Ancyromonas sigmoides is the most widely reported species in the genus, as well as being the type species. It has been reported from freshwaters, soil and marine habitats from around the world. Lee and Patterson (2000) give a recent account.
The cell outline is oval, and cells measure from 3 to 7 µm long; they are dorso-ventrally flattened. This species has a shallow groove ventrally near an antero-lateral margin of the cell. Cells may or may not have a thin stiff anterior flagellum emerging from an anterior depression. The anterior flagellum can be easily overlooked. The posterior flagellum is about 1.5 times the length of the cell and may have the same width along its length. The cell moves by gliding with the posterior flagellum trailing.
Description of Ancyromonas sigmoides