Editing Small intestine (section)

==Other animals==
The small intestine is found in all [[tetrapod]]s and also in [[teleost]]s, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of ''pyloric caeca'', small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the [[rectum]] being marked only by the end of the digestive epithelium.<ref name=VB/>

In tetrapods, the [[ileocaecal valve]] is always present, opening into the colon. The length of the small intestine is typically longer in tetrapods than in teleosts, but is especially so in [[herbivore]]s, as well as in mammals and [[bird]]s, which have a higher [[metabolic rate]] than [[amphibian]]s or [[reptile]]s. The lining of the small intestine includes microscopic folds to increase its surface area in all vertebrates, but only in mammals do these develop into true villi.<ref name=VB/>

The boundaries between the duodenum, jejunum, and ileum are somewhat vague even in humans, and such distinctions are either ignored when discussing the anatomy of other animals, or are essentially arbitrary.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 349–353|isbn= 978-0-03-910284-5}}</ref>

There is no small intestine as such in non-teleost fish, such as [[shark]]s, [[sturgeon]]s, and [[lungfish]]. Instead, the digestive part of the gut forms a '''spiral intestine''', connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.<ref name=VB/>

In [[lamprey]]s, the spiral valve is extremely small, possibly because their diet requires little digestion. [[Hagfish]] have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions.<ref name=VB/>