Who are Amphibians?

Amphibians, or amphibians, are what zoologists call the class of vertebrate animals, which includes the familiar frogs and toads. The name itself" amphibians" translated from Greek means “living a dual life,” which indicates an important feature of these animals. They lead a terrestrial lifestyle, but at the same time maintain a close connection with the aquatic environment. Amphibians, like fish, lay eggs, which, like fish, are called eggs. During spring "concerts" eggs are usually laid.

The class Amphibians includes approximately 2,600 species of animals. They originate from one of the groups of amphibians - stegocephali, who lived about 300 million years ago in swampy reservoirs. Stegocephalians breathed with lungs, had developed limbs, with the help of which they crawled from drying up reservoirs to others. Warm, humid climate At that time, a sufficient amount of food, the absence of competitors contributed to the fixation of the ancestors of amphibians on land. This is how the first terrestrial vertebrates appeared on land. The presence of two pairs of five-fingered limbs indicates the ability of animals to move along hard surface. In tailed amphibians, the limbs are poorly developed, so they do not walk well on land. In the water they swim like fish, bending their elongated body and a long tail and pressing the limbs to the body.

Features of Amphibians that distinguish them from other vertebrates

Amphibians have the peculiarity of adapting to environmental conditions, such as suspended animation - (from the Greek. anabiosis- revival). Anabiosis is a temporary state of the body in which life processes slow down dramatically, which contributes to its survival in adverse conditions of temperature, humidity, etc. It is also characteristic of microorganisms (spore-forming bacteria, microscopic fungi, protozoa) and invertebrates (hydras, worms, some molluscs and etc.). In some organisms it is part of the normal development cycle (seeds, spores).

When the temperature environment drops to 2°C, amphibians cease their vital activity and fall into torpor. In preparation for winter, green frogs leave shady and damp places and burrow into the mud. Far from water, in rodent burrows or in the underground passages of moles, toads hibernate. Tritons get out on land and sleep, climbing into dry rotten stumps, under tree roots or in large piles of brushwood.

The body of Amphibians is covered with thin, bare skin. Numerous glands of the skin abundantly secrete mucus, which, evaporating, lowers the body temperature by 5-8 ° C compared to the ambient temperature. This is why amphibians are cold to the touch. This feature of the skin structure has become a serious obstacle to their resettlement in the cold regions of the planet, where low temperatures they become numb from the cold.

The skin of tailless amphibians is connected to the muscles only in certain areas, so a lot of water can be stored in the spaces between the skin and muscles. For example, some species of African toads, once in a pond, absorb water with their skin and double their mass in just 10 minutes, while swelling greatly.

The males of many frog species have resonators, or vocal sacs, which, when croaking, inflate like bubbles, amplifying the sounds that attract females during the mating season.

Amphibians in temperate latitudes begin to reproduce after hibernation. So, frogs in springtime gather in large numbers in shallow reservoirs well warmed up by the sun, announcing the surroundings with croaking. After the breeding season, amphibians of most species leave water bodies.

In toads and frogs, eggs hatch after about a week, and in newts, larvae hatch after two to three weeks. They are very similar to fish: the body is elongated, there is a tail surrounded by a swimming membrane. The larvae of frogs and toads have a large head, due to the size of which they are called tadpoles. Like fish, they have gills, a lateral line organ, a two-chambered heart, and one circulation. Tadpoles feed on plant food, scraping it with the horny teeth of their jaws.

Class Amphibians = Amphibians.

The first terrestrial vertebrates that still retained contact with the aquatic environment. The class has 3,900 species and includes 3 orders: tailed (salamanders, newts), legless (tropical caecilians) and tailless (toads, tree frogs, frogs, etc.).

Secondary aquatic animals. Since the egg does not have an amniotic cavity (together with cyclostomes and fish, amphibians are anamnians), they reproduce in water, where they undergo the initial stages of their development. At different stages of their life cycle, amphibians lead a terrestrial or semi-aquatic lifestyle and are distributed almost everywhere, mainly in areas with high humidity along the banks of fresh water bodies and on damp soils. Among amphibians there are no forms that could live in salty sea ​​water. Various modes of movement are characteristic: species are known that make fairly long jumps, move at a walk or “crawl”, lacking limbs (caecilians).

Basic characteristics of amphibians.

Amphibians retained many of the features of their purely aquatic ancestors, but at the same time they acquired a number of features characteristic of true terrestrial vertebrates.

Tailed and tailless animals are characterized by larval development with gill breathing in fresh water (frog tadpoles) and their metamorphosis into an adult breathing with lungs. In legless animals, upon hatching the larva takes the form of an adult animal.

The circulatory system is characterized by two circles of blood circulation. The heart is three-chambered. It has one ventricle and two atria.

The cervical and sacral sections of the spine are separated, each having one vertebra.

Adult amphibians are characterized by paired limbs with articulated joints. The limbs are five-fingered.

The skull articulates movably with the cervical vertebra by two occipital condyles.

The pelvic girdle is tightly attached to the transverse processes of the sacral vertebra.

The eyes have movable eyelids and nictitating membranes to protect the eyes from clogging and drying out. Accommodation improves due to the convex cornea and flattened lens.

The forebrain enlarges and divides into two hemispheres. The midbrain and cerebellum are slightly developed. 10 pairs of cranial nerves depart from the brain.

The skin is bare, i.e. devoid of any horny or bone formations, permeable to water and gases. Therefore, it is always moist - oxygen first dissolves in the liquid covering the skin, after which it diffuses into the blood. The same thing happens with carbon dioxide, but in the opposite direction.

