Amphibians, or amphibians, belong to the most primitive of all existing land vertebrates. They occupy an intermediate place between aquatic and land animals for the reason that their reproduction and development takes place in one environment (aquatic), and the residence of adult individuals in another (on land). These are unusual and sometimes amazing creatures.

The article provides information about the origin of the class Amphibians, general characteristics of the class, structure and taxonomy.

Prerequisites for the occurrence

The conditions necessary for massive land development developed in the middle approximately 385 million years ago. This happened thanks to the establishment of warm and humid climate, the presence of a good food supply (invertebrate fauna). In addition, during this period, a lot of organic matter entered water bodies on Earth as a result of leaching, and this led to the oxidation of oxygen in water and, consequently, a decrease in its concentration. Fish have adapted to this by breathing atmospheric air.

Evolution: in brief

The ancestors of the modern class, which are given below in the text, appeared at the end of the Devonian in fresh water bodies, they are called ichthyostegids. They were a transitional form between lobe-finned fish and true amphibians.

The transition from an aquatic to a terrestrial lifestyle was accompanied by two decisive adaptive changes: movement on a solid substrate and breathing of atmospheric air. In other words, gill respiration gradually gave way to pulmonary respiration, and the fins transformed into five-fingered limbs. In parallel, there was a transformation of all other organs: the nervous system and senses.

In the Carboniferous, the second group of primitive amphibians appeared - lepospondyles. They were smaller in size and perfectly adapted to life in an aquatic environment, and some species lost their limbs for the second time. Modern amphibians took shape only at the end of the Mesozoic period.

Taxonomy

Modern taxonomy has divided amphibians broadly into three subclasses: Labyrinthodonts, Thin-vertebrates and Lamps. The first two groups of animals became extinct in the early Mesozoic and Paleozoic, respectively. All modern amphibians, which are more than 6,700 species, belong to the subclass Lamp-Armored. It, in turn, is divided into three orders that appeared in the Jurassic period.

  • Tailless. Currently there are 5602 species, united in 48 families. All known frogs and toads belong to this order.
  • Legless or caecilian (pictured above). The smallest order, represented by 190 species, united in 10 families.
  • This order includes salamanders and newts, with a total of about 570 species (10 families).

Class Amphibians: general characteristics and structural features

Amphibians have smooth and thin skin, which has relatively good permeability to gases and liquids. Its structure has features characteristic of vertebrates. The skin itself (corium) and the superficial multilayered epidermis are distinguished. It is abundantly supplied with mucus-secreting glands. The secret can be of two types: poisonous and improving gas exchange. Horny formations or ossifications on the skin of amphibians are rare.

The body has a tail (in caudates), a torso and a head (movable joint), as well as five-fingered limbs. The spine is divided into four sections: trunk, sacral, caudal and cervical. The number of vertebrae can vary: from 7 to 200.

Giving a brief general description of the class of amphibians, one cannot fail to mention the muscles of these vertebrates: the trunk and limbs. The first is segmented. Special muscles ensure the performance of complex movements with lever limbs. The lever and depressor muscles are located on the head. For example, the fire salamander, like other tailed representatives, has well-developed tail muscles.

Respiratory system

Many people probably remember some information about the structure of the respiratory system from a school biology course (they study the general characteristics of amphibians in 7th grade).

The main respiratory organ in amphibians is the lungs. Most species have them, with the exception of lungless salamanders and the Kalimantan barburula frog. The lungs are small in volume and look like thin-walled sacs, enveloped in a dense network of blood vessels. Each of them opens into the laryngeal-tracheal cavity with an independent opening. Breathing occurs by increasing and decreasing the volume of the oropharyngeal cavity.

Additional respiratory organs are the mucous membrane and skin lining the oropharyngeal cavity. Some aquatic species, as well as tadpoles, breathe through gills.

Circulatory system

Of course, the greatest attention when studying the general characteristics of amphibians in the 7th grade of school was paid to the circulatory system. By studying various groups of vertebrate animals, its evolution was traced from the most primitive to the highly developed, characteristic of mammals.

