Stentor, genus of trumpet-shaped, contractile, uniformly ciliated protozoans of the order Heterotrichida. They are found in fresh water, either free-swimming or attached to submerged vegetation. Stentor assumes an oval or pear shape while swimming. At its larger end, Stentor has multiple ciliary membranelles spiraling around the region that leads to the mouth opening. It uses these cilia to sweep food particles into its cytostome. The species S. coeruleus is large (sometimes up to 2 mm [0.08 inch] long) and is predominantly blue from a blue pigment, stentorin, found in its ectoplasm. Stentor is remarkable for its regenerative powers; a small fragment less than one-hundredth of the volume of an adult can grow back into a complete organism. This capability has made Stentor a favourite subject for studies of regeneration in protozoans.