Breathing in animals. Peculiarities of breathing in animals of different groups

Animal breathing – set of processes that providehit into the body from the environmentoxygen , hiscell use for the oxidation of organic substances andexcretion carbon dioxide from the body.This kind of breathing is calledaerobic , and organisms -aerobes .

OK. No. 28. Biology.

Green algae chlorella

Ciliate slipper

The breathing process in animals is conventionally divided into three stages :

External respiration = gas exchange. Thanks to this process, the animal receives oxygen and gets rid of carbon dioxide, which is the end product of metabolism.

Transport of gases in the body– this process is provided either by special tracheal tubes or internal body fluids (blood containing hemoglobin- a pigment that can attach oxygen and transport it into cells, as well as carry carbon dioxide out of cells).

Internal breathing- occurs in cells. Simple nutrients (amino acids, fatty acids, simple carbohydrates) with the help of cell enzymes are oxidized and broken down, during which the ENERGY necessary for the life of the body is released.

The main importance of respiration is the release of energy from nutrients with the help of oxygen, which takes part in oxidation reactions.

Some protozoa - anaerobic organisms, i.e. organisms, not requiring oxygen. Anaerobes There are facultative and obligate. Facultatively anaerobic organisms are organisms that can live both in the absence of oxygen and in its presence. Obligate anaerobic organisms are organisms for which oxygen is toxic. They can only live in the absence of oxygen. Anaerobic organisms do not need oxygen to oxidize nutrients.

Brachionella is an anaerobic ciliate

Intestinal Giardia

Human roundworm

By way of breathing and the structure of the respiratory apparatus in animals there are 4 types of respiration:

Skin breathing - This is the exchange of oxygen and carbon dioxide through the integument of the body. This process is based on the most important physical process - diffusion . Gases enter only in a dissolved state through the covers shallowly and at low speed. Such respiration occurs in organisms that are small in size, have moist integuments, and lead an aquatic lifestyle. This - sponges, coelenterates, worms, amphibians.

Tracheal breathing –

carried out using

connected systems

tubes – trachea , which

permeate the entire body, without

participation of liquids. WITH

their environment

connect special

holes – spiracles.

Organisms with a tracheal

breathing is also small in size (no more than 2 cm, otherwise the body will not have enough oxygen). This - insects, centipedes, arachnids.

Gill breathing – with the help of specialized formations with a dense network of blood vessels. These outgrowths are called gills . In aquatic animals - polychaetes, crustaceans, mollusks, fish, certain species of amphibians. In invertebrate animals, gills are usually external, while in chordates they are internal. Gill-breathing animals have additional forms of respiration through the skin, intestines, surface of the mouth, and swim bladder.

Polychaete with gills

Crustacean gills

Nudibranch

Pulmonary respiration – this is breathing with the help of internal specialized organs – lungs.

Lungs– These are hollow thin-walled bags, braided with a dense network of tiny blood vessels - capillaries. Diffusion of oxygen from the air into the capillaries occurs on the inner surface of the lungs. Accordingly, the larger the internal surface, the more active the diffusion.

Almost all terrestrial vertebrates breathe through their lungs. reptiles, birds, some terrestrial invertebrates - spiders, scorpions, pulmonary mollusks, and some aquatic animals - lungfish. Air enters the lungs through Airways.

Lungs of a mammal

Reptile lung

Respiratory system of birds

Breathing in animals is determined by their way of life and is carried out using the integument, trachea, gills and lungs.

Respiratory system – a set of organs for conducting air or water that contain oxygen and exchanging gases between the body and the environment.

The respiratory organs develop as outgrowths of the outer integument or walls of the intestinal tract. The respiratory system includes the airways and gas exchange organs. In vertebrates Airways – nasal cavity, larynx, trachea, bronchi ; A respiratory system -lungs .

Comparative characteristics of the respiratory organs.

