Amphibians(they are amphibians) - the first terrestrial vertebrates to appear in the process of evolution. However, they still maintain a close connection with the aquatic environment, usually living in it at the larval stage. Typical representatives of amphibians are frogs, toads, newts, and salamanders. Most diverse in tropical forests, because it is warm and damp there. There are no marine species among amphibians.

General characteristics of amphibians

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

Amphibians develop paired five-fingered limbs, which are multi-membered levers. The forelimb consists of the shoulder, forearm, and 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 lungs in the process of evolution was accompanied by the appearance of a second circulation and a three-chambered heart. Although there is a second circuit of blood circulation, due to the three-chambered heart there is no complete separation of venous and arterial blood. Therefore, most organs receive mixed blood.

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

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

The skin is bare, covered with mucus, and contains many glands. It does not protect against water loss, so they live near bodies of water. Mucus protects the skin from drying out and bacteria. The skin consists of epidermis and dermis. Water is also absorbed through the skin. Skin glands are multicellular, while 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 are also 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

Amphibians have fewer bones in their skeletons than fish, as many bones are fused while others remain cartilage. Thus, their skeleton is lighter than that of fish, which is important for living in the air, which is less dense than water.

The brain skull is fused 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 cervical vertebra, there is no neck in the external appearance of the body.

In amphibians, the spine consists of more sections than in fish. If fish have only two of them (trunk and caudal), then amphibians have four sections of the spine: cervical (1 vertebra), trunk (7), sacral (1), caudal (one tail bone in tailless amphibians or a number of separate 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, tibia and foot. The foot consists of the tarsus, metatarsus and phalanges.

The limb girdles serve as support for the skeleton of the limbs. The girdle of the forelimb of an amphibian consists of a scapula, clavicle, and crow bone (coracoid), common to the girdles 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 deep in the muscles and are in no way indirectly attached to the spine.

The hind limb girdles consist of the ischial and ilium bones, as well as pubic cartilage. Fusing together, they articulate with the lateral processes of the sacral vertebra.

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

In tailless amphibians, the ulna and radius bones are fused, and the bones of the tibia 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 down into individual muscles, which provide movement of some parts of the body relative to others. Amphibians not only swim, but also jump, walk, and crawl.

Digestive system of amphibians

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

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

Amphibians appear salivary glands. Their secretion moistens food, but does not digest it in any way, since it does not contain digestive enzymes. The jaws have conical teeth. They serve to hold food.

Behind the oropharyngeal cavity 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, into which the secretions of the liver, gallbladder and pancreas enter. In the small intestine, food digestion is completed and nutrients are absorbed into the blood.

Undigested food remains enter the large intestine, from where it moves to the cloaca, which is an extension 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 foods.

1 Right atrium, 2 Liver, 3 Aorta, 4 Oocytes, 5 Large intestine, 6 Left atrium, 7 Ventricle of the heart, 8 Stomach, 9 Left lung, 10 Gall bladder, 11 Small intestine, 12 Cloaca

Respiratory system of amphibians

Amphibian larvae (tadpoles) have gills and one circulation (like 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. Up to 50% of oxygen enters through it.

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

Gas exchange occurs in the lungs due to the difference in gas concentrations in the blood and air.

The lungs of amphibians are not well developed enough to fully ensure gas exchange. Therefore, skin breathing 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.

Underwater, frogs breathe only through their skin.

Circulatory system of amphibians

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

The heart of amphibians is three-chambered, consisting 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 arterial blood from the lungs. The vessel entering the left atrium is called pulmonary vein.

Contraction of the atria pushes blood into the common ventricle of the heart. Here the blood is partially mixed.

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

This division of 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 closest 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 is directed to the head.

Excretory system of amphibians

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

The excretion product is urea. To remove it you need less water than for the removal of ammonia (which is produced in fish).

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

Nervous system and sensory organs of amphibians

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

Air clearer than water Therefore, 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 an eardrum (visible as a pair of thin round films behind the eyes of a frog). From the eardrum, sound vibrations are transmitted through the auditory bone to the inner ear. The Eustachian tube connects the middle ear cavity to the oral cavity. This allows you to reduce pressure drops on the eardrum.

