In mammals, invertebrates, and most fish, the anus is the external body orifice at the exit of the digestive tract, enabling expulsion of waste such as flatus and feces. In amphibians, reptiles, and birds, a similar opening called the cloaca also serves for excretion, copulation, and egg-laying. The development of the anus was crucial in evolution, occurring independently in protostomes and deuterostomes, and linked to innovations like the bilaterian body plan and coelom. Some comb jellies even grow and lose an anus as needed.
Development
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 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 (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.5
See also
- Evolutionary biology portal
- The dictionary definition of anus at Wiktionary
- Anal canal – Functional segment of the large intestine
- Anorectal manometry – Medical functional test of the anus and rectum
- Imperforate anus – Birth defect of malformed rectum
External links
- Media related to Anus at Wikimedia Commons
References
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Langstroth, Lovell; Libby Langstroth; Todd Newberry; Monterey Bay Aquarium (2000). A living bay: the underwater world of Monterey Bay. University of California Press. p. 244. ISBN 978-0-520-22149-9. 978-0-520-22149-9 ↩
Chin, K.; Erickson, G.M.; et al. (1998-06-18). "A king-sized theropod coprolite". Nature. 393 (6686): 680. Bibcode:1998Natur.393..680C. doi:10.1038/31461. S2CID 4343329. Summary at Monastersky, R. (1998-06-20). "Getting the scoop from the poop of T. rex". Science News. 153 (25). Society for Science &: 391. doi:10.2307/4010364. JSTOR 4010364. Archived from the original on 2013-05-11. Retrieved 2009-04-24. https://zenodo.org/record/3943146 ↩
What is a warty comb jelly? | BBC Science Focus Magazine https://www.sciencefocus.com/nature/what-is-a-warty-comb-jelly/ ↩
Arendt, D.; Technau, U. & Wittbrodt, J. (4 January 2001). "Evolution of the bilaterian larval foregut". Nature. 409 (6816): 81–85. Bibcode:2001Natur.409...81A. doi:10.1038/35051075. PMID 11343117. S2CID 4406268. /wiki/Nature_(journal) ↩