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Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or...

Animals are a major group of mostly 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 for sustenance. Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. The word "animal" comes from the Latin word animal. In everyday colloquial usage, the word usually refers to non-human animals. Frequently only closer relatives of humans such as vertebrates or mammals are meant in colloquial use. The biological definition of the word refers to all members of the Kingdom Animalia including humans. Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and are multicellular (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae