1911] Michael: Chaetognatha of the San Diego Region. 133 



tribution leads me to suggest that the species migrates down- 

 ward after twilight because, the cause of upward migration being 

 removed, it seeks deeper water where the temperature is lower. 

 Unfortunately this suppositon cannot be adequately tested 

 because practically no closing net hauls were made with respect 

 to temperature. 



To summarize, it is demonstrated that, whether the data con- 

 cern day, night, time of maximum abundance with respect to 

 light, or time of minimum abundance, S. hipunctata is more 

 abundant and frequent on the surface when the temperature is 

 low than when it is high. It is further suggested that low 

 surface temperatures retard downward and aid upward migra- 

 tions, and that high surface temperatures retard upward and 

 aid downward migrations. 



VARIATION IN DISTRIBUTION OF S. BIPVNCTATA DUE TO 



SALINITY 



From June 12, 1908, to April 21, 1910, 101 surface hauls were 

 accompanied by water samples of which the salinities were 

 computed. The salinity varied mainly from 33.549 to 33.693, 

 although our data show a few instances outside these limits, the 

 extremes being 33.381 and 34.069. Of the 101 hauls, 31 were 

 scattered over the months of February, ]\Iarch, April, Septem- 

 ber, and November, while the remaining 70 were made during 

 June and July. Now the salinities corresponding to the 31 

 hauls just mentioned average above 33.693. It is therefore 

 advisable, owing to possible seasonal effects on distribution, to 

 confine the following discussion to June and July data. 



In attempting to correlate small changes in salinity with 

 variations in the distribution of S. hipunctata, our data are 

 not extensive enough to eliminate the effects of light and tem- 

 perature. Consequently I have segregated the 70 June and 

 July hauls into three groups according to whether the corre- 

 sponding salinities were less than 33.605, between 33.605 and 

 33.648, or greater than 33.648. The surface distribution witli 

 respect to these groups is given in the following table. 



