Give scientific reasons: Majority of fresh water organisms have contractile vacuoles while marine organisms lack them.

(N/A) The osmotic concentration of fresh water organisms like $Amoeba$ and $Paramoecium$ is hypertonic to the surrounding water,causing water to enter their bodies via endosmosis. This excess water must be expelled to prevent the cell from bursting,which is achieved by contractile vacuoles; thus,they play a crucial role in osmoregulation. In contrast,marine protozoans are isotonic to sea water,meaning there is no net movement of water into the cell,and therefore,they do not require contractile vacuoles.