252 University of California PaUications in Zoology [Vol. 18 



aggregate forms are most abundant between four and six fathoms, 

 decreasing from that depth until they also disappear below 100 

 fathoms. Here again, as in table 2, the maximum abundance of soli- 

 tary forms corresponds to a higher temperature than is the case with 

 aggregate forms. Is this correspondence merely consequent upon the 

 effects of random sampling, or does the temperature of the water play 

 an important and differential part in the distribution of the two 

 generations of this species ? 



B. TEMPERATURE AND SURFACE DISTRIBUTION 



1. Relations Revealed by Data 



The June and July data of the years 1908 and 1909 reveal a 

 variation in surface temperatures taken simultaneously with surface 

 net hauls of five degrees Centigrade, or from 15?9C to 20?8C. "When 

 these hauls are arranged in two groups according as the temperature 

 was 18?3C or less, or 18?4C or more, it is found (table 4) that an 

 average of 67 solitary forms per hour -and an average of 529 aggre- 

 gate forms per hour are associated with the lower temperatures, while 

 an average of 156 solitary forms and an average of 124 aggregate 

 forms are associated with the higher temperatures. In other words, 

 solitary forms were most abundant on the surface in the warmer 

 water, while aggregate forms were most abundant in the colder water. 



Table 4 



Eelation between surface temperature and surface distribution of Salpa 



democratica during the months of June and July, 



1908 and 1909 



Tempera- 

 ture in JC 

 centigrade as 



15?9-18?3 30 

 18?4-20?8 46 



Table 4 also shows that the frequency of both solitary forms and 

 aggregate forms was 92 when the temperature was 18?3C or less, 

 while that of solitary forms was 69 and that of aggregate forms 77, 

 when the temperature was 18?4C or more. In other words both soli- 

 tary forms and aggregate forms were obtained most frequently in the 

 colder water. To sum up, table 4 shows : 



