The spirotrichs are a large and distinctive class of ciliate protozoa. They typically have a prominent adoral zone of membranelles, beginning anterior to the oral cavity and running down to the left side of the mouth. Sometimes there is also an undulating membrane on the right. The body cilia may be sparse or absent, as in the oligotrichs and choreotrichs, or they may be modified into polykinetids, as in the hypotrichs and stichotrichs. These polykinetids, called cirri, are variously used in swimming, crawling, jumping, and aiding food capture. Most of the former are small and marine, while most of the latter are larger, and are common throughout marine, freshwater, and soil environments.
The spirotrichs were one of two orders in the first major classification of ciliates, by Otto BŁtschli in 1889, together with the now abandoned holotrichs. Until recently, the heterotrichs were included here, but they show a number of peculiarities that suggest they are not close relatives of the other groups and are now given their own class. However some of the smaller orders generally included with them, such as the Odontostomatida, Armophorida, and Clevelandellida may genuinely belong among the spirotrichs. Molecular studies support the unity of the group, but suggest that the subclasses may need revision.