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The skin of amphibians is covered. Hello student. Digestive system of amphibians

The skin of amphibians is literally riddled with blood vessels. Therefore, through it oxygen enters directly into the blood and is released carbon dioxide; The skin of amphibians is given special glands that secrete (depending on the type of amphibian) bactericidal, caustic, unpleasant-tasting, tear-producing, toxic and other substances. These unique skin devices allow amphibians with bare and constantly moist skin to successfully protect themselves from microorganisms, attacks by mosquitoes, mosquitoes, ticks, leeches and other blood-sucking animals.

In addition, amphibians, thanks to these protective abilities, are avoided by many predators; The skin of amphibians usually contains many different pigment cells, on which the general, adaptive and protective coloration of the body depends. So, bright color, characteristic of poisonous species, serves as a warning to attackers, etc.

As inhabitants of land and water, amphibians are provided with a universal respiratory system. It allows amphibians to breathe oxygen not only in the air, but also in water (although the amount there is approximately 10 times less), and even underground. Such versatility of their body is possible thanks to a whole complex of respiratory organs for extracting oxygen from the environment where they are located at a particular moment. These are the lungs, gills, oral mucosa and skin.

Skin respiration is of greatest importance for the life of most amphibian species. At the same time, the absorption of oxygen through the skin penetrated by blood vessels is possible only when the skin is moist. The skin glands are designed to moisturize the skin. The drier the surrounding air, the harder they work, releasing more and more new portions of moisture. After all, the skin is equipped with sensitive “devices”. They turn on emergency systems and modes of additional production of life-saving mucus in a timely manner.

U different types In amphibians, some respiratory organs play a major role, others play an additional role, and others may be completely absent. Thus, in aquatic inhabitants, gas exchange (oxygen absorption and carbon dioxide release) occurs mainly through the gills. The larvae of amphibians and adult tailed amphibians that constantly live in water bodies are endowed with gills. And lungless salamanders - inhabitants of land - are not provided with gills and lungs. They receive oxygen and expel carbon dioxide through moist skin and oral mucosa. Moreover, up to 93% of oxygen is provided by skin respiration. And only when individuals need particularly active movements, the system of additional oxygen supply through the mucous membrane of the bottom of the oral cavity is turned on. In this case, the share of its gas exchange can increase to 25%.

The pond frog, both in water and in air, receives the main amount of oxygen through the skin and releases almost all carbon dioxide through it. Additional breathing is provided by the lungs, but only on land. When frogs and toads are immersed in water, metabolic reduction mechanisms are immediately activated. Otherwise they would not have enough oxygen.

Representatives of some species of tailed amphibians, for example, the cryptobranch, which lives in the oxygen-saturated waters of fast streams and rivers, almost do not use their lungs. The folded skin hanging from its massive limbs, in which a huge number of blood capillaries are spread out in a network, helps it extract oxygen from the water. And so that the water washing it is always fresh and there is enough oxygen in it, the cryptobranch uses appropriate instinctive actions - it actively mixes the water using oscillatory movements of the body and tail. After all, his life is in this constant movement.

The versatility of the amphibian respiratory system is also expressed in the emergence of special respiratory devices during a certain period of their life. Thus, crested newts cannot stay in water for a long time and stock up on air, rising to the surface from time to time. It is especially difficult for them to breathe during the breeding season, since when courting females they perform mating dances underwater. To ensure such a complex ritual, Triton has mating season an additional respiratory organ grows - a skin fold in the form of a ridge. The trigger mechanism of reproductive behavior also activates the body's system for the production of this important organ. It is richly supplied with blood vessels and significantly increases the proportion of skin respiration.

Tailed and tailless amphibians are also endowed with an additional unique device for oxygen-free exchange. It is successfully used, for example, by the leopard frog. It can live in oxygen-deprived cold water for up to seven days.

Some spadefoots, the family of American spadefoots, are provided with cutaneous respiration not for staying in water, but underground. There, buried, they spend most of their lives. On the surface of the earth, these amphibians, like all other tailless amphibians, ventilate their lungs by moving the floor of the mouth and inflating the sides. But after the spadefoots burrow into the ground, their pulmonary ventilation system is automatically turned off and the control of skin respiration is turned on.

One of the necessary protective features of amphibian skin is the creation patronizing coloring. In addition, the success of a hunt often depends on the ability to hide. Usually the coloring repeats a specific pattern of an environmental object. Thus, the streaked color of many tree frogs blends perfectly with the background - the trunk of a tree covered with lichen. Moreover, the tree frog is also capable of changing its color depending on the general illumination, brightness and background color, and climatic parameters. Its color becomes dark in the absence of light or in the cold and brightens in bright light. Representatives of slender tree frogs can easily be mistaken for a faded leaf, and black-spotted frogs for a piece of bark of the tree on which it sits. Almost all tropical amphibians have a protective coloration, often extremely bright. Only bright coloring can make an animal invisible among the colorful and lush greenery of the tropics.

