44 JELLY-FISH, STAR-FISH, AND SEA-URCIIIXS. 



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 Medusai 

 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 aflected 

 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 w^as 

 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 duiing 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, 



