In vertebrates, common chamber and outlet into which the intestinal, urinary, and genital tracts open. It is present in amphibians, reptiles, birds, some fishes (e.g., sharks), and monotreme mammals but is absent in placental mammals and most bony fishes. Certain animals (e.g., many reptiles and some birds, including ducks) have an accessory organ (penis) within the cloaca that is used to direct the sperm into the female's cloaca. Most birds mate by joining their cloacas in a “cloacal kiss”; muscular contractions transfer the sperm from the male to the female.

This entry comes from Encyclopædia Britannica Concise.
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