Red-eyed tree frog (Agalychnis callidryas)

The combination of color and pattern often creates amazing camouflage. For example, a large toad is endowed with the ability to create a deceptive, camouflaging pattern with a certain optical effect. The upper part of her body resembles a lying one thin sheet, and the lower one is like the deep shadow cast by this leaf. The illusion is complete when the toad lurks on the ground, strewn with real leaves. Could all previous generations, even numerous ones, gradually create the pattern and color of the body (with an understanding of the laws of color science and optics) to accurately imitate its natural analogue - a browned leaf with a clearly defined shadow under its edge? To do this, from century to century, toads had to persistently change their coloring to the desired goal to get the top brown with a dark pattern, and the sides with abrupt change this color to chestnut brown.

The skin of amphibians is provided with cells that are wonderful in their capabilities - chromatophores. They look like a single-celled organism with densely branching processes. Inside these cells are pigment granules. Depending on the specific range of colors in the coloration of amphibians of each species, there are chromatophores with black, red, yellow and bluish-green pigment, as well as reflective plates. When the pigment granules are collected into a ball, they do not affect the color of the amphibian's skin. If, according to a certain command, pigment particles are evenly distributed over all processes of the chromatophore, then the skin will acquire the specified color.

Animal skin may contain chromatophores containing various pigments. Moreover, each type of chromatophore occupies its own layer in the skin. The different colors of the amphibian are formed by the simultaneous action of several types of chromatophores. An additional effect is created by reflective plates. They give colored skin an iridescent pearlescent sheen. Along with the nervous system, hormones play an important role in controlling the functioning of chromatophores. Pigment-concentrating hormones are responsible for the collection of pigment particles into compact balls, and pigment-stimulating hormones are responsible for their uniform distribution over numerous chromatophore processes.

And in this gigantic documentation volume, there is room for a program for the in-house production of pigments. They are synthesized by chromatophores and are used very sparingly. When the time has come for some pigment particles to participate in coloring and be distributed over all, even the most distant parts of the spread out cell, active work on the synthesis of pigment dye is organized in the chromatophore. And when the need for this pigment disappears (if, for example, the background color changes at the new location of the amphibian), the dye collects in a lump and the synthesis stops. Lean production also includes a waste disposal system. During periodic molting (for example, in lake frogs 4 times a year), particles of the frog's skin are eaten. And this allows their chromatophores to synthesize new pigments, freeing the body from additional collection of the necessary “raw materials”.

Some species of amphibians can change color, like chameleons, although more slowly. Thus, different individuals of grass frogs, depending on various factors can acquire different predominant colors - from red-brown to almost black. The color of amphibians depends on illumination, temperature and humidity, and even on emotional state animal. But still main reason changes in skin color, often local, patterned, is to “adjust” it to the color of the background or surrounding space. For this purpose, the work includes highly complex systems light and color perception, as well as coordination of structural rearrangements of color-forming elements. Amphibians are given the remarkable ability to compare the amount of incident light with the amount of light reflected from the background they are against. The lower this ratio, the lighter the animal will be. When exposed to a black background, the difference in the amount of incident and reflected light will be large, and the light of his skin will become darker.

Information about general illumination is recorded in the upper part of the amphibian’s retina, and information about background illumination is recorded in its lower part. Thanks to the system of visual analyzers, the received information is compared about whether the color of a given individual matches the nature of the background, and a decision is made in which direction it should be changed. In experiments with frogs, this was easily proven by misleading their light perception.

An interesting fact is that in amphibians, not only visual analyzers can control changes in skin color. Individuals completely deprived of vision retain their ability to change body color, “adjusting” to the color of the background. This is due to the fact that chromatophores themselves are photosensitive and respond to illumination by dispersing pigment along their processes. Only usually the brain is guided by information from the eyes, and suppresses this activity of skin pigment cells. But for critical situations, the body has a whole system of safety nets so as not to leave the animal defenseless. So in this case, a small, blind and defenseless tree frog of one of the species, taken from a tree, gradually acquires the color of the bright green living leaf on which it is planted. According to biologists, research into the mechanisms of information processing responsible for chromatophore reactions can lead to very interesting discoveries.

The skin secretions of many amphibians, for example, toads, salamanders, and toads, are the most effective weapon against various enemies. Moreover, these can be poisons and substances that are unpleasant, but safe for the life of predators. For example, the skin of some species of tree frogs secretes a liquid that burns like nettles. The skin of tree frogs of other species forms a caustic and thick lubricant, and when they touch it with their tongue, even the most unpretentious animals spit out the captured prey. The skin secretions of toaded toads living in Russia emit an unpleasant odor and cause lacrimation, and if it comes into contact with the skin of an animal, it causes burning and pain. skin amphibian amphibian fish

Studies of the poisons of various animals have shown that the palm in creating the most powerful poisons does not belong to snakes. For example, the skin glands of tropical frogs produce such a strong poison that it poses a danger to the lives of even large animals. The venom of the Brazilian aga toad kills a dog that catches it with its teeth. And Indian hunters lubricated arrow tips with the poisonous secretion of the skin glands of the South American bicolor leaf climber. The skin secretions of the cocoa plant contain the poison batrachotoxin, the most powerful of all known non-protein poisons. Its effect is 50 times stronger than cobra venom (neurotoxin), several times than the effect of curare. This poison is 500 times stronger than poison holothurian sea cucumber, and is thousands of times more toxic than sodium cyanide.

