ORIGIN AND DEVELOPMENT OF METACHRONISM 



191 



underneath, and which have an origin further back on the body 

 surface. These cilia have no fixed orientation of beat, and, by 

 changes in the direction of beat, the direction of metachronal 

 v^ave transmission may also be changed (see p. 193). Changes 

 in direction of movement that are caused by this means may be 

 much more difficult in a longer organelle, and, although the 

 viscous interaction is reduced in the shorter organelles, the cilia 

 of Opalina may have been shortened in order that the direction 

 of movement of this fairly rigid body can be changed. It is not 

 known, at present, how widespread the symplectic pattern of 

 ciliary metachronism may be, and no other examples of this 

 pattern are well enough known for any comparison to be made 

 with Opalina cilia. 



Anterior v Posterior 



Fig. 51. Diagram of flagella of the posterior region of a 

 trichonymphid flagellate. The flagella may be forced to beat 

 synchronously by viscous interaction, and flagella further back 

 on the body will start their beat later than those further forward, 



i.e. they show metachronism. 



The next stage in the development of metachronism is more 

 difficult to visualize, but it seems possible that the antiplectic 

 pattern preceded the diaplectic patterns, since the former is more 

 nearly related to the primitive symplectic type. If the cilia of 

 Opalina were shorter and spaced further apart, so that they could 

 move through a large angle in the eflPective stroke without 

 interfering with other cilia, then an antiplectic pattern could 

 easily be produced by a small adjustment of excitation intervals 

 (see p. 158). Thus, a change from a phase difference of about 

 one-sixth in the approximately symmetrical beat of Opalina cilia 

 to a much smaller phase difference of say one-twentieth in the 

 asymmetrical beat of the vestibular cilia of Paramecium (Fig. 52), 

 could result in a change from the symplectic to the antiplectic 

 patterns. Although antiplectic metachronism may require co- 

 ordination through the basal protoplasm, viscous interaction may 



