Excretory Products and Their Elimination - Notes | Class 11 | Part 1: Types of Excretion, Excretory Organs


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.

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