The kidneys, like those of fish, are primary = mesonephric.

To capture sound waves air environment The eardrum appears, followed by the middle ear (tympanic cavity), in which the auditory ossicle is located - the stirrup, which conducts vibrations to the inner ear. The Eustachian tube communicates with the middle ear cavity and the oral cavity. Choanae appear - internal nostrils, and the nasal passages become through.

Body temperature is not constant (poikilothermia) depends on the ambient temperature and only slightly exceeds the latter.

Aromorphoses:

Lungs and pulmonary breathing appeared.

Has become more complicated circulatory system, the pulmonary circulation has developed, i.e. Amphibians have two circles of blood circulation - large and small. The heart is three-chambered.

Paired five-fingered limbs were formed, representing a system of levers with articulated joints and intended for movement on land.

A cervical region has formed in the spine, which provides movement of the head, and a sacral region - the place of attachment of the pelvic girdle.

The middle ear, eyelids, and choanae appeared.

Muscle differentiation.

Progressive development of the nervous system.

Phylogeny.

Amphibians evolved from ancient lobe-finned fish in the Devonian period of the Paleozoic era approximately 350 million years ago. The first amphibians, Ichthyostegas, resembled modern tailed amphibians in appearance. Their structure had features characteristic of fish, including rudiments of the gill cover and lateral line organs.

Cover. Double layer. The epidermis is multilayered, the corium is thin, but abundantly supplied with capillaries. Amphibians have retained the ability to produce mucus, but not with individual cells, as in most fish, but with formed alveolar-type mucous glands. In addition, amphibians often have granular glands with a poisonous secretion of varying degrees of toxicity. The skin color of amphibians depends on special cells - chromatophores. These include melanophores, lipophores and iridocytes.

Under the skin of frogs there are extensive lymphatic lacunae - reservoirs filled with tissue fluid and allowing, under unfavorable conditions, to accumulate a supply of water.

Skeleton divided into axial and accessory, as in all vertebrates. The vertebral column is more differentiated into sections than in fish and consists of four sections: cervical, trunk, sacral and caudal. The cervical and sacral sections each have one vertebra. Anurans usually have seven trunk vertebrae, and all caudal vertebrae (about 12) merge into a single bone - the urostyle. Caudates have 13 - 62 trunk and 22 - 36 caudal vertebrae; in legless animals the total number of vertebrae reaches 200–300. Availability cervical vertebra important because Unlike fish, amphibians cannot turn their body so quickly, and the cervical vertebra makes the head mobile, but with a small amplitude. Amphibians cannot turn their heads, but they can tilt their heads.

The type of vertebrae in different amphibians may vary. In legless and lower caudate vertebrae are amphicoelous, with a preserved notochord, like in fish. In higher caudates, the vertebrae are opisthocoelous, i.e. The bodies are curved in front and concave in the back. In tailless animals, on the contrary, the anterior surface of the vertebral bodies is concave and the posterior surface is curved. Such vertebrae are called procoelous. The presence of articular surfaces and articular processes not only ensures a strong connection of the vertebrae, but also makes the axial skeleton mobile, which is important for the movement of tailed amphibians in water without the participation of limbs, due to the lateral bending of the body. In addition, vertical movements are possible.

The amphibian skull is a modified skull of a bony fish, adapted to terrestrial existence. The brain skull remains predominantly cartilaginous for life. The occipital region of the skull contains only two lateral occipital bones, which are carried along the articular condyle, with the help of which the skull is attached to the vertebrae. The visceral skull of amphibians undergoes the greatest transformations: secondary upper jaws appear; formed by the premaxillary and maxillary bones. The reduction of gill breathing led to a radical change in the hyoid arch. The hyoid arch is transformed into an element of the hearing aid and a sublingual plate. Unlike fish, the visceral skull of amphibians is directly attached by the palatoquadrate cartilage to the bottom of the brain skull. This type of direct connection of the components of the skull without the participation of elements of the hyoid arch is called autostyly. Amphibians lack elements of the operculum.

The accessory skeleton includes the bones of the girdles and free limbs. Like fish, the bones of the shoulder girdle of amphibians are located in the thickness of the muscles that connect them to the axial skeleton, but the girdle itself is not directly connected to the axial skeleton. The belt provides support for the free limb.

All land animals constantly have to overcome gravity, which fish do not have to do. The free limb serves as a support, allows you to lift the body above the surface and provides movement. The free limbs consist of three sections: proximal (one bone), intermediate (two bones) and distal (relatively large number of bones). Representatives of different classes of terrestrial vertebrates have structural features of one or another free limb, but all of them are of a secondary nature.

In all amphibians, the proximal part of the free forelimb is represented by the humerus, the intermediate part by the ulna and radius in caudates, and a single bone of the forearm (it is formed as a result of the fusion of the ulna and radius) in anurans. The distal section is formed by the wrist, metacarpus and phalanges of the fingers.

The girdle of the hind limbs articulates directly with the axial skeleton, with its sacral section. A reliable and rigid connection of the pelvic girdle with the spinal column ensures the functioning of the hind limbs, which are more important for moving amphibians.

