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.
Of the 60 lineages of protists identified among the eukaryotes, 27 satisfy the condition of being a protozoan flagellate. In addition, alveolates and stramenopiles include heterotrophic flagellates, as do several groups often thought of as algae (dinoflagellates, euglenids, and cryptomonads).Some of the amoebae revert to flagellated forms for part of their life history or have non-functional flagella attached to their bodies.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).