Water and terrestrial environment habitats have significant differences from each other. Therefore, the change in environment that occurred in amphibians led to significant changes in the structure of a number of organ systems.
Why did the fish “come out” to land? At that time, plants growing above water appeared. They were inhabited by insects and their larvae, which fish could feed on. For example, fallen dead insects or larvae living in water, or insects on the surface of the water. The more water a fish can get out of the water, the more full it will be. It is also possible that the reservoirs periodically dried up, and those ancient lobe-finned fish had to move to a neighboring reservoir. Thus, those who were able to do this, that is, those who had better developed lungs and limbs, received an advantage.
The main differences between water and air are related to the density of these media and the possibility of physical and chemical processes occurring. Water is a denser substance; it has a much greater buoyancy force compared to air. Despite the fact that there is more oxygen in the air than in water, oxygen can penetrate the body if there is moisture on the surface of the body, i.e. metabolic reactions occur only in solutions. A living organism living on land faces two problems - the inability to move by swimming and the body drying out in the air.
Fish swimming in the water column have a laterally compressed body. Thanks to this shape, they can swim while bending. However, in land animals, the body is more compressed from top to bottom due to gravity. In this case, it is more convenient to use lever limbs for movement. Parts of such limbs can bend and straighten relative to each other with the help of muscles attached to them. Amphibians have more of these specialized muscles than segmented musculature.
At the same time, on land, the limbs serve not only for movement, but also for supporting the body above the ground. For limbs to support body weight, they must be connected to something strong and not just end at soft tissues animal. Therefore, the limbs of amphibians, unlike the fins of fish, are attached to the spine with the help of well-developed limb girdles.
To reduce movement of the entire body during examination environment amphibians developed a neck.
The spine of amphibians has become more complex due to the acquisition of additional functions. It contains the cervical, trunk, sacral, and caudal sections.
Unlike fish, amphibians have bare, moist skin with many capillaries. Thanks to this, amphibians can carry out cutaneous respiration, that is, absorb oxygen from the air through the surface of the skin. Skin moisture is maintained by special glands that produce mucus.
However, skin respiration is not enough. Amphibians also receive oxygen through the walls of their mouth (which is quite large) and through their lungs. The larvae of many amphibians have gills, which dissolve in adults. Instead, from the invaginations of the wall of the pharynx, lungs are formed, which are folded bags. The inside of the mouth and lungs is always moist.
The nasal cavity in amphibians opens not only outward (like in fish), but also into the oral cavity. Penetrating through it, the air is also moistened.
Fish have a two-chambered heart, consisting of one atrium and one ventricle, one circle of blood circulation. Amphibians have a three-chambered heart, consisting of two atria and one ventricle. This structure is due to the appearance of pulmonary respiration and the need for a second circle of blood circulation.
Visibility in air is higher than in water. Therefore, amphibians have better developed organs of vision; they see further than fish. To prevent the eyes from drying out, eyelids and a nictitating membrane appear.
Most amphibians do not have a lateral line.
Sound travels worse in air than in water (due to low density). For Pisces to hear, it was enough to have only inner ear. In amphibians, an eardrum appears that can detect air vibrations, as well as bones that transmit vibrations of the membrane to the inner ear.
Amphibians have developed salivary glands that allow them to wet food. Fish living in water did not need them.
The excretion product ammonia in fish is released through the gills. In amphibians, ammonia is converted in the kidneys to urea, which is not toxic.
In amphibians, compared to fish, the cerebellum is less developed. This is due to the fact that amphibians do not need to move as difficult as fish.
Amphibians , or amphibians, are very different from all vertebrates. In their life, two periods must be distinguished: in their youth they are similar to fish, and then they gradually turn into animals with pulmonary breathing. Thus, in the development cycle of amphibians, a transformation takes place that is almost never found in other vertebrates, and, on the contrary, is widespread in lower, invertebrate animals.
General characteristics.
In their lifestyle and appearance, amphibians are very similar, on the one hand, to reptiles, and on the other, even more so, to fish; their larval stage constitutes, as it were, a transition between these two orders.
