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.
Explain the basic body plan of members of the phylum Platyhelminthes. Identify representatives of the classes Turbellaria, Trematoda, and Cestoda. Compare and contrast the anatomy and morphology of free-living and parisitic flatworms. Describe the symmetry of this group. Platyhelminthes is a phylum composed of three classes of flatworms. The class Turbellaria (planarians) is free-living, while the remaining classes including the Trematoda (flukes) and Cestoda (tapeworms) are parisitic. Members of the phylum Platyhelminthes are dorsoventrally flattened.