44. JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
occurrence of the stimulus. As this extraordinary 
difference in the latent period exhibited by the 
same animal towards different kinds of stimuli 
appeared to me a matter of considerable interest, 
I was led to reflect upon the probable cause of the 
difference. It occurred to me that the only respect 
in which luminous stimulation of the Medusz 
differed from all the other modes of stimulation I 
had employed consisted in this—that, as proved by 
my previous experiments on Sarsia, which I repeated 
on Tiaropsis, luminous stimulation directly affected 
the ganglionic tissues. Now, as in Tiaropsis poly- 
diademata luminous stimulation differed from all 
the other modes of stimulation in giving rise to an 
immensely longer period of latency, I seemed here 
to have an index of the difference between the 
rapidity of the response to stimuli by the contractile 
and by the ganglionic tissues respectively. The 
next question, therefore, which presented itself was 
as to whether the enormous length of time occupied 
by the process of stimulation in the ganglia was 
due to any necessity on the part of the latter to 
accumulate the stimulating influence prior to origi- 
nating a discharge, or to an immensely lengthened 
period of latent stimulation manifested by the 
ganglia under the influence of light.* This is an 
* The period of latent stimulation merely means the time after 
the occurrence of an excitation during which a series of physiolo- 
gical processes are taking place, which terminate in a contrac- 
tion; so that, whether the excitation is of a strong or of a weak 
intensity, the period of latent stimulation is not much affected. 
The above question, therefore, was simply this—Does the pro- 
longed delay on the part of these ganglia, in responding to light, 
