ZOOLOGY STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 15. SUPERCLASS TETRAPODS, OR TERRESTRIAL VERTEBRATES (Tetrapoda). CLASS AMPHIBIANS (Amphibia)

2. Class Amphibians (Amphibia)

Amphibians are a small group of the most primitive terrestrial vertebrates (4000 species), the vast majority of which, depending on the stage of their life cycle, live in Water or on land, but remain linked to the aquatic environment at any stage. The appearance of the first amphibians is dated to the late Devonian, and their heyday to the Carboniferous period.

Amphibians inhabit regions with high humidity and positive average annual environmental temperatures. Their habitats include the shores of freshwater bodies and moist soils of the tropics and subtropics, and less frequently, deserts. Some species can lead an arboreal lifestyle.

Batrachology (from Greek batrachos — frog, logos — word, study) is the branch of zoology that studies amphibians.

Evolutionary advances of amphibians:

1. The Development of a pentadactyl limb (forelimbs are four-digit, hind limbs are five-digit) as a system of levers connected by hinge joints, which enable movement on land.

2. The Skull is movably articulated with the cervical vertebra.

3. Differentiation of Muscles into Muscle bundles, enabling more advanced forms of locomotion.

4. The formation of air-breathing Organs — Lungs, which enabled gas exchange utilizing atmospheric pressure. Consequently, respiration in adult amphibians is cutaneous-pulmonary (in larvae, it is cutaneous-branchial).

5. The Emergence of a double Circulatory system. The Heart is three-chambered.

6. Progressive development of The Nervous System and refinement of the Sensory Organs, allowing effective adaptation to new terrestrial habitats. The Forebrain is clearly divided into two hemispheres, while the Cerebellum is poorly developed. Eyes have movable eyelids.

Structural Features of amphibians as the first terrestrial vertebrates:

✵ Two pairs of limbs for movement on land.

✵ Terrestrial respiratory organs — lungs.

✵ Distinct body regions.

✵ Increased complexity of the Skeletal Structure.

✵ Development of the muscles of free limbs and their girdles; muscle differentiation.

✵ Better-developed forebrain (compared to fish).

✵ Sensory organs:

✵ of Vision (eyes with eyelids and lacrimal glands that protect the eye from drying out; a biconvex lens that allows the eye to focus on more distant objects);

✵ of Hearing (Inner ear; Middle ear: auditory ossicle, tympanic membrane, which enable animals to perceive sound vibrations propagating through the air);

✵ of balance (represented by semicircular canals).

Physiological features of amphibians as the first terrestrial vertebrates:

✵ Pulmonary and cutaneous respiration. In various amphibian species, between 15 and 55% of consumed oxygen is absorbed through cutaneous capillaries. Cutaneous respiration becomes the sole source of oxygen when animals remain submerged in water for extended periods: during hibernation or when hiding in a body of water while being pursued by terrestrial predators. In this case, oxygenated Blood enters the right atrium via the cutaneous vein, while the left atrium becomes venous. Some amphibian species lack lungs (such as lungless salamanders and the Far Eastern newt), and their gas exchange occurs entirely through the Skin and the mucous membrane of the Oral Cavity.

Characteristics of amphibians as aquatic vertebrates:

✵ The bare skin, permeable to water and gases, contains A large number of mucous glands.

✵ In addition to the trunk Kidneys, the skin also serves as an excretory organ.

✵ Body Temperature depends on the ambient temperature, being only slightly higher than the latter (poikilothermy).

✵ The eggs (spawn) lack protective shells and can only develop in water.

✵ The larvae lead an aquatic lifestyle.

The Class Amphibia comprises only 3 orders:

1. Anura (Anura) — spadefoot toads, poison dart frogs, tree frogs, rhinodermas, leptodactylid frogs, toads, narrow-mouthed frogs, Glass frogs.

