Characteristics and diversity of crocodiles. Crocodile Nervous system of a crocodile diagram

Crocodiles are often ranked among the most interesting animals, and many zoologists and animal lovers spend their entire lives studying the characteristics and diversity of crocodiles.

Crocodiles (Crocodilia) are reptiles from the order of aquatic vertebrates. Their usual habitats are in Australia, Northern and South America, Asia and Africa. Today, they are considered the most powerful among reptiles. Allocate about 23 modern species that are in 3 families: crocodiles, alligators and gharials. As a rule, these animals live from 60 to 70 years, but there are individuals who have crossed the 125-year mark.

The sizes of crocodiles are very different, but the largest crocodile in the world, which is also the heaviest, is combed crocodile (Crocodylus porosus). Its length becomes 6.2 meters, and its weight is 1200 kg. Its habitats are the waters of eastern India, South-East Asia and Northern Australia.

(Osteolaemus tetraspis) is the smallest. Found in the waters of the tropical lowlands south of the Sahara of West Africa and West Central Africa. It is only 1-2 meters long.

Being hunters, all crocodiles have a very good vision. To begin with, they have the ability to keep their eyes open while immersed in water. This gives them the ability to lurk under water in anticipation of prey. On land, these reptiles are just as dangerous and fast, reaching speeds of up to 43.5 km/h.

Due to the specific appearance widely known is (Gavialis gangeticus). The largest males reach a length of 5-6 m, and females are slightly more than 4 m. They have a long and thin snout, designed to catch fish, their main food. Gharials live in the northern part of the Indian subcontinent, live in clean freshwater rivers with fast flow.

The second modern species in the gavial family gavial crocodile(Tomistoma schlegelii) also has a long snout and large size. It lives in Malaysia and Indonesia, very rarely, but still found in Thailand.

The most common member of the real crocodile family sharp-snouted crocodile(Crocodylus acutus). Enough large view: average length: 3.5 m, maximum ‒ 6 m, weight 180-450 kg. It has a stocky body with a long, powerful tail. Dwells along east coast Pacific Ocean from western Mexico south to Ecuador and along the west coast Atlantic Ocean from Guatemala north to the southern edge of Florida.

Siamese crocodile(Crocodylus siamensis) is a small, freshwater crocodile with a relatively broad, smooth muzzle. It is one of the most endangered species in wild nature, although it is widely bred in captivity. Adults feed mainly on fish, but may also eat amphibians, reptiles, and small mammals.

Crocodiles are among the most well-adapted hunters among reptiles and beyond. To begin with, there are about 24 teeth in one jaw, which are ideal for grasping prey, but not for chewing. Damaged and broken teeth are constantly replaced with new ones. After the crocodile has caught its prey, it does not let it go, pulls it into the water to drown it. After that, it swallows the victim without chewing, if it is too large, then it makes sharp movements from side to side to tear it into smaller pieces. All crocodiles are known as one of the most patient hunters because they can stay underwater right below the surface of the water, hiding for more than 8 hours waiting for prey.

One of the most dangerous animals in Africa and a famous cannibal is nile crocodile(Crocodylus niloticus). Only where hippos live, dies more people than from this terrible reptile. Among the very dangerous members of the family is also swamp crocodile(Crocodylus palustris) and Mississippi alligator(Alligator mississippiensis). About 100 people a year suffer from the first, and the second attacks only 10 a year, but almost all cases end in death.

(Alligator sinensis) has only about 200 individuals, so it has a very rare species. It hunts at night and its diet consists mainly of aquatic mollusks such as snails and mussels, as well as fish. It also feeds on occasional waterfowl and small mammals.

Male crocodiles grow much larger and faster than females. Depending on the species, a crocodile can live up to an average of 70 years, with some species the limit is as high as 130 years.

In the International Red Book is caiman black(Melanosuchus niger), which is found throughout most of the Amazon basin, but is much rarer than it was a few decades ago. This is the most large predator Amazon that grows up to 6 m in length. conservation status also have swamp crocodile(Crocodylus palustris), Orinoco(Crocodylus intermedius), sharp-nosed(Crocodylus acutus), narrow-nosed african(Crocodylus cataphractus) and gavial(Gavialis gangeticus).

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Features of the skeleton. The skeleton of crocodiles, both extinct and modern, is distinguished by a number of distinct features.

Firstly, it is a massive, strongly reinforced skull throughout. At the same time, the mouth is reinforced by a secondary bone palate, immobilely articulated with the cranium; in advanced forms it is more extensive than in mammals. This ensures the separation of the nasal passages from oral cavity, which is especially important for aquatic forms, however, this was primarily associated with the mechanical strengthening of the skull, which experiences large overloads during fast and strong bite. The top of the muzzle is formed by overgrown along middle line palate with prefrontal bones.

Apparently, for the same reason, the number and size of cranial foramens is minimized. The skull is characterized by a flat, low roof, the loss of the posterior frontal and posterior parietal bones and the epipterygoid, and the frontal and parietal bones fused along the midline.

Secondly, the skeleton of crocodiles is characterized by complex pneumatization of almost all bones of the skull. These are two systems of air channels. One is connected with the middle ear cavity and developed from the Eustachian tubes, and the second - with the nasal passages. This gives lightness to the massive skull without loss of strength. However, an exact explanation of such a pneumatization system has not yet been found.

Thirdly, most extinct and all modern crocodiles have 24 pre-sacral vertebrae, two sacral and 30-40 caudal. The bodies of all vertebrae, except for the atlas, the second sacral, and the first caudal, are procoelous, as in lizards and snakes that evolved later.

In modern crocodiles, the first nine vertebrae have ribs that do not reach the sternum - they form the neck. The cervical ribs, in addition to those associated with the atlas and epistrophy, have widely separated heads, while the dorsal (directed towards the back) is attached to the transverse process, and the main one is attached to the parapophysis in the lower part of the vertebral body.

Thus, a system of "arches" is formed, inside which the cervical artery passes. Trunk ribs, as a rule, ten to thirteen pairs. The ribs of the posterior trunk vertebrae gradually shorten and merge with the transverse process.

So, gharials do not have the last three, and crocodiles and alligators do not have four or five trunk ribs. Modern crocodiles have a sternum - a complex cartilaginous structure that connects the ends of the ribs, the bases of the coracoids and the interclavicle.

Fourthly, in the girdle of the forelimbs of crocodiles, the clavicle is lost, and the coracoids running in the dorsal-abdominal direction are elongated and equal in modern species to the length of the scapula. These signs indicate more effective work muscles that lift the body and take the limb back.

Fifth, it is characteristic for the hind limb girdle that two sacral vertebrae do not fuse into the sacrum, and the pubic bones do not participate in the formation of the acetabulum.

Sixth, an important feature of the skeleton of crocodiles is the biconvex first caudal vertebra, which performs the function of a ball joint. This ensures high mobility of the tail as the main locomotor organ in the water.

B-seventh, in the skeleton of the hind limbs of crocodiles, a well-developed calcaneal tuber is especially important, which is a powerful lever for bending the foot. This is due to the fact that when moving on land, crocodiles walk without lowering their bodies to the ground, which is much closer to the way mammals move.

Features of the central nervous system. Like all higher vertebrates, the crocodile brain is formed by five sections - the forebrain, diencephalon, midbrain, hindbrain, cerebellum and medulla oblongata. However, it has features that distinguish it from other groups of reptiles and bring it closer to the brains of birds and mammals.