Body shape varies greatly. Tailed amphibians are more similar to fish, have a laterally compressed body and a long paddle-shaped tail; in others the body is round or flat, and the tail is completely absent. Some amphibians have no limbs at all, in others they are very poorly developed, in others, on the contrary, they are highly developed.
Device skeleton amphibians are to some extent similar to fish. In fish-like amphibians, the vertebrae are exactly the same as those of fish; in others, vertebrae develop with an articular head in front and a dimple behind, which determines full articulation. The transverse processes of the vertebrae in all amphibians are well developed, but true ribs usually do not develop; instead of them there are only small bone or cartilaginous appendages. The above-mentioned transverse processes in some are very long and replace the missing ribs.
Device skulls can be varied; you can see here gradual complication and increase bone formations due to cartilaginous and connective tissue. A characteristic feature of the entire class of amphibians are two articular heads on the occipital part of the skull, which correspond to the two dimples of the first cervical vertebra. The skull is always flat, wide, the eye sockets are very large. The skull consists of two occipital bones, two frontal bones, and a sphenoid bone. In the lateral walls of the skull, for the most part, ossification does not occur at all, or the cartilage ossifies partially. The palatine bones are fixedly connected to the skull; On them, just like on the vomer and on the wedge bone, teeth sometimes sit. The lower jaw consists of two or more parts and never completely ossifies.
Brain amphibians have a simple structure. It has an elongated shape and consists of two anterior hemispheres, the midbrain and cerebellum, which is only a transverse bridge, and the medulla oblongata. The spinal cord is much more developed than the brain.
From feelings vision, hearing and smell are more developed. The tongue of most amphibians is well developed and in frogs it differs significantly from the tongue of other vertebrates in that it is attached not by the rear, but by the front end and can be thrown out of the mouth.
The teeth, like those of reptiles, are adapted only for grasping and holding prey, but cannot serve to chew it.
Alimentary canal relatively short and simply designed; it consists of a long esophagus, a simple thick-walled stomach and hindgut. All amphibians have lobed livers, gallbladder, pancreas, kidneys and bladder.
Circulatory and respiratory organs are of great importance in the life of amphibians and will be discussed further in connection with the history of development.
Feature of amphibians lies in the absence of any hard outer coverings, which is why they are called naked reptiles. Indeed, they have neither scales, like fish and reptiles, nor feathers, nor hair, like mammals; the majority are covered on the outside only with bare skin, and only a very few have some traces or semblance of horny formations on the skin. But in the skin of amphibians there are some formations that are not found in other vertebrates.
In the connective tissue layer of the skin of some amphibians there are small capsules filled with a gelatinous substance; in others, voluminous cavities are formed, adapted for the development and initial storage of embryos. Finally, in some people, ossifications or hard plates sometimes appear in the skin, somewhat similar to fish scales. The top layer of skin is very thin and often contains various dyes.
However, the color of some amphibians can change, as we saw in chameleons, and in most cases is determined by the relative position and state of the special pigments of the cells contained in the skin. Compression 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 skin, and in the internal part of all amphibians there are a lot of glands of various sizes and for various purposes. The most interesting of them are the poisonous glands. They are located in the lower layer of the skin, have a spherical or oval shape, and secrete a mucous fluid that contains a toxic substance. Amphibians, which have more developed such glands, can voluntarily increase the secretion of these glands and use it as a means of defense. It has now been established that the poisons of some amphibians are very strong, but they are not dangerous for humans and large animals because they are contained in the mucus only in a very minor impurity. However, experiments show that this poison can be fatal to many animals. Injecting toad venom into the blood of small birds quickly kills them; in the same way, the poisonous mucus of toads, introduced into the blood of puppies, guinea pigs, frogs and newts, has a deadly effect. Some toads, especially salamanders, have very developed mucous glands, from which they can at will cause very copious secretions, even splashing drops of poisonous liquid; that's where it happened popular belief as if the salamander does not burn in fire.
