THE MECHANICS OF CILIARY MOVEMENT 219 



spending increase in the offset of the elbow. This gives us a 

 possible explanation of the observed behaviour of the spirillum 

 when in motion. This force is not exerted when the spirillum 

 is at rest. The natural condition of a spirillum at rest is the 

 straight rod shape, and many are observed to have this shape ; 

 but we may suppose that the continuous assumption of the 

 spiral shape when in motion, accompanied probably by a 

 very slight increase in the outer membranous layers, has made 

 most of the organisms retain the spiral form permanently. 



When the ciliary impulse has been communicated to the 

 body of the organism, and the latter has assumed the spiral 

 form, there will be a similar torque between the body of the 

 organism and the surrounding liquid medium, and the extent 

 of this reaction will probably control the degree of spiral for- 

 mation. As the body of the spirillum moves through the 

 medium the reaction pressures of the liquid upon the organism 

 will have a resultant which will be tangential to the body, 

 and so a rotatory movement will be imparted to the body in 

 addition to its translatory motion. 



It is clear from the above dynamical treatment of the 

 subject that a helicoid movement of the cilium ought to 

 produce translatory and rotatory movements in the spirillum. 

 As such movements do, in fact, take place in the spirilla, the 

 observations of the helicoid whiplike action of the cilium 

 described by Buder (p. 2io) are supported. 



The following hypothesis is suggested in explanation of the 

 ciliary movement. The impulse, a vital action, comes in the 

 first case from the end of the body to which the cilium is 

 attached, and is similar in its nature to the elbow action in the 

 above experiment. The vital action is communicated to the 

 cilium, which then takes up the rotating helicoid form, thereby 

 giving to the organism a rotational and translatory movement. 



The following facts lend support to this hypothesis : — 



1. The cilium is not an independent unit, but is in pro- 

 toplasmic union with the cell. 



2. It has been proved that the sensitive seat of the organism 

 regulating movements in response to changes in the intensity of 

 light lies at the end of the body to which the cilium is attached. 



