Explanation

Contractile vacuoles are required foe osmoregulation, $i.e.,$ maintenance of water balance within the body. These are found in the cytoplasm of those organisms, which live in hypotonic water. $Amoeba$is a good example of such type of organisms. $Entamoeba$, an endoparasite, lives in the large intestine where the surrounding is isotonic. The osmotic concentration of its body protoplasm equals to that of the intestinal fluid of the host and hence no water enters the parasite by osmosis. So, this organism does not require contractile vacuoles.