Editing Gastrointestinal tract (section)

==Other animals==
{{See also|Equine_anatomy#Digestive_system|Ruminant#Digestive_system_of_ruminants}}
In most [[vertebrates]], including [[amphibian]]s, [[bird]]s, [[reptile]]s, [[monotreme|egg-laying mammals]], and some [[fish]], the gastrointestinal tract ends in a [[cloaca]] and not an [[anus]]. In the cloaca, the [[urinary system]] is fused with the genito-anal pore. [[Theria]]ns (all mammals that do not lay eggs, including humans) possess separate anal and uro-genital openings. The females of the subgroup [[Placentalia]] have even separate urinary and genital openings.

During [[Development of the digestive system|early development]], the asymmetric position of the bowels and inner organs is initiated (see also [[axial twist theory]]).

[[Ruminants]] show many specializations for digesting and [[fermenting]] tough plant material, consisting of [[Digestive system of ruminants|additional stomach compartments]], and the ability to regurgitate partially digested food material for further chewing (aka "chewing [[cud]]").<ref>{{Cite journal|last1=Sansone|first1=Randy A.|last2=Sansone|first2=Lori A.|date=2012|title=Rumination|journal=Innovations in Clinical Neuroscience|volume=9|issue=2|pages=29–34|issn=2158-8333|pmc=3312901|pmid=22468242}}</ref>

Many birds and other animals have a specialised stomach in the digestive tract called a [[gizzard]] used for grinding up food.<ref name=King>{{cite book|last1=King|first1=Anthony Stuart|last2=McLelland|first2=John|title=Birds: Their Structure and Function|date=1984|publisher=Baillière Tindall|location=London Philadelphia Toronto [etc.]|isbn=0702008729|edition=2nd|url=https://avian-vet.com/blog/resources-for-veterinarians-5/birds-structure-and-function-free-pdf-4|access-date=30 March 2025}}</ref>

Another feature found in a range of animals is the [[crop (anatomy)|crop]]. In birds this is found as a pouch alongside the esophagus.<ref name=King/>

In 2020, the oldest known fossil digestive tract, of an extinct wormlike organism in the [[Cloudinidae]] was discovered; it lived during the late [[Ediacaran]] [[Period (geology)|period]] about 550 million years ago.<ref name="NYT-20200110">{{cite news|last=Joel|first=Lucas|title=Fossil Reveals Earth's Oldest Known Animal Guts - The find in a Nevada desert revealed an intestine inside a creature that looks like a worm made of a stack of ice cream cones.|url=https://www.nytimes.com/2020/01/10/science/fossil-guts-intestines.html|date=10 January 2020|work=[[The New York Times]]|access-date=10 January 2020}}</ref><ref name="NAT-20200110">{{cite journal|author=Schiffbauer, James D.|display-authors=et al.|date=10 January 2020|title=Discovery of bilaterian-type through-guts in cloudinomorphs from the terminal Ediacaran Period|journal=[[Nature Communications]]|volume=11|page=205|bibcode=2020NatCo..11..205S|doi=10.1038/s41467-019-13882-z|pmc=6954273|pmid=31924764|doi-access=free|number=205}}</ref>

A through-gut (one with both mouth and anus) is thought to have evolved within the [[nephrozoa]]n clade of [[Bilateria]], after their ancestral ventral orifice (single, as in [[cnidaria]]ns and [[xenacoelomorpha|acoels]]; re-evolved in nephrozoans like [[flatworms]]) stretched antero-posteriorly, before the middle part of the stretch would get narrower and closed fully, leaving an anterior orifice (mouth) and a posterior orifice (anus plus [[Gonopore|genital opening]]). A stretched gut without the middle part closed is present in another branch of bilaterians, the extinct [[proarticulata|proarticulates]]. This and the [[embryological origins of the mouth and anus|amphistomic]] development (when both mouth and anus develop from the gut stretch in the embryo) present in some nephrozoans (e.g. [[roundworms]]) are considered to support this hypothesis.<ref>Nielsen, C., Brunet, T. & Arendt, D. Evolution of the bilaterian mouth and anus. Nat Ecol Evol 2, 1358–1376 (2018). https://doi.org/10.1038/s41559-018-0641-0</ref><ref>De Robertis, E. M., &amp; Tejeda-Muñoz, N. (2022). Evo-Devo of urbilateria and its larval forms. ''Developmental Biology'', '''487''', 10–20. https://doi.org/10.1016/j.ydbio.2022.04.003</ref>