INHIBITION AND ACTIVATION 197 



to beat when the beat of the somatic cilia is reversed. The 

 alternative suggestion that cirri are normally inactive because of 

 inhibition is also a possibility, for Taylor found that cirri isolated 

 v^ith small masses of protoplasm are continually active, but there 

 is no proof in this finding because the surface membrane and 

 protoplasm may have lost their normal constitution. It is worth 

 mentioning again that the co-ordination of the cirri is not 

 metachronal co-ordination, although the excitation of the beat of 

 cirri may employ the mechanism used in at least some types of 

 metachronism. 



7. Inhibition and Activation 



Almost all cilia and flagella seem to be under the control of the 

 organism, and such control may involve activation, inhibition and 

 sometimes reversal. Examples of these types of control were 

 quoted in Chapter III; the interesting example of the control of 

 ciliary beat in Euplotes has just been mentioned, and here as in 

 many other cases it is difficult to be certain whether activation or 

 inhibition is involved. 



The stoppage of cilia that are normally continuously active is 

 almost certain to be a genuine case of inhibition, and it is 

 interesting that inhibition of the movement of membranelles of 

 Stentor occurs at the same instant as reversal of beat of the 

 body cilia. It may be possible, therefore, that complete inhibition 

 of beat may be controlled by the same mechanism as reversal, 

 and that the membranelles respond to depolarization by ceasing 

 their beat, while the body cilia reverse. Perhaps the thigmotactic 

 inhibition of Paramecium similarly involves a depolarization or 

 hyperpolarization of the membrane, and may be an extension of 

 the change that causes reversal of beat. 



Inhibition of ciliary activity in metazoa is believed to result 

 from nervous activity, in gill cilia of Ostrea and locomotory cilia 

 of molluscan veliger larvae for example (see pp. 121-124). In 

 no case is the mechanism known. 



Activation of cilia by the stimulation of cilia that are normally 

 quiescent or increase in the activity of slowly beating cilia is 

 believed to occur in the cirri of hypotrichs and also in metazoans, 

 e.g. the lateral cilia of Mytilus gill (p. 122). This phenomenon 

 is again likely to bear a close relationship to the normal excitation 