Amphibians have a closed circulatory system with blood mixing occurring in the ventricle. The body temperature of amphibians depends on environment, they belong to cold-blooded animals.

The circulatory system of amphibians is characterized by two circles of blood circulation: small and large. The appearance of the first is due to the “acquisition” of breathing through the lungs. The heart is divided into two atria and one ventricle. Venous blood flows to the lungs and skin through the cutaneous pulmonary artery, and arterial blood flows to the head through the carotid artery. Mixed blood is supplied to the rest of the body by the aortic arches.

Digestive system

All representatives of the amphibian class, the general characteristics of which are given in the article, eat only mobile prey. The tongue is located at the bottom of the oropharyngeal cavity. In tailless species it is attached to the lower jaws. The tongue is used to catch insects, it is thrown out of the mouth and the prey sticks to its surface. The teeth present on the jaws serve only to hold food.

The secretion of the salivary glands, the ducts of which open into the oropharyngeal cavity, does not contain digestive enzymes. Food enters the duodenum through the stomach. It is into it that the ducts of the pancreas and liver are opened. The small intestine opens into the rectum, which forms an extension called the cloaca.

Excretory system

general characteristics class of amphibians, or amphibians, includes information about the excretory system. It is represented by paired kidneys, ureters extending from them and opening into the cloaca. There is a bladder in the cloaca, which is where the urine that gets into it accumulates. The mechanism for removing fluid is very specific. As soon as the bladder is filled, its walls contract and release concentrated urine into the cloaca, which is then thrown out. This complexity is due to the need for amphibians to retain a lot of moisture. Some metabolic products and a large number of water is released through the skin. These features of the body did not allow amphibians to completely transition to a terrestrial lifestyle.

Nervous system

A progressive evolutionary feature is the greater weight of the brain compared to fish in all representatives of the amphibian class. General characteristics:

  • The brain is characterized by the presence of five sections: middle, oblongata, intermediate, anterior (with two hemispheres) and a poorly developed cerebellum;
  • brain weight as a percentage of total body weight is 0.29-0.36 in tailed amphibians and 0.50-0.73 in tailless amphibians versus 0.06-0.44% in cartilaginous fish;
  • 10 pairs of head nerves depart from the brain of amphibians;
  • The sympathetic nervous system has received fairly good development, represented mainly by two nerve trunks located on the sides of the spine;
  • The spinal nerves form well-defined lumbar and brachial plexuses.

Amphibian sensory organs

The general characteristics of amphibians presented in the school course (7th grade, biology) provide superficial information about the sensory organs characteristic of them. The organ of hearing received a new section in the process of evolution - the middle ear. The eardrum is connected to the stapes (the auditory ossicle) and covers the external auditory opening. On both sides, the middle ear cavity is connected by the auditory tube to the oropharyngeal cavity.

The structure of the eye is adapted to life in air environment. And although the visual apparatus resembles a fish, it is still different and does not have a reflective and silvery shell, a crescent-shaped process. Higher amphibians have an upper and lower movable eyelid. The nictitating membrane is characteristic of lower amphibians. It performs a protective function instead of the lower eyelid. The shape of the cornea is convex, the lens is in the form of a biconvex lens, the diameter of which varies depending on the lighting. Many amphibians have color vision.

The skin of all representatives of the amphibian class, the general characteristics of which are presented in the text, performs the function of touch, as it contains numerous nerve endings. Aquatic species and tadpoles have a lateral line.

Paired olfactory sacs “work” exclusively in the air. They are lined from the inside with olfactory epithelium and open outwards with the nostrils, and the choanae - into the oropharyngeal cavity. The latter also houses the taste organs.

Reproductive system

Giving a general description of the class of amphibians, reproduction and life cycle development cannot be overlooked either. All amphibians are dioecious animals, and fertilization in most representatives occurs in the aquatic environment. Gonads are paired.

