60 DE. G. H. FOWLEE — BISCATAN PLANKTON : 



100 fathoms. The average expectation of specimens would seem to be greatest at the 

 surface and 50 fathoms, time of day and other variants being neglected. 



From the facts cited above it is obvious that the distribution of this species is 

 between the surface and 100 fathoms, possibly bat improbably, a little deeper. 



Reaction to Time of Day, Light, Main, Sfc. 



The hauls with ojien nets in the epiplankton were arranged, so far as circumstances 

 allowed, to test the alleged nocturnal rise and diurnal fall of the fauna of the upper 

 strata, and the effect on it of varying physical conditions. I have endeavoured — with 

 great elaboration, but very little success — to use serratodentata as a test-case, since it 

 was fairly plentiful at the higher levels. 



In the first place, when arranged on the time-depth tables *, the occurrences show no 

 indication whatever of a nocturnal rise and diurnal fall. 



In the second place, with regard to the reaction of this species to light, I adopted 

 Mr. E. W. L. Holt's suggestion of a photograplier's actinometer [cf. supra, p. 6). 



The numbers of specimens captured f were arranged by light-intensity at the various 

 depths down to 100 fathoms into six groups, according as the sensitive paper of the 

 actinometer reached the standard tint at 2", 3" to 6", 7" to 10", 11" to dusk or dawn, 

 or as the night was dark or brightly moonlit. The only thing that seemed definitely 

 to come out of this attempt was the negative proposition that serratodentata has no 

 special aflr'ection for mooiilight, since six hauls in bright moonlight produced in all 

 hut five specimens. On the other hand, the largest numbers of specimens taken at the 

 surface occurred in a 2'' light (18 siJecimens) and between 11" and dusk (51 specimens), 

 so that the actual light-intensity of the moment in daylight does not seem materially 

 to atfect its presence at the surface. 



Now, although the light-intensity of the moment i)i daytime does not seem to affect 

 the distribution of serratodentata, nevertheless it would seem that there exists an 

 oscillation of this species, but it is the exact contrary of that generally alleged to affect 

 the epiplankton. Dividing the upper hauls into three groups: — (1) daylight, 3 a.m. to 

 7 P.M. ; (2) night, 7 r.M. to 3 a.m. ; (3) hauls taken during or after rain, irrespective of 

 the time of day, and not reckoned under (1) and (2) : then we have the following 

 table, showing the average, the maximum, and the minimum number of specimens 

 captured : — 



*" Reproduced on p. 62 : the occurrences of the species are marked in heav}- type. The construction is described 

 on p. 7 (suprcl). 



t In these calculations, nine surface-hauls made with a net of ISO meshes per inch linear have been omitted, 

 as it was obvious from a comparison with other nets of wider mesh out at the same time that its small mouth and 

 small filtering area made it useless for catching Sar/itta. The numbers actually caught have been treated with a 

 time-factor to bring them to the standard one-hour haul, which was the usual period : for example, the numbers 

 taken in a half-hour haul have been multiplied by 2, those taken in a two-hour haul divided by 2, &c. Single 

 specimens, however, have not been divided. 



