendage to the testis, and ureters.
The reproductive system of sharks is represented, as we have already mentioned, by testes in males, from which seminiferous tubules with an extension at the end extend into the cloaca, and by ovaries in females.
Watch the video: Shark genitourinary system - structure and operation
It is worth noting a unique feature of the internal structure - the absence of kidneys and urinary tracts. Urine and the ammonia contained in it are washed out by the blood and excreted directly through the skin of the predator.
All types of sharks have a similar mechanism for thoroughly processing urine, but polar sharks treat their urine most carefully.
The fact is that, for example, the urine of land mammals contains many valuable microelements and fresh water, which they wastefully remove through the urinary tract.
Sharks in this regard are very economical. Every drop of fresh water, as well as a share of valuable microelements, is extracted from urea before everything unnecessary is released through the pores in the skin.
This careful attitude of the predator to its urea has led to the extreme saturation of polar shark meat with ammonia, giving it an unpleasant odor.
Sharks are characterized by internal fertilization. The mature egg falls into the abdominal cavity and rolls into the funnel of the oviduct, where fertilization occurs. The oviduct contains shell glands that form the shell of the egg.
At the end of the oviduct there is an extension - a kind of “uterus” in which the eggs ripen.
During ovoviviparity, sharks hatch from the eggs in these “wombs,” which may even be immature eggs.
If the shark is not viviparous, but oviparous, then the maturation of the embryo and its hatching from the egg will occur in the external environment.
Scientific discovery of 2016SUPERCLASS OF FISH (PISCES)
CLASS CARTILIATED FISHES (CHONDRICHTHYES)
Lesson 3. EXTERNAL AND INTERNAL STRUCTURE OF A SHARK
Systematic object position
Phylum Chordata
Subphylum Vertebrata
Section Gnathostomata
Superclass Pisces
Class Cartilaginous fish (Chondrichthyes)
Subclass Elasmobranchii
Superorder Sharks (Selachomorpha)
Representative - spiny shark, or katran (Squalus acanthias L.)
Equipment and materials
Ready-made preparations: 1) shark (stuffed animal and wet preparation); 2) dissected shark; 3) digestive system; 4) injected circulatory system; 5) excretory organs; 6) reproductive organs; 7) brain.
Tables: 1) appearance of shark and ray; 2) general location of internal organs; 3) digestive system; 4) circulatory system; 5) reproductive organs of the male and female; 6) brain.
Introductory Notes
Cartilaginous - relatively small (about 730 modern species) a group of fish whose morphophysiological organization combines primitive and evolutionarily progressive features. The primitive features are as follows. The skeleton of these fish remains cartilaginous for life. The skin is covered with primitive placoid scales. There are many gill slits (5 - 7), each of which opens with an independent opening. Progressive features include: the presence of nerve substance in the roof of the forebrain, internal insemination, and in many species, viviparity. The class Cartilaginous fish (Chondrichthyes) is divided into two subclasses: Elasmobranchii and Chimaera fish (Holocephali).
Study the features of the external and internal structure of a shark.
Consider:
External structure
Head; torso; tail; fins (paired - pectoral and abdominal); unpaired (dorsal, anal, caudal); mouth opening; eyes; nostrils; sprinklers; cloaca; copulatory organs; scales
Internal structure
Digestive system: oral cavity, teeth, pharynx, esophagus, stomach, small intestine, large intestine, spiral valve, liver, gallbladder, pancreas, rectal gland.
Respiratory system: gill slits, intergill septa, gill filaments.
Circulatory system: two-chambered heart (atrium and ventricle); conus arteriosus; venous sinus (venous sinus); abdominal aorta; five pairs of gill vessels. Using the drug, drawing and table, trace the blood circulation pattern.
Excretory organs: trunk kidneys, ureters.
Reproductive organs: testes, vas deferens, ovaries, oviducts.
Central nervous system: brain (forebrain, diencephalon, midbrain, medulla oblongata, cerebellum); head nerves; spinal cord.
