About animals

Amphibians: characteristics, structure, reproduction, and origin of the amphibian class


Find the three errors in the text above. Indicate the numbers of sentences in which errors were made, correct them.

(1) Amphibians are the first four-legged vertebrates that emerged from land to land. (2) The spine of amphibians is divided into departments: cervical, trunk, sacral and caudal. (3) The anterior and posterior pairs of limbs of amphibians are five-fingered. (4) Amphibians leading an aquatic life, for example, spur frogs, even in adulthood, have lateral line organs. (5) Adult amphibians have a two-chamber heart. (6) Fertilization in all tailless amphibians is internal. (7) The order Tailless amphibians includes pond frogs, toads, newts and salamanders.

Errors in sentences:

(5) Adult amphibians have three-chamber heart.

(6) Fertilization in tailless amphibians outside. (or, Most tailed amphibian insemination internal).

(7) The order Tailless amphibians include pond frogs, toads, and squad tailed amphibians include - newts and salamanders.

3 sentence can also be corrected:

The front pair of limbs of amphibians is four-fingered, the rear pair of limbs of amphibians is five-fingered. OR Extremities in amphibians - terrestrial type (five-fingered), with the exception of the forelimb - four fingers.


Amphibians (they are amphibians) are the first terrestrial vertebrates that appeared in the process of evolution. However, they still maintain a close relationship with the aquatic environment, usually living in it at the larval stage. Typical representatives of amphibians are frogs, toads, newts, salamanders. Most diverse in tropical forests, as it is warm and damp. 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 tailless, and newts belong to tailless.

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

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

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

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

The middle ear appears with an eardrum. (In fish, only the inside.) The eardrum is visible, located on the sides of the head behind the eyes.

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

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

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

Amphibians appeared about 350 million years ago (at the end of the Devonian period) from the ancient brushfish. Their heyday occurred 200 million years ago when huge swamps covered the Earth.

The musculoskeletal system

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

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

The musculoskeletal system 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. The cervical spine, consisting of one vertebra, first appears. However, the mobility of the head is not large, frogs can only tilt their heads. Although they have a cervical vertebra, there is no neck in the appearance of the body.

In amphibians, the spine consists of more departments than in fish. If fish have only two (trunk and caudal), then amphibians have four sections of the spine: cervical (1 vertebra), trunk (7), sacral (1), caudal (one caudal bone in tailless or a number of separate vertebrae in caudate amphibians) . In tailless amphibians, tail vertebrae grow together in one bone.

The limbs of amphibians are complex. The front consists of a shoulder, forearm and hand. The brush consists of wrists, metacarpals and phalanges of the fingers. The hind limbs are composed of thigh, lower leg and foot. The foot consists of tarsus, metatarsus and phalanx of the fingers.

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

Belts of the hind limbs are composed of sciatic and iliac bones, as well as pubic cartilage. Fused, they articulate with the lateral processes of the sacral vertebra.

Ribs, if any, do not form short, rib cages. Tailed amphibians have short ribs, while tailless amphibians do not.

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

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

The digestive system of amphibians

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

The front horses of the frogs tongue grows to the lower jaw, and the rear remains free. This structure of the tongue allows them to catch prey.

Amphibians have salivary glands. Their secret moistens food, but does not digest it, since it does not contain digestive enzymes. There are conical teeth on the jaws. 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 part of the small intestine is the duodenum. A single duct opens into it, where the secrets of the liver, gall bladder and pancreas get. In the small intestine, digestion is completed and nutrients are absorbed into the blood.

Undigested food residues enter the large intestine, from where they move to the cloaca, which is an expansion of the intestine. In the cesspool, the ducts of the excretory and reproductive systems also open. From it, undigested residues enter the external environment. Fish have no cesspools.

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

Amphibian Respiratory System

Amphibian larvae (tadpoles) have gills and one blood circulation (as in fish).

In adult amphibians, lungs appear, which are elongated sacs with thin elastic walls having a cellular structure. There is a network of capillaries in the walls. The respiratory surface of the lungs is small, so the naked skin of amphibians is also involved in the breathing process. Through it, up to 50% of oxygen is supplied.

The mechanism of inspiration and expiration is provided by raising and lowering the bottom of the oral cavity. When lowering, there is a breath through the nostrils, when lifting - air is pushed into the lungs, while the nostrils are closed. The exhalation is also carried out when raising the bottom of the mouth, but the nostrils are open, and the air exits through them. Also, as you exhale, the abdominal muscles contract.

