3i6 THE POPULAR SCIENCE MONTHLY. 



ized light, or any of the luminous rays of the spectrum employed sep- 

 arately. On the other ha-nd, neither the non-luminous rays beyond the 

 red, nor those beyond the violet, appear to exert the smallest degree 

 of stimulating effect. Hence, in all respects, the rudimentary eye of 

 jSarsia appears to be affected by the same qualities of light as are our 

 own. 



Not so, however, in the case of another species of medusa, which T 

 have called Tiaropsis polydiademata. This jelly-fish responds to lu- 

 minous stimulation in the same peculiar manner as it responds to all 

 other artificial — as distinguished from natural ganglionic — stimulation ; 

 that is to say, instead of giving a locomotor contraction of the bell, it 

 throws the bell into a violent contraction of a long-sustained character, 

 resembling cramp or tonic spasm. Now, in the case of this medusa, 

 the luminous stimulation requires to act for a comparatively long time 

 in order to produce a response. For, while in Sarsia the period of 

 latent stimulation appears to be as short in the case of luminous as it 

 is in the case of other modes of stimulation, in the ease of Tiaropsis 

 this is not so, although, as regards all modes of stimulation other than 

 luminous, the latent period is as brief in the case of Tiarojysis as it is 

 in the case of JSarsia. In other words, while this period is quite as in- 

 stantaneous in the case of Tiaropsis as it is in the case of Sarsia when 

 the stimulus employed is other than luminous, 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. Now, as my ex- 

 periments on Sarsia proved that the only respect in which luminous 

 stimulation differs from other modes of stimulation consists in its being 

 exclusively a stimulation of ganglionic matter, we have evidence, in the 

 case of Tiaropsis, of an enormous difference between the rapidity of 

 response to stimuli by the contractile and by the ganglionic tissues 

 respectively. The next question, therefore, is as to whether the enor- 

 mous length of time occupied by the process of stimulation in the 

 ganglia is due to any necessity on the part of the latter to accumulate 

 the stimulatina* influence of light prior to originating a discharge, or to 

 an immensely lengthened period of latent stimulation manifested by 

 the ganglia under the influence of light.* To answer this question, I 

 first allowed a continuous flood of light to fall on the medusid, and then 

 noted the time at which the responsive spasm first began. This time, 

 as already stated, was slightly more than one second. I next threw in 

 single flashes of light of measured duration, and found that, unless the 



' The period of latent stimulation merely means the time after the occurrence of an 

 excitation during which a series of physiological processes are taking place which termi- 

 nate in a contraction ; so that, whether the excitation is of a strong or of a weak inten- 

 sity, the period of latent stimulation is not much aifected. The above question, therefore, 

 was simply this : Does the prolonged delay on the part of these ganglia, in responding to 

 light, represent the time during which the series of physiological processes are taking 

 place in response to an adequate stimulus, or does it represent the time during which 

 light requires to act before it becomes an adequate stimulus ? 



