Excretory organs consist of paired mesonephric kidneys, which look like flat reddish bodies located on the sides of the spine, and a pair of ureters that open into the cloacal cavity and correspond to the Wolffian canals.
A large bladder (vesica urinaria) opens into the cloaca, into which urine constantly flows from the cloaca and from which, when the bladder is full, it is expelled out again through the cloaca. On the ventral surface of the kidneys are the adrenal glands, which are important endocrine glands.
Genitals. In male frogs, they are represented by a pair of round, whitish testes adjacent to the ventral surface of the kidneys. Above the testes lies a fat body characteristic of amphibians, which has an irregular shape and various sizes: it serves to nourish the testis and the sperm developing in it. Therefore, in the fall, when the testes are still small, the fat body is large, but by spring almost all of it is spent on the formation of greatly enlarging testes.
Numerous seminiferous tubules depart from the testes, which, after passing through the kidney, flow into the ureter (Wolffian canal). Before entering the cloaca, it forms an extension - the seminal vesicle, which serveslives as a reservoir for seed. The frog, like the vast majority of amphibians, lacks copulatory organs.
:
1 - testis, 2 - fat body, 3 - kidney, 4 - ureter (Wolffian canal), 5 - seminal vesicles, 6 - cloaca, 7 - bladder, 8 - posterior vena cava, 9 - seminiferous tubules, 10 - adrenal gland
:
1 - funnel of the oviduct, 2 - oviduct, 3 - uterine section of the oviduct, 4 - cloaca, 5 - bladder, 6 - right ovary, 7 - kidney, 8 - fat body
In females, the genital organs are represented by paired ovaries, which, unlike the testes, have a granular structure. Above them lies, as in males, the fat body. The size of the ovaries varies depending on the burden of the year: in summer and autumn they are small, but by spring they are very large and overflowing with round, dark-colored eggs. Ripe eggs fall into the body cavity, from where they enter the internal opening of the oviduct. The oviducts (Müllerian canals) are paired, highly convoluted tubes, the small internal openings of which are located nearby near the spine, near the root of the lungs, and the external ones open independently into the cloaca. The funnels of the oviducts grow to the heart sac so that when the heart contracts, they alternately contract and expand, sucking eggs from the body cavity. By the time of reproduction, the oviducts are greatly extendedshrink and acquire very thick walls. Thus, genitourinary system frogs, like all amphibians, are constructed according to the same type as those of cartilaginous and lungfishes.
Secondary sexual characteristics. Male frogs differ from females in external characteristics. In males, the inner toe of the front legs has a large tubercle at the base, which reaches a special development at the time of reproduction and helps the males hold on to the females during fertilization of eggs. In addition, most male frogs have vocal sacs, or resonators, that are located on the sides of the head and open into the oral cavity near the corners of the mouth. In their active state, the vocal sacs are filled with air and serve to amplify the sound produced when croaking. In male green frogs, when croaking, resonators protrude from the sides of the mouth in the form of large round bubbles; in male brown frogs they are internal and located under the skin in the submandibular lymph sac.
Amphibians are the first terrestrial vertebrates, most of which live on land and breed in water. These are moisture-loving animals, which determines their habitat.
Newts and salamanders living in water most likely once completed their life cycle at the larval stage and in this state reached sexual maturity.
Terrestrial animals - frogs, toads, tree frogs, spadefoots - live not only on the soil, but also on trees (frog), in the desert sands (toad, spadefoot), where they are active only at night, and lay eggs in puddles and temporary reservoirs, yes and not every year.
Amphibians feed on insects and their larvae (beetles, mosquitoes, flies), as well as spiders. They eat mollusks (slugs, snails) and fish fry. Toads are especially useful because they eat nocturnal insects and slugs that are inaccessible to birds. Grass frogs feed on garden, forest and field pests. One frog can eat about 1,200 harmful insects over the summer.
Amphibians themselves are food for fish, birds, snakes, hedgehogs, mink, ferrets, and otters. Birds of prey feed their chicks with them. Toads and salamanders, which have poisonous glands on their skin, are not eaten by mammals and birds.
Amphibians overwinter in shelters on land or in shallow water bodies, so snowless, cold winters cause their mass death, and pollution and drying out of water bodies leads to the death of their offspring - eggs and tadpoles. Amphibians must be protected.
9 species of representatives of this class are included in the Red Book of the USSR.
Class characteristics
The modern fauna of amphibians is not numerous - about 2,500 species of the most primitive terrestrial vertebrates. According to morphological and biological characteristics they occupy an intermediate position between the actual aquatic organisms and the actual terrestrial ones.
