The skin of amphibians is covered. hello student

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, lachrymal, poisonous and other substances. These unique skin devices allow amphibians with bare and constantly moist skin to successfully defend themselves against microorganisms, attacks from mosquitoes, mosquitoes, mites, leeches and other blood-sucking animals.

In addition, amphibians, due 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 coloration characteristic of poisonous species serves as a warning to attackers, etc.

As inhabitants of the earth 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 its amount is approximately 10 times less there), and even underground. Such versatility of their organism is possible thanks to a whole complex of respiratory organs for extracting oxygen from the environment where they are at a particular moment. These are the lungs, gills, oral mucosa and skin.

Skin respiration is of the 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. 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 saving mucus in time.

At different types In amphibians, some respiratory organs play a major role, others play an additional role, and still others may be completely absent. Yes, at aquatic life gas exchange (absorption of oxygen and release of carbon dioxide) occurs mainly through the gills. Gills are endowed with larvae of amphibians and adult tailed amphibians that constantly live in water bodies. And the lungless salamanders - the inhabitants of the land - are not provided with gills and lungs. They receive oxygen and remove carbon dioxide through moist skin and oral mucosa. Moreover, up to 93% of oxygen is provided by skin respiration. And only when individuals need especially 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 up 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, the mechanisms for reducing metabolism are immediately activated. Otherwise, they would not have enough oxygen.

Representatives of some species of tailed amphibians, for example, the cryptogill, which lives in the oxygenated waters of fast streams and rivers, hardly use their lungs. The folded skin hanging from the massive limbs, in which a huge number of blood capillaries are spread out in a network, helps him to extract oxygen from the water. And so that the water washing it is always fresh, and there is enough oxygen in it, the cryptogill uses expedient instinctive actions - actively mixes the water with the help of oscillatory movements of the body and tail. After all, this constant movement is his life.

The universality of the respiratory system of amphibians is also expressed in the emergence of special respiratory devices in a certain period of their life. So, crested newts cannot stay in the 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 under water. To ensure such a complex ritual, it is during the mating season that the newt grows an additional respiratory organ - a skin fold in the form of a comb. 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 endowed with an additional unique device for oxygen-free exchange. They are successfully used, for example, by the leopard frog. She can live in an oxygen-deprived cold water up to seven days.

Some spadefoot, a family of American spadefoot, are provided with skin respiration not to stay in the water, but underground. There, buried, they spend most of their lives. On the surface of the earth, these amphibians, like all other anurans, ventilate the lungs due to movements of the floor of the mouth and inflation of the sides. But after the spadelegs burrow into the ground, their lung ventilation system is automatically turned off and skin respiration control is turned on.

One of the necessary protective features The skin of amphibians is to create a protective coloration. In addition, the success of the hunt often depends on the ability to hide. Usually the coloring repeats some specific pattern of the object. environment. So, the coloration with stains in many tree frogs perfectly merges with the background - the trunk of a tree covered with lichen. Moreover, the tree frog is also able to change its color depending on the general illumination, brightness and background color, and on climatic parameters. Its color becomes dark in the absence of lighting or in the cold and brightens in bright light. Representatives of slender tree frogs are easily mistaken for a faded leaf, and black-spotted ones - for a piece of the bark of the tree on which she sits. Almost all tropical amphibians have a protective coloration, often extremely bright. Only bright colors can make the animal invisible among the colorful and lush greenery of the tropics.

Red-eyed tree frog (Agalychnis callidryas)

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

The skin of amphibians has at its disposal wonderful cells - 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 in a ball, they do not affect the color of the amphibian skin. If, on the other hand, pigment particles are uniformly distributed over all processes of the chromatophore according to a certain command, then the skin will acquire a given color.

The skin of an animal may contain chromatophores containing various pigments. Moreover, each type of chromatophore occupies its own layer in the skin. Different colors of amphibians are formed by the simultaneous action of several types of chromatophores. An additional effect is created by reflective plates. They give the painted skin an iridescent mother-of-pearl luster. An important role in controlling the work of chromatophores, along with nervous system hormones play. Pigment-concentrating hormones are responsible for collecting pigment particles into compact balls, and pigment-stimulating hormones are responsible for their uniform distribution over numerous processes of the chromatophore.