Group	Characteristic features of the respiratory system
Coelenterates	Gas exchange across the entire surface of the body. There are no special respiratory organs.
Annelids	External gills (polychaete worms) and entire body surface (oligochaete worms, leeches)
Shellfish	Gills (bivalves, cephalopods) and lungs (gastropods)
Arthropods	Gills (crustaceans), trachea and lungs (arachnids), trachea (insects)
Fish	Gills. Additional organs for breathing: lungs (lungfishes), parts of the oral cavity, pharynx, intestines, swim bladder
Amphibians	Lungs are cellular, gills (in larvae), skin (with a large number of vessels). Respiratory tract: nostrils, mouth, tracheal-laryngeal chamber
Reptiles	Light cellular. Respiratory tract: nostrils, larynx, trachea, bronchi
Birds	Lungs are spongy. Respiratory tract: nostrils, nasal cavity, upper larynx, trachea, lower larynx with voice box, bronchi. There are air bags.
Mammals	Alveolar lungs. Respiratory tract: nostrils, nasal cavity, larynx with vocal apparatus, trachea, bronchi.

Functions of the respiratory system:

Delivery of oxygen to body cells and removal of carbon dioxide from body cells and gas exchange(main function).

Body temperature regulation(because water can evaporate through the surface of the lungs and respiratory tract)

Purification and disinfection of incoming air(nasal mucus)

Questions for self-control.

Grade	Questions for self-control
	1.What is breathing? 2. The main stages of breathing? 3. Name the main types of animal respiration. 4. Give examples of animals that breathe using their skin, gills, trachea and lungs. 5. What is the respiratory system? 6. Name the main functions of the respiratory system.
	7. How important is respiration for the release of energy in animal cells? 8. What determines the type of breathing of animals? 9. What functions does the respiratory system perform?
	10. Describe the breathing methods of vertebrates.

Comparative characteristics of the respiratory organs of animals.

Respiratory system	Structural features	Functions	Examples
Gills	External(comb, filamentous and pinnate) or internal(always associated with the pharynx) thin-walled outgrowths of the body that contain many blood vessels	Gas exchange in the aquatic environment	In fish, almost all larvae of tailless amphibians, in most mollusks, some worms and arthropods
Trachea	Branched tubes that permeate the entire body and open outward with openings (stigmas)	Gas exchange in the air	In most arthropods
Lungs	Thin-walled bags that have an extensive network of vessels	Gas exchange in the air	In some mollusks and fish, terrestrial vertebrates

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

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

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

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

Reduction in the number of gills.

Increase in respiratory surface due to the formation of gill filaments.

Formation of gill capillaries.

In the lancelet, the lateral walls of the pharynx are pierced by numerous (up to 150 pairs) obliquely located gill slits. The afferent branchial arteries approach the interbranchial septa, and the efferent branchial arteries depart. When water washes the interbranchial septa, gas exchange occurs between the passing water and the blood that flows through the thin vessels of the septa. The branchial arteries do not branch into capillaries. In addition, oxygen enters the animal’s body through the capillaries of the skin.

In proto-aquatic vertebrates (jawless and fish), as well as in lower chordates, gill slits are formed that connect the pharyngeal cavity with the external environment. In cyclostomes, gill sacs are formed from the endoderm lining the gill slits (in fish, gills develop from ectoderm). The inner surface of the bags is covered with numerous folds - gill filaments, in the walls of which a dense network of capillaries branches. The bag opens with an internal narrow channel into the pharynx (in adult lampreys - into the respiratory tube), and with an external one - on the lateral surface of the animal’s body. Hagfish have from 5 to 16 pairs of gill sacs; in the bdellostomidae family, each of them opens outward with an independent opening, and in the hagfish family, all external gill passages on each side merge into one canal, which opens outward with one opening located far behind. Lampreys have 7 pairs of gill pouches, each of which opens outwards with an independent opening. Breathing is carried out through rhythmic contraction and relaxation of the muscular wall of the gill region. In non-feeding lampreys, water enters the respiratory tube from the oral cavity, then washes the lobes of the gill sacs, providing gas exchange, and is removed through the external gill passages. In feeding cyclostomes, water enters and exits through the external openings of the gill sacs.

