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.
Looking for phylum Mollusca? Find out information about phylum Mollusca. taxonomic name for the one of the largest phyla of invertebrate animals comprising more than 50,000 living mollusk species and about 35,000 fossil species... Explanation of phylum Mollusca