In females, granular ovaries, which fill almost the entire body cavity by spring, are suspended on the mesentery. Next to it are fat bodies that accumulate nutrients and thereby ensure the formation of reproductive products during the period hibernation. The oviducts are the long and thin Müllerian canals. Each of them opens into a body cavity. Ripe eggs, by rupturing the walls of the ovary, enter it and through the oviducts, gradually becoming covered with a mucous protein membrane, enter the cloaca, from where they are excreted.

Males have paired round testes and adjacent fat bodies. Both are attached to the mesentery near the anterior edges of the kidneys. The seminiferous tubules extend from the testes and open into the ureters, which also serve as vas deferens. The latter are excreted into the cloaca.

Life cycle

The life cycle of almost all amphibians is clearly divided into four stages.


Especially the amphibian lifestyle

Next, we present the final block of information about the class of amphibians - general characteristics and features of the habitat, lifestyle.

The vast majority of species are inhabitants of wet places that alternate their stay on land with life in the water. In addition, there are exclusively arboreal and aquatic amphibians. Insufficient adaptation to life on land gives rise to sudden changes associated with the change of seasons. Amphibians at unfavorable conditions environments go into long-term hibernation. They are active only under warm conditions; most species die at -1°C. The life rhythm of some species changes throughout the day.

Most amphibians are inhabitants of fresh water bodies and only a small part can live in salt water, for example, the sea toad (pictured below).

In progress adult all amphibians are predators. Their diet consists of small animals, mainly invertebrates and insects, juvenile fish, and in exceptional cases, waterfowl chicks and even small rodents.

Equipment: lizard skeletons, snakes, wet preparations of vipers, grass snakes, table “Type Chordata. Class Reptiles", dictionary of new words.

I. Test of knowledge

At the blackboard, one of the students fills out the table “Amphibians and Reptiles.” At this time, the others answer questions

1. Create a plan to talk about the features external structure sand lizard associated with life on land.
2. Why can the sand lizard reproduce on land?
3. What is the complexity of the structure of the circulatory system of a lizard compared to a frog?
4. What complications are observed in the structure of the respiratory system of a lizard compared to a frog?
5. How does the skeleton of a lizard differ from the skeleton of a frog?

Checking the completion of the table.

Amphibians and reptiles

Habitat

Wet places

Dry places

Coverings of the body

Bare mucous skin

Dry skin covered with horny scales

Respiratory system

Skin and lungs

Body temperature

Fickle

What kind of blood are the organs supplied with?

Mixed

Mixed, but to a lesser extent

Brain structure

Consists of five departments. The forebrain and cerebellum are poorly developed

Consists of five departments. The forebrain and cerebellum are better developed than in amphibians

Where does reproduction and development occur?

Reproduction and development occurs in water

Reproduction occurs on land

Is there a larval stage in development?

Larva – tadpole

No larvae

II. Learning new material

The bodies of lizards and snakes are covered with scales. Hence the name of this order - Scaly. The suborder of lizards includes geckos, agamas, flying Dragon, iguanas, serpentines, skinks, monitor lizards, real lizards, spindles, etc. ( Write new words on the board in advance.)

Spindles are harmless lizards. Their different types form a complete series of transitions from forms with developed limbs to legless ones, outwardly similar to snakes. We have two completely legless species: the spindle and the yellow-bellied one.

Amphibians are the first terrestrial vertebrates, most of which live on land and breed in water. These are moisture-loving animals, which determines their habitat.

Water-dwelling newts and salamanders most likely once completed their life cycle at the larval stage and reached sexual maturity in this state.

Terrestrial animals - frogs, toads, tree frogs, spadefoots - live not only on the soil, but also on trees (frog), in the desert sands (toad, spadefoot), where they are active only at night, and lay eggs in puddles and temporary reservoirs, yes and not every year.

Amphibians feed on insects and their larvae (beetles, mosquitoes, flies), as well as spiders. They eat mollusks (slugs, snails) and fish fry. Toads are especially useful because they eat nocturnal insects and slugs that are inaccessible to birds. Grass frogs feed on garden, forest and field pests. One frog can eat about 1,200 harmful insects over the summer.