In the lungs, gas is exchanged due to the difference in gas concentrations in the blood and air.

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

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

Under water, frogs breathe only skin.

Amphibian circulatory system

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

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

Venous blood from the organs of the body, as well as arterial from the skin, enters the right atrium. Arterial blood from the lungs enters the left atrium. A vessel that flows into the left atrium is called pulmonary vein.

Atrial contraction pushes blood into the common ventricle of the heart. Here the blood is partially mixed.

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

This separation of blood is achieved by a special arrangement of blood vessels emerging from the distribution chamber of the heart, where blood flows from the ventricle. When the first portion of blood is pushed, it fills the closest vessels. And this is the most venous blood that 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 that go to the organs of the body. Most arterial blood enters the distant pair of vessels (carotid arteries) and goes to the head.

Amphibian excretory system

The buds of amphibians are torso, have an oblong shape. Urine enters the ureters, then flows down the wall of the cloaca into the bladder. When the bladder contracts, urine is poured into the cloaca and further out.

The product of excretion is urea. Less water is needed to remove it than to remove ammonia (which is formed in fish).

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

The nervous system and sensory organs of amphibians

Amphibians in comparison with fish did not have key changes in the nervous system. However, the amphibian forebrain is more developed and divided into two hemispheres. But their cerebellum is less developed, since amphibians do not need to maintain equilibrium in water.

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

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 frog's eyes). From the eardrum, sound vibrations are transmitted through the auditory ossicle to the inner ear. The Eustachian tube connects the middle ear cavity with the oral cavity. This allows you to weaken the pressure drops on the eardrum.

Reproduction and development of amphibians

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

The eggs mature in the ovaries and then enter the oviducts, where they are covered with a transparent mucous membrane. Next, the eggs are in the cesspool and displayed outside.

Males secrete seminal fluid. In many frogs, males are fixed on the backs of the females, and while the female swallows eggs for several days, they water it with seminal fluid.

Amphibians toss less caviar than fish. Bunches of eggs attach to aquatic plants or swim.

The mucous membrane of the eggs in the water greatly swells, refracts sunlight and heats up, which contributes to a more rapid development of the embryo.

An embryo develops in each egg (usually about 10 days in frogs). The larva emerging from the eggs is called a tadpole. It has many signs similar to fish (a two-chamber heart and one circle blood circulation, breathing with the help of gills, an organ of the lateral line). At first, the tadpole has external gills, which then become internal. Hind limbs appear, then front ones. Lungs and a 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 foods.

Skeleton and Musculature

Amphibian body integument. The skin is naked and always covered with mucus, due to the large number of mucous multicellular glands. It performs not only a protective function and perceives external irritation, but also participates in gas exchange.

Amphibian skeleton. In the vertebral column, in addition to the trunk and tail sections, the cervical and sacral sections appear for the first time in the evolution of animals.

In the cervical region there is only one annular vertebra. Then follows 7 trunk vertebrae with lateral processes. In the sacral section, there is also one vertebra, to which the bones of the pelvis are attached. The tail section of the frog is represented by urostyle - a formation consisting of 12 fused tail vertebrae. Between the vertebral bodies remains of the chord are preserved, there are upper arches and the spinous process. Amphibians have no ribs and chest.

Significant cartilage remains have been preserved in the skull, which makes amphibians similar to cysterae fishes. The skeleton of the free limbs is divided into 3 sections. The limbs are connected to the spinal column through the bones of the limb belts. The belt of the forelimbs includes: the sternum, two crow bones, two clavicles and two shoulder blades. The hind limb is represented by fused pelvic bones.

Amphibian skeleton

Amphibian muscle. The skeletal muscles of the frog can provide movement of parts of the body due to contraction. Muscles can be divided into groups of antagonists: flexors and extensors, leading and abducting. Most muscles attach to the bones with tendons.

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

Amphibian Respiratory System

Amphibians breathe both lungs and skin.

The lungs are represented by thin-walled bags with a cellular inner surface. Air is pumped into the lungs as a result of pumping movements of the bottom of the oropharyngeal cavity. When a frog dives, its air-filled lungs act as a hydrostatic organ.

The arytenoid cartilages appear surrounding the laryngeal fissure and the vocal cords stretched over them, available only to males. Sound enhancement is achieved by voice bags formed by the mucous membrane of the oral cavity.