The olfactory bulbs (derivatives of the forebrain) are brought to the end of the long muzzle by a pair of olfactory processes. Unlike most higher vertebrates, crocodiles do not have an pineal gland (an endocrine gland associated with the hypothalamic-pituitary system that produces serotonin) and a parietal organ. Moreover, no traces of their laying are observed even at the embryonic stages. The hollow cerebellum is divided into three transverse segments, which is not found in other reptiles and which can be considered as the first step towards the folded cerebellum of birds and mammals. Twelve pairs of completely separated nerves depart from the brain of crocodiles, while in other reptiles the tenth and eleventh nerves are not completely separated.

The medulla oblongata passes into the spinal cord. The latter has a hypocentric spinal canal, two thickenings corresponding to the limb belts, and continues to the beginning of the caudal spine.

Sense organs. The sense organs of crocodiles are also characterized by a number of features. So, in the first place in importance in crocodiles are the organs of vision and hearing. The latter is atypical for most reptiles.

The upper eyelid is more developed than the lower; the third eyelid, which forms the nictitating membrane, can completely cover the eye from inner corner eyes to the outside. All three eyelids do not contain cartilaginous tissue, but all alligator species (A1ligatoridae), as well as the gharial crocodile (Tomistoma schlegelii) and the blunt-nosed crocodile (Osteolaemus tetraspis) have a small zone of ossification in the upper eyelid. The lower eyelid, on the contrary, is mobile and contains many muscle fibers.

The crocodilian eye is devoid of the sclerotic ring, which is formed in many other reptiles around the cornea and is formed from bone plates. The pupil is vertical. In connection with the nocturnal activity of these animals, rods predominate in the retina, which are responsible for night vision and form a cluster (the so-called Tapetum lucidum) that reflects light. This is also due to the glow of the eyes of crocodiles in the dark. The eyes of crocodilians are devoid of lacrimal glands, but have a well-developed Garder's gland, which opens with several ducts under the third eyelid. This gland secretes a fatty secret that lubricates the cornea - it is its secretions that form "crocodile tears". There are also small salt glands in the upper and lower eyelids of crocodiles.

Of all reptiles, crocodiles have the most developed hearing organ, which is second only to sight in importance.

Crocodiles have a well-developed, slit-like external auditory canal that closes with a valve when the crocodile dives. The middle ear of one side is connected with the middle ear of the other and with the pharynx by a whole system of adnexal cavities opening into the tympanic cavities. At the same time, it is still unknown what role the complex system of cavities in the skull of crocodiles, anatomically connected with the ear, plays for hearing. In the inner ear there is a cochlea similar to that of birds, which is not found in other reptiles. So we can say that the hearing of these reptiles is comparable to the hearing of birds and mammals.

The olfactory organs are well developed in most reptiles and are represented by the olfactory organ itself, located in the nasal cavity, and the Jacobson organ. In crocodiles, the features of the olfactory organs are associated with the structure of the skull, and the sense of smell itself is in third place in importance after sight and hearing. The nasal passages of crocodiles are elongated in accordance with the structure of the muzzle and are lined with three types of epithelium: keratinized epithelium lines the vestibule of the nasal cavity; it is followed by a region of the respiratory epithelium that lines the choanae, the ventral part of the nasal concha, and partially the paranasal sinuses; The olfactory epithelium lines the dorsal part of the nasal cavity. The Jacobson organ is absent in crocodiles, although it is formed at the embryonic stages.

The organs of taste in crocodiles are located on the mucous membrane covering the pterygoid bone and the ventral wall of the pharynx. At the same time, the integuments of the actual mouth are keratinized and do not have taste buds.

Despite the strong keratinization of the skin, reptiles have zones of skin sensitivity (touch), which are represented by tactile receptors located in the skin shields - these are the so-called sensitive kidneys. Such shields are often grouped into tactile patches. In crocodiles, the sense of touch is most developed among reptiles.

Most of the tactile receptors are located on the muzzle and in the mouth of crocodiles. In these areas there can be up to twenty on each shield. That is why female crocodiles, with the help of their mouths, manage to do very subtle manipulations - transfer eggs to the nest, lay them, break the shell to make it easier for the cubs to get out, and then transfer the cubs.

In other parts of the body, tactile receptors can be located one at a time on the shield. Young crocodiles have more of them than adults. So, in newborn crocodiles, they are on all shields, with the exception of the end of the tail and soles, and in large old animals from the genus Alligators (Alligator), they are preserved only on the muzzle and in the mouth. A separate organ of touch is a depression in the epidermis with a mound in the middle, under the cells of the epidermis in the depression is a gelatinous subepidermal connective tissue, which houses the tactile bodies connected to nerve fibers.

Features of the digestive system. . The digestive system of crocodiles is quite complex. The oral cavity is markedly separated from the pharynx. In crocodiles, the nasopharyngeal passages are separated from the oral cavity by a secondary bony palate, which arises from the growth of the palatine processes of the premaxillary and maxillary bones and the palatine and pterygoid bones themselves. This allows predators to completely separate the respiratory and digestive systems. A crocodile can safely catch fish and swallow it underwater without the risk of suffocation. At the bottom of the oral cavity is a movable muscular tongue. The mouth of crocodiles is equipped with thecodont-type teeth located in the alveoli - they are designed to grab and hold food. Teeth change regularly throughout life. The teeth sit on the maxillary, intermaxillary, pterygoid and mandibular bones. Unlike amphibians, the opener of crocodiles is devoid of teeth.

There are no salivary glands. The oral cavity ends with a muscular-cartilaginous valve that separates the mouth from the pharynx. The pharynx opens into a straight esophagus, which in turn opens into a two-chambered stomach, similar in structure to the stomach of birds.

The stomach is divided into two sections: muscular and pyloric. The first is equipped with external muscle cords attached to the tendon plates on both sides of the stomach.

The inner surface is provided with a cornified radula (grater). The second section of the stomach is intended for the digestion of food already pounded in the previous section. It has a folded mucosa. The epithelium is formed by prismatic cells, between which the sap glands open. Stones (gastroliths) and other foreign objects. According to one hypothesis, crocodiles swallow them in order to better grind their food, but there is no exact explanation for this phenomenon yet.

The fact is that the sizes of gastroliths are sometimes unreasonably large and do not allow them to be used for effective grinding of prey, in addition, in zoos, crocodiles are often deprived of the opportunity to swallow stones, which does not impair their digestion at all.

The intestines are divided into small and large, the large intestine ends with the rectum. The latter opens into the cloaca, where the ureters and ducts of the sex glands also open. The cecum of crocodilians is rudimentary and is located on the border of the large and small intestines. The total length of the intestine is one and a half to two times the total length of the body (from the tip of the muzzle to the tip of the tail).

The bilobed liver of crocodilians is located in the cranial direction from the diaphragm. The gallbladder is large and is located near the underside of the lower lobe of the right lung.

The pancreas of adult crocodiles varies greatly in size and shape. Topographically, it is adjacent to the first loop small intestine in the form of an elongated body.

The abdominal cavity of crocodiles is separated from the chest by a diaphragm, which is not observed in other reptiles. However, this diaphragm differs from the diaphragm of mammals - in crocodiles it is formed by connective tissue and muscle fusion between the stomach, liver and pleura of the lung. Peculiarities circulatory system. First and main distinctive feature The circulatory system of crocodiles is a four-chambered heart, while in all other representatives of the class of reptiles it is three-chambered. However, there is an opening establishing a connection between the two aortic arches, resulting in partial mixing of oxygenated blood and carbonated blood.

