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.
As a phylum the Protozoa were, historically, divided into four subphyla reflecting the means of locomotion: 1  Subphylum Sarcomastigophora Superclass Mastigophora (includes flagellates) Superclass Sarcodina. Superclass Opalinata. 2  Subphylum Sporozoa (includes apicomplexans) 3  Subphylum Cnidospora Class Microsporidea.