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, (or A urellia 

 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-' 0' 



10 



15" 20" 



Degrees Centigrade 



Fig. 8. 



25 



C 30 



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 A urellia 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. 



