PUI^ATION WITHOUT MARGINAL SENSE-ORGANS. 



29 



made. This can rarely be accomplished, however, for the returning 

 wave must usually be focused back upon the center in order to be 

 sustained ; and in a wide annulus it is dissipated and returns with 

 too little force to call forth the latent ability of the center to restimu- 

 late the wave. Similarly figures 20, 21, 22, 23, and 25 represent forms 

 which dissipate and confuse the contraction wave, setting up ' ' eddy 

 currents" which weaken the wave and prevent its returning definitely 



A 



C D 



Fig. 3 1 . — A, B, and C, disks having every annulus crossed by radial cuts, but which may 

 be set into sustained pulsation. D, a simple circuit which may be set into sustained 

 pulsation. 



and forcefully to the center. Hence these figures can not be set into 

 sustained pulsation. If, however, we cut partial rings, as in figures 

 2ia and 23a, or convert figure 21 into a shape such as is shown in 

 figure 24, we find no difficulty in setting them into sustained pulsa- 

 tion. In all of these cases the figures oblige the contraction wave to 

 return definitely and forcefully to the center. I could not obtain sus- 

 tained pulsation in a disk cut out of the side of a Medusa as in figures 



