82 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
them, be made along the other side of the contractile 
strip, the result is, of course, a number of interdigi- 
tating cuts; and it is easy to see that by beginning 
with a few such cuts and progressively increasing 
their number, a point must somewhere be reached 
at which one portion will become physiologically 
separated from the rest. The amount of such 
section, however, which contractile strips will some- 
times endure is truly surprising. I have seen such 
a strip twenty inches long by one and a half inches 
wide with ten such cuts along each side, and the 
contraction-waves passing without impediment from 
end to end. But what I wish more especially to 
observe just now is, that by progressively increasing 
the number of such interdigitating cuts up to the 
point at which the contraction-wave is blocked, and 
then leaving the tissue to recover itself, in many 
eases it will be observed that the blocking is sooner 
or later overcome; that on then adding more 
interdigitating cuts the blocking again supervenes; 
but that in time it may again be overcome, and so 
on. It is, however, comparatively rare to find cases 
in which blocking is overcome twice or thrice in 
succession. 
Section is not the only way in which blocking of 
waves may be caused in contractile strips. I find 
that pressure, even though very gentle, exerted on 
any part of a strip causes a blocking of the waves 
at that part, even after the pressure has been 
removed, If the pressure has been long continued, 
after its removal the blocking will probably be 
permanent; but if the pressure has been only of 
