Spirotrichea

The Spirotrichea are a 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. In some groups the body cilia are sparse or absent, and in others modified into polykinetids called cirri. These are coordinated groups of cilia, used variously in swimming, crawling, jumping, and aiding in food capture. Four subclasses are generally recognised.

The Oligotrichia[?] and Choreotrichia[?] include forms with sparse body cilia. Most of these are small, marine ciliates. Some oligotrichs are freshwater, however, including the well-known genera Halteria[?] and Strombidium[?], which swim with an obvious hopping motion. The choreotrichs include the Tintinnida, which characteristically form loricae (shells) of various sizes and shapes. These are the only ciliates with a fossil record, and many are known only as microfossils. They became common during the Jurassic, but are known from as early as the Ordovician period.

The Hypotrichia and Stichotrichia[?] include forms with cirri. These are generally large ciliates, common in marine, freshwater, and soil environments, with a few symbiotic forms. In most the body is flattened, with cirri on the ventral surface and borders, and in some hypotrichs dorsal cilia that function as sensory bristles. Hypotrichs usually have a rigid pellicle, and maintain their ciliature when encysted, as well as having some ultrastructural peculiarities - some had suggested removing them to the class Nassophorea. Euplotes[?] and Aspidisca[?] are representative genera. Typical stichotrichs include Stylonychia[?], Urostyla[?], and Oxytricha[?].

The Spirotrichea were first defined by Otto Bütschli in 1889. Traditionally the Heterotrichea have been included, but they show a number of peculiarities that suggest they are not close relatives of the other groups, and more recently are treated as a separate class. Molecular studies support the unity of the four groups mentioned above. Some smaller orders, such as the Odontostomatida[?], Armophorida[?], and Clevelandellida[?] may also belong here.