FXPERIMENTS IN STIMULATION. 45 
rays is present, and that this faculty is lodged 
exclusively in the marginal bodies; while from 
observations conducted on the covered-eyed Meduse, 
I have come to the same conclusion respecting them. 
But although I have tested many species of naked- 
eyed Medusze besides Sarsia, I have obtained indi- 
cations of response to luminous stimulation only 
in the case of one other. This is a species which 
I have called Tiaropsis polydiademata, and the 
response which it gives to luminous stimulation 
is even more marked and decided than that which 
is given by Sarsia ; for a sudden exposure to sun- 
light causes this animal to go into a kind of tonic 
spasm, the whole of the nectocalyx being drawn 
together in a manner resembling cramp. 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 (7.e. the time between 
the fall of the stimulus and the occurrence of the 
response) 1s, so far as the eye can judge, as instan- 
taneous in the case of response to luminous stimu- 
lation as it is in the case of response to any other 
kind of stimulation, such is far from being true 
with Tiaropsis polydiademata. 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 
