What are the three main modes of excretion in animals?

The three main modes of excretion in animals are as follows: 

i. Ammonotelism 

ii. Ureotelism 

iii. Uricotelism

1. Ammonotelism: 

• Elimination of nitrogenous wastes in the form of ammonia is called as ammonotelism. 
• Ammonia is basic in nature and hence it can disturb the pH of the body, if not eliminated immediately. 
• Any change in pH would disturb all enzyme catalyzed reactions in the body and would also make the plasma membrane unstable.
• Ammonia is readily soluble in water and needs large quantity of water to dilute and reduce its toxicity. 
• This is however an energy saving mechanism of excretion and hence all animals that have plenty of water available for dilution of ammonia, excrete nitrogenous wastes in the form of ammonia. 
• Animals that follow this mode of excretion are known as ammonotelic animals. 
• 1 gm ammonia needs about 300 – 500 ml of water for elimination.
•  Ammonotelic animals excrete ammonia through general body surface (skin), gills and kidneys. e.g. Ammonotelism is found in aquatic invertebrates, bony fishes, and aquatic / larval amphibians. Animals without excretory system (Protozoa) are also ammonotelic. 

2. Ureotelism: 

• Elimination of nitrogenous wastes in the form of urea is called as ureotelism. 
• Urea is comparatively less toxic and less water-soluble than ammonia. Hence, it can be concentrated to some extent in body. 
• The body requires less water for elimination. 
• Since it is less toxic and less water soluble, ureotelism is suitable for animals that need to conserve water to some extent. Hence, ureotelism is common in terrestrial animals, as they have to conserve water. 
• It takes about 50 ml H₂O for removal of 1 gm NH₂ in form of urea.
• Ureotelic animals generally convert ammonia to urea in the liver by operating ornithine / urea cycle in which 3 ATP molecules are used to produce one molecule of urea. e.g. Mammals, cartilaginous fishes (sharks and rays), many aquatic reptiles, most of the adult amphibians, etc. are ureotelic. 

3. Uricotelism:

• Elimination of nitrogenous wastes in the form of uric acid is called as uricotelism. 
• Uric acid is least toxic and hence, it can be retained in the body for some time in concentrated form. 
• It is least soluble in water. Hence there is minimum (about 5 – 10 ml for 1 gm) or no need of water for its elimination. 
• Those animals which need to conserve more water follow uricotelism. However, these animals need to spend more energy. 
• Ammonia is converted into uric acid by ‘inosinic acid pathway’ in the liver of birds, e.g. Birds, some insects, many reptiles, land snails, are uricotelic.