Reproduction and development of amphibians

Frogs begin to reproduce at about 3 years of age. Fertilization is external.

Males secrete seminal fluid. In many frogs, males attach themselves to the backs of females and, while the female spawns eggs over several days, waters them with seminal fluid.

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

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

Development of frog embryos in eggs

An embryo develops in each egg (in frogs it usually takes about 10 days). The larva that emerges from the egg is called a tadpole. It has many features similar to fish (two-chambered heart and one circulation, breathing with gills, lateral line organ). At first, the tadpole has external gills, which later become internal. The hind limbs appear, then the forelimbs. 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 feed on plant matter.

At the end Carboniferous period One of the representatives of large amphibians appeared on Earth - the Mastodonsaurus. It was large predator, feeding almost exclusively on fish, inhabiting freshwater bodies of water (lakes and swamps). He led an aquatic lifestyle. Its habits and behavior were very similar to the lifestyle of ordinary frogs. He also could not exist without water, only occasionally and briefly crawling onto land. Therefore, when in the Permian period the climate became less humid and water bodies, including big lakes, began to dry out and disappear, the mass death of mastodonsaurs began, and by the beginning of the Triassic this large predator disappeared from the face of the Earth.

The name of the described group - AMPHIBIDES - suggests that these animals, coming onto land, have not yet fully separated from life in the water. And in fact, many of them continued to lead an aquatic lifestyle, climbing onto land only for a short time, or if they lived on land, it was near the water with which they were constantly connected. They, like fish, laid eggs, the entire development cycle of which took place in water. Amphibians went through only the very first stages of land development, but that is why their biology is still a huge scientific interest, since the further evolution of these animals, their complete separation from aquatic environment, laid the foundation for the emergence of the next group - higher vertebrates (reptiles). For the first time, reptiles began to reproduce on land, away from water. They now have eggs with a dense outer shell, protecting them from drying out and mechanical damage. Thanks to this, new groups of higher vertebrates subsequently emerged - birds and mammals.

Amphibians, or amphibians, which means lungfish in Greek, are very different from other vertebrates. They appeared on Earth more than 300 million years ago. In their life, two periods must be distinguished: at the initial stage of development they are similar to fish, and then gradually turn into animals with pulmonary breathing. Thus, in the development cycle of amphibians, a transformation takes place that is almost never found in other vertebrates, and, conversely, is widespread in lower, invertebrates.

Amphibian habitats.

Amphibians , or amphibians, are very different from all vertebrates. These include: toads, frogs, salamanders and caecilians. In their life, two periods must be distinguished: in their youth they are similar to fish, and then gradually turn into animals with pulmonary breathing. Thus, in the development cycle of amphibians, a transformation takes place, which is almost never found in other vertebrates, and, on the contrary, is widespread in lower, invertebrate animals.

general characteristics

In their lifestyle and appearance, amphibians are very similar, on the one hand, to reptiles, and on the other, even more so, to fish; their larval stage constitutes, as it were, a transition between these two orders.

Body shape varies greatly. Tailed amphibians are more similar to fish, have a laterally compressed body and a long paddle-shaped tail; in others the body is round or flat, and the tail is completely absent. Some amphibians have no limbs at all, in others they are very poorly developed, in others, on the contrary, they are highly developed.

Device skeleton amphibians are to some extent similar to fish. Fish-like amphibians have vertebrae exactly the same as those of fish.

Feature of amphibians consists in the absence of any hard outer coverings.

The coloration of some amphibians can change, as we saw in chameleons, due to changes external conditions, and internal irritation.

How in top layer the skin, and in the internal part of all amphibians there are a lot of glands of various sizes and different purposes. The most interesting of them are the poisonous glands. They are located in bottom layer skin, have a spherical or oval shape, secrete a mucous liquid containing a toxic substance. Amphibians, which have more developed such glands, can voluntarily increase the secretion of these glands and use it as a means of protection. It has now been established that the poisons of some amphibians are very strong, but they are not dangerous for humans and large animals because they are contained in the mucus only in a very minor impurity.

