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The frog is a typical representative of amphibians. Using this creature as an example, you can easily study the most important characteristics of the class. To do this, we will look at the internal and external structure of the lake frog, which will further help us understand the structural features of all amphibians.

External structure

The external structure of a frog and its habitat are two inextricably linked links in a single chain. It’s no secret that the appearance and structure are determined by the place and way of life of the animal.

The lake frog lives on the banks and in reservoirs, which is why it got its name. However, a river, swamp, pond and other places can also serve as a place of residence.

The external structure of the frog is very simple. The wide flat head turns into a small body with a reduced tail. The frog has very short forelimbs equipped with four fingers. The hind limbs, on the contrary, are elongated and have five fingers.

Covers of an animal

Speaking about the external structure of a frog, it is necessary to pay attention to its skin. The body of an amphibian creature is covered with smooth skin, on the surface of which there is an impressive number of glands that constantly secrete mucus. The secretion lubricates the surface, helping to retain moisture and promoting gas exchange. Mucus also protects the animal from penetration of harmful microorganisms into the body.

The amphibian's thin skin protects the body and also participates in gas exchange. Features of the external structure of the frog are associated with its lifestyle. For example, water enters the animal’s body only through the skin. It is for this reason that the frog must spend most of its time in dampness or water.

Characteristics of the external structure of the frog: body parts

Speaking about the external structure of the frog, we can distinguish the following parts of the body - the hind and forelimbs, the head and the torso. One of the features of the amphibian is the almost complete absence of a neck. In general, the separation line between the body and the head is not clear, or rather, it is practically non-existent. The frog's body is slightly larger than its head. The animal completely lacks a tail.

The large head has large bulging eyes. They are covered by clear eyelids designed to prevent damage, drying out and clogging.

Below the eyes are the nostrils. In general, it is worth noting that the eyes and nostrils are located close to each other at the top of the head for a reason. The fact is that while swimming they are constantly above the surface of the water. Thanks to this, the animal can breathe and see everything that happens above the lake.

The upper jaw of frogs is armed with a row of small teeth. But they don't have ears. Their role is played by the eardrums located behind the animal’s eyes.

Animal color

Describing the external structure of the frog, let us pay attention to its color, which is largely determined by its habitat. Most amphibian representatives mimic external nature. And some varieties even have special cells that can change the color of the skin depending on environmental conditions. Therefore, the color of the animal often repeats the pattern of the places where the creature lives.

In the tropics there are amphibians with bright colors. This indicates that the animal is very poisonous. This is what scares away enemies from such frogs.

Amphibian limbs

When presenting the external structure of the lake frog, we mentioned the unusual limbs of the animal. The peculiarity of their structure is due to the fact that the front legs are used for landing and for support in a sitting position. The hind limbs are much stronger and longer than the forelimbs. It is the hind legs that are used to move through water or on land. There are membranes on the limbs of the animal, which greatly facilitate movement in the water. The external structure of the frog is such that it allows the animal to move not only in water, but also on land.

Some representatives can even climb trees or glide in the air. Frogs have a lot of adaptations depending on their habitat. Nature has endowed some with special suction cups, thanks to which they stick to any surface, while others can remain under layers of sand or soil for a long time. In all these cases, their strong limbs help amphibians move.

The skeleton of a frog, according to biologists, is very similar to the skeleton of a perch. However, due to the nature of the lifestyle, there are significant differences. As we know, frogs have limbs. The front ones are attached to the spine with the help of peculiar bones. But the rear ones are connected to the spine with the help of the hip bones. The amphibian representative is much simpler than that of fish, since it has fewer bones.

The frog's spine consists of a total of nine vertebrae. Moreover, it is divided into four sections: cervical, sacral, caudal and trunk. But the animal has no ribs.

Digestive system

Frogs and toads feed on insects and small invertebrates. Such animals are classified as predators. Sometimes they even disdain their relatives. Frogs guard their prey in a secluded place, motionless. As soon as the prey approaches, they shoot out their tongue and consume the prey.

Amphibian Blood

Blood is a connective tissue that plays a very important role in any organism. It is she who transports metabolic products and nutrients. As you know, blood consists of red blood cells and white blood cells.