The elastic, very thin and uncoated skin of amphibians has great importance in their lives. No amphibian drinks water in the usual way, and absorbs it exclusively through the skin. That is why they require proximity to water or dampness. Frogs removed from water quickly lose weight, become lethargic and soon die completely. If you put a wet rag near such frogs, exhausted by dryness, they begin to press their bodies against it and quickly recover. How large is the amount of water that frogs absorb through their skin can be seen from the following Thomson experiment. He took 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. The amphibian absorbs water through its skin and sweats it out. Gases are also exchanged through the skin. In a closed tin box, surrounded by a humid atmosphere, a frog can live for 20-40 days, even if the air supply to the lungs is cut off.
In most amphibians, the initial development of embryos occurs in the same way as in fish. Eggs are usually laid in water in the form of caviar, which is fertilized later, already in water. The eggs are surrounded by a thick layer of gelatinous substance. This shell is of great importance for the embryo, since in this way the egg is protected from drying out, from mechanical damage, and most importantly, it protects them from being eaten by other animals; Indeed, very few birds are able to swallow a gelatinous lump of frog eggs; The shell itself also protects the eggs from attacks by fish, shellfish and aquatic insects.
After the embryo completes the initial stages of its development, the larva breaks through the gelatinous membrane, feeding on it, and begins to swim in the water independent life. The larva has a flat, flattened head, a rounded body and a long paddle-shaped tail, trimmed at the top and bottom with a leathery fin. The original external gills grow on the head in the form of tree-like branched processes. After some time, these gills fall off and internal gills are formed instead. The body gradually narrows even more, the caudal fin increases, and little by little the limbs begin to develop; In frog tadpoles, the hind limbs grow first and then the forelimbs, while in salamanders it is the other way around. Tadpoles at first feed mainly on plant foods, but gradually switch to more and more animal foods. At the same time, changes occur in the organization of the entire body: the tail, which at first is the only organ of movement, loses its importance and shortens as the limbs develop; the intestines become shorter and adapt to the digestion of animal food; The horny plates with which the tadpole's jaws are armed become sharper, gradually disappear and are replaced by real teeth. The ever-shortening tail finally disappears completely - and the tadpole turns into adult frog.
In the development of the brain and sensory organs of amphibians, there is a great similarity with fish. The heart is formed in the larvae very early and immediately begins to act. Initially it represents a simple bag, which is subsequently divided into separate parts. The aorta passes into the branchial arches and branches first in the external gills, and later in the internal ones. The blood flows back through a vein running along the tail, and then branches on the surface of the yolk sac and returns through the yolk veins back to the atrium. Later, the portal systems of the liver and kidneys gradually form. At the end of the larval stage, gill respiration is gradually replaced by pulmonary respiration; the anterior branchial arches turn into the cephalic arteries, and the middle ones form the aorta.
Amphibians live in all parts of the world and in all zones, with the exception of the polar countries. Water, even more than warmth, is a necessary condition for their existence, since almost all amphibians spend their larval stages in water. They chew exclusively in fresh waters, avoiding salmon or even salt water. Almost half of amphibians spend their entire lives in water, while others settle on land as adults, although they always stay near water and in damp places; There are no completely dry amphibians in areas, but they can live where, with general dryness in known time It rains regularly. The dry season in such places is spent in hibernation, buried deep in the silt, in temperate zone They are also susceptible to hibernation. Tropical countries, abundant in forests and water, are the most favorable for their life. These are the central parts South America, Madagascar, islands of the Malay Archipelago, where virgin trees grow in abundance, rain forests; vice versa, middle Asia, Australia and much of inland Africa are very poor in amphibians. All amphibians swim excellently in water, not only in the larval state, but also in the adult state; on land, the tailed ones crawl like reptiles, and the tailless ones move in short, heavy jumps; many of them can even climb trees.
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 Carboniferous period One of the representatives of large amphibians appeared on Earth - Mastodonsaurus. It was a large predator that fed almost exclusively on fish and inhabited freshwater bodies of water (lakes and swamps). He led an aquatic lifestyle. Its habits and behavior were very similar to the lifestyle of ordinary frogs. He also could not exist without water, only occasionally and briefly crawling onto land. Therefore, when in the Permian period the climate became less humid, and bodies of water, including large lakes, began to dry out and disappear, 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. In the practical part I include observations of representatives of this class in natural environment habitats, frog anatomy (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 influence 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, about 3,000 species live on the globe. Amphibians are usually found in and near freshwater bodies. Here they feed on invertebrate animals. In case of danger, they quickly jump into the water.
