Nitrogen is a major constituent of amino acids and proteins. Generally animals receive excess of amino acids through their diet. The excess of amino acid is catabolized either for release of energy or is used for the synthesis of glycogen and fat. When proteins, amino acids or nucleic acids are catabolized, 3-nitrogen-containing predominant excretory end-products are formed ammonia, urea and uric acid.

(A) Ammonotelism. Animals that excrete excess nitrogen in the form of ammonia (NH₃) (the end product of protein metabolism) are called ammonotelic. Ammonia diffuses through the cell membrane extremely fast because of its high water solubility and small molecular size. Therefore, prompt excretion of ammonia occurs in aquatic animals (aquatic invertebrates, fishes, larvae, permanently aquatic amphibia). The route of ammonia diffusion in these animals is through skin, gills or kidneys.

(B) Ureotelism. Animals that excrete excess nitrogen in the form of urea are ureotelic. Terrestrial animals cannot use water freely for excretion (because of less availability in the environment), so ammonia is converted into a less toxic and easily soluble product, urea. In mammals and semi-terrestrial adult amphibians, urea is a major nitrogenous excretory product, therefore these animals are called ureotelic.

Click here to enlarge

(C) Uricotelism. Animals that excrete a major portion of nitrogenous waste in the form of semi-solid or solid uric acid are called uricotelic animals (e.g. birds, lizards, snakes, terrestrial insects, snails.) In these animals ammonia is converted into less toxic, relatively insoluble uric acid, which can be excreted with a relatively small amount of water. Arachnids (e.g. spiders, scorpions, etc.) excreted mostly guanine and hence are said to be guanotelic. The above terms apply to the predominant form of excretory product, but do not exclude the excretion of other forms in minor quantities.