The external structure and shape of frog red blood cells differs from human red blood cells. In the animal they are oval and have nuclei. But in humans, red blood cells are biconcave and lack nuclei. This increases the area occupied by oxygen molecules. This means that the human circulatory system is more perfect. In addition, frog red blood cells are larger and there are much fewer of them than in human blood. All this suggests that amphibians require oxygen in much smaller quantities than mammals. And the reason for this is the ability to absorb part of the necessary oxygen by the surface of the skin.

The respiratory system of frogs also has its own characteristics. Amphibians breathe not only with the help of their lungs. The skin also plays an active role in this process. As you can see, external integuments play a vital role in the life of amphibians. The lungs of an animal are thin-walled paired sacs that have a cellular internal structure and a highly branched network of blood vessels.

How does breathing occur? Amphibians use valves that close and open their nostrils. During inhalation, the nostrils open and the oropharyngeal cavity descends. This is how air gets inside. In order for it to get further into the lungs, the nostrils close, and the bottom of the oropharynx, on the contrary, rises. Exhalation is carried out due to the work of the walls of the lungs and the movement of the abdominal muscles.

Sense organs

The sense organs of amphibians have their own characteristics, due to the fact that animals also live on land. The lateral line organs help the frog navigate in space. The largest number of them is located on the head. Outwardly, they resemble two stripes that run along the body.

Temperature and pain receptors are located on the skin of amphibians. The nose as a tactile organ only works if it is located above the water. In water, the nostrils are always closed. Even the respiratory system is fully adapted to the amphibian’s habitat both in water and on land.

Nervous system of a frog

But the nervous system of amphibians is not very developed. It is in many ways similar to the nervous system of fish. In this regard, amphibians are not far along the evolutionary ladder. The amphibian brain consists of five sections: the anterior, middle, intermediate, medulla oblongata, and cerebellum. By the way, the latter is poorly developed due to the animal’s sedentary lifestyle. But the forebrain consists of two hemispheres and, on the contrary, is quite developed.

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, there are 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 that are poor in nutrients and unprotected from the external environment, as a result of which the 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(evidence of poor brain development) 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

Excretory organs of the frog - 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 of 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 melt water well-warmed by the sun. 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 the life cycle of amphibians. Thus, their annual cycle, due to the conditions of seasonal climate changes, is divided into the following periods: spring awakening, spawning period(reproduction), summer activity period And hibernation, hibernation can be terrestrial (newts) or underwater (frogs).

Frogs are one of the most numerous species of amphibians. Features of the external and internal structure of frogs are characteristic of most individuals from this class.

External structure of a frog

The frog's body is short, the large flat head without sharp boundaries merges into the body. Unlike fish, the head of amphibians is movably articulated with the body. Although the frog does not have a neck, it can tilt its head slightly.
Two large bulging eyes are noticeable on the head, protected by eyelids: leathery - upper and transparent movable - lower. The frog blinks frequently, while the moist skin of the eyelids moistens the surface of the eyes, protecting them from drying out. This feature developed in the frog in connection with its terrestrial lifestyle. (Fish, whose eyes are constantly in the water, do not have eyelids.) A pair of nostrils is visible on the head in front of the eyes. These are not only the openings of the olfactory organs. The frog breathes atmospheric air, which enters its body through its nostrils. The eyes and nostrils are located on the upper side of the head. When the frog hides in the water, it puts them out. At the same time, she can breathe atmospheric air and see what is happening outside the water. Behind each eye on the frog's head there is a small circle covered with skin. This is the outer part of the hearing organ - the eardrum. The inner ear of a frog, like that of fish, is located in the bones of the skull.
The frog has well-developed paired limbs - front and hind legs. Each limb consists of three main sections. The front leg is divided into: shoulder, forearm and hand. The frog's hand ends with four fingers (its fifth finger is underdeveloped). In the hind limb, these sections are called the thigh, tibia and foot. The foot ends in five toes, which in the frog are connected by a swimming membrane. The sections of the limbs are movably articulated with each other using joints. The hind legs are much longer and stronger than the front legs; they play a major role in movement. A sitting frog rests on slightly bent forelimbs, while the hind limbs are folded and located on the sides of the body. Quickly straightening them, the frog makes a jump. The front legs protect the animal from hitting the ground. The frog swims by pulling and straightening its hind limbs, while pressing its front limbs to its body.
All modern amphibians have naked skin. In a frog, it is always moist thanks to the liquid mucous secretions of the skin glands. Water from the environment enters the frog's body through the skin and through food. The frog never drinks.

