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.
Rhizopoda e.g. Amoeba), (2) ciliate â use cilia (Phylum Ciliophora e.g. Paramecium) and (3) flagellate â use flagella (Phylum Sarcomastigophora e.g. Trypanosoma). In addition, some protozoans also possess a food vacuole which is used to digest and absorb ingested materials and contractile vacuoles that function in expelling water.