Also called wheel animals and (more archaically) wheel animalcule, for they were among the first microscopic animals to be described, at the end of the 17th century. Mostly freshwater beasties, but there are a few saltwater species, with most under a half millimeter in length. The cilia are used for locomotion and feeding, and the name (from Latin meaning "wheel-bearer") comes from how when they're in motion, the ring looks like a rotating wheel. They're kinda cute, actually.
Rotifers are an important part of the freshwater zooplankton and a major foodsource for plankton-eating species.