Spirostomum

The heterotrichs are a class of ciliates. They typically have a prominent adoral zone of membranelles circling the mouth, used in locomotion and feeding, and shorter cilia on the rest of the body. Many species are highly contractile, and are typically compressed or conical in form. These include some of the largest protozoa, such as Stentor and Spirostomum, as well as many brightly pigmented forms, such as certain Blepharisma.

A number of ultrastructural details characterize the group. The cilia on the body are in dikinetids, in which either the anterior one or both kinetosomes may be ciliated, and which are associated with fibers composed of overlapping postciliary microtubules, called postciliodesmata and found only in this group and the closely related Karyorelictea. A series of oral polykinetids, each containing two or three rows of kinetosomes, support the membranelles. These run from the left to the anterior of the mouth, and often spiral out of the oral cavity. The macronucleus is divided by external microtubules, whereas in the Karyorelictea it forms by differentiation of micronuclei, and in all other ciliates it is divided by internal microtubules.