Amphibians have thin skin covered with mucus. Specific features of the skin of amphibians. Specific features of the skin

Batrachology -(from the Greek Batrachos - frog) studies amphibians, now it is part of herpetology.

Topic planning.

Lesson 1

Lesson 2

Lesson 3. Development and reproduction of amphibians.

Lesson 4

Lesson 5

Lesson 6

Basic terms and concepts of the topic.

Amphibians
Hip
legless
tailless
Shin
Sternum
toads
Brush
clavicle
Skin-pulmonary respiration
frogs
Brain
Cerebellum
Forearm
Bud
Medulla
salamanders
Triton
Worms.

Lesson 1

Tasks: on the example of a frog, to acquaint students with the features of the external structure and movement.

Equipment: wet preparation "internal structure of a frog". Table “Type Chordates. Amphibians class.

During the classes

1. Learning new material.

General characteristics of the class

The first terrestrial vertebrates that still retained a connection with the aquatic environment. In most species, eggs are devoid of dense shells and can only develop in water. The larvae lead an aquatic lifestyle and only after metamorphosis do they switch to a terrestrial lifestyle. Respiration is pulmonary and cutaneous. The paired limbs of amphibians are arranged in the same way as in all other terrestrial vertebrates - basically, these are five-fingered limbs, which are multi-membered levers (a fish fin is a single-membered lever). A new pulmonary circulation is formed. In adult forms, the lateral line organs usually disappear. In connection with the terrestrial way of life, the middle ear cavity is formed.

Appearance and dimensions.

Habitat

The larva (tadpole) lives in the aquatic environment (fresh water). An adult frog leads an amphibious lifestyle. Our other frogs (grass, moor) after the breeding season live on land - they can be found in the forest, in the meadow.

Motion

The larva moves with the help of the tail. An adult frog on land moves by jumping, in water it swims, pushing off with hind legs equipped with membranes.

Nutrition

The frog feeds on: air insects (flies, mosquitoes), grabbing them with the help of an ejected sticky tongue, terrestrial insects, slugs.

It is able to grasp (with the help of jaws, there are teeth on the upper jaw) even fish fry.

Enemies

Birds (herons, storks); predatory mammals (badger, raccoon dog); predatory fish.

2. Fixing.

• What animals are called amphibians?
• What living conditions and why limit the spread of amphibians on Earth?
• How do amphibians differ from fish in appearance?
• What features of the external structure of amphibians contribute to their life on land, in water?

3. Homework: 45.

Lesson 2

Tasks: on the example of a frog, to acquaint students with the structural features of organ systems and integuments.

Equipment: wet preparations, relief table "Internal structure of a frog".

During the classes

1. Testing knowledge and skills

• What environmental factors influence frog activity?
• What is the adaptation in the external structure of the frog to life on land?
• What are the structural features of a frog associated with life in water?
• What role do the front and hind legs of a frog play on land and in water?
• Tell us about the life of a frog according to your summer observations.

2. Learning new material.

Covers.

The skin is naked, moist, rich in multicellular glands. The secreted mucus protects the skin from drying out and thereby ensures its participation in gas exchange. The skin has bactericidal properties - it prevents the penetration of pathogenic microorganisms into the body. In toads, toads, some salamanders, the secret secreted by the skin glands contains poisonous substances - none of the animals eat such amphibians. Skin color acts as a camouflage - patronizing coloration. In poisonous species, the color is bright, warning.

Skeleton.

The spinal column is divided into 4 sections:

• cervical (1 vertebra)
• trunk
• sacral
• tail

In frogs, the tail vertebrae are fused into one bone - urostyle. The auditory ossicle is formed in the cavity of the middle ear. stapes.

The structure of the limbs:

Nervous system and sense organs.

The transition to a terrestrial way of life was accompanied by a transformation of the central nervous system and sensory organs. The relative size of the amphibian brain compared to fish is small. The forebrain is divided into two hemispheres. Accumulations of nerve cells in the roof of the hemispheres form the primary cerebral fornix - archipallium.

The sense organs provide orientation in water (larvae and some tailed amphibians have developed lateral line organs) and on land (vision, hearing), smell, touch, taste organs and thermoreceptors.

Respiration and gas exchange.

In general, amphibian milking is characterized by pulmonary and skin respiration. In frogs, these types of breathing are represented in almost equal proportions. In dry-loving gray toads, the proportion of pulmonary respiration reaches approximately 705; in newts leading an aquatic lifestyle, cutaneous respiration predominates (70%).

The ratio of pulmonary and skin respiration.

American lungless salamanders and Far Eastern newts have only pulmonary respiration. Some caudate (European Proteus) have external gills.

The lungs of frogs are simple: thin-walled, hollow, cellular sacs that open directly into the laryngeal fissure. Since the neck of the frog, as a department, is absent, there are no airways (trachea). The breathing mechanism is forced, due to the lowering and raising of the bottom of the oropharyngeal cavity. As a result, the frog's skull has a flattened shape.

Digestion.

There are no fundamental innovations in the structure of the digestive system, in comparison with fish, in frogs. But, salivary glands appear, the secret of which so far only wets the food, without exerting a chemical effect on it. The mechanism of swallowing food is interesting: swallowing is assisted by the eyes moving into the oropharyngeal cavity.

Circulatory system.
The heart is three-chambered, the blood in the heart is mixed (in the right atrium - into the venous, in the left - arterial, in the ventricle - mixed.

The regulation of blood flow is carried out by a special formation - an arterial cone with a spiral valve that directs the most venous blood to the lungs and skin for oxidation, mixed blood to other organs of the body, and arterial blood to the brain. A second circle of blood circulation appeared (lungfish also have a pulmonary circulation).

Selection.

Trunk or mesonephric kidney.

3. Fixing.

• What are the similarities in the structure of the skeletons of amphibians and fish?
• What features of the skeleton of amphibians distinguish it from the skeleton of fish?
• What are the similarities and differences between the digestive systems of amphibians and fish?
• Why can amphibians breathe atmospheric air, how do they breathe?
• How is the circulatory system of amphibians different?

