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Stentor are a group of filter feeders, a genus of ciliate protozoa, representative of the heterotrichs. The body is generally trumpet-shaped, hence the association with the Greek herald, with a ring of prominent membranelles around the anterior "bell" that sweep in food and aid in swimming. Stentor are common in freshwater lakes and streams, usually attached to algae and other detritus. Some reach several millimetres in length, making them among the largest single-celled organisms. Stentor can come in different colours, such as green, blue and amethyst. As in many freshwater protozoans, the stentor has a contractile vacuole. Because the concentration of salt inside the stentor and in the surrounding freshwater is different, the stentor must store water that enters it by osmosis and then discharge it from the vacuole. Stentor can regenerate, and small fragments can grow into full organisms. Stentor can live symbiotically with a species of green algae. The stentor ingests the algae and the algae converts the stentor's waste into nutrients. Stentor react to outside disturbances by contracting into a ball of protoplasm.
