1911] Michael: Chactognatha of the San Diego Region. 131 



tiire between 19 ?6 and 21? 5 were made between 10 a.m. and 

 2 p.m., only three of the twenty-one hauls in water having a 

 temperature between 17?6 and 19? 5, and six of the seventeen 

 hauls in water having a temperature between 15?9 and 17?5 

 were made during this period. Now we have learned that S. 

 bipunctata is on the surface in minimum numbers between 

 10 a.m. and 2 p.m. It will be interesting, therefore, to see what 

 effect low and high temperatures have on the abundance of the 

 species during this period of greatest light intensity. 



TABLE 42 



Distribution of S. hiyunctata during the time of Minimum Abundance 

 (10 a.m. to 2 p.m.) 



The number of hauls included in this table are too few to 

 determine fully the abundance of specimens with respect to 

 temperature. These data, however, indicate that, even when the 

 species migrates in maximum numbers from the surface, more 

 specimens remain when the temperature is low than when it is 

 high. However, the effect of higher temperatures in decreasing 

 the abundance and frequency of the species is not so great as 

 shown in the preceding table. I believe this means that, when 

 most intense, light has more effect in causing downward migra- 

 tions from the surface than low temperatures have in causing 

 the species to remain. This cannot be proven but, if true, it 

 suggests that the reverse is also true, namely, that subdued light 

 has more effect in causing upward migrations to the surface 

 than high temperatures have in preventing such migrations. In 

 examining the following two tables evidence of this fact is not 

 wanting. 



