Parsons, Thomas S. Ramsay Wright Zoological Laboratories, University of Toronto, Ontario, Canada.
Last reviewed:October 2020
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The tubular portion of the vertebrate digestive tract, usually found between the stomach and the cloaca or anus. The detailed functions of the intestine vary with the region, but are primarily digestion and absorption of food. The structure of the intestine varies greatly in different vertebrates (see illustration), but there are several common modifications. These modifications are mainly associated with increasing the internal surface area of the intestine. One modification, seen in many fishes, is the development of a spiral valve; this turns the intestine into a structure resembling a spiral staircase. Another modification, seen in some fish and most tetrapods, is simply elongating and then coiling the intestine. This can reach extremes in large herbivores. For example, oxen have intestinal lengths of more than 45 m (148 ft). In numerous forms, there are blind pouches, or ceca, off part of the intestine. In fish, these are commonly at the anterior end; in tetrapods, they generally lie at the junction between the large and small intestines. In all vertebrates, the inner surface of the intestine is irregular, with ridges and projections of various sorts; these structures reach their maximum development in the extremely fine and numerous finger-shaped villi found in mammals. See also: Digestion; Digestive system; Food; Vertebrata
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