THE DEVELOPMENT AND LIFE-HISTORY OF KRILL iiS 



each measuring 35 mm., occurred in October and December, the largest specimen measuring 60 mm. 

 in February ; all were at stage G externally. The majority of specimens (538) were fully developed 

 externally, but in 20 males growth of the copulatory organs was not complete, 19 being at stage F 

 and I at stage D. 



MATURE FEMALES 



I have classed all females in which the eggs are of measurable size as mature, i.e. likely to spawn 

 within the next 2 or 3 months. 



The eggs become large enough to be measured when the ovary fills about half of the thoracic 

 cavity (stage 4). The thelycum, though not necessarily very heavily chitinized as yet, has assumed the 

 adult shape, and is ready to hold the spermatophores, when they are transferred by the male. 



Table 9. Growth stages in adult females 



In stage 4 of the 307 specimens examined, 55 were externally at stage D, 246 were at stage E, 3 were 

 at stage F and 3 were at stage G. The majority had, therefore, fully mature though unfertilized 

 thelyca. In six of the specimens, those at stages F and G, copulation had taken place, and the 

 females were carrying full or empty spermatophores. The average monthly length varied from 39 mm. 

 in August through 31 mm. in November to 46 mm. in March, falling again in April to 42 mm. No 

 records were obtained in February, May, June or July. The smallest specimen measuring 27 mm. 

 occurred in November: it was externally at stage D. The largest specimen of 55 mm. occurred in 

 September and was at stage F. 



By the time stage 5 is reached, the eggs are growing rapidly, and consequently the ovary is beginning 

 to fill up the thoracic cavity. Externally, 117 specimens were at stage E, but the majority carried 

 spermatophores, 51 being at stage F and loi at stage G. One large specimen measuring 43 mm. was 

 at stage D, whereas the smallest specimen recorded, measuring 30 mm., was at stage G, and the 

 largest specimen of all, measuring 58 mm., was at stage F. The average monthly length varied from 

 46 mm. in August, through 43 mm. in November and 42 mm. in January back to 46 mm. in February. 

 Only one specimen occurred in March, and no records at all were obtained in April, May, June or 

 July. Stages 4 and 5 correspond to Ruud's stage 1. 



Stages 6, 7 A and 7 B may be regarded as covering the fully adult condition in the female and are 

 comparable with stages 2, 3 and 4 of Ruud. The three corresponding stages of external development, 

 F, G and H, show whether copulation has just taken place, or whether it occurred some time ago, 

 and lastly whether the female has spawned, although this is also readily seen from her altered shape. 

 When the spermatophores have been recently implanted in the thelycum, they still contain the sperm- 

 mass (stage F), but after a while this makes its way into the thelycum, leaving the spermatophores 

 empty (stage G). When the eggs are being laid, the spermatophores become loosened and break away 



