92 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



waves that course through the central zones, and 

 this is not the case. Indeed, it is surprising how 

 often the race is, as it were, neck and neck, thus 

 showing that the relative conductivity of all the 

 zones is precisely adjusted to their relative lengths ; 

 and forasmuch as in the unmutilated animal this 

 adjustment must clearly serve the purpose of 

 securing to the con tractii)n- wave a passage of 

 uniform rate over the whole radius of the umbrella, 

 I doubt not that, if it were possible to perform the 

 race-course section without interrupting any of 

 the lines of conduction-tissue, neck and neck races 

 would be of invariable occurrence. 



Interdigitating cuts, as might be expected, pro- 

 long the time of contraction-waves in their passage 

 through the tissue in which the cuts are interposed. 

 For example, in a spiral strip measuring twenty-six 

 inches in length, the time required for the passage 

 of a contraction -wave from one end to the other is 

 represented by the line a h in the annexed woodcut. 

 But after twenty interdigitating 

 cuts had been interposed, ten on 

 each side of the strip, the time 

 increased to c d, the line e f 

 Fig. 15. representing one second. And 



more severe forms of section are, of course, attended 

 with a still more retarding influence. 



The effects of temperature on the rate of con- 

 traction-waves are very striking. For instance, in 

 a rather narrow strip measuring twenty- eight inches 

 hmg and one and a half inches wide, the following 

 variations in rate occurred : — 



