Apusomonas proboscidea Alexieff, 1924. Cells are 4 - 12 microns long and dorso-ventrally flattened with a V-shaped structure on the ventral side of the cell. Cell outline is roundish to slightly ovate. The mastigophore emerges from a cavity about two thirds of the cell length from the anterior end, is highly flexible and may be withdrawn into the sheath when the cell is stressed - the mastigophore may become wrinkled. The anterior flagellum is partly in a sleeve, which arises at the distal end of the mastigophore. The length of the flagellum plus the mastigophore is longer the cell. On the dorsal side, there is no structure. The cells glide closely to the substrate.
