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.
Phylum Chordata is the only phylum where all the members have a  backbone.   However, there is more to it than that, especially given that there  are two invertebrate ch â¦ ordate groups-Urochordata (tunicates) and  Cephalachordata (lancelets) - whose backbone is not in the form of  actual vertebrae.