any member of the flagellate protozoan order Rhizomastigida, with features similar to both flagellates and sarcodines (protozoans having pseudopodia). Members are permanently amoeboid and may have from 1 to 50 flagella. Pseudopodia (cytoplasmic extensions) vary in number and appearance; some are axopodia (composed of an axial rod and a cytoplasmic envelope), others are lobopodia (tonguelike in form). Most members of the order (e.g., Mastigamoeba) are free-living in fresh and salt water, in soil, or in other organisms. An important parasitic form is Histomonas meleagridis, the cause of enterohepatitis (or blackhead) in poultry. Cell shape, although variable in this species, frequently is round; cells range from 10 to 14 micrometres (0.0004 to 0.0006 inch) in diameter. H. meleagridis is an active, amoeba-like organism with a flagellate stage and is passed to new hosts in feces or eggs of nematode worms.