Why do all fresh water organisms have contractile vacuoles whereas majority of marine organisms lack m? 

In fresh water organisms, the concentration of solute inside the cell is higher than that in external environment. This means that the external environment is hypotonic. This leads to movement of water inside the cells because of osmosis. Contractile vacuoles work like safety devices and prevent lysis of cell. In marine organisms, the concentration of solute inside the cell is lower than that in external environment. This means that the external environment is hypertonic. Hence, the cell does not have to face the risk of lysis. This explains the absence of contractile vacuoles in marine organisms.