Derivatives of skin are fish scales. Features of the structure and formation of scales, feathers, skin. Features of the structure and formation of scales

• Read: Variety of fish: shape, size, color

Melanophores and fish scales.

Fish are characterized by a variety of colors, which depend on the presence of pigment cells - chromatophores - in the skin. Chromatophores can lie at the border of the upper and lower layers of the dermis, in the lower layer of the dermis and in the subcutaneous connective tissue along with fat cells.

The following types of chromatophores are distinguished: 1) melanophores (include black and brown pigments); 2) xanthophores (yellow pigments); 3) erythrophores (red pigments); 4) leucophores, or guanophores (contain guanine crystals, which give the skin of fish a silvery color).

Melanophores, xanthophores, erythrophores are stellate in shape with processes, leucophores (or guanophores) are oval. The color of fish changes depending on age, sex and physiological state. Thus, salmon fry have transverse stripes, which disappear at the smoltification stage. The fish have a protective color (in pelagic fish the back is dark, the belly is light). The colors of the inhabitants of coral reefs are very diverse.

Some fish have the ability to change their color. Thus, flounders, stingrays and some other fish can change color in accordance with their environment.

In fish, color changes depend on the pigment located in the chromatophores, which can contract and expand. Light stimulation is perceived by the organs of vision, and under the influence of nerve impulses the color of the fish changes. Blinded fish lose the ability to change color. During the breeding season, the nuptial coloration of fish is the result of the influence of hormones from the pituitary gland and gonads.

In addition to mucous glands and pigment cells, the skin of fish contains scales, luminous organs and poisonous glands.

Scales. The body of most fish is covered with scales; slow-swimming fish usually lack scales (cyclostomes, catfish, some gobies, stingrays).

In modern fish, three types of scales are distinguished - placoid, ganoid and bony (Fig. 5). Placoid scales are the most ancient, and ganoid and bone scales are its derivatives. The placoid scale consists of a rhombic plate located in the dermis and a spine protruding outward. The spine may end in one or more points. It is characteristic of cartilaginous fish and is replaced several times during life.

Placoid scales consist of three layers: 1) vitrodentin (outer enamel-like substance); 2) dentin (organic substance impregnated with lime); 3) pulp (tooth cavity filled with loose connective tissue with blood vessels).

Ganoid scales have a rhombic shape and a lateral protrusion in the form of a tooth, with the help of which the scales are connected to each other. It consists of three layers: 1) ganoin (upper compacted); 2) cosmin (medium, containing numerous tubules); 3) isopedine (lower, consisting of bone substance).

These scales are characteristic of armored fishes, polyfins, and are preserved on the tail of sturgeons. A type of ganoid scale is cosmoid in lobe-finned fish (without the upper layer of ganoid). The bone scale was formed as a result of the transformation of the ganoid scale - the layers of ganoin and cosmin disappeared and only the bone substance remained.

Based on the nature of the surface, two types of bone scales are distinguished: 1) cycloid with a smooth posterior edge (herring, carp); 2) ctenoid, the posterior edge has spines (perciformes). Cycloid scales are more primitive, ctenoid scales are more progressive.

A distinctive feature of the scales of bony fish is the way they are formed. Introducing its base into a scaly pocket screwed into the dermis, its free end overlaps the next scale in a tiled manner.

The bone scales have three layers: 1) the upper one – transparent, structureless; 2) medium – integumentary, mineralized, with ribs or sclerites; 3) bottom – main.

The lower layer is composed of thin bone plates underlying one another. The growth of scales occurs in such a way that under the small first plate laid in the fry, the next year another larger one is laid, etc. Thus, the smallest and oldest plate is on top, and the largest and youngest is below. The number of plates in the lower layer corresponds to the age of the fish.

With intensive growth, wide and distant sclerites with high ridges are formed on the covering layer, and with slower growth, narrow and close sclerites with low ridges are formed. When determining the age of fish, the zones of convergence of sclerites (usually darker) are called annual rings. In some fish, keratinization of the skin is observed to protect against mechanical damage (cyclostomes, teleosts and lungfishes). During the mating season, many whitefish and carp fish develop so-called pearly tubercles (or rashes) and are the result of exposure to sex hormones.