The bright colors of amphibians usually indicate that their skin can secrete toxic substances. It is interesting that in some species of salamanders, representatives of certain races are poisonous and the most colored. In Appalachian forest salamanders, the skin of individuals secretes toxic substances, while in other related salamanders the skin secretions do not contain poison. At the same time, exactly poisonous amphibians endowed with brightly colored cheeks, and especially dangerous ones with red paws. Birds that feed on salamanders are aware of this feature. Therefore, they rarely touch amphibians with red cheeks, and generally avoid amphibians with colored paws.

From educational literature it is known that the skin of amphibians is bare, rich in glands that secrete a lot of mucus. On land, this mucus protects against drying out, facilitates gas exchange, and in water reduces friction when swimming. Through the thin walls of capillaries, located in a dense network in the skin, the blood is saturated with oxygen and gets rid of carbon dioxide. This “dry” information is, in general, useful, but is not capable of causing any emotions. Only with a more detailed acquaintance with the multifunctional capabilities of the skin does a feeling of surprise, admiration and understanding appear that amphibian skin is a real miracle. Indeed, largely thanks to it, amphibians successfully live in almost all parts of the world and zones. However, they do not have scales, like fish and reptiles, feathers, like birds, and fur, like mammals. The skin of amphibians allows them to breathe in water and protect themselves from microorganisms and predators. It serves as a fairly sensitive organ for perceiving external information and performs many other useful functions. Let's look at this in more detail.

Specific skin features

Like other animals, the skin of amphibians is the outer covering that protects body tissues from harmful influences. external environment: penetration of pathogenic and putrefactive bacteria (if the integrity of the skin is damaged, wounds suppurate), as well as toxic substances. It perceives mechanical, chemical, temperature, pain and other influences due to being equipped with a large number of skin analyzers. Like other analyzers, skin analyzing systems consist of receptors that perceive signal information, pathways that transmit it to the central nervous system, and higher nerve centers that analyze this information. cerebral cortex. The specific features of amphibian skin are as follows: it is endowed with numerous mucous glands that maintain its moisture, which is especially important for skin respiration. The skin of amphibians is literally riddled with blood vessels. Therefore, through it oxygen enters directly into the blood and carbon dioxide is released; The skin of amphibians is given special glands that secrete (depending on the type of amphibian) bactericidal, caustic, unpleasant-tasting, tear-producing, toxic and other substances. These unique skin devices allow amphibians with bare and constantly moist skin to successfully protect themselves from microorganisms, attacks by mosquitoes, mosquitoes, ticks, leeches and other blood-sucking animals. In addition, amphibians, thanks to these protective abilities, are avoided by many predators; The skin of amphibians usually contains many different pigment cells, on which the general, adaptive and protective coloration of the body depends. Thus, the bright color, characteristic of poisonous species, serves as a warning to attackers, etc.

Skin breathing

In different species of amphibians, some respiratory organs play a major role, others play an additional role, and others may be completely absent. Thus, in aquatic inhabitants, gas exchange (oxygen absorption and carbon dioxide release) occurs mainly through the gills. The larvae of amphibians and adult tailed amphibians that constantly live in water bodies are endowed with gills. And lungless salamanders - inhabitants of land - are not provided with gills and lungs. They receive oxygen and expel carbon dioxide through moist skin and oral mucosa. Moreover, up to 93% of oxygen is provided by skin respiration. And only when individuals need particularly active movements, the system of additional oxygen supply through the mucous membrane of the bottom of the oral cavity is turned on. In this case, the share of its gas exchange can increase to 25%. The pond frog, both in water and in air, receives the main amount of oxygen through the skin and releases almost all carbon dioxide through it. Additional breathing is provided by the lungs, but only on land. When frogs and toads are immersed in water, metabolic reduction mechanisms are immediately activated. Otherwise they would not have enough oxygen.

To help skin breathing

The versatility of the amphibian respiratory system is also expressed in the emergence of special respiratory devices during a certain period of their life. Thus, crested newts cannot stay in water for a long time and stock up on air, rising to the surface from time to time. It is especially difficult for them to breathe during the breeding season, since when courting females they perform mating dances underwater. To ensure such a complex ritual, the newt grows an additional respiratory organ, a crest-shaped fold of skin, during the mating season. The trigger mechanism of reproductive behavior also activates the body's system for the production of this important organ. It is richly supplied with blood vessels and significantly increases the proportion of skin respiration.

