Ciliates are heterotrophic organisms that lack cell wall and use numerous small cilia for locomotion. They have an outer tough membrane called as pellicle.
There are around 8000 species of ciliates and most of them occur in freshwater. The excess water from outside continuously moves inside the ciliates via osmosis and makes them hypotonic compared to outside solution. This excess water is removed actively via contractile vacuoles. Central ring of the vacuole collects water and actively transports it outside.
