A phylum of microscopic, mainly free-living aquatic animals, characterized primarily by an anterior ciliary apparatus, the corona. When the cilia of the corona are in motion the animals appear to have a pair of rapidly rotating wheels on their heads, thus the name “Rotifera,” Latin for “wheel bearers.” Rotifers were first observed by Anton Leeuwenhoek more than 300 years ago; because they are easy to find and of a convenient size for observation with a simple microscope, they have been avidly studied by amateur naturalists ever since.