Amphibians are direct descendants of cross-finned fishes.They appeared 380 million years ago and subsequently gave rise to the class of reptiles. What do amphibians look like? What is different from other animals and what kind of lifestyle do they lead?
According to the common version, crossfishwere the first inhabitants of the reservoirs, who managed to reach the land. Mastering a new space and adapting to other conditions, they gradually began to change, giving rise to new beings - amphibians.
“Amphibian” is an ancient Greek word thattranslates as "two kinds of life." In biology, it refers to animals living both on land and in water. In Russian terminology it is more and more clear, since amphibians are amphibians.
Previously, the concept also implied seals andotters, but later began to include only four-legged vertebrates, which do not belong to amniotes. The modern class of amphibians covers only salamanders, worms, newts and frogs. In total there are from 5 to 6.7 thousand species.
Amphibians are vertebrates that are in the kingdomanimals occupy an intermediate position between fish and reptiles. Many representatives alternate periods of life in water and on land. Reproduction and initial development in the majority occurs in the water, and growing up, they lead a terrestrial lifestyle. Some species live only in water.
Most amphibians can not tolerate coldprefer warm and humid places, but can live in arid areas. When adverse conditions occur, they can hibernate or change the activity time, for example, from night to day. However, some species settled far to the north, for example, the Siberian salaver.
Amphibians settle near freshwater bodies, andthe larvae sometimes lay even in deep pools. Only a few species live in seawater. Development, as a rule, is accompanied by four stages: an egg (caviar), a larva, a metamorphosis and an adult individual. Salamanders have a live birth.
У всех представителей класса слабый обмен substances, so they can not digest plant foods. Amphibians are predators and feed on insects, small invertebrates, and sometimes their fellows. Large individuals eat young fish, chicks and rodents. Only the larvae of the tailless order feed on plants.
The external structure of amphibians is very different.The group of caudates, which includes tritons and salamanders, resembles lizards in appearance. They grow to 20 centimeters. Their body is elongated and ends with a long tail. Neck, hind and front legs short.
Frogs are tailless amphibians.They have a wide, slightly flattened body and short neck. The tail is available only at the tadpole stage. Their limbs are elongated and bent, straightened at the time of jumping and swimming (basic modes of movement). The fingers of the frogs and the salamander are connected by a skin membrane.
Worms are amphibians of the "legless" order.Outwardly, they look like worms or snakes. Their sizes range from ten centimeters to a meter. In worms there are no limbs, and the tail is shortened. Their body is covered with lime scales and painted in dark black or brown tones, sometimes with spots or stripes.
Кожа этих позвоночных многослойная, но достаточно thin. It contains glands that secrete mucus that covers the whole body. Through it, breathing is partially effected. On the surface, the lungs help the amphibians to breathe, and the species that live predominantly in the water have gills.
The heart of amphibians is three-chambered, two cameras are observed only in salamanders. Circles of blood circulation two: small and large. Body temperature is variable and depends on the external environment.
The brain of amphibians is larger than that of fish, and isfrom 0.30% (in caudate) to 0.73% (in tailless) of body weight. Their vision is able to distinguish colors. Eyes cover transparent lower and leathery upper eyelids. They distinguish taste badly and can only identify salty and bitter.
The skin is the main organ of touch and contains many nerve endings. In tadpoles and aquatic species, the lateral line of fish is preserved, which is responsible for orientation in space.
У ряда бесхвостых слизь на коже содержит яд.In most cases, it is not harmful to humans and serves to disinfect the surface. However, the poison of some tropical species can be dangerous. So, the little yellow frog (see photo above) horrible leaf eye is one of the most poisonous creatures in the world.
