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.
flagellum-a lash-like appendage used for locomotion (e.g., in sperm cells and some bacteria and protozoa). eubacteria, eubacterium, true bacteria-a large group of bacteria having rigid cell walls; motile types have flagella.ntogenetic development of sensory structures on the antennules and ... by Henriques, Virginia Maria Cavalari; De Medeiros, Guilherme Fulgencio; New, Michael B.; Nakaghi, Laur / Journal of Shellfish Research.