The elastic, very thin and uncoated skin of amphibians has great importance in their lives. No amphibian drinks water in the usual way, and absorbs it exclusively through the skin. That's why they require proximity to water or dampness.. Frogs removed from water quickly lose weight, become lethargic and soon die completely. If you put a wet rag near such frogs, exhausted by dryness, they begin to press their bodies against it and quickly recover.How large is the amount of water that frogs absorb through their skin can be seen from the following Thomson experiment. He took the dried tree frog and, having weighed it, found that its weight was 95 grams. After that, he wrapped it in a wet rag, and after an hour it already weighed 152 grams. The amphibian absorbs water through its skin and sweats it out. Gases are also exchanged through the skin. In a closed tin box, surrounded by a humid atmosphere, a frog can live for 20-40 days, even if the air supply to the lungs is cut off.

In most amphibians, the initial development of embryos occurs in the same way as in fish. Eggs are usually laid in water in the form of caviar, which is fertilized later, already in water. The eggs are surrounded by a thick layer of gelatinous substance. This shell is of great importance for the embryo, since in this way the egg is protected from drying out, from mechanical damage, and most importantly, it protects them from being eaten by other animals. Also, the shell itself protects the eggs from attack by fish, shellfish and aquatic insects.

Amphibians live in all parts of the world and in all zones, with the exception of the polar countries. Water is even greater than heat a necessary condition their existence, this is how almost all amphibians spend larval stages in water. They live exclusively in fresh waters, avoiding sea or even salt water. Almost half of amphibians spend their entire lives in water, while others settle on land as adults, although they always stay near water and in damp places. Tropical countries, abundant in forests and water, are the most favorable for their life. These are the central parts South America, Madagascar, islands of the Malay Archipelago, where virgin, rain forests; vice versa, middle Asia, Australia and most inner Africa- very poor in amphibians. All amphibians swim perfectly in water, not only in the larval state, but also in the adult state; on land, tailed ones crawl like reptiles, and tailless ones move in short, heavy jumps; many of them can even climb trees.

The food of amphibians changes according to age. The larvae eat all sorts of small organisms, both plant and animal: ciliates, rotifers, microscopic crustaceans and small algae; As they grow, they have a greater and greater need for living food. Adult amphibians are already real predators and pursue all animals that they can overpower, starting with worms and insects and ending with small vertebrates; they even eat larvae of their own species if they are able to swallow them.

Based on their appearance and degree of organization, amphibians are divided into three orders:tailless, tailed And legless.

Order I. - Tailless amphibians

With the exception of the extreme polar regions, frogs and toads are distributed throughout the Earth, but they are most numerous in tropical America, where about half of all currently known species live. The lifestyle of frogs is varied, depending on their place of residence: they live in water, on the banks, in meadows, in forests and bushes and near human habitation, in a word, wherever they can find dampness and suitable food.

Squad II. - Tailed amphibians

Tailed amphibians appearance very similar to lizards. They have an elongated body, a long tail and 4 (very in rare cases 2) limbs equipped with 2 - 4 fingers. They all lead night image life, and hide during the day. Their movements on land are usually clumsy and slow, only very few can compare in their agility with lizards, but in water they are very agile, swim and dive well, like fish. Their food consists of small fish and shellfish, worms, spiders, insects and other small animals. For humans, almost all tailed amphibians are completely harmless; on the contrary, they are even useful, since they exterminate harmful animals. They are very easy to keep in captivity.

Tailed amphibians are divided into four families, of which the most important is the familysalamanders .

Since ancient times, salamanders have been considered terribly poisonous animals. The many glands that are contained in her skin can abundantly secrete mucus, which is completely harmless, but from ancient times, according to prejudice, was considered very poisonous. The same abundant secretion of mucus is the basis for the prejudice that the salamander does not burn in fire, since, thrown into the fire, it secretes a lot of mucus, but after some time it still burns

Squad III – Legless.

