Animals are the organisms that form a part of the organic world. All the animals belong to the heterotrophic kind as they are unable to make a nutrient synthesis out of the inorganic compounds. Their food constitutes only organic compounds. Another peculiar characteristic of theirs is the ability to move around.
The whole animal world represents two types of organisms: single-celled and multi-celled organisms. According to the up-to-date calculations the number of animal species ranges from 1,5 to 2 million. These numbers include all the big and small animals, water, land, and airspace animals. Among this huge animal kind diversity the biggest variety represent the insects.
Yet, there is still no specific structure as for the animal classification. For instance, the protozoan is no longer considered a type, but a subkingdom. All the animal representatives can be divided into cold-blooded (e.g. fish, amphibians, reptiles) and warm-blooded (e.g. mammals, birds). Under the influence of man-made factor, the number of animal species is constantly reducing and some of them are under extinction.