A number of features in the structure of the skin of amphibians show their relationship with fish. The integument of the amphibian is moist and soft and does not yet have such special features adaptive in nature, like feathers or hair. The softness and moisture of the skin of amphibians is due to the insufficiently advanced breathing apparatus, for the skin serves as an additional organ of the latter. This trait should have developed already in the distant ancestors of modern amphibians. This is what we actually see; Stegocephalians narrowly lose the bony skin armor they inherited from the ancestors of fish, remaining longer on the belly, where it serves as protection when crawling.
The integument consists of the epidermis and skin (cutis). The epidermis still retains features characteristic of fish: the ciliated cover of the larvae, which is preserved in Auura larvae until the onset of metamorphosis; ciliated epithelium in the lateral line organs of Urodela, which spend its entire life in water; the presence of unicellular mucous glands in the larvae and the same aquatic Urocleia. The skin itself (cutis) consists, like that of fish, of three mutually perpendicular systems of fibers. Frogs have large lymphatic cavities in their skin, so their skin is not connected to the underlying muscles. In the skin of amphibians, especially those that lead a more terrestrial lifestyle (for example, toads), keratinization develops, which protects the underlying layers of the skin from both mechanical damage and drying out, which is associated with the transition to a terrestrial lifestyle. The keratinization of the skin should, of course, impede cutaneous respiration, and therefore greater keratinization of the skin is associated with greater development of the lungs (for example, in Bufo compared to Rana).
In amphibians, molting is observed, i.e., periodic shedding of the skin. The skin is shed as one piece. In one place or another the skin breaks, and the animal crawls out and sheds it, and some frogs and salamanders eat it. Molting is necessary for amphibians, because they grow until the end of their lives, and the skin would restrict growth.
At the ends of the fingers, keratinization of the epidermis occurs most severely. Some stegocephalians had real claws.
Of modern amphibians, they are found in Xenopus, Hymenochirus and Onychodactylus. The spadefoot toad (Pelobates) develops a shovel-shaped outgrowth on its hind legs as a device for digging.
Stegocephalians had lateral sensory organs, characteristic of fish, as evidenced by the canals on the cranial bones. They are also preserved in modern amphibians, namely, best of all in the larvae, in which they are developed in a typical manner on the head and run in three longitudinal rows along the body. With metamorphosis, these organs either disappear (in Salamandrinae, in all Anura, except the clawed frog Xenopus from Pipidae), or sink deeper, where they are protected by keratinizing supporting cells. When Urodela is returned to water to reproduce, the lateral line organs are restored.
The skin of amphibians is very rich in glands. Unicellular glands characteristic of fish are still preserved in the larvae of Apoda and Urodela and in adult Urodela living in water. On the other hand, real multicellular glands appear here, developing phylogenetically, apparently from accumulations of unicellular glands, which are already observed in fish.

The glands of amphibians are of two kinds; smaller mucous glands and larger serous or protein glands. The former belong to the group of mesocryptic glands, the cells of which are not destroyed during the process of secretion, the latter are holocryptic, the cells of which are entirely used for the formation of secretion. Protein glands form wart-like elevations on the dorsal side, dorsal ridges in frogs, and ear glands (parotids) in toads and salamanders. Both glands (Fig. 230) are covered on the outside with a layer of smooth muscle fibers. The secretion of the glands is often poisonous, especially the protein glands.
The color of the skin of amphibians is determined, as in fish, by the presence of pigment and reflective iridocytes in the skin. The pigment can be either diffuse or granular, located in special cells - chromatophores. Diffuse pigment distributed in the stratum corneum of the epidermis, usually yellow; granular is black, brown and red. In addition to it, there are white grains of guanine. The green and blue color of some amphibians is a subjective coloration, caused by a shift in tones in the eye of the observer.
Studying the skin at low magnifications tree frog, tree frogs (Hyla arborea), we see that when examining the skin from below, it appears black due to the presence of anastomosing and branched black pigment cells, melanophores. The epidermis itself is colorless, but where light passes through the skin with contracted melanophores, it appears yellow. Leukophores, or interfering cells, contain guanine crystals. Xanthophores contain golden-yellow lipochrome. The ability of melanophores to change their appearance, sometimes curling up into a ball, sometimes extending processes, mainly determines the possibility of changing color. The yellow pigment in xanthophores is similarly mobile. Leucophores or interfering cells produce a blue-gray, red-yellow or silvery sheen. Cooperative play All these elements will create all types of amphibian coloring. Permanent black spots are caused by the presence of black pigment. Melanophores enhance its effect. White color caused by leucophores in the absence of melanophores. When melanophores coagulate and lipochrome spreads, a yellow color will be created. Green color is obtained by the interaction of black and yellow chromatophores.
Color changes depend on nervous system.
The skin of amphibians is richly supplied with blood vessels, serving for breathing. The hairy frog (Astyloslernus), which has greatly reduced lungs, has a body covered with hair-like outgrowths of the skin, abundantly supplied with blood vessels. The skin of amphibians also serves to perceive water and excrete. In dry air, the skin of frogs and salamanders evaporates so abundantly that they die. Toads with a more developed stratum corneum survive in the same conditions much longer. 0

External features of the skin

Skin and fat make up about 15% of the total weight of the grass frog.

The frog's skin is mucus-covered and moist. Of our forms, the skin of aquatic frogs is the most durable. The skin on the dorsal side of the animal is generally thicker and stronger than the skin on the belly, and also carries larger number various tubercles. In addition to a number of formations already described earlier, there is also a large number of permanent and temporary tubercles, especially numerous in the area of the anus and on the hind limbs. Some of these tubercles, which usually have a pigment spot at their apex, are tactile. Other tubercles owe their formation to glands. Usually at the top of the latter it is possible to distinguish the exit openings of the glands with a magnifying glass, and sometimes with the naked eye. Finally, the formation of temporary tubercles is possible as a result of contraction of smooth skin fibers.

IN marriage time In male frogs, “nuptial calluses” develop on the first toe of the forelimbs, which differ in structure from species to species.