The origin of amphibians is associated with a number of aromorphoses, such as the appearance of a five-fingered limb, the development of the lungs, the division of the atrium into two chambers and the appearance of two circulation circles, the progressive development of the central nervous system and sensory organs. Throughout their lives, or at least in the larval state, amphibians are necessarily associated with the aquatic environment. Adult forms for normal life They need constant skin hydration, so they live only near bodies of water or in places with high humidity. In most species, eggs (spawn) do not have dense shells and can only develop in water, like larvae. Amphibian larvae breathe through gills; during development, metamorphosis (transformation) occurs into an adult animal that has pulmonary respiration and a number of other structural features of terrestrial animals.
Adult amphibians are characterized by paired limbs of the five-fingered type. The skull is movably articulated with the spine. In addition to the internal hearing organ, the middle ear is also developed. One of the bones of the hyoid arch turns into the bone of the middle ear - the stapes. Two circles of blood circulation are formed, the heart has two atria and one ventricle. The forebrain is enlarged, two hemispheres are developed. Along with this, amphibians retained features characteristic of aquatic vertebrates. Amphibian skin has a large number of mucous glands, the mucus they secrete moisturizes it, which is necessary for skin respiration (oxygen diffusion can only occur through a water film). Body temperature depends on temperature environment. These features of body structure determine the richness of the amphibian fauna in humid and warm tropical and subtropical regions (see also Table 18).
A typical representative of the class is a frog, the example of which is usually used to characterize the class.
The structure and reproduction of a frog
lake frog lives in bodies of water or on their banks. Its flat, wide head smoothly transitions into a short body with a reduced tail and elongated hind limbs with swimming hind limbs. The forelimbs, unlike the hind limbs, are significantly smaller; they have 4, not 5 fingers.
Coverings of the body. The skin of amphibians is naked and always covered with mucus thanks to a large number of mucous multicellular glands. It not only performs a protective function (from microorganisms) and perceives external irritation, but also participates in gas exchange.
Skeleton consists of the spine, skull and skeleton of the limbs. The spine is short, divided into four sections: cervical, trunk, sacral and caudal. IN cervical spine there is only one ring-shaped vertebra. IN sacral region also one vertebra to which the pelvic bones are attached. The tail section of the frog is represented by the urostyle - a formation consisting of 12 fused caudal vertebrae. Between the vertebral bodies there are remains of the notochord, there are superior arches and a spinous process. There are no ribs. The skull is wide, flattened in the dorsal direction; in adult animals the skull retains much cartilage tissue, which makes amphibians similar to lobe-finned fish, but the skull contains fewer bones than fish. Two occipital condyles are noted. The shoulder girdle consists of the sternum, two coracoids, two clavicles and two scapulae. In the forelimb there is a shoulder, two fused bones of the forearm, several bones of the hand and four fingers (the fifth finger is rudimentary). The pelvic girdle is formed by three pairs of fused bones. The hind limb consists of a femur, two fused leg bones, several foot bones and five toes. The hind limbs are two to three times longer than the forelimbs. This is due to movement by jumping; in water, when swimming, the frog energetically works with its hind limbs.
Musculature. Part of the trunk muscles retains a metameric structure (similar to the muscles of fish). However, a more complex differentiation of muscles is clearly manifested, developed a complex system muscles of the limbs (especially the hind ones), chewing muscles and so on.
Internal organs of a frog lie in the coelomic cavity, which is lined with a thin layer of epithelium and contains a small amount of fluid. Most of the body cavity is occupied by the digestive organs.
Digestive system It begins with a large oropharyngeal cavity, at the bottom of which the tongue is attached at the anterior end. When catching insects and other prey, the tongue is thrown out of the mouth and the prey sticks to it. On the top and lower jaws frogs, and also on the palatine bones there are small conical teeth (undifferentiated), which serve only to hold prey. This expresses the similarity of amphibians with fish. Ducts open into the oropharyngeal cavity salivary glands. Their secretion moistens the cavity and food, making it easier to swallow prey, but it does not contain digestive enzymes. Further digestive tract passes into the pharynx, then into the esophagus and, finally, into the stomach, the continuation of which is the intestines. Duodenum lies under the stomach, and the rest of the intestine folds into loops and ends in the cloaca. There are digestive glands (pancreas and liver).
Food moistened with saliva enters the esophagus and then into the stomach. The glandular cells of the stomach walls secrete the enzyme pepsin, which is active in an acidic environment (hydrochloric acid is also released in the stomach). Partially digested food moves to the duodenum, into which it drains bile duct liver.
Pancreatic secretions also flow into the bile duct. The duodenum imperceptibly passes into small intestine where nutrient absorption occurs. Undigested food remains enter the wide rectum and are thrown out through the cloaca.
Tadpoles (larvae of frogs) feed mainly on plant foods (algae, etc.); they have horny plates on their jaws that scrape off soft plant tissue together with the unicellular and other small invertebrates found on them. The horny plates are shed during metamorphosis.
Adult amphibians (in particular, frogs) are predators that feed on various insects and other invertebrate animals; some aquatic amphibians catch small vertebrates.