This hole is important physiological function safety valve during prolonged immersion under water, when excess pressure in the lungs causes stagnation of blood in the pulmonary artery and high blood pressure in the pulmonary vein, which extends to the left ventricle and right aorta. At the same time, blood saturated with carbon dioxide flows from right to left, while normally, on the contrary, blood saturated with oxygen flows from left to right.

The heart rate at a body temperature of thirty degrees Celsius reaches twenty-five to thirty-five beats per minute. When immersed in water in case of danger, crocodiles first increase the heart rate, and then slow it down, while it can drop to two beats per minute.

The arterial system has a number of significant features. The arterial trunk is divided into 3 vessels, independently departing from various parts ventricle. From the right side of the ventricle (containing venous blood) departs the pulmonary vessel, which soon divides into the left and right pulmonary arteries. From the left side of the ventricle (containing arterial blood), the right aortic arch departs, from which, in turn, the carotid and subclavian arteries depart. Finally, the left aortic arch departs from the middle of the ventricle, which, having rounded the heart, connects with the right aortic arch and forms the dorsal aorta. In connection with this differentiation of arterial vessels, only venous blood enters the pulmonary arteries; into the right aortic arch, and therefore into the carotid and subclavian arteries - pure arterial blood. Only the left aortic arch receives mixed blood, and, consequently, the blood in the dorsal aorta is also mixed, but with a clear predominance of oxidized blood. The dorsal aorta stretches under the spine, branches depart from it to the internal organs and muscles. In the pelvic region, large iliac arteries depart from the dorsal aorta, carrying blood to the hind limbs.

The venous system does not have such significant features as the arterial one. From the caudal region, blood is collected into the caudal vein, which divides into 2 pelvic veins in the cloaca. Veins from the hind limbs enter the pelvic veins, after which, having separated 2 portal veins of the kidneys, they unite into the abdominal vein. The latter, having taken a series of veins from internal organs, flows into the liver, where it forms the portal circulatory system.

The veins that carry blood from the kidneys form the main venous vessel of the trunk region - the posterior vena cava, which stretches under the spine and pours blood into the right atrium. The hepatic vein also flows into the posterior vena cava, carrying blood to the liver, which got there through the abdominal vein and the vein from the intestine and passed through the portal vascular system of the liver.

From the head, blood is collected in paired jugular veins, which, having connected with paired subclavian veins, form 2 (left and right) anterior vena cava, which flow into the right atrium. The pulmonary veins drain blood into the left atrium.

The lymphatic system is better developed in crocodiles than in most reptiles, and has a vascular structure. Networks of lymphatic vessels are located at the attachment sites of the mesentery, at the base of the tail, under the armpits and at the back of the head.

The blood of crocodiles is not fundamentally different from the blood of other reptiles. Erythrocytes have nuclei, and the hemoglobin content ranges from 6.9 to 9.0 gram percent. White blood cells are represented by agranulocytes, large and small monocytes, and eosinophilic and neutrophilic granulocytes.

Respiratory system. The larynx lies on the wide shield-like body of the hyoid bone and is connected to it by a ligament. The straight trachea divides into two bronchi, each of which enters the lungs in the center on the underside. The lungs have a complex alveolar structure with tubular accessory chambers, which fits into general scheme buildings respiratory systems s reptiles. During embryonic development, the gill apparatus is not formed, and gas exchange in the embryo in the egg is carried out using the vessels of the allantois (primary bladder) and the yolk sac. Adult crocodiles breathe only with the lungs, since they have no skin respiration due to the appearance of the horny cover. All of the above plus higher general level life processes in reptiles cause them to have a more complex structure of the lungs.

Characterized by differentiation of the respiratory tract. The laryngeal fissure leads to the chamber of the same name, supported by the unpaired cricoid and paired arytenoid cartilages. A long trachea emerges from the larynx. The trachea divides into two bronchi that lead to the lungs.

The respiratory mechanism is different than that of amphibians. Air is not swallowed by the mouth, but is drawn into the lungs and pushed back out by the expansion, expansion and contraction of the chest due to the movement of the ribs. This type of breathing, characteristic of higher vertebrates, as well as a more complex structure of the lungs, also provide a more perfect gas exchange.

crocodile skeleton habitat reptile

Fig.2 The internal structure of a crocodile

Of interest is the structure of the initial sections of the respiratory tract. The nostrils, located at the end of the muzzle at the top of the nasal tubercle, are equipped with valves. The nasal cavity stretches along the entire muzzle and has large paranasal sinuses - they may be used as resonators. The long muzzle and the secondary bony palate carry the choanae (internal nasal openings) almost into the pharynx. The oral cavity is separated from the pharynx by a tightly closing valve. This structure allows crocodiles to manipulate prey or objects with the help of their mouths and at the same time breathe, exposing the tip of the muzzle to the surface. The respiratory rhythm normally consists of two to three respiratory cycles, followed by a pause of half a minute to five minutes.

Unlike birds and mammals, the unconditional signal to inhale is not an increase in the concentration of carbon dioxide in the blood, but a decrease in the concentration of oxygen. The maximum length of time that crocodiles can stay underwater without breathing is specific to different types and normally ranges from thirty minutes to one hour.

Urogenital system. The excretory organs of adult reptiles are represented by pelvic buds that develop behind the rudiments of the trunk bud from the rudimentary tissue common with it. Trunk kidneys arise as a germinal organ and function until the animals hatch from the egg or some time after hatching. With the development of the pelvic kidney, a canal departs from the back of the Wolffian canal, connecting with the excretory tubules of the kidney. Thus, the ureter is formed. The left and right ureters enter the cloaca from the dorsal side. From the ventral side, the bladder opens into the cloaca. In crocodiles, snakes and some lizards, the bladder is underdeveloped. There is a reduction of the trunk kidneys after the formation of the pelvic ones. In females, almost the entire primary kidney is reduced, in males, the anterior part of the kidney is preserved (the vas deferens pass through it) and represents the epididymis of the testis.

The number of nephrons in the kidney is very large, on average, reptiles have about 5000 of them. Crocodiles, due to their semi-aquatic lifestyle, have well-developed vascular glomeruli in the nephrons, which are responsible for filtering water. The filtration rate is much lower than that of amphibians. In turtles and crocodiles, it is 5-10, in scaly (lizards and snakes) - 4-5 ml / kg per hour (in tailless amphibians 34-35).

The main product of nitrogen metabolism is uric acid, a substance that is almost insoluble in water. In this regard, the urine of the vast majority of reptiles is mushy, its excretion requires a negligible amount of water consumption.

Reproductive organs. The sex glands lie in the body cavity on the sides of the spine. The testicles have an appendage, which, as already mentioned, is the remnant of the trunk kidney. The tubules of the appendage flow into the Wolf canal, which in male crocodiles (as well as in other higher vertebrates) serves only as a vas deferens. In crocodiles and turtles, the copulatory organ is unpaired and is an outgrowth of the cloacal wall.

In female wolfs, the channel is not preserved. The Muller canal functions as an oviduct, which is a paired thin-walled tube that opens with a funnel into the body cavity, and at the other end into the cloaca. Crocodiles, like turtles, have a gland in the middle section of the oviduct that secretes the protein shell of the egg. In the lower part of the oviduct there are glands that form the parchment-like (soaked in lime) shell of the egg.

The external structure of crocodiles

Leather and its derivatives. From modern reptiles crocodiles are one of the few that lead a semi-aquatic lifestyle. In this regard, their anatomical structure differs markedly from the structure of most reptiles, which is primarily due to the intensity of the development of the circulatory and respiratory systems, as well as the structure of the kidneys.

