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The water content in the organism has to be regulated as there is a constant inflow of excess water. This is because the cell membrane surrounding the animal is semipermeable and as the protoplasm contains a higher concentration of salts than the water outside, water enters the cell by osmosis. This water is more than what is actually required by the animal. To counter this, excess water collects into the contractile vacuole as fast as it enters the body. The vacuole slowly grows larger until it reaches the maximum size. Then the endoplasm in the area surrounding the vacuole contracts and the fluid contents which contain traces of urea and carbon dioxide are discharged out into the surrounding pond water. Thus the contractile vacuole functions effectively as an organelle that performs both excretion and osmoregualtion.

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∙ 17y ago
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