The respiratory system of fish has specialized gas exchange organs - ectodermal gills, which are either located on the interbranchial septa, as in cartilaginous fish, or directly extend from the gill arches, as in bony fish. The exchange of gases in the gills of vertebrates is built according to the type of “countercurrent systems”: during counter-flow, the blood comes into contact with oxygen-rich water, which ensures its effective saturation. An increase in the oxygen absorption surface due to the formation of gills was accompanied by a decrease in the number of gill slits in vertebrates compared to lower chordates. In whole-headed fish (from cartilaginous fish), a reduction in the interbranchial septa is observed and a leathery gill cover is formed, covering the outside of the gills. In bony fishes, a bony skeleton appears in the gill cover, and the intergill septa are reduced, which contributes to more intensive washing of the gill filaments with water. Along with gas exchange, fish gills participate in water and salt exchange, and in the removal of ammonia and urea from the body. The skin, swim bladder, suprapharyngeal labyrinths and specialized sections of the intestinal tube function as additional respiratory organs in certain groups of fish. Lungfishes and multi-feathered fish develop air breathing organs - lungs. The lungs arise as paired outgrowths of the abdominal part of the pharynx in the region of the last gill slit and are connected to the esophagus by a short canal. The walls of this outgrowth are thin and abundantly supplied with blood.

Directions of evolution of pulmonary type of breathing

Emergence and differentiation of the respiratory tract.

Differentiation of the lung and increase in respiratory surface.

Development of accessory organs (chest).

In amphibians, the following are involved in the absorption of oxygen and the release of carbon dioxide: in larvae - the skin, external and internal gills, in adults - the lungs, skin and mucous membrane of the oropharyngeal cavity. In some species of tailed amphibians (sirens, proteas) and in adults, the gills are retained and the lungs are underdeveloped or reduced. The ratio of pulmonary and other types of gas exchange is not the same: in species of humid habitats, skin respiration dominates in gas exchange; in inhabitants of dry places, most of the oxygen enters through the lungs, but the skin plays a significant role in the release of carbon dioxide. The respiratory system of adult amphibians includes the oropharyngeal, laryngeal-tracheal cavities and sac-like lungs, the walls of which are intertwined with a dense network of capillaries. Tailless amphibians have a common laryngeal-tracheal chamber; in caudate amphibians, it is divided into the larynx and trachea. The arytenoid cartilages appear in the larynx, which support its wall and vocal cords. The lungs of tailed amphibians are two thin-walled bags without partitions. In tailless animals, inside the lung sacs there are partitions on the walls that increase the surface of gas exchange (cellular lungs). Amphibians do not have ribs, and the act of breathing occurs by pumping air during inhalation (due to an increase and then decrease in the volume of the oropharyngeal cavity) and pushing out air during exhalation (due to the elasticity of the walls of the lungs and abdominal muscles).

In reptiles, there is further differentiation of the respiratory tract and a significant increase in the functional surface of gas exchange in the lungs. The airways are divided into the nasal cavity (it is combined with the oral cavity, but in crocodiles and turtles these cavities are separated by the bony palate), the larynx, the trachea and two bronchi. The walls of the larynx are supported by paired arytenoid and unpaired cricoid cartilages. In lizards and snakes, the inner walls of the lung sacs have a folded cellular structure. In turtles and crocodiles, a complex system of septa protrudes into the internal cavity of the lung so deeply that the lung acquires a spongy structure. The chest is formed: the ribs are movably connected to the spine and sternum, the intercostal muscles develop. The act of breathing is carried out due to a change in the volume of the chest (costal type of breathing). Turtles retain the oropharyngeal type of air injection. In aquatic turtles in water, additional respiratory organs are the capillary-rich outgrowths of the pharynx and cloaca (anal bladders). Reptiles do not have cutaneous respiration.

In birds, the airways are represented by the nasal cavity, the larynx, which is supported by the arytenoid and cricoid cartilages, the long trachea and the bronchial system. The lungs are small, dense and poorly extensible, attached to the ribs on the sides of the spinal column. The primary bronchi are formed by dividing the lower part of the trachea and enter the tissue of the corresponding lung, where they break up into 15–20 secondary bronchi, most of which end blindly, and some communicate with the air sacs. The secondary bronchi are interconnected by smaller parabronchi, from which many thin-walled cellular bronchioles arise. The bronchioles, entwined with blood vessels, form the morphofunctional structure of the lung. Associated with the lungs of birds are air sacs - transparent, elastic, thin-walled outgrowths of the mucous membrane of the secondary bronchi. The volume of the air sacs is approximately 10 times the volume of the lungs. They play a very important role in the implementation of the peculiar respiratory act of birds: air with a high oxygen content enters the lungs both during inhalation and exhalation - “double breathing”. In addition to intensifying breathing, air bags prevent the body from overheating during intense movement. An increase in intra-abdominal pressure during exhalation promotes defecation. Diving birds, by increasing the pressure in the air sacs, can reduce the volume and thereby increase the density, which makes it easier to dive into water. There is no skin respiration in birds.

