182 CO-ORDINATION OF BEAT 



chronal waves could start and stop at any part of an epithelium, 

 and did not necessarily start at any particular pacemaker. It 

 seems likely on more recent evidence that both of these 

 hypothetical mechanisms suggested by Gray may occur naturally ; 

 Kitching (1961) has also hinted that this may be true. 



It is evident from studies on Stentor (Sleigh, 1956a) that the 

 fundamental variables of a group of metachronally co-ordinated 

 cilia are the frequency and amplitude, which describe the beating 

 activity, and the metachronal wave velocity which describes the 

 co-ordination activity. Metachronal wavelength is probably not 

 a fundamental variable of the system. 



The rate of transmission of metachronal waves has been 

 accurately measured by two methods. The more direct of these 

 involves the use of a cine camera which exposes a known number 

 of frames per second; the distance travelled by a wave in a known 

 time can be measured from a film of the active cilia. The second 

 method is somewhat more convenient in use; it involves the 

 measurement of frequency with a stroboscope, and the simul- 

 taneous photographing of the metachronal waves, either using 

 the stroboscopic illumination or else a separate flash-light. The 

 wavelength of the metachronal waves may be measured from the 

 photographs, and the product of wavelength and frequency is the 

 metachronal wave velocity. Measurements made in these ways are 

 included in Table 13, where it is seen that our information is 

 rather limited. 



Let us consider two of these examples of metachronism in 

 detail, the slow symplectic metachronism of Opalina and the 

 fast dexioplectic metachronism of the membranelles of Stentor, 

 both of which are fairly well known. In Opalina the frequency 

 and the wave velocity of the body cilia vary in direct proportion, 

 so that variations in frequency along the length of the body are 

 accompanied by changes in metachronal wave velocity (Okajima, 

 1953). In Stentor, small changes in the frequency are not 

 necessarily accompanied by any changes in the metachronal wave 

 velocity. If the viscosity of the medium is slightly increased, 

 the frequency and metachronal wave velocity of Opalina cilia 

 are reduced (Potter, 1960; Kitching, 1961), while in Stentor the 

 frequency is reduced and the metachronal wave velocity remains 

 unchanged. A further increase in the viscosity around Stentor 