In crocodilians, the epidermis makes up 12-14% of all skin, while it contains from 82 to 98% of the pigments that determine the color of the skin of crocodiles. Most upper layer epidermis - the stratum corneum, consisting of dead cells. In it, most reptiles distinguish three zones: cuticular - external, then - alpha-keratin and betakeratin zones. However, the cuticular layer is absent in crocodiles, which excludes the possibility of molting, which is characteristic of most reptiles.

Their skin is constantly renewed as it wears out. The only apparent molt in crocodilians occurs immediately after hatching from the egg, which sheds the embryonic epitrichial layer. In all other cases, desquamation of pieces of skin is a sign of a metabolic disorder. On the body of crocodiles are horny shields, which, like bones, are derivatives of the epidermis.

The dermis (corium) or the skin itself is formed by two layers - spongy and compact. In crocodiles, both of these layers are thick and permeated with a large number of connective tissue fibers, which are arranged in layers. The number of layers increases with age: for example, in the Mississippi alligator (Alligator mississipiensis), embryos have two or three such layers, while adult alligators have twenty-four layers. Below are the connective tissue layer and fatty tissue, closely related to each other.

In the dermis of crocodiles there are zones of ossification, forming the so-called osteoderms - oval-shaped bone plates, elongated in the longitudinal direction, sometimes with a longitudinal rib. They are located under the horny shields. In all species of crocodiles, osteoderms are most pronounced in the dorsal (dorsal) scutes, however, in caimans (Caiman), smooth-fronted caimans (Paleosuchus), blunt-nosed crocodiles (Osteolaemus), Johnston's crocodile (Crocodylus johnstoni) and the Nile crocodile (Crocodylus niloticus), osteoderms are present in the lateral (lateral) shields. In the ventral (ventral) scutes, they are found in caimans (Caitan), black caimans (Melanosuchus), smooth-fronted caimans (Paleosuhus), blunt-nosed crocodiles (Osteolaemus), gharial crocodiles (Tomistoma), as well as narrow-nosed crocodile (Crocodylus cataphractus), Johnston's crocodile (Croc odylus johnstoni) and the Nile crocodile (Crocodylus niloticus).

Osteoderms form as the crocodile grows, and in adult animals their number increases, for example, in adult crocodiles, the scutes of the upper eyelid can ossify.

Osteoderms grow towards the edge of the skin shield and at the same time in thickness. They are either separated from each other (in most cases) or adjoin at the edges without forming seams, but seam joints can form between the longitudinal rows of dorsal scutes. Osteoderms are similar in bone structure to spongiosis (cancellous bone), have vertical depressions and branching canals associated with internal cavities. This structure of channels and cavities is supplied with blood vessels, has nerve cells, and also contains connective tissue and pigment cells.

Osteoderms perform several functions: protective reinforcing, thermoregulatory (conduct heat into the body and reduce its return) and cumulative (play the role of a possible depot of minerals)

Skin glands are derivatives of the skin. The widely held claim that reptile skin is devoid of glands is quite far from the truth. Three groups of glands are distinguished in crocodiles: dorsal (dorsal), mandibular (mandibular) and cloacal. All three groups of glands are considered to be modified sebaceous glands.

The dorsal glands were first described for the Nile crocodile (Crocodylus niloticus) in 1899. They are located under the anterior outer corners of the second row of dorsal scutes on either side of the midline and can be found along the entire length of the back from the shoulder blades to the base of the tail. These glands are invaginations (invaginations) of the epidermis that occur in the middle stages of embryogenesis, their walls are lined with a single-layer basal cell epithelium that produces an oil-fatty secret. These secretions lubricate the skin between the shields, giving it elasticity. With age, the number and size of the glands decrease, and their functions weaken. For example, in the Mississippian alligator (Alligator mississippiensis), one meter long, the glands are well defined and have a size of about three millimeters, while in large adult specimens their beneficial duct is glued and practically invisible.

There are two mandibular glands: they are located in the skin mandible close to the inner edge of the mandibular bones, approximately in their middle part. The gland is a powerful pin-shaped invagination of the epithelium and is able to turn outward like a finger of a glove. For this, the opening of the gland is provided with a sphincter and a muscle that runs from the base of the gland to the hyoid bone. The walls of the mandibular glands are lined with pigmented epithelium, which forms numerous papillae and folds and secretes a mucus-like sebaceous fat secretion. It serves for Western marks and identification of partners.

Paired cloacal glands lie in the side walls of the cloaca close to the anus. In structure, composition of the secret and functions, they are very close to the mandibular ones. During the mating season, the secretion of the mandibular and cloacal glands increases.

The internal structure of crocodiles

Features of the skeleton. The skeleton of crocodiles, both extinct and modern, is distinguished by a number of distinct features.

Firstly, it is a massive, strongly reinforced skull throughout. At the same time, the mouth is reinforced by a secondary bone palate, immobilely articulated with the cranium; in advanced forms it is more extensive than in mammals. This ensures the separation of the nasal passages from the oral cavity, which is especially important for aquatic forms, but it was primarily associated with the mechanical strengthening of the skull, which experiences large overloads during a fast and strong bite. The top of the muzzle is formed by prefrontal bones that have grown along the midline of the palate.

Apparently, for the same reason, the number and size of cranial foramens is minimized. The skull is characterized by a flat, low roof, the loss of the posterior frontal and posterior parietal bones and the epipterygoid, and the frontal and parietal bones fused along the midline.

Secondly, the skeleton of crocodiles is characterized by complex pneumatization of almost all bones of the skull. These are two systems of air channels. One is connected with the middle ear cavity and developed from the Eustachian tubes, and the second - with the nasal passages. This gives lightness to the massive skull without loss of strength. However, an exact explanation of such a pneumatization system has not yet been found.

Thirdly, most extinct and all modern crocodiles have 24 pre-sacral vertebrae, two sacral and 30-40 caudal. The bodies of all vertebrae, except for the atlas, the second sacral, and the first caudal, are procoelous, as in lizards and snakes that evolved later.

In modern crocodiles, the first nine vertebrae have ribs that do not reach the sternum - they form the neck. The cervical ribs, in addition to those associated with the atlas and epistrophy, have widely separated heads, while the dorsal (directed towards the back) is attached to the transverse process, and the main one is attached to the parapophysis in the lower part of the vertebral body.

Thus, a system of "arches" is formed, inside which the cervical artery passes. Trunk ribs, as a rule, ten to thirteen pairs. The ribs of the posterior trunk vertebrae gradually shorten and merge with the transverse process.

So, gharials do not have the last three, and crocodiles and alligators do not have four or five trunk ribs. Modern crocodiles have a sternum - a complex cartilaginous structure that connects the ends of the ribs, the bases of the coracoids and the interclavicle.

Fourthly, in the girdle of the forelimbs of crocodiles, the clavicle is lost, and the coracoids running in the dorsal-abdominal direction are elongated and equal in modern species to the length of the scapula. These signs indicate a more efficient work of the muscles that lift the body and take the limb back.

Fifth, it is characteristic for the hind limb girdle that two sacral vertebrae do not fuse into the sacrum, and the pubic bones do not participate in the formation of the acetabulum.

Sixth, an important feature of the skeleton of crocodiles is the biconvex first caudal vertebra, which performs the function of a ball joint. This ensures high mobility of the tail as the main locomotor organ in the water.

