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A Handbook of Biology

Excretion is the elimination of metabolic wastes like ammonia, urea, uric acid

etc. from the tissues.

TÝPËS ØF ËXÇRËTÏØÑ

Ammonotelism: Process of excretion of NH3.

Ammonotelic animals: Aquatic invertebrates, aquatic insects, bony fishes,

aquatic amphibians etc. (AIPMT 2012)

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.

Ureotelism: Process of excretion of urea.

Ureotelic animals: Cartilaginous fishes, terrestrial & semi aquatic amphibians

(frogs, toads etc.), aquatic or 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.

Uricotelism: Process of excretion of uric acid. It is water insoluble & less toxic.

So, water is not needed for excretion. (AIPMT 2011, 2012)

Uricotelic animals: Insects, some land crustaceans, land snails, terrestrial

reptiles & birds.



Ureotelism & uricotelism are needed for water conservation.

SØMË ËXÇRËTØRÝ ØRGÅÑS ÏÑ ÅÑÏMÅLS

Protonephridia (flame cells): In Flatworms Planaria, rotifers, some

annelids & cephalochordate (Amphioxus). Protonephridia are

primarily for osmoregulation.

Nephridia: In Annelids. Help in the removal of nitrogenous wastes

and osmoregulation.