§ 33. The emergence of amphibians on land The most probable biotope for the transition from water to land for cross-finned were coastal water-air labyrinths (Fig. II-32; II-33). They contained both sea water and fresh water flowing from the coast, numerous

Exit from the state of hibernation With the onset of spring, which is associated with warming and an increase in the length of daylight hours, hibernating mammals come out of a state of torpor, that is, they "wake up." It is obvious that an increase in body temperature upon awakening

12.3. Way out of the crisis - the transition to the noosphere The central theme of the doctrine of the noosphere is the unity of the biosphere and humanity. VI Vernadsky in his works reveals the roots of this unity, the importance of the organization of the biosphere in the development of mankind. This allows you to understand

About 385 million years ago, conditions favorable for the mass development of land by animals were formed on Earth. Favorable factors were, in particular, a warm and humid climate, the presence of an adequate food base (the formed abundant fauna of terrestrial invertebrates). In addition, at that time, a large amount of organic matter was washed into the water bodies, as a result of the oxidation of which the oxygen content in the water decreased. This contributed to the appearance in fish of devices for breathing atmospheric air.

Evolution

The rudiments of these devices can be found among various groups of fish. Some modern fish for one time or another are able to leave the water and their blood is partially oxidized due to atmospheric oxygen. Such is, for example, a creeper fish ( Anabas), which, coming out of the water, even climbs trees. Some representatives of the goby family crawl out onto land - mud jumpers ( Periophthalmus). The latter catch their prey more often on land than in water. The ability to stay out of water of some lungs is well known. However, all these adaptations are of a private nature and the ancestors of amphibians belonged to less specialized groups of freshwater fish.

Adaptations to terrestrialism developed independently and in parallel in several evolutionary lines of cross-finned fishes. In this regard, E. Jarvik put forward a hypothesis about the diphyletic origin of terrestrial vertebrates from two different groups of cross-finned fishes ( Osteolepiformes and Porolepiformes). However, a number of scientists (A. Romer, I. I. Shmalgauzen, E. I. Vorobyov) criticized Jarvik's arguments. Most researchers believe that the monophyletic origin of tetrapods from osteolepiform racemes is more likely, although this allows for the possibility of paraphilia, that is, reaching the level of organization of amphibians by several closely related phyletic lines of osteolepiform fish that evolved in parallel. Parallel lines are most likely extinct.

One of the most "advanced" cross-finned fishes was the Tiktaalik, which had a number of transitional features that brought it closer to amphibians. These features include a shortened skull, separated from the girdle of the forelimbs and a relatively mobile head, the presence of the elbow and shoulder joints. The fin of the tiktaalik could occupy several fixed positions, one of which was intended so that the animal could be in an elevated position above the ground (probably to “walk” in shallow water). Tiktaalik breathed through holes located at the end of the flat "crocodile" muzzle. Water, and possibly atmospheric air, was no longer pumped into the lungs by the gill covers, but by the buccal pumps. Some of these adaptations are also characteristic of the cross-finned fish Panderichthys.

The first amphibians that appeared in freshwater bodies at the end of the Devonian were ichthyostegids (Ichthyostegidae). They were true transitional forms between cross-finned fishes and amphibians. So, they had the rudiments of the operculum, a real fish tail, and the kleitrum was preserved. The skin was covered with small fish scales. However, along with this, they had paired five-toed limbs of terrestrial vertebrates (see the diagram of the limbs of cross-finned and ancient amphibians). Ichthyostegids lived not only in water, but also on land. It can be assumed that they not only multiplied, but also fed in the water, systematically crawling out onto land.

Later, in the Carboniferous period, a number of branches arose, which are given the taxonomic significance of superorders or orders. The superorder of Labyrinthodontia was very diverse. The early forms were relatively small in size and had a fish-like body. The later ones reached very large sizes (1 m or more) in length, their body was flattened and ended with a short thick tail. Labyrinthodonts existed until the end of the Triassic and occupied terrestrial, near-water and aquatic habitats. Some labyrinthodonts are relatively close to the ancestors of tailless - the orders Proanura, Eoanura, known from the end of the Carboniferous and from Permian deposits.

The second branch of primary amphibians, Lepospondyli, also emerged in the Carboniferous. They were small and well adapted to life in the water. Some of them lost their limbs for the second time. They existed until the middle of the Permian period. It is believed that they gave rise to the orders of modern amphibians - tailed (Caudata) and legless (Apoda). In general, all Paleozoic amphibians became extinct during the Triassic. This group of amphibians is sometimes called stegocephalic (shell-head) for a solid shell of skin bones that covered the cranium from above and from the sides. The ancestors of stegocephals were probably bony fish that combined primitive features of organization (for example, weak ossification of the primary skeleton) with the presence of additional respiratory organs in the form of pulmonary sacs.

The closest to stegocephalus are cross-finned fishes. They possessed pulmonary respiration, their limbs had a skeleton similar to that of stegocephals. The proximal segment consisted of one bone corresponding to the shoulder or thigh, the next segment consisted of two bones corresponding to the forearm or lower leg; then there was a section, consisting of several rows of bones, it corresponded to the hand or foot. Attention is also drawn to the clear similarity in the arrangement of the integumentary bones of the skull in the ancient cross-finned and stegocephals.

The Devonian period, in which stegocephals arose, was apparently characterized by seasonal droughts, during which life in many freshwater bodies was difficult for fish. The depletion of oxygen in the water and the difficulty of swimming in it was facilitated by the abundant vegetation that grew in the Carboniferous time along swamps and the shores of water bodies. Plants fell into the water. Under these conditions, fish could adapt to additional respiration with pulmonary sacs. In itself, the depletion of water in oxygen was not yet a prerequisite for going ashore. Under these conditions, cross-finned fish could rise to the surface and swallow air. But with a strong drying up of reservoirs, life for fish became already impossible. Unable to travel on land, they died. Only those aquatic vertebrates that, simultaneously with the ability to pulmonary respiration, acquired limbs capable of ensuring movement on land, could survive these conditions. They crawled out onto land and crossed into neighboring reservoirs, where water was still preserved.

At the same time, movement on land for animals covered with a thick layer of heavy bone scales was difficult, and the bony scaly shell on the body did not provide the possibility of skin respiration, which is so characteristic of all amphibians. These circumstances, apparently, were a prerequisite for the reduction of the bony shell on most of the body. In some groups of ancient amphibians, it was preserved (not counting the shell of the skull) only on the belly.

Stegocephals survived to the beginning of the Mesozoic. Modern orders of amphibians are formed only at the end of the Mesozoic.

Notes (edit)

Wikimedia Foundation. 2010.

It took a lot of work in search of fossil traces of extinct creatures to clarify this issue.

