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Video: Class Amphibians, Or Amphibians (Amphibia)
Amphibians, or amphibians, are the first terrestrial vertebrates that still retain significant connections with the aquatic environment. In most species, eggs (eggs) are devoid of dense membranes and can only develop in water. Each egg is surrounded by a transparent mucous membrane that swells in water. Embryos do not have embryonic membranes (anamnia). The larvae hatched from eggs lead an aquatic lifestyle and only then undergo metamorphosis (transformation), during which the characteristics of adults leading a terrestrial lifestyle are formed.
For adult amphibians, paired limbs with articulated joints are characteristic. The skull with two occipital condyles is movably articulated with the cervical vertebra. The pelvic girdle is attached to the transverse processes of the sacral vertebra. Two circles of blood circulation are formed, not completely separated: there are two atria in the heart, but one ventricle. The eyes have movable eyelids. The lateral line organs in adults usually disappear. The forebrain enlarges and divides into two hemispheres. Amphibians also retained the characteristics of aquatic vertebrates. Bare skin, which is permeable to water and gases, has a large number of mucous glands. The excretory organs are the kidneys and the skin. Body temperature in amphibians depends on the ambient temperature and only slightly exceeds the latter.
The appearance of amphibians is diverse. In tailed amphibians, the body is elongated, legs are short, of approximately the same length, a long tail is preserved throughout their life. In tailless amphibians, the body is short and wide, the hind legs are jumping, much longer than the front ones, the tail is absent in adults. Worms (legless) have a long, worm-like body without legs. In all amphibians, the neck is not expressed or is weakly expressed. Unlike fish, their head articulates with the spine in a flexible manner.
Covers of amphibians
The skin is thin, always devoid of scales (naked), but rich in glands that secrete abundant mucus, which makes it easy to distinguish amphibians from reptiles. In larvae, the mucous glands are unicellular, in adults, they are multicellular. The secreted mucus prevents the skin from drying out, which is necessary for skin respiration. In some amphibians, the skin glands secrete a poisonous or burning secret that protects them from predators. Skin is an important respiratory organ for amphibians.
The coloration of amphibians is often patronizing. Some, like the tree tree frog, are capable of changing it.
The skeletal musculature is represented by many individual muscles, the number of which in the frog exceeds 350. The ribs are not connected to the sternum. Unpaired fins, if any, do not have skeletal rays. The sacrum is formed by one vertebra.
Consists of the spine, skull, limb bones and their belts. The spine is divided into sections: cervical, consisting of one vertebra, trunk - from a number of vertebrae, sacral - from one vertebra and caudal. In tailless amphibians, the rudiments of the caudal vertebrae grow together into a long bone - the urostyle. In some caudate amphibians, the vertebrae are biconcave: the remnants of the notochord are preserved between them. In most amphibians, they are either convex in front and concave in the back, or, conversely, concave in front and convex in the back. The chest is missing.
The skull is mostly cartilaginous. With the transition from gill respiration of aquatic ancestors of amphibians to pulmonary respiration, the visceral skeleton has changed. The skeleton of the branchial region is partially modified into the hyoid bone. The upper part of the hyoid arch is the pendant, to which the jaws are attached in lower fish; in amphibians, due to the fusion of the primary upper jaw with the skull, it has turned into a small auditory ossicle - a stirrup located in the middle ear. The number of toes varies from species to species.
The nervous system of amphibians has undergone significant complications in comparison with that of fish. The brain is relatively larger. Due to the fact that amphibians are inactive, their cerebellum is poorly developed. The diencephalon has an appendage on top - the pineal gland, and a funnel departs from the bottom, with which the pituitary gland is connected. The midbrain is poorly developed. Nerves extend from the brain and spinal cord to all organs of the body. There are ten pairs of head nerves. The spinal nerves form the brachial and lumbosacral linkages that innervate the fore and hind limbs.
The sense organs of amphibians have received progressive development in the process of evolution. The nasal cavity communicates with the oral internal nostrils - the choans. The middle ear is bounded outside by the tympanic membrane. It communicates with the pharynx canal (Eustachian tube), which makes it possible to balance the air pressure in it with the pressure of the external environment. The cornea of the eye is convex, the lens is lenticular, there are eyelids that protect the eyes. The organs of smell have external and internal nostrils. The larvae and amphibians constantly living in water retained the lateral line organs characteristic of fish.
