1. 1.

    Vorticella contracted (i.e. shrinkage of the cell body and coiling of the stalk) in response to being touched with a microneedle.

  2. 2.

    The threshold excursion of the microneedle required to evoke a contraction was smallest on the cell body. On the stalk, it was larger in regions farther from the cell body.

  3. 3.

    Hitting the stalk did not evoke a contraction if the stalk was mechanically clamped in a region between the site of the hit and the cell body.

  4. 4.

    Rapidly drawing a small portion of the cell body into a micropipette by suction evoked a contraction, whereas a similar stimulus applied to the stalk did not.

  5. 5.

    The threshold depression of the surface membrane of the cell body required to evoke a contraction was inversely proportional to the rate of depression.

  6. 6.

    Tilting the stalk of a specimen detached from its substratum evoked a contraction. The threshold degree of tilting was inversely proportional to the angular velocity of tilting.

  7. 7.

    Tilting the stalk is assumed to cause a localized depression of the surface membrane of the cell body around the stalk.

  8. 8.

    We concluded (1) that the cell body is mechanosensitive and is the site where contractions are initiated; (2) that hitting the stalk evokes a contraction because the hit exerts a mechanical effect on the cell body; and (3) that the rate of expansion of the membrane of the cell body is responsible for activation of a hypothetical mechanoreceptor mechanism which initiates a contraction.

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