AbstractThe contraction and relaxation ofVorticella difficilis, V. campanulaandCarchesiumsp. were studied by high speed cinematography. InVorticellait was shown that coiling of the stalk usually started near the zooid and spread downwards; the point of initiation bore no relation to the position of the stimulating electrodes. Contraction took about 5 msec to complete, and the fully contracted animals were 29 ± 3.9% of their original lengths. The zooids were 66 ± 5.0% and the stalks 14 ± 6.0% of their original lengths (V. difficilis). The shortening of the stalk was mostly in the form of coiling. Measurement of the myoneme length demonstrated that its real shortening was less than 10%. Thus the contraction is virtually isometric, producing a helical deformation of the stalk. As the stalk contracts it takes the form of a steeply pitched helix. This change in shape should produce rotational forces on the zooid (torque). Physical models of similar proportions produced about 1.5 revolutions of torque for similar changes in pitch. However during contraction no turning of the zooid was detected, though rotation did occur after the completion of contraction.InCarchesiumthe contraction is not so isometric, the myoneme apparently shortening by 20%. While the coiled shape of the contractedVorticellastalk can be explained by its acentric structures, the stalk ofCarchesiumis much more symmetrical in cross‐section, demonstrating that a high acentricity is not necessary for helical coiling. In all three species there seems to be some separation of the control of zooid and stalk contrac
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