2. Caudata (Caudata, Urodela) — newts, salamanders.

3. Apoda (Apoda) — Ceylon caecilian, true caecilian.

The giants among anurans are:

✵ the goliath frog (length 25-40 cm, weight 3.5 kg, lives in Cameroon, Angola, Guinea);

✵ the American bullfrog (length 20 cm, weight 600 g, lives in North America);

✵ the cane toad (length 26 cm, weight 1 kg, lives in the Americas).

Dwarfs among amphibians:

✵ the kokoe frog (length 1.5 cm, lives in Colombia).

Giants among caudates:

✵ the giant salamander (length up to 1.6 m, lives in the rivers of Japan and China).

Giants among caecilians:

✵ caecilians (length up to 1.2 m).

Anatomical Features of amphibians

The body of amphibians is divided into a HEAD (Fig. 60) and a trunk.

Fig. 60. Head of a frog (after Naumov, Kartashev, 1979):

1 — external nares; 2 — glottis; 3 — vomerine Teeth;

4 — internal nares; 5 — tympanic membrane; 6 — auditory (Eustachian) tube;

7 — palate; 8 — Tongue.

Modern amphibians show minor variations in body shape:

1) in anurans — the head is flat, bearing eyes with eyelids and nostrils; the neck is barely distinct; the trunk is dorsoventrally flattened with a reduced tail; the hind limbs are 2–3 times longer than the forelimbs, a structure that enables locomotion by jumping; flying frogs capable of gliding flight inhabit Southeast Asia;

2) in caudates — a cylindrical, elongated, sometimes laterally compressed body with a small head and a well-developed tail; the fore- and hind limbs are equally developed; sometimes the hind pair of limbs is absent (sirens);

3) in caecilians — a worm-like body, limbs are absent, and the eyes are rudimentary; this structure is an ADAPTATION TO A burrowing lifestyle.

Amphibian skin (Fig. 61) is naked, consisting of a stratified epidermis and a thin corium (dermis) rich in blood capillaries and mucous glands. The mucous glands are sac-like and formed by glandular Cells. Each mucous gland is enclosed by a muscular sheath and has an excretory duct opening to The surface of the skin. Mucous glands secrete mucus, which keeps the skin constantly moist. This enables the skin to participate in gas exchange.

Fig. 61. Amphibian skin (after Naumov, Kartashev, 1979):

1 — epidermis; 2 — corium; 3 — pigment cells;

4 — glandular cells; 5 — muscular Sheath of the skin gland; 6 — excretory duct of the gland.

The secretion of Skin glands contains substances that serve a signaling function, influencing The behavior of other individuals. The skin secretions of amphibians may contain toxic substances (fire salamander, toads). The venom of the South American cane toad is so potent that a dog quickly dies after grabbing this toad. Indigenous people used the venom of these toads to make poisoned arrows.

Pigment cells are located in the lower layers of the epidermis and corium, determining species-specific coloration. Amphibian coloration serves various Functions: camouflage (cryptic, or protective, coloration); warning and deterrence in species with toxic glands (aposematic coloration with bright colored spots); and Sexual Dimorphism (in males, coloration becomes brighter before the breeding season, facilitating encounters between mature individuals and stimulating mating). A small number of species can change the intensity of their coloration depending on the Background color; this ability is most pronounced in certain tree frogs.

The amphibian Skeleton is divided into the Axial Skeleton, the skull (neurocranium and viscerocranium), the Skeleton of the limbs (forelimbs and hindlimbs), and their girdles.

The axial skeleton is formed by vertebrae. In more primitive amphibians (caecilians, some caudates, and anurans), the vertebrae are of the amphicoelous type; remnants of the notochord persist between and within the vertebral bodies. In true salamanders, most lungless salamanders, and some anurans, the vertebrae are of the opisthocoelous type (the vertebral bodies are convex anteriorly and concave posteriorly); in all other anurans, they are of the procoelous type (the vertebral bodies

are concave anteriorly and convex posteriorly). The development of opisthocoelous or procoelous vertebrae increases the strength of the vertebral articulation without reducing The flexibility of THE Vertebral Column. Above the vertebral body, the neural arch with a spinous process is well developed. The neural arches form a canal containing the Spinal Cord.

