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Describe the excretory organs in protozoans, sponges and coelentrates.
Answer
Protozoans : Protozoans have no excretory organs. The metabolic waste products i.e. ammonia is passed out by diffusion through the cell membrane in surrounding water. They carry out the process of excretion with the help of contractile vacuoles.

Sponges : Ammonia is the main excretory product in sponges. It diffuses from cells into the water filling the canals and surrounding the sponge. From the canal system, the waste matter is carried away by the outgoing current of water. 

Coelentrates : Coelentrates have no excretory organs. They pass out metabolic waste by diffusion from all cells. They are ammonotelic i.e. their excretory product is ammonia. The waste water entering coelentron leaves when its water is renewed.