This is the permanent condition of the terminal openings of the alimentary and genito-urinary tracts in the lower vertebrates. It is found in certain fishes, in all amphibia, reptiles and birds, and occurs also in one order of mammals, the monotremes. In man and mammals generally the anal orifice is separated from the genito-urinary opening, lying dorsad of the same and provided with special sphincters. Only in the monotremes do the anus and the genito-urinary tract open into a common cloaca surrounded by a sphincter common to the anal and genito-urinary openings (sphincter cloacae). In birds, reptiles, amphibia and many fishes (especially the Plagiostomata) this cloacal formation is the rule. In many fishes, especially the Teleosts, the anus and the genito-urinary openings are separate, as in mammals, but their position is reversed, the anus being ventral, while the genito-urinary opening is placed dorsally. [Illustration: FIG. 23.--Genito-urinary tract and cloaca of _Iguana tuberculata_, female. (Columbia University Museum, No. 1846.)] Fig. 23 shows the cloaca in a female specimen of _Iguana tuberculata_. The ventral wall of the cloaca has been divided to the left of the median line and turned over to the right, carrying with it the cloacal opening of the bladder. The termination of the alimentary canal opens into the cloaca from above. A transverse fold of the mucosa separates this upper compartment of the cloaca (_coprodaeum_) from a lower space (_urodaeum_) which receives in its dorsal wall the openings of the two oviducts and immediately above them--upon two papillae--the openings of the ureters, while the ventral wall contains the cloacal opening of the bladder. The right ovary has been removed--to show the abdominal opening of the right oviduct--by dividing the mesovarian peritoneal fold. [Illustration: FIG. 24.--Genito-urinary tract and cloaca of the hen, _Gallus bankiva_. (Columbia University Museum, No. 1208.)] Fig. 24--taken from a preparation of the hen--shows the typical arrangement of the female genito-urinary tract and cloaca in the birds. The terminal portion of the alimentary canal, in entering the cloaca, forms an expanded upper cloacal compartment for the accumulation of the excreta, called the _coprodaeum_. It is separated by a prominent mucous fold from the central compartment, or _urodaeum_ which receives the terminations of the two ureters and of the single (left) oviduct. A second fold forms the distal limit of the urodaeum and separates it from the lowest cloacal compartment, the _proctodaeum_. [Illustration: FIG. 25.--Genito-urinary tract and cloaca of _Platypus anatinus_, duck-billed platypus. (Columbia University Museum, No. 1802.)] Fig. 25 shows the male genito-urinary tract and the cloaca in the monotreme, _Platypus anatinus_. The cloaca is a spacious sac formed by the confluence of the rectum and the genito-urinary sinus.