Some species of amphibians, such as grass frogs, frogs, and common toads, live mainly 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 bodies of water.
All amphibians are active only in warm time of the year. When the ambient temperature drops to 4°C, they fall into torpor. 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).
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 noticeable. The eyes and nostrils are located on elevations in most species. Therefore, a frog, for example, can breathe atmospheric air and navigate the space around it without getting out of the water. Unlike fish, amphibians 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 help 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 - all give one color or another to the skin and are caused both by changes in external conditions and by internal irritation.
Both in the upper and inner layers 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, introduced into the blood of puppies, guinea pigs, frogs and newts, has a deadly effect. Some toads, and especially salamanders, have very developed mucous glands, from which they can arbitrarily cause copious secretions, even spraying drops of poisonous liquid; this is the reason for the popular belief that a salamander does not burn in fire.
The elastic, very thin and uncoated skin of amphibians is of 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. A characteristic feature of the entire class of amphibians are two articulated heads on the occipital part of the skull, which correspond to the two dimples of the first cervical vertebra. The skull is always flat, wide, the eye sockets are very large. The skull consists of 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 hind limbs are supported by pelvic girdle, consisting of pelvic bones that connect 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 movement. In tailed amphibians, the muscles of the tail are most developed.
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. Those who spend most of their lives in water have less developed lungs and better skin respiration. 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. A large artery departs from the ventricle; it is divided into branches that carry blood to all organs of the body (this is a systemic circulation), and branches through which blood is flowing 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.
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.
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. Poor development of the 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 on the developing embryo. 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 lead an independent life in the water. 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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On the head there is a pair of nostrils for breathing. There are eyes, they can be protected by eyelids. There is a mouth. The skin is bare, moistened with mucus. Amphibians can breathe through their lungs and also through their skin. Some species have gills.
The body temperature of these animals depends on the ambient temperature, so they are active only in warm weather. When the temperature drops, they immediately fall into torpor. Poisonous individuals are also found in nature.
Amphibians reproduce, like fish, by laying eggs. The eggs are not protected by shell or skin, so amphibians usually breed in water. The eggs of amphibians produce a larva that looks very similar to a fish. Further development occurs in water with transformation - metamorphosis. Metamorphosis- this is a deep transformation of the structure of the body, transformation. Then future amphibians lose their gills, and some individuals even lose their tail. Then they grow limbs and come out onto land in the form of an adult animal.
Amphibians feed exclusively on mobile living food. They destroy large numbers of insects and their larvae. They are found everywhere, excluding only too cold or hot zones of the Earth.
The most ancient and surviving to our times due to their underground lifestyle are legless amphibians. There are about 150 species of them in nature. These include all tropical and many aquatic caecilians. These amphibians are distinguished by their unusual body structure. These amphibians have a worm-like cylindrical body. The skin is bare, equipped with mucous poisonous glands. There are transverse rings, like earthworms. Animals have no limbs or tail. Their head is strong, small, and imperceptibly merges into the body. With it, worms make their own dungeons in the damp soil. Due to their burrowing lifestyle, their eyes ended up under their skin. Amphibians find food using their sense of smell and touch. They eat snails, worms, larvae, and insects. They lead a very hidden lifestyle and do not like sunlight. The most famous is the ringed caecilian (Fig. 2).
Rice. 2. Ringed worm ()
Unlike other amphibians, they lay their eggs on land. The female curls up around the clutch of eggs and moistens it with her mucus and incubates it.
The fish snake has small, invisible bony scales in its skin (Fig. 3).
Rice. 3. Fish snake ()
The Central American caecilian does not lay eggs; it immediately gives birth to live young.
Science knows about 350 species tailed amphibians. These animals are similar in appearance to lizards, only the skin is soft and completely devoid of scales. Tailed amphibians include newts and salamanders. These animals have an elongated spindle-shaped body, which imperceptibly turns into a long tail. Curving the tail left and right helps it move through the water. On land, amphibians move with the help of two pairs of underdeveloped limbs. The toes may be webbed and lack claws.
