Animals which excrete their nitrogenous wastes as ammonia are said to be ammonotelic. Excretion of ammonia, which is presumed to be the primitive material, is especially suited to an aquatic habit. Ammonia is highly soluble in water, with which it forms ammonium hydroxide (NH₄OH). The latter passes readily across the cell membranes and is dispersed into surrounding water. Ammonia is found as chief excretory product in the majority of primitively aquatic animals and certain protozoa, annelids, crustaceans, some mollusks, and teleost fishes. Anuran (amphibian) tad-poles excrete mainly ammonia, whereas the adults produce urea.