184 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
in answer to ganglionic stimulation.* Does not 
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 parenthetically 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 exclusively 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 rhythm, 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 
* Tt will not be forgotten that there are a multitude of 
ganglion-cells distributed throughout the contractile tissues of the 
Meduse; 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. 
