20 



P. T. CLEVE, PLANKTON-RESEARCHES. 



Deptli. 



50-0 m. 



200-0 m. 250-0 m. 



350-0 m. 



Podou Leockarti . . . 

 Spirialis retrovcrstt . . 

 Tomopteris helgolandica 

 Sagitta bipnnctata . . 

 Aglantha rosea .... 



r 

 rr 



In order to show iiiore clearly the seasonal changes in the nature of the plankton 

 I have calculated the percentage of its northern and southern forms and from these figu- 

 res constructed the curves tig. 1, where also a line indicates the total amount of dif- 

 ferent species. 



A similar curve for the North Sea has been laid down in fig. 2. 



* •■ rurl^tvit Kumj 



^Pvtll S<LQ 



— ■■ - narf^liirn fftrm/* 



Fig. 2. 



