190 CO-ORDINATION OF BEAT 



diaplectic or antiplectic metachronism in which the ciha are 

 widely spaced along the line of metachronal transmission, and the 

 transmission is rapid, then co-ordination is probably internal and 

 of the neuroid type. In addition to Stentor membranelles, it 

 seems that we can here include the metachronism of ctenophore 

 comb -plates, for Verworn (1891) found that metachronal waves 

 were transmitted past a plate whose movement was restrained so 

 that it could not touch any other plate. On the other hand, it 

 seems likely that cilia which are close together and show a slow 

 metachronal co-ordination depend on mechanical transmission 

 through the medium, especially where the metachronism is 

 symplectic. Experimentation with viscous media is a valuable 

 tool in these investigations, but it must be remembered that 

 different results must be expected from different metachronal 

 patterns, and it is in those cases where a change in viscosity does 

 not affect the metachronal transmission that an internal trans- 

 mission mechanism is likely. 



5. Some Thoughts on the Origin and Development 



of Metachronism 



Small groups of flagella may beat synchronously because of 

 viscous interaction betAveen their undulating shafts. Undulating 

 flagella may occur in large numbers over considerable surface 

 areas, where they are arranged so that they lie roughly parallel 

 to the surface which bears them, as in the trichonymphid 

 flagellates. It is not surprising that, in these cases, the same 

 viscous interaction which produces synchrony between the shafts 

 which lie side by side, also produces metachronism of the 

 flagellar beat in that flagella further back on the body start to 

 beat later than those further forward on the body (see Fig. 51). 

 These flagella are more or less fixed in their orientation on the 

 body, and can only produce forward movement; changes of 

 direction are caused by the '* steering " action of the anterior 

 end of the body. 



The cilia of Opalina are shorter than the flagella of tricho- 

 nymphids, yet the appearance of waves of beating organelles is 

 very similar in the two types, as can be seen by comparison of 

 Fig. 51 with Fig. 47. Waves of contraction passing along the 

 shafts of cilia must cause viscous interaction with those that lie 