Respiratory system. A frog's breathing involves not only the lungs, but also the skin, which contains a large number of capillaries. The lungs are represented by thin-walled bags, the inner surface of which is cellular. On the walls of the paired sac-like lungs there is an extensive network of blood vessels. Air is pumped into the lungs as a result of pumping movements of the bottom oral cavity when the frog opens its nostrils and lowers the floor of the oropharyngeal cavity. Then the nostrils close with valves, the bottom of the oropharyngeal cavity rises, and air passes into the lungs. Exhalation occurs due to action abdominal muscles and collapse of the pulmonary walls. U different types amphibians receive 35-75% of oxygen through the lungs, 15-55% through the skin, and 10-15% of oxygen through the mucous membrane of the oropharyngeal cavity. 35-55% of carbon dioxide is released through the lungs and oropharyngeal cavity, and 45-65% of carbon dioxide through the skin. Males have arytenoid cartilages surrounding the laryngeal fissure and stretched over them vocal cords. Sound amplification is achieved by the vocal sacs formed by the mucous membrane of the oral cavity.
Excretory system. Dissimilation products are excreted through the skin and lungs, but most of them are excreted by the kidneys located on the sides of the sacral vertebra. The kidneys are adjacent to the dorsal side of the frog's cavity and are oblong bodies. The kidneys contain glomeruli in which they are filtered from the blood. harmful products decay and some valuable substances. During the flow through the renal tubules, valuable compounds are reabsorbed, and urine flows through two ureters into the cloaca and from there into the bladder. For some time, urine can accumulate in the bladder, which is located on the abdominal surface of the cloaca. After filling Bladder the muscles of its walls contract, urine is discharged into the cloaca and thrown out.
Circulatory system. The heart of adult amphibians is three-chambered, consisting of two atria and a ventricle. There are two circles of blood circulation, but they are not completely separated, arterial and deoxygenated blood partially mixed due to a single ventricle. An arterial cone with a longitudinal spiral valve inside extends from the ventricle, which distributes arterial and mixed blood into different vessels. The right atrium receives venous blood from internal organs and arterial blood from the skin, i.e. mixed blood collects here. The left atrium receives arterial blood from the lungs. Both atria contract simultaneously and blood flows from them into the ventricle. Thanks to the longitudinal valve in the arterial cone, venous blood flows to the lungs and skin, mixed blood flows to all organs and parts of the body except the head, and arterial blood flows to the brain and other organs of the head.
The circulatory system of amphibian larvae is similar circulatory system fish: the heart has one ventricle and one atrium, there is one circle of blood circulation.
Endocrine system. In the frog, this system includes the pituitary gland, adrenal glands, thyroid, pancreas and gonads. The pituitary gland secretes intermedin, which regulates the color of the frog, somatotropic and gonadotropic hormones. Thyroxine, which produces thyroid, is necessary for the normal completion of metamorphosis, as well as for maintaining metabolism in an adult animal.
Nervous system characterized by a low degree of development, but along with this it has a number of progressive features. The brain has the same sections as in fish (forebrain, interstitial, midbrain, cerebellum and medulla). The forebrain is more developed, divided into two hemispheres, each of them has a cavity - the lateral ventricle. The cerebellum is small, which is due to its relatively in a sedentary manner life and monotony of movements. The medulla oblongata is much larger. There are 10 pairs of nerves leaving the brain.
The evolution of amphibians, accompanied by a change of habitat and emergence from water to land, is associated with significant changes in the structure of the sense organs.
The sense organs are generally more complex than those of fish; they provide orientation for amphibians in water and on land. In larvae and adult amphibians living in water, lateral line organs are developed; they are scattered on the surface of the skin, especially numerous on the head. The epidermal layer of the skin contains temperature, pain and tactile receptors. The organ of taste is represented by taste buds on the tongue, palate and jaws.
The olfactory organs are represented by paired olfactory sacs, which open outward through paired external nostrils, and into the oropharyngeal cavity through internal nostrils. Part of the walls of the olfactory sacs is lined with olfactory epithelium. The olfactory organs function only in the air; in water the external nostrils are closed. The olfactory organs of amphibians and higher chordates are part of the respiratory tract.
In the eyes of adult amphibians, movable eyelids (upper and lower) and a nictitating membrane are developed; they protect the cornea from drying out and contamination. Amphibian larvae do not have eyelids. The cornea of the eye is convex, the lens is shaped biconvex lens. This allows amphibians to see quite far. The retina contains rods and cones. Many amphibians have developed color vision.