4. Homework . 46, plan your response.

Lesson 3

Tasks: to reveal the features of reproduction and development of amphibians.

Equipment: relief table "Internal structure of a frog".

During the classes

I. Learning new material.

1. Organs of reproduction.

Amphibians are dioecious animals. The reproductive organs of amphibians and fish are similar in structure. The ovaries of females and the testes of males are located in the body cavity. In frogs, fertilization is external. Caviar is deposited in water, sometimes attached to aquatic plants. The shape of the egg clutches is different in different species. The rate of embryonic development is highly dependent on water temperature, so it takes from 5 to 15-30 days to hatch from a tadpole egg. The emerging tadpole is very different from the adult frog; he is dominated by fish features. As the larvae grow and develop, great changes occur: paired limbs appear, gill breathing is replaced by pulmonary breathing, the heart is three-chambered, the second circle of blood circulation. There is also a change in appearance. The tail disappears, the shape of the head and body changes, paired limbs develop.

Comparative characteristics of frogs and tadpoles

signs	Tadpole	Frog
body shape	Fish-like. Tail with a capitate membrane. At some stages of development there are no limbs.	The body is shortened. There is no tail. Two pairs of limbs are well developed.
Lifestyle		Terrestrial, semi-aquatic
Movement	Swimming with the tail	On land - jumping with the help of the hind limbs. In water - repulsion by hind limbs
	Algae, protozoa	Insects, mollusks, worms, fish fry
	Gills (first external, then internal). Through the surface of the tail (dermal)	Stucco, leather
Sense organs: Lateral line Hearing (middle ear)	There is no middle ear	Not Has a middle ear
Circulatory system	1 circle of blood circulation. Double chambered heart. Venous blood in the heart	2 circles of blood circulation. Three-chambered heart. The blood in the heart is mixed.

The duration of the larval period depends on the climate: in a warm climate (Ukraine) - 35-40 days, in a cold one (northern Russia) - 60-70 days

In newts, the larvae hatch more formed: they have a more developed tail, large external gills. The very next day they begin to actively hunt for small invertebrates.

The ability of larvae to reproduce sexually is called neoteny.

Some scientists suggest that the protea amphiums and sirens (all tailed amphibians) are neotenic larvae of some salamanders, in which the adult form completely disappeared during evolution.

The larva of a tailed amphibian - ambistoma, is called axolotl. She is able to reproduce.

2. Caring for offspring.

For a number of amphibian species, care for offspring is characteristic, which can manifest itself in a variety of ways.

A) Building nests (or using other shelters for eggs).

Phyllomedusa nest. South American phyllomedusa frogs make nests from plant leaves hanging over water. The larvae live in the nest for some time, and then fall into the water.

The female Ceylon fish snake builds a nest from her own body, wrapping around the eggs laid in the hole. The secretions of the skin glands of the female protects the eggs from drying out.

B) Carrying eggs on the body or in special formations inside.

In the midwife toad, the male winds the bundles of eggs around his hind legs and wears them until the tadpoles hatch.

The male rhinoderm frog hatches eggs in the vocal sac. Hatched tadpoles fuse with the walls of the sac: contact with the circulatory system of an adult individual occurs - this ensures the supply of nutrients and oxygen to the blood of the tadpole, and the decay products are carried away by the blood of the male.

In the Surinamese pipa, eggs (eggs) develop in leathery cells on the back. Small frogs that have completed metamorphosis emerge from the eggs.

Such care for offspring is caused primarily by a lack of oxygen in the water, as well as a large number of predators in tropical waters.

B) viviparity.

Known for caudates (alpine salamander), some legless and anurans (some desert toads).

II. Testing knowledge and skills.

• Oral survey.
• Students work with cards.

III. Homework:§ 47, answer the questions of the textbook.

Lesson 4

Tasks: prove the origin of amphibians from ancient lobe-finned fish.

Equipment: wet preparations, tables.

During the classes

I. Testing knowledge and skills.

1. Conversation with students on the following questions:

• When and where do amphibians breed?
• What are the similarities in the reproduction of amphibians and fish?
• What proves this similarity?
• What is the main difference between fish and amphibians?

2. Work with cards.

A close connection with water, similarities with fish in the early stages of development indicate the origin of amphibians from ancient fish. It remains to be clarified from which particular group of fish the amphibians originate and what force drove them out of the aquatic environment and forced them to switch to terrestrial existence. Modern lungfish were considered amphibious, and then they began to see them as a link between amphibians and real fish.

The appearance of the oldest amphibians dates back to the end of the Devonian period, and the heyday to the Carboniferous.

Initially, amphibians were represented by small forms. The oldest fossil amphibians of the Carboniferous period resemble our newts in general body shape, but differ from all modern amphibians in the strong development of the skin skeleton, especially on the head. Therefore, they were allocated to a special subclass stegocephalians.

The structure of the skull is the most characteristic feature of the stegocephalians. It consists of numerous bones, tightly closing with each other and leaving a hole only for the eyes, nostrils, and there is another unpaired hole on the crown of the head. In most stegocephalians, the ventral side of the body was covered with a shell of scales sitting in rows. The axial skeleton is poorly developed: the notochord was preserved and the vertebrae consisted of separate elements that were not yet soldered into one continuous whole.

According to the theory of Academician I.I. Schmalhausen, amphibians, and therefore all terrestrial vertebrates, descended from ancient freshwater lobe-finned fish. An intermediate form between fish and amphibians is called ichthyostegi.

III. Anchoring

Choose the correct answer option I

The teacher completes the students' answers.

IV. Homework:§ 47 to the end, answer questions.

Lesson 5

Tasks: To introduce students to the diversity of amphibians and their importance.

Equipment: tables.

During the classes

I. Testing knowledge and skills.

• Students work with cards.
• Conversation with students about the textbook.
• Oral responses.

