Rhythmical Pulsation in Scyphomedusce. 119 



in fig. 3, A-C, we see how a single broad ring may be finally divided by two 

 annular cuts into three separate rings, all of which remain in sustained pul- 

 sation. In this case ^ marks the stimulated point whence the contraction- 

 wave started, yet the two inner circles which are finally isolated from the 

 point ,? continue to transmit and maintain the wave. 



Similarly in fig. 3, D-I, we may completely isolate the stimulated point and 

 prevent its sending out any stimuli, yet the narrow inner and outer annuli, 

 made from the original broad ring, still remain in pulsation. 



Fig. 3, J-L illustrate the same point by showing that by a series of cuts 

 we may obtain two independent pulsating circuits in the place of the orig- 

 inal simple ring-circuit. In this case it is evident that the original center 

 of stimulation can be in but one of these circuits, yet both can remain in 

 pulsation. 



It is remarkable that these isolated circuit-waves, moving constantly in 

 one direction through a circuit, are not met tcvV/; in nature. Each pulsation 

 of the heart, or of the medusa, is a thing separate and distinct from the con- 

 traction which preceded or from that which is to follow it. Indeed, the 



FIG. 4. Showing that under normal conditions 

 interference of contraction-waves coming in 

 opposite directions prevents a rotary wave 

 from being entrapped in the circuit. 



heart, or pulsating medusa, contains within itself the means to prevent any 

 single pulsation-\vave from coursing constantly in one direction through the 

 tissue. In the scyphomedusa, for example, the pulsation-stimuli originate 

 in the marginal sense-organs, and the fastest-working sense-organ controls 

 the rate of the pulsations. For example, the course of events in the case 

 of each separate contraction is shown for Cassiopea in figure 4, where A is 

 the sense-organ which has originated a contraction-wave. The wave of 

 contraction spreads out on both sides of A and the wave of each side travels 

 half the wav around the subumbrella, where it meets with its fellow coming 



