URINARY SYSTEM
Amphibians have mesonephric kidneys that are unable to concentrate urine above the solute concentration of the plasma (Wright 2001c). A urinary bladder, which is bilobed in many caecilians, forms embryologically as an evagination of the cloaca.
Urine passes from the kidney tubules into the collecting duct, into the cloaca, and then into the bladder. Thus urine is not expected to be sterile.Amphibians excrete a variety of nitrogen wastes, based on habitat and the need to conserve water. Larvae and most aquatic adults excrete ammonia through the kidneys, skin, and gills, if present (Stebbins & Cohen 1995; Wright 2001c, 2001d). Terrestrial species convert toxic ammonia to less toxic urea in the liver. Urea can be stored in the bladder and excreted when water is readily available. Very specialized anurans, such as the waxy treefrog (Phyllomedusa sauvagii), are uricotelic, meaning they further conserve water by converting nitrogen wastes to uric acid. The clawed frog (Xenopus laevis) can convert from ammonia production to urea production based on the availability of water in the environment (Mitchell et al. 1988; Stebbins & Cohen 1995; Wright 2001c, 2001d).