168 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
greater alterations would be attended with still 
greater effects. Such, however, is not the case. 
Water at 70° or 80°, for instance, has the effect of 
permanently diminishing the rate of the rhythm, 
after having temporarily raised it for a few seconds. 
The following experiment will serve to convey a 
just estimation of these facts. 
An Aurelia whose rhythm in water at 40° was 
very regular at eighteen per minute, was suddenly 
transferred to water at 80°. In the immediately 
succeeding minutes the rhythm was 22, 20,14. The 
latter rate continued for nearly half an hour, when 
the observation terminated. 
The effect of very warm water, therefore, is to slow 
the rhythm, as well, I may add, as to enfeeble the 
vigour of the contractions. The case of Medusz 
thus differs, in the former respect, from that of the 
heart; and I think the reason of the difference is 
to be found in the following considerations. Even 
slight elevations of temperature are quickly fatal 
to the Medusz, so it becomes presumable that con- 
siderable elevations act very destructively on the 
neuro-muscular tissues of those animals. This de- 
structive effect of high temperatures may, therefore, 
very probably counteract the stimulating effect which 
such temperatures would otherwise exert on the 
natural rhythm, and hence a point would somewhere 
be reached at which the destructive effect would so 
far overcome the stimulating effect as to slow the 
rhythm. That this is probably the true, as it cer- 
tainly is the only explanation to be rendered, will, 
I think, be conceded when I further state that if 
