A class of ciliates characterized by the presence of a prominent anterior zone of membranelles, formed by cilia fused together, about the mouth. They are closely related to the Karyorelictea, united by the presence of dikinetids, simple oral kinetids, and postciliodesmata.
There are three subclasses of spirotrichs:
- The Heterotrichia contains forms with cilia all over the body, like Blepharisma, Stentor, Spirostomum, Metopus, Nyctotheroides, and Epalxella.
- The Choreotrichia contains forms where the body cilia are fused together into cirri, present in small groups. They include the oligotrichs, like Halteria and Strombidium, and the tintinnids, like Tintinnidium and Tintinnopsis. The latter are the only ciliates with a fossil record, becoming common in the Jurassic but dating back to at least the Ordovician.
- The Stichotrichia contains forms with groups of cirri present on the ventral surface, which they use to crawl along the substrate. They can be distinguished from the similar looking but unrelated Hypotricha (with which they were once grouped) by the presence of the AZM and by having a contractile or flexible body. Examples include Stylonychia, Oxyticha, Urostyla, Uroleptus, Gonostomum, and Holosticha.
Stichotrich - picture by JG