And in this gigantic documentation, in terms of information volume, there is a place for a program for our own production of pigments. They are synthesized by chromatophores and are used 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 cell, active work is organized in the chromatophore to synthesize the pigment dye. And when the need for this pigment disappears (when, for example, the background color changes at the new location of the amphibian), the dye is collected 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), the frogs eat skin particles. And this allows their chromatophores to synthesize new pigments, freeing the body from the additional collection of the necessary "raw materials".

Coloring in some amphibians can change, like chameleons, although more slowly. So, 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 the light, temperature and humidity, and even on emotional state animal. And yet, the main reason for the change in skin color, often local, patterned, is its “adjustment” to the color of the background or the surrounding space. For this, the work includes the most complex systems of light and color perception, as well as coordination by structural rearrangements of color-forming elements. Amphibians have been given the remarkable ability to compare the amount of incident light with the amount of light reflected from the background they are in. The smaller this ratio, the lighter the animal will be. When hit on a black background, the difference in the amount of incident and reflected light will be large, and the light of his skin becomes darker.

Information about the general illumination is recorded in the upper part of the retina of the amphibian, and about the illumination of the background - in its lower section. Thanks to the system of visual analyzers, the information received is compared about whether the color of a given individual corresponds to the nature of the background, and a decision is made in which direction it should be changed. In experiments with frogs, this was easily proved 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 sight retain their ability to change body color, "adjusting" to the background color. This is due to the fact that the chromatophores themselves have photosensitivity and react to illumination by dispersing the 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. In this case, too, a small, blind and defenseless tree frog of one of the species, taken from a tree, gradually acquires the color of a bright green living leaf on which it is planted. According to biologists, the study of the mechanisms of information processing responsible for chromatophore reactions can lead to very interesting discoveries.

The skin secretions of many amphibians, such as toads, salamanders, and toads, are the most effective weapons against various enemies. Moreover, it can be poisons and unpleasant, but safe substances for the life of predators. For example, the skin of some tree frogs exudes a liquid that burns like nettles. The skin of tree frogs of other species forms a caustic and thick lubricant, and, touching it with the tongue, even the most unpretentious animals spit out the seized prey. The skin secretions of the toads living in Russia emit an unpleasant odor and cause lacrimation, and if it comes into contact with the animal's skin, 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 a poison so strong that it poses a danger to the life of even large animals. From the poison of the Brazilian toad-aga, a dog dies, grabbing it with its teeth. And a poisonous secret skin glands Indian hunters lubricated the arrowheads of the South American bicolor leaf climber. The skin secretions of the cocoa leaf climber contain the poison batrachotoxin, the most powerful of all known non-protein poisons. Its action is 50 times stronger than cobra venom (neurotoxin), several times stronger than the effect of curare. This poison is 500 times stronger than poison holothurian sea cucumber, and it is thousands of times more toxic than sodium cyanide.

The bright coloration of amphibians usually indicates that their skin can release toxic substances. Interestingly, 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, skin secretions do not contain poison. At the same time, it is poisonous amphibians that are endowed with a bright color of their 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 them with colored paws.

External features of the skin

Skin and fat make up about 15% of the common frog's weight.

The frog's skin is covered with mucus and moist. Of our forms, the skin of aquatic frogs is the strongest. The skin on the dorsal side of the animal is generally thicker and stronger than the skin on the belly, and also bears a greater number of various tubercles. In addition to a number of previously described formations, there are still 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 bear a pigment spot at their apex, are tactile. Other tubercles owe their formation to the glands. Usually, at the top of the latter, it is possible to distinguish with a magnifying glass, and sometimes with a simple eye, the excretory openings of the glands. Finally, the formation of temporary tubercles is possible as a result of the contraction of smooth skin fibers.

During mating season, male frogs develop "nuptial calluses" on the first toe of their 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. One gland accounts for approximately 10 papillae. The glands are simple tubular and each is about 0.8 mm long and 0.35 mm wide. The orifice of each gland opens independently and is about 0.06 mm wide. It is possible that the papillae "corns" are modified sensitive tubercles, but main function"corns" are mechanical - it helps the male to hold the female firmly. It has been suggested that the secretions of the callus glands prevent inflammation of those inevitable scratches and wounds that form on the skin of the female during mating.