Amphibians themselves are food for fish, birds, snakes, hedgehogs, mink, ferrets, and otters. They feed the chicks predator birds. Toads and salamanders, which have poisonous glands on their skin, are not eaten by mammals and birds.

Amphibians overwinter in shelters on land or in shallow water bodies, so cold, snowless winters cause them mass death, and pollution and drying out of water bodies leads to the death of offspring - eggs and tadpoles. Amphibians must be protected.

9 species of representatives of this class are included in the Red Book of the USSR.

Class characteristics

The modern fauna of amphibians is not numerous - about 2,500 species of the most primitive terrestrial vertebrates. According to morphological and biological characteristics, they occupy an intermediate position between aquatic organisms and the actual ground ones.

The origin of amphibians is associated with a number of aromorphoses, such as the appearance of a five-fingered limb, the development of the lungs, the division of the atrium into two chambers and the appearance of two circulation circles, the progressive development of the central nervous system and sensory organs. Throughout their lives, or at least in the larval state, amphibians are necessarily associated with aquatic environment. Adult forms require constant skin hydration for normal functioning, so they live only near bodies of water or in places with high humidity. In most species, eggs (spawn) do not have dense shells and can only develop in water, like larvae. Amphibian larvae breathe through gills; during development, metamorphosis (transformation) occurs into an adult animal that has pulmonary respiration and a number of other structural features of terrestrial animals.

Adult amphibians are characterized by paired limbs of the five-fingered type. The skull is movably articulated with the spine. In addition to the internal hearing organ, the middle ear is also developed. One of the bones of the hyoid arch turns into the bone of the middle ear - the stapes. Two circles of blood circulation are formed, the heart has two atria and one ventricle. The forebrain is enlarged, two hemispheres are developed. Along with this, amphibians retained features characteristic of aquatic vertebrates. The skin of amphibians has a large number of mucous glands; the mucus they secrete moisturizes it, which is necessary for skin respiration (oxygen diffusion can only occur through a water film). Body temperature depends on the ambient temperature. These features of body structure determine the richness of the amphibian fauna in humid and warm tropical and subtropical regions (see also Table 18).

A typical representative of the class is a frog, the example of which is usually used to characterize the class.

The structure and reproduction of a frog

lake frog lives in bodies of water or on their banks. Its flat, wide head smoothly transitions into a short body with a reduced tail and elongated hind limbs with swimming hind limbs. The forelimbs, unlike the hind limbs, are significantly smaller; they have 4, not 5 fingers.

Coverings of the body. The skin of amphibians is naked and always covered with mucus thanks to a large number of mucous multicellular glands. It not only performs a protective function (from microorganisms) and perceives external irritation, but also participates in gas exchange.

Skeleton consists of the spine, skull and skeleton of the limbs. The spine is short, divided into four sections: cervical, trunk, sacral and caudal. There is only one ring-shaped vertebra in the cervical region. The sacral region also has one vertebra, to which the pelvic bones are attached. The tail section of the frog is represented by the urostyle - a formation consisting of 12 fused caudal vertebrae. Between the vertebral bodies there are remains of the notochord, there are superior arches and a spinous process. There are no ribs. The skull is wide, flattened in the dorsal-ventral direction; in adult animals, the skull retains a lot of cartilaginous tissue, which makes amphibians similar to lobe-finned fish, but the skull contains fewer bones than in fish. Two occipital condyles are noted. The shoulder girdle consists of the sternum, two coracoids, two clavicles and two scapulae. In the forelimb there is a shoulder, two fused bones of the forearm, several bones of the hand and four fingers (the fifth finger is rudimentary). Pelvic girdle formed by three pairs of fused bones. The hind limb consists of a femur, two fused leg bones, several foot bones and five toes. The hind limbs are two to three times longer than the forelimbs. This is due to movement by jumping; in water, when swimming, the frog energetically works with its hind limbs.

Musculature. Part of the trunk muscles retains a metameric structure (similar to the muscles of fish). However, a more complex differentiation of muscles is clearly manifested, developed a complex system muscles of the limbs (especially the hind ones), chewing muscles, etc.

