1. A method of analysing the parameters of ciliary beat as a function of membrane potential in free-swimming organisms is presented. The method obviates artefacts which may be produced on impaling small organisms with micro-electrodes.

2. Changes in the membrane potential of Paramecium caudatum produce two effects: an increase in frequency of beat of up to (23 ± 3) % mV-1 at 25 °C for both depolarizations and hyperpolarizations which falls to zero within a few minutes, and a change in the direction of ciliary beat of (9 ± 1) degree mV-1 for depolarizations only, which in a spiralling organism do not accommodate to the field.

3. The orientation of P. caudatum towards the cathode in an electric field is due principally to changes in the angle of ciliary beat on the cathodal side of the organism; changes in frequency seem to be relatively unimportant.

4. Axial spinning during normal locomotion in P. caudatum is satisfactorily accounted for by transverse components of ciliary thrust.

5. The oblique orientation of P. caudatum near boundaries is caused by an asymmetry (geometrical or electrical or both) over the surface of the organism.

6. Comparison with observations on extracted organisms indicates that a change in the direction of beat of 9° is associated with an increase in calcium concentration at the ciliary apparatus of about 25 %.

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