96 JELLY-FISH, STAR-FISH, AND SEA-URCHINS 



In normal water 



Quarter of an hour after 

 o.\posure to weak dose ... 



Two minutes after strength- 

 ening of dose 



Five minutes later, and just 

 before blocking of wave 



Fifteen seconds later, wave 

 continuing blocked 



Immediately after passage 

 of wave on restoration to 

 normal sea-water 



An hour later 



One second 



Fig. 20. 



From these experiments, however, it must not 

 be definitely concluded that it is the antesthesiating 

 property of such substances which exerts this slow- 

 ing and blocking influence on con traction- waves, 

 for 1 find that almost any foreign substance, whether 

 or not an anaesthetic, will do the same. That nitrite 

 of amyl, cafiein, etc., should do so, one would not 

 be very surprised to hear ; but it might not so readily 

 be expected that strychnine, for instance, should 

 block contraction- waves ; yet it does so, even in 

 doses so small as only just to taste bitter. Nay, 

 even fresh water completely blocks contraction- 

 waves after the strip has been exposed to its 

 influence for about half an hour, and exerts a per- 

 manently slowing efiect after the tissue is restored 

 to sea- water. These facts show the extreme sensi- 

 tiveness of the neuro-muscular tissues of the 

 Medusiie to any change in the character of their 

 surrounding medium, a sensitiveness which we 

 shall again have occasion to comment u})on when 

 treating of the effects of poisons. 



