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an order (or subclass) of protozoans of the class Infusoria. The cilia are either evenly distributed over the entire body or are developed predominantly on the ventral side. The Holotricha usually have special peristomial cilia, which often fuse into undulating membranes, of which there are usually three. There is no peristomial spiral of membranelles. More than 3,000 species are known. The Holotricha are particularly numerous in fresh and marine waters. There are parasitic forms, including the parasite of the fish Ichthyophthirius.