Four synapomorphies support the inclusion of Trepomonas, Hexamita, Spironucleus, Octomitus and Giardia in the suborder Diplomonadina (Fig.
Contributing to this is a transformation in trophic modes with the reduction (in Spironucleus species) and loss (in species of Octomitus and Giardia) of a cytostome, necessary for the ingestion of bacterial prey.