46 CELLY-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 
ganglionic 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 influence is 
necessary in order to admit of the summating 
influence of its vibrations on the molecules, so in 
the case of this ganglionic material, the decom- 
position which is effected in it by hght, 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 the 
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 effect 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- 
light, and to light reflected from a mirror inclined 
