82 JELLY-FISn, STAR-FlSir, AND SEA-URCHINS. 



them, be made along the other side of tlie 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, Avhich 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 

 cases 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 