Amphibian Respiratory System

Amphibian reproduction

Amphibian dioecious. The genitals are paired, consisting of slightly yellowish testes in the male and pigmented ovaries in the female. The efferent ducts extend from the testicles, penetrating into the anterior part of the kidney. Here they connect to the urinary tubules and open into the ureter, which functions in the same way as the vas deferens, and opens into the cloaca. Eggs from the ovaries enter the body cavity, from where they are brought out through the oviducts, which open into the cloaca.

Frogs have sexual dimorphism. Distinctive features of males are tubercles on the inner toe of the front legs and voice bags (resonators). Resonators amplify the sound when croaking. The voice first appears in amphibians: this is obviously connected with life on land.

The development of a frog, like other amphibians, occurs with metamorphosis. Amphibian larvae are typical inhabitants of water, which is a reflection of the ancestral lifestyle.

The development of amphibians, the example of a frog

The features of the tadpole morphology, which have adaptive value in accordance with environmental conditions, include:

  • a special apparatus on the underside of the head, used to attach the tadpole to underwater objects,
  • longer than the adult frog, intestines (compared with body size). This is due to the fact that the tadpole consumes vegetable, and not animal (like an adult frog) food.

The peculiarities of the organization of the tadpole, repeating the signs of their ancestors, should be recognized as a fish-shaped with a long tail fin, the absence of five-fingered limbs, external gills, a lateral line and one circle of blood circulation. In the process of metamorphosis, all organ systems are rebuilt:

  • Limbs grow
  • gills and tail dissolve
  • the bowels are shortened
  • the nature of food and the chemistry of digestion, the structure of the jaws and the whole skull, the skin,
  • there is a transition from gill breathing to pulmonary, profound transformations occur in the circulatory system.

The speed of development of tadpoles depends on temperature: the hotter the faster. It usually takes 2-3 months to turn a tadpole into a frog.

Amphibian diversity

Currently, 3 orders belong to the amphibian class:

Tailed Amphibians (newts, salamanders, etc.) are characterized by an elongated tail and paired short limbs. These are the least specialized forms. The eyes are small, without eyelids. Some retain gills and gill slits for life.

At tailless amphibians (toads, frogs) the body is short, without a tail, with long hind limbs. Among them there are a number of species that are eaten.

To squad legless amphibians Worms that live in tropical countries include. Their body is worm-shaped, devoid of limbs. Worms feed on rotting plant debris.

The largest of the European frogs is found in Ukraine and the Russian Federation - the lake frog, whose body length reaches 17 cm, and one of the smallest tailless amphibians is the common tree frog, having a length of 3.5-4.5 cm. Adult tree frogs usually live in trees and have special discs at the ends of their fingers for attachment to branches.

Four species of amphibians are listed in the Red Book: Carpathian newt, mountain newt, reed toad, quick frog.

Origin of Amphibians

Amphibians include forms whose ancestors are about 300 million. years ago came out of the water to land and adapted to new terrestrial living conditions. They differed from fish in the presence of a five-fingered limb, lungs and the characteristics of the circulatory system associated with them.

They were united with fishes:

  • The development of the larva (tadpole) in the aquatic environment,
  • the presence of gill slits in larvae,
  • the presence of external gills,
  • the presence of a side line,
  • lack of germinal membranes during embryonic development.

The ancestors of amphibians among ancient animals are considered to be carp fish.

All the data of comparative morphology and biology indicate that the ancestors of amphibians should be sought among the ancient brushfish. The transitional forms between them and modern amphibians were fossil forms - stegocephals, which existed in the Carboniferous, Permian and Triassic periods. Judging by the bones of the skull, these ancient amphibians were extremely similar to the ancient cysterae fishes. Their characteristic signs are: a shell of skin bones on the head, sides and abdomen, a spiral valve of the intestines, like in shark fish, and the absence of vertebral bodies.

Stegocephals were nocturnal predators living in shallow ponds. The release of vertebrates to land occurred in the Devonian period, characterized by an arid climate. During this period, those animals who could overland move from a drying up pond to a neighboring one acquired an advantage.

The heyday (period of biological progress) of amphibians falls on the Carboniferous period, the even, humid and warm climate of which was favorable for amphibians. Only due to land access did vertebrates get the opportunity to further progressively develop.