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.
Segmented wormsmake up the Phylum Annelida. The phylum includes earthworms, leeches, and a large number of mostly marine worms known as polychaetes.Polychaetes (meaning many bristles) have many bristles on the body, while earthworms and leeches have fewer bristles.Some are parasitic, as the leeches; others filter-feed or prey on other invertebrates.egmented wormsmake up the Phylum Annelida. The phylum includes earthworms, leeches, and a large number of mostly marine worms known as polychaetes.