Anus

The anus is a body opening at the end of the digestive system in many animals, including mammals, invertebrates, and most fish. It works by letting out waste material left after digestion. This waste includes gases, solid waste called feces, and other things the body can’t use or that could be harmful if left inside.

In many animals, passing feces through the anus is controlled by special muscles called sphincters. Some animals, like amphibians, reptiles, and birds, have a different opening called a cloaca that handles both waste and reproduction. Mammals have different arrangements; for example, female placental mammals have separate openings for waste and reproduction, while males have one opening for waste and another for both waste and reproduction.

The development of the anus was a key moment in the evolution of multicellular animals. It happened more than once in different animal groups and helped make possible important body plans and body structures in animals today. Some simple sea creatures, like comb jellies, can even grow and lose their anuses as needed.

Development

Main articles: Protostome and Deuterostome

In animals like earthworms, a tiny dent forms in the embryo called the blastopore. This dent deepens to create the gut. In deuterostomes, this dent becomes the exit for waste, called the anus, while the gut makes another opening to become the mouth. In protostomes, it was once thought the dent became the mouth first, with the anus forming later. New research shows that in protostomes, the edges of the dent meet in the middle, leaving openings at each end that become the mouth and anus.