In the organs of hearing except inner ear in place of the squirter of lobe-finned fishes, a middle ear is developed. It contains a device that amplifies sound vibrations. The external opening of the middle ear cavity is covered by an elastic eardrum, the vibrations of which are enhanced by sound waves. Through the auditory tube, which opens into the pharynx, the middle ear cavity communicates with the external environment, which makes it possible to reduce sudden changes in pressure on the eardrum. In the cavity there is a bone - the stirrup, one end of which rests against the eardrum, the other - against the oval window, covered by a membranous septum.
| Table 19. Comparative characteristics structures of larvae and adult frogs | ||
| Sign | Larva (tadpole) | Adult animal |
| Body Shape | Fish-like, with limb buds, tail with a swimming membrane | The body is shortened, two pairs of limbs are developed, there is no tail |
| Way to travel | Swimming with your tail | Jumping, swimming using hind limbs |
| Breath | Branchial (gills are first external, then internal) | Pulmonary and cutaneous |
| Circulatory system | Two-chambered heart, one circle of blood circulation | Three-chambered heart, two circles of blood circulation |
| Sense organs | The lateral line organs are developed, there are no eyelids in the eyes | There are no lateral line organs, eyelids are developed in the eyes |
| Jaws and feeding method | The horny plates of the jaws scrape off algae along with unicellular and other small animals | There are no horny plates on the jaws; the sticky tongue captures insects, mollusks, worms, and fish fry |
| Lifestyle | Water | Terrestrial, semi-aquatic |
Reproduction. Amphibians are dioecious. The genitals are paired, consisting of slightly yellowish testes in the male and pigmented ovaries in the female. Efferent ducts depart from the testes and penetrate into anterior section kidneys Here they connect to the urinary tubules and open into the ureter, which simultaneously performs the function of the vas deferens and opens into the cloaca. The eggs fall from the ovaries into the body cavity, from where they are released through the oviducts, which open into the cloaca.
Frogs have well-defined sexual dimorphism. Thus, the male has tubercles on the inner toe of the front legs ("nuptial callus"), which serve to hold the female during fertilization, and vocal sacs (resonators), which enhance the sound when croaking. It should be emphasized that voice first appears in amphibians. Obviously, this is related to life on land.
Frogs reproduce in the spring during their third year of life. Females spawn eggs into the water, and males irrigate them with seminal fluid. Fertilized eggs develop within 7-15 days. Tadpoles - the larvae of frogs - are very different in structure from adult animals (Table 19). After two to three months, the tadpole turns into a frog.
Development. In the frog, like in other amphibians, development occurs with metamorphosis. Metamorphosis is widespread in representatives of various types of animals. Development with transformation appeared as one of the adaptations to living conditions and is often associated with the transition of larval stages from one habitat to another, as is observed in amphibians.
Amphibian larvae are typical inhabitants of water, which is a reflection of the lifestyle of their ancestors.
Features of tadpole morphology that have adaptive significance in accordance with environmental conditions include:
- a special device on the underside of the head end, which is used for attachment to underwater objects - a suction cup;
- longer intestine than that of an adult frog (compared to body size); this is due to the fact that the tadpole consumes plant rather than animal (like an adult frog) food.
The organizational features of the tadpole, repeating the characteristics of its ancestors, should be recognized as a fish-like shape with a long caudal fin, the absence of five-fingered limbs, external gills, and one circle of blood circulation. During the process of metamorphosis, all organ systems are rebuilt: limbs grow, gills and tail dissolve, the intestines shorten, the nature of food and the chemistry of digestion, the structure of the jaws and the entire skull, the skin change, a transition from gill to pulmonary respiration occurs, deep transformations occur in the circulatory system .
The course of metamorphosis of amphibians is significantly influenced by hormones secreted by special glands (see above). For example, removal from a tadpole thyroid gland leads to an extension of the growth period, but metamorphosis does not occur. On the contrary, if thyroid preparations or thyroid hormone are added to the food of a tadpole of a frog or other amphibians, then metamorphosis is significantly accelerated and growth stops; As a result, you can get a frog only 1 cm long.
Sex hormones produced by the gonads determine the development of secondary sexual characteristics that distinguish males from females. In male frogs thumb the forelimbs do not form a “nuptial callus” when they are castrated. But if a castrate is transplanted with a testis or only injected with a male sex hormone, then a callus appears.
Phylogeny
Amphibians include forms whose ancestors about 300 million years ago (in the Carboniferous period) came out of the water onto land and adapted to new terrestrial living conditions. They differed from fish in the presence of a five-fingered limb, as well as lungs and associated features of the circulatory system. What they have in common with fish is the development of the larva (tadpole) in the aquatic environment, the presence in the larvae of gill slits, external gills, a lateral line, an arterial cone, and the absence of embryonic membranes during embryonic development. Data from comparative morphology and biology show that the ancestors of amphibians should be sought among ancient lobe-finned fish.
