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.
Heterotrophic flagellates are also found in the following taxa : The flagellates are a grade of organisation. Traditionally, they are those protozoa which spend most of their existence moving or feeding with a small number of flagella. This type of organization is the most widespread among protists.hese proteins include dynein, a molecular motor which can cause flagella to bend and propel the cell relative to its environment, or propel water or mucus relative to the cell. Many protists have embellishments within or on the outer surface of the flagellum (see electron-micrographs below).