A large order of the Spirotrichia containing many well-known, sizable species. The buccal ciliature, both membranes and the adoral zone of membranelles, is well developed, although in a number of families the somatic, or body, ciliature is really holotrichous in nature. Heterotrichs have become adapted to all sorts of habitats, including the digestive tracts of a variety of invertebrate and a few vertebrate hosts. Man is not involved. Some species contain pigments in their cytoplasm giving them such coloration as blue, green, pink, brown, or black.