192 CO-ORDINATION OF BEAT 



be present in the recovery phase in some cases (e.g. Fig. 52), and 

 this may be sufficient to maintain the rhythm. Where such 

 metachronism is found over large areas of ciliated surface, e.g. 

 on the phar}^ngeal epithelium of the frog and probably the 

 vestibular ciliature of Paramecium, it is important that cilia at 

 right angles to the direction of travel of the metachronal v^^aves 

 beat synchronously, or nearly so, presumxably under the influence 

 of mechanical interaction. 



M. W. 



Fig. 52. Diagram of a metachronal wave of the vestibular 

 cilia of Paramecium. The metachronal wave is moving towards 

 the right, and the effective stroke is towards the left. There 

 is a possibility of mechanical interaction between the cilia in 

 the recovery stroke, but this is less likely in the effective stroke. 



Narrow tracts of such ciliated surfaces could give either 

 antiplectic bands of cilia, if the tract is composed of short 

 diaplectic rov^s and long orthoplectic rows, e.g. the frontal cilia 

 of Mytiliis gill, or diaplectic bands of cilia with short orthoplectic 

 rows and long diaplectic rows, e.g. lateral cilia of Mytilus gill. 

 In both cases the metachronal waves could originally have been 

 mediated by mechanical interaction, and could have been modified 

 for co-ordination by neuroid impulses by some mechanism like 

 the stepwise conduction suggested earlier. 



The ideas expressed here are almost entirely speculative, but 

 may form some basis for experimentation. 



6. Ciliary Reversal 



Change in the beat direction of cilia is known to occur in the 

 ciliated protozoa and Opalina, and many experiments have been 

 carried out to discover the conditions under which it takes place. 



