UNEVENNESS OF OCEANIC PLANKTON 529 



of Copepoda at the approach of evening, but on record 26 there is a well-marked zone of 

 scarcity between two concentrations. The young of the hyperiid amphipod Parathemisto 

 form a dense concentration in each record at about the same time of the day ; this sug- 

 gests that perhaps they are migrating upwards at the same times and passing the path 

 of the recorder into the surface waters above it. Fluctuations in Globigerina are shown 

 in each record. Euphausiacean larvae occur and increase towards the end of each 

 record. 



Records 27, 28 and 29 



Dates: 16-22. iv. 26. 



Times: see Fig. 14. 



Position: Drake Strait: 6i° 15' 00" S, 64 42' 50" W to 56 11' 40" S, 67 32' 30" W. 



Distances: see Fig. 14. 



These three records form an almost complete line of observation between the Dis- 

 covery Stations 212-221 across the Drake Strait in the direction South Shetlands to 

 Cape Horn. The only gap in the series is a distance of 31 miles between Sts. 214 and 

 215, when the state of the sea was force 7 and the recorder was not used. A violent storm 

 caused a delay at St. 218, the working of which was unsatisfactory; it was repeated as 

 St. 219 when the weather had moderated. Thus the series is broken in time between 

 Sts. 218 and 219 but not in space. St. 218 was begun at noon on April 19 and St. 219 

 finished at 1800 on April 21. The total distance recorded in this series is 273-5 mi l es - 

 The fluctuations in the more important organisms, the times and the distances are 

 shown in Fig. 14. The positions are shown in the chart in Fig. 1. The line of the Ant- 

 arctic convergence, which separates the surface waters of the Antarctic from those of the 

 sub-Antarctic Zone, passes between Sts. 215 and 216. At about this point we see a 

 broad belt of the diatom Thalassiothrix antarctica giving place to one of Rhizosolenia 

 styliformis, the two occurring together for a distance of some 20 miles. 



The distribution of the pteropod Limacina extends over almost all the line of the 

 Antarctic Zone but also penetrates a little way into the sub-Antarctic water. Its num- 

 bers show very pronounced fluctuations. 



There is a remarkable general correspondence between the increase and decline in the 

 Chaetognatha and Copepoda. Both groups, however, show in detail considerable 

 fluctuations and often the numbers of the two fluctuate in reverse directions. This is 

 particularly so between Sts. 219 and 221, also to some extent between Sts. 215 and 216. 

 Since it is known that copepods often form an important element in the diet of chaeto- 

 gnaths, at first sight these reverse fluctuations might suggest that they are being grazed 

 down where the chaetognaths are more concentrated, whilst at other points, as between 

 Sts. 217 and 218, the Copepoda have outstripped the chaetognath numbers. But when 

 we consider the actual numbers involved such an interpretation would only be tenable 

 if the chaetognaths very rarely fed upon copepods of the size sampled. The numbers of 

 the two groups sampled are approximately equal. The chaetognaths must be subsisting 



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