An account of the structure of plants naturally begins with the cell which is the proximate unit of organic structure. The elementary unit of plant structure, as of animal structure, is the cell. Faunistically, although not geographically, the Nearctic and Palaearctic areas must form the two subdivisions of one great unit, for which the  Holarctic region  is now the generally accepted term.
Definition: a subclass considered a phylum of the kingdom Protista in some classifications; includes holozoic or saprozoic flagellates.
There are pathogenic and commensal species of flagellates. The flagellates which are encountered in the intestinal tract are Giardia lamblia, Dientamoeba fragilis, Chilomastix mesnili, Trichomonas hominis, Retortamonas intestinalis and Enteromonas hominis (the latter two being less common).orphology of Trophozoites. Trichomonas hominis do not have a cystic stage. The trophozoites measure from 5-15 m m in length by 7-10 m m in width. The shape is pyriform and has an axostyle which runs from the nucleus down the centre of the body and extends from the end of the body.