II. Learning new material.

Ancient amphibians were confined to bodies of water to a greater extent than their modern descendants. In the aquatic environment, they were kept by a heavy bone skull and a weak spine. As a result, the group of stegocephalians, which gave rise to both the later amphibians and the most ancient reptiles, ceased to exist, and the further development of the class went in the direction of unloading the bone skull, eliminating bone formations on the skin and ossifying the spine. At present, the process of historical development of amphibians has led to the formation of three sharply isolated groups - the orders of caudate and tailless amphibians already known to us and a very peculiar order of legless, or caecilians, in which there are about 50 species confined to humid tropical countries of both hemispheres. This is a specialized group, whose representatives "went underground": they live in the soil, feeding on various living creatures there, and in appearance resemble earthworms.

In the modern fauna, the most prosperous group is the tailless amphibians (about 2100 species). Within this group, further development went in different directions: some forms remained closely associated with the aquatic environment (green frogs), others turned out to be more adapted to terrestrial existence (brown frogs and especially toads), others switched to life on trees (tree frogs), dispersing thus in the living communities (biocenoses) of our modern nature.

Feeding on various small living creatures, amphibians exterminate a significant number of insects and their larvae. Therefore, frogs and toads can be included in the category of crop protectors and friends of gardeners and gardeners.

III. Homework: § 48, repeat §§ 45-47.

Offset. class amphibians

OPTION I

Choose the correct answer

1. Amphibians - the first vertebrates:

a) landed and became completely independent of water;

b) landed, but did not break the connection with water;

c) landed, and only a few of them cannot live without water;

d) become dioecious.

2. amphibians with skin:

a) they can drink water;

b) cannot drink water;

c) some can drink water, others cannot;

d) Distinguish between light and darkness.

3. During pulmonary breathing, inhalation in amphibians is carried out due to:

a) lowering and raising the bottom of the oral cavity;

b) change in the volume of the body cavity;

c) swallowing movements

d) diffusion.

4. Real ribs have amphibians:

a) only tailless;

b) only caudate;

c) both tailless and tailed;

d) only in the larval state.

5. Blood flows through the body of adult amphibians:

a) one circle of blood circulation;

b) in two circles of blood circulation;

c) in the majority in two circles of blood circulation;

d) in three circles of blood circulation.

6. In the cervical spine of amphibians there is:

a) three cervical vertebrae;

b) two cervical vertebrae;

c) one cervical vertebra;

d) four cervical vertebrae.

7. The forebrain in amphibians compared to the forebrain of fish:

a) larger, with complete division into two hemispheres;

b) larger, but without division into hemispheres;

c) has not changed;

d) smaller.

8. The hearing organ of amphibians consists of:

a) inner ear

b) inner and middle ear;

c) inner, middle and outer ear;

d) outer ear.

9. Urogenital organs in amphibians open:

a) in the cloaca;

b) independent holes;

c) in anurans - in the cloaca, in caudates - with independent external openings;

d) one independent outer hole,

10. Heart in tadpoles:

a) three-chamber;

b) two-chamber;

c) two-chamber or three-chamber;

d) four-chamber.

OPTION II

Choose the correct answer

1. Skin in amphibians:

a) all naked, mucous, devoid of any keratinized cells;

b) everyone has a keratinized layer of cells;

c) in the majority it is naked, mucous, in a few it has a keratinized layer of cells;

d) dry, devoid of any glands.

2. Amphibians breathe with:

a) skin only

b) lungs and skin;

c) only lungs;

d) only gills.

3. Heart in adult amphibians:

a) three-chamber, consisting of two atria and a ventricle;

b) three-chamber, consisting of an atrium and two ventricles;

c) two-chamber, consisting of an atrium and a ventricle;

d) four-chamber, consisting of two atria and two ventricles.

4. Cerebellum in amphibians:

a) everyone is very small;

b) very small, in some species of caudates it is practically absent;

c) larger than fish;

d) the same as in fish.

5. Vision in amphibians compared to vision in fish:

a) less farsighted;

b) more farsighted;

c) remained unchanged;

d) has almost lost its meaning.

6. Lateral line organs in adult amphibians:

a) are absent;

b) are present in most species;

c) are present in those species that constantly or spend most of their lives in water;

d) are present in those species that spend most of their lives on land.

7. Adult amphibians eat:

a) filamentous algae;

b) various aquatic plants;

c) plants, invertebrates and rarely vertebrates;

d) invertebrates, rarely vertebrates.

8. Teeth in amphibians:

a) are present in many species;

b) are available only in caudates;

c) available only in anurans;

d) absent in most species.

9. Fertilization in amphibians:

a) everyone has an internal;

b) all external;

c) in some species it is internal, in others it is external;

d) most internal.

10. The life of amphibians is associated with water bodies:

a) salty

b) fresh;

c) both salty and fresh.

11. Amphibians originated:

a) from coelacanths considered extinct;

b) extinct freshwater lobe-finned fish;

c) lungfish

Write down the numbers of the correct judgments.

1. Amphibians are vertebrates,
   reproduction of which is associated with water.
2. Amphibians have a middle ear, separated from the external environment by the tympanic membrane.
3. The skin of toads has keratinized cells.
4. Among amphibians, the largest animal is the Nile crocodile.
5. Toads live on land and breed in water.
6. In the skeleton of the belt of the forelimbs of amphibians there are crow bones.
7. The eyes of amphibians have movable eyelids.
8. The skin of a pond frog is always wet - it does not have time to dry out while the animal is on dry land for some time.
9. All amphibians have swimming membranes between the toes of their hind legs.
10. Amphibians, like fish, lack salivary glands.
11. The forebrain in amphibians is better developed than in fish.
12. The heart of tailless amphibians is three-chambered, while that of caudates is two-chambered.
13. Mixed blood enters the organs of the body in amphibians through the blood vessels.
14. Frogs are dioecious animals, newts are hermaphrodites.
15. Fertilization in most amphibians is internal - females lay fertilized eggs.
16. Development in most amphibians occurs with transformations according to the scheme: egg - larva of different ages - an adult animal.
17. Some of the amphibians are crepuscular and nocturnal and are of great help to humans in reducing the number of slugs and other plant pests.