Internal organs of a frog lie in the coelomic cavity, which is lined with a thin layer of epithelium and contains a small amount of fluid. Most of the body cavity is occupied by the digestive organs.

Digestive system It begins with a large oropharyngeal cavity, at the bottom of which the tongue is attached at the anterior end. When catching insects and other prey, the tongue is thrown out of the mouth and the prey sticks to it. On the upper and lower jaws of the frog, as well as on the palatine bones, there are small conical teeth (undifferentiated), which serve only to hold prey. This expresses the similarity of amphibians with fish. The ducts of the salivary glands open into the oropharyngeal cavity. Their secretion moistens the cavity and food, making it easier to swallow prey, but it does not contain digestive enzymes. Next, the digestive tract passes into the pharynx, then into the esophagus and, finally, into the stomach, the continuation of which is the intestines. The duodenum lies under the stomach, and the rest of the intestine folds into loops and ends in the cloaca. There are digestive glands (pancreas and liver).

Food moistened with saliva enters the esophagus and then into the stomach. The glandular cells of the stomach walls secrete the enzyme pepsin, which is active in an acidic environment (in the stomach it is also secreted hydrochloric acid). Partially digested food moves to the duodenum, into which the bile duct of the liver flows.

Pancreatic secretions also flow into the bile duct. The duodenum quietly passes into the small intestine, where nutrients are absorbed. Undigested food remains enter the wide rectum and are thrown out through the cloaca.

Tadpoles (larvae of frogs) feed mainly on plant foods (algae, etc.); they have horny plates on their jaws that scrape off soft plant tissues along with the unicellular and other small invertebrates found on them. The horny plates are shed during metamorphosis.

Adult amphibians (in particular, frogs) are predators that feed on various insects and other invertebrate animals; some aquatic amphibians catch small vertebrates.

Respiratory system. A frog's breathing involves not only the lungs, but also the skin, which contains a large number of capillaries. The lungs are represented by thin-walled bags, the inner surface of which is cellular. On the walls of the paired sac-like lungs there is an extensive network of blood vessels. Air is pumped into the lungs as a result of the pumping movements of the floor of the mouth when the frog opens the nostrils and lowers the floor of the oropharyngeal cavity. Then the nostrils close with valves, the bottom of the oropharyngeal cavity rises, and air passes into the lungs. Exhalation occurs due to action abdominal muscles and collapse of the pulmonary walls. U different types amphibians receive 35-75% of oxygen through the lungs, 15-55% through the skin, and 10-15% of oxygen through the mucous membrane of the oropharyngeal cavity. 35-55% of carbon dioxide is released through the lungs and oropharyngeal cavity, and 45-65% of carbon dioxide through the skin. Males have arytenoid cartilages surrounding the laryngeal fissure and the vocal cords stretched over them. Sound amplification is achieved by the vocal sacs formed by the mucous membrane of the oral cavity.

Excretory system . Dissimilation products are excreted through the skin and lungs, but most of them are excreted by the kidneys located on the sides of the sacral vertebra. The kidneys are adjacent to the dorsal side of the frog's cavity and are oblong bodies. The kidneys contain glomeruli in which they are filtered from the blood. harmful products decay and some valuable substances. During the flow through the renal tubules, valuable compounds are reabsorbed, and urine flows through two ureters into the cloaca and from there into the bladder. For some time, urine can accumulate in the bladder, which is located on the abdominal surface of the cloaca. After filling the bladder, the muscles of its walls contract, urine is discharged into the cloaca and thrown out.

Circulatory system. The heart of adult amphibians is three-chambered, consisting of two atria and a ventricle. There are two circles of blood circulation, but they are not completely separated; arterial and venous blood is partially mixed thanks to a single ventricle. An arterial cone with a longitudinal spiral valve inside extends from the ventricle, which distributes arterial and mixed blood into different vessels. The right atrium receives venous blood from internal organs and arterial blood from the skin, i.e. mixed blood collects here. The left atrium receives arterial blood from the lungs. Both atria contract simultaneously and blood flows from them into the ventricle. Thanks to the longitudinal valve in the arterial cone, venous blood flows to the lungs and skin, mixed blood flows to all organs and parts of the body except the head, and arterial blood flows to the brain and other organs of the head.

