1911] Michael: Chaetognatha of the San Diego Eegion. 117 



number of specimens obtained in each haul. Consequently, an 

 average based upon hauls irregularly distributed with respect 

 to these factors is necessarily misleading. 



These facts — insufficient number of hauls, lack of correlation 

 between surface and closing net hauls, and irregular distribution 

 of hauls — will explain the disagreement noted between fre- 

 quency and abundance. However, we may safely conclude : 



1. S. hipuiictata is typically epiplanktonic,^ occurring be- 

 tween the surface and 150 fathoms. 



2. It is rarely found below 75 fathoms. 



3. It is most frequent and abundant between the surface and 

 20 fathoms. 



EFFECT OF DAY AND NIGHT ON THE VERTICAL DISTRIBUTION 

 OF SAGITTA BIPUNCTATA 



It has just been noted that, owing to irregularity in distribu- 

 tion of hauls with respect to varying environmental conditons, 

 the average number of specimens per hour obtained from each 

 depth is misleading. Because our data regarding day and night 

 are more complete than those concerning other environmental 

 factors, it is desirable to consider the influence of this factor on 

 vertical distribution before undertaking to interpret the eifects 

 of variations in other factors. 



By separating all hauls made between 6 a.m. and 6 p.m. from 

 those made between 6 p.m. and 6 a.m., the general effect of day 

 and night on vertical distribution may be considered. Such data 

 are supplied in the following table : 



1 Fowler (1906, p. 47) defines the terms epiplaukton and mesoplankton as 

 follows: Epiplankton includes all plankton above the region of 100 fathoms. 

 Mesoplankton includes that between 100 fathoms and to within about 100 

 fathoms of the bottom. 



