188 



CO-ORDINATION OF BEAT 



The higher the viscosity, the faster the co-ordination, apparently 

 because the viscous interaction is increased. Since the amplitude 

 of beat is progressively decreased with increase of viscosity, 

 a point is eventually reached where the viscous interaction is no 

 longer effective and the co-ordination breaks down. It seems 

 probable that this explanation is the correct one, for slight 

 fluctuations of the frequency occur from time to time, and, while 

 slight decreases of frequency in the normal medium are accom- 

 panied by small ^ increases in wavelength (i.e. wave velocity 



Cilium 



Ciliutn 

 bate 



Fig. 50. A diagrammatic representation of the theory of 

 metachronal co-ordination proposed in the text. The spon- 

 taneous build-up of excitation in the pacemaker cilium 

 determines the frequency of beat of that cilium, and other cilia 

 beat at constant intervals after this as a result of the conducted 



impulses (from Sleigh, 1957). 



constant), similar decreases of frequency in a very viscous medium 

 are accompanied by a decrease in wavelength. 



Considerable evidence concerning the metachronism of the 

 lateral cilia of Mytilus and related lamellibranch molluscs has been 

 accumulated, particularly by Gray (1930), Lucas (1932a, b), 

 Aiello (1960) and Gosselin and associates (1958, 1961). Through- 

 out these studies it has been the common experience that any 

 factor which affects the frequency also changes the wave velocity. 

 In drugs like veratrine or serotonin for example the frequency 



