1914] Esterly: CtenopJiora of the San Diego Region 31 



Out of the sixty-nine hauls considered in table 7, twenty-one were 

 successful, all made in water below 17°. The frequency is 30, while 

 during the summer (table 5) it is about 20. In other words, Pleuro- 

 hrachia is found more often in this region when the water is below 17° 

 than when it is above that point, but the abundance is far greater dur- 

 ing the summer in warmer water. There is little more than this to be 

 said about the winter collecting as compared with that during the 

 summer, for the number of winter hauls is rather small. 



A consideration of the effect of salinity on abundance is based on 

 126 hauls of which 23 were successful. The August collections con- 

 tained most of the animals. Table 8 deals with the summer hauls for 

 which the salinities have been determined. 



TABLE 8 



Total Number of Surface Hauls, Number of Successful Hauls, Number of 

 Pleurobrachia, in Hauls During Each of the Summer Months at Different 

 Salinities. Left-Hand Figures, Total Hauls ; Center Figures, Successful 

 Hauls; Eight-Hand Figures, Number of Animals. 



Salinity 



less 



.June 





Jul^ 







Au|, 



ist 



September 



October 



33.60 or 



20 



4 



26 



11 



1 



8 



1 



1 



127 















10 



33.61-.65 





15 



1 



150 



11 



2 



21 



4 



3 



744 















2 



33.66-.70 





15 



2 



73 



4 



1 



3 



3 



2 



173 



1 



1 



250 



10 



33.71-.75 





8 



1 



13 



4 



1 



1 



3 



2 



1750 















5 



33.76-.80 





4 











2 











2 



1 



134 



1 















33.81-.85 





3 











1 











1 











1 











2 



During August the most favorable salinity is within the limits 33.71 

 and 33.75, but there is also the seasonal effect to reckon with as was 

 the case in the consideration of the effect of temperature. It will be 

 noted that more hauls were made in June, July and October than in 

 August at salinities between 33.71 and 33.75, but during the three 

 former months only 14 animals altogether were obtained. This seems 

 to indicate that the external conditions which favor great abundance of 

 Pleurohrachia are not alone those of certain salinities any more than 

 those of certain temperatures. The season has its effect ; but it is not 

 plain why August is so much more favorable than June, for example, 

 in table 8 or October in table 5. 



Table 9 is an arrangement of results with regard to salinity that 

 combines all the summer hauls. While the seasonal effect of August 

 collecting necessarily enters, this table shows, nevertheless, what may 