There are fish with different types of scales. Thus, some species of the goby family have cycloid and ctenoid scales in different parts of the body; in groupers above the lateral line it is ctenoid, and below it is cycloid; in polar flounders, males have a ctenoid, females have a cycloid, etc.

In some teleosts, the scales cannot be classified as either cycloid or ctenoid; it occupies an intermediate place between ordinary scales and a cutaneous tooth (knife fish). The size of the scales can vary greatly from microscopically small (eels) to 5 cm or more (tarpon, barbel).

N.V. ILMAST. INTRODUCTION TO ICHTHYOLOGY. Petrozavodsk, 2005

This is fully confirmed by embryonic development (Fig. 2). The embryonic feather rudiment is homologous to the rudiment of the horny scales. Later, it turns into a finger-like outgrowth, the epidermal sheath of which, at a certain stage of development, breaks up into separate longitudinal rods - barbs. Contour feathers are more difficult to develop due to the uneven growth of the barbs. Among other specific derivatives of the skin, birds have: a horny beak (of different structure depending on the nature of the food), claws on the toes, and in some birds on the first finger of the wing (in Archeopteryx, on the fingers of the wing) and, finally, various skin outgrowths - combs, beards, earrings, etc., playing the role of secondary sexual characteristics.

1a-1d - a series of stages of dentin formation in shark fish during the development of placoid scales with a typical enamel organ, 2a - 2d - development of bone denticles (2a-2b), dorsal bugs and plaques (2c-2d) in the skin of sturgeon fish with loss of dentin formation. For-Zv - development of cycloid scales in the skin of bony fish with complete rudimentation of the enamel organ, 4 - diagram of ganoid scales as an example of a complex of many generations of scales, 5 and 6 - replacement of the skeleton-forming function of the skin with a glandular function in the skin of fish. 7-10 - general rudiment of horny scales, feathers and hair in amniotes. 7a-7d - stages of development of horny scales in reptiles and birds, 8a - 8d - development of down feathers and 8d - development of contour feathers, 9a-9d - stages of stripe development with complete rudimentation of the connective tissue papilla and progressive development of the epidermal rudiment, 10a-10b - restoration horny scales in the skin of mammals

Rice. 2. Scheme of the phylogenetic development of scales, feathers and hair according to embryonic development data (according to Matveev). / - V- types of functions of the skin (/ - protective, through the formation of cilia and cuticle; // - secretory, through the formation of unicellular and multicellular glands, /// - sensitivity, through the separation of primary and secondary sensory cells; IV- coloring, by differentiation of pigment cells; V - protective function of the skin of vertebrates, through development from a common rudiment):

The skin of mammals, unlike other amniotes, is rich in various skin glands: sweat, sebaceous, mammary, as well as various glands for special purposes, for example: ungulates, musk, odorous, perianal, etc. All these glands are derivatives of the epidermis and are only secondarily deeply immersed into the corium. The presence of glands in the skin of mammals in the absence of them in reptiles and birds shows that the ancestors of mammals descended in the process of evolution from very ancient reptiles that still retained common characteristics with fossil amphibians. Mammary glands are typical skin glands, derived from simple sac-like tubular glands such as sweat glands through their complex branching.

Hair is a completely unique formation of the skin of mammals. Unlike horny scales and feathers, hair is a purely epidermal formation, and only the hair bulb has a connective tissue papilla. The presence of horny scales in some mammals (incomplete edentates, muskrat, beaver, nutria, etc.) simultaneously with hair shows that hair developed independently between the horny scales, only later replacing them. Therefore, the question of the origin of hair is less clear than the evolution of horny scales and feathers. Embryonic development of hair (Fig. 2) makes it possible to establish the homology of the primary hair rudiment with the scale rudiment. This shows that hair originated from scale-like organs by changing them in the early stages of development.

In addition to hair, various other horny formations are derived from the skin of mammals: claws, nails, hooves and, finally, the horns of bovids.

2. Features of the structure of the skin and their derivatives, taking into account the various living conditions of chordates

2.1 Features of the structure and formation of scales

Fish scales are always a derivative of the skin itself (corium), and only sometimes, in addition to the corium, the epidermis also takes a secondary part in its formation. There are four main types of fish scales: placoid Yu, cosmoid yu, ganoid and bone.