Sketch:
1) the appearance of the shark; 2) general location of internal organs; 3) diagram circulatory system(homework).
Make a table: “The head nerves of a shark”, indicating the number, name, place of origin, what they innervate, functions: sensory, motor, mixed (homework).
External structure
The shark's body is elongated, torpedo-shaped (Fig. 16). In the anterior part it is somewhat flattened in the dorso-ventral direction. Without clear boundaries, it is divided into three sections: head, body and tail. The last gill slit is considered the boundary between the head and body. The trunk section begins from the last gill slit and ends with the cloaca opening. Behind it is the tail section.
The head has an elongated snout - rostrum. On the underside of the head is a large arched mouth. Sharp, backward-pointing teeth are clearly visible on the jaws. They are modified placoid scales. In front of the mouth on the lower surface of the head there are paired nostrils. There are large eyes on the sides of the head. On the lateral surfaces of the head, five pairs of vertical gill slits are clearly visible, slightly covered in front by a fold of skin in a collapsed state.
Behind and slightly above each eye there is a small hole - a splash. The squirt is a rudimentary gill slit located between the maxillary (III pair) and hyoid (IV pair) arches.
The caudal peduncle ends in a powerful heterocercal caudal fin, into the large upper lobe of which the end of the spine passes. There are two unpaired dorsal fins on the dorsal side of the body.
Rice. 16. Shark body side view:
1 - mouth opening; 2 - gill openings; 3 - sprinkler; 4 - pectoral fin; 5 - ventral fin; 6 - dorsal fins; 7 - caudal heterocercal fin; 8 - rostrum
The paired limbs of a shark are represented by a pair of pectoral and a pair of ventral fins. In relation to the body, they are located horizontally and serve as depth and rotation rudders. In males, the internal parts of the ventral fins are transformed into copulatory organs.
The shark's body is covered with numerous small placoid scales bearing posteriorly directed denticles. These teeth can be easily felt if you run your finger over the shark's skin from the tail to the head. In some parts of the body, for example at the base of the fins, the scales are transformed into sharp spines that perform a protective function.
On the sides of the shark’s body, tail and head there is a noticeable lateral line - an organ characteristic of aquatic vertebrates. It consists of a series of small external holes leading into a special channel deeply immersed in the skin. It contains the skin sensory organs that perceive water vibrations.
Internal structure
General arrangement of the shark's internal organs
The abdominal cavity of the shark is separated in front by a septum from the pericardial cavity. The heart is visible in the pericardial cavity (Fig. 17). In front of it are internal gill openings leading into the pharynx. In the abdominal cavity, a mesentery is developed, on which the digestive organs are suspended. The large three-lobed liver partially covers the large curved stomach from the front and side and partially covers the anterior intestine. Near the stomach, a dark red spleen is suspended on the mesentery. departs from the stomach
Rice. 17. Dissected Shark:
1 - venous sinus; 2 - atrium; 3 - ventricle of the heart; 4 - arterial cone; 5 - abdominal aorta, 6 - afferent branchial arteries; 7 - nostril; 8 - gills; 9 - stomach; 10 - small intestine; 11 - large intestine (spiral valve visible); 12 - rectum; 13 - cloaca; 14 - rectal gland; 15 - liver; 16 - gallbladder; 17 - bile duct; 18 - pancreas; 19 - spleen; 20 - right kidney (left not shown)
intestine, differentiated into sections, ending in a cloaca. Near the cloaca, an outgrowth of the intestine is noticeable - the rectal gland, an organ of salt metabolism. Deep in the abdominal cavity, on both sides of the spine, there are oblong kidneys.