Previously, the transition of animals to land was explained as follows: in the water, they say, there are many enemies, and now the fish, fleeing from them, began to crawl out onto land from time to time, gradually developing the necessary adaptations and transforming into other, more advanced forms of organisms.

One cannot agree with this explanation. After all, even now there are such amazing fish that from time to time crawl ashore, and then return to the sea. But they do not throw the water at all for the sake of salvation from enemies. Let us also recall the frogs - amphibians, which, living on land, return to the water to produce offspring, where they spawn and where young frogs - tadpoles develop. Add to this that the most ancient amphibians were not at all defenseless, suffering from enemies. They were chained in a thick, hard shell and hunted other animals like fierce predators; it is unbelievable that they or their ilk would be driven out of the water by danger from their enemies.

They also expressed the opinion that aquatic animals that overflowed the sea seemed to be suffocating in sea water, felt the need for fresh air, and they were attracted by the inexhaustible reserves of oxygen in the atmosphere. Was it really so? Think of the flying sea fish. They either float near the surface of the sea, or with a strong splash they rise from the water and rush in the air. It would seem that it is easiest for them to start using the air of the atmosphere. But they just don't use it. They breathe with gills, that is, with respiratory organs adapted for life in water, and they are quite content with this.

But among the freshwater there are those that have special adaptations for air breathing. They are forced to use them when the water in the river or user becomes cloudy, clogged and becomes poor in oxygen. If the sea water is clogged by some streams of mud flowing into the sea, then the sea fish swim away to another place. Marine fish do not need special devices for air breathing. Freshwater fish find themselves in a different position when the water surrounding them becomes cloudy and rotten. It is worth observing some of the tropical rivers to get an idea of ​​what happens.

Instead of our four seasons in the tropics, the hot and dry half of the year gives way to the rainy and damp half of the year. During heavy rains and frequent thunderstorms, rivers overflow widely, the waters rise high and are saturated with oxygen from the air. But the picture is changing dramatically. The rain stops pouring. The waters subside. The scorching sun dries up the rivers. Finally, instead of running water, there are chains of lakes and swamps, in which standing water is overflowing with animals. They die in masses, corpses quickly decompose, and when rotting, oxygen is consumed, so that it becomes, therefore, less and less in these bodies of water filled with organisms. Who can survive such drastic changes in living conditions? Of course, only the one who has the appropriate adaptations: he can either go into hibernation, buried in the silt for the entire dry time, or switch to breathing atmospheric oxygen, or, finally, can do both. All the rest are doomed to extermination.

Fish have two kinds of adaptations to air breathing: either their gills have spongy outgrowths that retain moisture, and as a result, air oxygen easily penetrates into the blood vessels washing them; or they have an altered swim bladder, which serves to keep the fish at a certain depth, but at the same time can also act as a respiratory organ.

The first adaptation is found in some teleost fish, that is, those with a completely ossified skeleton, not cartilaginous. Their swim bladder is not involved in breathing. One of these fish - "crawling perch" - lives in tropical countries and now. Like some

other bony fish, it has the ability to leave the water and, with the help of fins, crawl (or jump) along the shore; sometimes it even climbs trees in search of slugs or worms that it feeds on. As surprising as the habits of these fish are, they cannot explain to us the origin of the changes that allowed aquatic animals to become inhabitants of the land. They breathe with the help of special devices 9 gill apparatus.

Let us turn to two very ancient groups of fish, those that lived on Earth already in the first half of the ancient era of Earth's history. We are talking about cross-finned and lung-breathing fish. One of the wonderful cross-finned fish, called the polypter, lives at the present time in the rivers of tropical Africa. During the day, this fish loves to hide in deep holes on the muddy bottom of the Nile, and at night it revives in search of food. She attacks both fish and crayfish, and does not disdain frogs. Trapping prey, the polypter stands at the bottom, leaning on its wide pectoral fins. Sometimes he crawls along the bottom on them, like on crutches. When pulled out of water, this fish can live for three to four hours if kept in wet grass. At the same time, her breathing occurs with the help of a swim bladder, into which the fish continually draws air. This bladder in cross-finned fish is double and develops as an outgrowth of the esophagus from the ventral side.

We do not know of a fossilized polypter. Another cross-finned fish, a close relative of the polypter, lived in very distant times and breathed with a well-developed swim bladder.

The lung-breathing fish are remarkable in that their swim bladder has become a respiratory organ and works like the lungs. Of these, only three genera have survived to this day. One of them, the cattletooth, lives in the slow flowing rivers of Australia. In the silence of summer nights, the grunting sounds of this fish are carried far away, floating to the surface of the water and releasing air from the swim bladder. But usually this large fish lies motionless on the bottom or slowly swims among the water thickets, plucking them and looking for crustaceans, worms, molluscs and other food there.

She breathes in a double way: with gills and swim bladder. Both the one and the other organ work simultaneously. When the river dries up in summer and small ponds remain of it, the cattle-toothed feels great in them, while the rest of the fish die in masses, their corpses rot and spoil the water, depriving it of oxygen. Travelers to Australia have seen these pictures many times. It is especially interesting that such pictures were extremely often unfolded at the dawn of the Carboniferous Age along the face of the Earth; they give an idea of ​​how, as a result of the extinction of some and the victory of others, a great event in the history of life became possible - the emergence of aquatic vertebrates on land.

The modern horntooth is not inclined to move to the shore for living. He spends the whole year in the water. Researchers have not yet been able to observe that he hibernated during hot weather.

Its distant relative, the ceratode, or fossil horntooth, lived on Earth in very distant times and was widespread. Its remains were found in Australia, Western Europe, India, Africa, North America.

Two other pulmonary fish of our time - the protopter and the lepidosiren - differ from the cattle-toothed by the structure of their swim bladder, which has turned into lungs. Namely, they have it double, while in the horntooth it is unpaired. The protopter is quite widespread in the rivers of tropical Africa. Rather, he does not live in the rivers themselves, but in the swamps that stretch next to the riverbed. It feeds on frogs, worms, insects, and crayfish. On occasion, the protopters attack each other as well. Their fins are not suitable for swimming, but serve to support the bottom when crawling. They even have a kind of elbow (and knee) joint at about the middle of the fin length. This remarkable feature shows that lung fishes, even before they left the water element, could develop adaptations that were very useful to them for life on land.

From time to time, the protopter rises to the surface of the water and draws air into the lungs. But this fish has a hard time in the dry season. There is almost no water left in the swamps, and the protopter is buried in silt to a depth of about half a meter in a special kind of hole; here he lies, surrounded by hardened mucus secreted by his skin glands. This mucus forms a kind of shell around the protopter and does not allow it to dry out completely, keeping the skin moist. A passage goes through the entire crust, which ends at the fish's mouth and through which it breathes atmospheric air. During this hibernation, the swim bladder serves as the only respiratory organ, since the gills then do not work. How is life going on in the body of a fish at this time? She is losing weight a lot, losing not only her fat, but also some of the meat, just as our animals live during hibernation due to the accumulated fat and meat - a bear, a marmot. Dry time in Africa lasts a good six months: in the homeland of the protopter - from August to December. When the rains come, life in the swamps will resurrect, the shell around the protopter dissolves, and it resumes its lively activity, now preparing for reproduction.