The surface of the callus is covered with pointed tubercles or papillae, arranged differently in different species. There is one gland for approximately 10 papillae. The glands are simple tubular and are about 0.8 mm long and 0.35 mm wide each. The opening of each gland opens independently and is about 0.06 mm wide. It is possible that the papillae of the “callus” are modified sensitive tubercles, but the main function of the “callus” is mechanical - it helps the male to firmly hold the female. It has been suggested that the secretions of the callus glands prevent inflammation of those inevitable scratches and wounds that form on the female's skin during mating.

After spawning, the “callus” decreases and its rough surface becomes smooth again.

During mating time, the female develops a mass of “nuptial tubercles” on her sides, in the back of her back and on the upper surface of her hind legs, which play the role of a tactile apparatus that excites the female’s sexual sense.

Rice. 1. Mating calluses of frogs:

a - pond, b - grass, c - sharp-faced.

Rice. 2. Cut through the callus:

1 - tubercles (papillae) of the epidermis, 2 - epidermis, 3 - deep layer of skin and subcutaneous tissue, 4 - glands, 5 - gland opening, 6 - pigment, 7 - blood vessels.

The skin color of different species of frogs is very diverse and is almost never the same color.

Rice. 3. Transverse section through the papillae of the nuptial callus:

A - grass frog, B - pond frog.

The majority of species (67-73%) have a brown, blackish or yellowish general background of the upper body. Rana plicatella from Singapore has a bronze back, and isolated areas of bronze color are found on our pond frog. A modification of the brown color is red. Our grass frog occasionally comes across red specimens; for Rana malabarica, a dark crimson color is the norm. Slightly more than a quarter (26-31%) of all frog species are green or olive in color on top. The large color (71%) of frogs lacks a longitudinal dorsal stripe. In 20% of species the presence of a dorsal stripe is variable. A clear permanent stripe is present in a relatively small number (5%) of species; sometimes there are three along the back. light stripes(South African Rana fasciata). The presence of a connection between the dorsal stripe and sex and age for our species has not yet been established. It is possible that it has a shielding thermal significance (it runs along the spinal cord). Half of all frog species have a single-colored belly, while the other half have a more or less spotted belly.

The coloring of frogs varies greatly both from individual to individual and within one individual, depending on conditions. The most permanent color element is black spots. In our green frogs, the general background color can vary from lemon yellow (in bright sun; rarely) through different shades of green to dark olive and even brown-bronze (in moss in winter). The general background color of the grass frog can vary from yellow, through red and brown, to black-brown. The color changes of the sharp-faced frog are smaller in amplitude.

During mating time, males of the sharp-faced frog acquire a bright blue color, and in males of the grass frog the skin covering the throat turns blue.

Albinotic adult grass frogs have been observed at least four times. Three observers saw albino tadpoles of this species. An albino sharp-faced frog was found near Moscow (Terentyev, 1924). Finally, an albino pond frog (Pavesi) was observed. Melanism has been noted for the green frog, the grass frog, and for Rana graeca.

Rice. 4. Mating tubercles of a female grass frog.

Rice. 5. Cross section of the belly skin of a green frog. 100x magnification:

1 - epidermis, 2 - spongy layer of skin, 3 - dense layer of skin, 4 - subcutaneous tissue, 5 - pigment, 6 - elastic threads, 7 - anastomoses of elastic threads, 8 - glands.

Skin structure

The skin consists of three layers: the superficial, or epidermis (epidermis), which has numerous glands, the deep, or skin proper (corium), which also contains a number of glands, and, finally, the subcutaneous tissue (tela subcutanea).

The epidermis consists of 5-7 different cellular layers, the top of which is keratinized. It is called accordingly the stratum corneum (stratum corneum), in contrast to others called germinal or mucous (stratum germinativum = str. mucosum).

The greatest thickness of the epidermis is observed on the palms, soles and, especially, on the joint pads. The lower cells of the germinal layer of the epidermis are tall and cylindrical. At their base there are tooth-like or spine-like processes that protrude into the deep layer of skin. Numerous mitoses are observed in these cells. The higher-lying cells of the germ layer are diversely polygonal and gradually flatten as they approach the surface. The cells are connected to each other by intercellular bridges, between which there are small lymphatic gaps. Cells immediately adjacent to the stratum corneum become keratinized to varying degrees. This process is especially intensified before molting, which is why these cells are called the replacement or reserve layer. Immediately after molting, a new replacement layer appears. The cells of the germ layer may contain grains of brown or black pigment. Especially many of these grains are contained in chrysmatophores, which are star-shaped cells. Most often, chromatophores are found in the middle layers of the mucous layer and are never found in the stratum corneum. There are stellate cells without pigment. Some researchers consider them to be a degenerating stage of chromatophores, while others consider them to be “wandering” cells. The stratum corneum consists of flat, thin, polygonal cells that retain their nuclei despite keratinization. Sometimes these cells contain brown or black pigment. The pigment of the epidermis generally plays a lesser role in coloring than the pigment of the deep layer of the skin. Some parts of the epidermis contain no pigment at all (the belly), while others give rise to permanent dark patches of skin. Above the stratum corneum, a small shiny stripe (Fig. 40)—the cuticle—is visible on the preparations. For the most part, the cuticle forms a continuous layer, but on the articular pads it breaks up into a number of sections. When molting, only the stratum corneum normally sheds, but sometimes the cells of the replacement layer also shed.