Muscular system different from the muscular system of fish. The trunk muscles retain their metameric structure only in the legless. In caudates, the metamerism of segments is disrupted, and in tailless amphibians, sections of muscle segments begin to separate, differentiating into ribbon-shaped muscles. The muscle mass of the limbs increases sharply. In fish, the movements of the fins are ensured mainly by muscles located on the body, while the five-fingered limb moves due to muscles located in itself. A complex system of muscles - antagonists - flexor and extensor muscles appears. Segmented muscles are present only in the region of the spinal column. The muscles of the oral cavity become more complex and specialized (masticatory, tongue, floor of the mouth), not only involved in the capture and swallowing of food, but also providing ventilation of the oral cavity and lungs.

Body cavity– in general. In amphibians, due to the disappearance of gills, the relative position of the pericardial cavity has changed. She was pushed to the bottom of the chest into the area covered by the sternum (or coracoid). Above it, in a pair of coelomic canals, lie the lungs. Cavities containing the heart and lungs. Separates the pleurocardial membrane. The cavity in which the lungs are located communicates with the main coelom.

Nervous system. The brain is of the ichthyopsid type, i.e. the main integrating center is the midbrain, but the amphibian brain has a number of progressive changes. The amphibian brain has five sections and differs from the fish brain mainly in the greater development of the forebrain and the complete separation of its hemispheres. In addition, the nerve substance already lines, in addition to the bottom of the lateral ventricles, also the sides and roof, forming the medullary vault - the archipallium. The development of the archipallium, accompanied by strengthening connections with the diencephalon and especially the midbrain, leads to the fact that associative activity regulating behavior in amphibians is carried out not only by the medulla oblongata and midbrain, but also by the forebrain hemispheres. The elongated hemispheres in front have a common olfactory lobe, from which two olfactory nerves originate. Behind the forebrain is the diencephalon. The epiphysis is located on its roof. On the underside of the brain there is an optic chiasm (chiasma). The infundibulum and the pituitary gland (lower medullary gland) extend from the bottom of the diencephalon.

The midbrain is represented as two round optic lobes. Behind the optic lobes lies the underdeveloped cerebellum. Immediately behind it is the medulla oblongata with the rhomboid fossa (fourth ventricle). The medulla oblongata gradually passes into the spinal cord.

In amphibians, 10 pairs of head nerves arise from the brain. The eleventh pair is not developed, and the twelfth pair extends outside the skull.

The frog has 10 pairs of true spinal nerves. The three anterior ones take part in the formation of the brachial plexus, which innervates the forelimbs, and the four posterior pairs take part in the formation of the lumbosacral plexus, which innervates the hind limbs.

Sense organs provide orientation for amphibians in water and on land.

All larvae and adults with an aquatic lifestyle have lateral line organs. They are represented by a cluster of sensitive cells with nerves corresponding to them, which are scattered throughout the body. Sensitive cells perceive temperature, pain, tactile sensations, as well as changes in humidity and chemical composition of the environment.

Olfactory organs. Amphibians have a small external nostril on each side of the head, which leads into an elongated sac that ends in the internal nostril (choana). The choanae open at the front of the roof of the oral cavity. In front of the choanae on the left and right there is a sac that opens into the nasal cavity. This is the so-called vomeronasal organ. In him a large number of sensory cells. Its function is to receive olfactory information about food.

The organs of vision have a structure characteristic of a terrestrial vertebrate. This is expressed in the convex shape of the cornea, the lens in the form of a biconvex lens, and movable eyelids that protect the eyes from drying out. But accommodation, as in fish, is achieved by moving the lens by contracting the ciliary muscle. The muscle is located in the annular ridge surrounding the lens, and when it contracts, the frog's lens moves forward somewhat.

The hearing organ is arranged according to the terrestrial type. A second section appears - the middle ear, in which the auditory bone, the stapes, which first appears in vertebrates, is located. The tympanic cavity is connected to the pharyngeal region by the Eustachian tube.

The behavior of amphibians is very primitive; conditioned reflexes are developed slowly and fade away quickly. The motor specialization of reflexes is very small, so the frog cannot form a protective reflex of withdrawing one leg, and when one limb is irritated, it jerks both legs.

Digestive system begins with the oral fissure leading into the oropharyngeal cavity. It houses a muscular tongue. Ducts open into it salivary glands. The tongue and salivary glands first appear in amphibians. The glands serve only to wet the bolus of food and do not participate in the chemical processing of food. On the premaxillary, maxillary bones, and vomer there are simple conical teeth, which are attached to the bone with their base. The digestive tube is differentiated into the oropharyngeal cavity, a short esophagus that carries food into the stomach, and a voluminous stomach. Its pyloric part passes into the duodenum - the beginning of the small intestine. The pancreas lies in the loop between the stomach and duodenum. The small intestine smoothly passes into the large intestine, which ends in a pronounced rectum that opens into the cloaca.

The digestive glands are the liver with the gallbladder and the pancreas. The liver ducts, together with the gallbladder duct, open into the duodenum. The pancreatic ducts empty into the gallbladder duct, i.e. This gland does not have independent communication with the intestines.

That. The digestive system of amphibians differs from the similar system of fish in the greater length of the digestive tract; the final section of the large intestine opens into the cloaca.

Circulatory system closed. Two circles of blood circulation. The heart is three-chambered. In addition, the heart has a venous sinus that communicates with the right atrium, and the conus arteriosus extends from the right side of the ventricle. Three pairs of vessels depart from it, homologous to the gill arteries of fish. Each vessel begins with an independent opening. All three vessels of the left and right sides first go through a common arterial trunk, surrounded by a common membrane, and then branch.