Worms

Caecilians are similar in appearance to snakes or legless lizards, but their internal structure and developmental history undoubtedly indicate their place in the system next to salamanders and the just described proteas. The body of caecilians is completely cylindrical, sometimes divided into segments by many ring-shaped folds of skin, the eyes are hidden under the skin or are completely absent, the teeth are the same as those of frogs and newts; There are no pelvic bones, only one lung develops. Development occurs in the same way as other amphibians, but very quickly: a tadpole develops from the egg, which gradually loses its gills and switches to pulmonary respiration. Some caecilians give birth to live young, and for most, almost the entire development cycle takes place in the egg shell, so that the larvae’s stay in the water lasts only a short time, after which the animal comes to land. Caecilians live in all tropical countries of Asia, Africa and America, but they are not found in Australia and Madagascar. In their lifestyle, caecilians are completely similar to earthworms, which they usually feed on, and live underground, where they constantly dig in search of prey.

Class amphibians (amphibians)

Representatives of this class are four-legged animals from the group anamnia, body temperature is unstable, skin is bare, with a large number iron Has internal nostrils choanae. The middle ear has one auditory ossicle. The cervical and sacral spine are formed by one vertebra. Amphibians have a three-chambered heart with two circulation circles. Fertilization occurs in water, and the larvae also develop in water. The class accommodates about 4,000 species.

Orders of amphibians:

1. Tailless (frog)

2. Tailed animals (newt, salamander)

3. Legless (caecilian)

The habitat of amphibians is located on the border between water and land. Their skin is bare and moist with glands that secrete mucus. Mucus has bactericidal properties, and also contains irritating substances, which, due to their properties, repel amphibian predators. Constantly moisturizing the skin is necessary for breathing, since amphibians suffocate the entire surface. For example, the ratio of the lengths of capillaries of the skin and lungs in a newt is 4:1, and in a toad it is 1:3.

The musculature of amphibians is highly differentiated due to the transition to life on land; four free limbs appear; amphibians have about 350 types of muscles.

Frog skeleton

Respiratory organs: internal nostrils (amphibians can breathe with their mouths closed) - larynx - trachea - two small lungs. Air absorption occurs through swallowing movements, because Amphibians do not have a chest.

Circulatory system: closed, 3-chambered heart.

Digestion: mouth x pharynx x esophagus x stomach x intestines x cloaca. Amphibians do not chew in the oral cavity; the tongue is used to catch prey, and the eyes are used to push food further.

Nervous system: the brain volume is larger than that of fish. Frogs have well-defined cerebral hemispheres and the complexity of behavior is higher than that of fish.

Sense organs: frogs have large eyes, a lens-shaped crystal, which allows them to change shape (accommodation). The external auditory canal appears, ending in the eardrum, connected to the stapes, which in turn connects to the inner ear. This system amplifies sound vibrations. The sense of smell does not play a big role.

Excretory system: two bean-shaped trunk kidneys, bladder, two ureters, cloaca.

Reproduction: all amphibians are dioecious; in many species, sexual dimorphism is well expressed (for example, female newts have a different coloration from the male). The process of reproduction in frogs: after mating, fertilization does not occur, but first hormones are released, when the period of fertilization occurs, eggs are formed from the egg and sperm, which are released into the water. The development of the larva proceeds with metamorphosis. Tadpole (the body is similar to a fish, there are external gills, 1 circle of blood circulation, a 2-chambered heart, there is a lateral line) - a frog (the gills are replaced by lungs, the back and front pair of limbs appear, the tail is shortened) - an adult frog.

The meaning of amphibians great. In an ecological sense, they are useful (they hunt blood-sucking insects, slugs, and worm-like larvae). In France they are eaten. Frogs are a traditional subject of research for biologists, veterinarians, and doctors (laboratory experiments).

AMNIOTES, HIGHER VERTEBRATES

True land animals contain the three highest classes of vertebrates - reptiles, birds and mammals. Reptiles and birds lay eggs on land; in mammals, the egg remains in the mother's body, and the embryo develops in utero (unlike fish and amphibians, whose embryos develop in water).

Class reptiles (reptiles)

Reptiles are terrestrial animals with variable body temperatures. Reptiles have a well-defined neck, dry skin with keratinized epidermis, and no glands. The thoracic spine contains ribs that form chest. The cortex appears in the cerebral hemispheres. The heart is 3-chambered with an incomplete septum in the ventricle. The excretory organs are the pelvic kidneys. Fertilization is internal. The class contains about 7,000 species.

