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Other animals

In most vertebrates, including fishes, amphibians, birds, reptiles, and egg-laying mammals, the gastrointestinal tract ends in a cloaca and not an anus. In the cloaca, the urinary system is fused with the genito-anal pore. Therians (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 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 additional stomach compartments.

Many birds and other animals have a specialised stomach in the digestive tract called a gizzard used for grinding up food.

Another feature found in a range of animals is the crop. In birds this is found as a pouch alongside the esophagus.

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 about 550 million years ago.

A through-gut (one with both mouth and anus) is thought to have evolved within the nephrozoan clade of Bilateria, after their ancestral ventral orifice (single, as in cnidarians and 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 genital opening). A stretched gut without the middle part closed is present in another branch of bilaterians, the extinct proarticulates. This and the 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.