B-seventh, in the skeleton of the hind limbs of crocodiles, a well-developed calcaneal tuber is especially important, which is a powerful lever for bending the foot. This is due to the fact that when moving on land, crocodiles walk without lowering their bodies to the ground, which is much closer to the way mammals move.

Features of the central nervous system. Like all higher vertebrates, the crocodile brain is formed by five sections - the forebrain, diencephalon, midbrain, hindbrain, cerebellum and medulla oblongata. However, it has features that distinguish it from other groups of reptiles and bring it closer to the brains of birds and mammals.

The olfactory bulbs (derivatives of the forebrain) are brought to the end of the long muzzle by a pair of olfactory processes. Unlike most higher vertebrates, crocodiles do not have an pineal gland (an endocrine gland associated with the hypothalamic-pituitary system that produces serotonin) and a parietal organ. Moreover, no traces of their laying are observed even at the embryonic stages. The hollow cerebellum is divided into three transverse segments, which is not found in other reptiles and which can be considered as the first step towards the folded cerebellum of birds and mammals. Twelve pairs of completely separated nerves depart from the brain of crocodiles, while in other reptiles the tenth and eleventh nerves are not completely separated.

The medulla oblongata passes into the spinal cord. The latter has a hypocentric spinal canal, two thickenings corresponding to the limb belts, and continues to the beginning of the caudal spine.

Sense organs. The sense organs of crocodiles are also characterized by a number of features. So, in the first place in importance in crocodiles are the organs of vision and hearing. The latter is atypical for most reptiles.

The upper eyelid is more developed than the lower; the third eyelid, forming the nictitating membrane, can completely close the eye from the inner corner of the eye to the outer. All three eyelids do not contain cartilaginous tissue, but all alligator species (A1ligatoridae), as well as the gharial crocodile (Tomistoma schlegelii) and the blunt-nosed crocodile (Osteolaemus tetraspis) have a small zone of ossification in the upper eyelid. The lower eyelid, on the contrary, is mobile and contains many muscle fibers.

The crocodilian eye is devoid of the sclerotic ring, which is formed in many other reptiles around the cornea and is formed from bone plates. The pupil is vertical. In connection with the nocturnal activity of these animals, rods predominate in the retina, which are responsible for night vision and form a cluster (the so-called Tapetum lucidum) that reflects light. This is also due to the glow of the eyes of crocodiles in the dark. The eyes of crocodilians are devoid of lacrimal glands, but have a well-developed Garder's gland, which opens with several ducts under the third eyelid. This gland secretes a fatty secret that lubricates the cornea - it is its secretions that form "crocodile tears". There are also small salt glands in the upper and lower eyelids of crocodiles.

Of all reptiles, crocodiles have the most developed hearing organ, which is second only to sight in importance.

Crocodiles have a well-developed, slit-like external auditory canal that closes with a valve when the crocodile dives. The middle ear of one side is connected with the middle ear of the other and with the pharynx by a whole system of adnexal cavities opening into the tympanic cavities. At the same time, it is still unknown what role the complex system of cavities in the skull of crocodiles, anatomically connected with the ear, plays for hearing. In the inner ear there is a cochlea similar to that of birds, which is not found in other reptiles. So we can say that the hearing of these reptiles is comparable to the hearing of birds and mammals.

The olfactory organs are well developed in most reptiles and are represented by the olfactory organ itself, located in the nasal cavity, and the Jacobson organ. In crocodiles, the features of the olfactory organs are associated with the structure of the skull, and the sense of smell itself is in third place in importance after sight and hearing. The nasal passages of crocodiles are elongated in accordance with the structure of the muzzle and are lined with three types of epithelium: keratinized epithelium lines the vestibule of the nasal cavity; it is followed by a region of the respiratory epithelium that lines the choanae, the ventral part of the nasal concha, and partially the paranasal sinuses; The olfactory epithelium lines the dorsal part of the nasal cavity. The Jacobson organ is absent in crocodiles, although it is formed at the embryonic stages.

The organs of taste in crocodiles are located on the mucous membrane covering the pterygoid bone and the ventral wall of the pharynx. At the same time, the integuments of the actual mouth are keratinized and do not have taste buds.

Despite the strong keratinization of the skin, reptiles have zones of skin sensitivity (touch), which are represented by tactile receptors located in the skin shields - these are the so-called sensitive kidneys. Such shields are often grouped into tactile patches. In crocodiles, the sense of touch is most developed among reptiles.

Most of the tactile receptors are located on the muzzle and in the mouth of crocodiles. In these areas there can be up to twenty on each shield. That is why female crocodiles, with the help of their mouths, manage to do very subtle manipulations - transfer eggs to the nest, lay them, break the shell to make it easier for the cubs to get out, and then transfer the cubs.

In other parts of the body, tactile receptors can be located one at a time on the shield. Young crocodiles have more of them than adults. So, in newborn crocodiles, they are on all shields, with the exception of the end of the tail and soles, and in large old animals from the genus Alligators (Alligator), they are preserved only on the muzzle and in the mouth. A separate organ of touch is a depression in the epidermis with a mound in the middle, under the cells of the epidermis in the depression is a gelatinous subepidermal connective tissue, which houses the tactile bodies connected to nerve fibers.

Features of the digestive system. . The digestive system of crocodiles is quite complex. The oral cavity is markedly separated from the pharynx. In crocodiles, the nasopharyngeal passages are separated from the oral cavity by a secondary bony palate, which arises from the growth of the palatine processes of the premaxillary and maxillary bones and the palatine and pterygoid bones themselves. This allows predators to completely separate the respiratory and digestive systems. A crocodile can safely catch fish and swallow it underwater without the risk of suffocation. At the bottom of the oral cavity is a movable muscular tongue. The mouth of crocodiles is equipped with thecodont-type teeth located in the alveoli - they are designed to grab and hold food. Teeth are replaced regularly throughout life. The teeth sit on the maxillary, intermaxillary, pterygoid and mandibular bones. Unlike amphibians, the opener of crocodiles is devoid of teeth.

There are no salivary glands. The oral cavity ends with a muscular-cartilaginous valve that separates the mouth from the pharynx. The pharynx opens into a straight esophagus, which in turn opens into a two-chambered stomach, similar in structure to the stomach of birds.

The stomach is divided into two sections: muscular and pyloric. The first is equipped with external muscle cords attached to the tendon plates on both sides of the stomach.

The inner surface is provided with a cornified radula (grater). The second section of the stomach is intended for the digestion of food already pounded in the previous section. It has a folded mucosa. The epithelium is formed by prismatic cells, between which the sap glands open. Stones (gastroliths) and other foreign objects are often found in the stomachs of crocodiles. According to one hypothesis, crocodiles swallow them in order to better grind their food, but there is no exact explanation for this phenomenon yet.

The fact is that the sizes of gastroliths are sometimes unreasonably large and do not allow them to be used for effective grinding of prey, in addition, in zoos, crocodiles are often deprived of the opportunity to swallow stones, which does not impair their digestion at all.

The intestines are divided into small and large, the large intestine ends with the rectum. The latter opens into the cloaca, where the ureters and ducts of the sex glands also open. The cecum of crocodilians is rudimentary and is located on the border of the large and small intestines. The total length of the intestine is one and a half to two times the total length of the body (from the tip of the muzzle to the tip of the tail).

The bilobed liver of crocodilians is located in the cranial direction from the diaphragm. The gallbladder is large and is located near the underside of the lower lobe of the right lung.

The pancreas of adult crocodiles varies greatly in size and shape. Topographically, it is adjacent to the first loop of the small intestine in the form of an oblong body.

