The class Amphibians, or Amphibians, includes a small group of the most primitive terrestrial vertebrates (about 2.5 thousand species).
Amphibians live in water and on land. Reproduction and development takes place in water.
The body is divided into head, trunk and limbs (five-fingered, lever-type). Covered with bare, slimy skin. Mucus is necessary for skin respiration (about 70% of oxygen penetrates through the skin).
Compared to fish, amphibians have the following distinctive features:
- the skeleton is divided into distinct sections, the cervical and sacral sections of the spine have appeared.
- In connection with the transition to life on land, they formed two pairs of five-fingered lever-type limbs.
- The brain consists of a trunk and two hemispheres. The organs of sight, hearing and smell are adapted to a terrestrial lifestyle (in addition to the inner one, the middle ear has appeared).
- Amphibians have a second – the pulmonary circulation and a three-chambered heart, consisting of two atria and one ventricle, which contains partially mixed blood.
- The respiratory organs are represented by the lungs and skin (in adult animals), the gill respiration is preserved in the larvae (like in fish).
With fish, amphibians are brought together by external fertilization, reproduction by eggs, the similarity of the larva (for example, the frog tadpole) with the fry of fish.
Classification of Amphibians
* Tailed squad (newts, salamanders and axolotls);
* Detachment Tailless (frogs, toads, garlic, toads, tree frogs);
* Squad Legless (tropical worms).
The origin and significance of amphibians
It is believed that amphibians descended from one of the groups of ancient freshwater cross-finned fish that lived about 300 million years ago in swampy bodies of water.
Cross-finned fish have powerful fins and double breathing. Crisp-finned fishes were the first fish in evolution to learn to crawl from a dry body of water to the nearest body of water on fins, which later became close to the limbs of terrestrial vertebrates. For these purposes, the ancestors of amphibians developed musculature at the base of the fins and a primitive pulmonary sac appeared.
Ichthyostega is considered an intermediate link between fish and amphibians. This is the first stegocephalic amphibian.
Amphibians are an important component of aquatic and terrestrial biocenoses. They regulate the number of invertebrates.