EXPERIMENTS IN STIMULATION. 47 



at the polarizing angle. It must also be stated that 

 responses are given to any of the luminous rays of 

 the spectrum when these are employed separately; 

 but that neither the non-luminous rays beyond 

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

 the smallest degree of stimulating influence. 



Electrical Stimulation. 



All the excitable parts of all the Medusae which I 

 have examined are highly sensitive to electrical 

 stimulation, both of the constant and of the induced 

 current. 



Exploration with needle-point terminals and in- 

 duction shocks of graduated strength showed that 

 certain parts or tracts of the nectocalyx are more 

 sensitive than others. The most sensitive parts are 

 those which correspond with the distribution of the 

 main nerve-trunks, i.e. round the margin of the 

 nectocalyx and along the course of the radial tubes. 

 The external or convex surface of a nectocalyx or 

 umbrella is totally insensitive to stimulation, and 

 the same statement applies to the whole thickness 

 of the gelatinous substance to which the neuro- 

 muscular sheet is attached. 



In all other respects the excitable tissues of the 

 Medusue in their behaviour towards electrical 

 stimulation conform to the rules which are followed 

 by excitable tissues of other animals. Thus, closure 

 of the constant current acts as a stronger stimulus 

 than does opening of the same, while the reverse is 

 true of the induction shock ; and exhaustion super- 