Frog skeleton

Unlike fish, the frog has a cervical vertebra. It is movably articulated with the skull. It is followed by the trunk vertebrae with lateral processes (the frog's ribs are not developed). The cervical and trunk vertebrae have superior arches that protect the spinal cord. At the end of the spine of the frog and all other tailless amphibians is a long tail bone. In newts and other tailed amphibians, this section of the spine consists of a large number of flexibly articulated vertebrae.
A frog's skull has fewer bones than a fish's skull. Due to pulmonary respiration, the frog does not have gills.
The skeleton of the limbs corresponds to their division into three sections and is connected to the spine through the bones of the limb girdles. The girdle of the forelimbs - the sternum, two crow bones, two clavicles and two shoulder blades - has the appearance of an arc and is located in the thickness of the muscles. The hind limb girdle is formed by fused pelvic bones and is attached tightly to the spine. It serves as a support for the hind limbs.

Internal structure of a frog

Muscles

The structure of the frog's muscular system is much more complex than that of fish. After all, the frog not only swims, but also moves on land. Through contractions of muscles or muscle groups, the frog can perform complex movements. Her limb muscles are especially well developed.

Digestive system

The digestive system of amphibians has almost the same structure as that of fish. Unlike fish, the hindgut does not open directly outward, but into a special extension called the cloaca. The ureters and excretory ducts of the reproductive organs also open into the cloaca.

Respiratory system

The frog breathes atmospheric air. The lungs and skin are used for breathing. The lungs look like bags. Their walls contain a large number of blood vessels in which gas exchange occurs. The frog's throat is pulled down several times per second, creating a rarefied space in the oral cavity. Then the air penetrates through the nostrils into the oral cavity, and from there into the lungs. It is pushed back under the action of the muscles of the body walls. The frog's lungs are poorly developed, and skin respiration is as important for it as pulmonary respiration. Gas exchange is possible only when the skin is wet. If a frog is placed in a dry vessel, its skin will soon dry out and the animal may die. Immersed in water, the frog completely switches to skin respiration.

Circulatory system

The frog's heart is located in the front of the body, under the sternum. It consists of three chambers: the ventricle and two atria. Both atria and then the ventricle contract alternately. In the heart of a frog, the right atrium contains only venous blood, the left - arterial, and in the ventricle the blood is mixed to a certain extent.
The special arrangement of the vessels originating from the ventricle leads to the fact that only the frog’s brain is supplied with pure arterial blood, while the whole body receives mixed blood.
In a frog, blood from the ventricle of the heart flows through the arteries to all organs and tissues, and from them through the veins it flows into the right atrium - this is a large circle of blood circulation. In addition, blood flows from the ventricle to the lungs and skin, and from the lungs back to the left atrium of the heart - this is the pulmonary circulation. All vertebrates, except fish, have two circles of blood circulation: small - from the heart to the respiratory organs and back to the heart; large - from the heart through the arteries to all organs and from them back to the heart.

Metabolism

Metabolism in amphibians is slow. The frog's body temperature depends on the ambient temperature: it rises in warm weather and falls in cold weather. When the air becomes hot, the frog's body temperature decreases due to the evaporation of moisture from the skin. Like fish, frogs and other amphibians are cold-blooded animals. Therefore, when it gets colder, frogs become inactive, and during the winter they go into hibernation.

Central nervous system and sensory organs

The forebrain is more developed than in fish, and two swellings can be distinguished in it - the cerebral hemispheres. Amphibians' bodies are close to the ground and they do not have to maintain balance. In this regard, the cerebellum, which controls the coordination of movements, is less developed in them than in fish.
The structure of the sense organs corresponds to the terrestrial environment. For example, by blinking its eyelids, a frog removes dust particles stuck to the eye and moistens the surface of the eye. Like fish, the frog has an inner ear. However, sound waves travel much worse in air than in water. Therefore, for better hearing, the frog also has a middle ear. It begins with the sound-receiving eardrum - a thin round film behind the eye. From it, sound vibrations are transmitted through the auditory bone to the inner ear.