The transitional forms between them and modern amphibians were fossil forms - stegocephals, which existed in the Carboniferous, Permian and Triassic periods. These ancient amphibians, judging by the skull bones, are extremely similar to ancient lobe-finned fish. Characteristic signs them: a shell of dermal bones on the head, sides and abdomen, a spiral intestinal valve, like in shark fish, the absence of vertebral bodies. Stegocephalians were nocturnal predators that lived in shallow bodies of water. The emergence of vertebrates onto land occurred during the Devonian period, which was characterized by an arid climate. During this period, those animals that could move overland from a drying up reservoir to another acquired an advantage. Heyday (period biological progress) of amphibians dates back to the Carboniferous period, whose even, humid and warm climate was favorable for amphibians. Only thanks to their access to land did vertebrates gain the opportunity to further progressively develop.
Taxonomy
The class of amphibians consists of three orders: legless (Apoda), tailed (Urodela) and tailless (Anura). The first order includes primitive animals adapted to a unique way of life in moist soil - caecilians. They live in the tropical zone of Asia, Africa and America. Tailed amphibians are characterized by an elongated tail and paired short limbs. These are the least specialized forms. The eyes are small, without eyelids. Some species retain external gills and gill slits throughout their lives. Tailed animals include newts, salamanders and amblystoma. U tailless amphibians(toads, frogs) the body is short, without a tail, with long hind limbs. Among them there are a number of species that are eaten.
The meaning of amphibians
Amphibians destroy large numbers of mosquitoes, midges and other insects, as well as mollusks, including pests of cultivated plants and carriers of diseases. The common tree frog feeds mainly on insects: click beetles, flea beetles, caterpillars, ants; green toad - beetles, bedbugs, caterpillars, fly larvae, ants. In turn, amphibians are eaten by many commercial fish, ducks, herons, fur animals(mink, ferret, otter, etc.).
The endocrine system of amphibians is no different from general type characteristic of vertebrates. Thyroid hormone plays an important role in embryonic development and may be the reason for its slowdown up to and including neoteny. Adrenal hormones regulate metabolism. General regulation and bringing the state of the body into line with environmental changes is ensured by pituitary hormones in interaction with adrenal corticosteroids and gonadal hormones. Pituitary hormones and neurosecrets of the hypothalamus regulate water and salt metabolism, ensuring water absorption through the skin.
Central nervous system and sense organs. The transition to a terrestrial lifestyle was accompanied by a transformation of the central nervous system and sensory organs. The relative sizes of the brain of amphibians compared to fish do not increase noticeably. Tailless animals have slightly larger brains than tailed animals. The weight of the brain as a percentage of body weight is in modern cartilaginous fish 0.06-0.44%, in bony fishes 0.02-0.94, in tailed amphibians 0.29-0.36, in tailless amphibians 0.50-0.73% (Nikitenko, 1969). It should be noted that in modern amphibians the brain is probably somewhat smaller compared to the brain of their stegocephalic ancestors (this is evidenced by a comparison of the sizes of the brain skulls).
In modern amphibians, the relative size of the forebrain has noticeably increased, divided into two hemispheres with an independent cavity - the lateral ventricle - in each of them. Clusters nerve cells form not only the striatal bodies (corpora striata) in the bottom of the lateral ventricles, but also a thin layer in the roof of the hemispheres - the primary medullary vault - archipallium (of modern fish, lungfish have it). The olfactory lobes are poorly delimited from the hemispheres. The diencephalon is only slightly covered by neighboring sections. The epiphysis is located on top of it. A funnel extends from the bottom of the diencephalon, to which the well-developed pituitary gland is adjacent. The midbrain is smaller than that of bony fishes. The cerebellum is small and looks like a small cushion lying behind the midbrain along leading edge rhomboid fossa - cavity of the fourth ventricle. From the brain of amphibians, like in fish, 10 pairs of head nerves depart; The XII pair (hypoglossal nerve) departs outside the skull, and the accessory nerve (XI pair) does not develop.
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. In tailed amphibians, the level of nervous activity is lower than in tailless amphibians; this is due to the relatively smaller size of the brain and the thinness of the archipallium (about 0.2 mm versus 0.6-0.8 mm in anurans). The weak development of the cerebellum in all amphibians corresponds to the simplicity (stereotype) of movements.
Slightly flattened spinal cord has brachial and lumbar thickenings associated with the origin of powerful nerve plexuses innervating the anterior and hind limbs. Compared to fish, the separation of gray and white matter increases, i.e., the nerve tracts become more complex. There are 10 pairs of spinal nerves in tailless amphibians, and several dozen pairs in caudate amphibians, depending on the number of vertebrae. The sympathetic nervous system in amphibians is represented by two trunks lying on the sides of the ventral side spinal column. The ganglia of these trunks are connected to the spinal nerves.
Sense organs provide orientation for amphibians in water and on land. In larvae and in adult amphibians leading an aquatic lifestyle, an important role is played by the lateral line organs (seismosensory system), touch, thermoception, taste, hearing and vision. In species with a predominantly terrestrial lifestyle, vision plays a major role in orientation.
