BEGINNING OF NERVES. 317 



flash was of slightly more than one second's duration, no response was 

 given. That is to say, the minimal duration of a flash required to pro- 

 duce a responsive spasm was just the same as the time during which a 

 continuous flood of light required to operate in order to produce a 

 similar spasm. From this, therefore, I conclude that the enormously 

 long period of latent excitation in the case of luminous stimuli is not, 

 properly speaking, a period of latent excitation at all ; but that it rep- 

 resents the time during which a certain summation of stimulating in- 

 fluence is taking place in the ganglia, which requires somewhat more 

 than a second to accumulate, and which then causes the ganglia to origi- 

 nate an abnormally powerful discharge. So that in the action of light 

 upon the ganglionic matter of this medusid 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 decomposition which is 

 efi'ected in it by light, and which terminates in an explosion of nervous 

 energy, can only be efi'ected by a prolonged exposure of the unstable 

 material to the summating influence of the luminous vibrations. Prob- 

 ably, 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 either 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 eff'ect the 

 needful molecular changes in response to a flash as instantaneous as 

 that of lightning ! 



Before leaving the case of luminous stimulation, I may observe that 

 some of the Mediism appear to be very fond of light. For, on placing 

 a number of Sarsim in a large bell-jar in a dark room, and then throw- 

 ing a beam of light through a part of the water in the bell-jar, the 

 MeduscB all crowded into the path of the beam, and dashed themselves 

 against the glass nearest to the light, very much as moths might do un- 

 der the influence of similar stimulation. On moving the lamp round 

 the jar, a cluster of MeduscB always followed it. This latter experi- 

 ment is important, because it proves that the marginal ganglia are 

 so far coordinated in their action that they can steer the animal in 

 any particular direction. 



Staurophora laciniata is a large species of naked-eyed medusa, 

 which responds to stimulation in two very difl"erent ways, according 

 as the stimulation is applied to the nervo-muscular sheet, or to the 

 marginal ganglia. For, if the stimulation is applied to the nervo-mus- 

 cular sheet, the response is an ordinary locomotor contraction ; Avhereas, 

 if the stimulation is applied to the marginal ganglia, the response is a 



