3i6 DISCOVERY REPORTS 



7. Certain differences in the distribution of the species in 1932-3 from the distribu- 

 tion in 193 1-2, such as the scarcity of the species in the western Drake Passage in 

 October 1932, have been ascribed to the fact that the investigations were carried out a 

 month earUer in the spring of 1932-3 than in the spring of 193 1-2, before the spring 

 ascent of the species had been completed. Otherwise the differences in distribution 

 during the two seasons were not very great. The catches generally were smaller in 1932-3 

 than in 1931-2 (pp. 308-11). 



8. As the season advances there is, so far as can be ascertained, a general diminution 

 in the catches of R. gigas, with a tendency for the area of maximum abundance to be- 

 come concentrated in Antarctic water around South Georgia rather than in the Drake 

 Passage (p. 312). 



9. The comparatively small Rhincalanus population in the Weddell Sea enters that 

 area by means of the southward-flowing warm deep water from the South Atlantic or 

 South Indian Oceans. In the latter case it is carried into the Weddell Sea in the deep 

 water of the East Wind Drift current flowing westwards into the Weddell Sea south of 

 66° S along the coast of Coats Land (pp. 304-7). 



10. The species appears to be carried upwards to the surface out of the deep water by 

 some unknown factor in the course of its passage around the Weddell Sea. It is suggested 

 that the upwelling of warm water in the centre of the cyclonic system may perhaps be 

 the cause of this (p. 306). 



LIFE HISTORY 

 APPROACH OF THE SPAWNING PERIOD 



The indications of the approach of the spawning period in any copepod population 

 are: 



(i) The appearance of large numbers of females with ripe gonads. 



(2) The appearance of male adults in large numbers in the catches. 



(3) The approach of the stock to maturity as indicated by the disappearance of 

 juveniles from the population. 



These indications may be discussed separately. 



Appearance of ripe females. No detailed observations of the numbers of ripe fe- 

 males in the catches were made during the season 193 1-2. During the season 1932-3, 

 however, a selection of samples of 50 or less adult females from a number of stations was 

 preserved and examined for the presence of ripening eggs in the gonads. The samples 

 selected cover the period from the end of October to the middle of March, but owing to 

 the fact that the ship was engaged on hydrographic survey during January there are no 

 observations for that month. 



R. gigas is transparent and in the female the gonads can be clearly seen occupying a 

 saddle-shaped area postero-dorsally beneath the carapace (Fig. 17). Two long oviducts 

 run back from the ovary on either side to open on the genital segment. In the yet im- 

 mature female the genital cells are transparent and almost invisible in the dorsal and 



