32 University of California Publications in Zoology [Vol. 13 



be regarded as the optimum salinity on the whole. The "best sea- 

 son" does not consist alone of a particular and favorable salinity, if 

 it may be assumed that abundance is an index of the favorable or 

 unfavorable character of the surroundings. For example, in table 

 8 we assume that August is the favorable season and 33.71-33.75 the 

 favorable salinities because the abundance of Pleurohrachia is so much 

 greater in that month and at those salinities. Yet that range of salin- 

 ity is not characteristic of August any more than of October judging 

 from the total number of hauls. So, while we should keep in mind 

 that there is a combination of the effects of season and salinity in 

 table 9, there is meaning in the general results with regard to salinity 

 shown in the table. 















TABLE 9 















DlSTRIBUTION OF 



Pleurobrachia at 



THE Surface According to Salinity, 

















Summer Months 

















No. 



of 



hauls : 



No. of 



hours : 



No. of 



animals : 



Frequen 



cy: 





Salinity 



Total. 

 A 





Success. 

 B 



Total. 

 C 



Success. 

 D 



Total. 

 E 



Per hour. 

 P 



Haul. 

 G 



Timi 

 H 



1. 



33.60 



or less 



33 





6 



25.4 



5.1 



161 



6.4 



18 



20 



2. 



33.61- 



-.65 



32 





6 



25.2 



4.3 



915 



36. 



19 



17 



3. 



33.66- 



-.70 



24 





6 



17.7 



3.3 



499 



28.2 



25 



19 



4. 



33.71- 



-.75 



20 





4 



18.9 



3.4 



1764 



94.3 



20 



18 



5. 



33.76- 



-.80 



9 





1 



9.5 



.8 



134 



14. 



11 



8 



6. 



33.81- 



-.85 



8 









8.9 



0. 







0. 











The largest number of animals per hour is found in line 4, column 

 F ; that is, the organisms are most abundant at salinities between 33.71 

 and 33.75. Out of 3474 specimens, 3178 were obtained at salinities 

 ranging from 33.61 to 33.75, and of the latter number, more than half 

 w^ere taken in hauls made at salinities from 33.71 to 33.75. 



As has been shoMai in the preceding account of the effects of tem- 

 perature and salinity, August is the month during which Pleuro- 

 hrachia is particularly abundant. In table 5 it appears that August 

 temperatures are all above 19 ?1, so that high temperatures are char- 

 acteristic. The salinities at which the abundance is greatest range 

 from 33.71 to 33.75, but it cannot be said that they are specially 

 characteristic of the month, as can be seen in table 8. Furthermore, 

 we find that the numbers of animals taken during the others of the 

 summer months do not nearly approach those obtained during Aug-ust, 

 though as many or more hauls were made at the same temperatures 

 and salinities that were so favorable when collecting was done in 

 August. 



