682 
ME. G-. J. EOMANES ON THE LOCOMOTOE SYSTEM OF MEDUSAE. 
every flash. There can thus be no doubt that a sudden transition from darkness to 
light acts upon Sarsia as a stimulus, and this even though the transition be but of 
momentary duration. The question therefore arises as to whether the stimulus consists 
in the presence of light, or in the occurrence of the sudden transition from darkness 
to light and from light to darkness. To answer this question I tried the converse expe- 
riment of placing a vigorous specimen in sunlight, waiting till the middle of one of the 
quiescent stages in the swimming-motions had come on, and then suddenly darkening. 
In no case, however, under these circumstances, did I obtain any response ; so that I 
cannot doubt it is the light per se, and not the sudden nature of the transition from 
darkness to light, which in the former experiment acted as the stimulus. Indeed the 
effect of the converse experiment just described 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 Sarsia are more active in the light than they are in the dark, 
the comparative duration of the quiescent stages being less in the former than in the 
latter case. Light thus appears to act towards these animals as a constant stimulus. 
Nevertheless, when the flashing method of experimentation is employed, it is observable 
that the stimulating effect of the flashes progressively declines with their repetition — 
supposing, of course, that the intensity of the light employed remains constant, and that 
the rate at which the flashes are thrown in is not so slow as to cause the ganglia to 
forget, as it were, the occurrence of the previous flash before its successor is thrown in. 
It is difficult to measure with any exactness the limit of time during which the effect of 
the previous flash remains, but it appears to be about a quarter of a minute ; for if the 
flashes are thrown in with a greater frequency than this, the progressive decline in their 
efficiency as stimulants first becomes observable. In all cases the response of Sarsia 
to luminous stimulation appears to be quite as rapid as is their response to any other 
mode of stimulation, although, as I have not in the case of this genus been able to 
measure the period of latent stimulation, this statement rests only on the estimate formed 
by the eye. It is further to be observed that when the efficacy of luminous stimulation 
is being diminished — either in consequence of frequent repetition at short intervals, or 
in consequence of the slow dying of the animal on an object-glass — the period of latency 
is very much prolonged. In this case the first effect of the flash is to cause retraction of 
the tentacles, to be followed immediately afterwards by a contraction of the bell. (This 
is also the case in other modes of stimulation of feeble specimens.) Lastly, it may be 
stated that when the marginal bodies of Sarsia are removed, the swimming-bell, although 
still able to contract spontaneously, no longer responds to luminous stimulation of any 
kind or degree. But if only one body be left in situ , or if the severed margin alone be 
experimented upon, the same unfailing response may be obtained to luminous stimu- 
lation as that which is obtained from the entire animal. 
Although I have tested many of the Medusae, I have obtained indications of response 
to luminous stimulation only in the case of one other species of the naked-eyed division. 
