Why is contractile vacuole absent in parasitic protozoans but present in free living protozoans?

Question

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.

Answer 1

Parasitic protozoans are living in a environment that is isotonic to their cytoplasm (internal environment). Free living protozoans (fresh water) live in a hypotonic environment.

Answer 2

The solute concentration of the parasite and the host's cells are approximately the same. So the parasite is existing in an isotonic solution and is in equilibrium with the host's cells' cytoplasm. Thus there is no need for a contractile vacuole and so they evolved without one.