Reproduction and development of amphibians

Reproductive organs

The reproductive organs of amphibians are very similar in structure to the reproductive organs of fish. All amphibians are dioecious.

Spawning

After spending the winter in a state of torpor, amphibians wake up with the first rays of the spring sun and soon begin to reproduce. Males of some species of frogs croak loudly. The amplification of sounds is facilitated by special bags - resonators, which, when croaking, swell on the sides of the male’s head. When breeding, animals split into pairs. The germ cells enter the cloaca through tubular ducts and are thrown out from there. Female amphibians lay eggs in the water, similar to fish eggs. Males release fluid containing sperm onto her.

Development

After some time, the shell of each egg swells and turns into a gelatinous transparent layer, inside of which the egg is visible. The upper half is dark and the lower half is light: the dark part of the egg uses the sun's rays better and heats up more. Clumps of eggs in many frog species float to the surface where the water is warmer.
Low temperature retards development. If the weather is warm, the egg divides repeatedly and develops into a multicellular embryo. After one or two weeks, the frog larva, a tadpole, hatches from the egg. Outwardly, it resembles a small fish with a large tail. The tadpole first breathes through external gills (in the form of small tufts on the sides of the head). Soon they are replaced by internal gills. The tadpole has one circulation and a two-chambered heart; a lateral line is visible on the skin. Thus, amphibian larvae have some structural features of fish.
During the first days, the tadpole lives off the nutritional reserves of the eggs. Then a mouth appears, equipped with horny jaws. The tadpole begins to feed on algae, protozoa and other aquatic organisms. The hotter the weather, the faster the tadpole changes. First his hind legs appear, then his front legs. The lungs are developing. The tadpole begins to rise to the surface of the water and swallow air. The tail gradually shortens, the tadpole becomes a young frog and comes ashore. From the moment the eggs are laid until the end of the transformation of the tadpole into a frog, about 2-3 months pass. Baby frogs, like adult frogs, eat animal food. They can reproduce from the third year of life.

The frog is a typical representative of amphibians. Using this animal as an example, you can study the characteristics of the entire class. This article describes in detail the internal structure of a frog.

The digestive system begins with the oropharyngeal cavity. At its bottom is attached a tongue, which the frog uses to catch insects. Thanks to its unusual structure, it is capable of being thrown out of its mouth at high speed and sticking its victim to itself.

On the palatine bones, as well as on the lower and upper jaws of the amphibian, there are small conical teeth. They do not serve for chewing, but primarily for holding prey in the mouth. This is another similarity between the amphibian and fish. The secretion secreted by the salivary glands moistens the oropharyngeal cavity and food. This makes it easier to swallow. Frog saliva does not contain digestive enzymes.

The frog's digestive tract begins with the pharynx. Next comes the esophagus, and then the stomach. Behind the stomach is the duodenum, the rest of the intestine is laid out in the form of loops. The intestine ends in the cloaca. Frogs also have digestive glands - the liver and pancreas.

The prey caught with the help of the tongue ends up in the oropharynx, and then through the pharynx enters the esophagus into the stomach. Cells located on the walls of the stomach secrete hydrochloric acid and pepsin, which help digest food. Next, the semi-digested mass follows into the duodenum, into which the secretions of the pancreas also flow and the bile duct of the liver flows.

Gradually, the duodenum passes into the small intestine, where all useful substances are absorbed. The remains of food that has not been digested end up in the last section of the intestine - the short and wide rectum, ending in the cloaca.

The internal structure of the frog and its larvae are different. Adults are predators and feed mainly on insects, but tadpoles are true herbivores. On their jaws there are horny plates, with the help of which the larvae scrape off small algae along with the single-celled organisms living in them.

Respiratory system

Interesting features of the internal structure of the frog also concern breathing. The fact is that, along with the lungs, the capillary-filled skin of the amphibian plays a huge role in the process of gas exchange. The lungs are thin-walled paired bags with a cellular inner surface and an extensive network of blood vessels.