All larvae and adults with an aquatic lifestyle have lateral line organs. They are scattered throughout the body (more densely on the head) and, unlike fish, lie on the surface of the skin. Tactile corpuscles (clusters of sensory cells with nerves approaching them) are scattered in the superficial layers of the skin. All amphibians have free endings of sensory nerves in the epidermal layer of the skin. They perceive temperature, pain and tactile sensations. Some of them apparently respond to changes in humidity and, possibly, to changes in the chemistry of the environment. In the oral cavity and on the tongue there are clusters of sensory cells intertwined with nerve endings. However, they apparently do not perform the function of “taste” receptors, but serve as organs of touch, allowing one to sense the position of a food object in the oral cavity. The weak development of taste in amphibians is evidenced by their eating insects with pungent odor and caustic secretions (ants, bedbugs, ground beetles, etc.).
Spotted poison dart frog (Dendrobates tinctorius)
The sense of smell apparently plays a significant role in the life of amphibians. Olfactory sacs are paired. The external nostrils open and close by the action of special muscles. Each sac communicates with the oral cavity through the internal nostrils (choanae). The surface of the olfactory sacs is increased by the longitudinal folding of their walls and lateral protrusions. The tubular glands of the walls secrete a secretion that wets the mucous membrane of the olfactory sacs. Only part of the walls of the olfactory sacs is lined with a special olfactory epithelium, the cells of which are approached by the endings of the olfactory nerve. The volume of the olfactory sacs and the area occupied by the olfactory epithelium are especially large in legless animals (caecilians) and some tailless animals (toads, some tree frogs). The organ of smell functions only in the air; in water the external nostrils are closed. The role of smell in orientation and search for food is great in burrowing caecilians. Tailed and tailless amphibians recognize the smell of their habitat, the smell of “their” or “alien” species, and the smell of food. The sensitivity of smell changes with different seasons; it is especially high in spring. Amphibians manage to develop conditioned reflexes to smells.
In all amphibians, small blind recesses are formed in the area of the choanae, the walls of which are lined with sensory epithelium and nerved by the branches of the olfactory nerve. The cavity of these depressions is filled with the secretion of special glands. These organs are called Jacobson's and it is believed that they serve to perceive the smell of food in the oral cavity. In caecilians, in a hole on the head there is a movable tentacle, which the Animals constantly stick out, as if feeling the space around the head. It is believed that it performs the function of not only touch, but also smell.
The organs of vision are well developed in the vast majority of amphibians; only in caecilians living in the soil and permanent inhabitants of underground reservoirs - European proteus, underground salamander - Typhlotriton spealaeus and several other species - small eyes slightly visible through the skin or not visible. Compared to fish, the cornea of amphibian eyes is more convex, and the lens is shaped like a biconvex lens with a flatter front surface. Accommodation is carried out only by moving the lens with the help of muscle fibers of the ciliary body. The eyes of larvae, like fish, do not have eyelids. During metamorphosis, movable eyelids are formed - upper and lower - and a nictitating membrane (separated from the lower eyelid). Secret of the glands inner surface the eyelids and nictitating membrane protect the cornea from drying out; When the eyelids move, settled foreign particles are removed from the surface of the eye.
The retina contains rods and cones; in species with crepuscular and nocturnal activity, the former predominate. Total number photoreceptor cells in tailed amphibians range from 30-80 thousand per 1 mm2 of retina, and in tailless amphibians ( Rana etc.) - up to 400-680 thousand. Many amphibians have developed color perception. It has been shown that color discrimination is provided in the so-called Bellonzi nucleus (diencephalon), while the main information enters visual cortex(tectum opticum). In the retina, groups of receptors (rods and cones) communicate with bipolar cells through transverse and amacrine neurons; groups of bipolars transmit the received information to detectors - ganglion cells. It was found that retinal ganglion cells of frogs are represented by several functional types. Some react to small round objects that come into the field of view - food (shape detectors), others contrast the image, highlighting it against the general background (contrast detectors), others (motion detectors) react to the movement of “food”, and others - to fast and general shading of the field of view (regarded as a danger signal - the approach of an enemy). There are also “directional” neurons that register the direction of movement of “food”; they are connected to the basal nucleus of the diencephalon. Thus, primary processing(classification) of visual signals, unlike other vertebrates, in amphibians occurs already in the retina. The information collected is sparse. Fixed amphibians perceive only the movement of small objects or the approach of an enemy; everything else appears to them as an indifferent “gray background.” When moving, they begin to distinguish between stationary objects. Due to the position of the eyes, many tailless amphibians have a total field of view of 360° with a significant sector binocular vision, which allows one to estimate the distance to a moving food object, which makes it possible to successfully catch small moving prey. Based on the study of frog vision mechanisms, phototechnical devices have been created that recognize small objects.
