THE REPRODUCTIVE SYSTEM OF 

 EUPHAUSIA SUPERB A 



By Helena E. Bargmann, Ph.d. 

 (Plates I-V ; Text-figs. 1-26) 



INTRODUCTION 



THIS account of the reproductive system oi Euphausia superba forms the introduction 

 to a further paper on the development and distribution of post-larval and adult krill. 



The composition of any specific population is estimated not merely by length 

 measurements, but by the degree of maturity to which the individual members of each 

 sex have attained, and before the life history and distribution of a species can be described, 

 it is essential that these facts should be ascertained. 



In E. superba, external sexual characters appear first in half-grown specimens, but 

 in a great number of such individuals these characters are not yet developed. In this 

 investigation, therefore, to ensure correct sex determination, all the specimens both 

 large and small were measured and dissected, and the degree of development of the 

 external and internal genitalia was noted. Some new anatomical details resulting from 

 this intensive investigation are set out in this short paper. 



MATERIAL AND METHODS 



John (1936), in his paper referring to the distribution of the genus Euphausia, places 

 E. superba in his Southern Group, which is circumpolar in range. This species, which 

 can occur in enormous numbers, is found in the colder water of the Antarctic Zone, 

 and along the edge of and under the pack-ice. No detailed list of stations from which 

 material was obtained is appended here, since it belongs more appropriately to the 

 main paper, where it will be included. 



The post-larval and adult specimens in the Discovery Collections were preserved 

 in alcohol, and in consequence the varying degrees of shrinkage due to the use of 

 different preservatives, which Eraser (1936) encountered in his work on the larval forms, 

 did not exist. Some specimens were, of course, more flexed than others, but each was 

 straightened out as completely as possible on an opal glass scale, and its length from the 

 anterior margin of the eyes to the end of the telson determined to the nearest millimetre. 

 The anterior edge of the eyes, rather than the tip of the rostrum, was selected as one 

 Hmit of measurement, being readily distinguished and less liable to damage. 



The carapace and body muscles were then dissected from the left side under a 

 binocular microscope, and the reproductive system was examined. The sex and the 

 approximate age of the specimen having been thus determined, external sexual cha- 



