460 Birge — Vertical Distribution qf Pelagic Crustacea. 



find that the upper level contained somewhat more Cyclops by- 

 day than by night, the percentage found in the 0-3 m. level 

 being 44.75 per cent, by day and 39. 64 per cent, by night. 

 The percentage found in the 3-6 m. level was almost the same 

 in the two cases, being 30.81 per cent, by day and 30.05 per 

 cent, by night. The 6-9 m. level showed, of course, a smaller 

 number by day, 15.08 per cent., and a larger number at night, 

 18.55 per cent. These numbers are, however, not large enough 

 to show decisively any migration of the Crustacea. In the 

 different periods much the same differences in diurnal distribu- 

 tion are noticed as for Diaptomus. In Period I, the number 

 found by night in the 0-3 m. level is slightly greater than by 

 day, but the difference is so small as to come well within the 

 limits of error. In Period II, the day observations show a very 

 considerable excess, having 52 per cent, of the total catch, while 

 the night observations show only 44.5 per cent. A difference 

 still greater was found in Period III, where the day observa- 

 tions gave 42.75 per cent, in the upper level, and 33.09 per cent, 

 by night. This last difference is greater than the difference of 

 Diaptomus for the same period. In Period IV, the night 

 catches show a slight excess over the day catches, the upper 

 level containing 39.44 per cent, by day and 43.7 per cent, by 

 night. It is clear that here are no indications of a descent of 

 the Crustacea by day and also that they do not indicate an as- 

 cent by day as plainly as does Diaptomus. If there is any 

 diurnal migration of the Crustacea, Cyclops would probably not 

 show it as distinctly as Diaptomus, since its power of loco- 

 motion are so much smaller. In Period I, while the number in 

 the 0-3 m. level at night was somewhat larger than that by 

 day, the number in the 9-12 m. level was also considerably 

 larger by night than by day. The number of specimens and 

 the number of observations in this period were both so small 

 that chance would play a considerable part in the apparent dis- 

 tribution of the Crustacea, and the night observations were 

 all made earlier than 10 p. m. 



Center of distribution. — The level above and below which lay 

 50 per cent, of the Cyclops varied somewhat during the month. 

 In Period I it lay at 4.54 m. below the surface. In Period II it 



