Paramecia that live in freshwater would have contractile vacuole because they will need to get rid of excess water unlike those that live in salt water.
The vacuole primarily functions to get rid of excess water from the cell.
Freshwater is hypotonic to the cytosol of paramecia. Thus, water will always enter the paramecia cells by osmosis. If the excess water is not excreted from the cell, turgidity will result and the cell may burst.
On the other hand, salt water is likely hypertonic to paramecia cells. Thus, instead of having vacuoles for water excretion, a mechanism would have to be put in place so as not to lose too much water to the surrounding salt solution. In other words, a vacuole would be totally redundant in the hypertonic solution.
More on the contractile vacuole can be found here: https://brainly.com/question/13137778
