THE MECHANISM OF METACHRONISM 183 



membranelles results in a large increase in the metachronal wave 

 velocity to well over 2 mm/sec in some cases. 



When studying the movement of Opalina cilia, the observer 

 gains the impression that their beating activity could be forced 

 more or less into phase by the viscous interaction between cilia 

 moving in the same direction, and that this interaction may act 

 throughout the beat. This has been pointed out by Kitching 

 (1961), who also believes that the observations made on Opalina 

 cilia are those that one would expect if the cilia were co-ordinated 

 mechanically by viscous interaction. Thus, when the beat is 

 slowed down, the wave velocity is also reduced. Where the 

 frequency is reduced in viscous media, the amplitude of beat is 

 also reduced, so that the wave velocity is reduced by more than 

 the frequency, and the wavelength is decreased. One surprising 

 feature of this metachronal transmission in Opalina is that the 

 rate of transmission is of the same order whatever its direction, 

 e.g. the rate of propagation of the waves across the ciliary rows 

 may be about twice that along the rows, while the separation of 

 the cilia is some six times as great; the faster transmission across 

 the body probably results because the cilia beat faster in this 

 direction, and not because of the greater separation of the cilia. 

 It appears that the metachronal wave velocity is directly controlled 

 by the rate of beat of the cilia, and that even when they are at 

 their maximum separation they will still be close enough to excite 

 each other mechanically. 



The undulatory waves of various organelles are known to be 

 forced into phase, e.g. Gray (1928) reported this from swimming 

 spirochaetes and it is well known in the tails of spermatozoa; it 

 has been calculated by Taylor (1951) that it is much easier for 

 such tails to beat in unison. Examination of a metachronal wave 

 of Opalina (Fig. 47 and PL XVI I lb) shows that a similar effect 

 could be operating here, where, although each cilium of the 

 metachronal wave lags or leads on its neighbours in the plane 

 of beat, the cilia at right angles to this plane beat synchronously. 

 It may be a little difficult to visualise why mechanical interaction 

 should be able to give synchrony at right angles to the beat rather 

 than metachrony, when one might expect the viscous or other 

 mechanical interaction to be less at right angles to the beat. 

 After disturbance of the animal, waves appear simultaneously 