How does a frog breathe? The amphibian uses valves capable of opening and closing its nostrils and movements of the floor of the oropharynx. In order to inhale, the nostrils open, and the bottom of the oropharyngeal cavity drops, and the air ends up in the frog's mouth. To allow it to pass into the lungs, the nostrils close and the floor of the oropharynx rises. Exhalation occurs due to the collapse of the pulmonary walls and movements of the abdominal muscles.

In males, the laryngeal cleft is surrounded by special arytenoid cartilages, on which the vocal cords are stretched. High sound volume is ensured by the vocal sacs, which are formed by the mucous membrane of the oropharynx.

Excretory system

The internal structure of the frog, or rather, it is also very curious, since the waste products of the amphibian can be excreted through the lungs and skin. But still, most of them are secreted by the kidneys, which are located at the sacral vertebra. The kidneys themselves are oblong bodies adjacent to the back. These organs have special glomeruli that are capable of filtering waste products from the blood.

Urine is discharged through the ureters into the bladder, where it accumulates. After the bladder is filled, the muscles at the ventral surface of the cloaca contract and fluid is expelled through the cloaca.

Circulatory system

The internal structure of the frog is more complex than that of an adult frog; it is three-chambered, consisting of a ventricle and two atria. Due to the single ventricle, arterial and venous blood are partially mixed, the two circulation circles are not completely separated. The conus arteriosus, which has a longitudinal spiral valve, extends from the ventricle and distributes mixed and arterial blood into different vessels.

Mixed blood collects in the right atrium: venous blood comes from the internal organs, and arterial blood comes from the skin. Arterial blood enters the left atrium from the lungs.

The atria contract simultaneously, and blood from both enters a single ventricle. Due to the structure of the longitudinal valve, it enters the organs of the head and brain, mixed - to organs and parts of the body, and venous - to the skin and lungs. Students may have a hard time understanding the internal structure of a frog. A diagram of the amphibian circulatory system will help you visualize how blood circulation works.

The circulatory system of tadpoles has only one circulation, one atrium and one ventricle, like in fish.

The structure of the blood of a frog and a person is different. have a core, oval shape, and in humans they have a biconcave shape, with no core.

Endocrine system

The endocrine system of the frog includes the thyroid, reproductive and pancreas glands, adrenal glands and pituitary gland. The thyroid gland produces hormones necessary to complete metamorphosis and maintain metabolism; the gonads are responsible for reproduction. The pancreas is involved in the digestion of food, the adrenal glands help regulate metabolism. The pituitary gland produces a number of hormones that affect the development, growth and coloring of the animal.

Nervous system

The nervous system of the frog is characterized by a low degree of development; it is similar in characteristics to the nervous system of fish, but has more progressive features. The brain is divided into 5 sections: midbrain, diencephalon, forebrain, medulla oblongata and cerebellum. The forebrain is well developed and is divided into two hemispheres, each of which has a lateral ventricle - a special cavity.

Due to monotonous movements and a generally sedentary lifestyle, the cerebellum is small in size. The medulla oblongata is larger. In total, ten pairs of nerves emerge from the frog's brain.

Sense organs

Significant changes in the sensory organs of amphibians are associated with the exit from the aquatic environment to land. They are already more complex than those of fish, since they must help navigate both in water and on land. Tadpoles have developed lateral line organs.

Pain, tactile and temperature receptors are hidden in the epidermis layer. Papillae on the tongue, palate and jaws serve as taste organs. The olfactory organs consist of paired olfactory sacs, which open through both the external and internal nostrils into the environment and the oropharyngeal cavity, respectively. In water, the nostrils are closed, the sense of smell does not function.

As a hearing organ, the middle ear is developed, in which there is an apparatus that amplifies sound vibrations thanks to the eardrum.

The structure of a frog's eye is complex, because it needs to see both underwater and on land. The eyes of adults are protected by movable eyelids and a nictitating membrane. Tadpoles do not have eyelids. The cornea of ​​a frog's eye is convex, the lens is biconvex. Amphibians can see quite far and have color vision.

