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 contractiun-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 b 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 
long and one and a half inches wide, the following 
variations in rate occurred :— 