Subclasses of reptiles:

1. Lizards (beaked)

2. Scaly

3. Turtles

4. Crocodiles

General skeleton (crocodile)

Circulatory system

Amphibians , or amphibians, are very different from all vertebrates. In their life, two periods must be distinguished: in their youth they are similar to fish, and then they gradually turn into animals with pulmonary breathing. Thus, in the development cycle of amphibians, a transformation takes place that is almost never found in other vertebrates, and, on the contrary, is widespread in lower, invertebrate animals.

general characteristics

In their lifestyle and appearance, amphibians are very similar, on the one hand, to reptiles, and on the other, even more so, to fish; their larval stage constitutes, as it were, a transition between these two orders.

Body shape varies greatly. Tailed amphibians are more similar to fish, have a laterally compressed body and a long paddle-shaped tail; in others the body is round or flat, and the tail is completely absent. Some amphibians have no limbs at all, in others they are very poorly developed, in others, on the contrary, they are highly developed.

Device skeleton amphibians are to some extent similar to fish. In fish-like amphibians, the vertebrae are exactly the same as those of fish; in others, vertebrae develop with an articular head in front and a dimple behind, which determines full articulation. The transverse processes of the vertebrae in all amphibians are well developed, but true ribs usually do not develop; instead of them there are only small bone or cartilaginous appendages. The above-mentioned transverse processes in some are very long and replace the missing ribs.

Device skulls can be varied; here you can notice a gradual complication and increase in bone formations due to cartilaginous and connective tissue. A characteristic feature of the entire class of amphibians are two articular heads on the occipital part of the skull, which correspond to the two dimples of the first cervical vertebra. The skull is always flat, wide, the eye sockets are very large. The skull consists of two occipital bones, two frontal bones, and a sphenoid bone. In the lateral walls of the skull, for the most part, ossification does not occur at all, or the cartilage ossifies partially. The palatine bones are fixedly connected to the skull; On them, just like on the vomer and on the wedge bone, teeth sometimes sit. Lower jaw consists of two or more parts and never completely ossifies.

Brain amphibians have a simple structure. It has an elongated shape and consists of two anterior hemispheres, the midbrain and cerebellum, which is only a transverse bridge, and the medulla oblongata. The spinal cord is much more developed than the brain.

From feelings vision, hearing and smell are more developed. The tongue of most amphibians is well developed and in frogs it differs significantly from the tongue of other vertebrates in that it is attached not by the rear, but by the front end and can be thrown out of the mouth.

The teeth, like those of reptiles, are adapted only for grasping and holding prey, but cannot serve to chew it.

Alimentary canal relatively short and simply designed; it consists of a long esophagus, a simple thick-walled stomach and hindgut. All amphibians have a lobed liver, gall bladder, pancreas, kidneys and bladder.

Circulatory and respiratory organs are of great importance in the life of amphibians and will be discussed further in connection with the history of development.

Feature of amphibians lies in the absence of any hard outer coverings, which is why they are called naked reptiles. Indeed, they have neither scales, like fish and reptiles, nor feathers, nor hair, like mammals; the majority are covered on the outside only with bare skin, and only a very few have some traces or semblance of horny formations on the skin. But in the skin of amphibians there are some formations that are not found in other vertebrates.

In the connective tissue layer of the skin of some amphibians there are small capsules filled with a gelatinous substance; in others, voluminous cavities are formed, adapted for the development and initial storage of embryos. Finally, in some people, ossifications or hard plates sometimes appear in the skin, somewhat similar to fish scales. The top layer of skin is very thin and often contains various dyes.

However, the color of some amphibians can change, as we saw in chameleons, and is determined in most cases relative position and the state of special pigments of cells contained in the skin. Contraction or expansion, change in shape, approaching the outer surface of the skin or moving away from it - all give one color or another to the skin and are caused both by changes in external conditions and by internal irritation.

