(i) Excretory waste is ammonia (NH3).
(ii) It is characteristic of aquatic animals such as protozoans, sponges, coelenterates, crustaceans,bony fishes and
(iii) Excess of water is required to eliminate ammonia. It is highly toxic.
(iv) Water loss is about 300-500 ml to remove one gram of ammonia.
(v) Ammonia is formed in liver cells by the deamination of the amino acids.
1.Excretory waste is urea (NH- CO-NHj).
2.It is characteristic of mammals, cartilaginous fishes (e.g., dogfish) and amphibians etc.
3.Less water is needed, urea is less toxic than ammonia. It can be retained for some time in body without any harmful effect.
4.Water loss is about 50 ml to eliminate one gram of urea.
5.Urea is formed in liver by detoxification of ammonia.
1.Excretory waste is uric acid.
2.It is characteristic of terrestrial reptiles and insects. All the birds are uricotelic.
3.Much less water is needed. Uric acid is least toxic.
4.Water loss is about 10 ml to excrete out one gram of uric acid.
5.Uric acid is formed mainly from the purines in liver cells. In insects, uric acid is formed in malpighian tubules.