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.
The phylum is Platyhelmenthus   The three classes are   1. turbellaria   2. trematoda   3. cestoda   Platyhelminthes is a phylum composed of three classes of fl â¦ atworms. Members of the phylum Platyhelminthes are dorsoventrally flattened   with a triploblastic body composed of three different tissue layers:   ectoderm, endoderm, and the mesoderm. These animals are bilaterally   symmetrical, meaning that a slice directly through the middle can produce   two mirror images.