The vessels of the first pair (counting from the head), homologous to the vessels of the first pair of gill arteries of fish, are called carotid arteries, which carry blood to the head. Through the vessels of the second pair (homologous to the second pair of gill arteries of fish) - the aortic arches - blood is directed to the back of the body. The subclavian arteries depart from the aortic arches, carrying blood to the forelimbs.

Through the vessels of the third pair, homologous to the fourth pair of gill arteries of fish - the pulmonary arteries - blood is sent to the lungs. Each pulmonary artery gives rise to a large cutaneous artery, which carries blood into the skin for oxidation.

Venous blood 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, forms two anterior vena cava. They carry mixed blood into the venous sinus, since arterial blood moves through the skin veins.

Amphibian larvae have one circulation; their circulatory system is similar to the circulatory system of fish.

Amphibians develop a new circulatory organ - the red bone marrow of the long bones. Red blood cells are large, nuclear, white blood cells are not identical in appearance. There are lymphocytes.

Lymphatic system. In addition to the lymphatic sacs located under the skin, there are lymphatic vessels and hearts. One pair of lymphatic hearts is placed near the third vertebra, the other - near the cloacal opening. The spleen, which looks like a small round red body, is located on the peritoneum near the beginning of the rectum.

Respiratory system. Fundamentally different from the respiratory system of fish. In adults, the respiratory organs are the lungs and skin. The airways are short due to the absence of the cervical spine. Represented by the nasal and oropharyngeal cavities, as well as the larynx. The larynx opens directly into the lungs with two openings. Due to the reduction of the ribs, the lungs are filled by swallowing air - according to the principle of a pressure pump.

Anatomically, the respiratory system of amphibians includes the oropharyngeal cavity (upper airways) and the laryngeal-tracheal cavity (lower airways), which directly passes into the sac-like lungs. During embryonic development, the lung is formed as a blind outgrowth of the anterior (pharyngeal) section of the digestive tube, and therefore remains connected to the pharynx in adulthood.

That. The respiratory system in terrestrial vertebrates is anatomically and functionally divided into two sections - the airway system and the respiratory section. The airways carry out two-way transport of air, but do not participate in gas exchange itself; the respiratory department carries out gas exchange between the internal environment of the body (blood) and atmospheric air. Gas exchange occurs through the surface liquid and occurs passively in accordance with the concentration gradient.

The system of gill covers becomes unnecessary, therefore the gill apparatus in all terrestrial animals is partially modified, its skeletal structures are partially included in the skeleton (cartilage) of the larynx. Ventilation of the lungs is carried out due to forced movements of special somatic muscles during the respiratory act.

excretory system, as in fish, it is represented by primary, or trunk buds. These are compact bodies of a reddish-brown color, lying on the sides of the spine, and not ribbon-shaped, like those of fish. From each kidney a thin Wolffian canal stretches to the cloaca. In female frogs it serves only as a ureter, and in males it serves as both a ureter and a vas deferens. In the cloaca, the Wolffian canals open with independent openings. It also opens separately into the cloaca and bladder. The final product of nitrogen metabolism in amphibians is urea. In aquatic amphibian larvae, the main product of nitrogen metabolism is ammonia, which is excreted in solution through the gills and skin.

Amphibians are hyperosmotic animals in relation to fresh water. As a result, water constantly enters the body through the skin, which does not have mechanisms to prevent this, like other terrestrial vertebrates. Sea water is hyperosmotic in relation to the osmotic pressure in the tissues of amphibians; when they are placed in such an environment, water will leave the body through the skin. This is why amphibians cannot live in sea water and die in it from dehydration.

Reproductive system. In males, the reproductive organs are represented by a pair of round, whitish testes adjacent to the ventral surface of the kidneys. 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 to the Wolffian canals and through them to the cloaca. Before flowing into the cloaca, the Wolffian canals form a small expansion - seminal vesicles, which serve for the temporary storage of sperm.

The reproductive organs of females are represented by paired ovaries of a granular structure. Above them are the fat bodies. They accumulate nutrients that ensure the formation of reproductive products during hibernation. In the lateral parts of the body cavity there are strongly convoluted light oviducts, or Müllerian canals. Each oviduct into the body cavity in the region of the heart opens with a funnel; the lower uterine part of the oviducts is sharply expanded and opens into the cloaca. Ripe eggs through the rupture of the walls of the ovary fall into the body cavity, then are captured by the funnels of the oviducts and move along them to the cloaca.

Wolffian canals in females perform only the functions of the ureters.

In tailless amphibians, fertilization is external. The eggs are immediately irrigated with seminal fluid.

External sexual characteristics of males:

Males have a genital wart on the inner finger of the forelimbs, which reaches a special development at the time of reproduction and helps the males to hold the females during the fertilization of eggs.

Males are usually smaller than females.

Development amphibians is accompanied by metamorphosis. The eggs contain relatively little yolk (mesolecithal eggs), so radial crushing occurs. A larva emerges from the egg - a tadpole, which in its organization is much closer to fish than to adult amphibians. It has a characteristic fish-like shape - a long tail surrounded by a well-developed swimming membrane, on the sides of the head it has two to three pairs of external feathery gills, there are no paired limbs; There are lateral line organs; the functioning kidney is the pronephros (pre-kidney). Soon the external gills disappear, and in their place three pairs of gill slits with their gill filaments develop. At this time, the similarity of the tadpole with a fish is also a two-chambered heart, one circle of blood circulation. Then, by protrusion from the abdominal wall of the esophagus, paired lungs develop. At this stage of development, the arterial system of the tadpole is extremely similar to the arterial system of lobe-finned and lungfishes, and the only difference is that due to the absence of the fourth gill, the fourth afferent gill artery passes into the pulmonary artery without interruption. Even later, the gills are reduced. In front of the gill slits, a fold of skin is formed on each side, which, gradually growing back, tightens these slits. The tadpole switches entirely to pulmonary breathing and swallows air through its mouth. Subsequently, the tadpole develops paired limbs - first the front ones, then the hind ones. However, the anterior ones remain hidden under the skin longer. The tail and intestines begin to shorten, mesonephros appears, the larva gradually moves from plant food to animal food and turns into a young frog.