Sirens have only forelimbs (Fig. 4).
Amphibians that live constantly in water breathe through gills. There is a tongue in the mouth, its shape is varied. There are small teeth. Many tailed animals have the ability to grow a new one if they lose a tail or leg. Amphibians do not know how to chew; they swallow food whole. Amphibians grab everything that moves and do not take stationary food that is completely edible. Tailless amphibians feed on insects, grabbing them in motion using a long sticky tongue. Tailed animals feed on worms and arthropods.
Legless amphibians find food by touch or use their sense of smell. They feed on insect larvae and worms.
The Siberian salamander is one of the few amphibians that is not afraid to live in conditions permafrost(Fig. 5).
Rice. 5. Siberian salamander ()
The most well-known tailed amphibian is the newt (Fig. 6). They look like little dragons. Newts love to hunt at night.
The fire salamander is famous for its bright color (Fig. 7). It is interesting that the shape, size, and pattern on the body of salamanders are unique to each individual.
Rice. 7. Salamander ()
The axolotl looks like an adult larva (Fig. 8).
Rice. 8. Axolotl ()
In nature, there is the most numerous order of amphibians - these are tailless amphibians. There are about 3 thousand species. This is the most known to man squad. These include toads, frogs, tree frogs, toads, and spadefoots. Their body is short and squat. The head is wide, without a neck, and goes into the body. There is no tail. The skin is bare, moisturized with secretions. There is a pair of moving eyes on the head. Amphibians use their vision to find prey. There is a pair of nostrils. The forelimbs are shorter than the hind limbs. They have membranes that help them swim. Amphibians jump on land and lead an active lifestyle. They are helped to catch prey by small teeth and a sticky tongue that folds in the mouth.
The bullfrog is a predator (Fig. 9). She even attacks chickens and eats young ducklings. Her cry resembles the roar of a bull.
Rice. 9. Bullfrog ()
The Surinamese pipa is famous for carrying tadpoles in cells on its back (Fig. 10). Adult frogs emerge from them.
Rice. 10. Surinamese pipa ()
The hairy frog defends itself with sharp claws, like those of a cat (Fig. 11).
Rice. 11. Hairy frog ()
The tiny Colombian frog (Fig. 12) fits in a teaspoon, and its poison is the most powerful of all animal poisons.
Rice. 12. Colombian frog ()
Flying frogs deftly jump from trees, straightening their membranes (Fig. 13). This helps keep them in the air.
Rice. 13. Flying frogs ()
Amphibians play big role in a person's life. They destroy a huge number of insects, thereby benefiting agriculture. They also eat insects that carry diseases. Amphibians are also used in medical laboratory research. Man even breeds amphibians as pets. In some countries they are even eaten.
Bibliography
- Samkova V.A., Romanova N.I. The world 1. - M.: Russian word.
- Pleshakov A.A., Novitskaya M.Yu. The world around us 1. - M.: Enlightenment.
- Gin A.A., Faer S.A., Andrzheevskaya I.Yu. The world around us 1. - M.: VITA-PRESS.
- Worldofnature.ru ().
- Floranimal.ru ().
- Zoodrug.ru ().
Homework
- What are amphibians?
- How do amphibians reproduce?
- What are the three orders of amphibians? Describe each order of amphibians.
- * Prepare a story about the most unusual and interesting, in your opinion, representative of the Amphibian class.
Amphibians
(amphibians), a class of vertebrates. The skin is bare and rich in glands. Heart with 2 atria and 1 ventricle. Amphibians are the first vertebrates to transition from an aquatic to an aquatic-terrestrial lifestyle. Larvae breathe with gills, adults with lungs. Eggs are laid in water, some are viviparous. Development with transformation (metamorphosis). Body temperature is not constant (heterothermic animals). 3 modern orders: legless, tailed and tailless; over 3400 species. Most amphibians destroy forest pests and Agriculture. Fossil amphibians are numerous, the main branch being labyrinthodonts. 41 species and subspecies in the IUCN Red List.
AMPHIBIDESencyclopedic Dictionary. 2009 .
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