In the upper part of the body cavity lies a three-chambered heart (Fig. 38). The dark-colored atria and lighter ventricle are clearly visible.

On the sides of the heart lie dark gray thin-walled lungs. As a rule, they collapse when opened and are therefore difficult to see. It is necessary to note the thin-walledness of the lung sacs, the weak cellularity of their surface and the network of blood vessels in their walls.

Below the heart is a large three-lobed liver. A greenish-brown gall bladder is visible between the lobes of the liver.

Figure 38 – Dissected frog

1 – heart; 2 – lung; 3 – liver; 4 – gallbladder; 5 – stomach; 6 – pancreas; 7 – duodenum; 8 – small intestine; 9 – rectum; 10 – spleen; 11 – cloaca; 12 – bladder; 13 – kidney; 14 – ureter; 15 – right ovary (left ovary removed); 16 – fat body; 17 – right oviduct; 18 – uterine section of the oviduct;

19 – dorsal aorta; 20 – posterior vena cava; 21 – carotid artery;

22 – left aortic arch; 23 – pulmonary artery.

Under the liver, on the left side of the body, is the stomach, which passes into the duodenum. In the loop between the duodenum and stomach, a small orange-yellow pancreas is attached to the mesentery.

The duodenum passes into the small intestine, which is curled into a ball. The rectum is very clearly defined. On the mesentery, approximately at the level of the anterior edge of the rectum, lies the burgundy-colored spleen. Above the rectum, at the point where it exits into the cloaca, there is a transparent, two-lobed bladder (often upon opening it is damaged, collapses and is difficult to see).

The kidneys are located on the dorsal side of the abdominal cavity and are covered by the intestines, and in female frogs, by the genitals.

Lifting the intestines (and ovaries in females) with tweezers, we will see the kidneys and the fat bodies (genital appendages) lying in front of them, which are represented by multi-lobed flat formations.

If the male is opened, then under the intestines we find a pair of oval testes. In a sexually mature female, the entire posterior part of the body cavity is occupied by ovaries filled with eggs (spawn) and long oviducts rolled into a complex ball. It should be emphasized that the reproductive system of females is usually so developed that it even covers the intestines, so to examine the latter it is necessary to move the ovaries and oviducts to the sides.

Compared to bony fish, the digestive system of amphibians is characterized by further complexity and differentiation.

The digestive tube begins with an oral slit leading into the oropharyngeal cavity (the latter was studied during an external examination of the frog). It should only be recalled that the tongue is placed in this cavity. The ducts of the salivary glands, which first appear in amphibians, open into it. However, in frogs these glands serve only to wet the food bolus and are not yet involved in the chemical processing of food.

The oropharyngeal cavity passes into a short but wide esophagus (Fig. 39), and the latter into a relatively voluminous stomach, which has a slightly curved shape.

The pyloric part of the stomach, bending strongly, passes into the duodenum, which is the beginning of the small intestine. As already indicated, the pancreas lies in the loop between the stomach and duodenum. The small intestine forms many bends and loops and smoothly passes into the large intestine, which ends in a clearly visible rectum. The rectum opens into the cloaca. The entire intestine is suspended from the walls of the cavity on special folds of the peritoneum - the mesentery.

The digestive glands - liver with gall bladder and pancreas - are well developed. The liver ducts, together with the gallbladder duct, open into the duodenum. The pancreatic ducts flow into the gallbladder duct, so this gland does not have independent communication with the intestines.

Figure 39 – Frog intestine

1 – esophagus; 2 – stomach; 3 – duodenum; 4 – small intestine; 5 – rectum; 6 – cloaca; 7 – the place where the rectum flows into the cloaca; 8 – bladder.

The respiratory organs of amphibians are of a completely different type than those of fish. They are represented by light ones - two thin-walled oval-shaped bags with narrow lower ends (Fig. 38). The inner surface of the lungs is slightly honeycombed. However, as a result of the imperfection of the lungs (small oxidation surface), the skin plays an important role in breathing. For example, in green frogs, over 50% of the oxygen necessary for blood oxidation passes through the skin. In connection with pulmonary breathing, internal nostrils, or choanae, appear, connecting the nasal cavity with the oropharyngeal cavity.

