Description of Petalomonas labrum:
Cells are ovate and elliptical, 10 to 17 microns long, 6 to 10 microns wide, dorso-ventrally flattened, and with a rim surrounding a wide apical opening of the flagellar canal. The cells have very indistinct or smooth pellicle and the ventral face of the cell is slightly concave. The cells taper slightly towards the wedge-shaped posterior end. One flagellum emerges from the long narrow canal and is about as long as the cell or slightly longer. The flagellum only moves a little when the cell glides forwards. The reservoir is situated equatorially in the right hand side of the cell. The nucleus is located in the left side of the cell.