MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
683 
This is a species which I have called Tiaropsis polydiademata, and the response which it 
gives to luminous stimulation is even more decided than is that which is given by Sarsia. 
In the opening paragraphs of my former paper, I described a peculiar spasmodic movement 
that is performed by Staurophom laciniata when subjected to marginal stimulation of any 
kind — a movement quite distinct from the ordinary locomotor contractions. A precisely 
similar movement is exhibited under similar circumstances by Tiaropsis polydiademata, and 
it is very interesting to observe that a sudden increase of light causes this peculiar spasm 
in this species quite as invariably and quite as decidedly as does any other kind of sti- 
mulation. Now in one remarkable particular this response to luminous stimulation on 
the part of Tiaropsis polydiademata differs from that given by Sarsia tubulosa ; and the 
difference consists in the fact that while with Sarsia the period of latency is, so far as 
the eye can judge, as instantaneous in the case of response to luminous stimulation as 
it is in the case of response to any other kind of stimulation, such is far from being true 
with Tiaropsis poly diademata. The period of latency in the last-named species is, so far 
as the eye can judge, quite as instantaneous as it is in the case of Sarsia , when the 
stimulus employed is other than luminous ; but 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. As this extraordinary difference in the latent period exhi- 
bited 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 Medusae 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 stimula- 
tion directly affected the central nervous tissues alone, while all the other modes of sti- 
mulation directly affected the contractile 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 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 originating a discharge, or to an immensely lengthened period of 
latent stimulation manifested by the ganglia under the influence of light. This is an 
interesting question, because if such a lengthened period of latent stimulation occurs in 
this case, it would stand in curious antithesis to the very short period of latent stimu- 
lation manifested by the contractile tissues of the same animal under other modes of 
irritation. To test these alternative hypotheses, I employed the very simple method 
of first allowing a continuous flood of light to fall suddenly on the Medusid, and then 
noting the time at which the responsive spasm first began. This time, as already stated, 
was slightly more than one second. I next allowed the animal to remain for a few 
minutes in the dark to recover shock, and, lastly, proceeded to throw in single flashes 
MDCCCLXXVII. 5 D 
