PULSATION OF JKLLYFISHES. 



through the circuit. Thus there is normally but one contraction wave 

 which proceeds from its center, travels through the labyrinth of sub- 

 umbrella tissue, and returns to the center whence it came, only to be 

 again augmented and sent forth. 



It is thus the function of the center to reinforce and maintain the 

 contraction wave. This is well shown in a long circuit such as is 

 shown in figure 30, i-in ; where on account of the great length of 

 the circuit the course of the wave may readily be followed by the eye. 

 The outer annuli of the sub-umbrella tissue are more sensitive, and 

 conduct contraction waves * better than do the inner parts of the disk; 



27 a 



26 



Figs. 20, 21, 22, 23, 25, 27a, disks cut so that they can not be set into continuous pulsation. 

 Figs. 2 la, 23a, 24, 26 can be set into sustained pulsation in sea- water. 



and if we touch the disk at A, figure 30, i, the greater part of the con- 

 traction wave takes the short path of least resistance into the interior of 

 the labyrinth, as is shown by the full arrow, and only a very weak 

 wave goes in the direction of the dotted arrow. The strong contraction 



* The sub-umbrella tissue is a good conductor of electricity, but the gelatinous sub- 

 stance of the Medusa is a poor conductor. 



