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 possess 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.
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Cloacal opening in an Australian brushtail possum