56 JELLY-FISH, STAR-FLSH, AND SEA-URCHINS. 



an induction stimulus of slightly less than minimal 

 intensity in relation to the original irritability oi 



the tissue, no response is given to the first two or 

 three shocks of a series ; but at the third or fourth 

 shock a slight response is given, and from that 

 point onward the staircase is built up as usual. 

 This was the case in the experiment of which Fig. 2 

 is a record, no response having been given to the 

 first two shocks. 



With regard to this interesting staircase action, 

 two questions naturally present themselves. In 

 the first place, we are anxious to know whether the 

 arousing effect which is so conspicuous in a stair- 

 case series is due to the occurrence of the previous 

 stimulations, or to that of previous contractions; 

 and, in the next place, we should like to know 

 whether, during the natural rhythm of the tissue, 

 each contraction exerts a beneficial influence on its 

 successor, analogous to that which occurs in the case 

 of contractions which are due to artijicial stimuli. To 