Both in the upper layer of the skin and in the inner layer of all amphibians there are a lot of glands of various sizes and different purposes. The most interesting of them are the poisonous glands. They are located in the lower layer of the skin, have a spherical or oval shape, and secrete a mucous fluid that contains a toxic substance. Amphibians, which have more developed such glands, can voluntarily increase the secretion of these glands and use it as a means of defense. It has now been established that the poisons of some amphibians are very strong, but they are not dangerous for humans and large animals because they are contained in the mucus only in a very minor impurity. However, experiments show that this poison can be fatal to many animals. Injecting toad venom into the blood of small birds quickly kills them; in the same way, the poisonous mucus of toads injected into the blood of puppies, guinea pigs, frogs and newts, has a lethal effect. Some toads, especially salamanders, have very developed mucous glands, from which they can at will cause very copious secretions, even splashing drops of poisonous liquid; that's where it happened popular belief as if the salamander does not burn in fire.

In most amphibians, the initial development of embryos occurs in the same way as in fish. Eggs are usually laid in water in the form of caviar, which is fertilized later, already in water. The eggs are surrounded by a thick layer of gelatinous substance. This shell is of great importance for the embryo, since in this way the egg is protected from drying out, from mechanical damage, and most importantly, it protects them from being eaten by other animals; indeed, very few birds are able to swallow a gelatinous lump of frog caviar; The shell itself also protects the eggs from attacks by fish, shellfish and aquatic insects.

After the embryo completes the initial stages of its development, the larva breaks through the gelatinous membrane, feeding on it, and begins to lead an independent life in the water. The larva has a flat, flattened head, a rounded body and a long paddle-shaped tail, trimmed at the top and bottom with a leathery fin. The original external gills grow on the head in the form of tree-like branched processes. After some time, these gills fall off and internal gills are formed instead. The body gradually narrows even more, the caudal fin increases, and little by little the limbs begin to develop; In frog tadpoles, the hind limbs grow first and then the forelimbs, while in salamanders it is the other way around. Tadpoles at first feed mainly on plant foods, but gradually switch to more and more animal foods. At the same time, changes occur in the organization of the entire body: the tail, which at first is the only organ of movement, loses its importance and shortens as the limbs develop; the intestines become shorter and adapt to the digestion of animal food; The horny plates with which the tadpole's jaws are armed become sharper, gradually disappear and are replaced by real teeth. The ever-shortening tail finally disappears completely - and the tadpole turns into an adult frog.

In the development of the brain and sensory organs of amphibians, there is a great similarity with fish. The heart is formed in the larvae very early and immediately begins to act. Initially it represents a simple bag, which is subsequently divided into separate parts. The aorta passes into the branchial arches and branches first in the external gills, and later in the internal ones. The blood flows back through a vein running along the tail, and then branches on the surface of the yolk sac and returns through the yolk veins back to the atrium. Later, the portal systems of the liver and kidneys gradually form. At the end of the larval stage, gill respiration is gradually replaced by pulmonary respiration; the anterior branchial arches turn into the cephalic arteries, and the middle ones form the aorta.

Amphibians live in all parts of the world and in all zones, with the exception of the polar countries. Water, even more than warmth, is a necessary condition for their existence, since almost all amphibians spend their larval stages in water. They chew exclusively in fresh waters, avoiding salmon or even salt water. Almost half of amphibians spend their entire lives in water, while others settle on land as adults, although they always stay near water and in damp places; There are no completely dry amphibians in areas, but they can live in places where, with general dryness, it rains regularly at certain times. The dry season in such places is spent in hibernation, buried deep in the silt, in temperate zone They are also susceptible to hibernation. Tropical countries, abundant in forests and water, are the most favorable for their life. These are the central parts of South America, Madagascar, the islands of the Malay Archipelago, where virgin, moist forests grow in abundance; on the contrary, Central Asia, Australia and most of inland Africa are very poor in amphibians. All amphibians swim excellently in water, not only in the larval state, but also in the adult state; on land, the tailed ones crawl like reptiles, and the tailless ones move in short, heavy jumps; many of them can even climb trees.

In contrast to reptiles, almost all amphibians are vocal; many of them can even be called songbirds, although their voice is not nearly as pleasant as that of birds. However, only adult males, and not females, can scream and sing, just as all young amphibians can be called mute. The mental abilities of amphibians are no more developed than those of reptiles. According to some researchers, in general they should be ranked among the stupidest of all vertebrates.

