An order of the Holotrichia which contains a large, widely distributed group of what are believed to be the most primitive ciliate protozoa. These organisms occur abundantly in sands of intertidal zones, as well as in the more usual fresh- and salt-water habitats. The body size is frequently large; ciliation is simple and plentiful; and the oral area lacks buccal ciliature, as the name of the order implies. The cytopharynx, however, is reinforced by fibrillar rods known as trichites, or nemadesmata. Members of two families exist as harmless commensals in such herbivores as horses and camels and in a few other mammals. Prorodon, Holophrya, Didinium, and Dileptus are examples of carnivorous gymnostomes. Chilodonella and Nassula are herbivorous and anatomically more complex. Ichthyophthirius, a fish parasite, is often classified erroneously as a gymnostome, but belongs in the order Hymenostomatida. It is found on fish in home aquaria and causes the disease known as the ich.