Animals are a major group of multicellular, eukaryotic (complex internal structure with surrounding membrane) 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. Animals are also heterotrophs, meaning they are dependent on other organisms (e.g., plants) for sustenance.
Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.
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