Cloaca

In animal anatomy, a cloaca /kloʊˈeɪkə/ kloh-AY-kə (plural cloacae /kloʊˈeɪsi/ kloh-AY-see or /kloʊˈeɪki/ kloh-AY-kee) is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals, opening at the vent. All amphibians, reptiles, and a few mammals (monotremes, tenrecs, golden moles, and marsupial moles) have this orifice, from which they excrete both urine and feces; this is in contrast to most placental mammals, which have two or three separate orifices for evacuation. Excretory openings with analogous purpose in some invertebrates are also sometimes referred to as cloacae. Mating by cloaca is known as cloacal copulation, mostly referred to as cloacal kiss. In animal anatomy, a cloaca /kloʊˈeɪkə/ kloh-AY-kə (plural cloacae /kloʊˈeɪsi/ kloh-AY-see or /kloʊˈeɪki/ kloh-AY-kee) is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals, opening at the vent. All amphibians, reptiles, and a few mammals (monotremes, tenrecs, golden moles, and marsupial moles) have this orifice, from which they excrete both urine and feces; this is in contrast to most placental mammals, which have two or three separate orifices for evacuation. Excretory openings with analogous purpose in some invertebrates are also sometimes referred to as cloacae. Mating by cloaca is known as cloacal copulation, mostly referred to as cloacal kiss. The cloacal region is also often associated with a secretory organ, the cloacal gland, which has been implicated in the scent-marking behavior of some reptiles, marsupials, amphibians, and monotremes. The word is from the Latin verb cluo, 'to cleanse', thus the noun cloaca, 'sewer, drain'. Birds reproduce using their cloaca; this occurs during a cloacal kiss in most birds. Birds that mate using this method touch their cloacae together, in some species for only a few seconds, sufficient time for sperm to be transferred from the male to the female. For some birds, such as ostriches, cassowaries, kiwi, geese, and some species of swans and ducks, the males do not use the cloaca for reproduction, but have a phallus. In those, the penis helps ensure water does not wash away the male's sperm during copulation. One study has looked into birds that use their cloaca for cooling. Among fish, a true cloaca is present only in elasmobranchs (sharks and rays) and lobe-finned fishes. In lampreys and in some ray-finned fishes, part of the cloaca remains in the adult to receive the urinary and reproductive ducts, although the anus always opens separately. In chimaeras and most teleosts, however, all three openings are entirely separated. With a few exceptions noted below, mammals have no cloaca. Even in those that have one, the cloaca is partially subdivided into separate regions for the anus and urethra. The monotremes (egg-laying mammals) possess a true cloaca. In marsupials (and a few birds), the genital tract is separate from the anus, but a trace of the original cloaca does remain externally. This is one of the features of marsupials (and monotremes) that suggest their basal nature, as the amniotes from which mammals evolved possessed a cloaca, and the earliest animals to diverge into the mammalian class would most likely have had this feature, too.