I8 



Papers from the Marine Biological Laboratory at Tortugas. 



rapidly as it does on cooling; the case being in general similar to that of 

 Cassiopea xamachana, where the rate of decline upon heating is about 8 

 times as rapid as in cooling. 



Hitherto we have considered only tropical forms or animals living in 

 the tropics, and in these we see that a definite temperature of maximum 

 activity is well shown in their temperature reactions. Thus for C. xama- 

 chana the optimum is about 33° C, for C.frondosa 28.5° to 32.5°, ior Aurellia 

 aurita 29°, for the movements of the branchial arms of Lepas fascicularis it 

 is about 32° C, and for the reef corals it ranges between 30° and 35.7° C. 

 Thus the optimum temperature is very close to the usual temperature of 

 the sea-water itself, this being about 28° to 31° in summer. Even a slight 

 degree of cooling or heating beyond the optimum causes a decided falling- 

 off in rate. 



A very different picture is presented by the arctic scyphomedusa Cyanea 

 arctica, from Halifax, Nova Scotia, in September, for here the optimum 



A-' 



15 20 



Degrees Centigrade 



Fig. 8. 



35 T 



temperature ranges from about 2° to 21° C, the animal's rate of pulsation 

 increasing only slightly as the temperature rises, and with an ill-defined 

 maximum at about 19° C. Upon cooling below 2° C, the medusa pulsates 

 until the ice imprisons it, although its rate declines rapidly. Similarly, if 

 we heat it above 21° C. the rate declines at about the same rate as it does 

 upon cooling from 2° C. All movements cease at about 27° C. 



The same general conditions are also shown by Aurellia aurita from the 

 cold waters of Nova Scotia, in marked contrast with its behavior at Tor- 

 tugas. For example, at Halifax, Nova Scotia, in September, when the 

 harbor water is about 14° C, the optimum temperature for Aurellia is 

 anywhere from 2° to 18° C. and it ceases to pulsate at — 1.4° and at 29.4° C. 