During the development of the larva, its internal systems are reconstructed: respiratory, circulatory, excretory, digestive. Metamorphosis ends with the formation of a miniature copy of the adult individual.

Ambystomas are characterized by neoteny, i.e. They reproduce with larvae, which for a long time were mistaken for an independent species, which is why they have their own name - axolotl. This larva is larger than the adult. Another interesting group are proteas that constantly live in water, which retain external gills throughout their lives, i.e. signs of a larva.

The metamorphosis of a tadpole into a frog is of great theoretical interest, since not only proves that amphibians evolved from fish-like creatures, but makes it possible to restore in detail the evolution of individual organ systems, in particular the circulatory and respiratory systems, during the transition of aquatic animals to terrestrial ones.

Meaning amphibians is that they eat many harmful invertebrates and themselves serve as food for other organisms in food chains.

These are vertebrates, one of the most ancient that appeared on Earth in the Devonian period. They originated from Rhipidistia, predatory lobe-finned fish that emerged from the water onto land. There are not so many amphibians, about six thousand species, they are divided into caudate, tailless and legless.

IN ordinary life The easiest way to find a frog or toad. And it’s unlikely that anyone would want to face a giant Chinese salamander, the weight of which can reach 100 kg.

Reptiles

Cold-blooded vertebrates. They are at a higher stage of development compared to amphibians. They are divided into four orders: crocodiles (various types of alligators, caimans, crocodiles), turtles, scaly (snakes, chameleons, s) and beakheads.

The main differences between amphibians and reptiles

1. The appearance of offspring.

Amphibians lay eggs, glued together, in water or damp burrows. Tadpoles emerge. They do have a tail. As they grow older, tadpoles lose their tail, but acquire eyelids, which gives them the ability to see both in water and on land. Among reptiles, only a small part is viviparous. The rest build nests and lay eggs. The offspring of reptiles are completely independent, as often the parent leaves the clutch and does not return to it. But crocodiles take care of both the hatched and the hatched.

2. Skin.

The skin of amphibians is smooth and moist. It’s not for nothing that they were once called naked bastards. The skin of amphibians is literally permeated with glands that secrete poisonous mucus to protect against the effects of the external environment and enemies. Some amphibians are harmless and, in order to protect themselves from attack, are forced to imitate their war color. poisonous frogs and toads. Between the skin and muscles of amphibians there are cavities with watery fluid.

In reptiles, or scaly reptiles, the skin is practically devoid of glands. It is impermeable to liquids and gases. The skin on top becomes keratinized and scales form on them. Periodically, reptiles molt, shedding their skin. Some get rid of the old skin at once, others in parts. The pattern on the shed skin is practically invisible, and the skin itself (the crawl) is colorless.

3. Diet.

Amphibians feed on insects, snails, worms, small invertebrate animals, rodents, and slugs that are harmful to plants. They do not disdain eggs laid by other amphibians and even attempt to kill their own kind. Sea toads eat dead animals and plants.

Among the reptiles you can find both insectivores and carnivores. The diet of reptiles includes fish, algae, birds and their eggs, and rodents. There are known cases of attacks by such a reptile as the Komodo dragon, even on children. Some of the reptiles are poisonous and before biting the victim, they inject poison into its body.

4. Life expectancy.

Under natural conditions, amphibians cannot boast of longevity. Although in captivity individual species salamanders can live to be half a century old. The lifespan of snakes and small lizards is from 2 to 20 years. But reptiles such as turtles live up to 100-200 years. So reptiles are the elders of the animal world.

Features of amphibians as a class

Introduction

The oldest amphibians - Ichthyostegas - lived in the Upper Devonian about 300 - 320 million years ago. These primitive amphibians still retained many of the original and even common features(signs) with lobe-finned fish. Therefore, the origin of amphibians from lobe-finned fish is beyond any doubt. The abundance and prosperity of these animals was noted in the Carboniferous, Triassic and Cenozoic, when they were represented by a variety of various forms. At the same time, in the Jurassic and Cretaceous periods, their development slowed down, their numbers and species diversity decreased. However, from the beginning of the Upper Carboniferous (in the Paleozoic) to the end of the Triassic (in the Mesozoic), amphibians predominated in the fauna of that time.

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

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

It was probably this close relationship between amphibians and other life forms that determined my choice of topic. The purpose of my work is to study this class.

My research can be divided into two parts. The first is theoretical. This part includes a search for various information about amphibians (encyclopedias, textbooks, brochures, specialized Internet sites, forums, magazines), recording on storage devices, as well as its analysis. Analyzing the information, I was faced with the problem of confirming or refuting it, for this I had to move on to the second part of the research - the practical one. The practical part includes observations of representatives of this class in their natural habitat, dissection of a frog (summer 2001), as well as keeping clawed frogs (albino form) at home.