Organ systems
Digestive system
The shark's mouth is equipped with movable cartilaginous jaws. On the skin covering the jaws there are large conical teeth, curved at the tops back - transformed placoid scales. The oral fissure leads into the oral cavity, which without a noticeable border passes into the large pharynx. The internal gill openings and openings leading to the squirts open into the pharyngeal cavity. The tongue of sharks is poorly developed. The esophagus begins at the back of the pharynx. The esophagus, without noticeable boundaries, passes into an easily distensible, voluminous U-shaped stomach. The stomach forms two knees: a long descending one and a short ascending one. In the middle lobe of the liver there is a large gallbladder, the ducts of which open into the intestines. A short small intestine arises from the short ascending section of the stomach. The anterior section of the small intestine is called the duodenum. The small intestine becomes the wide intestine, followed by the rectum, or hind intestine. The rectum opens into the cloaca. A hollow finger-like outgrowth extends from the middle part of the rectum - the rectal gland - an organ of salt metabolism. A special fold of the mucous membrane appears through the wall of the colon, making a series of revolutions in the intestinal cavity. This is a spiral valve that slows down the passage of food and increases the internal absorption surface of the intestine.
In the mesentery, at the bend of the stomach, lies a massive spleen. In the loop formed by the ascending part of the stomach and the intestine, there is a small, light-colored pancreas, the ducts of which, like the gallbladder, flow into the duodenum.
Respiratory system
The respiratory organs of cartilaginous fish are gills. The gill apparatus consists of three elements: the gill arch, the interbranchial septum and the gill filaments. The shark's throat is pierced by five pairs of gill slits that open outward. Cartilaginous gill arches are located between the gill slits. To them
interbranchial septa are attached, the leathery edges of which cover the gill slits lying behind them.
On the anterior and posterior sides of the interbranch septa there are numerous lamellar gill filaments. Unlike cyclostomes, gill filaments in cartilaginous fish are of ectodermal origin. The gill filaments, located on each side of the interbranch septum and directed into different gill slits, are semi-gills (see Fig. 27). On the anterior gill arch, the hyoid, the gill filaments are located only on the posterior side, i.e., there is one semi-gill. Four intact gills are located on the I - IV gill arches; The V branchial arch does not bear gills. Thus, cartilaginous fish have four gills and one semi-gill, or nine semi-gills. The rudimentary gill filaments also sit on the wall of the squirt - a rudiment of the gill slit.
The act of breathing in sharks occurs as a result of swallowing movements of the mouth, when water penetrates into the pharynx cavity to the gills, washes them and exits through the external gill openings. Saturation of the blood with oxygen and release of carbon dioxide occurs in the capillaries of the gill filaments.
Circulatory system
The two-chamber heart is located in the pericardial cavity, separated from the body cavity by a vertical partition - the pericardium. The thick-walled ventricle is directed forward with its apex. The atrium is located above the ventricle and partially covers it from the sides. A small arterial cone is adjacent to the apex of the ventricle. The conus arteriosus is part of the ventricle of the heart; its walls are formed by striated muscles. Another part of the heart - the venous sinus - is a voluminous thin-walled venous sinus. The abdominal aorta extends forward from the conus arteriosus (Fig. 18). From the abdominal aorta, five pairs of afferent branchial arteries with venous blood are sent to the gills. In the gill filaments, the afferent gill arteries break up into a network of capillaries. Here the blood gives off carbon dioxide and is saturated with oxygen. Oxygen-enriched blood is collected in the efferent branchial arteries. The efferent branchial arteries of the left and right sides merge and form two aortic roots, which merge closer to the body into the dorsal aorta. Two carotid arteries extend forward from the anterior efferent branchial arteries and the roots of the aorta to the head.
The dorsal aorta is located under the spine, passing at the end of the body into the caudal artery. Along branches from the dorsal
Rice. 18. Diagram of the circulatory system of a shark:
1 - heart; 2 - abdominal aorta; 3 - afferent branchial artery; 4 - efferent branchial artery; 5 - left aortic root; 6 - left carotid artery; 7 - dorsal aorta; 8 - tail vein; 9 - left posterior cardinal vein; 10 - left anterior cardinal vein; 11 - left Cuvier duct; 12 - portal vein of the liver; 13 - hepatic vein; 14 - left lateral vein; 15 - portal vein of the kidneys; 16 - subclavian vein. Veins are painted black, arteries are shaded
The aorta carries blood to all organs and parts of the body. The subclavian arteries carry blood to the pectoral fins. The intestinal artery gives branches to the stomach, liver and anterior part of the intestine. The anterior mesenteric artery supplies blood to the posterior intestine and genitals. The renal arteries carry blood to the kidneys, the iliac arteries carry blood to the ventral fins, and the posterior mesenteric artery carries blood to the walls of the body cavity.