In mammals, further differentiation of the respiratory tract is observed. The nasal cavity, nasopharynx is formed, the entrance to the larynx is covered by the epiglottis (in all terrestrial vertebrates except mammals, the laryngeal fissure is closed by special muscles), thyroid cartilage appears in the larynx, then comes the trachea, which branches into two bronchi going into the right and left lungs. In the lungs, the bronchi branch repeatedly and end in bronchioles and alveoli (the number of alveoli is from 6 to 500 million), this significantly increases the respiratory surface. Gas exchange occurs in the alveolar ducts and alveoli, the walls of which are densely intertwined with blood vessels. The morphofunctional unit of the mammalian lung is the pulmonary acinus, which is formed as a result of the branching of the terminal bronchiole. The chest is formed, which is separated by the diaphragm from the abdominal cavity. The number of respiratory movements is from 8 to 200. Respiratory movements are carried out in two ways: due to changes in the volume of the chest (costal breathing) and due to the activity of the diaphragmatic muscle (diaphragmatic breathing). Higher mammals have developed cutaneous respiration through a system of skin capillaries, which plays an important role in gas exchange.

The gill apparatus of chordates has evolved in the direction of the formation of gill filaments. In particular, fish have developed 4-7 gill sacs, which are slits between the gill arches and contain a large number of petals, which are penetrated by capillaries (Fig. 190). In fish, an air bladder is also involved in respiration.[...]

Gill respiration is typical aquatic respiration. The physiological purpose of the gills is to supply the body with oxygen. They transfer oxygen from the external environment to the blood.[...]

Cutaneous respiration, as the most primary one in phylogenesis and ontogenesis, is then replaced by special, gill respiration, but still continues to play a known role until the end of the fish’s life.[...]

Respiratory system. Gills are respiratory organs. They lie on both sides of the head. Their basis is the gill arches. In the vast majority of cases, in our freshwater fish, with the exception of lampreys, the gills are covered on the outside with covers, and their cavity communicates with the oral cavity. On the gill arches there are double-rowed gill plates. Each gill plate is oblong, pointed, tongue-shaped, and has at its base a cartilaginous stamen, enclosed in a bone sheath and reaching to its free end. Along the inner edge of the gill plate there is a branch of the branchial artery, which brings venous blood, and along the outer edge there is a branch of the branchial vein, which drains arterial blood. Hair vessels extend from them. On both flat sides of the gill plate there are leaf-shaped plates, which actually serve for respiration or gas exchange. If there is only one row of plates on the gill arch, then it is called a semi-gill.[...]

In gobies, respiration in moist air is provided by the scalp, mouth and gill cavities. The mucous membrane of these cavities is well supplied with blood vessels. Air is taken in through the mouth, oxygen is absorbed in the mouth or gill cavity, and the remaining gas is expelled back through the mouth. Interestingly, many gobies do not have a swim bladder, and other organs are adapted for air breathing.[...]

In a number of fish, gill respiration in the early stages of development does not fully satisfy the needs of the body. As a result, accessory organs develop (intestinal, superior caudal and dorsal veins), which serve as a significant addition to gill respiration. With the development and improvement of gill respiration, embryonic respiration is gradually reduced.[...]

In addition to breathing frequency, changes in the depth of breathing are also observed. Fish in some cases (at low P02, elevated temperature, increased CO2 content in water) breathes very often. The breathing movements themselves are small. Such shallow breathing is especially easy to observe at elevated temperatures. In some cases, the fish takes deep breaths. The mouth and gill covers open and close widely. With shallow breathing the respiratory rhythm is high, with deep breathing it is small.[...]

Observing the breathing rhythm of fish, M. M. Voskoboynikov came to the conclusion that the passage of water in one direction through the mouth, gill filaments and gill openings is ensured by the work of the gill covers and the special position of the gill filaments. [...]

