Most are microscopic, freshwater, and free swimming. These tiny animals superfically resemble ciliate protozoans in size and behavior but are multicellular, with about 1000 cells each. They are common but largely unnoticed animals that can be found in almost every conceivable freshwater habitat including such obvious locations as lakes and rivers but also in places that are moist periodically, such as moss, soil, and leaf litter. They are the most abundant metazoans in freshwater benthic habitats and are one of the dominant groups of metazoans in the freshwater plankton. Those that inhabit temporarily moist habitats are tolerant of desiccation.
Rotifers occur at all except polar latitudes.
Almost all are freshwater and only two genera are exclusively marine. Rotifers are pseudocoelomate animals with a body cavity containing fluid and a loose syncytium of amoeboid cells. There is no peritoneum. These animals are eutelic and individuals of each species have a characteristic and constant number of cells. Tissues are often syncytial.