Rhythmical Pulsation in Scyphomeduse. 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 s marks the stimulated point whence the contraction- 
wave started, yet the two inner circles which are finally isolated from the 
point s continue to transmit and maintain the wave. 
Similarly in fig. 3, D-1, 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-1 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 reniain in 
pulsation. 
It is remarkable that these isolated circuit-waves, moving constantly in 
one direction through a circuit, are not met with 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 
Fic. 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-wave 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 way around the subumbrella, where it meets with its fellow coming 
