The Animal Kingdom
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms or their products for sustenance. Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.
Animals belong to the largest and most diverse of the five kingdoms of living things. So far over two million animal species have been identified. All animals share certain features. Unlike plants, animals get the energy they need by eating food. They are all made up of many cells and many animals are highly mobile. Most reproduce sexually and have sense organs that allow them to react quickly to their surroundings.
Though there is great diversity in the animal kingdom, animals can be distinguished from the other kingdoms by a set of characteristics. Though other types of life may share some of these characteristics, the set of characteristics as a whole provide a distinction from the other kingdoms.