SEA-FISHERIES LABORATORY. 393 



reliable and appreciable addition to the summer zoo- 

 plankton. The nutritive value of the various forms in 

 the plankton at the different seasons is a matter of great 

 practical importance upon which we hope shortly to have 

 fuller information. 



Sagitta. 



Sagitta bifunctata was again present throughout the 

 year, and had its maximum (over 2,000 on May 17th and 

 2,600 on May 20th) a little earlier than usual. The 

 second increase, which in former years has taken place in 

 October or November, was not manifest in 1912 until 

 December (347 on December 23rd). 



We feel, however, in regard to this organism that 

 our Bay surface nets may fail adequately to represent the 

 facts. It has been shown by E. L. Michael at San Diego, 

 California, that Sagitta performs very considerable 

 vertical migrations in the twenty-four hours, and tends 

 to seek what are called "twilight conditions." It 

 migrates to that zone in which the intensity of light is 

 similar to that occurring during the day in from 15 to 20 

 fathoms, sinking before the brightest light, but rising 

 also before the time of greatest darkness, and being most 

 abundant on the surface during morning and evening 

 twilight. So that horizontal hauls at various depths, 

 such as we obtain with the weighted net, the shear- 

 net and closing zone-nets, are necessary in order to 

 sample adequately the Sagitta population. 



It is evident from our work that this holds good for 

 various organisms of the plankton — if not indeed for all, 

 and that in the case of groups affected by light, tempera- 

 ture and other environmental influences there is an 

 optimum which is neither the maximum nor the 

 minimum. 



