It appears to have happened at least twice, following different paths in protostomes and deuterostomes. The Structure and Function of the Digestive System The article provides an overview of the digestive system, with a brief description of the body part components from the top the mouth to the bottom the anus. Additional Content Medical News. The development of the anus was an important stage in the evolution of multicellular animals. When the storage site becomes full, the intrarectal pressure causes the anal canal walls to dilate and expand. Anatomical terminology [ edit on Wikidata ]. Water, bile, enzymes, and mucous contribute to the change in consistency.
The anus from Latin anus meaning "ring", "circle" is an opening at the opposite end of an animal's digestive tract from the mouth.
The Structure and Function of the Digestive System
More recent research, however, shows that in protostomes the edges of the dent close up in the middle, leaving openings at the ends which become the mouth and anus. If they can, the sphincters relax and the rectum contracts, disposing its contents. Contents of the small intestine start out semi-solid, and end in a liquid form after passing through the organ. If the contents cannot be disposed, the sphincter contracts and the rectum accommodates so that the sensation temporarily goes away. The expansion of the rectal walls causes the stretch receptors within the walls to stimulate the urge to defecate. Its function is to control the expulsion of fecesunwanted semi-solid matter produced during digestion, which, depending on the type of animal, may include: