An animal is any organism belonging to the kingdom Animalia. Animals are multicellular heterotrophs and generally mobile so that they can seek out food. Specialized sense organs allow them to detect changes in their surroundings, while a nervous system processes information from the senses and enables an appropriate response to be made to environmental stimuli.
Higher animals, such as the vertebrates, are easily distinguishable from plants (kingdom Plantae), but the distinction becomes blurred with the lower forms. Some one-celled organisms could easily be assigned to either category. About a million species of animal have been categorized in more than 20 phyla (see section on classification below). The simplest (least highly evolved) include the protozoa, sponges, jellyfish, and worms. Other invertebrate phyla include arthropods, molluscs, and echinoderms. Vertebrates belong to the chordate phylum, which includes fish, amphibians, reptiles, birds, and mammals.
Animals are unable to manufacture foodstuffs because they don't possess the light-harvesting chemical chlorophyll, which id found almost all plants. For this reason, animals are ultimately dependent on plants for their food.
Classification of animals
The first major classification of animals was drawn by Aristotle. The method now used is based on that devised by Carolus Linnaeus in the 1750s. Each animal is given a two-part Latin name, the first part indicating its genus, the second its species. The family takes in all related genera, and an order is made up of all related families. Similar orders are grouped in a class, and related classes make up a phylum.