After spawning, the "corn" decreases, and its rough surface becomes smooth again.

In the female, on the sides, in the back of the back and on the upper surface of the hind limbs during mating season, a mass of "nuptial tubercles" develops, playing the role of a tactile apparatus that arouses the sexual feeling of the female.

Rice. 1. Marriage calluses of frogs:

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

Rice. 2. Cut through the bridal 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 color of the skin of different types of frogs is very diverse and almost never the same color.

A - herbal, B - pond frogs.

The majority of species (67-73%) have a brown, blackish or yellowish general background of the upper body. Rana pplicatella from Singapore has a bronze back, and patches of bronze 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 above. The large suit (71%) of frogs is devoid of a longitudinal dorsal stripe. In 20% of the species, the presence of the dorsal stripe is variable. A relatively small number (5%) of species has a clear permanent stripe, sometimes three light stripes run along the back (South African Rana fasciata). The presence of a relationship between the dorsal stripe and sex and age for our species has not yet been established. It is possible that it has a thermal screening value (it runs along the spinal cord). Half of all frog species have a solid belly, while the other half is more or less spotted.

The coloration of frogs is highly variable both from individual to individual, and in 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 various shades of green to dark olive and even brown-bronze (in moss in winter). General background The common frog's coloration can vary from yellow, through red and brown, to black-brown. Color changes in the moored frog are smaller in their amplitude.

At mating time, male moor frogs acquire a bright blue color, and in males, the skin covering the throat turns blue.

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

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

Rice. 5. Transverse section of the skin of the abdomen of a green frog. 100 times magnification:

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

Skin structure

The skin consists of three layers: the superficial, or epidermis (epidermis), which has numerous glands, deep, or the skin itself (sorium), in which a certain amount of glands is also found, and, finally, subcutaneous tissue (tela subcutanea).

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

The greatest thickness of the epidermis is observed on the palms, feet and, especially, on the articular pads. The lower cells of the germ layer of the epidermis are high, cylindrical. At their base are tooth-like or spiky processes protruding into the deep layer of the skin. Numerous mitoses are observed in these cells. The cells of the germ layer located above are manifold polygonal and gradually flatten as they approach the surface. Cells are connected to each other by intercellular bridges, between which small lymphatic gaps remain. Cells directly adjacent to the stratum corneum become keratinized to varying degrees. This process is especially enhanced before molting, due to which these cells are called a replacement or reserve layer. Immediately after the molt, a new replacement layer appears. Germ layer cells may contain granules of brown or black pigment. Especially many of these grains are found in star-shaped chrzmatophore cells. Most often, chromatophores are found in the middle layers of the mucous layer and never come across in the stratum corneum. There are stellate cells and 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 nuclei despite keratinization. Sometimes these cells contain a brown or black pigment. The pigment of the epidermis as a whole plays a lesser role in color 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 on the preparations, a small shiny strip (Fig. 40) is visible - the cuticle (cuticula). 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 comes off, but sometimes the cells of the replacement layer also come off.

In young tadpoles, the cells of the epidermis 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 ontogeny only with the development of the glands, and before that the dense layer adjoins 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 itself with the germinal layer of the epidermis in some places represents a flat surface, while in other places (for example, "marital calluses") one can speak of papillae of the spongy layer of the skin. The basis of the spongy layer is connective tissue with incorrectly curled thin fibers. It includes glands, blood and lymphatic vessels, pigment cells and nerves. Directly below the epidermis is a light, poorly pigmented border plate. Under it lies a thin layer, penetrated by the excretory channels of the glands and richly supplied with vessels - the vascular layer (stratum vasculare). It contains numerous pigment cells. On the colored parts of the skin, two varieties 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 (stratum glandulare). The basis of the latter is the connective tissue, permeated with lymphatic slits containing numerous stellate and fusiform fixed and mobile cells. This is where the skin glands meet. 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 juts out dome-like into the spongy layer. Experiments with feeding frogs with krappa (Kashchenko, 1882) and direct observations make it necessary 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 is permeated with vertically extending elements, among which two categories can be distinguished: isolated thin bundles of connective tissue that do not penetrate the lattice layer, and "penetrating bundles" consisting of vessels, nerves, connective tissue and elastic filaments, but as well as smooth muscle fibers. Most of these penetrating bundles extend from the subcutaneous tissue to the epidermis. In the bundles of the skin of the abdomen, connective tissue elements predominate, while in the bundles of the skin of the back, muscle fibers predominate. When folded into small muscle bundles, smooth muscle cells can, when contracting, give the phenomenon of "goosebumps" (cutis anserina). Interestingly, it appears when the medulla oblongata is transected. Elastic threads in frog skin were first discovered by Tonkov (1900). They go inside penetrating bundles, often giving arcuate connections with elastic connections of other bundles. The elastic threads in the belly area are especially strong.

