Known for displaying the fastest contraction of any living cell, the relatively giant ciliates in the genus Spirostomum are the true speed demons of the microscopic world. With some species large enough to be observed without the aid of a microscope, these protozoans can reduce their body sizes to 25 percent of their normal lengths in less than 8 milliseconds, using very strong myonemes.

Reaching lengths greater than 4 millimeters, a tremendous size for a single-celled animal, the entire body is covered with spiral rows of cilia. The cilia are used in unison for locomotion and to aid the ingestion of bacteria, algae, and other protozoans suitable as prey. The cilia create currents that flow toward the cytostome, or mouth, and drive food through the peristome. As ingested food fills a membranous sac at the end of the peristome, it closes off and breaks away, creating a large food vacuole. Digestion progresses and the vacuoles get progressively smaller. A water-expelling vesicle completes the notable anatomical characteristics of Spirostomum.

As representatives of the phylum Ciliophora (the ciliates) in the class Spirotrichia, Spirostomum species have well-developed fused mouth cilia that lead clockwise into the mouth. Unlike their relatives, the Paramecium, Spirostomum are elongated, cylindrical, and flattened in body shape. In addition to swimming with a coordinated ciliate movement, this large unicellular organism can move in a snake-like undulating manner. As occurs in other large single-celled organisms such as the giant Amoeba and Stentor, these ciliates often possess multiple nuclei forming a long strand, which may resemble a miniature string of pearls.

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