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cloaca(klōā`kə), in biology, enlarged posterior end of the digestive tract of some animals. The cloaca, from the Latin word for sewer, is a single chamber into which pass solid and liquid waste materials as well as the products of the reproductive organs, the gametes. Cloacas are found in amphibians, reptiles, birds, and lower mammals; higher mammals have a separate rectal outlet, the anus. The term cloaca is also used for analogous chambers in many invertebrates, such as worms of the phylum NematodaNematoda
, phylum consisting of about 12,000 known species, and many more predicted species, of worms (commonly known as roundworms or threadworms). Nematodes live in the soil and other terrestrial habitats as well as in freshwater and marine environments; some live on the deep
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the broadened extremity of the hindgut of some vertebrate animals. The wall of the cloaca is covered with a many-layered epithelium. The ureter, the genital ducts (sperm ducts or oviducts), and the urinary bladder open into the cloaca. It is found in certain cyclostomes (hagfish) and fishes (sharks, skates, dipnoans and pipefish) and in all amphibians, reptiles, and birds. The cloaca is found in mammals of the subclass Prototheria. In other mammals a cloaca is found only in the early embryonic stage of development; it subsequently divides into the urogenital sinus and the terminal part of the rectum, which have separate urogenital and anal openings. In amphibians the urinary bladder is formed from an evagination of the abdominal wall of the cloaca. Allantoides appear in the embryo of amniotes.