1911] Michael: Chaetognatha of the San Diego Region. 137 



during twilight, and a maximum number corresponding to salini- 

 ties outside these limits during darkness. Now of the ten hauls 

 corresponding to salinities less than 33.605 and temperatures 

 between 17?6 and 19?5, four were made during evening ticilight 

 (6 to 10 p.m.), and six during morning twilight (4 to 6 a.m.). 

 Concerning salinities in the second group, all seven hauls were 

 made during evening twilight. Finally of the eleven hauls cor- 

 responding to the third salinity group, four were made during 

 evening twilight, four during morning twilight, and three during 

 darkness (10 p.m. to 4 a.m.). 



The distribution of hauls does show irregularity with respect 

 to twilight, which may partly explain why the maximum abund- 

 ance and frequency corresponds to the second salinity group. 

 However, because of the three hauls made during darkness, 

 which correspond to salinities more than 33.648, and the fact that 

 the species maintains a relative minimum on the surface during 

 darkness, we should expect both abundance and frequency to be 

 less than that corresponding to salinities below 33.605. This is 

 obviously not the case. It w^ould seem, therefore, that while 

 irregular distribution of hauls with respect to twilight has prob- 

 ably affected the relation between abundance and frequency of 

 the species and salinity of the water, the data included in the 

 above tables cannot be adequately explained without involving 

 the effect of variations in salinity. 



Finally, it is well to consider data based on daylight hauls. 

 Such data are tabulated below. 



TABLE 49 

 Effect of Salinity on Distribution of -S. bipunctata with respect to Daylight 



(6 a.m. to 6 p.m.) 



Number of hauls ' Number Number of specimens Average 



l^ ^ of ,• '" \ Fre- tem- 



Salinity Total Successful hours Total Per hour quency perature 



33.604 or less ' 12 10 9.0 949 105 83 17?2 



33.605-33.648 6 6 3.3 178 54 100 18?9 



33.649 or more 11 5 8.9 145 16 22 18? 9 



The distribution with respect to salinity, as shown in this 

 table, is not parallel with that shown in the preceding tables. 

 However, since the temperature average corresponding to the 