Everything that has been said about the relatively insignificant degree of life activity of reptiles is quite applicable to amphibians, which also have cold blood. Public life they have just as little development; however, their concern for offspring is somewhat more noticeable than among reptiles.

Most amphibians are nocturnal, from sunset until morning. During the day, many of them crawl somewhere into cracks or under stones and sit motionless, others take advantage of the sun's warmth and spend the day half asleep.

The food of amphibians changes according to age. The larvae eat all sorts of small organisms, both plant and animal: ciliates, rotifers, microscopic crustaceans and small algae; As they grow, they have a greater and greater need for living food. Adult amphibians are already real predators and pursue all animals that they can overpower, starting with worms and insects and ending with small vertebrates; they even eat larvae of their own species if they are able to swallow them. Most of them are distinguished by great gluttony, which increases with increasing temperature. environment; Thus, in the spring, frogs eat less than in the summer, although they wake up after hibernation very emaciated; in the same way, tropical species are more gluttonous than the inhabitants of temperate countries.

At the beginning of their life they grow very quickly, but over time their growth slows down greatly. Frogs become mature only at 4-5 years of age, but continue to grow for up to 10 years; others reach their present size only after 30 years.

Amphibians are able to endure starvation no less than reptiles; a toad planted in a damp place can remain without food for more than two years.

In the same way, amphibians also have the ability to restore lost parts (regeneration): a broken tail, a cut off finger, and even an entire leg grow back; however, this ability in more highly organized forms noticeably decreases and even completely disappears. Their wounds heal as easily as those of reptiles. In general, the vitality of some amphibians is amazing; tailed amphibians are especially distinguished by this quality. A salamander or newt can be completely frozen in water; in this state they become brittle and show absolutely no signs of life; but only the ice will melt; These animals awaken again and, as if nothing had happened, continue to live. When taken out of water and placed in a dry place, the newt shrinks and becomes a completely lifeless mass. But as soon as this dead lump is thrown into the water, a living newt appears again in complete well-being.

Based on their appearance and degree of organization, amphibians are divided into three orders: tailless, tailed And legless.

Amphibians- a small group of vertebrates, occupying an intermediate position between fish and true terrestrial chordates. The vast majority of amphibians live, depending on the stages of their life cycle, either in water or on land, therefore amphibians are classified as semi-aquatic, semi-terrestrial chordates. This class of terrestrial animals has maintained a very close connection with the aquatic environment.

Adaptation to a land lifestyle is evidenced by paired five-fingered limbs, characteristic of terrestrial animals. Their limbs consist of three sections (the front limb consists of a shoulder, forearm and bone, the hind limb has a thigh, lower leg, and foot). The hand and foot end with fingers. Breathe lightly and with moist skin. They have two circles of blood circulation and a three-chambered heart. They reproduce and develop in water. The larva is equipped with gills. Adult amphibians retain a number of traits inherited from their fish-like ancestors. First of all, this a large number of mucous glands in the skin that help keep it moist. The skin is an important respiratory organ for amphibians, but in a dry state it cannot perform a respiratory function, since oxygen diffusion occurs only through the water film. This explains the richness of the amphibian fauna in warm and humid regions of the globe.

The origin of amphibians from fish is also evidenced by the method of reproduction. Amphibians lay eggs, poor nutrients and unprotected from exposure external environment, as a result of which eggs can only develop in water. Just like fish, amphibians are characterized by external fertilization of eggs. An even greater resemblance to fish is found in the larvae of amphibians - tadpoles. Their respiratory organs are gills, first external, then internal; The larvae have a two-chamber heart and one circulation. The body retains the lateral line organ, the organ of locomotion - the tail, surrounded by a swimming membrane.

pond frog

Adult amphibians, the typical representative of which is pond frog, has a short and wide body. The neck is not expressed. The nostrils are located above the mouth, and slightly behind are the eyes, which have eyelids that protect the eyes from drying out (adaptation to life on land). Behind the eyes are the hearing organs, consisting of the middle ear, closed by the eardrum, and the inner ear. The body rests on two pairs of limbs. The hind ones are the most developed. With their help, the frog moves by jumping on land and swims well. This is also facilitated by the presence of a swimming membrane between the fingers.

