1911] Michael: Chaetognatha of the San Diego Region. 135 



33.605 is much less in table 46 than in table 45. Now the 

 average temperature corresponding to this salinity group is 

 17?7, which means tha! more hauls were made when the tem- 

 perature was below 17?6 than when it was above 19?5. In fact 

 during only two of the 27 hauls was the temperature above 

 19?5. while during 11 it was below 17?6. The effect of low tem- 

 peratures, then, affords an explanation of why a reduction 

 occurred in abundance from 373 to 219, and in frequency from 

 81 to 71. 



Comparison of the two tables also shows that the frequency 

 and abundance corresponding to salinities between 33.605 and 

 33.648 are much greater in table 46 than in table 45. As the 

 temperature average corresponding to this salinity group is 19 ?4, 

 we should infer that more hauls were made when the tempera- 

 ture was over 19?5 than when it was under 17?6. In fact the 

 temperature during eight of the 19 hauls was above 19?5, while 

 during only two hauls was it below 17?6. Hence, because high 

 temperatures are detrimental to the accumulation of specimens 

 on the surface, the reason is evident why the frequency increased 

 from 84 to 89, and the abundance from 555 to 753. 



Again, comparison of the two tables shows that the abund- 

 ance and frequency corresponding to salinities greater than 

 33.648 increased respectively from 352 and 67 as given in table 

 45 to 421 and 72 as given in table 46. The temperature during 

 four of the 24 hauls was more than 19?5, while during two hauls 

 it was less than 17? 6. This not only explains why the abundance 

 and frequency increased, but also why the amount of increase is 

 less than that corresponding to the second salinity group, and 

 less than the amount of decrease corresponding to the first salinity 

 group. 



Finally, these tables demonstrate that salinity has a marked 

 effect on the distribution of -S. bipunctata. While the effect is 

 evident in table 45, it is much greater when the antagonistic 

 effects of variable temperatures are eliminated from the data, as 

 in table 46. This means, then, that the species is most abundant 

 and frequent on the surface when the salinity of the water varies 

 between 33.605 and 33.648, that it is less when the salinity is 



