Spirostomum is a very large ciliate measuring from 300-600 microns long (almost big enough to see with the naked eye). It has a long opaque, cylindrical body.

The ectoplasm in the Spirostomum has strong myonemes which contract the body to one fourth its length in 6-8 miliseconds. Because of these contractions, a thick culture looks like a plate of wiggling spaghetti.