As the gill type of respiration develops, salmon uses oxygen more easily, even if the latter is in low concentration (decrease in the threshold concentration of O2).[...]

The ratio of main and additional respiration varies in different fish. Even in the loach, intestinal respiration has turned from additional to almost equal to gill respiration. The loach still needs. intestinal respiration, even if it is in well-aerated water. From time to time it rises to the surface and swallows air, and then sinks to the bottom again. If, for example, in a perch or carp, due to a lack of oxygen, the respiratory rhythm becomes more frequent, then the loach c. In such conditions, it does not increase the breathing rate, but uses intestinal respiration more intensively.[...]

Water is pumped through the gill cavity using the movement of the mouthparts and gill covers. Therefore, the respiratory rate of fish is determined by the number of movements of the gill covers. The breathing rhythm of fish is primarily affected by the oxygen content in water, as well as the concentration of carbon dioxide, temperature, pH, etc. Moreover, the sensitivity of fish to a lack of oxygen (in water and blood) is much higher than to an excess of carbon dioxide (hypercapnia) . For example, at 10 °C and normal oxygen content (4.0-5.0 mg/l), trout makes 60-70, carp -30-40 respiratory movements per minute, and at 1.2 mg 02/l the respiratory rate increases 2-3 times. In winter, carp's breathing rhythm slows down sharply (up to 3-4 respiratory movements per minute).[...]

With the mouth open and the gill covers closed, the zod enters the oral cavity and passes between the gill filaments into the gill cavity. This is a breath. Then the mouth closes, the gill cover opens slightly and the water comes out. This is an exhalation. Consideration of this process in detail led to two different ideas about the mechanism of respiration.[...]

In some fish, the pharynx and gill cavity are adapted for air breathing.[...]

Gills are the main respiratory organ in most fish. However, examples can be given where in some fish the role of gill respiration is reduced, and the role of other organs in the respiration process is increased. Therefore, it is not always possible to answer the question of what the fish is breathing at the moment. Having significantly expanded Bethe's table, we present the ratios of different forms of respiration in fish under normal conditions (Table 85).[...]

The inhibitory effect of excess CO2 on gill respiration and stimulation of pulmonary respiration in lungfishes has been noted repeatedly. The transition of lungfish from aquatic to air respiration is accompanied by a decrease in arterial p02 and an increase in pCO2. It should be especially noted that stimulation of air respiration and inhibition of aquatic respiration in lungfish occurs under the influence of a decrease in the level of 02 in water and an increase in the level of CO2. True, during hypoxia in lungfishes ((Cheosegagosk) both pulmonary and gill respiration increase, and during hypercapnia, only pulmonary respiration. It is curious that with the combined action of hypoxia and hypercapnia, ventilation of the lungs increases, and gill ventilation decreases. According to the authors, chemoreceptors are localized in the area of ​​the gills or in the efferent gill vessels.[...]

Underdevelopment or complete absence of the gill cover makes breathing difficult and leads to disease of the gills. A slanted snout interferes with food intake. An arched back and pug-shaped head lead to significant stunting.[...]

The most common type of intestinal respiration is one in which air is forced through the intestine, and gas exchange occurs in the middle or posterior part of it (loaches, some catfish). In another type, for example in Hippostomos and Acarys, the air, after remaining for some time in the intestines, does not escape through the anus, but is squeezed back into the oral cavity and then thrown out through the gill slits. This type of intestinal respiration is fundamentally different from the first; subsequently in some fish it developed into pulmonary respiration.[...]

A more complex device for air breathing is the epibranchial organ. The epibranchial organ is found in Ory-ocephalus (snakehead), living in the river. Cupid, in Luciocephalus, in Anabas, etc. This organ is formed by the protrusion of the pharynx, and not the gill cavity itself, as in labyrinthine fish.[...]

Breathing movements, breathing rhythm. In fish, the operculum periodically opens and closes. These rhythmic movements of the operculum have long been known as respiratory movements. However, a correct understanding of the breathing process was achieved relatively recently.[...]

It is quite obvious that the intensity of skin respiration is an expression of the fish’s adaptation to life in conditions of oxygen deficiency, when gill respiration is not able to provide the body with oxygen in the required quantity.[...]

