1911] Michael: ChaetognatJia of tJie San Diego Begion. 159 



surface when the temperature is below 17?5, and possibly why 

 it is more abundant when the salinity is between 33.605 and 

 33.648. 



Concerning S. serratodentata we found : 



1. It is typically mesoplanktouic, being most abundant and 

 frequent in the region of 200 fathoms. 



2. It is rarely taken above 40 fathoms and practically never 

 above 10 fathoms, and is obtained from these depths only at 

 night. 



3. It is probable that the species migrates toward the surface 

 when twilight is very subdued. 



4. Its exceptional occurrence on the surface is undoubtedly 

 correlated with some unusual surface conditions, of which low 

 temperature may be one. 



5. Its center of migration, or the depth in which the greatest 

 number of optimum conditions are found, is probably between 

 100 and 200 fathoms. 



Concerning S. lyra we found : 



1. It is typically mesoplanktouic, rarely occurring above 25 

 fathoms, and irregularly increasing in abundance and frequency 

 at least to 250 fathoms. 



2. It occurs in greater numbers within 35 fathoms of the sur- 

 face during the night than during the day, which indicates that 

 nocturnal migrations toward the surface probably occur. 



3. Its center of migration is probably below 200 fathoms. 



Concerning the remaining species we found : 



1. 8. planktoiiis is entirely confined to the mesoplankton 

 because it was not obtained above 100 fathoms. Its center of 

 migration is probably in the region of 300 fathoms. 



2. yS". hexaptera probably belongs in the mesoplankton or 

 lower epiplankton, although it occurs to a certain extent in 75, 

 50, and 45 fathoms, and on the surface. 