The airways are very short due to the absence of the cervical spine. They are represented by the nasal and oropharyngeal cavities, as well as the larynx. The larynx opens directly into the lungs with two openings.

The circulatory system of amphibians has undergone significant transformations and is significantly different from that of fish. In connection with the appearance of the lungs, a second circle of blood circulation and a three-chambered heart arose (Fig. 40).

The frog has a three-chambered heart; it consists of the right and left atria and a ventricle. The right atrium is more voluminous - blood from the whole body collects through the veins into it, while the left atrium receives blood only from the lungs.

The ventricle is thick-walled, its inner surface is covered with numerous projections, between which there are pocket-like depressions.

Figure 40 – Diagram of an opened frog heart

1 – right atrium; 2 – left atrium; 3 – ventricle; 4 – valves that close the common opening leading from both atria to the ventricle; 5 – arterial cone; 6 – common arterial trunk;

7 – pulmonary cutaneous artery; 8 – aortic arch; 9 – common carotid artery; 10 – carotid gland; 11 – spiral valve of the arterial cone.

In addition to the indicated main parts of the heart, there is a venous sinus (sinus), which communicates with the right atrium, and an arterial cone extending from the right side of the ventricle.

Three pairs of arterial vessels (arterial arches) depart from the conus arteriosus. All three vessels (arches) of the left and right sides first go through a common arterial trunk, surrounded by a common membrane, and then branch (Fig. 40 and 41).

The vessels of the first pair (counting from the head) are called the carotid arteries. The carotid arteries carry blood to the head. These vessels depart from the common arterial trunk in the form of the common carotid arteries, each of which almost immediately splits into the external and internal carotid arteries (Fig. 41). At the site of their separation lies the carotid gland, which regulates blood pressure in the carotid arteries.

Figure 41 – Diagram of the arterial system of a frog

1 – ventricle; 2 – right atrium; 3 – left atrium;

4 – arterial cone; 5 – common carotid artery; 6 – aortic arch;

7 – subclavian artery; 8 – dorsal aorta; 9 – iliac artery; 10 – femoral artery; 11 – sciatic artery; 12 – enteromesenteric artery; 13 – pulmonary artery; 14 – cutaneous arteries;

15 – carotid gland; 16 – external carotid artery;

17 – internal carotid artery (arteries with venous blood are painted black, arteries with arterial and mixed blood are shaded).

Through the vessels of the second pair - the aortic arches - blood is directed to the back of the body. The arches go around the heart on the right and left sides, respectively, and merge under the spine into a common trunk - the dorsal aorta. Subclavian arteries depart from the aortic arches, carrying blood to the forelimbs (Fig. 41).

Through the vessels of the third pair - the pulmonary arteries - blood is sent to the lungs. A large cutaneous artery branches off from each pulmonary artery, through which blood is directed to the skin for oxidation (Fig. 41).

From the dorsal aorta, blood is carried through a series of arteries to the internal organs and hind limbs.

Venous blood (Fig. 42) from the anterior end of the body is collected through two pairs of jugular veins. The latter, merging with the cutaneous veins, which have already absorbed the subclavian veins, form two anterior vena cava. These veins carry mixed blood into the venous sinus, since oxygenated arterial blood moves from the skin through the cutaneous veins.

Blood from the hind limbs and back of the body moves through the iliac veins to the kidneys, where it passes through the portal system. The vessels leaving the kidneys merge to form the powerful posterior vena cava. Through the posterior vena cava, blood is directed to the venous sinus, from which it then enters the right atrium (Fig. 42).

From the intestines, blood is collected by the subintestinal vein, which flows into the liver, where the portal system functions. Blood also passes through the portal system of the liver from the abdominal vein, which carries it from the hind limbs. From the liver, blood flows through the hepatic veins into the posterior vena cava (Fig. 42).

From the lungs, blood moves through the pulmonary veins to the left atrium.

