About 2200 species of rotifers have been described. They are placed in the phylum Rotifera. This phylum is subdivided into three classes, "Monogononta", "Bdelloidea", and "Seisonidea". The largest group is the Monogononta, with about 1500 species, followed by the Bdelloidea, with about 350 species. There are only two known species of Seisonidea.