Type chordates. Class Reptiles or Reptiles.

herpetology- (from the Greek. Herpeton - reptiles) - studies reptiles and amphibians.

Theme Planning

Lesson 1 (Annex 6)

Lesson 2. Features of the internal structure. (Annex 7)

Lesson 3 (

CLASS Amphibians (AMRNIVIA)

General characteristics. Amphibians - four-legged vertebrates from the group Anamnia. Their body temperature is variable, depending on the temperature of the external environment. The skin is naked, with a large number of mucous glands. The forebrain has two hemispheres. The nasal cavity communicates with the oral internal nostrils - choanae. There is a middle ear, in which one auditory ossicle is located. The skull is articulated with a single cervical vertebra by two condyles. The sacrum is formed by one vertebra. The respiratory organs of larvae are gills, while adults are lungs. The skin plays an important role in respiration. There are two circles of blood circulation. The heart is three-chambered and consists of two atria and one ventricle with an arterial cone. Trunk kidneys. They reproduce by spawning. The development of amphibians takes place with metamorphosis. Caviar and larvae develop in water, have gills, they have one circle of blood circulation. Adult amphibians after metamorphosis become terrestrial pulmonary-breathing animals with two circles of blood circulation. Only a few amphibians spend their entire lives in the water, retaining gills and some other signs of larvae.

More than 2 thousand species of amphibians are known. They are widely distributed on the continents and islands of the globe, but are more numerous in countries with a warm, humid climate.

Amphibians serve as valuable objects of physiological experiments. During their study, many outstanding discoveries were made. So, I. M. Sechenov discovered the reflexes of the brain in experiments on frogs. Amphibians are interesting as animals phylogenetically related, on the one hand, to ancient fish, and v the other - with primitive reptiles.

Structure and life functions. The appearance of amphibians is varied. In tailed amphibians, the body is elongated, the legs are short, approximately the same length, and a long tail is preserved throughout life. In tailless amphibians, the body is short and wide, the hind legs are jumping, much longer than the front ones, and the tail is absent in adults. Worms (legless) have a long, worm-like body without legs. In all amphibians, the neck is not expressed or is weakly expressed. Unlike fish, their head is movably articulated with the spine.

Covers. The skin of amphibians is thin, naked, usually covered with mucus secreted by numerous skin glands. In larvae, the mucous glands are unicellular, in adults they are multicellular. The secreted mucus prevents the skin from drying out, which is necessary for skin respiration. In some amphibians, the skin glands secrete a poisonous or burning secret that protects them from predators. The degree of keratinization of the epidermis in different species of amphibians is far from the same. In larvae and those adults that lead a mainly aquatic lifestyle, keratinization of the surface layers of the skin is poorly developed, but in toads on the back the stratum corneum makes up 60% of the entire thickness of the epidermis.

The skin is an important respiratory organ in amphibians, as evidenced by the ratio of the length of skin capillaries to the length of these vessels in the lungs; in the newt it is 4:1, and in toads, which have drier skin, it is 1:3.

The coloration of amphibians is often protective. Some, like the tree frog, are able to change it.

The skeleton of amphibians consists of the spine, skull, bones of the limbs and their belts. The spine is divided into sections: cervical, consisting of one vertebra, trunk - from a number of vertebrae, sacral - from one vertebra and tail. In tailless amphibians, the rudiments of the caudal vertebrae fuse into a long bone - the urostyle. In some tailed amphibians, the vertebrae are biconcave: remnants of the notochord remain between them. In most amphibians, they are either convex in front and concave in the back, or, conversely, concave in the front and convex in the back. The chest is missing.

Scull mostly cartilaginous, with a small number of overhead (secondary) and main (primary) bones. With the transition from gill breathing of aquatic ancestors of amphibians to pulmonary respiration, the visceral skeleton changed. The skeleton of the gill region has partially changed into the hyoid bone. The upper part of the hyoid arch - pendants, to which the jaws are attached in lower fish, in amphibians, due to the fusion of the primary upper jaw with the skull, has turned into a small auditory bone - a stirrup located in the middle ear.

Skeleton limbs and their belts consists of elements characteristic of the five-fingered limbs of terrestrial vertebrates. The number of toes varies across species. . musculature amphibians, due to more diverse movements and the development of limbs adapted to movement on land, to a large extent loses its metameric structure and acquires greater differentiation. The skeletal muscles are represented by many individual muscles, the number of which in a frog exceeds 350.

nervous system has undergone significant complications compared to that of fish. The brain is relatively larger. The progressive features of its structure should be considered the formation of the forebrain hemispheres and the presence of nerve cells not only in the side walls, but also in the roof of the hemispheres. Due to the fact that amphibians are inactive, their cerebellum is poorly developed. The diencephalon from above has an appendage - the epiphysis, and a funnel departs from its bottom, with which the pituitary gland is connected. The midbrain is poorly developed. Nerves extend from the brain and spinal cord to all organs of the body. There are ten pairs of head nerves. The spinal nerves form the brachial and lumbosacral clutches that innervate the fore and hind limbs.

sense organs amphibians have received progressive development in the process of evolution. Due to the fact that the air environment is less sound-conducting, the structure of the inner ear became more complicated in the hearing organs of amphibians and the middle ear (tympanic cavity) with the auditory ossicle was formed. The middle ear is bounded externally by the tympanic membrane. It communicates with the pharynx by a canal (Eustachian tube), which allows you to balance the air pressure in it with the pressure of the external environment. In connection with the peculiarities of vision in the air, amphibians have undergone changes in the structure of the eyes. The cornea of ​​the eye is convex, the lens is lenticular, there are eyelids that protect the eyes. Organs The sense of smell has external and internal nostrils. The larvae and amphibians permanently living in the water retained the lateral line organs characteristic of fish.

