Anus

 ( Digestive System ) 

In , and most fish,

Langstroth, Lovell (2025). 9780520221499, University of California Press. . ISBN 9780520221499

the anus (: 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 of wastes that remain 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 , hair pellets, endozoochory and gastrolith; Summary at residual food material after the digestible have been extracted, for example cellulose or lignin; ingested matter which would be toxic if it remained in the digestive tract; excretion like bilirubin-containing bile; and dead epithelia or excess gut bacteria and other . Passage of feces through the anus is typically controlled by muscular , and failure to stop unwanted passages results in fecal incontinence.

, and use a similar 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 . 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 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 and . This accompanied or facilitated other important evolutionary developments: the bilaterian, 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.

In Ctenophora, there are species with one and sometimes two permanent anuses, species like the Mnemiopsis grows an anus, which then disappear when it is no longer needed. What is a warty comb jelly? | BBC Science Focus Magazine

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Development 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. 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 ( 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.

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