The body is horn-shaped, with a ring of prominent cilia around the anterior "bell" that sweeps in food and helps swimming. Some reach several millimeters in length, making them among the largest single celled organisms. As in many freshwater protozoans, Stentor has a contractile vacuole. Because the concentration of salt inside the cell and in the surrounding freshwater is different, Stentor must store water that enters it by osmosis and then discharge it from the vacuole. They can regenerate, and small fragments can grow into full organisms. Each cell has one (often elongated) macronucleus and several micronuclei. Having two types of nuclei is charcteristic of ciliates.