Placoid scales give rise not only to ganoid and bone scales, but also to teeth. In a shark, the teeth are real placoid scales. These teeth are completely homologous to the teeth of all higher classes, up to mammals, whose teeth also consist of dentin, are covered with a substance of ectodermic origin - enamel, and contain an internal cavity filled with pulp.

Cosmoid scales are a special type of scales that are found in some fossil bony fish and are found in modern coelacanths. It is devoid of ganoine and its surface layer consists of cosmina, which in its structure consists of many individual dentin teeth fused with each other. Ganoid scales are characteristic of only a very few modern fish (polyfinned and caimans), but they were very widespread in fossil fish. Typically, ganoid scales look like flat rhombic plates, which are arranged in oblique rows and connected to each other using special joints, so that a continuous shell is formed that covers the entire body of the animal. The outer layer of ganoid scales consists of a special very hard substance - ganoin, the lower layer is made of bone tissue. Ganoid scales are formed in the connective tissue and, therefore, are never covered with enamel. The lower bony layer of the ganoid scale appears to be formed from dentin, into which bone cells penetrate. In contrast to placoid scales, ganoid scales are not replaced and are formed for life. The evolution of scales in fossil fish undoubtedly proves that ganoid scales arose by fusion of the main plates of individual placoid scales with underlying bony plates. The top of these scales is covered with ganoin.

Bony scales are characteristic of all modern bony fish, with the exception of polyfin fish, coelacanths and caiman fish. Bone scales are bone plates of various sizes, tiled-like overlapping each other with their edges. They constantly grow, forming annual rings along the periphery of the plate (Fig. 3).

Ichthyologists use these annual rings to determine the age of fish. Perciformes are characterized by ctenoid scales with spines along the posterior edge of the scales; cyprinids and salmonids are characterized by smooth cycloid scales without denticles. In many bottom fish (catfish, eels), the scales are completely reduced. Sturgeons have special bony scales that form five longitudinal rows of bugs with small star-shaped scales between them. In many fish, bony scales form bony spines on their fins.

Rice. 3. Scales of bony fish. / - ctenond (perch); // cycloid (cyprinid fish): a− annual rings

The color of fish depends on a number of reasons. For example, a silvery sheen, characteristic not only of scales, but also of many internal organs of fish (swim bladder, peritoneum), is determined by the presence of guanine.

Guanine from the scales of some fish (bleak) is used for technical purposes (for example, to make artificial pearls). The color of fish is due to the presence of chromatophores. Under the influence of nervous irritation, they can shrink and expand, which accounts for the ability of many fish to change their color to match the color of the surrounding background.

2.2 Features of the structure and formation of the feather

Birds are characterized by thin skin, a complete absence of any bone formations, and a kind of horny cover consisting of feathers. On the upper and lower jaws, derivatives of the skin are horny sheaths characteristic of birds, on the tarsus and fingers there are horny scales, and at the ends of the fingers there are claws. Feathers do not completely cover the body of birds (for example, a pigeon), but are located only on certain areas of the skin - pterilia, between which there are areas devoid of feathers - apteria (Fig. 4). This arrangement of feathers is associated with flight, since the arrangement of feathers in sections is convenient for muscle contraction during flight. Only a very few, mostly flightless, birds lack apteria, and feathers evenly cover the entire body.

The feather (Fig. 4) consists of an elastic trunk and softer side plates - outer and inner webs. The upper part of the trunk, to which the fans are attached, is called the rod and has a quadrangular shape in cross section, with the upper surface of the trunk being convex, while the lower one bears a longitudinal groove. The lower part of the trunk, devoid of webs, is called the edge and has a round cross-section, and is equipped with a hole at the base. While the inner part of the trunk is occupied by a cellular core, the cavity of the horn contains a chain of delicate horny caps inserted into each other - the soul of the feather, which is a dead papilla that fed blood to a young, growing feather.

Rice. 4. Outline pen. The quill is opened to show the bow of the feather: 1 - shaft, 2 - outer blade, 3 - internal blade, 4 - trunk, 5 - blade, 6 - blade opening, 7 - stem of the feather.

Each fan is formed by numerous beards of the first order, on which sit smaller beards of the second order. Each of them is equipped with elongated triangular plates at the end with hooks that interlock with the same hooks of neighboring barbs of the 2nd order, which already belong to another barb of the 1st order (Fig. 5).

