Kingdom Protista is a part of domain (also called "superkingdom") eukarya, which includes all organism composed of eukaryotic cells. Protista, the most diverse kingdom, includes more than 200,000 species (Corliss, 2001). It consists of eukaryotic microogranisms and their immediate descendants; some organisms in this kingdom may be more closely related to eukaryotes in other kingdoms (Plantae, Fungi, Animalia) than they are to one another (Tree of Life 1998). Recently is has been referred to as "protoctista" rather than "protista" because the term protist has come to suggest a unicellular organism (Margulis 1998). .
Archaeprotists, a Greek term which in English means "old protists," are all anaerobic and lack mitochondria. These organisms were once in a phylum called Zoomastigina, but this phylum has been divided into Archaeprotista and Discominochondriates, which have mitocondria. .
Archamoebae, one of three classes of phylum Archaeprotista, is grouped into two subclasses and includes free-living, marine and freshwater protists. Subclass Pelobiontae includes one genus, Pelomyxa, large cells which lack almost every characteristic of eukaryotic cells except the existence of a membrane-bound nucleus; these cells' nuclei divide like bacterial nucleoids. Members of the Mastigamoeba subclass are unicellular, asexual heterotrophs that may be either parasitic or free-living. .
Class Metamonada consists of unicellular organisms in which the nuclei are attached to the undulipodia (a cilium or flagellum whose purpose is locomotion or feeding) by rhizoplasts (also known as nuclear connectors. There are three subclasses of Metamonada: Diplomonadida, Retortamonadida, and Oxymonadida. Giardia lamblia, which adheres to the intestines and causes Giardias, is included in Diplomonadida. Many members of Retortamonadida live in the gastrointestinal tracts of mammals as well; they use an undulipodium for feeding.