Digestive organs. A wide mouth leads into a vast oral cavity: many amphibians have small teeth on the jaws, as well as on the palate, which help to hold prey. Amphibians have a tongue of various shapes; in frogs, it is attached to the front of the lower jaw and can be thrown out of the mouth; animals use this to catch insects. The internal nostrils, the choanae, open into the oral cavity, and the Eustachian tubes open into the pharynx. Interestingly, in a frog, the eyes take part in swallowing food; having captured the prey with its mouth, the frog, by contraction of the muscles, draws its eyes deep into the oral cavity, pushing the food into the esophagus. Through the esophagus, food enters the bag-shaped stomach, and from there into the relatively short intestine, which is divided into thin and thick sections. The bile produced by the liver and the secretion of the pancreas enter the beginning of the small intestine through special ducts. In the final part of the large intestine - the cloaca - the ureters, the duct of the bladder and the genital ducts open.

Respiratory system change with the age of the animal. Amphibian larvae breathe with external or internal gills. Adult amphibians develop lungs, although some tailed amphibians retain gills for life. The lungs look like thin-walled elastic bags with folds on the inner surface. Since amphibians do not have a chest, air enters the lungs by swallowing: when lowering the bottom of the oral cavity, air enters it through the nostrils, then the nostrils close, and the bottom of the oral cavity rises, pushing air into the lungs. role played by gas exchange through the skin.

Circulatory system. Amphibians in connection with air breathing have two circles of blood circulation. The amphibian heart is three-chambered, it consists of two atria and a ventricle. The left atrium receives blood from the lungs, and the right atrium receives venous blood from the whole body with an admixture of arterial blood coming from the skin. Blood from both atria flows into the ventricle through a common opening with valves. The ventricle continues into a large arterial cone, followed by a short abdominal aorta. In tailless amphibians, the aorta divides into three pairs of symmetrically outgoing vessels, which are modified afferent branchial arteries of fish-like ancestors. The anterior pair - carotid arteries, carry arterial blood to the head. The second pair - the aortic arches, curving to the dorsal side, merge into the dorsal aorta, from which the arteries depart, carrying blood to various organs and parts of the body. The third pair is the pulmonary arteries, through which venous blood flows to the lungs. On the way to the lungs, large cutaneous arteries branch off from them, heading to the skin, where they branch into many vessels, causing skin respiration, which is of great importance in amphibians. From the lungs, arterial blood moves through the pulmonary veins to the left atrium.

Venous blood from the back of the body passes partly to the kidneys, where the renal veins break up into capillaries to form the portal system of the kidneys. The veins leaving the kidneys form the unpaired posterior (inferior) vena cava. Another part of the blood from the back of the body flows through two vessels, which, merging, form the abdominal vein. It goes, bypassing the kidneys, to the liver and participates, together with the portal vein of the liver, which carries blood from the intestines, in the formation of the portal system of the liver. Upon leaving the liver, the hepatic veins flow into the posterior vena cava, and the latter into the venous sinus (venous sinus) of the heart, which is an expansion of the veins. The venous sinus receives blood from the head, forelimbs and skin. From the venous sinus, blood flows into the right atrium. Tailed amphibians retain cardinal veins from aquatic ancestors.

excretory organs in adult amphibians, they are represented by trunk kidneys. A pair of ureters depart from the kidneys. The urine they excrete first enters the cloaca, from there - into the bladder. With the reduction of the latter, urine again finds itself in the cloaca, and is released from it. Amphibian embryos have functioning head kidneys.

Reproductive organs. All amphibians have separate sexes. Males have two testes located in the body cavity near the kidneys. The seminiferous tubules, passing through the kidney, flow into the ureter, represented by the wolf channel, which serves to remove urine and sperm. In females, large paired ovaries lie in the body cavity. Ripe eggs enter the body cavity, from where they enter the funnel-shaped initial sections of the oviducts. Passing through the oviducts, the eggs are covered with a transparent thick mucous membrane. The oviducts open into

Development in amphibians takes place with a complex metamorphosis. From the eggs emerge larvae, which differ both in structure and lifestyle from adults. Amphibian larvae are true aquatic animals. Living in the aquatic environment, they breathe with gills. The gills of the larvae of tailed amphibians are external, branched; in larvae of tailless amphibians, the gills are initially external, but soon become internal due to the fouling of their skin folds. The circulatory system of amphibian larvae is similar to that of fish and has only one circulation. They have lateral line organs, like most fish. They move mainly due to the movement of a flattened tail trimmed with a fin.

When a larva turns into an adult amphibian, profound changes occur in most organs. Paired five-fingered limbs appear, tailless amphibians have a reduced tail. Gill respiration is replaced by pulmonary respiration, gills usually disappear. Instead of one circle of blood circulation, two develop:

large and small (pulmonary). In this case, the first pair of afferent branchial arteries turns into carotid arteries, the second becomes aortic arches, the third is reduced to one degree or another, and the fourth is converted into pulmonary arteries. In the Mexican amphibian amblistoma, neoteny is observed - the ability to reproduce at the larval stage, that is, to reach sexual maturity while maintaining larval structural features.

Ecology and economic importance of amphibians. The habitats of amphibians are diverse, but most species stick to wet places, and some spend their entire lives in the water without going to land. Tropical amphibians - worms - lead an underground lifestyle. A peculiar amphibian - the Balkan Proteus lives in the reservoirs of caves; his eyes are reduced, and his skin is devoid of pigment. Amphibians belong to the group of cold-blooded animals, that is, their body temperature is not constant and depends on the ambient temperature. Already at 10 ° C, their movements become sluggish, and at 5-7 ° C, they usually fall into a stupor. In winter, in a temperate and cold climate, the vital activity of amphibians almost stops. Frogs usually hibernate at the bottom of reservoirs, and newts - in minks, in moss, under stones.

