100 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
the isthmus serves to connect, the resulting con- 
traction-waves will at first pa-s freely across the 
isthmus ; but after a time it may happen in some 
preparations that every now and then a contraction- 
wave fails to pass across the isthmus. When this 
is the case, if the stimulation is still continued, a 
greater and greater proportion of waves fail to pass 
across the isthmus, until perhaps only one in every 
five or six becomes propagated from the one area to 
the other. If single induction-shocks be then sub- 
stituted for the faradaic stimulation, it may be found 
that by leaving an interval of four or five seconds 
between the successive shocks, every wave that 
is started in the one area will be propagated across 
the isthmus to the other area. But if the interval 
between the successive shocks be reduced to two or 
three seconds, every new and then a wave will fail 
to pass across the isthmus; and if the interval be 
still further reduced to one second, or half a second, 
comparatively few of the waves will pass across. 
Now, however, if the tissue be allowed five minutes’ 
rest from stimulation, and the single shocks be 
thrown in at one second’s intervals, all the first six 
or ten waves will pass across the isthmus, after 
which they begin to become blocked as before. It 
may be observed also that when the waves are thus 
blocked, owing to exhaustion of the connecting 
isthmus, they may again be made to force a passage 
by increasing the intensity of the stimulation, and 
so giving rise to stronger waves having a greater 
power of penetration. Thus, on re-enforcing the 
electrical stimulus with the simultaneous application 