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

Subcutaneous tissue (tela subcutanea \u003d 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 the intermuscular tissue. In most places of the body, the skin lies over extensive lymph sacs. Each lymphatic 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. Section through the epidermis of the skin of the belly of a green frog:

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

Inside the plate adjacent to the skin, cells with a gray granular content are observed, especially in the belly area. They are called "interfering cells" and are considered to impart a slight silvery sheen to the color. 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 coloration of the frog is created primarily due to the elements that are in the skin itself.

Frogs have four types of dyes: 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 dye of brown frogs. These pigments are found separately, and the chromatophores that carry them are called melanophores, xanthophores, or lipophores, respectively (in brown frogs they also contain a red dye) and leukophores (guanophores). However, often lipochromes, in the form of droplets, are found together with guanine grains in one cell - such cells are called xantholeukophores.

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 a weak alcoholic extract from the skin of a green frog has a greenish color (the color of the concentrated extract is yellow - an extract of lipochromes). All of the listed types of pigment cells are found in the skin itself, while only stellate, light-scattering cells are found in the subcutaneous tissue. In ontogeny, chromatophores differentiate very early from primitive connective tissue cells and are called melanoblasts. The formation of the latter is related (in time and causally) to the appearance of blood vessels. Apparently, all varieties of pigment cells are derivatives of melanoblasts.

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

Rice. 8. Cut through the skin of the back of a common frog:

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. Hole of the mucous gland. View from above:

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

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

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

The cells of the epithelium of the mucous glands secrete a flowing liquid without being destroyed, while the release of the caustic juice of the granular glands is accompanied by the death of some of the cells of their epithelium. The secretions of the mucous glands are alkaline, and those of the granular glands are acidic. Considering the distribution of glands on the body of the frog described above, it is not difficult to beat why litmus paper turns red from the secretion of the glands of the lateral fold and turns blue from the secretions of the belly glands. There was an assumption that the mucous and granular glands are the age stages of the same formation, but this opinion, apparently, is incorrect.

The blood supply to the skin goes through a large cutaneous artery (arteria cutanea magna), which breaks up into a number of branches that go 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 (retésub epidermal) in the spongy layer of the skin proper. 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 of the nerves approach the skin, like vessels, inside the partitions between the lymphatic sacs, forming a subcutaneous deep network (plexus nervorum interiog = pl. profundus) and in the spongy layer - a superficial network (plexus nervorum superficialis). The connection of these two systems, as well as similar formations of the circulatory and lymphatic systems, occurs through penetrating bundles.

Skin functions

The first and main function of the frog skin, like any skin in general, is to protect the body. Because 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 helping to slip out 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 well-known protective role of these secretions cannot be denied.

Injection of the skin secretions of a green frog causes the death of a goldfish in a minute. In white mice and frogs, immediate paralysis of the hind limbs was observed. The effect was also noticeable in rabbits. Skin secretions of some species can cause irritation when they get on the human mucosa. The American Rana palustris often kills other frogs planted with it with its secretions. However, a number of animals calmly eat frogs. Perhaps the main significance of the secretions of the granular glands lies in their bactericidal action.

Rice. 11. Granular gland of frog skin:

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

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

The following experience gives an idea of ​​the amount of liquid that the skin of a frog can secrete: you can repeatedly dump a frog in gum arabic powder, and it will be dissolved by skin secretions until the frog dies from excessive loss of water.

Constantly moist skin allows gas exchange. In a frog, the skin releases 2 / 3 - 3 / 4 of all carbon dioxide, and in winter - even more. For 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 smearing them with paraffin kills them faster than removing lungs. If sterility was observed during the removal of 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. Long ago (Townson, 1795) it was described that a frog, deprived of lung activity, can live at temperatures from +10° to +12° in a box with moist air 20-40 days. On the other hand, at a temperature of +19°, the frog dies in a vessel of 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 fingers of the hind limb. In the first days after hatching, larvae can move due to the ciliated cilia of the skin epidermis.