I also address environmental issues in my work. modern world, in particular the impact of the anthropogenic factor on the environment.

General information about amphibians

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

Amphibian habitats

Currently on globe There are about 3000 species. Amphibians are commonly found in and around fresh water bodies. Here they feed on invertebrates. In case of danger, they quickly jump into the water.

Some types of amphibians, such as grass and sharp-faced frog, the common toad, live mostly away from bodies of water. During the day they hide among clods of soil and other shelters, and at dusk they go out hunting. Only during the breeding season do they move to live in water bodies.

All amphibians are active only in the warm season. When the ambient temperature drops to 4 ° C, they fall into a stupor. In a state of suspended animation, vital processes do not stop, but occur at a very low level, for example, the heart makes 1 - 2 contractions per minute. Amphibians usually winter at the bottom of reservoirs (frogs) or in various shelters on land (newts, toads).

External structure

The body shape of amphibians is different. Tailed amphibians are more similar to fish, have a laterally compressed body and a long, paddle-shaped tail; others, tailless or jumping (more than 75% of all amphibians), have a round or flat body, and no tail.

On the head of amphibians, two large bulging eyes and a pair of nostrils are visible. The eyes and nostrils are found on elevations in most species. Therefore, a frog, for example, can, without getting out of the water, breathe atmospheric air and orientate itself in its surroundings. Unlike fish, amphibian eyes have eyelids. The upper eyelid is mobile, the lower one has the appearance of a translucent nictitating membrane. The eyelids protect the eyes from clogging and keep them moist.

In frogs, toads and most other tailless amphibians, eardrums are visible on the head, separating the middle ear cavity from the external environment.

The limbs of some amphibians are absent (caecilians - Apoda), others are underdeveloped or, conversely, highly developed. Most amphibians have two pairs of legs (tailed amphibians may have only one pair). In tailless amphibians, the hind legs are longer and stronger than the front ones, which allows these animals to move by jumping. Swimming membranes are developed between the toes of the hind legs of tailless animals.

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

In the connective tissue layer of the skin of some amphibians there are small capsules filled with a gelatinous substance; in others, voluminous cavities are formed, adapted for the development and initial storage of embryos. Finally, some people sometimes develop ossifications or hard plates in their skin, somewhat similar to fish scales.

The coloring of some amphibians can change and is determined in most cases by the relative position and state of the special pigments of the cells contained in the skin. Contraction or expansion, change in shape, approaching the outer surface of the skin or moving away from it - everything gives one color or another to the skin and is caused by both changes external conditions, and internal irritation.

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

The elastic, very thin and uncoated skin of amphibians has great importance in their lives. Not a single amphibian drinks water in the usual way, but absorbs it exclusively through the skin. That is why they require proximity to water or dampness. Frogs removed from water quickly lose weight, become lethargic and soon die. If you put a wet rag near such frogs, exhausted by dryness, they begin to press their bodies against it and quickly recover. How large is the amount of water that frogs absorb through their skin can be seen from the following Thomson experiment. He took a dried tree frog and, after weighing it, found that its weight was 95 grams. After that, he wrapped it in a wet rag, and after an hour it already weighed 152 grams. Amphibians absorb water through their skin and sweat it out. Also, gas exchange occurs through the skin, which contains a huge number of capillaries. In a closed tin box, surrounded by a humid atmosphere, a frog can live for 20–40 days, even if the air supply to the lungs is cut off. Breathing through the skin occurs both in water and on land. Skin respiration is of particular importance at a time when the amphibian is in the water for a long time (hibernation, long stay in a reservoir in case of danger).

Features of the amphibian skeleton

The skeletal structure of amphibians is to some extent similar to that of fish. The skeleton of the head, torso and limbs is distinguished. The skeleton of the head is formed by fewer bones than in fish. The structure of the skull is varied. Here you can notice a gradual increase in bone formations due to cartilaginous and connective tissue. characteristic feature of the entire class of amphibians are two articulated heads on the back of the skull, which correspond to the two dimples of the first cervical vertebra. The skull is always flat, wide, the eye sockets are very large. The skull consists of the occipital bones, two frontal bones, and the main bone. In the lateral walls of the skull, for the most part, ossification does not occur at all, or the cartilage ossifies partially. The palatine bones are immovably connected to the skull; teeth sometimes sit on them, just like on the vomer and on the sphenoid bone. The lower jaw consists of two or more parts and never completely ossifies. The facial region is much larger than the cerebral region.

The spine of amphibians, due to their semi-terrestrial lifestyle, is more dissected compared to fish. It consists of the cervical, trunk, sacral and caudal sections. In fish-like amphibians, the vertebrae are exactly the same as in fish; in others, vertebrae develop with an articulated head in front and a dimple in the back, which results in full articulation. The transverse processes of the vertebrae in all amphibians are well developed, but real ribs usually do not develop; instead, there are only bone or cartilaginous appendages. The transverse processes of some are very long and replace the ribs.

The cervical region is formed by one vertebra that connects to the skull. The number of trunk vertebrae varies among amphibians. In some species, such as newts, poorly developed ribs articulate with the trunk vertebrae.

Most amphibians have a sacral section; it consists of one vertebra. The tail section of tailless amphibians is small (the vertebrae are fused into one bone). In tailed amphibians this section is well expressed.

