1911] Estcrhj: Vertical Distrihution of Eucalanus. 5 



the hourly averages at all levels [represented in oiir collections] 

 from 6 to about 9 p.m. At one level (Table 2, column C, line 3), 

 there seems to be an increase about midnight, but this does not 

 l)ring up the hourly average to what it was during the day. The 

 hourly averages in line 6 of columns A, B, and D indicate a 

 return in the earl^y morning to the daylight plurimum, but in the 

 light of the other figures it is difficult to see what significance 

 this can have. 



To repeat. Table 2 shows that Eucalanus, as a whole, keeps 

 well down during the day (column A), the plurimum being at 

 200 fathoms. During the early part of the night (column B, lines 

 3-7), the population at all depths is materially decreased. There 

 is no corresponding increase, therefore, at any of the levels and 

 accordingly there does not seem to be a vertical migration. The 

 fact of the decreased hourly averages, however, deserves empha- 

 sis, because the decrease is in evidence at a time when the light 

 is much reduced, and also at the time (6-9 p.m.) that collections 

 with the surface nets show an increase (columns A and B, line 1). 



It does not seem likely that the smaller opening of the closing 

 net can be the reason for this apparent inconsistency, because in 

 another species {Calanus fimnarcliicus) the collections with the 

 same kinds of nets bore out each other. Likewise, it does not 

 seem probable that collections at other levels would show w^here 

 the animals are at night; there is no reason to think that the 

 depths at which collections were made do not give a fair enough 

 indication of the vertical distribution of the species. A third 

 suggestion is that the distribution shown in Table 2 may be due 

 to horizontal migration, such as described by Juday (1904, p. 

 558) for some of the fresh water Crustacea; however, the fact 

 that the decrease in hourly averages takes place over the entire 

 area does not bear out this idea. Lastly, the animals may spread 

 over a wider horizontal area, as night comes on, than that which 

 they occupy during the day, on account of active wandering or a 

 "nocturnal habit." Even if this were known to be the case it 

 would not add much to our understanding of the movements of 

 the plankton organisms. 



A study of the movements or distribution of an organism so 

 perfectly adapted on account of transparency and lack of color 