Frog skeleton

Frog skeleton consists of a small brain box(certificate underdevelopment brain) and short spine. The skeletons of the limbs consist of three sections, which are mobile due to the connection with the help of joints. The forelimb is attached to the shoulder girdle, which consists of infant, two crow bones, collarbone and two shoulder blades. The hind limbs are connected to the spine using pelvic girdle, formed by fused pelvic bones. The frog's muscles are especially developed in the area of ​​the belts and especially the free limbs.

Digestive system of a frog

Digestive system of a frog very similar to that of fish, only in amphibians hindgut does not open outward, but into its special expansion - cloaca. The cloaca opens ureters And excretory ducts reproductive organs. The frog catches prey using sticky tongue, which is attached to the mouth at the anterior end. The frog usually swallows captured food (insects) whole

Respiratory organs of a frog

Respiratory organs of a frog - lungs And wet skin. Through nostrils air enters the oral cavity, and from there into lungs. Exhalation occurs as a result of contractions of the muscles of the abdominal side of the frog. Mucus-covered skin with a well-developed capillary system promotes cutaneous respiration.

Circulatory system of a frog

Circulatory system of a frog has a more complex structure. Appearance two circles of blood circulation led to the complexity of the structure hearts. It consists of three chambers: ventricle and two atria. The right atrium contains only venous blood saturated with carbon dioxide, and the left atrium contains only arterial blood; the blood mixes in the ventricle. The frog's brain is supplied with arterial, oxygenated blood, and the whole body receives mixed blood. Through the systemic circulation, blood from the ventricle is sent through the arteries to all organs and tissues, and from them flows through the veins into the right atrium. Through the pulmonary circulation, blood from the ventricle enters the lungs and skin, and from the lungs returns to the left atrium.

Frog excretory organs

Frog excretory organs - kidneys, ureters, bladder. The kidneys produce urine, which flows through the ureters into the cloaca, and from it into the bladder. As it fills, urine is removed through the cloaca to the outside.

Nervous system of a frog

Central nervous system amphibians consists of the same sections as in fish, but the forebrain is more developed, it is possible to distinguish cerebral hemispheres. The cerebellum is less developed than in fish, which is due to the simpler and more uniform movements of amphibians.

Reproduction and development of the frog

After awakening from hibernation, frogs leave deep bodies of water, moving to shallow ponds, ditches, puddles and spills well warmed by the sun. melt water. Here the females lay eggs, very similar to fish eggs, and the males water them with seminal fluid. Sperm penetrate the eggs and fertilize them. The shells of the eggs in the water swell greatly, become transparent, stick together, forming lumps, and float to the surface or attach to underwater objects. After fertilization, the larvae begin to develop rapidly, resulting in the formation of multicellular embryo. After 12-25 days, a larva appears from the egg - tadpole.

The tadpole initially has a tail and resembles a fish fry. Its tail is surrounded by a thin swimming membrane. The tadpole breathes with three pairs of feathery gills located on the sides of the head. It has lateral line organs in its skin. The mouth and limbs are initially absent. After some time, a mouth begins to erupt with two horny plates and teeth on the lips, with which the tadpole scrapes off the plants that serve it as food. Then the external gills disappear and internal ones develop. At this stage of development, the tadpole is especially similar to a fish. At this time, he has developed a notochord, a two-chambered heart and one circulation. IN further development lungs, a three-chambered heart, and two circles of blood circulation appear. Next, the hind and fore limbs appear. First it thins out and then shortens, then the tail completely disappears, and the tadpole turns into a small frog. This process lasts 3-4 months and is called metamorphosis. Sexual maturity in frogs occurs in the third year of life.

Seasonal natural phenomena influence life cycle amphibians. So, they have an annual cycle due to seasonal conditions climate change is divided into the following periods: spring awakening, spawning period(reproduction), summer activity period And hibernation , hibernation can be terrestrial (newts) or underwater (frogs).