EXPERIMENTS IN STIMULATION. 4o 



rays is present, and that this faculty is lodged 

 exclusively in the marginal bodies; while from 

 observations conducted on the covered-eyed Medusa, 

 I have come to the same conclusion respecting them. 

 But although I have tested many species of naked- 

 eyed Medusie 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 tliis 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 (i.e. the time between 

 the fall of the stimulus and the occurrence of the 

 response) is, 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 beino- 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 