Venous blood from the head collects into the paired anterior cardinal veins, and from the body and tail into the posterior cardinal veins (Fig. 19). The anterior and posterior cardinal veins of each side merge into the ducts of Cuvier, which flow into the venous sinus, and from it the blood enters the atrium.
The caudal vein, which carries blood from the back of the body, enters the abdominal cavity
Rice. 19. Diagram of the venous system of a shark:
1 - duct of Cuvier; 2 - cardinal sines; 3 - hepatic sinus; 4 - tail vein; 5 - anterior cardinal (jugular) vein; 6 - posterior cardinal vein; 7 - hepatic vein; 8 - inferior jugular vein; 9 - lateral vein; 10 - portal vein of the liver; 11 - portal vein of the kidneys; 12 - subclavian vein
and forms the renal portal veins, which drain into the posterior cardinal veins.
The veins that collect blood from the digestive tract and spleen form the portal vein of the liver. Then the blood from this organ is collected in short paired hepatic veins, and from them into the venous sinus.
From the paired fins there are lateral veins that flow into the ducts of Cuvier.
From the cephalic end of the body, venous blood is directed back through a pair of anterior cardinal (jugular) veins, which pass over the gill arches and, merging at the level of the heart with the posterior cardinal veins, form the above-mentioned ducts of Cuvier.
Excretory organs
Sharks have trunk, or mesonephric, kidneys with ureters, the function of which is performed by Wolffian canals. The shark's kidneys are paired, highly elongated, looking like two flat, elongated dark bodies lying on the sides of the spine (Fig. 20). They extend approximately from the level of the pectoral fins to the cloaca. The kidney of cartilaginous fish is divided into two sections: a narrow anterior one and a widened compact posterior one. In the posterior part of the kidneys, thin tubes called ureters are visible on their abdominal surface.
In males, seminiferous tubules pass through the anterior part of the kidney. Thus, this part of the kidney does not have an excretory function. The ducts of the posterior dilated part of the kidney flow into the Wolffian canal, which opens into the cloaca.
In the female, the kidneys are not connected to the genitals and the Wolffian canal is only a ureter that opens into the cloaca.
Reproductive organs
Females. In females, paired ovaries in the form of short grape-shaped bodies are suspended on the mesentery on the sides of the esophagus (see Fig. 20, A). Paired oviducts (their role is played by the Müllerian canals) are located on the ventral surface of the kidneys. The oviducts are usually divided into two sections. The anterior ends of the right and left oviducts are connected into a common funnel of the oviducts. It lies on the ventral surface of the central lobe of the liver. In the upper part, the oviducts form small extensions - shell glands. Their highly thickened glandular walls secrete substances that form dense egg shells. The greatly expanded posterior part of each oviduct is called the "uterus", in
Rice. 20. Genitourinary system of a female (A) and male (B) shark:
1 - segment of the esophagus; 2 - oviduct funnel; 3 - shell gland; 4 - oviduct; 5 - ovary; 6 - kidney; 7 - posterior part of the kidney; 8 - urogenital papilla; 9 - cloaca; 10 - abdominal time; 11 - ventral fins; 12 - epididymis and vas deferens; 13 - testis; 14 - ureter; 15 - seminal vesicle; 16 - seminal receptacle; 17 - rectum; 18 - copulatory organ
in which the embryo develops in viviparous species. The openings of the “uterus” open in the cloaca on the sides of the urinary papilla.
The ovary does not have a direct connection with the oviducts. Mature eggs fall through a gap in the ovarian wall into the body cavity and enter the oviduct funnel. The fusion of the egg with the sperm occurs in the upper parts of the oviduct. Peristaltic contractions of the oviduct walls move the egg towards the “uterus”. In viviparous forms, eggs are retained in the uterine sections of the oviduct until the embryo is fully formed; In oviparous species, the eggs, covered with a dense shell, are released outward.
Males. Paired testes are loose, elongated bodies (see Fig. 20, B). They are located on the sides of the esophagus above the lobes of the liver and suspended on the mesentery to the wall
abdominal cavity. The thinnest seminiferous tubules extend from the testes. They are visible as thin whitish threads running along the mesentery on which the testis is suspended. The seminiferous tubules penetrate the upper edge of the kidney, which in males has almost lost its significance as an excretory organ and actually serves as an appendage to the testis. The tubules of this part of the kidney merge into the vas deferens, which run along the inner edges of the ventral surface of the kidneys. The vas deferens are Wolffian canals in origin. The vas deferens form expansions, seminal vesicles, then on the urogenital papilla they open into the cloaca. The formation of male germ cells occurs in the tubules of the testis. Still immature spermatozoa, through the seminiferous tubules, enter the anterior part of the kidney, which functionally corresponds to the epididymis. Here they mature. Mature spermatozoa accumulate in the seminal vesicles. During fertilization, contractions of the walls of the seminal vesicles push sperm into the male's cloaca, from where, with the help of copulatory organs, they are introduced into the female's cloaca
central nervous system
Brain cartilaginous fish (Fig. 21) compared to the brain of cyclostomes is more developed, which is expressed primarily in the large size of the forebrain and cerebellum. The olfactory lobes of the forebrain are relatively large. The nervous tissue of the forebrain lines not only its sides, bottom, but also roof. The surface of the cerebellum forms a system of convolutions. In a shark, like in other vertebrates, the brain consists of five sections
Forebrain large. It is vaguely demarcated into right and left halves. In the anterior part of each half, highly developed olfactory lobes are clearly visible, ending at the olfactory chambers
Rice. 21. Shark brain from above:
1 - forebrain; 2 - olfactory lobes; 3 - diencephalon; 4 - pineal gland; 5 - midbrain; 6 - cerebellum; 7 - medulla oblongata; 8 - rhomboid fossa Roman numerals indicate the head nerves
large swellings. Rear end the forebrain smoothly passes into the diencephalon. On the preparation it is visible as a depression between the swellings of the forebrain and midbrain.
Diencephalon has an epithelial roof and is relatively small. On the roof of the brain there is a small thickening - the pineal gland, below the pituitary gland - the endocrine glands.
Midbrain well developed and has the appearance of large paired oval swellings, which are called optic lobes. They cover the rest of this part of the brain.
Cerebellum oval in shape, well developed. In front it covers a significant part of the optic lobes of the midbrain, and behind it overlaps part of the medulla oblongata. On the surface of the cerebellum there are several deep grooves that clearly separate its folds (gyri).
The last part of the brain is medulla, has an elongated shape. At the top, in its central part, a diamond-shaped fossa is visible. The medulla oblongata smoothly passes into the spinal cord, which stretches along the entire spinal canal.
Head nerves. Ten pairs of cerebral (cranial) nerves depart from the shark’s brain symmetrically on both sides of the brain.
I pair- olfactory nerves - depart from the olfactory lobes of the forebrain and are visible as its outgrowths. The olfactory nerve fan-shapedly breaks up into branches in the epithelium of the inner surface of the olfactory sac. Unlike other head nerves, the fibers of the olfactory nerve are processes of sensory cells, i.e., a purely sensory nerve.
II pair- optic nerves - originate at the bottom of the diencephalon. The right nerve goes to the left eye, and the left nerve goes to the right eye. In this way, the optic chiasm is formed. The optic nerve penetrates the orbit, enters the eyeball, where it branches into the retina. The nerve is purely sensory.