The abdominal cavity of crocodiles is separated from the chest by a diaphragm, which is not observed in other reptiles. However, this diaphragm differs from the diaphragm of mammals - in crocodiles it is formed by connective tissue and muscle fusion between the stomach, liver and pleura of the lung. Features of the circulatory system. The first and main distinguishing feature of the circulatory system of crocodiles is a four-chambered heart, while in all other representatives of the reptile class it is three-chambered. However, there is an opening establishing a connection between the two aortic arches, resulting in partial mixing of oxygenated blood and carbonated blood.

This opening has an important physiological function as a safety valve during prolonged submergence, when excess pressure in the lungs causes congestion of blood in the pulmonary artery and increased pressure in the pulmonary vein, which extends to the left ventricle and right aorta. At the same time, blood saturated with carbon dioxide flows from right to left, while normally, on the contrary, blood saturated with oxygen flows from left to right.

The heart rate at a body temperature of thirty degrees Celsius reaches twenty-five to thirty-five beats per minute. When immersed in water in case of danger, crocodiles first increase the heart rate, and then slow it down, while it can drop to two beats per minute.

The arterial system has a number of significant features. The arterial trunk is divided into 3 vessels, independently departing from various parts of the ventricle. From the right side of the ventricle (containing venous blood) departs the pulmonary vessel, which soon divides into the left and right pulmonary arteries. From the left side of the ventricle (containing arterial blood), the right aortic arch departs, from which, in turn, the carotid and subclavian arteries depart. Finally, the left aortic arch departs from the middle of the ventricle, which, having rounded the heart, connects with the right aortic arch and forms the dorsal aorta. In connection with this differentiation of arterial vessels, only venous blood enters the pulmonary arteries; into the right aortic arch, and therefore into the carotid and subclavian arteries - pure arterial blood. Only the left aortic arch receives mixed blood, and, consequently, the blood in the dorsal aorta is also mixed, but with a clear predominance of oxidized blood. The dorsal aorta stretches under the spine, branches depart from it to the internal organs and muscles. In the pelvic region, large iliac arteries depart from the dorsal aorta, carrying blood to the hind limbs.

The venous system does not have such significant features as the arterial one. From the caudal region, blood is collected into the caudal vein, which divides into 2 pelvic veins in the cloaca. Veins from the hind limbs enter the pelvic veins, after which, having separated 2 portal veins of the kidneys, they unite into the abdominal vein. The latter, having received a number of veins from the internal organs, flows into the liver, where it forms the portal circulatory system.

The veins that carry blood from the kidneys form the main venous vessel of the trunk region - the posterior vena cava, which stretches under the spine and pours blood into the right atrium. The hepatic vein also flows into the posterior vena cava, carrying blood to the liver, which got there through the abdominal vein and the vein from the intestine and passed through the portal vascular system of the liver.

From the head, blood is collected in paired jugular veins, which, having connected with paired subclavian veins, form 2 (left and right) anterior vena cava, which flow into the right atrium. The pulmonary veins drain blood into the left atrium.

The lymphatic system is better developed in crocodiles than in most reptiles, and has a vascular structure. Networks of lymphatic vessels are located at the attachment sites of the mesentery, at the base of the tail, under the armpits and at the back of the head.

The blood of crocodiles is not fundamentally different from the blood of other reptiles. Erythrocytes have nuclei, and the hemoglobin content ranges from 6.9 to 9.0 gram percent. White blood cells are represented by agranulocytes, large and small monocytes, and eosinophilic and neutrophilic granulocytes.

Respiratory system. The larynx lies on the wide shield-like body of the hyoid bone and is connected to it by a ligament. The straight trachea divides into two bronchi, each of which enters the lungs in the center on the underside. The lungs have a complex alveolar structure with tubular adnexal chambers, which fits into the general structure of the respiratory system of reptiles. During embryonic development, the gill apparatus is not formed, and gas exchange in the embryo in the egg is carried out using the vessels of the allantois (primary bladder) and the yolk sac. Adult crocodiles breathe only with the lungs, since they have no skin respiration due to the appearance of the horny cover. All of the above, as well as a higher general level of life processes in reptiles, cause them to have a more complex structure of the lungs.

Characterized by differentiation of the respiratory tract. The laryngeal fissure leads to the chamber of the same name, supported by the unpaired cricoid and paired arytenoid cartilages. A long trachea emerges from the larynx. The trachea divides into two bronchi that lead to the lungs.

The respiratory mechanism is different than that of amphibians. Air is not swallowed by the mouth, but is drawn into the lungs and pushed back out by the expansion, expansion and contraction of the chest due to the movement of the ribs. This type of breathing, characteristic of higher vertebrates, as well as a more complex structure of the lungs, also provide a more perfect gas exchange.

Of interest is the structure of the initial sections of the respiratory tract. The nostrils, located at the end of the muzzle at the top of the nasal tubercle, are equipped with valves. The nasal cavity stretches along the entire muzzle and has large paranasal sinuses - they may be used as resonators. The long muzzle and the secondary bony palate carry the choanae (internal nasal openings) almost into the pharynx. The oral cavity is separated from the pharynx by a tightly closing valve. This structure allows crocodiles to manipulate prey or objects with the help of their mouths and at the same time breathe, exposing the tip of the muzzle to the surface. The respiratory rhythm normally consists of two to three respiratory cycles, followed by a pause of half a minute to five minutes.

Unlike birds and mammals, the unconditional signal to inhale is not an increase in the concentration of carbon dioxide in the blood, but a decrease in the concentration of oxygen. The maximum time that crocodiles stay under water without breathing is specific to different species and normally ranges from thirty minutes to one hour.

Urogenital system. The excretory organs of adult reptiles are represented by pelvic buds that develop behind the rudiments of the trunk bud from the rudimentary tissue common with it. Trunk kidneys arise as a germinal organ and function until the animals hatch from the egg or some time after hatching. With the development of the pelvic kidney, a canal departs from the back of the Wolffian canal, connecting with the excretory tubules of the kidney. Thus, the ureter is formed. The left and right ureters enter the cloaca from the dorsal side. From the ventral side, the bladder opens into the cloaca. In crocodiles, snakes and some lizards, the bladder is underdeveloped. There is a reduction of the trunk kidneys after the formation of the pelvic ones. In females, almost the entire primary kidney is reduced, in males, the anterior part of the kidney is preserved (the vas deferens pass through it) and represents the epididymis of the testis.

The number of nephrons in the kidney is very large, on average, reptiles have about 5000 of them. Crocodiles, due to their semi-aquatic lifestyle, have well-developed vascular glomeruli in the nephrons, which are responsible for filtering water. The filtration rate is much lower than that of amphibians. In turtles and crocodiles, it is 5–10, in scaly ones (lizards and snakes) it is 4–5 ml/kg per hour (in tailless amphibians, 34–35).

The main product of nitrogen metabolism is uric acid, a substance that is almost insoluble in water. In this regard, the urine of the vast majority of reptiles is mushy, its excretion requires a negligible amount of water consumption.

Reproductive organs. The sex glands lie in the body cavity on the sides of the spine. The testicles have an appendage, which, as already mentioned, is the remnant of the trunk kidney. The tubules of the appendage flow into the Wolf canal, which in male crocodiles (as well as in other higher vertebrates) serves only as a vas deferens. In crocodiles and turtles, the copulatory organ is unpaired and is an outgrowth of the cloacal wall.

