Animals (lat. Animalia) - traditionally (since Aristotle) allocated category of organisms, is now regarded as a biological kingdom. Animals are the main object of study zoology. Animals are eukaryotes (cells have a nucleus). The classic signs of animals are: heterotrophy (food prepared with organic compounds) and the ability to actively move. However, there are many animals, the leading fixed way of life, but also characterized by heterotrophy mushrooms and some parasitic plants. In everyday life, the terms "wild animals", "pets" often refers only to mammals, sometimes - only four-legged terrestrial vertebrates (mammals, reptiles and amphibians). Russian word "animal" is derived from the "belly", formerly meant "life and property." In science, the term for the animal secured greater value corresponding to the Latin Animalia (See above.).