anus
Opening at the end of the alimentary canal which allows undigested foods and other waste materials to pass out of the body in the form of faeces. In humans the term is also used to describe the last 4 cm/1.5 in of the alimentary canal. It is normally kept closed by a ring of muscle called a sphincter. A common medical condition in humans associated with the anus is haemorrhoids (piles).
The anus is found in all types of multicellular animal except the coelenterates (sponges) and the platyhelminths (flatworms), which have a mouth only.
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