Overview
The cloaca is a single posterior chamber found in many land-dwelling vertebrates. It serves as an exit for urine and solid waste, and it also accommodates various reproductive functions in the species that possess it.
Distribution among animal groups
This arrangement is characteristic of early-diverging lineages within the group of tetrapods. For example, members of the group of sauropsids—including both reptiles and birds—retain a cloaca. Certain egg-laying mammals, the monotremes, also have this single opening, reflecting an ancestral body plan among these mammals.
Contrast with therian mammals
Most mammals are part of the therian group and show a different arrangement. In therians (which include marsupials and placental mammals), the digestive tract ends separately at an anus, used for defecation, while excretory and reproductive openings are distinct from that posterior opening.