Young protopters hatched from eggs look more like salamanders than fish. They have long external gills, like those of tadpoles, and the skin is covered with multi-colored spots. There is no swim bladder at this time. It develops when the external gills fall off, in exactly the same way as it happens in young frogs.

The third pulmonary fish, lepidosiren, lives in South America. She spends her life in much the same way as her African relative. And their offspring develops very similarly.

No more lungfish survived. And those that still remain - the cattle-toothed, protopter and lepidosiren - are approaching the end of their century. Their time is long gone. But they give us an idea of ​​the distant past and are especially interesting for us.

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• Introduction
• 6. The emergence of amnion
• 9. Live birth
• Conclusion

Introduction

The emergence of vertebrates from water to land was the most important stage in the history of the development of the animal world, and therefore the discussion of the origin of amphibians is of particular interest. Amphibians were the first of the vertebrates that had dissected limbs that carried fingers, switched to pulmonary respiration and, thus, began to master the terrestrial environment.

The arid climate of the continental regions, characteristic of the Devonian period, put the inhabitants of drying up reservoirs or reservoirs with oxygen-poor water in the most unfavorable conditions. In such conditions, the life advantage remained with those fish that could use their swim bladder as a respiratory organ and thus endure temporary drying out and survive until a new period of rains in order to return to the fish lifestyle.

This was the first step towards getting out of the aquatic environment. But the real development of the conditions of terrestrial life was still far away. The most that pulmonary fish could achieve then was the ability to passively survive the unfavorable season, hiding in the silt.

But then the Devonian period was replaced by the Carboniferous period. Its very name speaks of a huge mass of plant residues that formed coal seams in shallow water conditions. Both the lush development of tree-like spore plants, and the fact that these plants did not decay on the surface, but charred under water, all this testifies to the damp and hot climate that prevailed at that time over vast areas of the Earth.

The changed climate also created new conditions for the descendants of the Devonian lung fishes. One of them, the ability to breathe air came in handy in connection with life in warm swampy water bodies with decaying vegetation (these are approximately the same conditions in which the Amazonian flake now lives); others, in whom the internal changes in the process of metabolism, the action of natural selection developed the ability to temporarily do without water, in the damp atmosphere of the coal forests could already lead a more active life - to move and get food for themselves.

The emergence of vertebrates on land took place in the late Devonian epoch, approximately 50 million years after the first land conquerors - psilophytes. At this time, the air was already assimilated by insects, and descendants of cross-finned fish began to spread over the Earth. The new method of movement allowed them to move away from the water for a while. This led to the emergence of vertebrates with a new way of life - amphibians. Their most ancient representatives - ichthyostegs - were found in Greenland in Devonian sedimentary rocks. The short five-toed paws of ichthyostegs, thanks to which they could crawl over land, were more like flippers. The presence of the caudal fin, a body covered with scales, indicates the aquatic lifestyle of these animals.

The flourishing of ancient amphibians is confined to the Carboniferous. It was during this period that stegocephals (shell-headed) were widely developed. Their body shape was reminiscent of newts and salamanders. Reproduction of stegocephals, like modern amphibians, took place with the help of eggs, which they threw into the water. Larvae with gill respiration developed in the water. Because of this feature of reproduction, amphibians have forever remained associated with their cradle - water. They, like the first land plants, lived only in the coastal part of the land and could not conquer inland areas located far from water bodies.

vertebrate land air breathing

1. Prerequisites for the emergence of vertebrates on land

A thick helophyte "brush" (one can call it "rhinophyte reeds") that has arisen in coastal amphibiotic landscapes begins to act as a filter regulating raincoat runoff: it intensively filters out (and settles) debris carried from land and thus forms a stable coastline. The formation of "alligator ponds" by crocodiles can serve as some analogue of this process: animals constantly deepen and expand the swamp reservoirs inhabited by them, throwing soil onto the shore. As a result of their many years of "irrigation activity", the swamp turns into a system of clean deep ponds, separated by wide forested "dams". So the vascular vegetation in the Devonian divided the notorious amphibiotic landscapes into "real land" and "real freshwater reservoirs".

The appearance of the first tetrapods (tetrapods) in the Late Devonian, a group of vertebrates with two pairs of limbs, is associated with the newly arisen freshwater bodies of water; it unites in its composition amphibians, reptiles, mammals and birds (simply put, tetrapods are all vertebrates, except for fish and fish-like). It is now generally accepted that tetrapods are descended from cross-finned fish (Rhipidistia); this relict group has now the only living representative, coelacanth. The once popular hypothesis of the origin of tetrapods from another relict group of fish - dipnoi, now has practically no supporters.

The Devonian period, in which stegocephals arose, was apparently characterized by seasonal droughts, during which life in many freshwater bodies was difficult for fish. The depletion of oxygen in the water and the difficulty of swimming in it was facilitated by the abundant vegetation that grew in the Carboniferous time along swamps and the shores of water bodies. Plants fell into the water. Under these conditions, fish could adapt to additional respiration with pulmonary sacs. In itself, the depletion of water in oxygen was not yet a prerequisite for going ashore. Under these conditions, cross-finned fish could rise to the surface and swallow air. But with a strong drying up of reservoirs, life for fish became already impossible. Unable to travel on land, they died. Only those aquatic vertebrates that, simultaneously with the ability to pulmonary respiration, acquired limbs capable of ensuring movement on land, could survive these conditions. They crawled out onto land and crossed into neighboring reservoirs, where water was still preserved.

At the same time, movement on land for animals covered with a thick layer of heavy bone scales was difficult, and the bony scaly shell on the body did not provide the possibility of skin respiration, which is so characteristic of all amphibians. These circumstances, apparently, were a prerequisite for the reduction of the bony shell on most of the body. In some groups of ancient amphibians, it was preserved (not counting the shell of the skull) only on the belly.