Amphibians breed in most cases in the spring. Female frogs, toads, and many other anurans spawn into the water, where the males fertilize it with sperm. In tailed amphibians, a kind of internal fertilization is observed. So, the male newt lays sperm clods in mucous sacs-spermatophores on aquatic plants. The female, finding a spermatophore, captures it with the edges of the cloacal opening.

The fertility of amphibians varies widely. An ordinary grass frog spawns 1-4 thousand eggs in the spring, and a green frog - 5-10 thousand eggs. The development of common frog tadpoles in eggs lasts from 8 to 28 days, depending on the water temperature. The transformation of a tadpole into a frog usually occurs at the end of summer.

Most amphibians, having laid their eggs in the water and fertilizing it, do not show concern for it. But some species take care of their offspring. So, for example, the male midwife toad, widespread in our country, winds the cords of fertilized eggs around its hind legs and swims with it until tadpoles hatch from the eggs. In the female of the South American (Surinamese) pipa toad, during spawning, the skin on the back strongly thickens and softens, the cloaca stretches and becomes an ovipositor. After spawning and fertilization, the male lays it on the back of the female and presses them into the swollen skin with his belly, where the development of the young takes place.

Amphibians feed on small invertebrates, primarily insects. They eat many pests of cultivated plants. Therefore, most amphibians are very useful for crop production. It is estimated that one grass frog can eat about 1.2 thousand insects harmful to agricultural plants during the summer. Toads are even more useful, because they hunt at night and eat a lot of nocturnal insects and slugs that are inaccessible to birds. In Western Europe, toads are often released into greenhouses and greenhouses to exterminate pests. Newts are useful because they eat mosquito larvae. At the same time, it is impossible not to note the harm that large frogs bring by the extermination of juvenile fish. In nature, many animals feed on frogs, including commercial ones.

The class Amphibians is divided into three orders: Tailed amphibians , Tailless amphibians , Legless amphibian .

Detachment Tailed amphibians (Urodela). The most ancient group of amphibians, represented in the modern fauna by about 130 species. The body is elongated, valky. The tail is preserved throughout life. The fore and hind limbs are about the same length. Therefore, tailed amphibians move by crawling or walking. Fertilization is internal. Some forms retain gills for life.

In our country, tailed amphibians are widespread newts(Triturus). The most common are the large crested newt (males are black with an orange belly) and the smaller common newt (males are usually light spotted). In summer, newts live in the water, where they breed, and spend the winter on land in a state of stupor. In the Carpathians you can meet quite a large fire salamander (Salamandra), which is easily recognizable by its black coloration with orange or yellow spots. Giant Japanese salamander reaches 1.5 m in length. To the Proteus family (Proteidea) applies Balkan Proteus, living in the reservoirs of caves and retaining gills all his life. Its skin has no pigment, and its eyes are rudimentary, as the animal lives in darkness. In laboratories for physiological experiments, the larvae of American amblistoms, called axolotls. These animals, like all tailed amphibians, have a remarkable ability to restore lost body parts.

Order Tailless amphibians(Anura) - frogs, toads, tree frogs. They are characterized by a short, wide body. The tail is absent in adults. The hind legs are much longer than the front legs, which determines the movement in jumps. fertilization external,

At lagunis(Ranidae) the skin is smooth, mucous. There are teeth in the mouth. Mostly diurnal and crepuscular animals. At toad (Bufonidae) the skin is dry, bumpy, there are no teeth in the mouth, the hind legs are relatively short. Towakshi(Hylidae) differ in small size, thin slender body and paws with suction cups at the ends of the fingers. The suction cups make it easier to move through the trees where tree frogs hunt for insects. The color of tree frogs is usually bright green, and may vary depending on the color of the surrounding environment.

Order Legless amphibians(Apoda) -tropical amphibians, leading an underground lifestyle. They have a long, valky body with a short tail. In connection with life in minks underground, their legs and eyes have undergone reduction. Fertilization is internal. They feed on soil invertebrates.

Literature: "Course of Zoology" Kuznetsov et al. M-89

"Zoology" Lukin M-89

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 of an adaptive nature as 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 lives 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.

The glands of amphibians are of two kinds; smaller mucous glands and larger serous, or proteinaceous. The former belong to the group of mesocryptic glands, the cells of which are not destroyed in the process of secretion, the latter are holocryptic, the cells of which are entirely used to form a secret. Protein glands form warty elevations on the dorsal side, dorsal ridges of frogs, ear glands (parotids) in toads and salamanders. Both those and other glands (Fig. 230) are dressed on the outside with a layer of smooth muscle fibers. The secret of the glands is often poisonous, especially the protein glands.
The color of the skin of amphibians is determined, as in fish, by the presence of pigment and reflective iridocytes in the skin. The pigment is either diffuse or granular, located in special cells - chromatophores. Diffuse pigment distributed in the stratum corneum of the epidermis, usually yellow; granular is black, brown and red. In addition to it, there are white grains of guanine. The green and blue coloration of some amphibians is a subjective coloration due to shifting tones in the eye of the observer.
Studying at low magnifications the skin of tree frogs, tree frogs (Hyla arborea), we see that when looking at the skin from below, it appears black due to the presence of anastomosing and branched black pigment cells, melanophores. The epidermis itself is colorless, but where light passes through the skin with reduced melanophores, it appears yellow. Leukophori, or interfering cells, contain crystals of guanine. Xanthophores contain golden yellow lipochrome. The ability of melanophores to change their appearance, either by rolling into a ball, or by stretching out processes, and determines mainly the possibility of color change. The yellow pigment in xanthophores is mobile in the same way. Leukophores or interfering cells give a blue-gray, red-yellow or silver sheen. Playing all these elements together will create all kinds of amphibian coloration. Permanent black spots are due to the presence of black pigment. Melanophores enhance its action. The white color is caused by leucophores in the absence of melanophores. When the melanophores collapse and the lipochrome spreads, a yellow color will be created. Green color is obtained by the interaction of black and yellow chromatophores.
Color changes are dependent on the nervous system.
The skin of amphibians is richly supplied with vessels, serving for respiration. In the hairy frog (Astyloslernus), which has greatly reduced lungs, the body is covered with hair-like outgrowths of the skin, abundantly supplied with blood vessels. The skin of amphibians also serves for the perception of water and for excretion. In dry air, the skin of frogs and salamanders evaporates so profusely that they die. Toads with a more developed stratum corneum survive much longer under the same conditions.

