Spirostomum is a genus of free-living ciliate protists, belonging to the class Heterotrichea. Species of Spirostomum are found in both salt and freshwater. These ciliates are important because they work with the bacteria and feed on it, helping to clarify waste in the water. All are elongated, flexible and highly contractile. Although unicellular, members of some species can grow as long as 4 mm (0.16 in). The body of the cell is long and worm-like. In cross section it is mainly cylindrical, but may be flattened at the tail end. The posterior excretory vacuole is large, and may fill the whole "tail." Cilia on the cell body are short and arranged in longitudinal rows. The length of the peristome varies between species, from about 1/4 to as much as 2/3 the length of the cell. The peristome is fringed with membranelles, which are used to channel particles of food into the creature's oral cavity. The nucleus may be moniliform (like a string of beads) or compact and oval, depending on the species. Spirostomum reproduces by binary fission. Reproduction may be purely asexual, or it may follow conjugation, during which compatible mating individuals come together and transfer genetic material across a cytoplasmic link. Members of the genus are extremely contractile. When startled, Spirostomum can contract its body length by more than 60% within a few milliseconds (a contraction speed similar to that of the ciliate Vorticella). As it contracts, the cortex of the cell twists and widens, and its spiral structure becomes visible.