In female wolfs, the channel is not preserved. The Muller canal functions as an oviduct, which is a paired thin-walled tube that opens with a funnel into the body cavity, and at the other end into the cloaca. Crocodiles, like turtles, have a gland in the middle section of the oviduct that secretes the protein shell of the egg. In the lower part of the oviduct there are glands that form the parchment-like (soaked in lime) shell of the egg.

Crocodiles occupy a special position among modern reptiles, being closer relatives of extinct dinosaurs, which they survived by almost 60 million years, and modern birds than other reptiles of our time. A number of features of the organization of crocodiles, and first of all the perfection of the nervous, circulatory and respiratory systems, allows us to consider them the most highly organized of all living reptiles. The evolution of crocodiles, starting from the appearance of this group about 150 million years ago, went in the direction of ever greater adaptation to the aquatic lifestyle and predation. The fact that crocodiles have survived to our time is often explained by their life in various fresh water bodies of the tropical and subtropical zones, that is, in places whose conditions have changed little since the appearance of crocodiles.

The general body shape of a crocodile is lizard-like. They are characterized by a long, laterally compressed, high tail, membranes between the fingers of the hind limbs, a long muzzle and a head flattened in the dorsal-abdominal direction. There are five fingers on the forelimbs, four on the hind limbs (there is no little finger). The nostrils, located at the front end of the muzzle, and the eyes are raised and located on the upper side of the head, which allows crocodiles to stay in the water near its surface, exposing only their eyes and nostrils to the air. External auditory openings are closed with movable valves that protect the eardrums from mechanical damage when immersed in water.

The body, tail and limbs of crocodiles are covered with large correct form horny shields located on the back and abdomen in regular rows. Dorsal shields bear longitudinal ridges. In the inner layer of the skin (corium), under the horny scutes of the outer layer on the back and in some species on the belly, bone plates (osteoderms) develop that are firmly connected with the horny scutes, forming a shell that protects the body of the crocodile well; on the head, the osteoderms fuse with the bones of the skull.

In the deep layer of the skin on the lower surface of the head, inside from the branches of the lower jaw, as well as on the sides of the cloaca, there are paired large glands that secrete a brownish secret with a strong smell of musk. The secretion of the musk glands is especially intense during the breeding season, and probably serves animals of different sexes to find each other.

Scull crocodiles has two temporal arches (diapsid): the upper one, formed by the postorbital and squamous bones, and the lower one, consisting of the zygomatic and square-zygomatic bones. These temporal arches limit the superior and lateral temporal pits, respectively, along the edges of which the masticatory muscles begin. Extinct dinosaurs, winged lizards, modern to ANY fish (tuatara) had a similar skull structure, and the skull of birds, lizards and snakes comes from this type of skull.

Characteristic of the skull of crocodiles is the development of a secondary palate - an additional bone "bridge" under the primary palate, separating the nasopharyngeal passage from the oral cavity. The secondary palate in modern crocodiles is formed by fusion along the midline of the palatine processes of the premaxillary, maxillary, palatine, and pterygoid bones. The secondary internal nostrils - the choans - are located far behind the external nostrils under the occipital region of the skull. The secondary palate is of great functional importance for an air-breathing aquatic predator, allowing it to keep its mouth open underwater and at the same time continue to breathe through its nostrils exposed above the water. At the same time, the entrance to the pharynx is closed in front of the choanae by a palatine veil, tightly pressed against the arcuate ridge on the posterior edge of the tongue, in which the anterior edge of the hyoid apparatus lies. The elongated jaws of crocodiles can be compared to tweezers, making it easier to grasp small and mobile prey. Gharials, narrow-nosed crocodiles and other fish-feeding species have the longest jaws.

The square and articular bones of the skull of crocodiles are permeated with air-bearing outgrowths of the middle ear cavity. Most of the posterior bones of the skull contain cavities of a very overgrown and complexly branching system of the Eustachian tubes. Perhaps this whole system of passages and cavities associated with the middle ear and pharynx serves to equalize the pressure in the middle ear when immersed in water.

Teeth in crocodiles of a simple conical shape, they sit on the premaxillary, maxillary and dentary bones. They are reinforced in separate cells (thecodont), which allows crocodiles to have huge "fangs", up to 5 cm long. The bases of the teeth are hollow inside, in these cavities new, replacing teeth develop. The change of teeth during the life of a crocodile is multiple. The largest teeth are located at the tops of the convex arches of the scalloped lateral edge of the jaws, and the teeth of the upper and lower jaws are so aligned that the smallest teeth of the upper jaw fall against the largest teeth of the lower jaw, and vice versa. This allows the crocodile to more securely hold the prey seized by the jaws.

vertebral column crocodiles consists of 9 cervical, 15-16 trunk, 2 sacral and about 40 tail vertebrae. The vertebral bodies are concave anteriorly and convex posteriorly. The 2-5 posterior trunk vertebrae do not have ribs and are sometimes regarded as the lumbar spine. 6-8 trunk ribs articulate with the sternum - an oval cartilaginous plate, bifurcated behind. The belly of crocodiles is protected by 7-8 pairs of abdominal ribs that are not connected to the spine and located between the sternum and pelvis.

Crocodiles don't have collarbones and shoulder girdle consists of a scapula, a coracoid, and connecting the left and right half belt (together with the cartilaginous sternum) of the interclavicle.

IN pelvic girdle the pubic bone does not enter the acetabulum and in its place is the anterior process of the ischium. In the skeleton of the forelimb, the elongation of the two proximal bones of the hand, which form an additional section of the limb, is remarkable.

In the brain of crocodiles, the cerebellum is highly developed, which indicates a high degree of coordination of movements. The sense organs are very perfect. The retina of the eye consists mainly of rods, the pupil is in the form of a vertical slit, capable of greatly expanding. The inner ear has a well developed cochlea.

IN oral cavity no salivary glands.

Stomach has thick muscular walls, and almost always in adult crocodiles there is a more or less significant amount of stones in it. The weight of stones in the stomach of Nile crocodiles reaches 5 kg (1% of body weight). The role of these stones is not entirely clear; suggest that they increase the specific gravity of the crocodile or. which is more likely to move the animal's center of gravity forward and downward, giving greater stability when swimming.

Circulatory system crocodile is the most perfect among reptiles, it is characterized by a complete separation of the left and right ventricles of the heart. But at the same time, both aortic arches are preserved in crocodiles. The right aortic arch passes into the dorsal aorta, which carries arterial blood to the head, muscles of the body and to all organs that need especially oxygen-rich blood. The left aortic arch passes into the celiac artery, which supplies blood to the intestines. At the intersection of two aortic arches, there is an anastomosis between them (panizzi foramen), through which, due to the difference in blood pressure in the arches, arterial blood flows from the right arch to the left. When the crocodile breathes atmospheric air, venous blood from the right ventricle comes, apparently, only to the pulmonary artery. When diving, the crocodile in the vessels of the pulmonary circle increases the resistance to blood flow, as a result of which venous blood from the right ventricle begins to flow into the left aortic arch.

Lungs Crocodiles have a complex structure and are able to accommodate a large supply of air.

Bladder no, which is possibly related to life in the water. The cloaca has the form of a longitudinal slit, in the back of which the males have an unpaired genital organ, equipped with a deep longitudinal groove.

Modern crocodiles inhabit various fresh water bodies. Relatively few species are tolerant of brackish water and are found in river estuaries (African narrow-nosed crocodile, Nile crocodile, American sharp-snouted crocodile). Only the combed crocodile swims far into the open sea and has been observed at a distance of 600 km from the nearest shore.

