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 Medusee 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, ze. 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 
Medusze 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- 