Frogs molt 4 or more times during the year, with the first molt occurring after waking up from hibernation. When shedding, 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 molting. There is no doubt that molting is connected with 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 process of molting during metamorphosis and probably also affects it in the adult animal.

An important adaptation is the ability of the frog to change its color somewhat. A slight accumulation of pigment in the epidermis can form only dark permanent spots and stripes. The general black and brown color (“background”) of frogs is the result of the accumulation of melanophores in deeper layers in a given place. In the same way, yellow and red (xanthophores) and white (leukophores) are explained. The green and blue color of the skin is obtained by a combination of different chromatophores. If xanthophores are located superficially, and leucophores and melanophores lie under them, then the light incident on the skin is reflected in the form of 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, then a blue color is obtained. Previously, it was believed that the change in color occurs due to amoeba-like movements of the processes of chromatophores: 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, there is a redistribution of black pigment granules inside the pigment cell by plasma currents.

If the melanin granules are dispersed throughout the pigment cell, the color darkens and, conversely, the concentration of all the granules in the center of the cell gives a lightening. Xanthophores and leucophores apparently retain the ability of amoeboid movements in adult animals as well. Pigment cells, and therefore coloration, are controlled by a significant number of both external and internal factors. Melanophores are the most sensitive. For coloring frogs environmental factors temperature and humidity are the most important. Heat(+20° and above), dryness, strong light, hunger, pain, circulatory arrest, lack of oxygen and death cause lightening. On the contrary, 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 coloration was observed. When an animal is placed on a black background, its back quickly darkens, the underside is much later. When placed on White background the head and fore limbs brighten most rapidly, the trunk and hind limbs lighten the slowest. Based on blinding experiments, it was believed that light acts on color through the eye, however, after a certain period of time, a 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 the melanophores.

After the destruction of the central nervous system and the transection of the nerves, the chromatophores still retain some reactivity to mechanical, electrical, and light stimuli. The direct effect of light on melanophores can be observed on fresh cut pieces of skin, which lighten on a white background and darken (much more slowly) on a black one. The role of internal secretion in changing the color of the skin is exceptionally great. 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 (1: 1,000 solution) results in darkening in 30-40 minutes. A similar injection of adrenaline acts much faster; after 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 the mode of their work and thereby the amount of adrenaline in the blood, which, in turn, affects the color.

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

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, performed experiments on frogs lacking the pituitary gland (1937). The endocrine factor of prenatal colostrum and colostrum on the first day after birth gave a clear melanophoric reaction when injected into the pond frog and had no effect on the lake frog melanophores.

The general correspondence of the coloration of frogs to the colored background on which they live is beyond doubt, but no particularly striking examples of protective coloration have yet been found among them. Perhaps this is a consequence of their relatively high mobility, in which a strict correspondence of their coloration to any one 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 not yet clear. On the contrary, the role of individually very 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.

References: P. V. Terentiev
Frog: Study Guide / P.V. Terentiev;
ed. M. A. Vorontsova, A. I. Proyaeva. - M. 1950

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

Specific features of the skin

Like other animals, the skin of amphibians is the outer covering that protects body tissues from harmful influence external environment: the penetration of pathogenic and putrefactive bacteria (if the integrity of the skin is violated, suppuration of wounds occurs), as well as toxic substances. It perceives mechanical, chemical, temperature, pain and other influences due to the equipment 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 also analyze this information from higher nerve centers in the cerebral cortex. The specific features of the skin of amphibians are as follows: it is endowed with numerous mucous glands that maintain its moisture, which is especially importance for skin respiration. The skin of amphibians is literally riddled with blood vessels. Therefore, oxygen enters directly into the blood through it and carbon dioxide is released; The skin of amphibians is given special glands that secrete (depending on the type of amphibian) bactericidal, caustic, unpleasant, lachrymal, poisonous and other substances. These unique skin devices allow amphibians with bare and constantly moist skin to successfully defend themselves against microorganisms, attacks from mosquitoes, mosquitoes, mites, leeches and other blood-sucking animals. In addition, amphibians, due 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 coloration characteristic of poisonous species serves as a warning to attackers, etc.