What gives the scales the hardness they need? It's all about the specialized tissues composing them, containing a large number of crystals of inorganic salts, mainly calcium in the form of hydroxyapatite (if you don't go into details, this salt is a phosphate). In addition to the inorganic component, the share of which in some cases reaches an unimaginable 95% or more, these tissues also contain organic matter, in particular, the protein collagen. Collagen molecules, connecting with each other, form fibers that give the scales elasticity.

Below, in the main part of the text, I will give the names of the tissues that make up the scales, and in order not to get completely confused in the terms, I will talk about them, albeit in passing, but in a little more detail.

Depending on the relative position of collagen fibers, the composition and proportion of inorganic salts, the presence (or absence) of living cells, their shape and location, the presence (or absence) of blood vessels, several types of hard tissues are distinguished. The mechanism of development also plays an important role in their classification.

The most mineralized (up to 96%), and therefore the hardest tissues cover the outer part of the scales. These are relatively thin tissues, usually containing no (or almost no) cells, collagen fibers, or blood vessels, which is why they are often called substances rather than tissues. These include, for example, enamel, vitrodentin (enameloid), ganoin, hyaloin.

The next hardest tissue is dentin. It contains the same inorganic components as enamel, but their share does not exceed 75%. In addition, dentin is distinguished from enamel by the presence of a large amount (about 20%) of collagen fibers and the presence of living cells located not inside the tissue, but on its periphery. Numerous long and thin processes extend from these cells deep into the tissue, forming characteristic radial tubules.

A type of dentin is elasmodin, which is distinguished by the presence of thin layers of collagen, forming so-called plywood-like structures (literally: structures resembling plywood) - in elasmodine, collagen fibers in adjacent layers are oriented perpendicularly (or at different angles) to each other.

Bone is the most common and perhaps most diverse tissue that makes up fish scales. It is probably its most ancient component. Bone tissue, like dentin, contains inorganic salts (no more than 70%) and collagen fibers (about 30%), otherwise its structure can vary significantly.

There are several types of bone tissue found in fish scales. One of the first to appear in evolution was probably an acellular bone devoid of blood vessels, called aspidin. Younger in evolutionary terms (but now more common) types of bone have living cells in their composition. By depositing collagen fibers and inorganic salts around themselves, these cells form thin bone plates. Depending on the relative position of such plates, as well as the blood supply pattern, dense and spongy bone tissue is distinguished. In addition, the scales of some fish contain lamellar bone (=lamellar bone =isopedine), in which collagen fibers are packaged, as in elasmodine, in plywood-like structures.

The functions of the skin are very diverse. Along with protecting the body from harmful environmental influences, it takes an active part in metabolism. Water, ammonia, some salts, carbonic acid, and oxygen penetrate through it, which is important for skin respiration and osmoregulation. In addition, the skin of fish is characterized by sensitive cells of various types.

The skin of fish consists of two layers: the upper - the epidermis of ectodermal origin, and the lower - the dermis (cutis, corium) of mesodermal origin. The boundary between these layers is the basement membrane. The skin is underlain by subcutaneous connective tissue containing fat cells.

In cyclostomes and fish, the epidermis is soft and thin, consisting of several (2-15) rows of epithelial cells. The cells located in the upper layers are flattened, somewhat keratinized and constantly rejected without interfering with the secretion of mucus (Fig. 1).

The epidermis is rich in various sensitive cells and free nerve endings, and there are no blood vessels in it. The dermis contains both nerves and blood vessels.

The skin of cyclostomes and fish differs in structure. Lampreys have always bare skin, covered with a thin layer of cuticle, devoid of scales. The epidermis contains a large number of cells that secrete mucus. Hagfishes also have multicellular mucous glands located along the lateral line, secreting a significantly larger amount of mucus than the skin of lampreys.

The epidermis is underlain by the dermis, consisting of connective tissue, the fibers of which are located in the longitudinal and transverse directions. Unlike fish, in cyclostomes the pigment cells are not located in the dermis, but deeper, above the layer of subcutaneous connective tissue.

