Metabolic waste

[citation needed] Thus, the marine organisms excrete ammonia directly into the water and are called ammonotelic.

[2] Ammonotelic animals include crustaceans, platyhelminths, cnidarians, poriferans, echinoderms, and other aquatic invertebrates.

Land animals, mainly amphibians and mammals, convert ammonia into urea, a process which occurs in the liver and kidney.

[3] Urea is a less toxic compound than ammonia; two nitrogen atoms are eliminated through it and less water is needed for its excretion.

[citation needed] Uricotelism is the excretion of excess nitrogen in the form of uric acid.

Though requiring more metabolic energy to make than urea, uric acid's low toxicity and low solubility in water allow it to be concentrated into a small volume of pasty white suspension in feces, compared to the liquid urine of mammals.

[3] Notably however, great apes and humans, while ureotelic, are also uricotelic to a small extent, with uric acid potentially causing problems such as kidney stones and gout, but also functioning as a blood antioxidant.