Skin respiration

As inhabitants of the earth 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 its amount is approximately 10 times less there), and even underground. Such versatility of their organism is possible thanks to a whole complex of respiratory organs for extracting oxygen from the environment where they are at a particular moment. These are the lungs, gills, oral mucosa and skin.

Skin respiration is of the 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. 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 saving mucus in time.

In different types of amphibians, some respiratory organs play a major role, others play an additional role, and still others may be completely absent. So, in aquatic inhabitants, gas exchange (the absorption of oxygen and the release of carbon dioxide) occurs mainly through the gills. Gills are endowed with larvae of amphibians and adult tailed amphibians that constantly live in water bodies. And the lungless salamanders - the inhabitants of the land - are not provided with gills and lungs. They receive oxygen and remove carbon dioxide through moist skin and oral mucosa. Moreover, up to 93% of oxygen is provided by skin respiration. And only when individuals need especially 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 up 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, the mechanisms for reducing metabolism are immediately activated. Otherwise, they would not have enough oxygen.

Helps skin breathe

Representatives of some species of tailed amphibians, for example, the cryptogill, which lives in the oxygenated waters of fast streams and rivers, hardly use their lungs. The folded skin hanging from the massive limbs, in which a huge number of blood capillaries are spread out in a network, helps him to extract oxygen from the water. And so that the water washing it is always fresh, and there is enough oxygen in it, the cryptogill uses expedient instinctive actions - actively mixes the water with the help of oscillatory movements of the body and tail. After all, this constant movement is his life.

The universality of the respiratory system of amphibians is also expressed in the emergence of special respiratory devices in a certain period of their life. So, crested newts cannot stay in the 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 under water. To ensure such a complex ritual, an additional respiratory organ grows in the newt during the mating season - a skin fold in the form of a comb. 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 endowed with an additional unique device for oxygen-free exchange. They are successfully used, for example, by the leopard frog. She can live in oxygen-deprived cold water for up to seven days.

Some spadefoot, a family of American spadefoot, are provided with skin respiration not to stay in the water, but underground. There, buried, they spend most of their lives. On the surface of the earth, these amphibians, like all other anurans, ventilate the lungs due to movements of the floor of the mouth and inflation of the sides. But after the spadelegs burrow into the ground, their lung ventilation system is automatically turned off and skin respiration control is turned on.

vital coloration

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

But how could amphibians develop and gradually dress in protective coloration without knowledge of color science and optics? After all, most often they have such a color when the coloring creates the illusion of a broken continuous surface of the body. At the same time, when joining the parts of the pattern located on the body and legs (when they are pressed against each other), an apparent continuity of the composite pattern is formed. The combination of coloration and pattern often creates amazing camouflage. For example, a large toad is endowed with the ability to create a deceptive, masking pattern with a certain optical effect. The upper part of her body resembles a lying thin leaf, and the lower part is like a deep shadow cast by this leaf. The illusion is complete when the toad lurks on the ground strewn with real leaves. Could all previous, even if numerous, generations have gradually created the pattern and color of the body (with an understanding of the laws of color science and optics) to accurately imitate the natural counterpart - a browned leaf with a clearly defined shadow under its edge? To do this, from century to century, the toads had to persistently lead their color to the desired goal in order to get the top - brown with a dark pattern, and the sides - with a sharp change in this color to chestnut brown.

How skin creates color?

The skin of amphibians is provided with cells, miraculous 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 in a ball, they do not affect the color of the amphibian skin. If, on the other hand, pigment particles are uniformly distributed over all processes of the chromatophore according to a certain command, then the skin will acquire a given color. The skin of an animal may contain chromatophores containing various pigments. Moreover, each type of chromatophore occupies its own layer in the skin. Different colors of amphibians are formed by the simultaneous action of several types of chromatophores. An additional effect is created by reflective plates. They give the painted skin an iridescent mother-of-pearl luster. Along with the nervous system, hormones play an important role in controlling the work 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 processes of the chromatophore.

