117 



THE DEVELOPMENT AND LIFE-HISTORY OF KRILL 



exceptional and suggests that the net passed through a dense swarm of them, which might well 

 account for a concentration of eggs. Some of the females in this particular catch subsequently spawned 

 in the ship's laboratory. The great increase in size of the ovary, when the females become fully gravid, 

 may affect their specific gravity, and this may account for their presence in the deeper layers and their 

 absence from the surface nets. In the most recent analyses made on board ship, so-called gravid 

 females were obtained at depths varying between looo m. and the surface, but the majority, however, 

 occurred in the 0-5 m. layer. These females may not have been fully gravid: measurements of the 

 eggs in the ovaries could not be systematically attempted in the ship's laboratory, and it is possible 

 that females have been classed as gravid which more properly belong to stage 6. On the other hand, 

 if these analyses are correct, the depths at which gravid females are found extend over a very wide 

 range. This problem really depends for its solution on a discussion of the factors influencing the 

 general distribution of E. superba, and is rather outside the scope of this paper. 



Comparison of measurements of length with stages of development shows clearly that undue 

 importance must not be attached to size as an indication of maturity. The adolescent class contains 

 specimens, which judged by length alone would be regarded as fully aduh, but which on internal 

 evidence are far from mature. The smallest specimens in the adult class, on the other hand, might 

 well be regarded as being still adolescent. 



It is interesting to note that although external and internal development do not necessarily keep 

 pace with one another, there is a very fair degree of coincidence of development in the majority of 

 cases. Thus most specimens of stage 1 are also at stage A, those at stage 2 are also at stage B, and so on 

 (Table 6). I would again emphasize the fact that the developmental stages are clearly defined: there 

 is no difficulty in determining to which stage a specimen belongs, so that evidence based on internal 

 and external anatomy, though laborious to acquire, is reliable. 



PAIRING 

 The first records of fully adult males were obtained in September, when five specimens were found. 

 By October they appeared in larger numbers, and in this month the first females carrying spermato- 

 phores in the thelycum were noticed. These six females were not yet gravid, but were at stages 4 and 5, 

 the eggs being still in course of development. Pairing evidently takes place as soon as the females 

 have a fully developed thelycum, and it is generally possible to determine whether or not it has occurred 

 recently, because shortly after the spermatophores have been implanted, the sperm-mass passes into 

 the thelycum, leaving the spermatophores empty. The following table shows that of 556 females 

 carrying spermatophores, only 102 were full, showing that the migration of the sperm-mass into the 

 thelycum must be a rapid process. 



Table 1 1 . Number of females with spermatophores 



556 



