1. In forward-swimming Paramecium the direction of metachronal wave propagation is turned progressively clockwise from forward-right to backward-left if the viscosity of the medium is increased to more than 100 cP.2. With increasing viscosity the direction of the power stroke is turned clockwise at a lower rate than the direction of waves. This leads to a gradual transformation of the dexioplectic metachrony toward a symplectic pattern.3. As viscosity is raised the polarization of the ciliary cycle in time and space is Progressively reduced, so that the beat becomes increasingly helicoidal.4. Metachronal coordination gradually breaks down at viscosities of more than about 100 cP, but is retained better at the anterior end of the cell than in more posterior regions.5. At viscosities above 12 cP the left-handed swimming helix of Paramecium is changed into a right-handed helix. This is produced primarily by the viscositydependent clockwise shift in the direction of the power stroke from backward-right to backward-left.6. The frequency of peristomal cilia (32/s. at 20°C) decreases with rising viscosity. Under constant conditions, a posteriorly directed gradient of decreasing frequency can be observed with the stroboscope.7. Raising the viscosity leads to an increase of the average wavelength from 10.7 µm at 1 cP to 14.3 µm at 40 cP. In the same range of viscosity the wave velocity, which is the product of frequency and wavelength, is reduced from 340 to 200 µm/s, since the drop in frequency exceeds the increase in wavelength.8. The wave velocity tends to be stabilized by reciprocal relations between frequency and wavelength, if all other factors are kept constant. However, the wavelength is found to be different in forward-swimming and backward-swimming animals at 40 cP without a change in frequency (14.1 bps; 14.3 compared to 12.7 µm). This is explained if the metachronal wavelength is increased by decreasing polarization of the ciliary cycle.9. A working hypothesis is put forward which explains the origin of a metachronal system by the distribution of forces parallel to the cell surface produced by polarized or unpolarized cycles of ciliary movement.
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