MATERIAL AND METHODS 313 



MATERIAL AND METHODS 



This paper is based upon material contained in the plankton samples of the Discovery Collections. 

 The samples used were largely those collected by means of oblique hauls with the 100 cm. diameter 

 net (NiooB) but, for the purpose of studying vertical distribution, samples obtained from vertical 

 hauls with the 70 cm. closing net (N70V) have also been used and a number of samples from oblique 

 hauls with the young fish trawl (TYFB) have been examined where it was necessary to fill in gaps in 

 the distribution chart. 



The N lOoB was fished for 20 min. from approximately 100 m. to the surface and, at some stations, 

 also from 250 to 100 m. and 500 to 250 m. During these deeper hauls the net was closed at the upper 

 horizon before being brought to the surface. At the majority of stations the N70V was fished at six 

 depths: 50-0, 100-50, 250-100, 500-250, 750-500, and 1000-750 m. ; an additional haul from 1500- 

 1000 m. was also fished at a good many stations. Details of the construction and the methods employed 

 in working these nets can be found in Kemp, Hardy and Mackintosh (1929). 



lOmm 



Fig. I. Adult female E. triacantha. 



By no means all the available material has been used for this study. The samples have been chosen 

 largely from certain lines or groups of stations in order to give adequate coverage of the Southern 

 Ocean at diflFerent times of year and to represent the principal features of the distribution of the species. 

 It is very unlikely that the method of selection has introduced any accidental bias which affects the 

 results. A great many samples had already been analysed by other workers for different purposes 

 before this present work was started, and these analyses, which consist of counts, or both counts and 

 measurements, have been used where possible. 



Where a large number of samples have to be examined the time involved depends to a great extent 

 on the ease with which the species can be identified. Aduh E. triacantha can usually be picked out 

 quite readily on its general appearance alone, but in cases of doubt it can be distinguished from the 

 other related species with mid-dorsal spines on the third to fifth abdominal segments, i.e. E. hanseni, 

 E. spinifera and E. longirostris, by the absence of post-ocular spines (Fig. i). The larvae are also fairly 

 easily identified, either by the strong median spine on the posterior margin of the carapace in the 

 first calyptopis to first furcilia, or by the presence, in the later stages, of the mid-dorsal abdominal 

 spines. They can be distinguished from the larvae of E. spinifera and E. longirostris in all these stages 

 by the absence of denticles on the margin of the frontal plate. 



The lengths given in this paper have been measured from the anterior margin of the eyes to the tip 



of the telson. 



Except for those specimens collected during the earlier years of the Discovery Investigations, when 

 Crustacea picked out of samples were preserved in alcohol, all the material is preserved in neutralized 

 sea-water formalin. 



