56 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
an induction stimulus of slightly less than minimal 
intensity in relation to the original irritability of 
Fig. 5. 
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 artificial stimuli. To 