2. The emergence of a five-toed limb

In most fish, in the skeleton of paired fins, a proximal section is distinguished, consisting of a small number of cartilaginous or bony plates, and a distal section, which includes a large number of radially segmented rays. The fins are connected to the girdles of the limbs inactive. They cannot support the body when moving along the bottom or on land. In cross-finned fish, the skeleton of paired limbs has a different structure. The total number of their bony elements is reduced and they are larger in size. The proximal section consists of only one large bone element corresponding to the humerus or femur of the fore or hind limbs. This is followed by two smaller bones, homologous to the ulna and radius or tibia and fibula. 7-12 radially located rays rest on them. Only homologues of the humerus or femur are involved in connection with the girdles of the extremities of such a fin; therefore, the fins of cross-finned fish are actively mobile (Fig. 1 A, B) and can be used not only to change the direction of movement in water, but also to move along a solid substrate ... The life of these fish in small, drying up water bodies in the Devonian period contributed to the selection of forms with more developed and mobile limbs. The first representatives of Tetrapoda - stegocephals - had seven- and five-toed limbs, which remain similar to the fins of cross-finned fishes (Fig. 1, B)

Rice. 1. Skeleton of the limb of a cross-finned fish (A), its base (B) and the skeleton of the front paw of a stegocephalus (C): I-humerus, 2-ulna, 3-radius.

In the skeleton of the wrist, the correct radial arrangement of bone elements in 3-4 rows is preserved, 7-5 bones are located in the wrist, and then the phalanges of 7-5 fingers also lie radially. In modern amphibians, the number of fingers in the limbs is equal to five or their oligomerization occurs up to four. Further progressive transformation of the limbs is expressed in an increase in the degree of mobility of the joints of the bones, in a decrease in the number of bones in the wrist, first to three rows in amphibians and then to two in reptiles and mammals. In parallel, the number of phalanges of the fingers also decreases. The lengthening of the proximal parts of the limb and the shortening of the distal ones are also characteristic.

The location of the limbs also changes during evolution. If in fish the pectoral fins are at the level of the first vertebra and are turned to the sides, then in terrestrial vertebrates, as a result of the complication of orientation in space, a neck appears and head mobility occurs, and in reptiles and especially in mammals, in connection with raising the body above the ground, the forelimbs move posteriorly and are oriented not horizontally, but vertically. The same goes for the hind limbs. The variety of habitats provided by the terrestrial lifestyle provides a variety of forms of movement: jumping, running, crawling, flying, digging, climbing rocks and trees, and when returning to the aquatic environment, swimming. Therefore, in terrestrial vertebrates, one can find both an almost unlimited variety of limbs and their complete secondary reduction, and many similar adaptations of limbs in various environments have repeatedly arisen convergently (Fig. 2).

However, in the process of ontogenesis, most terrestrial vertebrates show common features in the development of extremities: the formation of their primordia in the form of poorly differentiated folds, the formation of six or seven finger buds in the hand and foot, the outermost of which are soon reduced and only five develop later.

Rice. 2 Skeleton of the forelimb of terrestrial vertebrates. A-frog - B-salamander; B-crocodile; G-bat; D-man: 1-humerus, 2-radius, 3-bones of the wrist, 4-metacarpus, 5-phalanges of the fingers, 6-ulna

3. Reduction of cutaneous mucus glands and the appearance of horny formations

In amphibian larvae, the epidermis also contains a large number of glandular cells, but in adult animals, the latter disappear and are replaced by multicellular glands.

In legless amphibians, in the anterior half of each segment of their annular body, in addition to glands of the usual type, there are also special giant skin glands.

In reptiles, the skin is devoid of glands. As an exception, they have only separate large glands that carry special functions. So, crocodiles have a pair of musky glands on the sides of the lower jaw. In turtles, similar glands are found at the junction of the dorsal and abdominal shields. In lizards, special femoral pores are also observed, but they push out of themselves in the form of a papilla only a mass of keratinized cells and therefore can hardly be attributed to glands (some authors compare these formations with hair).

Reptile skin, freed from the respiratory function, undergoes significant changes aimed at protecting the body from drying out. Skin glands in reptiles are absent, since the need to wet the skin has disappeared. The evaporation of moisture from the surface of the body has decreased, since the entire body of these animals is covered with horny scales. Complete rupture with the aquatic environment leads to the fact that the osmotic pressure in the body of reptiles becomes independent of the environment. Keratinization of the skin, which makes it impervious to water, removes the threat of a change in osmotic pressure even when reptiles switch to aquatic life for the second time. Since water enters the body of reptiles only voluntarily along with food, the osmoregulatory function of the kidneys almost completely disappears. Reptiles have no need, like amphibians, to remove the constantly arising excess of water from the body. On the contrary, they, like land animals, need to economically use the water in the body. Trunk buds (mesonephros) of amphibians are replaced in reptiles by pelvic buds (metanephros).

Birds also lack skin glands, with the exception of only one paired gland, which carries a special function. This is the coccygeal one - usually opening with a pair of holes above the last vertebrae. It has a rather complex structure, consists of numerous tubes, radially converging to the excretory channel, and secretes an oily secret that serves to lubricate the feathers.

Mammals share the abundance of skin glands with amphibians. The skin of mammals contains multicellular glands of both main types - tubular and alveolar. The first include sweat glands, which look like a long tube, the end of which is often rolled into a ball, and the rest is usually curved in the form of a corkscrew. In some lower mammals, these glands are almost saccular in shape.

4. The appearance of the respiratory organs

The similarity of the lungs of lower terrestrial vertebrates with the swim bladder in which fish has long led researchers to the idea of ​​the homology of these formations. In this general form, however, this widespread opinion encounters considerable difficulties. The swimbladder in most fish is an unpaired organ that develops in the dorsal mesentery. It is supplied with intestinal arterial blood and gives off venous blood partly to the cardinal ones, partly to the portal vein of the liver. These facts, undoubtedly, speak against the indicated theory. However, in some fish, a paired swim bladder is observed, which communicates with the abdominal wall of the esophagus and, moreover, further in front. This organ is supplied, like the lungs of terrestrial vertebrates, with blood from the fourth pair of branchial arteries and delivers it directly to the heart (to the venous sinus in lungs and to the adjacent part of the hepatic vein in Polyptorus). It is quite clear that we are dealing here with formations of the same kind as the lungs.

Thus, the above hypothesis about the origin of the lungs can be accepted with certain limitations - the lungs of terrestrial vertebrates are the result of further specialization (as a respiratory organ) of the pulmonary bladder.

Based on the fact that the lungs in amphibians are laid in the form of paired saccular outgrowths behind the last pair of gill sacs, Goette suggested that the lungs are the result of the transformation of a pair of gill sacs. This theory can be approximated to the first, if we assume that the swim bladder has the same origin. Thus, some authors believe that the swim bladder of fish and the lungs of terrestrial vertebrates developed independently (divergently) from a pair of the last gill sacs.

At present, it can be considered that Goette's theory of the origin of the lungs is most consistent with the facts. As for the question of the origin of the swim bladder of fish, we can accept only for the paired bladder of many-feather ganoids and lung-breathing fish the same occurrence as for the lungs. In this case, there is also no need to accept a completely independent development of these bodies. The lungs of terrestrial vertebrates are specialized paired swim bladders. The latter arose by transformation from a pair of gill sacs.