And how is your own production for the manufacture of pigments carried out? The fact is that the body creates all the most complex macromolecules and other substances in a miraculous way for itself. He quickly and confidently, as it were, "weaves" from the air, light and from timely delivered to him necessary elements- own own body. These elements are absorbed through digestive system, come by inhalation, diffuse through the skin. There is a comprehensive genetic "documentation" for this "weaving production" in the focal point of each cell and in the control system of the whole organism. It includes a huge databank and program of actions for each molecule, molecular complexes, systems, organelles, cells, organs, etc. up to the whole body. And in this gigantic documentation, in terms of information volume, there is a place for a program for our own production of pigments. They are synthesized by chromatophores and are used 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 cell, active work is organized in the chromatophore to synthesize the pigment dye. And when the need for this pigment disappears (when, for example, the background color changes at the new location of the amphibian), the dye is collected 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), the frogs eat skin particles. And this allows their chromatophores to synthesize new pigments, freeing the body from the additional collection of the necessary "raw materials".

Ability to perceive light and color

Coloring in some amphibians can change, like chameleons, although more slowly. So, different individuals of common frogs, depending on various factors, can acquire different predominant colors - from red-brown to almost black. The color of amphibians depends on the light, temperature and humidity, and even on the emotional state of the animal. And yet, the main reason for the change in skin color, often local, patterned, is its “adjustment” to the color of the background or the surrounding space. To do this, the work includes the most complex systems of light and color perception, as well as coordination by structural rearrangements of color-forming elements. Amphibians have been given the remarkable ability to compare the amount of incident light with the amount of light reflected from the background they are in. The smaller this ratio, the lighter the animal will be. When hit on a black background, the difference in the amount of incident and reflected light will be large, and the light of his skin becomes darker. Information about the general illumination is recorded in the upper part of the retina of the amphibian, and about the illumination of the background - in its lower part. Thanks to the system of visual analyzers, the information received is compared about whether the color of a given individual corresponds to the nature of the background, and a decision is made in which direction it should be changed. In experiments with frogs, this was easily proved by misleading their light perception. If they painted over the cornea and blocked the light from entering the lower part of the pupil, then the animal had the illusion that they were on a black background, and the frogs became darker. In order to change color scheme coloring of their skin, amphibians need not only to compare the intensity of lighting. They must also estimate the wavelength of the reflected light, i.e. define the background color. Scientists know very little about how this happens.

An interesting fact is that in amphibians, not only visual analyzers can control changes in skin color. Individuals completely deprived of sight retain their ability to change body color, "adjusting" to the background color. This is due to the fact that the chromatophores themselves have photosensitivity and react to illumination by dispersing the 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. In this case, too, a small, blind and defenseless tree frog of one of the species, taken from a tree, gradually acquires the color of a bright green living leaf on which it is planted. According to biologists, the study of the mechanisms of information processing responsible for chromatophore reactions can lead to very interesting discoveries.

Skin protection

Skin protects against predators

The skin secretions of many amphibians, such as toads, salamanders, and toads, are the most effective weapons against various enemies. Moreover, it can be poisons and unpleasant, but safe substances for the life of predators. For example, the skin of some tree frogs exudes a liquid that burns like nettles. The skin of tree frogs of other species forms a caustic and thick lubricant, and, touching it with the tongue, even the most unpretentious animals spit out the seized prey. The skin secretions of the toads living in Russia emit an unpleasant odor and cause lacrimation, and if it comes into contact with the animal's skin, it causes burning and pain. Having tasted the toad at least once, the predator remembers the lesson given to it well and no longer dares to touch the representatives of this amphibian species. There is a widespread belief among many people that warts appear on the skin of a person who picks up a toad or a frog. These are prejudices that have no basis, but it must be borne in mind that if the secretions of the skin glands of frogs get on the mucous membranes of the mouth, nose and eyes of a person, they will cause irritation.

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 a poison so strong that it poses a danger to the life of even large animals. From the poison of the Brazilian toad-aga, a dog dies, grabbing it with its teeth. And with the poisonous secret of the skin glands of the South American bicolor leaf climber, Indian hunters lubricated arrowheads. The skin secretions of the cocoa leaf climber contain the poison batrachotoxin, the most powerful of all known non-protein poisons. Its action is 50 times stronger than cobra venom (neurotoxin), several times stronger than the effect of curare. This poison is 500 times more potent than that of the sea cucumber holothurian, and it is thousands of times more toxic than sodium cyanide.

