136 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
tween A and B will not respond to the discharge 
of B; while the tissue included between B and C, 
not having been just previously in contraction, 
will respond. And conversely, of course, if the con- 
traction-wave had been travelling in the opposite 
direction. 
Seeing that this explanation is the only one 
possible, and that it moreover follows as a deduc- 
tive necessity from my experiments on stimulation, 
I think there is no need to detail any of the further 
experiments which I made with the view of con- 
firming it. But the following experiment, devised 
to confirm this explanation, is of interest in itself, 
and on this account I shall state it. Having pre- 
pared a contractile strip with a single remaining 
lithocyst at one end, I noted the rhythm exhibited 
by this lithocyst, and then imitated that rhythm 
by means of single induced shocks thrown in with 
a key at the other end of the strip. The effect of 
these shocks was, of course, to cause the contraction- 
waves to pass in the direction opposite to that in 
which they passed when originated by the litho- 
cyst. Now I found, as I had expected, that so long 
as I continued exactly to imitate the rate of gang- 
lionic rhythm, so long did the waves always pass 
in the direction B A—A being the lithocyst, and B 
the other end of the strip. I also found that if I 
allowed the rate of the artificially caused rhythm - 
to sink slightly below that of the natural rhythm, 
after every one to six waves (the number depend- 
ing on the degree in which the rate of succession of 
my induction shocks approximated to the rate of 
