470 



Mr. G. J. Eomanes on the 



[Jan. 11, 



stimulating influence of light is exerted solely through the sense-organs, 

 is proved by the fact that when these are removed the swimming-bell, 

 though still able to contract spontaneously, no longer responds to lumi- 

 nous stimulation ; but if only one marginal body be left in situ, or if the 

 severed margin alone be experimented upon, unfailing response to this 

 mode of stimulation may be obtained. 



Tiaropsis ]Dolycliademata responds to luminous stimulation in the same 

 peculiar manner as it responds to all other kinds of stimulation, viz. by 

 performing the spasmodic movements described in my previous paper. 

 But the period of latency in this species is very much longer in the case 

 of luminous than in that of other modes of stimulation ; for while this 

 period is, so far as the eye can judge, quite as instantaneous as it is in 

 the case of Sarsia when the stimulus supplied is other than luminous, in 

 response to light the characteristic spasm does not take place till slightly 

 more than a second has elapsed after the first occurrence of the stimulus. 

 Now, as my experiments on Sarsia proved that the only respect in which 

 luminous stimulation differs from other modes of stimulation consists in its 

 being exclusively a stimulation of central nervous matter, we have evidence, 

 in the case of Tiaroi^sis, of an enormous difference between the rapidity of 

 response to stimuli by the contractile and by the ganghonic tissues re- 

 spectively. The next question, therefore, is as to whether the enormous 

 length of time occupied by the process of stimulation in the ganglia is 

 due to any necessity on the part of the latter to accumulate the stimu- 

 lating influence prior to originating a discharge, or to an immensely 

 lengthened period of latent stimulation manifested by the ganglia under 

 the influence of light. To answer this question, I first allowed a con- 

 tinuous flood of light to fall on the Medusid, and then noted the time 

 at which the responsive spasm first began. This time, as already stated, 

 was slightly more than one second. I next threw in single flashes of light 

 of measured duration, and found that, unless the flash was of slightly 

 more than one second's duration, no response was given. That is to say, 

 the minimal duration of a flash required to produce a responsive spasm 

 was just the same as the time during which a continuous flood of light 

 required to operate in order to produce a similar spasm. From this, 

 therefore, I conclude that the enormously long period of latent excitation 

 in the case of luminous stimuli is not, properly speaking, a period of 

 latent excitation at all, but that it represents the time during which a 

 certain summation of stimulating influence is taking place in the ganglia, 

 which requires somewhat more than a second to accumulate, and which 

 then causes the ganglia to originate an abnormally powerful discharge *. 



Responses to luminous stimulation occur in all cases equally well 

 whether the light employed be direct sunlight, diffused daylight, light 



* Tliis summation of stimtilating influence in central nervous matter (electrical 

 stimuli employed) has recently been described by Dr. Sterling, in the case of reflex 

 action in the frog. — [Jan. 10, 1877.] 