The circulatory system of amphibian larvae is similar to the circulatory system of fish: there is one ventricle and one atrium in the heart, there is one circle of blood circulation.

Endocrine system. In the frog, this system includes the pituitary gland, adrenal glands, thyroid, pancreas and gonads. The pituitary gland secretes intermedin, which regulates the color of the frog, somatotropic and gonadotropic hormones. Thyroxine, which is produced by the thyroid gland, is necessary for the normal completion of metamorphosis, as well as for maintaining metabolism in an adult animal.

Nervous system characterized by a low degree of development, but along with this it has a number of progressive features. The brain has the same sections as in fish (forebrain, interstitial, midbrain, cerebellum and medulla oblongata). The forebrain is more developed, divided into two hemispheres, each of them has a cavity - the lateral ventricle. The cerebellum is small, which is due to a relatively sedentary lifestyle and monotony of movements. The medulla oblongata is much larger. There are 10 pairs of nerves leaving the brain.

The evolution of amphibians, accompanied by a change of habitat and emergence from water to land, is associated with significant changes in the structure of the sense organs.

The sense organs are generally more complex than those of fish; they provide orientation for amphibians in water and on land. In larvae and adult amphibians living in water, lateral line organs are developed; they are scattered on the surface of the skin, especially numerous on the head. The epidermal layer of the skin contains temperature, pain and tactile receptors. The organ of taste is represented by taste buds on the tongue, palate and jaws.

The olfactory organs are represented by paired olfactory sacs, which open outward through paired external nostrils, and into the oropharyngeal cavity through internal nostrils. Part of the walls of the olfactory sacs is lined with olfactory epithelium. The olfactory organs function only in the air; in water, the external nostrils are closed. The olfactory organs of amphibians and higher chordates are part of the respiratory tract.

In the eyes of adult amphibians, movable eyelids (upper and lower) and a nictitating membrane are developed; they protect the cornea from drying out and contamination. Amphibian larvae do not have eyelids. The cornea of ​​the eye is convex, the lens has the shape of a biconvex lens. This allows amphibians to see quite far. The retina contains rods and cones. Many amphibians have developed color vision.

In the organs of hearing except inner ear in place of the squirter of lobe-finned fishes, a middle ear is developed. It contains a device that amplifies sound vibrations. The external opening of the middle ear cavity is covered by an elastic eardrum, the vibrations of which are enhanced by sound waves. Through the auditory tube, which opens into the pharynx, the middle ear cavity communicates with the external environment, which makes it possible to reduce sudden changes in pressure on the eardrum. In the cavity there is a bone - the stirrup, one end of which rests against the eardrum, the other - against the oval window, covered by a membranous septum.

Table 19. Comparative characteristics structures of larvae and adult frogs
Sign Larva (tadpole) Adult animal
Body Shape Fish-like, with limb buds, tail with a swimming membrane The body is shortened, two pairs of limbs are developed, there is no tail
Way to travel Swimming with your tail Jumping, swimming using hind limbs
Breath Branchial (gills are first external, then internal) Pulmonary and cutaneous
Circulatory system Two-chambered heart, one circle of blood circulation Three-chambered heart, two circles of blood circulation
Sense organs The lateral line organs are developed, there are no eyelids in the eyes There are no lateral line organs, eyelids are developed in the eyes
Jaws and feeding method The horny plates of the jaws scrape off algae along with unicellular and other small animals There are no horny plates on the jaws; the sticky tongue captures insects, mollusks, worms, and fish fry
Lifestyle Water Terrestrial, semi-aquatic

Reproduction. Amphibians are dioecious. The genitals are paired, consisting of slightly yellowish testes in the male and pigmented ovaries in the female. Efferent ducts extend from the testes and penetrate into the anterior part of the kidney. Here they connect to the urinary tubules and open into the ureter, which simultaneously performs the function of the vas deferens and opens into the cloaca. The eggs fall from the ovaries into the body cavity, from where they are released through the oviducts, which open into the cloaca.

