2 Answers
Terrestrial animals are either ureotelic or uricotelic, and not ammonotelic. This is because of the following two main reasons:
(a) Ammonia is highly toxic in nature. Therefore, it needs to be converted into a less toxic form such as urea or uric acid.
(b) Terrestrial animals need to conserve water. Since ammonia is soluble in water, it cannot be eliminated continuously. Hence, it is converted into urea or uric acid. These forms are less toxic and also insoluble in water. This helps terrestrial animals conserve water.
Many aquatic creatures are ammonotelic which excrete ammonia as waste product. Ammonia is generally excreted by diffusion across body surfaces as ammonium ions. Kidneys do not play any significant role in its removal. Terrestrial adaptation necessitated the production of lesser toxic acid for conservation of water. Hence terrestrial animals are mostly ureotelic (excrete urea) and uricotelic (excrete uric acid).