The axial skeleton of amphibians is divided into 4 regions:

1) cervical — has 1 vertebra;

2) trunk — has a varying number of vertebrae: 7 in anurans, 13-62 in caudates, and up to 200 in caecilians;

3) sacral — has 1 vertebra;

4) caudal — has a varying number of vertebrae: in anurans, approximately 12 vertebrae fuse into a single bone — the urostyle; in caudates, 22-36; in caecilians, up to 100.

Trunk vertebrae have well-developed transverse processes, to which very short Ribs are articulated in caudates; in most anurans, the ribs fuse with the transverse processes. Caudal vertebrae in caudates bear hemal arches, which form the hemal canal.

The amphibian skull is predominantly cartilaginous and consists of fewer bones compared to the skull of bony Fishes. The skull is formed by the frontoparietal and occipital bones, orbits, and jaw bones. The gill arches are partially modified into the auditory apparatus, and opercula are absent.

The limb skeleton has a structure typical of terrestrial vertebrates. Each forelimb consists of the upper arm (humerus), forearm (ulna and radius), and hand (carpals, metacarpals, and Phalanges). The hindlimb consists of the thigh (Femur), shin (Tibia and Fibula), and FOOT (tarsals, metatarsals, and phalanges). In the joint between the thigh and shin, There is a small bone — the Patella. In caecilians, the fore- and hindlimbs and their girdles are reduced. In proteids (a suborder of caudates), the forelimb has only 3 digits, and the hindlimb has only 2. In anurans, there are 4 digits on the forelimb and 5 on the hindlimb. There is webbing between the digits of the hindlimbs.

The Pectoral Girdle of amphibians consists of paired bones—the scapulae, clavicles, and coracoid bones (coracoids)—and the unpaired Sternum. Attached to the scapula is a broad suprascapular Cartilage, to which the back muscles attach. In front of the coracoid lies a thin, rod-like cartilage—the procoracoid. Below it is a thin bone—the clavicle. The inner ends of the coracoids fuse with each other. Behind the fusion site of the coracoids lies the sternum with an expanded cartilaginous posterior tip. In front of the clavicle is a small presternum, the anterior part of which is cartilaginous and the posterior part is bony. Amphibians lack a rib cage. The short ribs of tailed amphibians do not reach the sternum.

The hind limb girdle (Pelvic Girdle) consists of three paired elements—the ilium, ischium, and pubic cartilage, which form the pelvis.

The Muscular System of amphibians differs significantly from that of fish. Part of the trunk musculature retains a metameric structure. However, there is a distinct differentiation: regions of muscle segments fuse into band-like muscles. The mass of the limb musculature increases dramatically, represented by a complex system of antagonist muscles (muscle groups performing opposing functions) and synergist muscles (muscle groups performing cooperative functions).

The Digestive System of amphibians consists of the Mouth, the buccopharyngeal cavity with small conical teeth (intended only for holding prey; in tailless amphibians, teeth are partially reduced and persist only on the BONES OF THE upper jaw), the tongue, and the ducts of Salivary Glands (the secretion contains no digestive Enzymes and only moistens food), the Esophagus, The Stomach (its glandular cells secrete the enzyme Pepsin, which actively acts on food under METABOLISM/18.html">The Influence of Hydrochloric acid), and the intestine, which consists of the duodenum (where the ducts of The Liver and Pancreas open), the Small Intestine, and the rectum, ending in an expansion—the cloaca.

Amphibians feed on small invertebrates. In frogs, the tongue is attached to the floor of the buccopharyngeal cavity by its anterior end, while its posterior part is free (Fig. 62).

Fig. 62. Successive Stages of the green frog's tongue movement during prey capture (after Naumov, Kartashev, 1979).

