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Animals (lat. Animalia) is a traditionally (since the time of Aristotle) distinguished category of organisms, currently considered as a biological kingdom. Animals are the main object of study of zoology.

Animals are eukaryotes (they have nuclei in their cells). The classic signs of animals are: heterotrophy (nutrition of ready-made organic compounds) and the ability to actively move. However, there are many animals that lead an attached lifestyle, and heterotrophy is characteristic of fungi and some parasitic plants.

The Russian word "animal" is derived from "belly", which in the past meant "life, property". In everyday life, the terms "wild animals", "domestic animals" are often understood only as mammals or four-legged terrestrial vertebrates (mammals, reptiles and amphibians). However, in science, the term "animals" has a broader meaning, corresponding to the Latin Animalia (see above). In the scientific sense, animals, in addition to mammals, reptiles and amphibians, include a huge variety of other organisms: fish, birds, insects, arachnids, mollusks, starfish, worms and others. Man also belongs to the animal kingdom, the order of placental mammals, primates [1], but is traditionally studied separately.

At the same time, many heterotrophic protists were previously assigned to this kingdom and animals were divided into subkingdoms: unicellular Protozoa and multicellular Metazoa. Now the name "animals" in the taxonomic sense has been assigned to multicellular organisms. In this sense, animals as a taxon have more specific features - they are characterized by oogamy, a multi-tissue structure, the presence of at least two germ layers, the stages of blastula and gastrula in embryonic development. The vast majority of animals have muscles and nerves, and the groups that do not have them - sponges, lamellar, Mesozoic, cnidosporidia - may have lost them a second time.

At the same time, in science, the term "animals" is sometimes proposed to be used in an even broader sense, meaning by animals not a taxon, but a type of organization - a life form based on mobility.

Currently (Zhang, 2013), scientists have described more than 1.6 million animal species (including more than 133 thousand fossil species; Zhang, 2013), most of which are arthropods (more than 1.3 million species, 78%), molluscs ( more than 118 thousand species) and vertebrates (more than 42 thousand species)[2][3].