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Animalia" redirects here. For other uses, see Animalia (disambiguation).
For other uses, see Animal (disambiguation).
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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 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 animalis, meaning "having breath".[1] In everyday colloquial usage, the word usually refers to non-human animals.[2] Frequently, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.[3] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.[4]
Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and mostly multicellular,[5] which separates them from bacteria and most protists. They are heterotrophic,[6] generally digesting food in an internal chamber, which separates them from plants and algae.[7] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.[8] All animals are motile,[9] if only at certain life stages. In most animals, embryos pass through a blastula stage,[10] which is a characteristic exclusive to animals.
Structure
With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve... [continues]

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