Rice. 1. Structure of fish skin (sturgeon)

1 – epidermis; 2 – dermis; 3 – subcutaneous tissue; 4 – goblet mucous cells; 5 – round cells; 6 – pigment cells

The structure of fish skin depends on their lifestyle. Typically, in fish with higher swimming speeds, the thickness of the skin increases and its structure and the degree of development of individual layers change.

In fish (as in cyclostomes), the lower germinal layer of the epidermis is represented by one row of cylindrical cells, and the upper layer is represented by several rows of flattened ones. The middle layer consists of rows of epithelial cells, the shape of which gradually changes from cylindrical to flattened. It is here that the glandular cells that produce mucus are located - goblet, round (serous) and flask-shaped (Fig. 2).

Rice. 2. The structure of the skin of fish (barbel) (according to Stroganov, 1962)

1 – goblet cells; 2 – granular, 3 – basal, 4 – flask-shaped

Fish, like cyclostomes, have the same types of glandular cells. Under the layer of flat cells there are goblet-shaped mucous cells, somewhat deeper - rounded (serous), and in the lowest part of the epidermis, adjacent to the basal layer, there are flask-shaped cells. In flask-shaped cells that have no connection with the surface of the skin, the so-called fear substance is produced, which is released from the skin when wounded and causes a feeling of fear in all fish in the school. In fish that do not have flask-like cells, the “fright substance” is apparently located in other cells that are not connected to the surface. Sharks have only round mucous cells developed. Slow-swimming bony fish have 2-3 types of mucous cells, fish swimming at an average speed have 1-2 types (usually goblet-shaped and round), and fast-swimming ones (swordfish) have only round cells. In slow-swimming fish, mucous cells are located evenly over the entire surface of the body in one row. As swimming speed increases, the maximum number of mucous cells shifts to the middle and end parts of the body. This arrangement of cells has adaptive significance and helps reduce hydrodynamic resistance.

Mucus reduces the friction of fish on water, has bactericidal properties, and also takes part in blood clotting during injury, coagulation of particles suspended in water, protecting the gills from clogging. The biochemical composition of the mucus of different fish species is different. There is a correlation between the protein content of mucus and swimming speed. Fish that swim at high speeds have more protein in their mucus than slow-swimming fish.

The epidermis is underlain by the dermis, which consists of connective tissue with a large number of collagen fibers and primarily performs a supporting function. In most fish, the dermis consists of two layers: the upper, formed by a thin layer of loose connective tissue surrounding the scales, and the lower, consisting of dense connective tissue. The blades of this layer move between the scales, forming scale pockets.

In slow-swimming fish, the dermis is poorly developed, the fibers are loosely arranged, without forming thick layers. In fast-swimming fish, the thickness of the dermis increases, especially in the caudal region. In some sharks, the dermis consists of several layers of collagen fibers located at an angle to each other.

Individual layers of collagen fibers are interconnected by transverse fibers. Below the dermis is the subcutaneous layer, consisting of loose connective tissue with fat cells.

The subcutaneous layer is well developed in bony fish and weaker in sharks, in which it is even absent over most of the body and the trunk muscles are in direct contact with the skin.

Fish are characterized by a variety of colors, which is due to the presence of pigment cells in the skin - chromatophores and leukophores, which can lie on the border of the upper and lower layers of the dermis, in the lower layer and subcutaneous connective tissue along with fat cells. The following types of chromatophores are distinguished:

melanophores with black pigment grains; erythrophores and xanthophores, which have red or yellow inclusions in the cytoplasm; leucophores, or guanophores, containing guanine crystals, which give the skin of fish a silvery color.

Fish have a protective coloring, making them invisible in appropriate conditions. Pelagic fish have a dark back and a light belly. Pike, pike perch, and river perch living among aquatic vegetation have dark transverse stripes on their bodies. The variegated coloring of salmon fry in the river, which hides them against the background of pebbly soil, disappears when they slide into the sea. The color of benthic fish of shallow depths, especially coral reefs, is very diverse.

Some fish have the ability to change their color. Males of many species of gobies turn black during the mating season. Male sticklebacks acquire different shades when irritated, sea cocks turn pale when irritated, and scorpion fish darken when irritated. Flounder and some other fish can change color to match their environment.

The change in color in fish is due to the fact that the pigment located in the chromatophores can contract and expand. Light stimulation is perceived by the organs of vision, and under the influence of nerve impulses the color of the fish changes.

