1) A very heterogeneous group (35 phyla!); include some important parasites of humans: malaria, sleeping sickness, beaver fever
2) Actual phylogenetic relations & classification hotly debated; four 'groupings of convenience' based mainly on locomotion:
a) Flagellates (~6,900 spp.)- use 1 or more flagella; includes both phytoflagellates (have chloroplasts*) and zooflagellates (lack chloroplasts)
EXAMPLES: dinoflagellates*, Euglena*, Volvox (colonial)*, diatoms*, choanoflagellates, Trypanosoma, Giardia
b) Sarcodines (~12,000 spp.)- use pseudopodia (lobe- or spine-like outgrowths) of ectoplasm; some produce spectacular skeletons
EXAMPLES: amoebas, slime molds, foramenifera, radiolarians
c) Sporozoans (~4,000 spp.)- most move by body flexion (some use flagella or pseudopods); all are parasitic
EXAMPLES: Plasmodium (malarial parasite)
d) Ciliates (~7,200 spp.)- use cilia; includes the most structurally complex protozoans that come in a remarkable variety of forms
EXAMPLES: Paramecium, Stentor
3) Defining characters: a) unicellular (some colonial); b) eukaryote (distinct nucleus & membrane-bound organelles);
4) Other notable features: small size (1µm - 5mm); symmetry (all types); locomotory structures (cilia, flagella & pseudopodia); some rigid skeletal structures (cellulose plates, carbonate or silicate shells, attached bits); osmoregulatory structure (contractile vacuole)