Most of the day crocodiles spend in the water. They come out to the coastal shallows in the morning and in the late afternoon - to warm themselves in the sun. The most favorable body temperature for life in Mississippi alligators is 32-35 °; fatal for this type of temperature above 38 °. The lower threshold of activity is about 20°. On land, crocodiles often lie with their mouths wide open, which is apparently associated with thermoregulation: some heat transfer occurs when water evaporates from the mucous membranes of the oral cavity.

Crocodiles hunt at night. An essential component in the diet of all crocodiles is fish, but crocodiles devour any prey they can handle. Therefore, the set of feeds changes with age: various invertebrates serve as food for the young - insects, crustaceans, mollusks, worms; larger animals prey on fish, amphibians, reptiles and water birds. Adult crocodiles are able to cope with large mammals. There is a known case of finding the remains of a rhinoceros in the stomach of a Nile crocodile. In many species of crocodiles, cannibalism is noted - devouring larger individuals of smaller ones. Often crocodiles eat carrion; some species hide the uneaten remains of the victim under the overhanging shore and later devour them half-decomposed.

Crocodiles move in water with the help of their tail. On land, crocodiles are slow and clumsy, but sometimes they make significant transitions, moving several kilometers away from water bodies. When moving quickly, crocodiles put their legs under the body (usually they are widely spaced), which rises high above the ground. Young Nile crocodiles can run at a gallop at a speed of about 12 km per hour.

Crocodiles lay eggs the size of chicken or goose, covered with calcareous shells. The number of eggs in a clutch varies from 10 to 100 in different species. Some species bury their eggs in the sand, while others lay them in nests made by the female from rotting vegetation. The female remains close to the clutch, protecting it from enemies. Young crocodiles are still inside the eggs, by the time they hatch, they make croaking sounds, after which the mother digs up the masonry, helping the offspring to get out.

Crocodiles grow rapidly in the first 2-3 years of life, during which they reach a size of 1-1.5 m. With age, the growth rate decreases, and they add only a few centimeters in length per year.

Sexual maturity is reached at the age of 8-10 years. Crocodiles live up to 80 - 100 years.

Enemies in adult crocodiles are few, if we exclude humans. Cases of attacks by elephants and lions on crocodiles, making transitions by land from one reservoir to another, have been noted. In addition, many animals, especially monitor lizards, some turtles, birds (marabou), hyenas, eat the eggs of young crocodiles.

Crocodiles are dangerous to humans to varying degrees. Some never attack a person (small blunt-nosed crocodiles, fish-eating narrow-nosed crocodiles, etc.), others attack systematically (combed crocodile), others (Nile crocodile) are dangerous only in some areas.

People everywhere exterminate crocodiles, mainly for the sake of the skin. In the United States, the extermination of the Mississippi alligators has reached such proportions (up to 50,000 alligators are killed per year) that the government has taken special measures to protect these animals, as they are threatened with complete destruction. In some countries of Indochina, crocodile meat is used for food.

21 species of modern crocodiles are known (with subspecies, the number of modern forms reaches 28), which are grouped into 3 families.

Animal life: in 6 volumes. - M.: Enlightenment. Edited by professors N.A. Gladkov, A.V. Mikheev. 1970 .

Features of the skeleton. The skeleton of crocodiles, both extinct and modern, is distinguished by a number of distinct features.

Firstly, it is a massive, strongly reinforced skull throughout. At the same time, the mouth is reinforced by a secondary bone palate, immobilely articulated with the cranium; in advanced forms it is more extensive than in mammals. This ensures the separation of the nasal passages from the oral cavity, which is especially important for aquatic forms, but it was primarily associated with the mechanical strengthening of the skull, which experiences large overloads during a fast and strong bite. The top of the muzzle is formed by prefrontal bones that have grown along the midline of the palate.

Apparently, for the same reason, the number and size of cranial foramens is minimized. The skull is characterized by a flat, low roof, the loss of the posterior frontal and posterior parietal bones and the epipterygoid, and the frontal and parietal bones fused along the midline.

Secondly, the skeleton of crocodiles is characterized by complex pneumatization of almost all bones of the skull. These are two systems of air channels. One is connected with the middle ear cavity and developed from the Eustachian tubes, and the second - with the nasal passages. This gives lightness to the massive skull without loss of strength. However, an exact explanation for such a system of pneumatization has not yet been found. Thirdly, most extinct and all modern crocodiles have 24 pre-sacral vertebrae, two sacral and 30-40 caudal. The bodies of all vertebrae, except for the atlas, the second sacral, and the first caudal, are procoelous, as in lizards and snakes that evolved later.

In modern crocodiles, the first nine vertebrae have ribs that do not reach the sternum - they form the neck. The cervical ribs, in addition to those associated with the atlas and epistrophy, have widely separated heads, while the dorsal (directed towards the back) is attached to the transverse process, and the main one is attached to the parapophysis in the lower part of the vertebral body.

Thus, a system of "arches" is formed, inside which the cervical artery passes. Trunk ribs, as a rule, ten to thirteen pairs. The ribs of the posterior trunk vertebrae gradually shorten and merge with the transverse process.

So, gharials do not have the last three, and crocodiles and alligators do not have four or five trunk ribs. Modern crocodiles have a sternum - a complex cartilaginous structure that connects the ends of the ribs, the bases of the coracoids and the interclavicle. Fourth, the clavicle is lost in the girdle of the forelimbs of crocodiles, and the coracoids running in the dorsal-abdominal direction are elongated and in modern species equal to the length of the shoulder blade. These signs indicate a more efficient work of the muscles that lift the body and take the limb back. Fifth, it is characteristic for the hind limb girdle that two sacral vertebrae do not fuse into the sacrum, and the pubic bones do not participate in the formation of the acetabulum. Sixth, an important feature of the skeleton of crocodiles is the biconvex first caudal vertebra, which performs the function of a ball joint. This ensures high mobility of the tail as the main locomotor organ in water. Seventh, in the skeleton of the hind limbs of crocodiles, a well-developed calcaneal tuber is especially important, which is a powerful lever for bending the foot. This is due to the fact that when moving on land, crocodiles walk without lowering their bodies to the ground, which is much closer to the way mammals move.

Features of the central nervous system. Like all higher vertebrates, the crocodile brain is formed by five sections - the forebrain, diencephalon, midbrain, hindbrain, cerebellum and medulla oblongata. However, it has features that distinguish it from other groups of reptiles and bring it closer to the brains of birds and mammals.

The olfactory bulbs (derivatives of the forebrain) are brought to the end of the long muzzle by a pair of olfactory processes. Unlike most higher vertebrates, crocodiles do not have an pineal gland (an endocrine gland associated with the hypothalamic-pituitary system that produces serotonin) and a parietal organ. Moreover, no traces of their laying are observed even at the embryonic stages. The hollow cerebellum is divided into three transverse segments, which is not found in other reptiles and which can be considered as the first step towards the folded cerebellum of birds and mammals. Twelve pairs of completely separated nerves depart from the brain of crocodiles, while in other reptiles the tenth and eleventh nerves are not completely separated.

The medulla oblongata passes into the spinal cord. The latter has a hypocentric spinal canal, two thickenings corresponding to the limb belts, and continues to the beginning of the caudal spine.

The significance of the works of Copernicus and Galileo for the development of modern science and natural science
Introduction Modern science arose in Europe during the period of the 15th-16th centuries. At this time, in the field of economics, feudal relations were disintegrating and the rudiments of capitalist production were developing ...