Frogs have well-defined sexual dimorphism. Thus, the male has tubercles on the inner toe of the front legs ("nuptial callus"), which serve to hold the female during fertilization, and vocal sacs (resonators), which enhance the sound when croaking. It should be emphasized that voice first appears in amphibians. Obviously, this is related to life on land.

Frogs reproduce in the spring during their third year of life. Females spawn eggs into the water, and males irrigate them with seminal fluid. Fertilized eggs develop within 7-15 days. Tadpoles - the larvae of frogs - are very different in structure from adult animals (Table 19). After two to three months, the tadpole turns into a frog.

Development. In the frog, like in other amphibians, development occurs with metamorphosis. Metamorphosis is widespread in representatives of various types of animals. Development with transformation appeared as one of the adaptations to living conditions and is often associated with the transition of larval stages from one habitat to another, as is observed in amphibians.

Amphibian larvae are typical inhabitants of water, which is a reflection of the lifestyle of their ancestors.

Features of tadpole morphology that have adaptive significance in accordance with environmental conditions include:

  • a special device on the underside of the head end, which is used for attachment to underwater objects - a suction cup;
  • longer intestine than that of an adult frog (compared to body size); this is due to the fact that the tadpole consumes plant rather than animal (as adult frog) food.

The organizational features of the tadpole, repeating the characteristics of its ancestors, should be recognized as a fish-like shape with a long caudal fin, the absence of five-fingered limbs, external gills, and one circle of blood circulation. During the process of metamorphosis, all organ systems are rebuilt: limbs grow, gills and tail dissolve, the intestines shorten, the nature of food and the chemistry of digestion, the structure of the jaws and the entire skull, the skin change, a transition from gill to pulmonary respiration occurs, deep transformations occur in the circulatory system .

During the metamorphosis of amphibians significant influence have hormones secreted by special glands (see above). For example, removal of the thyroid gland from a tadpole leads to an extension of the growth period, but metamorphosis does not occur. On the contrary, if thyroid preparations or thyroid hormone are added to the food of a tadpole of a frog or other amphibians, then metamorphosis is significantly accelerated and growth stops; As a result, you can get a frog only 1 cm long.

Sex hormones produced by the gonads determine the development of secondary sexual characteristics that distinguish males from females. Male frogs do not develop a “nuptial callus” on the big toe of their forelimbs when they are castrated. But if a castrate is transplanted with a testis or only injected with a male sex hormone, then a callus appears.

Phylogeny

Amphibians include forms whose ancestors about 300 million years ago (in Carboniferous period) came out of the water onto land and adapted to new terrestrial living conditions. They differed from fish in the presence of a five-fingered limb, as well as lungs and associated features of the circulatory system. They are united with fish by the development of the larva (tadpole) in the aquatic environment, the presence in the larvae of gill slits, external gills, lateral line, arterial cone and the absence of embryonic membranes during embryonic development. Data from comparative morphology and biology show that the ancestors of amphibians should be sought among ancient lobe-finned fish.

The transitional forms between them and modern amphibians were fossil forms - stegocephals, which existed in the Carboniferous, Permian and Triassic periods. These ancient amphibians, judging by the skull bones, are extremely similar to ancient lobe-finned fish. Characteristic signs them: a shell of dermal bones on the head, sides and abdomen, a spiral intestinal valve, like in shark fish, the absence of vertebral bodies. Stegocephalians were nocturnal predators that lived in shallow bodies of water. The emergence of vertebrates onto land occurred during the Devonian period, which was characterized by an arid climate. During this period, those animals that could move overland from a drying up reservoir to another acquired an advantage. The heyday (period of biological progress) of amphibians occurred during the Carboniferous period, whose even, humid and warm climate was favorable for amphibians. Only thanks to their access to land did vertebrates gain the opportunity to further progressively develop.

