Paramecia regulate their water content by using contractile vacuoles, special vacuoles that collect and actively pump water out of their cytoplasm. They are especially necessary in the freshwater environments where paramecia are primarily found, as in these environments they are constantly taking in water via osmosis. They also possess special salt crystals that dissolve or solidify as needed to maintain the solute balance within the cell.
The contractile vacuoles paramecia used to eliminate excess water are physical pumps. They fill with water and then contractile fibers squeeze the vacuole and force the water out. This method of removing excess water is shared by many other single-celled organisms such as amoebas.
Paramecia are typified by their use of cilia, tiny hairs on the outside of their cell membranes, for use in locomotion. Their primary food source is bacteria in the stagnant water, which is their most common habitat. They also consume other organisms such as yeasts and algae. At least one species of paramecium forms a symbiotic relationship with photosynthetic algae, which gain protection from the paramecium and provide it with food. Paramecia are most famously prey to another ciliated cell type known as Didinium, which, despite being far smaller than a paramecium, can consume several of them in a day.