Aquatic animals are ammonotelic as the ammonia is generally excreted by diffusion across the body surface. Whereas in terrestrial animals ammonia which is toxic cannot circulate in the blood till it reach the excretory system, it is converted into lesser toxic nitrogenous wastes like urea and uric acid by the liver and released into the blood from where it is filtered and excreted out by the kidneys. Hence terrestrial animals are either ureotelic or uricotelic.