a subclass of protozoans of the class Infusoria. Spirotricha are characterized by the presence of an adoral zone of membranelles—laminar structures formed as a result of the fusion of cilia arranged in a row. The zone, which is coiled from left to right, provides locomotion and directs food to the mouth opening.
There are approximately 2,000 species of Spirotricha. The major orders are Heterotricha (approximately 450 species), Oligotricha (approximately 100 species), Tintinnida (approximately 1,000 species), Odontostomatida (approximately 40 species), and Hypotricha (approximately 400 species). The protozoans inhabit sea and fresh waters. Most are microbenthic or benthic. Spirotricha include endoparasitic species of the genus Nyctotherus and species of the genus Licnophora, which live as commensals on the surface of the body of marine invertebrates.