During prey capture, the tongue is projected from the mouth to a certain distance by specialized muscles. When swallowing food, the eyeballs retract, pressing on the walls of the buccopharyngeal cavity and pushing the food into the esophagus, from where it enters the stomach.

Some aquatic species (Surinam toad) can catch fish fry. Large species (marsh frog, sirens) even catch amphibians and their larvae, waterfowl chicks, and small rodents. The daily ration of amphibians can be up to 10-30% of their body weight. At low ambient temperatures, amphibians easily tolerate prolonged starvation (up to a year in experiments).

The excretory system consists of paired trunk kidneys (mesonephroi), Ureters, the cloaca, and the Urinary Bladder. On the ventral surface of the kidneys are the Adrenal Glands—Endocrine glands. In the kidneys, the blood is cleared of

excess water, salts, and urea, resulting in the formation of urine. From the kidneys, urine is carried via the ureters into the cloaca. For some time, urine can accumulate in the urinary bladder, which is located on the ventral surface of the cloaca and communicates with it. When the urinary bladder is full, the contraction of its muscular walls expels the concentrated urine into the cloaca, from where it is discharged outside.

Some Metabolic waste products are excreted through the skin.

The Respiratory system of amphibians consists of the respiratory tract (paired nostrils with Valves, the nasopharyngeal cavity, and the Larynx) and paired lungs, which have thin, cellular walls penetrated by blood capillaries where gas exchange occurs. Cutaneous respiration plays an important role in amphibians, which is why their skin is always moist (this increases its permeability to gases). In different amphibian species, 15-55% of oxygen is absorbed through the skin, 35-75% through the lungs, and 10-15% through the mucosa of the buccopharyngeal cavity. Through the lungs and the buccopharyngeal cavity, 35-55% of Carbon dioxide is excreted, while 45-65% is excreted through the skin.

Male tailless amphibians have specific vocal sacs—resonators, the inflation of which produces the calls used to attract females.

Among tailed amphibians, there are lungless salamanders that live in North America. They lack lungs and a Pulmonary Circulation loop. Gas exchange occurs through the skin. How does this happen? In order to breathe through the skin, it must be moist (dry skin is incapable of gas exchange). A film of mucus is secreted by specialized skin glands, which is why amphibian skin is always moist. Atmospheric oxygen dissolves in this mucus film on the skin surface and can diffuse into the blood. Therefore, all lungless salamanders are small. Cutaneous respiration allows the animal to remain underwater for a long time.

The CIRCULATORY SYSTEM OF amphibians is characterized by A number of advancements compared to fish. The amphibian heart is three-chambered (two atria and one ventricle). When the atria contract, they pump blood into the ventricle. Upon ventricular contraction, blood is prevented from returning to the atria by the atrioventricular valve; instead, it enters the main Arteries and is distributed throughout the body. There are two circulation loops—systemic (large) and pulmonary (small). Both loops begin at the ventricle, and As a result of its contraction, blood of varying composition enters three different arteries. The pulmonary (small) circulation loop: during ventricular contraction, a portion of venous blood is expelled into the pulmonary arteries and lungs, where it becomes oxygenated (arterial); it then flows into the Pulmonary Veins and returns to the left atrium. The systemic (large) circulation loop: during ventricular contraction, mixed blood (the most oxygen-rich blood goes to the Brain) is forced into the aorta, through which it travels to all Organs of the body and returns via veins that bring venous blood to the right atrium; some of the mixed blood goes to the skin, where gas exchange occurs during cutaneous respiration; this oxygen-enriched (arterial) blood returns via veins that also enter the right atrium. Thus, blood mixes in the right atrium.

The Hematopoietic organs are the Spleen and Cytology/practical/86.html">Red Bone Marrow, where Blood Cells—erythrocytes, leukocytes, and platelets—are formed. The total blood volume accounts for 1.2-7.2% of the total body mass.

