The demarcation of protist kingdoms is reviewed, a complete revised classification
down to the level of subclass is provided for the kingdoms Protozoa, Archezoa, and
Chromista, and the phylogenetic basis of the revised classification is outlined. Removal
of Archezoa because of their ancestral absence of mitochondria, peroxisomes, and Golgi
dictyosomes makes the kingdom Protozoa much more homogeneous: they all either have
mitochondria and peroxisomes or have secondarily lost them. Predominantly phagotrophic,
Protozoa are distinguished from the mainly photosynthetic kingdom Chromista (Chlorarachniophyta,
Cryptista, Heterokonta, and Haptophyta) by the absence of epiciliary retronemes (rigid
thrust-reversing tubular ciliary hairs) and by the lack of two additional membranes
outside their chloroplast envelopes. The kingdom Protozoa has two subkingdoms: Adictyozoa,
without Golgi dictyosomes, containing only the phylum Percolozoa (flagellates and
amoeboflagellates); and Dictyozoa, made up of 17 phyla with Golgi dictyosomes. Dictyozoa
are divided into two branches: (i) Parabasalia, a single phylum with hydrogenosomes
and 70S ribosomes but no mitochondria, Golgi dictyosomes associated with striated
roots, and a kinetid of four or five cilia; and (ii) Bikonta (16 unicellular or plasmodial
phyla with mitochondria and bikinetids and in which Golgi dictyosomes are not associated
with striated ciliary roots), which are divided into two infrakingdoms: Euglenozoa
(flagellates with discoid mitochondrial cristae and trans-splicing of miniexons for
all nuclear genes) and Neozoa (15 phyla of more advanced protozoa with tubular or
flat [usually nondiscoid] mitochondrial cristae and cis-spliced spliceosomal introns).
Neozoa are divided into seven parvkingdoms: (i) Ciliomyxa (three predominantly ciliated
phyla with tubular mitochondrial cristae but no cortical alveoli, i.e., Opalozoa [flagellates
with tubular cristae], Mycetozoa [slime molds], and Choanozoa [choanoflagellates,
with flattened cristae]); (ii) Alveolata (three phyla with cortical alveoli and tubular
mitochondrial cristae, i.e., Dinozoa [Dinoflagellata and Protalveolata], Apicomplexa,
and Ciliophora); (iii) Neosarcodina (phyla Rhizopoda [lobose and filose amoebae] and
Reticulosa [foraminifera; reticulopodial amoebae], usually with tubular cristae);
(iv) Actinopoda (two phyla with axopodia: Heliozoa and Radiozoa [Radiolaria, Acantharia]);
(v) Entamoebia (a single phylum of amoebae with no mitochondria, peroxisomes, hydrogenosomes,
or cilia and with transient intranuclear centrosomes); (vi) Myxozoa (three endoparasitic
phyla with multicellular spores, mitochondria, and no cilia: Myxosporidia, Haplosporidia,
and Paramyxia); and (vii) Mesozoa (multicells with tubular mitochondrial cristae,
included in Protozoa because, unlike animals, they lack collagenous connective tissue).