Rotifer

Rotifers make up a phylum, Rotifera, of microscopic and near-microscopic pseudocoelomate animals, typically around 0.1 to 0.5 millimeters long. They are common in freshwater environments throughout the world, with a few saltwater species, and take their name from the corona around the mouth, whose concerted sequential motion resembles a wheel. Rotifers form an important part of freshwater zooplankton, serving as a major food source, and many species also contribute to the decomposition of soil organic matter.