The nervous system of amphibians consists of the brain, spinal cord, and nerves.

The brain consists of five divisions: the forebrain (divided into two hemispheres), Diencephalon, Midbrain, Medulla Oblongata, and a poorly developed cerebellum. The brain in tailed amphibians accounts for 0.29-0.36% of body mass, and in tailless amphibians, 0.5-0.73%.

The spinal cord is located in the spinal canal of the vertebral column. Compared to fish, the conducting nerve tracts are more complex. There are 10 pairs of Spinal Nerves in tailless amphibians, and in tailed and limbless amphibians, depending on the number of vertebrae, several dozen pairs.

The sensory organs of amphibians include:

✵ eyes protected by upper and lower eyelids; unlike in fish, the cornea is more convex, and the lens is biconvex with a flatter anterior surface; accommodation is achieved solely by moving the lens using the muscle fibers of the ciliary body; the retina contains rods and cones; most amphibians have Color Vision; the eyes of larvae, like those of fish, lack movable eyelids; stationary amphibians perceive only the movement of small objects or the approach of an enemy, while everything else is an indifferent "gray background" to them; when they move, they begin to distinguish stationary objects as well; due to THE POSITION OF the eyes in many tailless amphibians, the total field of vision is 360°; based on The Study of frog vision mechanisms, phototechnical devices have been created to recognize small objects;

✵ hearing organs consisting of an auditory opening covered by a tympanic membrane; the middle ear (an air-filled cavity containing a rod-like auditory ossicle—the stapes, which connects at one end to the tympanic membrane and transmits sound vibrations in the air) and the inner ear (membranous labyrinth) protected by the skull bones; the membranous labyrinth is filled with fluid (endolymph); a narrow canal—the Eustachian tube—connects the air cavity of the middle ear with the oral cavity, equalizing pressure and preventing damage to the tympanic membrane during loud sounds; amphibians perceive sounds with frequencies from 30 to 15 000 Hz; in some tailless amphibians (spadefoot toads, fire-bellied toads), and in all tailed and limbless amphibians, the middle ear cavity and tympanic membrane are reduced.

✵ organs of balance represented by three semicircular canals communicating with the inner ear; when the body changes position, the fluid in the canals moves. This movement is detected by sensory cells.

✵ olfactory organs (paired sacs) that communicate with the external environment via paired nostrils; the external nostrils open and close through the actions of specialized muscles; each olfactory sac connects to the oral cavity via internal nostrils (choanae); olfactory organs

function only in the air; in water, the external nostrils are closed;

✵ in larvae and aquatic adults, the lateral line organs (seismosensory system) are scattered all over the body (more densely on the head); unlike in fish, they lie On the surface of the skin;

✵ tactile corpuscles scattered in the superficial layers of the skin;

taste receptors are poorly developed, as evidenced by their consumption of insects with a pungent odor and acrid secretions (ants, bugs).

Amphibians are dioecious animals. Females have paired Ovaries, and males have Testes. By spring, the granular ovaries fill almost the entire body cavity. Located next to the ovaries are multilobed fat bodies, which accumulate nutrients that ensure the development of Gametes during hibernation. The long, thin oviducts are Müllerian ducts. Each oviduct opens into the body cavity via a funnel located in the cardiac region; the lower uterine portion of the oviducts is dilated and opens into the cloaca. Mature oocytes break through the ovarian wall into the body cavity, are captured by the edges of the funnel, and move down the oviducts, becoming coated with mucous albuminous envelopes.

Rounded testes, with fat bodies located next to them, lie near the anterior margins of the kidneys. Several thin efferent ductules emerge from each Testis, entering the Kidney where they open into the Wolffian duct. In amphibians, this duct functions as both a Ureter and a vas deferens. In sexually mature males, the lower part of the Wolffian duct expands to form a Seminal Vesicle, which serves as a reservoir for sperm. The Wolffian ducts open into the cloaca via urogenital apertures.