The paired limbs of amphibians differ sharply from the paired fins of fish. If the fins of fish are single-membered levers with their own muscles, then in the skeleton of the fore limb of amphibians the humerus, bones of the forearm and hand are distinguished, and in the skeleton of the hind limb - the femur, bones of the lower leg and bones of the foot. The support for the forelimbs is the skeleton of the shoulder girdle, consisting of paired shoulder blades, crow bones, and in most cases, also clavicles. Connected to the girdle of the forelimbs is the sternum, or sternum. The support of the hind limbs is the pelvic girdle, consisting of the pelvic bones, which are connected to the processes of the sacral vertebra or to the processes of the last trunk vertebra.

Amphibian musculature

The muscular system of amphibians is more complex than that of fish. It consists of different groups muscles. Tailless animals have the most developed muscles of the limbs, which are attached by tendons to the bones and cause their movements. In tailed amphibians, the muscles of the tail are most developed.

Digestive system

Consists of the oral cavity, pharynx, esophagus, stomach and intestines. Amphibians have a more developed stomach, and the duodenum, small and large intestines are prominent in the intestines. The liver ducts open into the duodenum along with the gallbladder duct, into which the pancreatic ducts open. The final digestion of food and absorption into the blood occurs in the small intestine. nutrients. Undigested food remains accumulate in the colon. The large intestine ends in the rectum, called the cloaca. The bladder also opens here (in defense, the frog can release a stream of accumulated urine), ureters and oviducts (in females).

Amphibians are able to endure fasting for a very long time; a toad planted in a damp place can remain without food for more than two years.

Respiratory system

Most amphibians breathe using the lungs and skin (skin respiration was discussed above). The lungs of amphibians are poorly developed and imperfect. They have a small internal area of ​​contact with the inhaled air. The lungs look like oblong sacs with thin elastic walls, in which many capillaries branch. Such lungs cannot fully supply the body with oxygen. Amphibians do not suck, but swallow air. The animal increases the volume of the oral cavity, and air enters it through the nostrils. As the floor of the mouth rises toward the roof of the mouth, the nostrils close and air is forced through the larynx into the lungs. Gas exchange occurs in the lungs: oxygen penetrates the capillaries, and carbon dioxide from the blood passes into the air, which is then released outside.

Pulmonary and skin respiration in amphibians is unequally developed. For those who most spends its life in water, the lungs are less developed, and skin respiration is better. Amphibian larvae breathe through gills. Some tailed amphibians retain gills for life.

Circulatory system

Due to the presence of lungs, the circulatory system of amphibians has a more complex structure than that of fish. The heart of amphibians consists of three chambers: two atria and one ventricle. Blood from all organs collects in the veins and enters the right atrium. This blood contains a lot of carbon dioxide and nutrients coming from the intestines. Blood from the lungs enters the left atrium. It is rich in oxygen.

When the atria contract, blood is pushed into the ventricle. Here it is partially mixed. Leaves the stomach major artery, it is divided into branches that carry blood to all organs of the body (this big circle blood circulation), and the branches through which blood goes to the lungs and skin (this is the pulmonary circulation). Thus, amphibians, unlike fish, have not one, but two circles of blood circulation - large and small.

The red blood cells of amphibians still have nuclei. Due to their presence, less oxygen reaches the organs.

Excretory system

The excretory system of amphibians includes oblong red-brown kidneys, which are located in the body cavity on the sides of the spine, ureters and bladder. Substances that are unnecessary for the body, released from the blood, enter the cloaca through the ureters and are removed outside.

Metabolism

Underdeveloped lungs, a circulatory system with mixed blood and red blood cells containing nuclei, limit the supply of oxygen to the organs. Therefore, oxidative processes in tissues proceed slowly and little energy is released. As a result, the body temperature of amphibians is variable. Amphibians are cold-blooded animals.

These factors also influence the lifestyle of amphibians. All amphibians are sedentary.

Nervous system

The amphibian brain is simple. It has an elongated shape and consists of two anterior hemispheres, the midbrain and the cerebellum, which is only a transverse bridge, as well as the medulla oblongata. In amphibians, the forebrain is more developed (later in evolution, the development of the forebrain will be observed), but there is still no cerebral cortex, gray matter, nerve cells are scattered over the entire surface, the cerebellum is weaker. Weak development cerebellum is associated with the uniformity of motor reactions of amphibians. The spinal cord is much better developed than the brain.

The behavior of amphibians is based on unconditioned reflexes, and conditioned ones are developed after a long combination of unconditioned and conditioned stimuli.

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

The teeth are adapted only for grasping and holding prey, but cannot be used for chewing it.

Reproductive organs of amphibians

Amphibians are dioecious animals. The ovaries of females and the testes of males are located in the body cavity.

Time and place of breeding of amphibians

After hibernation, all amphibians (with rare exceptions) accumulate in fresh water bodies. Soon the females begin to lay eggs. Some of them, for example brown frogs, deposit it near the shore of a reservoir in shallow, warm areas. Others, such as green frogs, lay their eggs at great depths, most often among aquatic plants. In frogs, the eggs are glued together into large clumps, in toads - into long cords. Newts place single eggs (eggs) on the leaves or stems of aquatic plants. Fertilization in most amphibians is external. At the same time, the males release liquid with sperm into the water. After fertilization, embryos develop in the eggs.

