any member of the ciliated protozoan order Heterotrichida. Complete ciliation is typical, although there is a tendency toward loss of the cilia, which are minute, hairlike processes, in several families (Peritromidae, Licnophoridae). Heterotrichs are considered the most primitive of the subclass Spirotrichia because of their uniform ciliation. Heterotrichida have a zone of membranelles near the peristome (a groove leading to the mouth) and often an undulating membrane close by. Representative genera include Bursaria, an oval freshwater form; Spirostomum, a long, cylindrical protozoan; Stentor (q.v.), a trumpet-shaped freshwater genus; and Nyctotheroides, a common parasite of frogs and toads.