In anurans and some caudates (Hynobiidae), Fertilization is external, occurring in water. The male clasps the female with his forelimbs; many species have nuptial pads on their hands that help grasp the female. The female releases spawn (eggs) resembling fish roe, and males release seminal fluid containing sperm over it.

In most caudate amphibians, fertilization is internal. For instance, in newts, the male deposits a spermatophore, which the female picks up with the Lips of her cloaca; inside the cloaca, the envelope dissolves, and the sperm fertilize the eggs located in the lower sections of the oviducts. In some salamanders, the male presses his cloacal opening against that of the female, transferring the spermatophore directly. Caecilians also have internal fertilization.

In most amphibians, eggs are deposited in water. The embryo develops inside the egg, and the hatched larva leads an aquatic lifestyle until metamorphosis. In most anurans, each egg is covered by a gelatinous envelope that swells in water. Eggs laid at the same time stick together into a clump and float on the water surface. The firm adhesion of the eggs makes it difficult for small predators to eat them, and the translucent spherical envelopes of the eggs act as converging lenses that concentrate light rays. As a result, in sunny weather, the temperature inside the clump can be 5-7°C higher than the temperature of the surrounding air and water.

The amphibian egg contains yolk. Cleavage of the egg begins 3-4 hours after fertilization. The rate of embryonic development depends heavily on water temperature: the higher the temperature, the faster the development. Therefore, it usually takes from 5 to 15-30 days from egg-laying to larval hatching. Development in amphibians occurs with metamorphosis (Fig. 63).

Fig. 63. Metamorphosis of amphibians (using anuran frogs as an example):

1 — adult anuran frog; 2 — clutch of eggs (spawn); 3 — larva at the moment of hatching;

4 — larva at the stage of resorption of external gills and fin fold;

5 — larva at the stage of operculum development and limb emergence;

6 — larva at the stage of completed limb formation and tail resorption;

7 — emergence of the young frog onto land.

In anurans, the developed larvae (tadpoles) hatch from their envelopes and, using a specialized organ—an adhesive disc (sucker)—attach themselves to aquatic plants or empty egg envelopes. At

this time, the larva's mouth has not yet broken through, the cutaneous fin fold of the tail is poorly developed, and external gills are present. The larva shares common features with fish: an elongated body shape, a caudal fin, a notochord, a single circulatory loop, a two-chambered heart, opercula, gills (first external, then internal), and a visible lateral line on the skin.

During the first few days, amphibian larvae survive on the remaining yolk of the egg. Later, their mouth breaks through and they begin to feed: first, they consume the gelatinous envelopes to which they were attached, and then they start feeding on Algae, Protozoans, and other aquatic organisms. Horny denticles hidden under fleshy lips help the larvae secure food. The larva's tail elongates, functioning not only as an organ of locomotion but also for respiration (a branched capillary network develops within it). On days 20-25, limb buds appear as small bumps (the forelimbs are covered by the operculum). During this period, the choanae break through, the glottis forms, lungs begin to develop, the circulatory system undergoes transformation, mesonephric kidneys form, cartilaginous vertebrae develop, and the skull enlarges. The limbs form: joints are established, and digits develop. Pulmonary respiration begins to function, and the internal gills degenerate. In the final phases of metamorphosis, the forelimbs break through to the outside, the gills disappear, the gill slits close, the eyes enlarge, skeletal formation is completed, the tail is gradually resorbed, and the tadpole (larva) transforms into a tiny froglet.

A comparative characterization of adult anurans and their larvae is presented in Table 11.

Table 11

COMPARATIVE CHARACTERIZATION OF ADULT ANURAN AMPHIBIANS AND THEIR LARVAE

In caudate amphibians, the larvae hatch more developed: the tail is better developed, and the external gills are larger. The following day

gill slits break through, external gills begin to branch, the mouth forms, and the larva begins to feed, catching small invertebrates. At 2-3 weeks of age, the forelimbs appear, followed by the hindlimbs. Metamorphosis occurs gradually: lungs develop, the circulatory system undergoes reorganization, and the external gills regress.

