Amphibians are the direct descendants of lobe-finned fish. 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 kind of lifestyle do they lead?
Amphibian - what is it?
According to the widespread version, the lobe-finned fish were the first inhabitants of water bodies who managed to go to 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 that live both on land and in water. In Russian terminology, everything is more clear, since amphibians are amphibians.
Earlier, the concept also included seals and otters, but later it began to include only four-legged vertebrates that do not belong to amniotes. The modern class of amphibians includes only salamanders, caecilians, newts, and frogs. In total, there are from 5 to 6, 7 thousand species.
Brief description of the class
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 most occurs in the water, and growing up, they lead a terrestrial lifestyle. Some species live only in water.
Most amphibians do not tolerate cold weather, prefer warm and humid places, but can also 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 to the north, for example, the Siberian salamander.
Amphibians settle near fresh water bodies, and the larvae sometimes lay even in deep puddles. Only a few species live in sea water. Development, as a rule, is accompanied by four stages: egg (caviar), larva, metamorphosis and adult. Salamanders also have live birth.
All representatives of the class have a weak metabolism, so they cannot digest plant foods. Amphibians are predators and feed on insects, small invertebrates, and sometimes their own brethren. Large individuals eat young fish, chicks and rodents. Only larvae of the anurans order feed on plants.
What do they look like?
The external structure of amphibians is very different. The caudate group, which includes newts and salamanders, resembles lizards in appearance. They grow up to 20 centimeters. Their body is elongated and ends with a long tail. The neck, hind and fore limbs are short.
Frogs are tailless amphibians. They arehave a broad, slightly flattened body and a short neck. The tail is present only at the stage of tadpoles. Their limbs are elongated and bent, straightening at the moment of jumping and swimming (the main methods of movement). The 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. Worms have no limbs, and the tail is shortened. Their body is covered with calcareous scales and is colored in dark black or brown tones, sometimes with spots or stripes.
Building features
The skin of these vertebrates is multi-layered, but quite thin. It contains glands that secrete mucus that covers the entire body. Through it, breathing is partially carried out. On the surface, amphibians use lungs to breathe, while species that live primarily in water have gills.
The heart of amphibians is three-chambered, two chambers are observed only in salamanders. There are two circulation circles: small and large. Body temperature is unstable and depends on the external environment.
The brain of amphibians is larger than that of fish, and ranges from 0.30% (for caudates) to 0.73% (for anurans) of body weight. Their vision is able to distinguish colors. The eyes are covered with a transparent lower and leathery upper eyelids. They taste badly and can only detect s alty and bitter.
Skin is the main organ of touch and contains many nerve endings. In tadpoles and aquatic species from fishthe lateral line, which is responsible for orientation in space, has been preserved.
In a number of anurans, the 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, the little yellow frog (see photo above) the terrible leaf-eye is one of the most poisonous creatures in the world.