In all vertebrates, movements of the eyeball are carried out using six eye muscles. These muscles are innervated by three pairs of cephalic nerves (III, IV, VI). All these nerves are purely motor.
III pair- oculomotor nerves - depart from the bottom of the midbrain, penetrate the orbit, branch and innervate most of the eye muscles. Motor nerve.
IV pair- trochlear nerves - in the form of thin cords extend from the postero-superior part of the midbrain. In the preparation, the trochlear nerve is visible emerging from under the cerebellum. It passes through the anterior wall of the orbit and innervates only one oblique eye muscle. Motor nerve.
All other head nerves arise from the medulla oblongata.
V napa- trigeminal nerves - depart from the anterolateral surface of the medulla oblongata and are immediately divided into three branches (which is why these nerves are called trigeminal): orbital, maxillary, mandibular. The orbital branch, together with the orbital branch of the facial nerve (VII), passes through the orbit and branches in the anterior part of the snout, innervating the cutaneous sensory organs. The buccal branch, together with the branch of the facial nerve of the same name, passes along the bottom of the orbit, innervating the cutaneous sensory organs of the abdominal surface of the head. The maxillary and mandibular branches begin with a common trunk and are separated in the area of the jaw joint. They innervate the muscles of the jaw arch, teeth, and oral mucosa.
The function of the trigeminal nerve is mixed: it has purely sensory branches (orbital, buccal) and mixed sensory and motor branches (maxillary and mandibular).
VI pair- abducens nerves - motor, depart from the lower part of the medulla oblongata and immediately go to the bottom of the cranium. The origin of this nerve is not visible on the specimen. The abducens nerve innervates the external rectus (abductor) muscle of the eye.
VII pair- facial nerves - originate from the anterior part of the medulla oblongata and depart immediately behind the trigeminal (it is difficult to distinguish their bases on the preparation). The facial nerve splits into several branches.
The orbital branch goes along with the branch of the same name trigeminal nerve. Behind the orbital branch there is a rather large trunk, almost immediately splitting into two branches: palatine and sublingual. The palatine branch goes to the bottom of the skull and branches in the mucous membrane of the oral cavity. The hyoid branch passes behind the sputter and innervates the muscles of the hyoid arch and the cutaneous sensory organs of the sides of the head.
The facial nerve is mixed, its orbital and palatine branches are sensory, and the hypoglossal nerve is mixed: sensory and motor.
VIII pair- auditory nerves - originate from the anterior lateral part of the medulla oblongata. Almost immediately they enter the wall of the skull, innervating the inner ear. These are only sensory nerves.
IX pair- glossopharyngeal nerves - approach the first gill slit and here they are divided into two branches that innervate its anterior and posterior surfaces, as well as the tongue and pharynx. The nerves are mixed in function: they have sensory and motor fibers.
X pair- The vagus nerves are the most powerful of all the brain nerves. Unlike other brain nerves, the vagus nerve innervates a large area of the body. It begins from the posterolateral surface of the medulla oblongata with several roots, almost immediately merging into a thick nerve cord, which then exits through the opening between the auditory chamber and the foramen magnum. Then the vagus nerve splits into branches going to the pharynx, esophagus, stomach, heart, lateral line organs and the posterior end of the body. Its main branches are: four gills, visceral and lateral.
Four branchial branches innervate the second to fifth gill slits. The splanchnic branch runs as a continuation of the main trunk of the vagus nerve. It enters the abdominal cavity, innervating the internal organs. More superficially lies the lateral branch, which innervates the cutaneous sensory organs of the trunk and tail.
The vagus nerves are mixed: they include both sensory and motor fibers.
In higher vertebrates, two more pairs of head nerves are distinguished. XI pair - accessory nerves - are present only in mammals. XII pair - hypoglossal nerves - first appear in reptiles.
The spinal nerves emerge from the spinal cord segment by segment. They innervate the corresponding segments of the body and, in their function, are mixed nerves consisting of sensory and motor fibers.
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