In young tadpoles, epidermal cells bear ciliated cilia.

The deep layer of the skin, or the skin itself, is divided into two layers - spongy or upper (stratum spongiosum = str. laxum) and dense (stratum compactum = str. medium).

The spongy layer appears in ontogenesis only with the development of the glands, and before that the dense layer is adjacent directly to the epidermis. In those parts of the body where there are many glands, the spongy layer is thicker than the dense one, and vice versa. The border of the spongy layer of the skin proper with the germinal layer of the epidermis in some places represents a flat surface, while in other places (for example, “nuptial calluses”) we can talk about papillae of the spongy layer of the skin. The basis of the spongy layer is connective tissue with irregularly curled thin fibers. It includes glands, blood and lymph vessels, pigment cells and nerves. Directly below the epidermis there is a light, weakly pigmented border plate. Underneath it lies a thin layer, penetrated by the excretory canals of the glands and richly supplied with vessels - the vascular layer (stratum vasculare). It contains numerous pigment cells. On colored parts of the skin, two types of such pigment cells can be distinguished: more superficial yellow or gray xantholeukophores and deeper, dark, branched melanophores, closely adjacent to the vessels. The deepest part of the spongy layer is the glandular layer (stratum glandulare). The basis of the latter is connective tissue, penetrated by lymphatic slits containing numerous stellate and spindle-shaped immobile and motile cells. This is where the skin glands are found. The dense layer of the skin itself can also be called a layer of horizontal fibers, because it consists mainly of connective tissue plates running parallel to the surface with slight wavy bends. Under the bases of the glands, the dense layer forms depressions, and between the glands it protrudes dome-shaped into the spongy layer. Experiments with feeding frogs with crappies (Kashchenko, 1882) and direct observations force us to contrast the upper part of the dense layer with its entire main mass, called the lattice layer. The latter does not have a lamellar structure. In some places, the bulk of the dense layer turns out to be pierced by vertically running elements, among which two categories can be distinguished: isolated thin bundles of connective tissue that do not penetrate the ethmoidal layer, and “piercing bundles” consisting of vessels, nerves, connective tissue and elastic threads, and also smooth muscle fibers. Most of these piercing bundles extend from the subcutaneous tissue to the epidermis. Connective tissue elements predominate in the abdominal skin tufts, while muscle fibers predominate in the dorsal skin tufts. Composed into small muscle bundles, smooth muscle cells can, when contracting, give the phenomenon of “goose bumps” (cutis anserina). Interestingly, it appears when the medulla oblongata is cut. Elastic threads in frog skin were first discovered by Tonkov (1900). They go inside the piercing bundles, often giving arc-like connections with elastic connections of other bundles. Elastic threads are especially strong in the abdominal area.

Rice. 6, Epidermis of the palm with chromatophores. 245x magnification

Subcutaneous tissue (tela subcutanea = subcutis), which connects the skin as a whole with muscles or bones, exists only in limited areas of the frog’s body, where it directly passes into intermuscular tissue. In most places on the body, the skin lies over large lymphatic sacs. Each lymph sac, lined with endothelium, splits the subcutaneous tissue into two plates: one adjacent to the skin, and the other covering the muscles and bones.

Rice. 7. Cut through the epidermis of the belly skin of a green frog:

1 - cuticle, 2 - stratum corneum, 3 - germinal layer.

Inside the plate adjacent to the skin, cells with gray granular content are observed, especially in the abdominal area. They are called "interfering cells" and are considered to give the color a slight silvery sheen. Apparently, there are differences between the sexes in the nature of the structure of the subcutaneous tissue: in males, special white or yellowish connective tissue ribbons are described that encircle some muscles of the body (lineamasculina).

The frog's coloring is created primarily by elements found in the skin itself.

Four types of coloring matter are known in frogs: brown or black - melanins, golden yellow - lipochromes from the group of fats, gray or white grains of guanine (a substance close to urea) and the red coloring matter of brown frogs. These pigments are found separately, and the chromatophores that carry them are called, respectively, melanophores, xanthophores or lipophores (in brown frogs they also contain a red dye) and leucophores (guanophores). However, often lipochromes, in the form of droplets, are found together with guanine grains in the same cell - such cells are called xantholeucophores.

Podyapolsky's (1909, 1910) indications of the presence of chlorophyll in the skin of frogs are doubtful. It is possible that he was misled by the fact that the weak alcoholic extract from the skin of a green frog has a greenish color (the color of the concentrated extract is yellow - lipochrome extract). All of the listed types of pigment cells are found in the skin itself, while in the subcutaneous tissue only stellate, light-scattering cells are found. In ontogenesis, chromatophores differentiate from cells of primitive connective tissue very early and are called melanoblasts. The formation of the latter is connected (in time and causally) with the appearance of blood vessels. Apparently, all varieties of pigment cells are derivatives of melanoblasts.

