Think and discuss: Paramecia that live in freshwater have contractile vacuoles, while those that live in salt water do not. Why do you think this is the case

See the answer below

Explanation:

The contractile vacuole primarily functions for the excretion of excess water in organisms that possess the organelle. Paramecia that lives in freshwater would need to constantly get rid of excess water that is likely to be entering the cell due to the fact that the ambient water is hypotonic to their cells. The cytosols of paramecia that live in freshwater have a higher solute concentration and hence, lower water potential than the surrounding freshwater.

The paramecia that live in salt water do not have a need for the contractile vacuole because the problem of excess water entering their cells may not arise. The salt water may not be hypotonic to the cytosol of the paramecia.