A general rule is observed: with the development of air respiration, a decrease in gill breathing occurs (Suvorov). Anatomically, this is expressed in the shortening of the gill filaments (in Polypterus, Ophiocephalus, Arapaima, Electrophorus) or in the disappearance of a whole number of petals (in Monopterus, Amphipnous and lungfishes). In Protopterus, for example, there are almost no petals on the first and second arches, and in Lepidosirene the gill filaments are poorly developed.[...]

Fish of warm waters have a device for air breathing in the form of a labyrinth. The labyrinthine organ is formed by the protrusion of the gill cavity itself and sometimes (as in Anabas) is equipped with its own muscles. The inner surface of the “labyrinth cavity” has various curvatures due to curved bone plates covered with mucous membrane. Many blood vessels and capillaries approach the surface of the “labyrinth cavity”. Blood enters them from the branch of the fourth afferent branchial artery. Oxygenated blood flows into the dorsal aorta. The air captured by the fish in the mouth enters the labyrinth from the mouth and releases oxygen into the blood there. [...]

Recently, more detailed studies of cutaneous respiration on 15 species of fish were carried out by S. V. Streltsova (1949). It determined both general breathing and specifically skin breathing. Gill respiration was switched off by placing a sealed rubber mask over the gills. This technique allowed her to determine the share of skin respiration in the overall respiration of fish. It turned out that this value is very different in different fish and is associated with the lifestyle and ecology of the fish.[...]

Experiments have shown that the V, VII, IX and X pairs of cephalic nerves are necessary for normal breathing. The branches from them innervate the upper jaw (V pair), the operculum (VII pair) and the gills (IX and X pairs).[...]

In practice, all cyclostomes and fish have a “morphofunctional reserve” for increasing respiration power in the form of certain “vzbmgochshh” gas exchange structures. It has been experimentally established that under normal conditions in fish no more than 60% of the gill filaments function. The rest are turned on only under conditions of advancing hypoxia or when the need for oxygen increases, for example, when swimming speed increases.[...]

In the larval stage (tadpoles), amphibians are very similar to fish: they retain gill breathing, have fins, a two-chambered heart and one circulation. Adult forms are characterized by a three-chambered heart, two circles of blood circulation, and two pairs of limbs. Lungs appear, but they are poorly developed, so additional gas exchange occurs through the skin (Fig. 81). Amphibians live in warm, humid places, especially common in the tropics, where they are most numerous.[...]

Larvae and fry of sturgeon fish are transported in the first two days after hatching from eggs before switching to gill respiration, since gill respiration requires more oxygen. The oxygen saturation of water should be at least 30% of normal saturation. At a water temperature of 14-17 °C and constant aeration, the planting density, depending on the mass of the larvae, can be increased to 200 pcs. per 1 liter of water.[...]

At the age of 15 days, the larva has enlarged intestinal veins that entwine the intestines (already performing the function of breathing), and a pectoral fin with densely branched vessels. At the age of 57 days, the larvae's external gills have shrunk and are completely closed by the operculum. All. the fins, except for the preanal one, are well supplied with vessels. These fins serve as respiratory organs (ri£.-67).[...]

In a carefully performed test work on the same type of fish - brook trout, it was shown that already at pH 5.2, hypertrophy of the mucous cells of the gill epithelium occurs, and mucus accumulates on the gills. Subsequently, with an increase in water acidity to 3.5, destruction of the gill epithelium and its rejection from supporting cells was noted. The accumulation of mucus on the gills during periods when breathing is especially difficult has also been noted in other species of salmon fish.[...]

It is necessary to increase pO2, at which HbO2 is formed. For the most part, gill respiration and heart rate increase in fish. In this case, not only p02 is maintained at a higher level, but also pCO2 decreases. However, the body can achieve this only within certain temperature limits, since in a reservoir the water is less saturated with oxygen at elevated temperatures than at lower temperatures. In laboratory conditions and when transporting live fish in closed vessels, the condition of the fish can be improved by; that with increasing temperature, the POg in water increases artificially, through aeration.[...]

