22 University of California Puhlications in Zoology. [Vol.8 



PAGE 



General Discussion of the Problems of Plankton Distribution 99 



Discussion of the Vertical Distribution of the San Diego Chaetog- 



natha 112 



Vertical Distribution of S. bipunctata 115 



Effect of Day and Night on Vertical Distribution of S. 



bipunctata 117 



Variation in Distribution of S. bipunctata due to Temperature 128 



Variation in Distribution of S. bipimctata due to Salinity 133 



Seasonal Variation in the Distribution of S. bipunctata, 139 



Effect of Locality on the Distribution of S. bipunctata 141 



Significance of the Data concerning the Distribution of S. 



bipunctata 143 



Distribution of S. serratodentata 145 



Distribution of S. lyra 150 



Distribution of Other Species 153 



General Summary and Conclusions 157 



Bibliography 161 



INTRODUCTION 



This paper is based upon material collected by the San Diego 

 Marine Biological Station from 1904 to 1909. During this 

 period the station has explored a region lying, in the main, 

 between the parallels of 32° 20' and 33° 30' N, and extending 

 from the coast to 119° W. In this material we recognize seven 

 out of eighteen valid species of Sagitta, two of the three species 

 of Eukrohnia (= KroJniia Langerhans), and one of the two 

 species of Spadella. The seven species of Sagitta are S. enflata, 

 8. hexaptera, S. lyra, S. serratodentata, S. bipunctata, S. plank- 

 tonis, and S. neglecta. Eukrohnia is represented by E. hamata 

 and E. suhtilis. The one species of Spadella is S. draco. Though 

 I have spent much time in detailed and critical examination of 

 this material, I have failed to discover any indication of new 

 species. 



The species of Eul-rohnia and Spadella are readily recog- 

 nized, but those of Sagitta are more difficult to identify. The 

 latter naturally fall into two groups, those with and those with- 

 out collarettes. Although, in the species of each group, the diag- 

 nostic characters are annoyingly alike, each species has a definite 

 and distinctive appearance. 



It is my aim to give (pis. 1, 2) some idea of this distinctive 

 appearance, in addition to the usual taxonomic characters. 



