1. 1.

    The carnivorous ciliate Didinium nasutum discharged its extrusive organelles when a strong inward current was injected into the cell in the presence of external Ca2+ ions.

  2. 2.

    In the absence of external Ca2+ ions, the strong inward current produced fusion of the apex membrane of the proboscis.

  3. 3.

    External application of Ca2+ ions after the fusion of the apex membrane produced discharge of the organelles.

  4. 4.

    An increase in Ca2+ concentration around the organelles seems to cause discharge of the organelles.

  5. 5.

    Ca2+ concentration threshold for the discharge of the pexicysts seems to be lower than that for the toxicysts.

  6. 6.

    External CaCa2+ ions were not necessary for discharge of the organelles upon contact with Paramecium.

  7. 7.

    Chemical interaction of the apex membrane with Paramecium membrane may cause intracellular release of Ca2+ ions from hypothetical Ca2+ storage sites around the organelles.

  8. 8.

    A small hyperpolarizing response seen before the discharge upon contact with Paramecium seems to correlate with the chemical interaction.

  9. 9.

    The depolarizing spike response associated with discharge of the organelles is caused by the depolarizing mechano receptor potential evoked by mechanical stimulation of the proboscis membrane by the discharging organelles.

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