The epibranchial and labyrinth organs are found in snakeheads and tropical fish (bettas, gourami, macropods). They are sac-like protrusions of the gill cavity (labyrinth organ) or pharynx (epibranchial organ) and are intended mainly for air respiration.[...]

In the European bitterling, the vessels of the respiratory network reach greater development than in our other cyprinid fish. This is the result of the organism’s adaptation to life in the gill cavity of mollusks in the early stages of development in poor oxygen conditions. With the transition to life in water, all these adaptations disappear and only developed gill respiration remains.[...]

Fish are divided into cartilaginous and bony. The habitat of fish is water bodies, which shaped the features of their body and created fins as organs of movement. Breathing is gill, and the heart is two-chambered and has one circulation.[...]

According to R. Lloyd, the leading point in this case is an increase in the flow of water passing through the gills, and, as a consequence, an increase in the amount of poison reaching the surface of the gill epithelium with subsequent penetration into the body. Moreover, the concentration of poison on the surface of the gill epithelium is determined not only by the concentration of poison in the bulk of the solution, but also by the rate of respiration. Let's add to this that according to data obtained by M. Shepard, with a decrease in oxygen concentration in water, the hemoglobin content in the blood increases and, most importantly, the rate of blood circulation through the gills increases.[...]

By the way, the same ability was used to explain the cases of CGRPs with overgrown mouths. And here, studies have shown that these carps eke out their existence for some time, having adapted to absorb water for breathing and along with it a certain number of crustaceans through the gill openings.[...]

Chordates are also characterized by the presence of a nerve bundle in the form of a tube above the notochord and a digestive tube under the notochord. Further, they are characterized by the presence in the embryonic state or throughout life of numerous gill slits that open outward from the pharyngeal region of the digestive tube and are respiratory organs. Finally, they are characterized by the location of the heart or its replacement vessel on the abdominal side.[...]

Summarizing the numerous experimental data available today on the effect of long-term or short-term oxygen deficiency on fish of different ecology, a number of general conclusions can be drawn. The primary reaction of fish to hypoxia is to increase respiration by increasing its frequency or depth. The volume of gill ventilation increases sharply. The heart rate falls and the stroke volume increases, causing blood flow to remain constant. During the development of hypoxia, oxygen consumption initially increases slightly, then returns to normal. As hypoxia deepens, the efficiency of oxygen absorption begins to decrease, while oxygen consumption by tissues increases, which creates additional difficulties for fish in meeting the oxygen demand under conditions of low oxygen content in water. The oxygen tension in arterial and venous blood, the utilization of oxygen from water, the efficiency of its transfer and the efficiency of blood oxygenation are reduced.[...]

An electrocardiogram is recorded as follows. Electrodes soldered onto thin flexible conductors are inserted: one into the heart area on the ventral side of the body, and the other between the dorsal fin and the head on the dorsal side. To record the respiratory rate, electrodes are inserted into the operculum and rostrum. Recording of breathing rate and heart rate can be carried out simultaneously through two independent channels of an electrocardiograph or any other device (for example, a two-channel electroencephalograph). In this case, the fish can be either in a free state in the aquarium or in a fixed state. Recording an electrocardiogram is only possible under conditions of complete screening of the aquarium water. Shielding can be done in two ways: by immersing a galvanized iron plate in water or by soldering a conductor to the bottom of the aquarium. If the aquarium is plexiglass, it should be installed on a sheet of iron.[...]

Comparing these data for juveniles with Kuptsis's data for adult roaches, it is easy to see that the threshold value for juvenile roaches on the 49th day after hatching is very close to the threshold value for adults (1 and 0.6-1 mg/l, respectively). Consequently, after the establishment of gill respiration, the ability to use oxygen quickly reaches its limit.[...]

Gills play a significant role in removing excess salts. If divalent ions are excreted in significant quantities through the kidneys and digestive tract, then monovalent ions (mainly N and SG) are excreted almost exclusively through the gills, which perform a dual function in fish - respiration and excretion. The gill epithelium contains special large goblet cells containing a large number of mitochondria and a well-developed eudoplasmic reticulum. These "chloride" (or "salt") cells are located in the primary gill filaments and, unlike respiratory cells, are associated with the vessels of the venous system. The transfer of ions through the gill epithelium has the character of active transport and involves the expenditure of energy. The stimulus for the excretory activity of chloride cells is an increase in blood osmolarity.[...]