All skin glands of the frog belong to the simple alveolar type, are equipped with excretory ducts and, as mentioned above, are located in the spongy layer. The cylindrical excretory duct of the cutaneous gland opens on the surface of the skin with a triradiate opening, passing through a special funnel-shaped cell. The walls of the excretory duct are two-layered, and the rounded body of the gland itself is three-layered: the epithelium is located on the inside, and then there are the muscular (tunica muscularis) and fibrous (tunica fibrosa) membranes. Based on the details of structure and function, all skin glands of the frog are divided into mucous and granular, or poisonous. The former are larger in size (diameter from 0.06 to 0.21 mm, more often 0.12-0.16) smaller than the latter (diameter 0.13-0.80 mm, more often 0.2-0.4). There are up to 72 mucous glands per square millimeter of skin on the extremities, and in other places 30-40. The total number of them for the frog as a whole is approximately 300,000. The granular glands are distributed very unevenly throughout the body. Apparently, they exist everywhere, excluding the nictitating membrane, but they are especially numerous in the temporal, dorsolateral, cervical and humeral folds, as well as near the anus and on the dorsal side of the leg and thigh. On the belly there are 2-3 granular glands per square centimeter, while in the dorsal-lateral folds there are so many of them that the cells of the skin proper are reduced to thin walls between the glands.

Rice. 8. Cut through the skin of the grass frog's back:

1 - border plate, 2 - places of connection of the muscle bundle with the superficial cells of the epidermis, 3 - epidermis, 4 - smooth muscle cells, 5 - dense layer.

Rice. 9. Opening of the mucous gland. View from above:

1 - opening of the gland, 2 - funnel-shaped cell, 3 - nucleus of the funnel-shaped cell, 4 - cell of the stratum corneum of the epidermis.

Rice. 10. Section through the dorsolateral fold of a green frog, magnified 150 times:

1 - mucous gland with high epithelium, 2 - mucous gland with low epithelium, 3 - granular gland.

The epithelial cells of the mucous glands secrete a fluid liquid without being destroyed, while the secretion of the caustic juice of the granular glands is accompanied by the death of some of their epithelial cells. The secretions of the mucous glands are alkaline, and the granular ones are acidic. Considering the above-described distribution of glands on the frog’s body, it is not difficult to understand why litmus paper turns red from the secretion of the glands of the lateral fold and turns blue from the secretions of the abdominal glands. There was an assumption that the mucous and granular glands are age-related stages of one formation, but this opinion is apparently incorrect.

The blood supply to the skin goes through the large cutaneous artery (arteria cutanea magna), which splits into a number of branches running mainly in the partitions between the lymphatic sacs (septa intersaccularia). Subsequently, two communicating capillary systems are formed: subcutaneous (rete subcutaneum) in the subcutaneous tissue and subepidermal (retesub epidermale) in the spongy layer of the skin itself. There are no vessels in the dense layer. The lymphatic system forms two similar networks in the skin (subcutaneous and subepidermal), standing in connection with the lymphatic sacs.

Most nerves approach the skin, like vessels, inside the partitions between the lymphatic sacs, forming a deep subcutaneous network (plexus nervorum interог = pl. profundus) and in the spongy layer - a superficial network (plexus nervorum superficialis). The connection between these two systems, as well as similar formations of the circulatory and lymphatic systems, occurs through threading bundles.

Skin functions

The first and main function of frog skin, like any skin in general, is to protect the body. Since the frog's epidermis is relatively thin, the deep layer, or skin itself, plays the main role in mechanical protection. The role of skin mucus is very interesting: in addition to the fact that it helps to slip away from the enemy, it mechanically protects against bacteria and fungal spores. Of course, the secretions of the granular skin glands of frogs are not as poisonous as, for example, toads, but the known protective role of these secretions cannot be denied.

Injecting the green frog's skin secretions causes the goldfish to die within a minute. Immediate paralysis of the hind limbs was observed in white mice and frogs. The effect was also noticeable on rabbits. Skin secretions of some species can cause irritation when they come into contact with the human mucous membrane. The American Rana palustris with its secretions often kills other frogs planted with it. However, a number of animals quietly eat frogs. Perhaps the main significance of the secretions of the granular glands lies in their bactericidal effect.

Rice. 11. Granular gland of frog skin:

1 - excretory duct, 2 - fibrous membrane, 3 - muscular layer, 4 - epithelium, 5 - secretion granules.

The permeability of frog skin to liquids and gases is of great importance. The skin of a living frog more easily conducts fluids from the outside to the inside, while in dead skin the fluid flow goes in reverse direction. Substances that depress vitality can stop the current and even change its direction. Frogs never drink with their mouths; we can say that they drink with their skin. If the frog is kept in a dry room and then wrapped in a wet rag or placed in water, it will soon gain noticeable weight due to the water absorbed by the skin.

The amount of liquid that the skin of a frog can secrete is given by the following experiment: you can repeatedly dump a frog in gum arabic powder, and it will continue to be dissolved by skin secretions until the frog dies from excessive loss of water.

Constantly moist skin allows gas exchange. The frog's skin releases 2/3-3/4 of all carbon dioxide, and in winter - even more. In 1 hour, 1 cm 2 of frog skin absorbs 1.6 cm 3 of oxygen and releases 3.1 cm 3 of carbon dioxide.

