Formation of anus in proto- and deuterostomes

The anus 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 be one or more of: 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 for excreting liquid and solid wastes, and for copulation and egg-laying; this orifice is known as the cloaca. Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliest amniotes via the therapsids. Marsupials have two nether orifices: one for excreting both solids and liquids; the other for reproduction, which appears as a vagina in females and a penis in males. 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. In fact it appears to have happened at least twice, following different paths in protostomes and deuterostomes. This accompanied or facilitated other important evolutionary developments: the bilaterian body plan; the coelom, an internal cavity that provided space for a circulatory system and, in some animals, formed a hydrostatic skeleton which enables worm-like animals to burrow; metamerism, in which the body was built of repeated "modules" which could later specialize, for example the heads of most arthropods are composed of fused, specialized segments.



First attested in 1658, from Latin anus (“ring, anus”), from Proto-Indo-European *ano- (“ring”). See also anal, annular, annelid.


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 used to be thought that in their embryos the dent formed the mouth while 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. ^ 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:10.2307/4010364. JSTOR 4010364. 
  2. ^ Arendt, D., Technau, U., and Wittbrodt, J. (4 January 2001). "Evolution of the bilaterian larval foregut". Nature 409 (6816): 81–85. doi:10.1038/35051075. PMID 11343117. Retrieved 2008-07-14. 

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  • anus — anus …   Dictionnaire des rimes

  • anus — [ anys ] n. m. • 1314; mot lat. ♦ Orifice du rectum qui donne passage aux matières fécales. ⇒ fondement; proct(o) ; fam. cul, trou (de balle, du cul). Sphincters de l anus. Fistule à l anus. De l anus. ⇒ anal. ♢ Chir. Anus artificiel : orifice… …   Encyclopédie Universelle

  • anus — ÁNUS, anusuri, s.n. Orificiu terminal al intestinului gros, care comunică cu exteriorul. – Din fr., lat. anus. Trimis de ana zecheru, 07.03.2004. Sursa: DEX 98  ÁNUS s. (anat.) (pop.) cur, (reg.) găoază. Trimis de siveco, 14.10.2008. Sursa:… …   Dicționar Român

  • anus — (n.) inferior opening of the alimentary canal, 1650s, from O.Fr. anus, from L. anus ring, anus, from PIE root *ano ring. So called for its shape; Cf. Gk. daktylios anus, lit. ring (for the finger), from daktylos finger …   Etymology dictionary

  • ANUS — ANUS, лат. название в анатомии, обозначающее наружное отверстие прямой кишки; врачи под словом А. обычно понимают заднепроходный канал (pars analis recti, canalis analis), представляющий собой узкий проход, соединяющий нижний отдел прямой кишки… …   Большая медицинская энциклопедия

  • anus — ánus m DEFINICIJA anat. otvor na kraju debelog crijeva; čmar, šupak SINTAGMA umjetni anus med. kirurški otvor koji izvodi fekalije na površinu kože; nužna mjera kod raznih bolesti (tumor i sl.) ETIMOLOGIJA lat. anus …   Hrvatski jezični portal

  • anus — ANUS. s. masc. (On prononce l S.) Terme d Anatomie. On appelle ainsi Le fondement, ou l extrémité de l intestin nommé Rectum, qui se rétrécit et se termine par un orifice étroitement plissé, Avoir une fistule à l anus …   Dictionnaire de l'Académie Française 1798

  • Anus — der; , Ani <aus gleichbed. lat. anus; vgl. ↑anal> After …   Das große Fremdwörterbuch

  • anus — ANUS. s. m. L orifice du fondement. Il ne se dit qu en Medecine. Cet homme avoit une fistule dans l anus …   Dictionnaire de l'Académie française

  • anús — anus m. anus. voir ulhet, fondament, trauc dau cuou …   Diccionari Personau e Evolutiu

  • anus — [ā′nəs] n. pl. anuses or ani [ā′nī΄] [L, ring, anus < IE base * āno , ring] the opening at the lower end of the alimentary canal …   English World dictionary

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