The blood circulation in the heart of a frog can be schematically represented as follows. Mixed blood enters the right atrium, and arterial blood (from the lungs) enters the left atrium. When the atria contract, blood flows through the common opening into the ventricle. This is where further mixing of the blood occurs. However, venous blood predominates in the right part of the ventricle, and arterial blood predominates in the left. The opening leading from the ventricle to the conus arteriosus is located on the right side of the ventricle. Therefore, when the ventricle contracts, the first portion of blood, containing more venous blood, enters the opening of the nearest pulmonary arch, the next portion, with a predominance of arterial blood, enters the aortic arch, and the portion with the least content of venous blood enters the carotid arteries.

Figure 42 – Diagram of the venous system of a frog

1 – venous sinus; 2 – external jugular vein; 3 – internal jugular vein; 4 – great cutaneous vein; 5 – subclavian vein;

6 – anterior vena cava; 7 – posterior vena cava; 8 – femoral vein;

9 – sciatic vein; 10 – iliac vein; 11 – renal portal system; 12 – subintestinal vein; 13 – portal system of the liver;

14 – hepatic veins; 15 – abdominal vein; 16 – pulmonary vein (veins with arterial blood are shaded).

The excretory organs (Fig. 43, 44) are represented in amphibians, as well as in fish, by the trunk kidneys (mesonephros). They are elongated, compact, reddish-brown bodies that lie on the sides of the spine. From each kidney a thin Wolffian canal stretches to the cloaca. In female Wolffrogs, the canal serves only as an excretory duct, or ureter, while in males it simultaneously serves as the genital duct, or vas deferens. In the cloaca, the Wolffian canals open with independent openings. It also opens separately into the cloaca and bladder. Urine enters first into the cloaca, and from it into the bladder. After filling the latter, through the same hole, urine is discharged again into the cloaca, and then out.

The reproductive organs of amphibians are represented by paired gonads. In males, these are oval-shaped testes, attached by a special mesentery to the anterior part of the kidneys (Fig. 43). Thin seminiferous tubules stretch from the testes to the kidneys. Sexual products from the testis are sent through these tubules to the bodies of the kidneys, then into the already known Wolffian canals and through them to the cloaca. Before flowing into the cloaca, the Wolffian canals form small expansions - seminal vesicles, which serve to temporarily reserve sperm.

The ovaries of females (Fig. 44) are thin-walled sacs, filled with eggs in adults. In the lateral parts of the body cavity there are highly convoluted light oviducts, or Müllerian canals. These genital canals are not directly connected to the ovaries; they open through small funnels near the lungs into the body cavity. Before flowing into the cloaca, each oviduct expands into the so-called “uterus”. Mature eggs fall out through breaks in the walls of the ovary into the body cavity, then are captured by the funnels of the oviducts and move along them to the cloaca. Thus, in females the excretory and reproductive ducts are completely separated.

The central nervous system in amphibians is represented, like in all vertebrates, by the brain and spinal cord (Fig. 45).

Figure 45 – Frog brain top and bottom

1 – cerebral hemispheres of the forebrain; 2 – olfactory lobe;

3 – olfactory nerve; 4 – diencephalon; 5 – visual chiasm; 6 – funnel; 7 – pituitary gland; 8 – optic lobes of the midbrain;

9 – cerebellum; 10 – medulla oblongata; 11 – spinal cord.

Compared to fish, the amphibian brain has a number of progressive features. This mainly concerns the forebrain, which is relatively larger than that of fish, its hemispheres are completely separated, and the nervous substance lines, in addition to the bottom of the lateral ventricles, also the sides and the roof, i.e. Amphibians have a true primary medullary vault, the archipallium.

The frog's brain consists of five sections (Fig. 45). In front is the forebrain, consisting of two elongated hemispheres separated by a deep fissure. In front of the hemispheres there is a common olfactory lobe, from which two olfactory nerves originate. Behind the forebrain is the diencephalon. On its roof is the epiphysis (endocrine gland). The midbrain is presented in the form of two rounded optic lobes. Behind the optic lobes lies the underdeveloped cerebellum. Immediately behind it is the medulla oblongata with the rhomboid fossa (the fourth cerebral ventricle). The medulla oblongata gradually transforms into the spinal cord.

On the lower side of the brain we find the optic chiasm, or chiasm, extending from the bottom of the diencephalon, the infundibulum and the pituitary gland.

Amphibian skeleton

Theoretical information: