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.
There are about 15,000 species in this phylum. The phylum includes earthworms, leeches, and marine worms known as polychaetes. Species in this phylum have metameric bodies. Metameric bodies are divided into segments called annuli.The annuli are covered in tiny hairs called setae. Setae help the worms move.he phylum includes earthworms, leeches, and marine worms known as polychaetes. Species in this phylum have metameric bodies.