5. The emergence of homeothermy

Homeothermia is a fundamentally different way of temperature adaptations, which arose on the basis of a sharp increase in the level of oxidative processes in birds and mammals as a result of the evolutionary improvement of the circulatory, respiratory and other organ systems. Oxygen consumption per 1 g of body weight in warm-blooded animals is tens and hundreds of times higher than in poikilothermic animals.

The main differences between homeothermic animals and poikilothermic organisms:

1) a powerful flow of internal, endogenous heat;

2) the development of an integral system of effectively working thermoregulatory mechanisms, and as a result 3) the constant flow of all physiological processes in the optimal temperature regime.

Homeothermal keep a constant heat balance between heat production and heat transfer and, accordingly, maintain a constant high body temperature. The body of a warm-blooded animal cannot be temporarily "suspended" as it happens during hypobiosis or cryptobiosis in poikilothermic animals.

Homeothermic animals always produce a certain minimum of heat production, which ensures the work of the circulatory system, respiratory system, excretion, and others, even when at rest. This minimum is called basal metabolism. The transition to activity enhances heat production and, accordingly, requires an increase in heat transfer.

Warm-blooded animals are characterized by chemical thermoregulation - a reflex increase in heat production in response to a decrease in the temperature of the environment. Chemical thermoregulation is completely absent in poikilotherms, in which, in the event of additional heat release, it is generated due to the direct motor activity of animals.

In contrast to poikilothermic, under the action of cold in the body of warm-blooded animals, oxidative processes do not weaken, but intensify, especially in skeletal muscles. In many animals, muscle tremors are first observed - an inconsistent muscle contraction leading to the release of heat energy. In addition, the cells of muscle and many other tissues emit heat even without performing working functions, coming into a state of a special thermoregulatory tone. With a further decrease in the temperature of the environment, the thermal effect of the thermoregulatory tone increases.

With the production of additional heat, lipid metabolism is especially enhanced, since neutral fats contain the main store of chemical energy. Therefore, the fat reserves of animals provide better thermoregulation. Mammals even have specialized brown adipose tissue, in which all the released chemical energy is dissipated in the form of heat, i.e. goes to heating the body. Brown adipose tissue is most developed in animals - inhabitants of cold climates.

Maintaining the temperature due to the increase in heat production requires a large expenditure of energy, therefore animals, with increased chemical thermoregulation, either need a large amount of food, or spend a lot of fat reserves accumulated earlier. For example, the tiny shrew has an exceptionally high metabolic rate. Alternating between very short periods of sleep and activity, it is active at any hour of the day and eats 4 times its own weight per day. The heart rate in shrews is up to 1000 bpm. Also, birds that stay for the winter need a lot of food: they are afraid not so much of frost as for lack of food. So, with a good harvest of spruce and pine seeds, crossbills even breed chicks in winter.

The enhancement of chemical thermoregulation, therefore, has its limits, due to the possibility of obtaining food. With a lack of food in winter, this way of thermoregulation is ecologically disadvantageous. For example, it is underdeveloped in all animals living in the Arctic Circle: Arctic foxes, walruses, seals, polar bears, reindeer, etc. For the inhabitants of the tropics, chemical thermoregulation is also not typical, since they practically do not need additional heat production.

Within a certain range of external temperatures, homeotherms maintain body temperature without spending additional energy on it, but using effective mechanisms of physical thermoregulation, which make it possible to better preserve or remove the heat of basal metabolism. This temperature range, within which animals feel most comfortable, is called the thermoneutral zone. Beyond the lower threshold of this zone, chemical thermoregulation begins, and beyond the upper threshold, energy consumption for evaporation.

Physical thermoregulation is ecologically beneficial, since adaptation to cold is carried out not due to additional heat production, but due to its preservation in the body of the animal. In addition, protection against overheating is possible by enhancing heat transfer to the external environment.

There are many ways of physical thermoregulation. In the phylogenetic series of mammals - from insectivores to bats, rodents and predators, the mechanisms of physical thermoregulation are becoming more sophisticated and diverse. These include reflex narrowing and expansion of blood vessels of the skin, which changes its thermal conductivity, changes in the heat-insulating properties of fur and feathers, countercurrent heat exchange through contact of blood vessels during the blood supply of individual organs, regulation of evaporative heat transfer.

The thick fur of mammals, feathers and especially the down cover of birds allow maintaining a layer of air around the body with a temperature close to the body temperature of the animal, and thereby reduce heat radiation to the external environment. Heat dissipation is regulated by the slope of the hair and feathers, the seasonal change of fur and plumage. The exceptionally warm winter fur of mammals in the Arctic allows them to do without a significant increase in metabolism in cold weather and reduces the need for food. For example, Arctic foxes on the coast of the Arctic Ocean consume even less food in winter than in summer.

In marine mammals - pinnipeds and whales - a layer of subcutaneous adipose tissue is distributed throughout the body. The thickness of subcutaneous fat in some species of seals reaches 7-9 cm, and its total weight is up to 40-50% of the body weight. The thermal insulating effect of such a "fat stocking" is so high that the snow does not melt under the seals lying in the snow for hours, although the body temperature of the animal is maintained at 38 ° C. In animals of a hot climate, such a distribution of fat reserves would lead to death from overheating due to the impossibility of removing excess heat, therefore, their fat is stored locally, in individual parts of the body, without interfering with heat radiation from the common surface (camels, fat-tailed sheep, zebu, etc. ).

Countercurrent heat exchange systems that help maintain a constant temperature of internal organs are found in the paws and tails of marsupials, sloths, anteaters, semi-monkeys, pinnipeds, whales, penguins, cranes, etc. Moreover, the vessels through which heated blood moves from the center of the body are in close contact with the walls of the vessels that direct the cooled blood from the periphery to the center, and give them their heat.

Of no small importance for maintaining the temperature balance is the ratio of the body surface to its volume, since, ultimately, the scale of heat production depends on the mass of the animal, and heat exchange occurs through its integuments.

