634 



PHOTOPERIODISM IN INVERTEBRATES 



therefore be more effective than old ones with a high proportion of 

 rather difficultly digestible polysaccharides. Unfortunately, it is not 

 possible at present to assess the full importance of this factor. 



An attempt has been made to determine whether correlation exists 

 between the temperature and the breeding cycle. The most extensive 

 data, those upon the purple sea urchin and the ochre star, are given in 

 Figs. 1 and 2. It will be noted that the temperature changes are slight 

 but definite, and that in each case a decline in temperature is accom- 

 panied by growth of the gonads, while a rise in temperature is ac- 

 companied by spawning. Mere correlation does not constitute proof of 



28 



24 



ao 



X 



UJ 16 

 O 



O 



< 



z 

 o 

 o 



12 



STRONGYLOCENTROTUS 

 PURPURATUS 



SEPT. '54 - OCT. '55 



1- 



13 





CO 



«;ll 



LONGEST 



DAY LENGTH 



SHORTEST 



03 



I 



s 



'54 



I 

 



I 

 N 



I 

 D 



J 

 "55 



F M A M 







Fig. 1. Gonad index of the purple sea urchin, Strongylocentrotus pur- 

 piiratus, as correlated with temperature at the Hopkins Marine Station 

 fH.M.S.) and with day length. The vertical lines on the gonad index graph 

 indicate the spread of data for determinations at any one time (95% 

 confidence limits). 



