Cloaca

The cloaca is a common rear opening in land vertebrates. Through this opening, the animals expel urine and faeces, and carry out reproduction.

The cloaca is a primitive or 'basal' feature of tetrapods. All amphibia, all sauropsids (reptiles and birds) and even monotreme mammals have a cloaca.

A different method is used by therian mammals, such as marsupials and placentals. They have an anus for defecation, and another opening for urine and sexual reproduction.