6. The emergence of amnion

All vertebrates are subdivided into primary aquatic - Anamnia and primary terrestrial - Amniota, depending on the conditions under which their embryonic development occurs. The evolutionary process in animals was associated with the development of a new habitat - land. This can be traced both in invertebrates, where the upper class of arthropods (insects) became an inhabitant of the terrestrial environment, and in vertebrates, where the land was mastered by higher vertebrates: reptiles, birds and mammals. The landfall was accompanied by adaptive changes at all levels of organization - from biochemical to morphological. From the standpoint of developmental biology, adaptation to a new environment was expressed in the appearance of adaptations that preserve the living conditions of the ancestors for the developing embryo, i.e. aquatic environment. This applies both to ensuring the development of insects and higher vertebrates. In both cases, the egg, if development takes place outside the mother's body, is dressed with shells that provide protection and preservation of the macrostructure of the semi-liquid egg contents in the air. Around the embryo itself, which develops inside the egg membranes, a system of embryonic membranes is formed - amnion, serosa, allantois. The embryo membranes in all Amniota are homologous and develop in a similar way. Development up to the exit from the egg takes place in the aquatic environment, which is preserved around the embryo with the help of the amniotic membrane, by the name of which the entire group of higher vertebrates is called Amniota. Insects also have a functional analogue to the amnion of vertebrates. Thus, the problems find a common solution in two so different groups of animals, each of which can be considered the highest in its evolutionary branch. The amniotic membrane forms an amniotic cavity around the embryo, filled with a liquid, the salt composition is close to the composition of the cell plasma. In reptiles and birds, the embryo rising above the yolk is gradually enclosed in front, from the sides and behind by a double fold formed by the ectoderm and parietal mesoderm. The folds close over the embryo and grow together in layers: the external ectoderm with the external ectoderm, the underlying parietal mesoderm with the parietal mesoderm of the opposite fold. In this case, the entire embryo and its yolk sac are covered from above with ectoderm and the underlying parietal mesoderm, which together form the outer shell - serosa. The ectoderm of the inner part of the folds, facing the embryo, and the parietal mesoderm covering it, close over the embryo, forming the amniotic membrane, in the cavity of which it develops. Later, an outgrowth of its ventral wall (endoderm with visceral mesoderm) develops in the embryo in the hindgut region, which increases and occupies the exocole between the serosa, amnion and yolk sac.

This outgrowth is the third germinal membrane called allantois. In the visceral mesoderm of allantois, a network of vessels develops, which, together with the vessels of the serous membrane, come close to the shell membranes and the pored shell of the egg, providing gas exchange of the developing embryo.

The preadaptations preceding the formation of the embryonic membranes of Amniota (their common "promising standard") in the course of evolution can be illustrated by two examples.

1. Fish notobranchia (Notobranchius) and aphiosemion (Aphiosemion) in Africa and Cynolebias in South America live in drying up water bodies. The eggs are still deposited in the water, and their development occurs during a drought. Many adult fish die in drought, but the laid eggs continue to develop. In the rainy season, fry hatch from the eggs, immediately capable of active feeding. The fish grow quickly and at the age of 2 - 3 months they themselves lay eggs. At the same time, at first there are only a few eggs in the clutch, but with age and growth, the size of the clutches increases. It is interesting that adaptation to reproduction in periodically drying up water bodies led to the dependence of development on this factor: without preliminary drying, the caviar loses its ability to develop. So, for the development of the golden-striped aphiosemion, its caviar must go through six months of drying in the sand. In the eggs of these fish, the yolk liquefies under the embryo and the embryo begins to sink into it, dragging the upper wall of the yolk sac with it. As a result, folds from the outer walls of the yolk sac are closed around the embryo, forming a chamber that retains moisture and in which the embryo experiences drought. This example shows how the embryonic membranes of Amniota could have arisen and, as it were, imitates and anticipates the method and path of formation of the amnion and serosa in higher vertebrates.

2. The embryo of primitive reptiles, whose eggs are devoid of protein, grows in the process of development, separates from the yolk and rests against the shell. Unable to change the shape of the shell, the embryo drowns in the yolk, and the extraembryonic ectoderm (according to actual data, it was at first) closes in double folds over the immersed embryo. Later, the parietal mesoderm grows into the folds.

Comparison of these two examples suggests a possible evolutionary scheme for the origin of two of the three embryonic membranes - serosa and amnion.

The origin of allantois is initially associated with the excretion of nitrogen metabolism products in the embryogenesis of higher vertebrates. In all amniotes, allantois has one common function - the function of a kind of embryonic bladder. In connection with the early functioning of the embryonic kidney, it is believed that allantois arose as a result of "premature" development of the bladder. The urinary bladder is also present in adult amphibians, but it is not developed to any appreciable extent in their embryos (A. Romer, T. Parsons, 1992). In addition, allantois has a respiratory function. Connecting with the chorion, the vascularized chorioallantois acts as a respiratory system, absorbing oxygen entering through the shell and removing carbon dioxide. In most mammals, allantois is also located under the chorion, but already as an integral part of the placenta. Here, the vessels of the allantois also deliver oxygen and nutrients to the embryo and carry carbon dioxide and metabolic end products to the mother's bloodstream. In various manuals, allantois is called a derivative of the visceral mesoderm and ectoderm or endoderm. The discrepancy is explained by the fact that anatomically it is close to the cloaca, which, according to G. J. Romeis, is the primary sign of vertebrates. The very same cloaca in embryogenesis has a dual origin. In the embryos of all vertebrates, it is formed by the expansion of the posterior end of the endodermal hind gut. Until relatively late stages of development, it is fenced off from the external environment by a membrane, outside of which is the invagination of the ectoderm (proctodeum) - the hindgut. With the disappearance of the membrane, the ectoderm is included in the composition of the cloaca, and it becomes difficult to distinguish which part of the cloacal lining originates from the ectoderm and which from the endoderm.

All reptiles and birds have large, polylecitic, telolecital eggs with a meroblastic type of cleavage. A large amount of yolk in oocytes in animals of these classes serves as the basis for the lengthening of embryogenesis. Their postembryonic development is direct and not accompanied by metamorphosis.

7. Changes in the nervous system

The role of the nervous system became especially significant after the emergence of vertebrates on land, which put the former primary aquatic animals in an extremely difficult situation. They perfectly adapted to life in the aquatic environment, which did not resemble terrestrial habitats. New requirements for the nervous system were dictated by the low resistance of the environment, an increase in body weight, good diffusion of odors, sounds and electromagnetic waves in the air. The gravitational field made extremely stringent demands on the somatic receptor system and on the vestibular apparatus. If it is impossible to fall in the water, then such troubles are inevitable on the surface of the Earth. Specific organs of movement - limbs - were formed on the border of the media. A sharp increase in the requirements for the coordination of the work of the muscles of the body led to the intensive development of the sensorimotor parts of the spinal cord, posterior cord, and medulla oblongata. Breathing in the air, changes in the water-salt balance and the mechanisms of digestion caused the development of specific control systems for these functions from the side of the brain and the peripheral nervous system.

The main structural levels of the organization of the nervous system

As a result, the total mass of the peripheral nervous system increased due to the innervation of the limbs, the formation of skin sensitivity and cranial nerves, and control over the respiratory organs. In addition, there was an increase in the size of the control center of the peripheral nervous system - the spinal cord. Special spinal thickenings and specialized centers for controlling limb movements in the hind and medulla oblongata have been formed. In large dinosaurs, these sections exceeded the size of the brain. It is also important that the brain itself has become larger. The increase in its size is caused by an increase in the representation of various types of analyzers in the brain. First of all, these are motor, sensorimotor, visual, auditory and olfactory centers. The system of connections between different parts of the brain was further developed. They became the basis for a quick comparison of information coming from specialized analyzers. In parallel, an internal receptor complex and a complex effector apparatus developed. To synchronize the control of receptors, complex muscles, and internal organs, associative centers have arisen in the course of evolution on the basis of different parts of the brain.