Amphibians(they are amphibians) - the first terrestrial vertebrates that appeared in the process of evolution. At the same time, they still retain a close relationship with the aquatic environment, usually living in it at the larval stage. Typical representatives of amphibians are frogs, toads, newts, salamanders. The most diverse in tropical forests, as it is warm and damp there. There are no marine species among amphibians.

General characteristics of amphibians

Amphibians are a small group of animals with about 5,000 species (according to other sources, about 3,000). They are divided into three groups: Tailed, Tailless, Legless. The frogs and toads familiar to us belong to the tailless ones, the newts belong to the tailed ones.

Amphibians have paired five-fingered limbs, which are polynomial levers. The forelimb consists of the shoulder, forearm, hand. Hind limb - from the thigh, lower leg, foot.

Most adult amphibians develop lungs as respiratory organs. However, they are not as perfect as in more highly organized groups of vertebrates. Therefore, skin respiration plays an important role in the life of amphibians.

The appearance of the lungs in the process of evolution was accompanied by the appearance of a second circle of blood circulation and a three-chambered heart. Although there is a second circle of blood circulation, due to the three-chambered heart, there is no complete separation of venous and arterial blood. Therefore, mixed blood enters most organs.

The eyes have not only eyelids, but also lacrimal glands for wetting and cleansing.

The middle ear appears with a tympanic membrane. (In fish, only the internal.) The eardrums are visible, located on the sides of the head behind the eyes.

The skin is naked, covered with mucus, it has many glands. It does not protect against water loss, so they live near water bodies. Mucus protects the skin from drying out and bacteria. The skin is made up of the epidermis and dermis. Water is also absorbed through the skin. The skin glands are multicellular, in fish they are unicellular.

Due to the incomplete separation of arterial and venous blood, as well as imperfect pulmonary respiration, the metabolism of amphibians is slow, like that of fish. They also belong to cold-blooded animals.

Amphibians breed in water. Individual development proceeds with transformation (metamorphosis). The frog larva is called tadpole.

Amphibians appeared about 350 million years ago (at the end of the Devonian period) from ancient lobe-finned fish. Their heyday occurred 200 million years ago, when the Earth was covered with huge swamps.

Musculoskeletal system of amphibians

In the skeleton of amphibians, there are fewer bones than in fish, since many bones grow together, while others remain cartilage. Thus, their skeleton is lighter than that of fish, which is important for living in an air environment that is less dense than water.

The brain skull fuses with the upper jaws. Only the lower jaw remains mobile. The skull retains a lot of cartilage that does not ossify.

The musculoskeletal system of amphibians is similar to that of fish, but has a number of key progressive differences. So, unlike fish, the skull and spine are movably articulated, which ensures the mobility of the head relative to the neck. For the first time, the cervical spine appears, consisting of one vertebra. However, the mobility of the head is not great, frogs can only tilt their heads. Although they have a neck vertebra, they do not appear to have a neck in appearance.

In amphibians, the spine consists of more sections than in fish. If fish have only two of them (trunk and tail), then amphibians have four sections of the spine: cervical (1 vertebra), trunk (7), sacral (1), caudal (one tail bone in anurans or a number of individual vertebrae in tailed amphibians) . In tailless amphibians, the caudal vertebrae fuse into one bone.

The limbs of amphibians are complex. The anterior ones consist of the shoulder, forearm and hand. The hand consists of the wrist, metacarpus and phalanges of the fingers. The hind limbs consist of the thigh, lower leg and foot. The foot consists of the tarsus, metatarsus and phalanges of the fingers.

Limb belts serve as a support for the skeleton of the limbs. The belt of the forelimb of an amphibian consists of the scapula, clavicle, crow bone (coracoid), common to the belts of both forelimbs of the sternum. The clavicles and coracoids are fused to the sternum. Due to the absence or underdevelopment of the ribs, the belts lie in the thickness of the muscles and are not indirectly attached to the spine in any way.

The belts of the hind limbs consist of the ischial and ilium bones, as well as the pubic cartilages. Growing together, they articulate with the lateral processes of the sacral vertebra.

The ribs, if present, are short and do not form a chest. Tailed amphibians have short ribs, tailless amphibians do not.

In tailless amphibians, the ulna and radius are fused, and the bones of the lower leg are also fused.

The muscles of amphibians have a more complex structure than those of fish. The muscles of the limbs and head are specialized. Muscle layers break up into separate muscles, which provide movement of some parts of the body relative to others. Amphibians not only swim, but also jump, walk, crawl.

Digestive system of amphibians

The general plan of the structure of the digestive system of amphibians is similar to that of fish. However, there are some innovations.

The anterior horse of the tongue of frogs adheres to the lower jaw, while the posterior one remains free. This structure of the tongue allows them to catch prey.

Amphibians have salivary glands. Their secret wets food, but does not digest it, as it does not contain digestive enzymes. The jaws have conical teeth. They serve to hold food.

Behind the oropharynx is a short esophagus that opens into the stomach. Here the food is partially digested. The first section of the small intestine is the duodenum. A single duct opens into it, where the secrets of the liver, gallbladder and pancreas enter. In the small intestine, food digestion is completed and nutrients are absorbed into the blood.

Undigested food remnants enter the large intestine, from where they move to the cloaca, which is an expansion of the intestine. The ducts of the excretory and reproductive systems also open into the cloaca. From it, undigested residues enter the external environment. Fish do not have a cloaca.

