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



in answer to ganolionic stimulation.* Does not 



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this tend to show that for the production of the 

 natural rhythm the presence of the ganglionic 

 element is non-essential ; that if we merely suppose 

 the function of this element to be that of supplying 

 a constant stimulus of a low intensity, without in 

 addition supposing the presence of any special 

 resistance-mechanism to regulate the discharges, 

 the periodic sequence of systole and diastole would 

 assuredly result ; and, therefore, that the rhythmical 

 character of the natural swimming motions is 

 dependent, not on the peculiar relations of the 

 ganglionic, but on the primary qualities of the 

 contractile tissue ? Or, if we do not go so far as 

 this (and, as I may parentis etically observe, I am 

 not myself inclined to go so far), must we not at 

 least conclude that the natural rhythm of these 

 tissues is not exchisively due to any mechanism 

 whereby the discharges of the ganglia are inter- 

 rupted at regular intervals; but that whether these 

 discharges are supposed to be interrupted or 

 continuous, the natural rhythm is probably in a 

 large measure due to the same cause as the 

 artificial rliythm, viz. in accordance with our 

 previous hypothesis, to the alternate exhaustion 

 and recovery of the excitable tissues ? This much, 

 at least, must be allowed even by the most cautious 



* It will not be forgotten that there are a mnltitnde of 

 ganglion-cells distributed throughout the contractile tissues of the 

 Medusae ; but forasmuch as these are comparatively rarely- 

 instrumental in originating stimulation, I think it is probable 

 that artificial stimulation acts directly on the contractile tissues, 

 and not through the medium of these scattered cells. 



