Why is contractile vacuole absent in parasitic protozoans but present in free living protozoans? I know that contractile vacuole helps in osmoregulation of protozoans and some nitrogenous wastes are also excreted through it. Since contractile vacuole plays an important role in protozoans, why is it absent in parasitic protozoans? I could not find the logic behind this.

No, you are wrong. Parasitic protozoans too have contractile vacuole, for example trypanosoma. Also Wikipedia says:

> Not all species that possess a contractile vacuole are freshwater organisms; some marine, soil microorganisms and **parasites** also have a contractile vacuole

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