It would seem, why are amphibians provided with the ability to produce such an effective poison? But in living organisms, everything is arranged expediently. After all, its injection occurs without special devices (teeth, harpoons, thorns, etc.), which other poisonous animals are provided with, so that the poisonous substance enters the blood of the enemy. And the venom of amphibians is released from the skin mainly when the amphibian is squeezed in the teeth of a predator. It is absorbed mainly through the mucous membrane of the mouth of the animal that attacked it.

Frightening coloration
The bright coloration of amphibians usually indicates that their skin can release toxic substances. Interestingly, 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, skin secretions do not contain poison. At the same time, it is poisonous amphibians that are endowed with a bright color of their 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 them with painted paws.

An interesting fact is connected with the red-bellied American newts, which are brightly colored and completely inedible. The mountain false and non-poisonous red newts that live near them, called "harmless deceivers", are provided with the same bright paint (mimicry). However, false red newts usually outgrow their venomous counterparts considerably and become less like them. Perhaps for this reason, bright colors are specially given to them only for the first 2-3 years. After this period, the grown-up "deceivers" begin to synthesize pigments for a species-typical dark, brown-brown color, and they become more careful.

Experiments were carried out with chickens, which clearly demonstrated the clear effect of warning coloring on them. The chickens were offered brightly colored red-bellied, false red, and false mountain newts as food. As well as dim lungless salamanders. The chickens ate only the “simple-dressed” salamanders. Since the chickens had no experience of meeting amphibians before, then from these unambiguous results of the experiments there should be only one conclusion: “knowledge” about the dangerous coloration is innate. But maybe the parents of the chickens, having learned an unpleasant lesson when they encountered brightly colored poisonous prey, passed this knowledge on to their offspring? Scientists have established that the development, improvement of the instinctive mechanisms of behavior does not occur. There are only successive age stages of its realization, which replace each other at a given moment. Therefore, in a complex set of protective instinctive behavioral reactions, this fear of bright creatures that carry a potential danger was laid down from the very beginning.

A number of features in the structure of the skin of amphibians show their relationship with fish. The integuments of an amphibian are moist and soft and do not yet have such special features adaptive nature, like a feather or hair. The softness and moisture of the skin of amphibians are due to the insufficiently perfect apparatus for breathing, because the skin serves as an additional organ of the latter. This feature should have developed already in the distant ancestors of modern amphibians. This is what we actually see; narrowly in stegocephals, the bone skin armor inherited from the ancestors of fish is lost, 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 ciliary cover in larvae, which persists in Auura larvae until metamorphosis; ciliary epithelium in the lateral line organs of Urodela, which spend their whole life in water; the presence of unicellular mucous glands in larvae and the same aquatic Urocleia. The skin itself (cutis) consists, like in fish, of three mutually perpendicular systems of fibers. Frogs have large lymphatic cavities in their skin, due to which the 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, protecting 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 must, of course, impede skin 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 bursts, and the animal crawls out of it and throws it off, and some frogs and salamanders eat it. Moulting is necessary for amphibians, because they grow until the end of their lives, and the skin would hamper growth.
At the ends of the fingers, the keratinization of the epidermis occurs most strongly. Some stegocephalians had real claws.
Of modern amphibians, they are found in Xenopus, Hymenochirus and Onychodactylus. In the spade toad (Pelobates), a shovel-like outgrowth develops on its hind legs as a device for digging.
Lateral sense organs, characteristic of fish, were present in stegocephalians, as evidenced by canals on the cranial bones. They are also preserved in modern amphibians, namely, they are best preserved in larvae, in which they are developed in a typical way on the head and run along the body in three longitudinal rows. With metamorphosis, these organs either disappear (in Salamandrinae, in all Anura, except for the clawed frog Xenopus from Pipidae), or sink deeper, where they are protected by keratinizing supporting cells. When the Urodela is returned to the breeding water, the lateral line organs are restored.
The skin of amphibians is very rich in glands. The unicellular glands characteristic of fish are still preserved in the larvae of Apoda and Urodela and in the adult Urodela living in the water. On the other hand, real multicellular glands appear here, which developed phylogenetically, apparently from accumulations of unicellular glands, which are already observed in fish.