The Tetrahymena reference article from the English Wikipedia on 24-Jul-2004
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Tetrahymena

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Tetrahymena are ciliate protozoa, common in aquatic habitats. They are pear-shaped and 30-100 μm in length, belonging among the hymenostomes and with cilia typical of that group. Tetrahymena thrive at temperatures of 18-22 degrees Celsius. Because of their short generation time and because they can be grown to high cell density in inexpensive media, they are common subjects of scientific study. Species include: