1C8 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 diminisJihig 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 

 vio'our of the contractions. The case of Medusie 

 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 Med usee, 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 tlie 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 