Amphibians are anamniac, that is, their eggs do not have amniotic fluid, this is due to development in an aquatic environment. However, the eggs are surrounded by a thick layer of a transparent gelatinous substance. This shell is of great importance for the embryo. It protects the embryo from drying out, mechanical damage, prevents the eggs from approaching each other, thereby improving the access of oxygen, and it also protects them from being eaten by other animals; indeed, very few birds are able to swallow a gelatinous lump of frog spawn; The shell itself also protects the eggs from attacks by fish, shellfish and aquatic insects. In addition, this shell, like a lens, collects the sun's rays onto developing fetus. The eggs themselves are black, so they absorb heat well sun rays necessary for the development of the embryo.

Development of the embryo

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

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

At the beginning of their lives, amphibians grow very quickly, but over time their growth slows down greatly. Frogs become mature only at 4–5 years of life, although the larvae of some amphibians (axolotls) reach maturity before metamorphosis, but continue to grow for another 10 years; others reach their present size only after 30 years.

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Among the animals that humans encounter, there are many that, despite their external similarity, differ from each other. These include amphibians and reptiles.

Where do amphibians live

Amphibians belong to primitive vertebrates living on earth. They have the qualities of terrestrial and aquatic animals. Most breed and develop in fresh water. Growing up, they live on land. Such amphibians include salamanders, newts, frogs and caecilians. Science knows up to seven thousand amphibians. Of these, 90% are frogs. Most amphibians live in humid and warm environments. The name "amphibian" is of ancient Greek origin and refers to creatures capable of living in water and on land.

Amphibians originate from ancient lobe-finned fish. As a result of evolution, amphibians developed limbs with five fingers, lungs and a three-chambered heart. They formed two circles of blood circulation and the middle ear. There are amphibians without a tail and legs. In amphibians, the head is connected to the body, most with a tail and four five-fingered paws. Amphibians alternate between being on land and in water. There are known species that live mainly in water or on trees. When it is warm, they move in search of food, hunt.

React to seasonal changes, during cold weather or droughts, they become numb and hibernate. They die at sub-zero temperatures. However, amphibians are known that can survive prolonged drying out or freezing. The abilities of some of them are extraordinary. For example, the sea toad is able to live in salt water. Some amphibians are able to restore the lost parts of the body themselves. Amphibians are cold-blooded animals with a low metabolic rate. Body temperature is related to the state of the environment.

The body is provided blood and lymph. The respiratory organs are the lungs, and in some aquatic inhabitants - gills. Additional respiratory organs are the oral mucosa and skin. The brain is larger than most fish. nerve fibers permeate the body.. Smooth thin skin facilitates gas exchange. The skin glands secrete mucus, often poisonous. Complex excretory organs retain water in the body of amphibians. They have developed sense organs. Adult amphibians are predators that hunt primarily insects.

Could dinosaurs be their relatives? These animals were called reptiles because of the way the first reptiles moved. When they moved, their belly dragged along the ground.

Reptiles are primarily vertebrate animals and live on land. This applies to crocodiles, lizards, turtles and snakes. The distant ancestors of reptiles ruled the earth in time immemorial, but eventually died out for unknown reasons. Today scientists know over nine thousand species of reptiles.

Reptiles have both features of developed vertebrates and primitive amphibians. Metabolism level is not high. Inactivity is interrupted sporadically for short periods sudden movements and throws. The strong and dry skin cover is covered with keratinized elements on the outside. Thus, the fused scutes of turtles form durable shell protecting these animals. And the horny scales of lizards resemble tiles, overlapping each other.

The outer cover of reptiles changes periodically during partial or complete molting. The skin of reptiles contains glands that secrete a characteristic odor. And some reptiles, for example, chameleons, contain substances that instantly change color. They have a developed skeleton and muscles, which are able to feed on energy without oxygen. This encourages reptiles to make short-distance throws. After this, as a result of the accumulation of lactic acid, the reptile's muscles become tired and require several hours of rest.

The developed brain of reptiles compares favorably with the brain of amphibians. Sense organs help to confidently navigate space and get food. Reptiles are sensitive to heat and detect the source. Hearing does not play a decisive role in the life of reptiles, but the sense of touch is developed. Reptiles breathe with their lungs skin covering is not involved in this. These cold-blooded animals have a three-chambered heart, with the exception of crocodiles, which have a four-chambered heart.

Body temperature is regulated by moving into the sun or shade. They can make the color darker to keep warm, and lighter to cool. The embryos of reptiles develop mainly in a shell-covered egg. Most of them are carnivores. Some have a mixed or herbivorous diet. The only reptiles known to be predators are snakes, crocodiles and some lizards. Reptiles are able to run, crawl, swim, and some even glide in the air.

How are they different?

Amphibians and reptiles are distinguished by the following characteristics.

1. Amphibians descended from animals that lived in water; the ancestors of reptiles were land dinosaurs.
2. Amphibians are born in an aquatic environment with gills that develop into lungs. Reptiles are born with lungs.
3. Amphibians are able to breathe using their own skin. Reptiles do not have such qualities.
4. Amphibians live near water bodies and in damp places. Reptiles need mainly dry and hot places.
5. The skin of amphibians is thin and without scales with a large number of mucus-secreting glands. Reptiles have dry skin, without glands, and shed periodically.
6. Amphibians have simple brains and sensory organs. In reptiles, the life support of the body is more developed.
7. Amphibians are capable of living in cold conditions, even freezing. Reptiles need warmth. They die in the cold.
8. Fertilization of amphibians occurs in water. In reptiles it is internal. Reptiles hatch from eggs.
9. The diet of amphibians consists primarily of invertebrates. Reptiles are carnivores and also feed on plant foods.
10. The lifespan of amphibians is shorter than that of reptiles.