Adult amphibians feed on animal food, most often various insects. Tadpoles feed on plankton and plant matter.

1 Right atrium, 2 Liver, 3 Aorta, 4 Oocytes, 5 Large intestine, 6 Left atrium, 7 Heart ventricle, 8 Stomach, 9 Left lung, 10 Gallbladder, 11 Small intestine, 12 Cloaca

Respiratory system of amphibians

Amphibian larvae (tadpoles) have gills and one circle of blood circulation (like in fish).

In adult amphibians, lungs appear, which are elongated sacs with thin elastic walls that have a cellular structure. The walls contain a network of capillaries. The respiratory surface of the lungs is small, so the bare skin of amphibians also participates in the breathing process. Through it comes up to 50% oxygen.

The mechanism of inhalation and exhalation is provided by raising and lowering the bottom of the oral cavity. When lowering, inhalation occurs through the nostrils, when raised, air is pushed into the lungs, while the nostrils are closed. Exhalation is also carried out when the bottom of the mouth is raised, but at the same time the nostrils are open, and the air exits through them. Also, when exhaling, the abdominal muscles contract.

In the lungs, gas exchange occurs due to the difference in the concentrations of gases in the blood and air.

The lungs of amphibians are not well developed to fully provide gas exchange. Therefore, skin respiration is important. Drying out amphibians can cause them to suffocate. Oxygen first dissolves in the fluid covering the skin, and then diffuses into the blood. Carbon dioxide also first appears in the liquid.

In amphibians, unlike fish, the nasal cavity has become through and is used for breathing.

Under water, frogs breathe only through their skin.

The circulatory system of amphibians

The second circle of blood circulation appears. It passes through the lungs and is called the pulmonary, as well as the pulmonary circulation. The first circle of blood circulation, passing through all organs of the body, is called large.

The heart of amphibians is three-chambered, consists of two atria and one ventricle.

The right atrium receives venous blood from the organs of the body, as well as arterial blood from the skin. The left atrium receives blood from the lungs. The vessel that empties into the left atrium is called pulmonary vein.

Atrial contraction pushes blood into the common ventricle of the heart. This is where the blood mixes.

From the ventricle, through separate vessels, blood is directed to the lungs, to the tissues of the body, to the head. The most venous blood from the ventricle enters the lungs through the pulmonary arteries. Almost pure arterial goes to the head. The most mixed blood entering the body is poured from the ventricle into the aorta.

This separation of the blood is achieved by a special arrangement of vessels emerging from the distribution chamber of the heart, where blood enters from the ventricle. When the first portion of blood is pushed out, it fills the nearest vessels. And this is the most venous blood, which enters the pulmonary arteries, goes to the lungs and skin, where it is enriched with oxygen. From the lungs, blood returns to the left atrium. The next portion of blood - mixed - enters the aortic arches going to the organs of the body. The most arterial blood enters the distant pair of vessels (carotid arteries) and goes to the head.

excretory system of amphibians

The kidneys of amphibians are trunk, have an oblong shape. Urine enters the ureters, then flows down the wall of the cloaca into the bladder. When the bladder contracts, urine flows into the cloaca and out.

The excretion product is urea. It takes less water to remove it than to remove ammonia (which is produced by fish).

In the renal tubules of the kidneys, water is reabsorbed, which is important for its conservation in air conditions.

Nervous system and sense organs of amphibians

There were no key changes in the nervous system of amphibians in comparison with fish. However, the forebrain of amphibians is more developed and is divided into two hemispheres. But their cerebellum is worse developed, since amphibians do not need to maintain balance in the water.

Air is more transparent than water, so vision plays a leading role in amphibians. They see further than fish, their lens is flatter. There are eyelids and nictitating membranes (or an upper fixed eyelid and a lower transparent movable one).

Sound waves travel worse in air than in water. Therefore, there is a need for a middle ear, which is a tube with a tympanic membrane (visible as a pair of thin round films behind the eyes of a frog). From the tympanic membrane, sound vibrations are transmitted through the auditory ossicle to the inner ear. The Eustachian tube connects the middle ear to the mouth. This allows you to weaken the pressure drops on the eardrum.

Reproduction and development of amphibians

Frogs start breeding at about 3 years of age. Fertilization is external.

Males secrete seminal fluid. In many frogs, the males attach themselves to the backs of the females, and while the female spawns for several days, she is poured with seminal fluid.

Amphibians spawn less eggs than fish. Clusters of caviar are attached to aquatic plants or float.

The mucous membrane of the egg swells greatly in water, refracts sunlight and heats up, which contributes to the faster development of the embryo.

Development of frog embryos in eggs

An embryo develops in each egg (usually about 10 days in frogs). The larva that emerges from the egg is called a tadpole. It has many features similar to fish (two-chambered heart and one circle of blood circulation, breathing with the help of gills, lateral line organ). At first, the tadpole has external gills, which then become internal. The hind limbs appear, then the front. The lungs and the second circle of blood circulation appear. At the end of metamorphosis, the tail resolves.

The tadpole stage usually lasts several months. Tadpoles eat plant foods.

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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 region 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 of the "corns" are modified sensitive tubercles, but the main function of the "corns" is mechanical - it helps the male to hold the female tightly. 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 female's sexual feeling.

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.

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

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; rare) through various shades of green to dark olive and even brown-bronze (in moss in winter). The general background color of the common frog 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 germinal 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 one. Experiments with feeding frogs with krapp (Kashchenko, 1882) and direct observations force us to oppose the upper part of the dense layer to 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 running elements, among which two categories can be distinguished: isolated thin bundles of connective tissue that do not penetrate the cribriform 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 age stages of the same formation, but this opinion is apparently wrong.

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 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 cloth or put 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. For a long time (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 for 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 a special 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. From environmental factors, temperature and humidity are of the greatest importance for coloring frogs. High temperature (+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 a white background, the head and forelimbs brighten most quickly, the trunk and, last of all, the hind limbs lighten more slowly. 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 melanophores of the lake frog.

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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