Amphibians are the direct descendants of brushfish. They appeared 380 million years ago and subsequently gave rise to the reptile class. What do amphibians look like? How are they different from other animals and what lifestyle do they lead?
Amphibian - what is it?
According to a common version, cysterae fish were the first inhabitants of water bodies that managed to reach land. Mastering a new space and adapting to other conditions, they gradually began to change, giving rise to new creatures - amphibians.
"Amphibian" is an ancient Greek word that translates as "two kinds of life." In biology, it refers to animals living on land and in water. In Russian terminology, it is becoming increasingly clear, since amphibians are amphibians.
Previously, the concept also meant seals and otters, but later it began to include only four-legged vertebrates that are not related 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.
Class Brief
Amphibians are vertebrates that in the animal kingdom 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 water, and when growing up, they lead a terrestrial way of life. Some species live only in water.
Most amphibians do not tolerate cold, prefer warm and humid places, but can live in arid areas. When adverse conditions occur, they can hibernate or change the time of activity, for example, from night to day. However, some species settled far in the north, for example, the Siberian lugfish.
Amphibians settle near fresh water bodies, and larvae sometimes even lay in deep puddles. Only a few species live in seawater. Development, as a rule, is accompanied by four stages: egg (caviar), larva, metamorphosis and adult. The salamanders also have a live birth.
All members of the class have a weak metabolism, so they can not digest plant foods. Amphibians are predators and feed on insects, small invertebrates, and sometimes their brethren. Large individuals eat young fish, chicks and rodents. Plants feed only on tailless larvae.
How do they look?
The external structure of amphibians is very different. The group of caudate, which includes newts and salamanders, in appearance resembles lizards. They grow up to 20 centimeters. Their body is extended and ends with a long tail. The neck, hind and forelimbs are short.
Frogs belong to tailless amphibians. They have a wide, slightly flattened body and a short neck. The tail is present only at the tadpole stage. Their limbs are elongated and bent, straighten at the moment of jumping and swimming (the main ways of moving). Fingers of frogs and salamanders are connected by a skin membrane.
Worms are amphibians of the legless squad. Outwardly, they look like worms or snakes. Their sizes range from ten centimeters to a meter. The worms have no limbs, and the tail is shortened. Their body is covered with calcareous scales and painted in dark black or brown tones, sometimes with spots or stripes.
Structural features
The skin of these vertebrates is multi-layered, but thin enough. It contains glands that secrete mucus that covers the entire body. Through it, breathing is partially carried out. Lungs help breathe amphibians on the surface, while species that live mostly in water have gills.
The heart of amphibians is three-chambered, two chambers are observed only in salamanders. There are two circles of blood circulation: small and large. Body temperature is unstable and depends on the environment.
The amphibian brain is larger than that of fish, and ranges from 0.30% (in caudate) to 0.73% (in tailless) of body weight. Their vision can distinguish colors. The eyes cover the transparent lower and leathery upper eyelids. They distinguish taste poorly and can determine only salty and bitter.
The skin is the main organ of touch and contains many nerve endings. In tadpoles and aquatic species from fish, a lateral line has been preserved, which is responsible for spatial orientation.
A number of tailless mucus on the skin contains poison. 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, a small yellow frog (see photo above) a terrible leaf-eyed one of the most poisonous creatures in the world.