The coloration of fish depends on the different combinations of pigment cells and on the distribution of pigment in these cells. Pigment grains can accumulate in the center of the cell (pigment contraction), and the fish becomes lighter, or, conversely, pigment grains can spread along the processes of the cell (pigment expansion) - the fish darkens. There are two types of cytoplasm in the chromatophore.

Rice. 3. Schematic representation of the melanophore of crucian carp (on the left – expansion, on the right – contraction of pigment grains).

1 – grains; 2 – fibrils; 3 – cores.

The surface layer - ectoplasm - determines the shape of the cell; this layer is secured by solid skeletal formations - radial fibrils. The inner layer - cinema plasma - is mobile, it contains pigment grains with a diameter of 0.5 microns. During expansion, kinoplasma spreads throughout the cell. During contraction, the cinema plasma together with pigment granules is collected into one drop (Fig. 3).

Blinded fish lose the ability to change color. The mating coloration of fish is the result of the influence of hormones from the pituitary gland and gonads.

In addition to mucous glands and pigment cells, luminous organs, poisonous glands, and scales are formed in the skin of fish.

Scales. The body of most fish is covered with scales, but catfish and some other fish, as well as cyclostomes, do not have scales.

The scales provide a smooth surface of the body and prevent skin folds on the sides. Fish that swim at low speeds usually lack scales.

LEATHER AND ITS DERIVATIVES

The functions of the skin are very diverse. Along with protecting the body from harmful environmental influences, it takes an active part in metabolism. Water, ammonia, some salts, carbonic acid, and oxygen penetrate through it, which is important for skin respiration and osmoregulation. In addition, the skin of fish is characterized by sensitive cells of various types.

The skin of fish consists of two layers: the upper - the epidermis of ectodermal origin, and the lower - the dermis (cutis, corium) of mesodermal origin. The boundary between these layers is the basement membrane. The skin is underlain by subcutaneous connective tissue containing fat cells.

In cyclostomes and fish, the epidermis is soft and thin, consisting of several (2-15) rows of epithelial cells. The cells located in the upper layers are flattened, somewhat keratinized and constantly rejected without interfering with the secretion of mucus.

The epidermis is rich in various sensitive cells and free nerve endings, and there are no blood vessels in it. The dermis contains both nerves and blood vessels.

The skin of cyclostomes and fish differs in structure. Lampreys have always bare skin, covered with a thin layer of cuticle, devoid of scales. The epidermis contains a large number of cells that secrete mucus. Hagfishes also have multicellular mucous glands located along the lateral line, secreting a significantly larger amount of mucus than the skin of lampreys.

The epidermis is underlain by the dermis, consisting of connective tissue, the fibers of which are located in the longitudinal and transverse directions. Unlike fish, in cyclostomes the pigment cells are not located in the dermis, but deeper, above the layer of subcutaneous connective tissue.

The structure of fish skin depends on their lifestyle. Typically, in fish with higher swimming speeds, the thickness of the skin increases and its structure and the degree of development of individual layers change.

Structure of fish skin (sturgeon)

1 – epidermis; 2 – dermis; 3 – subcutaneous tissue; 4 – goblet mucous cells; 5 – round cells; 6 – pigment cells

In fish (as in cyclostomes), the lower germinal layer of the epidermis is represented by one row of cylindrical cells, and the upper layer is represented by several rows of flattened ones. The middle layer consists of rows of epithelial cells, the shape of which gradually changes from cylindrical to flattened. It is here that the glandular cells that produce mucus are located - goblet, round (serous) and flask-shaped.

Rice. 5. The structure of the skin of fish (barbel) (according to Stroganov, 1962):
1 – goblet cells; 2 – granular, 3 – basal, 4 – flask-shaped

Fish, like cyclostomes, have the same types of glandular cells. Under the layer of flat cells there are goblet-shaped mucous cells, somewhat deeper - rounded (serous), and in the lowest part of the epidermis, adjacent to the basal layer, there are flask-shaped cells. In flask-shaped cells that have no connection with the surface of the skin, the so-called fear substance is produced, which is released from the skin when wounded and causes a feeling of fear in all fish in the school. In fish that do not have flask cells, the “fright substance” is apparently located in other cells that are not connected to the surface. Sharks have only round mucous cells developed. Slow-swimming bony fish have 2-3 types of mucous cells, fish swimming at an average speed have 1-2 types (usually goblet-shaped and round), and fast-swimming ones (swordfish) have only round cells. In slow-swimming fish, mucous cells are located evenly over the entire surface of the body in one row. As swimming speed increases, the maximum number of mucous cells shifts to the middle and end parts of the body. This arrangement of cells has adaptive significance and helps reduce hydrodynamic resistance.

