NATURAL HHYTHM. 



159 



of the rhythm manifested by the smaller than by 

 the larger segments in tlie stale water, and the 

 fact is presumably to be accounted for by the con- 

 sideration that the ganglia in tlie smaller segments 

 were more potent than those in the larger. 



With rc-ard, therefore, to the original point 

 imder consideration, I conclude that, although the 

 size of the segments is doubtless one factor in 

 determining the relative frequency of contraction, 

 there are at least two other factors quite as impor- 

 tant, viz. the relative potency of the lithocysts, and 

 the length of time that elapses between performing 

 the operation and observing the rhythm. Hence it 

 is that in my experience I have found but very few 

 examples of Dr. Elmer's rule. 



Effects of Other Forms of Mutilation on the Rhythn. 



The next point I have to dwell upon is one of 

 some interest. If the manubrium of Aurelia, or of 

 any other covered-eyed Medusa, be suddenly cut off 

 at its base, the swimming motions of the umbrella 



Next day, the temperature unfortunately rose sufficiently to 

 cause the death of the single surviving segment, which otherwise 

 would probably have lived for one or two days longer. 



