(Latin: “sewer”), in vertebrates, common chamber and outlet into which the intestinal, urinary, and genital tracts open. It is present in amphibians, reptiles, birds, elasmobranch fishes (such as sharks), and monotremes. A cloaca is not present in placental mammals or in most bony fishes. Certain animals have, within the cloaca, an accessory organ ( penis) that is used to direct the sperm into the female's cloaca. This structure occurs in many reptiles and in a few birds (bird), including ducks. Most birds, however, mate by joining their cloacas in a “cloacal kiss”; muscular contractions transfer the sperm from the male to female.