148 MOVEMENT OF CILIA AND FLAGELLA 



usually present, and this may resist compression within a modified 

 flagellar organelle; some people regard this additional material 

 as being contractile, and its position does suggest this. 



As an alternative hypothesis, it is possible that rigidity is 

 provided by all of the fibrils of the axial bundle itself. If these 

 have a spiral or tubular sub -structure, then they may be resilient 

 and elastic, except during contraction when they may be under 

 the influence of a plasticiser (e.g. ATP) and capable of shortening 

 and extension. 



Carter (1924) found that the eflFective stroke of Mytilus latero- 

 frontal cilia could be arrested by a needle, while in the recovery 

 stroke the cilium slipped past the needle. This led to the idea 

 that cilia were stiflF in the effective stroke and limp in the recovery 

 stroke. However, Harris (1961) has pointed out that Carter's 

 results could also be explained if it was assumed that the cilium 

 was stiff throughout the cycle. This, on the whole, seems the 

 more reasonable hypothesis, and no good evidence is available to 

 refute it. 



Several authors, e.g. Bradfield (1955) have suggested that the 

 two central fibrils may be responsible for the conduction of the 

 stimulus for the contraction of the peripheral fibrils, and yet 

 others have suggested that the membrane may be responsible for 

 this conduction. While either of these ideas could be correct, 

 it seems that the nine contractile fibrils themselves are the most 

 likely candidates. In many sensory cilia the two central fibrils 

 are missing, and yet the cilia are still capable of carrying informa- 

 tion, probably by the means normally used to conduct the 

 contraction stimulus (and involving fibrillar contraction ?). The 

 sensory cilia possess an outer membrane, and, in view of the fact 

 that the membrane is the important conducting structure in nerve, 

 it might hvae the same function here. In many sensory cilia 

 the conducted impulse seem.s to arise in the peripheral fibrils or 

 in structures continuous with them, e.g. in the retinal rod the 

 flattened sacs of the photoreceptor region appear to be in 

 continuity with the peripheral fibrils of the connecting cilium, 

 while in cilia which detect mechanical vibrations distortion of the 

 peripheral fibrils may initiate the sensory impulse (see pp. 32 ff.). 



What function can then be allocated to the two central fibrils ? 

 We know that in most motile cilia the two central fibrils are 



