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EXCRETION IN ANIMALS

Excretion is the process where metabolic wastes are eliminated from an organism. In humans, this function is performed through kidneys, lungs and skin. In animals, the main excretory products are:

1. Ammonia
2. Carbon Dioxide
3. Urea
4. Uric Acid
5. Guanine
6. Creatine
   

Modes of Excretion

Based on the excretory product, five modes of excretion are known in animals. They are:

1. Ammonotelism (Type of excretion- ammonia)
2. Ureotelism (Type of excretion – urea)
3. Uricotelism (Type of excretion – uric acid)
4. Aminotelism (Type of excretion – amino acids)
5. Guanotelism (Type of excretion – guanine)

Ammonotelism

The process of eliminating ammonia from the body is known as ammonotelism , and the organisms which exhibit this nature are called ammonotelic. Most fish, protozoans, echinoderms, poriferans and crustaceans fall into this category. Aquatic animals excrete ammonia directly into the environment; where the compound is quickly diluted. It is also very toxic to tissues.

Ureotelism

In some mammals and amphibians, urea is excreted as the metabolic waste products. Such organisms are called ureotelic. In these organisms, ammonia that is produced is converted to urea in the liver of animals and is released back into the blood. The kidneys filter the urea and are expelled outside the body. Some of the urea is retained in the matrix of the kidney to maintain a desired osmolarity in the organisms. Humans are ureotelic as we expel the urea through urine. Moreover, urea is comparatively less toxic than ammonia.

Uricotelism

Uricotelic animals remove nitrogenous wastes as uric acid in the form of pellets or paste. Metabolically, this process is quite costly; however, the water loss is minimal, and it is the least toxic. Moreover, since uric acid is not readily soluble in water, the excrements form pasty white suspensions. Most reptiles, birds, and insects are classified as uricotelics.

Aminotelism

Certain molluscs and echinoderms excrete excess amino acids. This feature is called aminotelism.

Guanotelism

Spiders convert the ammonia into guanine before excretion. This characteristic is also found in some reptiles, birds and earthworms. It is also insoluble in water; hence no water is required for its excretion.

Excretion Examples & Structures

1. All vertebrates have kidneys – Excretory product is urea
2. Flame cells in planaria
3. Earthworms have Nephridia
4. Cockroaches have malpighian tubules
5. Prawns have antennal glands or green glands