Mucus reduces the friction of fish on water, has bactericidal properties, and also takes part in blood clotting during injury, coagulation of particles suspended in water, protecting the gills from clogging. The biochemical composition of the mucus of different fish species is different. There is a correlation between the protein content of mucus and swimming speed. Fish that swim at high speeds have more protein in their mucus than slow-swimming fish.

The epidermis is underlain by the dermis, which consists of connective tissue with a large number of collagen fibers and primarily performs a supporting function. In most fish, the dermis consists of two layers: the upper, formed by a thin layer of loose connective tissue surrounding the scales, and the lower, consisting of dense connective tissue. The blades of this layer move between the scales, forming scale pockets.

In slow-swimming fish, the dermis is poorly developed, the fibers are loosely arranged, without forming thick layers. In fast-swimming fish, the thickness of the dermis increases, especially in the caudal region. In some sharks, the dermis consists of several layers of collagen fibers located at an angle to each other.

Individual layers of collagen fibers are interconnected by transverse fibers. Below the dermis is the subcutaneous layer, consisting of loose connective tissue with fat cells.

The subcutaneous layer is well developed in bony fish and weaker in sharks, in which it is even absent over most of the body and the trunk muscles are in direct contact with the skin.

Fish are characterized by a variety of colors, which is due to the presence of pigment cells in the skin - chromatophores and leukophores, which can lie on the border of the upper and lower layers of the dermis, in the lower layer and subcutaneous connective tissue along with fat cells. The following types of chromatophores are distinguished:

melanophores with black pigment grains; erythrophores and xanthophores, which have red or yellow inclusions in the cytoplasm; leucophores, or guanophores, containing guanine crystals, which give the skin of fish a silvery color.

Fish have a protective coloring, making them invisible in appropriate conditions. Pelagic fish have a dark back and a light belly. Pike, pike perch, and river perch living among aquatic vegetation have dark transverse stripes on their bodies. The variegated coloring of salmon fry in the river, which hides them against the background of pebbly soil, disappears when they slide into the sea. The color of benthic fish of shallow depths, especially coral reefs, is very diverse.

Some fish have the ability to change their color. Males of many species of gobies turn black during the mating season. Male sticklebacks acquire different shades when irritated, sea cocks turn pale when irritated, and scorpion fish darken when irritated. Flounder and some other fish can change color to match their environment.

The change in color in fish is due to the fact that the pigment located in the chromatophores can contract and expand. Light stimulation is perceived by the organs of vision, and under the influence of nerve impulses the color of the fish changes. Blinded fish lose the ability to change color. The mating coloration of fish is the result of the influence of hormones from the pituitary gland and gonads.

In addition to mucous glands and pigment cells, luminous organs, poisonous glands, and scales are formed in the skin of fish.

Scales. The body of most fish is covered with scales, but catfish and some other fish, as well as cyclostomes, do not have scales.

The scales provide a smooth surface of the body and prevent skin folds on the sides. Fish that swim at low speeds usually lack scales.

Modern fish have three types of scales: placoid,

ganoid and bone, and ganoid and bone are derivatives of the most ancient placoid scales.

Placoid scale, consisting of a rhombic plate located in the dermis and a spine protruding outward, covers the body of cartilaginous fish and is replaced several times during their life.

The structure of various types of fish scales:

A - placoid; B - ganoid; B - bone: a - herring; b - bream; c - perch; d - scales (in section).

The scales consist of an organic substance impregnated with lime - dentin, which does not contain cellular elements. The outside of the spine is covered with a dense enamel-like substance - vitrodentin. The tooth cavity is filled with dental pulp, formed by loose connective tissue with blood vessels.

Some of the placoid scales grow greatly, forming placoid plaques, for example in the sea fox. All spines in cartilaginous fish are transformed placoid scales.