The main centers of the nervous system of vertebrates on the example of a frog.

Important evolutionary events leading to a change in the habitat required qualitative changes in the nervous system.

Detailed description of illustrations

In animals of different groups, the comparative sizes of the spinal cord and brain differ greatly. In the frog (A), the brain and spinal cord are almost equal, in the green monkey (B) and the marmoset (C), the mass of the brain is much greater than the mass of the spinal cord, and the spinal cord of the snake (D) is many times larger than the brain in size and mass.

In the metabolism of the brain, three dynamic processes can be distinguished: the exchange of oxygen and carbon dioxide, the consumption of organic substances and the exchange of solutions. In the lower part of the figure, the proportion of consumption of these components in the primate brain is indicated: the top line is in a passive state, the bottom line is during strenuous work. Consumption of aqueous solutions is calculated as the time it takes for all body water to pass through the brain.

The main structural levels of the organization of the nervous system. The simplest level is a single cell that perceives and generates signals. A more complex option is the accumulation of bodies of nerve cells - ganglia. The formation of nuclei or layered cellular structures is the highest level of cellular organization in the nervous system.

The main centers of the nervous system of vertebrates on the example of a frog. The brain is colored red, and the spinal cord is colored blue. Together they make up the central nervous system. The peripheral ganglia are green, the head ganglia are orange, and the spinal ganglia are blue. There is a constant exchange of information between the centers. Generalization and comparison of information, control of effector organs occur in the brain.

Important evolutionary events leading to a change in the habitat required qualitative changes in the nervous system. The first event of this kind was the emergence of chordates, the second was the emergence of vertebrates on land, and the third was the formation of the associative brain region in archaic reptiles. The emergence of the bird brain cannot be considered a fundamental evolutionary event, but mammals went much further than reptiles - the associative center began to perform the functions of controlling the work of sensory systems. The ability to predict events has become a tool for mammals to dominate the planet. A-G - origin of chordates in silty shallow waters; D-J - access to land; Z, P - the emergence of amphibians and reptiles; К-Н - formation of birds in the aquatic environment; P-T - the appearance of mammals in the crowns of trees; I-O is a specialization of reptiles.

8. Changes in water-salt metabolism

Trunk (mesonephric) kidneys have developed in amphibians. These are elongated compact bodies of reddish-brown color, lying on the sides of the spinal column in the region of the sacral vertebra (Fig. 3). A ureter (wolf's canal) stretches from each kidney and each independently flows into the cloaca. An opening at the bottom of the cloaca leads to the bladder, where urine enters and where water is reabsorbed and concentrated urine is excreted from the body. In the renal tubules, water, sugars, vitamins, sodium ions are also absorbed (reabsorption or reabsorption), some of the decay products are excreted through the skin. Head buds function in amphibian embryos.

Rice. 3. Genitourinary system of the male frog: 1 - kidney; 2 - the ureter (aka the vas deferens); 3 - cavity of the cloaca; 4 - urogenital opening; 5 - bladder; 6 - the opening of the bladder; 7 - testis; 8 - vas deferens; 9 - seminal vesicle; 10 - fatty body; 11 - adrenal gland

At the front edge of each kidney, in both sexes, there are finger-shaped yellowish-orange fat bodies that serve as a reserve of nutrients for the gonads during the reproductive period. A narrow, barely noticeable yellowish strip stretches along the surface of each kidney - the adrenal gland - the endocrine gland (Fig. 3).

In reptiles, the kidneys have no connection with the wolf channel, they have developed their own ureters connected to the cloaca. The Wolffian canal is reduced in females, and in males it serves as a vas deferens. In reptiles, the total filtration area of ​​the glomeruli is less, and the length of the tubules is greater. With a decrease in the area of ​​the glomeruli, the intensity of water filtration from the body decreases, and in the tubules most of the water filtered in the glomeruli is absorbed back. Thus, a minimum of water is excreted from the body of reptiles. In the bladder, water is additionally absorbed, which cannot be removed. Sea turtles and some other reptiles, forced to use salt water for drinking, have special salt glands to remove excess salts from the body. In turtles, they are located in the orbit of the eyes. Sea turtles do "cry bitter tears" as they free themselves from excess salts. Marine iguanas have salt glands in the form of so-called "nasal glands" that open into the nasal cavity. Crocodiles do not have a bladder, and salt glands are found near their eyes. When the crocodile grabs the prey, the muscles of the visceral skeleton are triggered and the lacrimal glands open, therefore there is an expression "crocodile tears" - the crocodile swallows the victim and "sheds tears": this is how salts are excreted from the body.

Rice. 4.1 The genitourinary system of the female Caucasian agama: 1 - kidney; 2 - bladder; 3 - urinary opening; 4 - ovary; 5 - oviduct; 6 - funnel of the oviduct; 7 - genital opening; 8 - cavity of the cloaca; 9 - rectum

Rice. 4.2 The genitourinary system of the male Caucasian dragon agama: 1 - kidney; 2 - bladder; 3 - testis; 4 - the epididymis of the testis; 5 - seed tube; 6 - urogenital opening; 7 - copulation sac; 8 - cavity of the cloaca; 9 - rectum

The development of reptiles is not associated with the aquatic environment, the testes and ovaries are paired and lie in the body cavity on the sides of the spine (Fig. 4.1 - 4.2). Fertilization of eggs is carried out in the body of the female, development takes place in the egg. The secretions of the secretory glands of the oviduct form around the egg (yolk) a albuminous membrane, underdeveloped in snakes and lizards and powerful in turtles and crocodiles, then outer membranes are formed. During embryonic development, the embryonic membranes are formed - serous and amnion, allantois develops. A relatively small number of reptile species have ovoviviparity (common viper, viviparous lizard, spindle, etc.). True viviparity is known in some skinks and snakes: they form a true placenta. Parthenogenetic reproduction is assumed in a number of lizards. A case of hermaphroditism was found in a snake - island botrops.

Excretion of metabolic products and regulation of water balance in birds are carried out mainly by the kidneys. In birds, metanephric (pelvic) kidneys, located in the depressions of the pelvic girdle, the ureters open into the cloaca, there is no bladder (one of the adaptations for flight). Uric acid (the final product of excretion), which easily falls out of the solution in crystals, forms a mushy mass that does not stay in the cloaca and is quickly released outward. The nephrons of birds have a middle section - the loop of Henle, in which water is reabsorbed. In addition, water is sucked into the cloaca. Thus, osmoregulation is carried out in the body of birds. All this allows you to remove decay products from the body with minimal water loss. In addition, most birds have nasal (orbital) glands (especially seabirds that drink salt water) that serve to remove excess salt from the body.