Anatomy, physiology and ecology of tailless amphibians
Sense organs
Organs of hearing. Behind each eye on the frog's head there is a small circle covered with skin. This is the outer part organ of hearing- eardrum. The inner ear of a frog, like that of fish, is located in the bones of the skull. In addition to the inner ear, there is also a middle ear with an eardrum, sometimes hidden under the skin. In some aquatic forms it is reduced, for example, in fire toads.
The frog's auditory system allows it to perceive and then analyze sound signals. via three channels.
- In the air sound waves are captured by cells in the inner ear, through the eardrum and ear bone.
- Sounds propagated in the soil, are perceived by the bones and muscles of the limbs and are transmitted through the bones of the skull to the inner ear.
- In water sound waves easily penetrate the body of an individual and quickly reach the inner ear without special channels.
The main participant in the perception and transmission of signal information in the auditory system of amphibians is the sound analyzer, which is endowed with an amazing sensitivity. It is able to monitor very small but rapid fluctuations in environmental pressure. The analyzer records instantaneous, even microscopic compression and expansion of the medium, which spread in all directions from the place of their origin.
The upper limit of hearing for a frog is 10,000 Hz.
Voice. Tailless amphibians have voice and often resort to signaling system sounds. These are mating calls, distress calls, warning calls, territorial calls, release calls, etc. Other individuals hear these signals very well and react to them accordingly. An example is the imitative reaction of frogs to a warning signal - the sound of a slap that is heard when one of them jumps into the water in case of danger. Other frogs that sit aside and are not directly attacked, when they hear the sound of a frog jumping from the shore, react to it as an alarm signal. They immediately jump into the water and dive, as if they themselves noticed the approaching danger. Frogs also perceive warning calls - sound signals emitted by individuals in a state of fear.
Organs of vision. The frogs' eyes are positioned so that they can see almost 360 degrees around them. In the African clawed frog (Xenopus), the eyelids are also reduced and the lateral line organ is preserved. Most anurans have two eyelids - the upper one and the nictitating membrane, and in toads, in addition, there is a rudiment of the lower eyelid. Nictitating membrane(instead of the lower eyelid in most anurans) performs a protective function. The frog blinks frequently, while the moist skin of the eyelids moistens the surface of the eyes, protecting them from drying out. This feature developed in the frog in connection with its terrestrial lifestyle. (Fish, whose eyes are constantly in the water, do not have eyelids). By blinking its eyelids, the frog also removes dust particles adhering to the eye and moistens the surface of the eye.
Olfactory organs. In front of the eyes on the head there is a visible pair nostrils. These are not only the openings of the olfactory organs. The frog breathes atmospheric air, which enters its body through its nostrils. The eyes and nostrils are located on the upper side of the head. When the frog hides in the water, it puts them out. At the same time, she can breathe atmospheric air and see what is happening outside the water.
Of the olfactory organs, amphibians are endowed with olfactory sacs. Thanks to the receptors located in them, the bags have the ability to chemorecept both air and water. For example, air enters there through the nostrils and then goes to the lungs. Such an olfactory system is quite expedient. It is an integral part respiratory system, so all the air consumed during breathing is analyzed. Amphibians often use their sense of smell to orient themselves in space during hunting. Representatives individual species it helps to find and eat even motionless prey. Some salamanders guarding their eggs are able to smell and eat unfertilized eggs. They do this instinctively, obeying their internal innate program. Otherwise, the eggs, not having received a continuation of life, die, and the infection that develops on them spreads to newborn tadpoles.
The sense of smell allows amphibians to sense not only familiar odors, but also aromas such as anise or geranium oil, cedar balsam, vanillin, etc. Amphibians are able to sense chemicals not only through their sense of smell, but also thanks to the chemical analyzers of their skin.
The sense of smell also plays a role in behavior amphibians. For this, amphibians use pheromones. These are biologically active substances at the right moment they are automatically released by the animal’s body. A olfactory system, for example, a female or a fellow tribesman, with the help of its receptors, perceives information about the traces left. Then the obtained data is compared with the odor standards stored in memory. And only then does the animal receive a command for certain purposeful actions - say, the female approaching a place prepared by the male for laying eggs, etc. Many amphibians mark and protect their territory. The sense of smell can play an important role in the orientation of amphibians in the area when they search for their permanent spawning reservoir in the spring.
Organs of taste poorly developed. Amphibians are able to distinguish well between four types of taste substances - sweet, bitter, sour and salty. The taste organs of amphibians, which are bulbous bodies, are concentrated in their nasal cavity, in the mucous membrane of the palate and tongue. They are a peripheral part of the complex taste analyzer system. At the level of chemoreceptors that perceive chemical stimuli, the primary coding of taste signals occurs. And taste sensations are determined by the central “brain” structures of the analyzer. Each taste bud is responsible for the perception of 2-4 types. For example, a frog, thanks to the most complex system of its taste analyzers, will instantly and accurately distinguish a beetle that has entered its mouth, despite its chitinous shell, from a dry leaf or sliver. She will immediately spit out inedible objects. As experiments have shown, the ability to distinguish an edible from an inedible object by taste is better in terrestrial amphibians than in aquatic ones.