Immersing frogs in oil or covering them with paraffin kills them faster than removing the lungs. If sterility was maintained when removing the lungs, the operated animal can live for a long time in a jar with a small layer of water. However, temperature must be taken into account. It was described long ago (Townson, 1795) that a frog, deprived of lung activity, can live at a temperature of +10° to +12° in a box with moist air for 20-40 days. On the contrary, at a temperature of +19° the frog dies in a vessel with water after 36 hours.

The skin of an adult frog does not take much part in the act of movement, with the exception of the skin membrane between the toes of the hind limb. In the first days after hatching, the larvae can move due to the ciliated cilia of the epidermis of the skin.

Frogs molt 4 or more times during the year, with the first moult occurring after awakening from hibernation. When molting, the surface layer of the epidermis comes off. In sick animals, molting is delayed, and it is possible that this very circumstance is the cause of their death. Apparently, good nutrition can stimulate shedding. There is no doubt that there is a connection between molting and the activity of the endocrine glands; hypophysectomy delays molting and leads to the development of a thick stratum corneum in the skin. Thyroid hormone plays an important role in the molting process during metamorphosis and probably affects it in the adult animal.

An important adaptation is the ability of the frog to slightly change its color. A slight accumulation of pigment in the epidermis can form only dark, permanent spots and stripes. General black and Brown color The (“background”) of frogs is the result of the accumulation of melanophores in a given place in deeper layers. Yellow and red (xanthophores) and white (leucophores) are explained in the same way. Green and blue skin colors are obtained through a combination of different chromatophores. If xanthophores are located superficially, and leukophores and melanophores lie under them, then the light falling on the skin is reflected as green, because long rays are absorbed by melanin, short rays are reflected by guanine grains, and xanthophores play the role of light filters. If the influence of xanthophores is excluded, a blue color is obtained. Previously, it was believed that color changes occur due to amoeba-like movements of the chromatophore processes: their expansion (expansion) and contraction (contraction). It is now believed that such phenomena are observed in young melanophores only during the development of the frog. In adult frogs, redistribution of black pigment grains within the pigment cell takes place by plasma currents.

If melanin grains are dispersed throughout the pigment cell, the color darkens and, conversely, the concentration of all grains in the center of the cell gives lightening. Xanthophores and leukophores apparently retain the ability of amoeboid movements in adult animals. Pigment cells, and therefore coloration, are controlled by a significant number of both external and internal factors. Melanophores exhibit the greatest sensitivity. For coloring frogs from environmental factors highest value have temperature and humidity. Heat(+20° and above), dryness, strong light, hunger, pain, circulatory arrest, lack of oxygen and death cause lightening. Against, low temperature(+ 10° and below), as well as humidity cause darkening. The latter also occurs in carbon dioxide poisoning. In tree frogs, the sensation of a rough surface gives darkening and vice versa, but this has not yet been proven in relation to frogs. In nature and under experimental conditions, the influence of the background on which the frog sits on its color has been observed. When an animal is placed against a black background, its back quickly darkens, and its underside lags significantly. When placed on a white background, the head and forelimbs lighten the fastest, the torso slowest, and the hind limbs the last. Based on blinding experiments, it was believed that light acts on color through the eye, however, after a certain period of time, the blinded frog begins to change its color again. This, of course, does not exclude the partial significance of the eyes, and it is possible that the eye may produce a substance that acts through the blood on melanophores.

After the destruction of the central nervous system and the cutting of nerves, the chromatophores still retain some reactivity to mechanical, electrical and light stimulation. The direct effect of light on melanophores can be observed in fresh cut pieces of skin, which lighten on a white background and darken (much more slowly) on a black background. The role of internal secretion in changing skin color is extremely important. In the absence of the pituitary gland, the pigment does not develop at all. Injecting a frog into the lymphatic sac with 0.5 cm 3 of pituitrin (solution 1: 1,000) gives darkening after 30-40 minutes. A similar injection of adrenaline works much faster; 5-8 minutes after injection of 0.5 cm 3 solution (1: 2,000), lightening is observed. It was suggested that part of the light falling on the frog reaches the adrenal glands, changes their mode of operation and thereby the amount of adrenaline in the blood, which, in turn, affects the coloration.

Rice. 12. Melanophores of a frog with darkening (A) and lightening (B) of color.

There are sometimes quite subtle differences between species with regard to their response to endocrine influences. Vikhko-Filatova, working on the endocrine factors of human colostrum, conducted experiments on frogs lacking a pituitary gland (1937). The endocrine factor of prenatal colostrum and colostrum on the first day after birth gave a clear melanophore reaction when injected pond frog and had no effect on lake melanophores.

The general correspondence of the color of frogs to the colored background on which they live is beyond doubt, but no particularly striking examples of protective coloring have yet been found among them. Perhaps this is a consequence of their relatively high mobility, in which strict correspondence of their color to one particular color background would be rather harmful. The lighter color of the belly of green frogs fits the general “Thayer’s rule,” but the color of the belly of other species is still unclear. On the contrary, the role of the individually highly variable large black spots on the back is clear; merging with the dark parts of the background, they change the contours of the animal’s body (the principle of camouflage) and mask its location.

Literature used: P. V. Terentyev
Frog: Tutorial/ P.V. Terentyev;
edited by M. A. Vorontsova, A. I. Proyaeva. - M. 1950

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