Stentor, sometimes called trumpet animalcules, are a genus of filter-feeding, heterotrophic ciliates. They are usually horn-shaped, and reach lengths of two millimeters; as such, they are among the biggest known extant unicellular organisms. They reproduce asexually through binary fission. Stentor have a ring of prominent cilia around the anterior "bell" that sweep in food and aid in swimming. Stentor can come in different colors. For example, S. coeruleus can appear blue due to the presence of Stentorin, a natural pigment. 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. These protists are common worldwide in freshwater lakes and streams, but few have been recorded from marine and even terrestrial biotopes. They are usually attached to algal filaments or detritus. Some Stentor species, such as S. polymorphus, can live symbiotically with certain species of green algae (Chlorella). After being ingested, the algae live on while their host absorbs nutrients produced, whereas the algae, in turn, absorb and feed on the Stentor's metabolic wastes. Stentor species react to outside disturbances by contracting into a ball.