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Amphibians (Amphibia) are a particularly primitive terrestrial vertebrate, occupying an intermediate position between terrestrial and aquatic vertebrate animals: reproduction and establishment occurs in an aquatic environment, while adults live on land.
Skin All amphibians have smooth, thin skin, which is relatively easily permeable to fluids and gases. Skin structure is characteristic of vertebrate animals: multi-layered epidermis and skin (corium) is prominent. The skin is rich in skin glands that secrete mucus. Some mucus can be poisonous or facilitate gas exchange. The skin is an additional organ of gas exchange and has a dense network of capillaries.
The body is divided into head, body, tail (in caudal animals) and five-toed limbs. Head is mobile and united with the body. The skeleton is divided into segments: the axial skeleton (backbone); the head skeleton (skull); and the paired limb skeleton.
The spine is divided into four divisions: cervical, trunk, sacrum and caudal. The number of vertebrae ranges from 7 in tailless amphibians to 200 in legless amphibians.
Musculature is subdivided into trunk and limb musculature. The trunk muscles are segmented. Specific muscle groups provide the difficult movements of the lever limbs. The head has lifting and descending muscles.
In the frog, say, the muscles of the jaw and limbs are better developed than each. Tailed amphibians (fire salamanders) have similarly powerful tail muscles.
The respiratory system of amphibians includes: lungs (special organ of air breathing); skin and mucous lining of oropharyngeal cavity (additional respiratory organs); gills (some water dwellers and tadpoles).
The circulatory system is closed; the heart is three-chambered with blood mixing in the ventricle (apart from the lungless salamanders, which have a two-chambered heart). The body temperature depends on the temperature of the environment.
The circulatory system consists of a large and a small circulatory system. The emergence of the second circle is associated with receiving pulmonary respiration. The heart consists of 2 atria (the right atrium has mixed blood, preferably venous blood and the left atrium has arterial blood) and one ventricle. Inside the ventricle walls form folds that prevent arterial and venous blood from mixing. An arterial cone with a spiral valve exits the ventricle.
All amphibians feed only on mobile prey. The tongue is located at the bottom of the oropharyngeal cavity. In tailless animals, the tongue is attached to the front end of the lower jaw, and when catching insects, the tongue protrudes from the mouth and the prey sticks to it. The jaws have teeth, which serve only to retain prey. Frogs have them on the upper jaw only.
The oropharyngeal cavity is opened by the ducts of salivary glands, whose secretion does not contain digestive enzymes. From the oropharyngeal cavity food enters the stomach and from there into the duodenum. This is where the ducts of the liver and pancreas open. Digestion takes place in the stomach and duodenum. The small intestine passes into the rectum, which forms an extension, the cloaca.
The eyes are similar to those of fish, but lack the silvery and reflecting membranes, as well as the sickle-shaped process. Only Proteus have underdeveloped eyes. There are adaptations to function in the aerial environment. Higher amphibians have upper (leathery) and lower (transparent) mobile eyelids. The blinking membrane (replacing the lower eyelid in most tailless amphibians) has a guarding function. There are no lacrimal glands, but there is a Gardner’s gland, the secret of which moistens the cornea and prevents it from drying out. The cornea is convex. The crystalline lens is a biconvex lens, the diameter of which varies with the light; accommodation is due to the metamorphosis of the distance between the crystalline lens and the retina. Many have developed colour vision.
The olfactory organs work only in an airy environment and are represented by paired olfactory sacs. Their walls are lined with olfactory epithelium. They open outward with nostrils and enter oropharyngeal cavity with choana.
The middle ear is the newest part of the organ of hearing. The external auditory orifice is closed by the tympanic membrane connected with the auditory ossicle – the stirrup. The stapes rest against the oval window leading into the inner ear cavity, transmitting the vibrations of the eardrum to it. The middle ear cavity is connected to the oropharyngeal cavity by an auditory tube to equalise the pressure on either side of the eardrum.
The organ of touch is the skin, which contains tactile nerve endings. Aquatic solvers and tadpoles have lateral line organs.
The life cycle of amphibians has four distinct formation stages: egg, larva, change period, and imago.
Many amphibians spend their lives in damp places, alternating between being on land and in water, but there are some purely aquatic species, as well as species that live extraordinarily in trees. Unsatisfactory adaptation of amphibians to living in terrestrial environment determines abrupt metamorphosis of their lifestyle in connection with seasonal changes in living conditions. Amphibians are able to hibernate for long periods under unfavourable conditions (cold, drought, etc.). Some species may change their activity from nocturnal to diurnal with decreasing temperatures at night. Amphibians are energetic only in warm conditions. At +7 – +8 °C many species fall into a stupor, while at -2 °C they die. However, some amphibians are able to tolerate long periods of freezing, drying out, and regenerate significant lost body parts.