BEGINNING OF NERVES. 315 



jelly-fish and a frog, up even to the most complex of our psychological 

 processes, we have in this recently discovered principle of the summa- 

 tion of stimuli a very remarkable uniformity of occurrence. 



Hitherto light has never been actually proved to act as a direct 

 stimulus to ganglionic matter. It is therefore of interest to note that 

 it thus acts in the case of some species of Medusce. JSarsice, for instance, 

 almost invariably respond to a single flash by giving one or more con- 

 tractions. If the animal is vigorous, the effect of a momentary flash 

 thrown upon it during one of the natural pauses is immediately to 

 originate a bout of swimmipg ; but if the animal is non-vigorous, it 

 usually gives only one contraction in response to every flash. That it is 

 light per se, and not the sudden transition from darkness to light, which 

 here acts as the stimulus, is proved by the result of the converse experi- 

 ment, viz., placing a vigorous specimen in sunlight, waiting till the 

 middle of one of the natural pauses, and then suddenly darkening. In 

 no case did I thus obtain any response. Indeed, the effect of this con- 

 verse experiment is rather that of inhibiting contractions ; for if the 

 sunlight be suddenly shut off during the occurrence of a swimming- 

 bout, it frequently happens that the quiescent stage immediately sets 

 in. Again, in a general way, it is observable that Sarsice are more 

 active in the light than they are in the dark : it appears as though 

 light acts toward these animals as a constant stimulus. Nevertheless, 

 when the flashing method of experimentation is employed, it is observ- 

 able that the stimulating effect of the flashes progressively declines 

 with their repetition. The time during which the deleterious effect of 

 one such stimulus on its successor lasts appears to be about a quarter 

 of a minute. The period of latent stimulation is, judging by the eye, 

 as short in the case of luminous as in that of other stimulation ; but 

 when the efiicacy of luminous stimulation is being diminished by fre- 

 quent repetition, the period of latency is very much prolonged. 



The question as to what part of the organism it is which is thus sus- 

 ceptible of luminous stimulation, was easily determined by detaching 

 various parts of the organism and experimenting with them separately. 

 I thus found that it is the marginal bodies alone which are thus affected 

 by light ; for, when these are removed, the swimming-bell, though still 

 able (in the case of Sarsia) * to contract spontaneously, no longer re- 

 sponds to luminous stimulation ; whereas, if only one marginal body be 

 left in situ, or if the severed margin, or even a single excised marginal 

 body, be experimented on, unfailing response to this mode of stimula- 

 tion may be obtained. 



Responses to luminous stimulation occur in all cases equally well, 

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



' In all th'e waiei-eyed division of Medusce, to which Sarsia belonps, total paralysis of 

 the bell can only be obtained by removing the entire margin ; but in all the covered-eyed 

 . division, to which Anrelia belongs, paralysis of the bell ensues on removing the marginal 

 bodies alone. 



