• 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 (~6,900 spp.)- use 1 or more flagella and includes:

    • phytoflagellates (have chloroplasts);
      EXAMPLES: dinoflagellates, Euglena, Volvox (colonial), diatoms

    • zooflagellates (lack chloroplasts)
      EXAMPLES: 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 & reproduce via spores
    EXAMPLES: Plasmodium (malarial parasite)

  • d) Ciliates (~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 (distinct nucleus & membrane-bound organelles);

• 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 & flagella 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 complex osmoregulatory structure: contractile vacuole

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