Larvae are an extremely vulnerable developmental stage of amphibians; therefore, frogs and toads that do not care for their spawn lay many thousands of eggs. However, many tropical species exhibit parental care. The blacksmith tree frog builds a circular mud nest up to 30 cm in diameter in shallow water, where it lays its eggs. This barrier protects the eggs from predators. Another tree frog species lines tree hollows with resin. These hollows retain rainwater, in which the frogs then lay their eggs. In South American marsupial frogs, tadpoles develop in a pouch—a skin fold that opens on the back. The female marsupial frog lays only 4-20 eggs. The male midwife toad (found in Western Europe) carries a string of eggs covered with thick membranes on his body for one and a half to two months. When the eggs are ready to hatch, he releases the tadpoles into a body of water. During spawning, the male Surinam toad (native to South America) places large eggs onto the female's back. The eggs embed into the skin of her back, where the development and metamorphosis of the larvae take place. Female Surinam toads lay 40-100 eggs. In Darwin's frog, tadpoles develop and undergo metamorphosis inside the male's vocal sac, which the eggs enter through an opening under the tongue. During development, they fuse with the wall of the male's vocal sac, obtaining nutrients from his blood. The female Darwin's frog lays only 20-30 eggs. In the Australian gastric-brooding frog, the male swallows the fertilized eggs, and the tadpoles develop in the stomach, feeding on secretions from its walls. The mucus of the tadpoles inhibits the secretion of gastric juice, preventing them from being digested. The froglets are "born" through the father's mouth. Scientists have discovered that some species of tailed amphibians exhibit neoteny—the ability of larvae to reproduce sexually. This phenomenon is well-studied in the tiger salamander, a terrestrial amphibian native to North America. In shallow, warm waters with low oxygen content, the larvae complete metamorphosis relatively quickly and leave the water upon reaching a length of 8-9 cm. In cold, deep waters with high oxygen content, the larvae grow well, reaching 20-25 cm in length, but do not undergo metamorphosis. Their Reproductive System develops fully (ahead of other body systems), enabling them to reproduce while remaining in the larval stage. These larvae are bred in laboratory conditions under the name axolotls. In experiments, metamorphosis in axolotls can be induced by administering Thyroid Gland preparations. Thus, it has been proven that the thyroid hormone, thyroxine, regulates the normal course of metamorphosis.

Some amphibians exhibit ovoviviparity. In salamanders, fertilized eggs are retained in the oviducts, where a significant part of larval development takes place. The salamander gives birth to fully formed larvae that rupture the egg membrane and transition to an independent life. In the fire salamander, eggs are also retained in the oviducts. When the young are born, they are fully prepared for life on land. In the African viviparous toad, the eggs develop in the lower PARTS OF THE oviducts, and fully metamorphosed young froglets are born.

In seasonal climates, the annual Life Cycle of amphibians is divided into the following periods: spring awakening, reproduction (spawning), summer activity, and hibernation (winter torpor).

The Role of amphibians in nature

Amphibians are an essential link in the food chains of both aquatic and terrestrial ecosystems. In the wild, they serve as prey for many animals, while they themselves consume large numbers of invertebrates, thereby regulating their populations in the environment.

The Importance of amphibians in human life

Amphibians feed on many invertebrates that are harmful to humans, such as mosquitoes and their larvae.

The meat of certain amphibian species (such as the giant salamander, tiger frog, goliath frog, and moor frog) is consumed by humans. There are frog farms dedicated to breeding these animals, and they are a commodity in international trade.

Certain amphibian species (such as the fire salamander and green toad) are sources of toxins (bufotoxin, salamandratoxin) used in the manufacture of Pharmaceuticals.

Amphibians serve as subjects for laboratory research by scientists, biology students, and medical professionals. All amphibian species are in need of conservation.