 Dear students, in this topic we shall discuss the excretion of urea and ureotylic animals. The animals which excrete urea as their major nitrogenous waste are called ureotylic animals. Excreting urea has certain advantages. A urea molecule contains two nitrogen atoms, which means two nitrogen atoms are being excreted from the body. Excreting urea has less toxicity as compared to ammonia. This is why urea can be retained in the body for some time compared to ammonia. Moreover, urea requires much less water for excretion than ammonia. Dear students, now we shall discuss the formation of urea. Urea is formed from ammonia. Ureotylic animals utilize one of the two available pathways for urea formation. That is, one pathway is called ornithine urea cycle and the other pathway is called uricolytic pathway. The ornithine urea cycle is used by all vertebrates except the tilioste fishes. In this method, synthesis of urea occurs primarily in the liver. Two molecules of ammonia and one molecule of carbon dioxide are added to an ornithine molecule which is converted into arginine. Arginine acts on it and cleaves it to produce a molecule of urea and regenerate the ornithine. Now we shall discuss the uricolytic pathway. Most tiliostes and many invertebrates utilize this pathway. Urea is produced from uric acid which is coming from transamination of aspartate and from nucleic acid metabolism. In the uricolytic pathway, uric acid is first converted to allantoin by an enzyme uricase. This allantoin is converted to allantoic acid by the enzyme allantoinase. Another enzyme which is called allantoi case converts this allantoic acid into urea. These three enzymes are not present in mammals such as uricase, allantoi case and allantoinase. This is why they release this uric acid as a end product of nucleic acid. They do not convert uric acid back into urea.