An order of the class Zoomastigophorea, phylum Protozoa. These are small, colorless flagellates, some of which are free-living and some parasitic. The body of this protozoan is bilaterally symmetrical, composed of two mirror halves, each with a nucleus and a full set of organelles. There are four flagella to a side, not all of the same length. In division of the cell the two nuclei each form a spindle, so that each daughter cell receives two nuclear complexes.