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.
1 Loricifera: a new phylum, discovered in the 1970s. 2  They are microscopic, 100â485Î¼m; ~100 species. 3  They have an exoskeleton called a lorica, and an introvert which can be withdrawn into the trunk. 4  They live at the bottom of the water column attached to gravel.