|MadSci Network: Anatomy|
Birds, reptiles, amphibians, fish, and monotremes (platypuses and echidnas) all have their urogenital and alimentary tracts joined internally to form the cloaca (Klow-ay-kuh). The cloaca is a muscular sinus analogous to the rectum in higher mammals which allows the urine and fecal material to mix prior to excretion, which is why bird droppings contain both solid and liquid components. Since the oviducts which carry the eggs join the urethra as it enters the cloaca, the eggs must pass through the cloaca as well, as they are being layed. A cloaca forms during early human development, but is lost during later remodelling of the urogenital tract.
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