Sphenomonadine euglenids, colourless, phagotrophic and/or osmotrophic, rigid, fusiform or triangular, flattened cells, usually very flat and leaf-shaped, mostly with strong ribs or keels; no special ingestion organelle visible by light microscopy but some species, at least, have a simple feeding apparatus visible by electron-microscopy; canal opening subapical with one emergent flagellum, directed anteriorly, straight, during swimming; second flagellum non-emergent; paramylon abundant; freshwater and marine, mainly benthic, in mud sediments, cosmopolitan; Scytomonas was purportedly distinguished by having a single flagellum and no reduced second flagellum in the reservoir although a reduced basal body may have been overlooked; cells assigned to Scytomonas have a simple feeding pocket and are known to indulge in sexuality - two cells fuse as isogametes, one flagellum is lost and the zygote swims by means of the other; the gamete nuclei fuse but meiosis and further stages have not been seen, type species P. abscissa (Dujardin, 1841) Stein, 1859.