io6 DISCOVERY REPORTS 



a period covering the daylight, dusk and darkness of the approaching end of the day. Clearly, however, 

 as Fig. 6 shows, when co-existent with Metanauplii in the plankton, it is by no means always 

 massed so conspicuously then at this particular level, but may in fact be encountered in equally 

 conspicuous concentration at any level from the surface down to looo m. Fig. 6 in fact reveals 

 a vertical distribution for this period so haphazard that I can only conclude that its motley pattern 

 springs from the fact that sometimes our nets are striking deep concentrations of First Calyptopes 

 that have but recently cast off their MetanaupUar husks and at other times shallower concentrations 

 sprung from earlier moultings. In other words, wherever the deep Metanauplii are moulting and 

 feeding the upper layers the expected level or levels at which the main body of the First Calyptopes 

 will occur, at any time it seems of the day or night, will generally be unpredictable, since in many 

 instances it must be dependent upon how far the larvae may have journeyed upwards towards the 

 surface from the level of their first deep appearance. 



But let us look more closely at the graphs in Fig. 6. The massing of the First Calyp topis in the 

 50-0 m. layer at Station 647, for instance, does it is true suggest a pronounced diurnal movement up 

 to the surface during the darkest hours of the night.^ It is also possible, however, that their appearance 

 at the surface then coincided with the culmination of their long climb from deep water, and if this be 

 true we are not confronted with an instance of diurnal movement here. But what of the deep and by 

 no means insignificant occurrences of First Calyptopes at this and other stations that have been 

 recorded both by night and by day? Are we to conclude that they too spring from diurnal movement? 

 Surely the actively moulting Metanauplii^ with which they are so closely associated does not permit 

 such a view. On the contrary, it can only be concluded that they are deep concentrations, recently 

 sprung from their Metanaupliar parents, either at the outset of, or in some instances well embarked 

 upon, their journey towards the surface, and that they have never in fact been near the surface at all. 



At Station 647 the developmental ascent is in full swing and large numbers of First Calyptopes, 

 springing from the Metanauplii massed below 500 m., are climbing towards the surface. It is unlikely, 

 however, that all of these climbers will actually reach the surface here, since the majority of them, 

 being still in the warm deep current, must be moving away from Station 647 in a direction with a 

 southerly trend. It is equally unlikely, I think, that the 361 Calyptopes at the surface sprang from 

 the same batch of Metanauplii as gave rise to the climbers, for if they had, having regard to the move- 

 ment of the warm layer in which earlier they must have spent some considerable time climbing, they 

 would have appeared at the surface somewhere to the south of Station 647. They could only in fact 

 have sprung from the same batch, or rather part of the same batch, if the Metanauplii in this locality 

 had been spread horizontally over a considerable distance to the north. 



Some further evidence of the more or less unpredictable subsurface levels at which the First 

 Calyptopis may be encountered when co-existent with Nauplii or Metanauplii in the plankton is 

 provided by Fig. 7 presenting data drawn from (i) oceanic stations where only one Metanauplius 

 occurred, (2) oceanic stations where Nauplii and Metanauplii alone occurred, and (3) stations in shelf 

 or slope^ water where Nauplii or Metanauplii were also taken. Even with such small numbers as 

 these the vertical distribution of the First Calyptopis from dawn to dawn presents, in the presence of 

 Nauplii and Metanauplii, the same haphazard pattern as it does in Fig. 6. It is evident too, having 

 regard to the exclusive occurrences of Nauplii and Metanauplii at Stations 2594, 1099, 762 and 764, 

 that had our nets struck these organisms at the precise moment when they were moulting and not, as 



1 This large catch was actually made at about 0130 hr. in 59° 29J' S. The major part of the vertical work at this station, 

 however, took place between 0200 and 0600 hr. 



^ See again p. 97 and footnotes to Tables 13 and 14. 



' The seemingly anomalous near-surface occurrences of Nauplii and Metanauplii in shallow water, several instances of 

 which appear in Fig. 7, are discussed on p. 205 and illustrated in Figs. 29 and 30. 



