336 DISCOVERY REPORTS 



and calculating the mean length for each month. The o- and i-year groups are readily distinguished 

 and the points at which they have been separated are indicated in Fig. 15 by the lower pecked line. 

 The I- and 2-year groups can also be separated in September, October and November as all the 

 males above the upper pecked line have fully developed spermatophores and nearly all the females 

 have well-developed ovaries with large eggs scattered about the surface, whereas these characters are 



40- 

 36- 

 32- 



28- 

 i 24. 



£ 2o^ 



o 



z 



_) 



12' 

 8- 

 4- 

 O- 



X XI XII I II III IV V VI VII VIII IX X XI XII I II Ml IV V VI VII VIII IX X XI Xil 



MONTHS 



Fig. 16. Growth curve for E. triacantha taken from Fig. 15. 



— I 1 — 



I II III 



Table 9. Mean monthly lengths of Euphausia triacantha 



absent in the smaller size groups. For the purposes of calculating the mean monthly lengths for Fig. 16 

 it has been assumed that after the end of November all those animals in size groups above the pecked 

 line in Fig. 15 belong to the i-year group. The mean lengths and the number of observations on which 

 they are based are shown in Table 9. The mean lengths for September to December do not take 

 account of larvae not yet hatched and the estimated curve for these months is shown in Fig. 16 as a 

 pecked line which starts at the estimated mean time of hatching (see p. 333) and joins the rest of 

 the curve in January when no more larvae are being hatched. 



During the first six and a half months after hatching, that is from mid-October to the end of April, 

 E. triacantha grows fairly steadily at a rate of about 2.5 mm. per month. With the onset of winter the 

 rate of growth slows down to less than half this, the curve giving an average of i mm. per month for 

 May to August. From September to February of the second year the rate is again increased though 



