Only the location of one nitrogen is different. It inhibits xanthine oxidase and causes the concentration of xanthine and hypoxanthine to rise, while preventing or limiting the formation of uric acid. Uric acid is the end of purine metabolism in birds, reptiles, insects and primates. This is not true in other mammals and in molluscs. In these organisms, the purine ring is opened by urate oxidase to form allantoin Fig 22-21, p. 717.
Bony fish continue the ring breakage by forming allantoic acid.
Crustaceans can break down urea to CO2 and NH3 by the enzyme urease. Land animals cannot tolerate ammonia since it is toxic, but when you live in the water it can be continuously excreted and diluted so it is not harmful as a waste product.