46 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



than a second to accumulate, and which then caused 

 the ganglia to originate an abnormally powerful 

 discharge. So that in the action of light upon the 

 o'anMionic matter of this Medusa we have some 

 analogy to its action on certain chemical compounds 

 in this respect, that, just as in the case of those 

 compounds which light is able to split up, a more 

 or less lengthened exposure to its intluence is 

 necessary in order to admit of the sum mating 

 influence of its vibrations on the molecules, so in 

 the case of this ganglionic material, the decom- 

 position which is effected in it by light, and which 

 terminates in an explosion of nervous energy, can 

 only be effected by a prolonged exposure of the 

 unstable material to the summating influence of 

 the luminous vibrations. Probably, therefore, we 

 have here the most rudimentary type of a visual 

 organ that is possible ; for it is evident that if tlie 

 ganglionic matter were a very little more stable 

 than it is, it would altogether fail to be thrown 

 down by the luminous vibrations, or would occupy 

 so long a time in the process that the visual sense 

 would be of no use to its possessor. How great is 

 the contrast between the excitability of such a 

 sense-organ and that of a fully evolved eye, which 

 is able to efTect the needful molecular changes in 

 response to a flash as instantaneous as that of 

 lightning. 



With regard to luminous stimulation, it is only 

 necessary further to observe that responses were 

 given equally well to direct sunlight, diffused day- 

 liglit, and to light reflected from a mirror inclined 



