Contractile vacuoles take part in _____.

Contractile vacuoles take part in osmoregulation and excretion both. 

In unicellular organisms like bacteria and amoeba, contractile vacuoles act as specialized structures that regulate the water content in the cell by osmoregulation. Since there is no other way to excrete out waste, the contractile vacuoles expel the waste matter dissolved in water along with the extra water.