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Animalia are a Kingdom of the Domain Eukaryota. 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. Most 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 MYA