Excretion is the elimination of metabolic wastes like ammonia, urea, uric acid etc. from the tissues.
Types of Excretion
1. Ammonotelism
- Process of excretion of Ammonia (NH3).
- Ammonotelic animals: Aquatic invertebrates, aquatic insects, bony fishes, aquatic amphibians etc.
- NH3 is highly toxic. So, excretion needs excess of water.
- NH3 is readily soluble in water and is excreted by diffusion through body surface or gill surfaces (in fishes) as ammonium ions.
- Kidneys do not play any significant role in its removal.
2. Ureotelism
- Process of excretion of urea.
- Ureotelic animals: Cartilaginous fishes, terrestrial & semi-aquatic amphibians (frogs, toads etc.), aquatic & semi-aquatic reptiles (alligators, turtles), mammals etc.
- In liver, NH3 is converted into less toxic urea. So, it needs only moderate quantity of water for excretion.
- Some amount of urea may be retained in the kidney matrix of some animals to maintain a desired osmolarity.
3. Uricotelism
- Process of excretion of uric acid.
- It is water insoluble & less toxic. So, water is not needed for excretion.
- Uricotelic animals: Insects, some land crustaceans, land snails, terrestrial reptiles & birds.
- Ureotelism & uricotelism are needed for water conservation.
Some Excretory Organs in Animals
- Protonephridia (flame cells): In Flatworms, rotifers, some annelids & cephalochordate (Amphioxus). Protonephridia are primarily for osmoregulation.
- Nephridia: In Annelids. Help in the removal of nitrogenous wastes and osmoregulation.
- Malpighian tubules: In Insects. Help in the removal of nitrogenous wastes and osmoregulation.
- Antennal or green glands: In Crustaceans (prawn etc.)
- Kidneys: In higher animals.