The Mechanism of Ciliary Movement. 107 



exactly as is the case in a fishing line during the backward movement of a 

 cast. As the cilium moves back it loses its limpness, and at the end of the 

 recovery stroke possesses a considerable degree of rigidity (fig. 3). 



When movement is taking place fairly quickly the cilium does not appear 



b 



Fig. 3. — Diagram illustrating the form of the terminal cilia of Mytilus during («) the 

 effective and (b) recovery beats. 



to straighten out completely at the end of the recovery stroke, but moves 

 forward in the form of a sickle. During the final phase of the effective beat 

 the hooked shape is always lost and re-develops during the recovery stroke. 



It should be mentioned that the effective stroke is always quicker than the 

 recovery stroke. When movement is very rapid it is impossible to see the 

 cilium during the effective beat. 



The change in the consistency of the cilium during the two phases of its 

 beat appears to be an observation of considerable significance, but does not 

 appear to have been commented on by other observers. The illustrations 

 given in Verworn's (31) text-book of the cilia of Urostyla grandis appear to 

 indicate the same phenomenon. The only detailed description of the move- 

 ment of large cilia is that by Williams (32) of the cilia of a molluscan larva, 

 which clearly indicates a difference in the elastic properties of the cilia 

 during the two phases of the beat (fig. 4). 



The effect of stimulation on a muscle fibre has been compared by 

 Bayliss (1) to the conversion of a stretched lead spring to a stretched steel 

 spring, so that the excited fibre is capable of expending energy in the form 

 of work. The cilia of Mytilus, and to a still greater extent the cilia of 

 Ctenophores, can be compared, with equal justice, to bent strips of lead and 

 steel wire. It seems fairly certain that the energy which is expended by the 

 cilium is stored as tension energy. 



Let us now consider the point of origin of the stimulus to which the 



