abed 



A A A A 



DORSAL DORSAL LEFT VENTRAL 



Fig. 2. Diagram illustrating the effect of axial spin on the movement 

 of helical waves. The movement of the waves is indicated relative to a 

 smooth cylinder which maintains a constant distance behind the animal's 

 head, which is flattened dorsoventrally. Dotted areas show regions of the 

 body lying over the cylinder. The wavelength is four units. 



In Fig. 2a, the organism is at rest and presents its dorsal surface to the 

 observer; the most anterior wave crest A is at zero on the cylinder. If each 

 element of the body executes one-half of a complete contractile cycle while 

 the body is completely restrained from spinning, each wave crest travels 

 one-half wavelength backwards along the cylinder (Fig. 2b); if the organ- 

 ism as shown in Fig. 2a spins through 90° whilst undergoing one-half of 

 a contractile cycle, the wave crests travel only one-quarter of a wavelength 

 as in Fig. 2c; if the body spins 180° during each half-contractile cycle, the 

 wave crests remain stationary as in Fig. 2d. 



Note that for any given element the phase of contraction is exactly the 

 same in Fig. 2b-d, viz., half-cycle ahead of that in Fig. 2a; in Fig. 2b, the 

 organism still presents its dorsal surface to the observer at the end of the 

 half-cycle contraction, whereas it presents its left surface in Fig. 2c, and 

 its ventral surface in Fig. 2d. In each case the direction of "spin" is opposite 

 to that in which each element of the system moves round the circumscribed 

 cylinder. 



