Contractile vacuole is responsible to throw excess water that comes from outside to inside the cell. In presence of marine water, due to exosmosis cell start losing water. So, there is a need to preserve water. In that situation, the presence of contractile vacuole will dehydrate the cell. So, contractile vacuole will be disappeared.

Firstly when the fresh water protozoan is placed in marine water exosmosis occur due to concentration gradient and then slowly and slowly vacuole shrinks and finally disappears.

**Explanation:**
When a fresh-water protozoan possessing a contractile vacuole is placed in a glass containing marine water, the vacuole will disappear.

**Contractile Vacuole:**
A contractile vacuole is an organelle found in many freshwater protozoans and some freshwater algae. It is responsible for maintaining osmotic balance by collecting excess water and expelling it from the cell. The vacuole acts like a pump, filling with water and then contracting to expel the excess.

**Osmosis and Osmotic Balance:**
Osmosis is the movement of water molecules across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration. In a freshwater environment, the concentration of solutes is higher inside the cell than in the surrounding water. As a result, water molecules move into the cell by osmosis. To prevent excess water intake, freshwater protozoans have contractile vacuoles.

**Marine Water Environment:**
Marine water has a higher concentration of solutes (salts) compared to freshwater. When a freshwater protozoan is placed in marine water, the concentration of solutes outside the cell becomes higher than inside the cell. This creates a hypertonic environment for the protozoan.

**Effects on the Contractile Vacuole:**
When a freshwater protozoan is placed in a hypertonic environment (marine water), the following changes occur in the contractile vacuole:

1. **Decreased Water Influx:** In a hypertonic environment, water molecules move out of the cell by osmosis. The decreased water influx results in a decrease in the amount of water collected by the contractile vacuole.

2. **Reduced Vacuole Filling:** With less water entering the cell, the contractile vacuole will have less water available to fill itself.

3. **Lack of Vacuole Contraction:** In the absence of sufficient water to fill the vacuole, it will be unable to contract and expel any water from the cell.

4. **Disappearance of the Vacuole:** As the contractile vacuole cannot function properly in a hypertonic environment, it will eventually disappear.

Therefore, when a freshwater protozoan possessing a contractile vacuole is placed in a glass containing marine water, the vacuole will disappear.