A wide mouth leads into a vast oral cavity: in many amphibians, small teeth are located on the jaws, as well as on the palate, which help to retain prey. Amphibians have tongues of various shapes; in frogs, it is attached to the front of the lower jaw and can be thrown out of the mouth; animals use this to catch insects. In the oral cavity, the internal nostrils - the choanae - open, and the Eustachian tubes - into the pharynx. In the frog, the eyes take part in swallowing food; seizing prey with its mouth, the frog draws its eyes deep into the oral cavity by muscle contraction, pushing food into the esophagus. Through the esophagus, food enters the sac-like stomach, and from there into the relatively short intestine, which is divided into a thin and thick sections. Bile produced by the liver and pancreatic secretions enter the beginning of the small intestine through special ducts.In the final part of the large intestine - the cloaca - the ureters, the bladder duct and the reproductive ducts open.
change with the age of the animal. Amphibian larvae breathe with external or internal gills. In adult amphibians, lungs develop, although in some tailed amphibians the gills persist for life. The lungs look like thin-walled elastic bags with folds on the inner surface.
In connection with air breathing, amphibians have two circles of blood circulation.The amphibian heart is three-chambered, it consists of two atria and a ventricle. The left atrium receives blood from the lungs, and the right atrium receives venous blood from the whole body with an admixture of arterial blood coming from the skin. Blood from both atria flows into the ventricle through a common valve opening. The ventricle continues into a large arterial cone, followed by a short abdominal aorta. In tailless amphibians, the aorta is divided into three pairs of symmetrically departing vessels, which are modified gill arteries of fish-like ancestors. The anterior pair - the carotid arteries, carry arterial blood to the head. The second pair - the arches of the aorta, bending to the dorsal side, merge into the dorsal aorta, from which arteries that carry blood to different organs and parts of the body depart. The third pair is the pulmonary arteries, through which venous blood flows into the lungs.On the way to the lungs, large cutaneous arteries branch off from them, heading into the skin, where they branch into many vessels, causing cutaneous respiration, which is of great importance in amphibians. From the lungs, arterial blood flows through the pulmonary veins into the left atrium.
Venous blood from the back of the body passes partially to the kidneys. The veins leaving the kidneys form an unpaired posterior (inferior) vena cava. Another part of the blood from the posterior part of the body flows through two vessels, which, merging, form the abdominal vein. It goes, bypassing the kidneys, to the liver and participates, together with the portal vein of the liver, which carries blood from the intestine, in the formation of the portal system of the liver. Upon exiting the liver, the hepatic veins flow into the posterior vena cava, and the latter into the venous sinus (venous sinus) of the heart, representing the expansion of the veins. The venous sinus receives blood from the head, forelimbs, and skin. From the venous sinus, blood flows into the right atrium.
The excretory organs in adult amphibians are represented by the trunk kidneys. A pair of ureters leaves the kidneys. The urine excreted by them first enters the cloaca, and from there - into the bladder. Head buds function in amphibian embryos.
All amphibians are dioecious. Males have two testicles located in the body cavity near the kidneys. The vas deferens, passing through the kidney, flow into the ureter, represented by the wolf channel, which serves to excrete urine and sperm. In females, large paired ovaries lie in the body cavity. Ripe eggs enter the body cavity, from where they enter the funnel-shaped initial sections of the oviducts. Passing through the oviducts, the eggs are covered with a transparent thick mucous membrane. The oviducts open into the cloaca.
Development in amphibians takes place with a complex metamorphosis. Larvae emerge from the eggs, which differ both in structure and in lifestyle from adults. Amphibian larvae are real aquatic animals. Living in the aquatic environment, they breathe with gills. The gills of the larvae of tailed amphibians are external, branched; in the larvae of tailless amphibians, the gills are at first external, but soon become internal. The circulatory system of amphibian larvae is similar to that of fish and has only one circle of blood circulation. They have lateral line organs. They move mainly due to the movement of a flattened tail, trimmed with a fin.
With the transformation of the larva into an adult amphibian, profound changes occur in most organs. Paired five-fingered limbs appear, tailless amphibians have a reduced tail. Gill respiration is replaced by pulmonary respiration, and the gills usually disappear. Instead of one circle of blood circulation, two develop: large and small (pulmonary). In the Mexican ambistoma, neoteny is observed - the ability to reproduce at the larval stage, that is, to reach sexual maturity while maintaining the larval features of the structure.
The habitats of amphibians are diverse, but most species adhere to wet places, and some spend their entire life in the water, without going out on land. Tropical amphibians - worms - lead an underground lifestyle. A kind of amphibian - the Balkan Proteus lives in caves; his eyes are reduced, and his skin is devoid of pigment. Amphibians belong to the group of cold-blooded animals, that is, their body temperature is unstable and depends on the ambient temperature. Already at 10 ° C, their movements become sluggish, and at 5-7 ° C, they usually fall into a daze. In winter, in a temperate and cold climate, the vital activity of amphibians almost stops. Frogs usually hibernate at the bottom of water bodies, and newts - in burrows, in moss, under stones.