Taxonomy

The class of amphibians consists of three orders: legless (Apoda), tailed (Urodela) and tailless (Anura). The first order includes primitive animals adapted to a unique way of life in moist soil - caecilians. They live in the tropical zone of Asia, Africa and America. Tailed amphibians are characterized by an elongated tail and paired short limbs. These are the least specialized forms. The eyes are small, without eyelids. Some species retain external gills and gill slits throughout their lives. Tailed animals include newts, salamanders and amblystoma. Tailless amphibians (toads, frogs) have a short body, no tail, and long hind limbs. Among them there are a number of species that are eaten.

The meaning of amphibians

Amphibians destroy large numbers of mosquitoes, midges and other insects, as well as shellfish, including pests cultivated plants and disease vectors. The common tree frog feeds mainly on insects: click beetles, flea beetles, caterpillars, ants; green toad- beetles, bedbugs, caterpillars, fly larvae, ants. In turn, amphibians are eaten by many commercial fish, ducks, herons, fur-bearing animals (mink, ferret, otter, etc.).

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 cervical vertebra, in appearance there is no neck 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, lower leg 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

General plan of the building digestive system amphibians are similar to 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 develop 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 matter.

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 Gallbladder, 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 the 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

Key changes in nervous system amphibian compared to fish did not happen. 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, 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.

Class amphibians- these are cold-blooded animals associated with both water and terrestrial environment; there are about 5000 species. They are also called amphibians.

The structure of the amphibian class

Amphibian organ

What parts does it consist of?

SKELETON

Head skeleton

Skull box;

Jaws - upper and lower

Brain protection

Capturing food

Spine

Vertebrae (there is a cervical vertebra); tail bone

Body support, protection of internal organs

Forelimb belt

Sternum, two crow bones, two collarbones and two shoulder blades

They connect the limbs with the spine

Hind limb belt

Fused pelvic bones attached to the spine

Hind leg support

Forelimb

Humerus, two fused bones of the forearm, small bones of the hand, bones of the four fingers

Support while moving

Hind limb

Femur, two fused bones of the lower leg, bones of the foot and five toes

Push-off while moving

NERVOUS SYSTEM

Brain

Divisions: anterior (better developed than in fish), middle, intermediate, oblongata, cerebellum (due to the uniformity of motor reactions, less developed than in fish)

Movement control, unconditioned and conditioned reflexes

Spinal cord

implementation of simple reflexes, conduction of nerve impulses

perception and conduction of signals

Sense organs

The organ of vision is the eyes, protected by eyelids; The organ of hearing is the ear (consists of the middle and inner ear, the organs of smell and balance are located in the brain

Perception of signals from the external environment

ORGANS OF THE BODY CAVITY

Digestive system

1. Digestive tract (mouth, pharynx, esophagus, stomach, intestines, anus)

2. Digestive glands (pancreas, liver)

1. Capturing, chopping, moving food

2. secretion of juices that promote food digestion

Respiratory system (may be pulmonary and cutaneous respiration)

Lungs (sacs with elastic walls in which many capillaries branch)

Gas exchange

Circulatory system

Three-chambered heart (two atria and one ventricle), arteries, veins, capillaries; two circles of blood circulation

Supplying all body cells with oxygen and nutrients, removal of decomposition products

Origin of the class Amphibians

Amphibians or amphibians appeared about 375 million years ago. The first amphibians descended from ancient lobe-finned fish, which were of enormous size, which in turn also reached gigantic sizes.

Classification of Amphibians

Amphibians are divided into 3 main orders:

Representatives

Features and numbers

Squad Tailed

representatives are newts, salamanders, ambistoms, sirens

All of them have an elongated body, which turns into a tail, and the limbs are short and weak. A feature of tailed animals is the high regeneration of body parts, which happens when animals restore up to the half of their body. This order includes approximately 500 species of amphibians.

Tailless Squad

toads, frogs, toads, tree frogs and others

Representatives of this order have well-developed hind limbs for jumping movement and lack a tail. Includes approximately 4,000 species of amphibians

Legless Squad

these include worms

Primitive amphibians, which have neither tails nor limbs, resemble earthworms.

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A source of information: Biology in tables and diagrams./ Edition 2, - St. Petersburg: 2004.