Suspended solids tend to form unstable or stable suspensions and include both inorganic and organic components. When their content increases, light transmission deteriorates, photosynthetic activity decreases, the appearance of water deteriorates, and gill respiration may be impaired. As solid particles settle to the bottom, the activity of benthic flora and fauna decreases.[...]

In the ontogenesis of fish, a certain sequence of roles of individual oxygen-receiving surfaces is observed: the stellate sturgeon egg breathes over the entire surface; in the embryo, oxygen supply occurs mainly through a dense network of capillaries on the yolk sac; after hatching, approximately on the 5th day, gill breathing appears, which then becomes the main one.[...]

The loach rises to the surface of the water to swallow air at: t = 10° 2-3 times per hour, and at 25-30° already 19 times. If you boil the water, i.e. reduce P02, then the loach rises to the surface at t = 25-2.7°’ once an hour. At t=5° in running water it did not rise to the surface for 8 hours. These experiments quite clearly show that intestinal respiration, which is a complement to gill respiration, copes with its function quite satisfactorily with low demands of the body at 02 (at t = 5°) or with a high concentration of oxygen in the environment (running water). But gill breathing is not enough if the metabolism in the body is increased (t == 25-30°) or the P02 in the environment (boiled water) has greatly decreased. In this case, intestinal respiration is additionally activated, and the loach receives the required amount of oxygen.[...]

In the Devonian, the climate was sharply continental, arid, with sharp fluctuations in temperature throughout the day and between seasons; extensive deserts and semi-deserts appeared. The first glaciations were also observed. During this period, fish flourished, populating the seas and fresh waters. At that time, many land-based reservoirs dried up in the summer, froze in the winter, and the fish that inhabited them could be saved in two ways: burying in silt or migrating in search of water. The first path was taken by lungfishes, which, along with gill breathing, developed pulmonary respiration (the lung developed from the swim bladder). Their fins looked like blades, consisting of individual bones with muscles attached to them. With the help of fins, fish could crawl along the bottom. In addition, they could also have pulmonary respiration. Lobe-finned fish gave rise to the first amphibians - stegocephalians. On land in the Devonian, the first forests of giant ferns, horsetails and mosses appeared.[...]

Among the general clinical changes in fish, the following are noted: depression of the general condition, suppression and distortion of reactions to: external irritations; darkening, pallor, hyperemia and hemorrhages on the skin of the body; ruffled scales; disturbance of the sense of balance, orientation, coordination of movements and coordinated work of the fins; conjunctivitis, keratitis, cataracts, corneal ulcerations, bulging eyes, loss of vision; complete or partial refusal to eat food; swelling of the abdomen (acute cases of poisoning); changes in the rhythm of breathing and the amplitude of vibration of the gill covers; periodic spasms of the trunk muscles, tremors of the gill covers and pectoral fins. With chronic intoxication, signs of increasing exhaustion develop. In severe processes, toxic dropsy develops. In case of death, poisoned fish: sink from the surface of the water to the bottom, they develop a coma, breathing becomes shallow, then stops - death occurs.[...]

The localization of peripheral receptors that perceive changes in CO2 content and the pathways for conducting impulses from these receptors to the respiratory center are less clear. For example, after cutting the IX and X pairs of cranial nerves innervating the gills, the impulses remained weakened. In lungfishes, inhibition of gill respiration with an increase in pCO2 in water was noted, which can be relieved by atropine. The effect of suppression of pulmonary respiration under the influence of excess carbon dioxide in these fish was not noted, which suggests the presence of receptors sensitive to CO2 in the gill area.

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 the 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 from the air, the eardrum appears, followed by the middle ear (tympanic cavity), in which the auditory ossicle is located - the stapes, 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.

The circulatory system has become more complex, 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. The presence of a cervical vertebra is 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. It contains 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. The ducts of the salivary glands open into it. 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 the same 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 highly 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 fall out into the body cavity through a rupture in the ovarian walls, 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 males hold females during fertilization of eggs.

Males are usually smaller than females.

Development amphibians are 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 live permanently in water and 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, because not only proves that amphibians descended from fish-like creatures, but makes it possible to reconstruct 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 the food chain.