The ducts that drain the paired kidneys are usually two, although some fishes may have six or eight supplementary ducts when the nephridial organs extend into the extensive tail region. In higher vertebrates the urinary ducts are termed ureters. These tubes with muscular walls and a comparatively small bore forward the continuous products of the kidneys, not by gravity alone, but by peristalsis regardless of the position of the body.
The length of the ureters depends upon the position of the kidneys within the body cavity. They are very long in snakes and extremely short in birds. In adult man they average from eleven to fourteen inches in length.
Urinary ducts terminate at the outside in various ways. In bony fishes, male amphibians, and monotremes, they unite with the sexual ducts into a common channel, or urogenital canal, opening at the genital aperture. In elasmobranchs, most reptiles, and birds, they debouch into the cloaca, while in mammals they open into a reservoir, the bladder, whence by a second duct, the urethra, the outside is finally reached (Fig. 367).