In real frogs and tree frogs teeth only available on upper jaw. Toads have no teeth. May be reduced in aquatic species language(peep, spur). In terrestrial forms, the tongue, projecting outward, plays an important role in capturing food. The shape of the tongue of toads is given by the so-called genioglossus - muscles attached to the chin. In a calm state, the long and soft tongue of the toad lies curled in the throat. At the right moment, the muscle comes into tension and forms a rigid bridge at the base of the tongue. At the same time, another muscle, the submentalis, running from cheek to cheek across the jaw, swells under this bridge, and a lever is created that forcefully throws the tongue out of the mouth.
In most tailless amphibians, the tongue is located in the mouth in a rather peculiar way - backwards. The root of the tongue is located in front, and the free part of the end of the tongue is facing inward. The tip of their tongue is sticky and the prey sticks to it and is pulled into the predator’s mouth. Less than one tenth of a second after the tongue ejection mechanism is launched, the hyoglossus, a muscle connected to the Adam's apple, is activated. He tenses and pulls his tongue, along with the stunned prey, into his mouth.
The tongue helps catch prey, but does not help swallow. Eyeball large and not limited by bony partitions from the oral cavity; when closing the eyes, the lower part is squeezed into the oral cavity. From time to time, the eyes disappear from the frog’s face and are pulled somewhere inside the head: they push another portion of food into the esophagus.
Toaded toads do not use their tongues to catch prey; they have a thick, disc-like tongue, which is why these amphibians are called round-tongued. And pond frogs, after catching a large insect with their tongue, push it into their mouth with their front paws. Toads that catch insects with their tongues can be trained to grab large foods with their mouths. Amphibians also have salivary glands.
Tailless amphibians are the first of the vertebrates endowed with vocal cords. Also, many frogs and toads (but only males) have resonators- sound amplifiers. Resonators can be external or internal.
At the Ecosystem Ecological Center you can purchase color identification table " Amphibians and reptiles of central Russia"and a computer identification of amphibians (amphibians) of Russia, as well as others teaching materials on aquatic fauna and flora(see below).
Any living organism is an ideal system, and if the circulatory, nervous and others allow us to exist, then the sense organs are exactly what the body uses to know and perceive external environment. Moreover, each class of animal organisms has its own characteristics.
Sense organs of fish
Representatives of this class of animals have quite developed eyes, which consist of the retina, lens and cornea. The fundamental difference between these organs is that when perceiving an image, the lens does not change curvature, like in other vertebrates - it simply moves relative to the cornea, thereby focusing the gaze.
They are found in fish and consist of three semicircular, mutually perpendicular canals. Some representatives have the so-called Weber's organ, which connects the cavity of the inner ear with the working in this case like a sound resonator. Receptors for taste and smell can be located not only in the mouth and nostrils, but also scattered throughout the body.
Another interesting organ is lateral line, which is a collection of channels associated with nerve fibers. The lateral line is especially developed in those fish that do not have eyes - it is thanks to it that they can perceive the outside world and maintain balance.
It's no secret that some fish can respond to electric fields and even generate electrical impulses with the help of special cells and nerve fibers.
Sense organs of amphibians
The sense organs of representatives of this class are already more adapted to existence in the air. For example, their eyes already have eyelids, as well as a nictitating membrane, which performs moisturizing and protective functions. The lens can change its size depending on the lighting.
In addition, amphibians have olfactory sacs that open outward through the nostrils. An animal can perceive odors only in the air. As for the hearing organs, amphibians are already developing a small bone called the stapes.
All mechanical receptors are located in skin tissues. In primitive aquatic amphibians, as well as in tadpoles, the lateral line is still preserved.
Sense organs of reptiles
Representatives of this class have more developed senses and are adapted to life in the air. Very important for these animals are the eyes, which are more developed than those of amphibians - there are developed muscles that are attached to the lens and can change its curvature to focus the image. In addition, reptiles develop real secretions that protect the animal’s eyes from drying out. There are also movable eyelids.
Such animals have choanae (internal nostrils), which are located closer to the throat, which greatly facilitates breathing while eating. It has been proven that reptiles are much more sensitive to odors than representatives of the amphibian class.
The taste organs are represented by specific structures - taste buds, which are located in the pharynx. And between the eyes and nose there is the so-called facial fossa, which allows you to react to temperature changes. For example, in some snakes it is this organ that allows them to quickly find food.
The hearing organs are not very well formed and resemble the hearing aid of amphibians. Reptiles have a middle and eardrum, as well as a stapes - a small bone that transmits vibrations to the eardrum. Hearing is not particularly important in the life of these animals. For example, in snakes it is practically not developed.
As can be seen, the sense organs gradually changed during evolution, adapting to survival in certain conditions and becoming more complex and functional.
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