Monogononta Digononta Seisonidea The rotifers, or rotifaers, make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696 and other forms were described by Anton van Leeuwenhoek in 1703. Most rotifers are around 0.1-0.5 mm long (although their size can range from 50μm to over 2 millimeters) , and are common in freshwater environments throughout the world with a few saltwater species. Some rotifers are free swimming and truly planktonic, others move by inchworming along the substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate.
About 25 species are colonial (e.g., Sinantherina semibullata), either sessile or planktonic. Rotifers play an important part of the freshwater zooplankton, being a major foodsource and with many species also contributing to decompositioning of soil.