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.
Mollusks (British spelling: molluscs) are a large and diverse phyla of invertebrate animals, featuring over 110,000 species. Their phylum name, Mollusca, means thin-shelled, though many species lack shells entirely.ollusks (British spelling: molluscs) are a large and diverse phyla of invertebrate animals, featuring over 110,000 species. Their phylum name, Mollusca, means thin-shelled, though many species lack shells entirely.