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{{short description|Digestive tract waste expulsion opening}}{{Redirect|Butthole|the slang word|Asshole}}
{{Redirect|Pooper|the fan nickname of the fictional Pokémon species|Wooper}}{{About|the anuses of animals in general|details specific to humans|Human anus|other uses}}
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{{Infobox anatomy
| Name = Anus
| Latin = anus
| Image = File:Dog’s anus.jpg
| Caption = Anus of a [[dog]]
| Width =
| Image2 = Protovsdeuterostomes.svg
| Caption2 = Formation of anus in proto- and deuterostomes
| Precursor = [[Proctodeum]]
| System = [[Gastrointestinal tract|Alimentary]]
| Artery = [[Inferior rectal artery]]
| Vein = [[Inferior rectal vein]]
| Nerve = [[Inferior rectal nerves]]
| Lymph = [[Superficial inguinal lymph nodes]]
}}

In [[mammal]]s, [[invertebrate]]s and most [[fish]],<ref>{{cite book|last1=Helms|first1=Doris R.|last2=Helms|first2=Carl W.|last3=Kosinski|first3=Robert J.|last4=Cummings|first4=John C.|title=Biology in the Laboratory With BioBytes 3.1 CD-ROM|url=https://books.google.com/books?id=Fw54Ce6DfPYC&pg=SA36-PA12|publisher=W. H. Freeman|page=36-12|date=1997|isbn=978-0-7167-3146-7 }}</ref><ref>{{cite book|title=A living bay: the underwater world of Monterey Bay|year=2000|publisher=University of California Press|isbn=978-0-520-22149-9|author=Langstroth, Lovell|author2=Libby Langstroth|author3=Todd Newberry|author4=Monterey Bay Aquarium|page=[https://archive.org/details/livingbayunderwa0000lang/page/244 244]|url=https://archive.org/details/livingbayunderwa0000lang/page/244}}</ref> the '''anus''' ({{plural form}}: '''anuses''' or '''ani'''; from [[Latin]], 'ring' or 'circle') is the external [[body orifice]] at the ''exit'' end of the [[digestive tract]] ([[bowel]]), i.e. the opposite end from the [[mouth]]. Its function is to facilitate the [[defecation|expulsion]] of waste that remains after [[digestion]].

Bowel contents that pass through the anus include the gaseous [[flatus]] and the semi-solid [[feces]], which (depending on the type of animal) include: indigestible matter such as [[bone]]s, [[hair]] [[pellet (ornithology)|pellet]]s, [[endozoochory|endozoochorous]] [[seed]]s and [[gastrolith|digestive rock]]s;<ref name="ChinEtal1998KingSizeCoprolite">{{cite journal |date=1998-06-18 |title=A king-sized theropod coprolite |journal=[[Nature (journal)|Nature]] |volume=393 |pages=680 |author=Chin, K. |author2=Erickson, G.M. |doi=10.1038/31461 |issue=6686 |bibcode=1998Natur.393..680C |s2cid=4343329 |url=https://zenodo.org/record/3943146 |display-authors=etal }} Summary at {{cite journal |author=Monastersky, R. |title=Getting the scoop from the poop of ''T. rex'' |journal=Science News |volume=153 |issue=25 |date=1998-06-20 |pages=391 |url=http://www.sciencenews.org/pages/sn_arc98/6_20_98/fob2.htm |doi=10.2307/4010364 |publisher=Society for Science &#38 |jstor=4010364 |access-date=2009-04-24 |archive-date=2013-05-11 |archive-url=https://web.archive.org/web/20130511121022/http://www.sciencenews.org/pages/sn_arc98/6_20_98/fob2.htm |url-status=dead |url-access=subscription }}</ref> residual [[food]] material after the digestible [[nutrient]]s have been extracted, for example [[cellulose]] or [[lignin]]; ingested matter which would be toxic if it remained in the digestive tract; [[excretion|excreted]] [[metabolite]]s like [[bilirubin]]-containing [[bile]]; and dead [[mucosa]]l [[epithelia]] or excess [[gut bacteria]] and other [[endosymbiont]]s. Passage of feces through the anus is typically controlled by [[muscular]] [[sphincter]]s, and failure to stop unwanted passages results in [[fecal incontinence]].

[[Amphibian]]s, [[reptile]]s and [[bird]]s use a similar orifice (known as the [[cloaca]]) for excreting liquid and solid wastes, for [[copulation (zoology)|copulation]] and [[egg-laying]]. [[Monotreme]] mammals also have a cloaca, which is thought to be a feature inherited from the earliest [[amniote]]s. [[Marsupial]]s have a single orifice for excreting both solids and liquids and, in females, a separate [[vagina]] for reproduction. Female [[placental]] mammals have completely separate orifices for [[defecation]], [[urination]], and reproduction; males have one opening for defecation and [[external urethral orifice (male)|another for both urination and reproduction]], although the channels flowing to that orifice are almost completely separate.

The development of the anus was an important stage in the evolution of multicellular animals. It appears to have happened at least twice, following different paths in [[protostome]]s and [[deuterostome]]s. This accompanied or facilitated other important evolutionary developments: the [[bilaterian|bilaterian body plan]], the [[coelom]], and [[Metamerism (biology)|metamerism]], in which the body was built of repeated "modules" which could later specialize, such as the heads of most [[arthropods]], which are composed of fused, specialized segments.

In [[Ctenophora|comb jellies]], there are species with one and sometimes two permanent anuses, species like the [[Mnemiopsis|warty comb jelly]] grows an anus, which then disappear when it is no longer needed.<ref>[https://www.sciencefocus.com/nature/what-is-a-warty-comb-jelly/ What is a warty comb jelly? | BBC Science Focus Magazine]</ref>

==Development==
{{main|Protostome|Deuterostome}}
In animals at least as complex as an [[earthworm]], the [[embryo]] forms a dent on one side, the [[blastopore]], which deepens to become the [[archenteron]], the first phase in the growth of the [[Gut (zoology)|gut]]. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it was thought that in their embryos the dent formed the mouth first (''[[wikt:proto-|proto–]]'' meaning "first") and the anus was formed later at the opening made by the other end of the gut. Research from 2001 shows the edges of the dent close up in the middles of protosomes, leaving openings at the ends which become the mouths and anuses.<ref>{{cite journal|title=Evolution of the bilaterian larval foregut |author=Arendt, D. |author2=Technau, U. |author3=Wittbrodt, J. |name-list-style=amp |journal=[[Nature (journal)|Nature]] |volume=409 |pages=81–85 |date=4 January 2001 |doi=10.1038/35051075 |pmid=11343117 |issue=6816|bibcode=2001Natur.409...81A |s2cid=4406268 }}</ref>

==See also==
{{Portal|Evolutionary biology}}
*{{wiktionary-inline|anus}}

* {{annotated link|Anal canal}}
* {{annotated link|Anorectal manometry}}
* {{annotated link|Imperforate anus}}


==References==
{{reflist}}

{{Authority control}}

[[Category:Anus| ]]

Anus - Revision history

imported>Scratchykit5743 at 21:04, 17 September 2025