REACTION-TIME OF GONIONEMUS MURBACHII. 89 



subumbrellar, instead of the exumbrellar, surface toward the light, 

 a difference in sensitiveness is apparently the cause of the dif- 

 ference in reaction-time. Certain organs which are especially 

 sensitive to light are found on the subumbrellar surface of the 

 margin, and it is when they are most fully exposed to the action 

 of light that the organism responds most promptly to the stim- 

 ulus. The rate of impulse transmission is probably the same no 

 matter which surface is stimulated by light, but the latency period 

 of stimulation is far greater for stimulation of the exumbrella. 



These experimentally demonstrable facts clearly prove that the 

 nervous system of the medusa Gonionemus consists of cells which 

 possess irritability and conductivity in high degrees. And they 

 further show that rapidity of reaction is directly dependent upon 

 specially differentiated paths of conduction. 



REACTION-TIME OF TENTACLES. 



A study of the reaction-time of the various parts of Gonionemus 

 (tentacles, manubrium, margin, bell), when normally functioning, 

 and when isolated by operation, throws interesting light upon 

 certain problems in the physiology of the nervous system. 



Cutting off the tentacles close to the bell causes Gonionemus a 

 severe shock. If only one tentacle is cut off the usual response 

 is a contraction of the bell, which may occasionally lead to a 

 swimming bout. The severity of the shock, or as we would 

 commonly say, the strength of the stimulus which Gonionemus 

 receives from tentacle excision, evidently varies directly with the 

 size of the organ. Small tentacles frequently may be cut with- 

 out causing any visible reaction except slight contractions of the 

 adjacent tentacles; large tentacles cause one or more contractions, 

 and in general the larger the organ the greater the number and 

 force of the contractions. Excision of the primary tentacles, 

 those at the bases of the radial canals, apparently causes the 

 most severe shock, for when one of them is suddenly clipped off 

 the animal frequently swims about rapidly for a considerable 

 length of time. In every way its reaction is more vigorous than 

 those caused by excision of smaller tentacles or of large tentacles 

 in other positions. 



It would appear from this that the radial canal tentacle is of 