Ganoid scale It has a rhombic shape and a lateral protrusion in the form of a tooth, with the help of which the scales are connected to each other, forming a kind of shell. This scale is characteristic of bony ganoids, multi-feathers, is preserved on the tail of sturgeons and consists of three layers: the upper, compacted (ganoine), the middle, containing numerous tubules (cosmina), and the bottom, consisting of bone substance (isopedine). A variety of ganoid scales is cosmoid in lobe-finned fish (without the upper layer of ganoid).

Bone scale formed as a result of the transformation of the ganoid - the layers of ganoin and cosmin disappeared and only the bone substance remained. Based on the nature of the surface, two types of bone scales are distinguished:

cycloid with a smooth rear edge (herring, carp) and ctenoid, the posterior edge of which is armed with spines (perciformes).

There are three layers in the bone scales - the upper transparent shiny structureless one, the middle integumentary one and the lower main one. The lower layer is composed of thin bone plates underlying one another. The growth of scales occurs in such a way that under the small first plate laid in the fry, the next year another larger one is laid, etc. Thus, the smallest and oldest plate is on top, and the largest and youngest is below. The number of plates in the lower layer corresponds to the age of the fish. Above the lower main layer is a covering, mineralized layer with ribs, or sclerites.

With intensive growth, wide and distant sclerites with high ridges are formed on the covering layer, and with slower growth, narrow and close sclerites with low ridges are formed.

To determine the age of a fish, the surface layer of scales with sclerites is studied. Zones of convergence of sclerites (usually darker) are called annual rings and counting them allows you to determine the age of the fish.

Venom glands . Some fish have venom-secreting glands in the epidermis, located mainly at the base of the spines or spiny rays of the fins. Sometimes poison-secreting cells are formed and function only during reproduction, in other cases - constantly. There are three types of poison glands in fish. The most primitive of them are individual epidermal cells containing poison and scattered at the base of the spines of the fins and the spines of the gill cover (stargazer).

In other fish species, a complex of poisonous cells (stingray) is formed in the epidermis near the spines and spines. And finally, in many species, poisonous cells form an independent multicellular poisonous gland with strong poison near the thorns and spines (sea dragon, terrible wart, sea bass)

In a stingray, when pricked, the poison enters the wound through the thorn groove and causes acute pain, severe swelling, chills, nausea and vomiting, and in some cases death occurs.

The most powerful poison is produced in the poisonous glands of the terrible wart. It destroys red blood cells, affects the nervous system and leads to paralysis. When poison enters the bloodstream, death soon occurs. Fish that have a specialized venomous apparatus are called poisonous, and fish with poisonous organs and tissues are poisonous. The most poisonous are considered to be fish from the order of fused-jawed fish, whose gonads, liver, intestines, and skin contain a neurotoxin (tetrodotoxin), which can cause rapid death, since it is 10 times more toxic than curare poison. The meat of these fish is edible and in some countries (Japan) is highly valued, which often leads to fatal poisoning.

The main types of poison glands in fish:

A - unicellular glands of the epidermis of the fin spine; B - complex of unicellular glands of the epidermis of the tail spine of the stingray; 5 - compact multicellular gland of the operculum of the sea dragon; 1 - epidermis; 2 - mucous cells; 3 - glandular cells; 4 - supporting cells; 5 - false excretory duct; 6 - poison protruding outward; 7-spike; 8 - poisonous gland.

Of the fish that live in the waters of the USSR, the caviar and milt of the marinka and osman are poisonous. Lamprey mucus is also poisonous. However, those fish that, as a result of injury or poor-quality storage, are infected with toxic microbes (including botulinus) and the consumption of which can lead to poisoning should not be classified as poisonous.

Glowing organs. The luminous organs (photophores) of many deep-sea fish consist of luminous cells (photocytes) containing a special substance, luciferin. Luminous photophore cells are derivatives of the glandular epidermis.

The structure of photophores, their location and the light emitted are different. In luminous anchovies, for example, a cluster of luminous cells located in the muscular cavity is underlying black pigment cells, covered with a shiny layer that acts as a reflector. In front of the luminous cells there is a transparent, modified scale that acts as a lens. Some photophores have a diaphragm that allows you to change the direction and intensity of light.-