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==Origin==
[[Latin]], from ''animale'', neuter of ''animalis'' animate, from ''anima'' [[soul]]
*[http://en.wikipedia.org/wiki/14th_century 14th_century]
The word "animal" comes from the [[Latin]] word ''animalis'', meaning "having [[breath]]". In everyday [[colloquial]] usage, the word often refers to non-human members of kingdom ''Animalia''. Sometimes, only closer relatives of humans such as [[mammals]] and other vertebrates are meant in colloquial use. The [[biological]] [[definition]] of the word refers to all members of the kingdom ''Animalia'', encompassing [[creatures]] as diverse as sponges, jellyfish, insects and [[humans]].
==Definitions==
*1: any of a kingdom (''Animalia'') of living things including many-celled [[organisms]] and often many of the single-celled ones (as protozoans) that typically differ from [[plants]] in having [[cells]] without cellulose walls, in lacking chlorophyll and the capacity for [http://en.wikipedia.org/wiki/photosynthesis], in requiring more complex [[food]] materials (as [http://en.wikipedia.org/wiki/proteins]), in being organized to a greater degree of [[complexity]], and in having the capacity for [[spontaneous]] movement and rapid motor [[responses]] to [[stimulation]]
*2a : one of the lower animals as distinguished from [[human beings]]
*b : [[mammal]]; broadly : vertebrate
==Description==
'''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 [http://en.wikipedia.org/wiki/Heterotroph heterotrophs], meaning they must ingest other organisms or their products for sustenance.
Most known animal [http://en.wikipedia.org/wiki/Phylum phyla] appeared in the [[fossil]] record as marine species during the [http://en.wikipedia.org/wiki/Cambrian_explosion Cambrian explosion], about 542 million years ago.
Animals have several characteristics that set them apart from other living [[things]]. Animals are [http://en.wikipedia.org/wiki/Eukaryote eukaryotic] and mostly [http://en.wikipedia.org/wiki/Multicellular multicellular], which separates them from [[bacteria]] and most protists. They are [http://en.wikipedia.org/wiki/Heterotroph heterotrophic], generally digesting [[food]] in an internal chamber, which separates them from [[plants]] and algae. They are also distinguished from [[plants]], algae, and fungi by lacking rigid cell walls. All animals are motile, if only at certain [[life]] [[stages]]. In most animals, [[embryos]] pass through a [http://en.wikipedia.org/wiki/Blastula blastula stage], which is a characteristic exclusive to animals.
With a few exceptions, most notably the [http://en.wikipedia.org/wiki/Sponge 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]] tissues, which send and [[process]] [[signals]]. Typically, there is also an internal [http://en.wikipedia.org/wiki/Digestion digestive] chamber, with one or two openings. Animals with this sort of organization are called metazoans, or [http://en.wikipedia.org/wiki/Eumetazoan eumetazoans] when the former is used for animals in general.
All animals have [http://en.wikipedia.org/wiki/Eukaryotic eukaryotic] cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form [[structures]] like shells, bones, and [http://en.wikipedia.org/wiki/Spicule spicules]. During development, it forms a relatively flexible framework upon which cells can move about and be reorganized, making [[complex]] structures possible. In [[contrast]], other multicellular organisms, like [[plants]] and fungi, have cells held in place by cell walls, and so develop by [[progressive]] [[growth]]. Also, unique to animal cells are the following intercellular junctions: [http://en.wikipedia.org/wiki/Tight_junction tight junctions], [http://en.wikipedia.org/wiki/Gap_junction gap junctions], and [http://en.wikipedia.org/wiki/Desmosome desmosomes].[http://en.wikipedia.org/wiki/Animals]
[http://en.wikipedia.org/wiki/Aristotle Aristotle] divided the living world between ''animals'' and [[plants]], and this was followed by [http://en.wikipedia.org/wiki/Carolus_Linnaeus Carolus Linnaeus] (Carl von Linné), in the first [[hierarchical]] [[classification]]. Since then [[biologists]] have begun emphasizing [[evolutionary]] [[relationships]], and so these groups have been restricted somewhat. For instance, microscopic [http://en.wikipedia.org/wiki/Protozoa protozoa] were originally considered animals because they move, but are now treated separately.
In Linnaeus's original scheme, the animals were one of three kingdoms, divided into the classes of [http://en.wikipedia.org/wiki/Vermes_in_the_10th_edition_of_Systema_Naturae Vermes], [http://en.wikipedia.org/wiki/Insecta_in_the_10th_edition_of_Systema_Naturae Insecta], [http://en.wikipedia.org/wiki/Pisces_in_the_10th_edition_of_Systema_Naturae Pisces], [http://en.wikipedia.org/wiki/Amphibia_in_the_10th_edition_of_Systema_Naturae Amphibia], [http://en.wikipedia.org/wiki/Aves_in_the_10th_edition_of_Systema_Naturae Aves], and [http://en.wikipedia.org/wiki/Mammalia_in_the_10th_edition_of_Systema_Naturae Mammalia]. Since then the last four have all been subsumed into a single phylum, the [http://en.wikipedia.org/wiki/Chordate Chordata], whereas the various other forms have been separated out. The above lists represent our current [[understanding]] of the group, though there is some variation from source to source.
==See also==
*'''''[[Animal nature]]'''''
==External Links==
*[http://www.eol.org/pages/1 Animal] at the Encyclopedia of Life
*[http://tolweb.org/ Tree of Life Project]
*[http://animaldiversity.ummz.umich.edu/site/index.html Animal Diversity Web] – University of Michigan's database of animals, showing taxonomic classification, images, and other information.
*[http://www.arkive.org/ ARKive] – multimedia database of worldwide endangered/protected species and common species of UK.
[[Category: Biology]]
