THE FORM OF BEAT OF CILIA 



141 



cilium returns to the position required to commence a new 

 effective phase. For maximum efficiency of the effective stroke, 

 the cilium should present as large a surface as possible perpendi- 

 cular to, and yet moving parallel with, the required direction of 

 movement of the medium. In the recovery phase the cilium 

 should present the smallest possible surface to the moving medium, 

 and is therefore trailed through the water (see Fig. 37). 



The beating of the membranelles of Stentor is easily observed 

 under stroboscopic illumination, especially at the edge of the 

 peristome where the cilia appear as in Fig. 37. The sequence 



Fig. 38. The sequence of movements of a membranelle of 



Stentor (from Sleigh, 1960). 



of movements of these cilia in one complete cycle is shown in 

 Fig. 38. The effective phase starts with stage 1, and has nearly 

 ended by stage 3. The wave of flexure which causes the recovery 

 phase has started to move up the cilium before the effective phase 

 has been completed, and, by the time it has reached the tip of 

 the cilium, the cycle of beat is complete. It is interesting that 

 the effective and recovery phases are not separate parts of the 

 beating cycle, but that they overlap, merging together in a single 

 movement that could be caused by a single wave of contraction 

 passing up the cilium from the base. 