Water-salt metabolism in mammals is carried out mainly through the kidneys and is regulated by hormones of the posterior lobe of the pituitary gland. The water-salt metabolism involves the skin with its sweat glands and the intestines. Metanephric kidneys are bean-shaped and located on the sides of the spine. The ureters drain into the bladder. The duct of the urinary bladder in males opens into the copulatory organ, and in the female - on the eve of the vagina. In oviparous (cloacal) ureters, the ureters flow into the cloaca. The reabsorption of water and sodium ions occurs in the Henle loop, the reverse absorption of substances beneficial to the body (sugars, vitamins, amino acids, salts, water) occurs through the walls of different sections of the nephron tubules. In the water balance, the rectum also plays a certain role, the walls of which absorb water from the feces (typical for semi-desert and desert animals). Some animals (for example, camels) during the feeding period are able to store fat consumed in low-feeding and dry times: when fat is broken down, a certain amount of water is formed.

9. Live birth

Viviparity is a way of reproducing offspring, in which the embryo develops inside the mother's body and an individual is born, already free of egg membranes. Viviparous some coelenterates, cladocerans, molluscs, many roundworms, some echinoderms, salps, fish (sharks, rays, as well as aquarium fish - guppies, swordtails, mollienesia, etc.), some toads, worms, salamanders, turtles, lizards snakes, almost all mammals (including humans).

Among reptiles, viviparity is quite widespread. It is found only in forms with soft egg membranes, thanks to which the eggs retain the ability to exchange water with the environment. In turtles and crocodiles, whose eggs have a developed protein membrane and shell, viviparity is not observed. The first step to live birth is the retention of fertilized eggs in the oviducts, where partial development takes place. So, in a quick lizard, eggs can linger in the oviducts for 15-20 days. The snake may be delayed for 30 days, so that a half-formed embryo is found in the laid egg. Moreover, the further north the region, the more prolonged egg retention in the oviducts, as a rule, occurs. In other species, for example, in a viviparous lizard, spindle, copperhead, etc., eggs are retained in the oviducts until the embryos hatch. This phenomenon is called ovoviviparity, since development takes place at the expense of reserve nutrients in the egg, and not at the expense of the mother's body.

True live birth is often considered only the birth of individuals in placentals.

Fertilized eggs of lower vertebrates are retained in the oviducts of the female, and the embryo receives all the necessary nutrients from the egg reserves. In contrast, small mammalian eggs have negligible amounts of nutrients. Fertilization in mammals is internal. Ripe egg cells enter the paired oviducts, where they are fertilized. Both oviducts open into a special organ of the female reproductive system - the uterus. The uterus is a muscular sac, the walls of which can be strongly stretched. The fertilized egg attaches to the wall of the uterus where the fetus develops. At the place of attachment of the egg to the wall of the uterus, the placenta or baby's place develops. The embryo is connected to the placenta by the umbilical cord, inside which its blood vessels pass. In the placenta, through the walls of blood vessels from the mother's blood, nutrients and oxygen enter the blood of the embryo, carbon dioxide and other waste products harmful to the embryo are removed. At the moment of birth in higher animals, the placenta is separated from the wall of the uterus and pushed out in the form of an afterbirth.

The position of the embryo in the uterus

The features of reproduction and development of mammals make it possible to divide them into three groups:

Oviparous

Marsupials

Placental

Oviparous animals

The platypus and echidna, found in Australia, are oviparous. In the structure of the body of these animals, many features characteristic of reptiles have been preserved: they lay eggs, and their oviducts open into the cloaca, like the ureters and intestinal canal. Their eggs are large, containing a significant amount of nutritious yolk. In the oviduct, the egg is also covered with a layer of protein and a thin parchment-coated shell. In the echidna, during the egg-laying period (up to 2 cm long), the skin on the ventral side forms a brood sac, where the ducts of the mammary glands open without forming nipples. This bag holds an egg and is hatched

Marsupial animals

In marsupials, the embryo first develops in the uterus, but the connection between the embryo and the uterus is insufficient, since the placenta is absent. As a result, babies are born underdeveloped and very small. After birth, they are placed in a special bag on the mother's belly, where the nipples are located. Cubs are so weak that at first they are unable to suck milk themselves, and it is periodically injected into their mouth under the influence of the muscles of the mammary glands. Cubs remain in the pouch until they are capable of independent feeding and movement. Marsupials include animals that have a variety of adaptations to living conditions. For example, the Australian kangaroo moves by jumping, having for this very elongated hind legs; others are adapted to climbing trees - the koala bear. The marsupials also include the marsupial wolf, marsupial anteaters and others.

These two groups of animals are classified as lower mammals, and taxonomists distinguish two subclasses: the subclass oviparous and the subclass marsupials.

Placental animals

The most highly organized mammals belong to the subclass of placental animals, or real animals. Their development completely occurs in the uterus, and the shell of the embryo grows together with the walls of the uterus, which leads to the formation of the placenta, hence the name of the subclass - placental. It is this method of development of the embryo that is the most perfect.

It should be noted that mammals have a well-developed care for offspring. Females feed their young with milk, warm them with their bodies, protect them from enemies, teach them to look for food, etc.

Conclusion

The emergence of vertebrates on land, like any large expansion of the adaptive zone, is accompanied by the transformation of mainly four morphofunctional systems: locomotion, orientation (sense organs), nutrition, and respiration. The transformations of the locomotor system were associated with the need to move over the substrate, subject to an increase in the action of gravity in the air. These transformations were expressed primarily in the formation of walking limbs, strengthening the girdle of the limbs, reducing the connection between the shoulder girdle and the skull, as well as strengthening the spine. Transformations of the food grasping system were expressed in the formation of an autostyle of the skull, the development of head mobility (which was facilitated by the reduction of posttemporale), as well as in the development of a mobile tongue, which ensures the transport of food inside the oral cavity. The most complex rearrangements were associated with adaptation to air breathing: the formation of lungs, a pulmonary circulation, and a three-chambered heart. Of the less significant changes in this system, it should be noted the reduction of the branchial clefts and the separation of the digestive and respiratory tracts - the development of the choanas and the laryngeal cleft.

The whole range of adaptations associated with the use of air for breathing has developed in cross-finned fishes (and their ancestors) in water (Shmalhausen, 1964). Breathing out of water entailed only a reduction of the gills and the ophthalmic apparatus. This reduction was associated with the release of hyomandibulare and its transformation into stapes - with the development of the orientation system and the emergence of language mobility. The transformation of the orientation system was expressed in the formation of the middle ear, reduction of the seismosensory system, and in the adaptation of sight and smell to functioning outside the water.

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