428 University of California Publications in Zoology [Vol. 22 



relate to actual diurnal differences. But it is of interest to note the 

 rather marked significance of the difference between numbers obtained 

 at 8 a.m. as compared with noon. As is shown in table 6 the differences 

 between 8 p.m. and midnight, and between midnight and 4 a.m., are of 

 slight significance, but those between 8 p.m. and 4 p.m. and 4 a.m. and 

 8 a.m. are very strongly significant. On the whole the results show 

 that there is a marked excess of numbers at night, and this excess is 

 not due to chance in the cases of Acartia tonsa and clausi where the 

 probability has been determined. 



TABLE 6 



To Show the Probability That the Differences Between Numbers of 



Animals per Haul at Different Times of Day Are Significant 



The figures are based on the corrected values and deal with the two-year 



averages given in table 5. 



Probability that 



8 a.m. 4 p.m. S p.m. 4 a.m. Midnight 4 a.m. 



exceeds exceeds exceeds exceeds exceeds exceeds 



noon noon 4 p.m. S a.m. 8 p.m. midnight 



A. tonsa 0045 .0087 .0005 .00012 .13 see note 



A. clausi 0022 .057 .00037 .0002 .13 039 



Small calanids 0007 042 .10 044 015 



Podoplea 0495 02S .02 .045 



Note: The number of animals per haul for the two years was greater at mid- 

 night than at 4 a. m., and the probability is .165 that the difference, is significant. 



It will be noted in table 6 that the values under the headings 

 "small calanids" and "Podoplea" are usually larger than those 

 under Acartia. This difference is especially evident in the third and 

 fourth columns. The probable reason for the larger values is that 

 the small calanids and Podoplea are mixed groups. It is expected 

 that when the species for those groups have been worked up, the day 

 and night differences will be as great as for the species of Acartia. 



DISCUSSION OF BESULTS 



Since this paper is a preliminary one and in the nature of a report 

 of progress, an extended discussion of the results obtained is hardly 

 necessary. But some facts of general importance have been learned as 

 well as some suggestions regarding methods, and these may well be 

 pointed out. The tables show that there is considerable seasonal varia- 

 tion in the copepod plankton, and the question is always present whether 

 such variability is due to changes in the sea water. It is possible that 

 correlations may be found between the relative abundance of the 

 animals and hydrographic conditions, but, as has been pointed out 

 by Herdman and Scott (1908), hydrographic changes will not deter- 



