Anus

The anus is a body opening at the end of the digestive system in many animals, including mammals and fish. It lets out waste material left after digestion. This waste includes gases and solid waste called feces, as well as other things the body can’t use.

In many animals, special muscles called sphincters help control the passing of feces through the anus. 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 important in the evolution of multicellular animals. It happened more than once in different animal groups and helped shape important body plans and structures in animals today. Some simple sea creatures can even grow and lose their anuses as needed.

Development

Main articles: Protostome and Deuterostome

In animals like earthworms, a small dent forms in the embryo called the blastopore. This dent deepens to make the gut. In deuterostomes, this dent becomes the exit for waste, called the anus. The gut then makes another opening to become the mouth. In protostomes, the dent was thought to become the mouth first. But new research shows that in protostomes, the edges of the dent meet in the middle. This leaves openings at each end that become the mouth and anus.