1) Very heterogeneous group (35 phyla!). Includes important human parasites (malaria, sleeping sickness, beaver fever, dysentery) and some of the most structurally complex cells on the planet
2) Actual phylogenetic relations & classification hotly debated; four 'groupings of convenience' based mainly on locomotion:
a) Flagellates (approx. 6,900 spp.)- use 1 or more flagella and includes:
b) Sarcodines (approx. 12,000 spp.)- use pseudopodia (lobe- or spine-like outgrowths) of ectoplasm; some produce spectacular skeletons
EXAMPLES: amoebas, slime molds, foraminifera, radiolarians
c) Sporozoans (approx. 4,000 spp.)- most move by body flexion (some use flagella or pseudopods); all are parasitic & reproduce via spores
EXAMPLES: Plasmodium (malarial parasite)
d) Ciliates (approx. 7,200 spp.)- use cilia; includes the most structurally complex protozoans; come in a remarkable variety of forms
EXAMPLES: Paramecium, Stentor
3) Defining characters: a) unicellular (some colonial); b) eukaryote (membrane-bound nucleus & 1 or more organelles, mitosis)
4) Other notable features of these single-celled organisms: small size (0.001 - 5mm); symmetry (all types); some rigid skeletal structures (cellulose plates, carbonate or silicate shells, attached bits); also:
a) locomotory structures include: pseudopodia (e.g., rhizopodia or axopodia), cilia, flagella
cilia and flagella are ultrastructurally identical ("9 doublet +2 singlet" microtubule arrangement, basal body)
flagella: long, few per cell, continuous regular undulation
cilia: short, many per cell, separate power & recovery stroke
b) may have a complex osmoregulatory structure: contractile vacuole