 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, so excreting urea has less toxicity as compared to ammonia. Therefore, urea can be retained for some time in the body 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 urecolytic pathway. The ornithine urea cycle is used by all vertebrates except the teleoste 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. The arginine on it acts as an arginase and cleaves it to produce a molecule of urea and regenerate the ornithine. Now we shall discuss the urecolytic pathway. Most teleostes 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 urecolytic 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 allantoic acid is converted into urea. These three enzymes are not present in mammals such as uricase, allantoicase 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.