From Wikipedia, the free encyclopedia This article is about the bodily orifice. For other uses, see Anus (disambiguation).For details specific to the human anus, see Human anus.[1]


Formation of anus in proto- and deuterostomes

Latin Anus
Gray's p.1184
System Alimentary, sometimes reproductive
Artery Inferior rectal artery
Vein Inferior rectal vein
Nerve Inferior rectal nerves
Lymph Superficial inguinal lymph nodes
Precursor Proctodeum

The anus (from Latin anus, meaning "ring, anus", which is from the Proto-Indo-European ano–, meaning "ring" ) is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may include: matter which the animal cannot digest, such as bones;[1] food material after all the nutrients have been extracted, for example cellulose or lignin; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess gut bacteria and other endosymbionts.

Amphibians, reptiles, and birds use the same orifice (known as the cloaca) for excreting liquid and solid wastes, for copulation and egg-laying.Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliest amniotes via the therapsidsMarsupialshave 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 defecationurination, and reproduction; males have one opening for defecation and 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 protostomes and deuterostomes. This accompanied or facilitated other important evolutionary developments: thebilaterian body plan, the coelom, and 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.


Main articles: Protostome and Deuterostome

In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become thearchenteron, the first phase in the growth of the 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 (proto– meaning "first") and the anus was formed later at the opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the dent close up in the middle, leaving openings at the ends which become the mouth and anus.[2]


  1. Jump up^ Chin, K., Erickson, G.M. et al. (1998-06-18). "A king-sized theropod coprolite"Nature 393 (6686): 680. doi:10.1038/31461. Summary at Monastersky, R. (1998-06-20). "Getting the scoop from the poop of T. rex"Science News (Society for Science &#38) 153 (25): 391. doi:10.2307/4010364JSTOR 4010364.
  2. Jump up^ Arendt, D., Technau, U., and Wittbrodt, J. (4 January 2001). "Evolution of the bilaterian larval foregut"Nature 409 (6816): 81–85. doi:10.1038/35051075PMID 11343117. Retrieved 2008-07-14.

See alsoEdit

Ad blocker interference detected!

Wikia is a free-to-use site that makes money from advertising. We have a modified experience for viewers using ad blockers

Wikia is not accessible if you’ve made further modifications. Remove the custom ad blocker rule(s) and the page will load as expected.