DEVELOPMENT OF EUPHAUSIA SUPERBA 



35 



It has escaped notice that of the stages which are dominant in the six species just 

 mentioned, the earher dominant stage bears a direct relationship to the later. This 

 relationship is significant in the consideration of Eiiphaiisia siiperba which follows. 



The less advanced dominant stage becomes in each species the more advanced by the 

 provision of setae on the non-setose pleopods present in the earlier stage and the addi- 

 tion of one or more pairs of non-setose pleopods. In other words, each pleopod appears 

 first in the non-setose form, and is provided with setae at the next ensuing moult. Thus, 

 in Nematoscelis as indicated in Fig. 7 below, the one pair of simple pleopods gives rise 



NEMATOSCELIS MICROPS 



IT 



5?^ 



□ 



roS? 



/K/V/T\yX A\ 



EUPHAUSIA KROHNM 



■^^ ^W^ 9BS 



THYSANOPODA AEQUALIS 



corn (^qrrn 55^ 555? 5gg 



EUPHAUSIA FRIGIDA 



W^ 9ffl? SSS 



THYSANOESSA VICINA AND MACRURA 



WWv 



^^ ^ ^ /K 



MEGANYCTIPHANES NORVEGICA (DOMINANT STAGES ONLY) 



Fig. 7. Diagram to show the relationship of pleopod development and successive dominant stages. In 

 euphausians where the development is known the pleopods appear first in the non-setose form and become 

 setose in the ensuing dominant stage. 



to a form having one pair of setose pleopods and three pairs simple, and in the next 

 conspicuous stage these three simple become setose and one non-setose pair is added. 

 A similar method of development takes place in Eiiphaiisia krohnii, Thysanopoda aeqtialis, 

 Euphausia frigida and Thysanoessa macnira and vicina. Meganyctiphanes norvegica 

 shows by its dominant stages that a similar process of development is taking place in it 

 also, for the first dominant stage, that having three pairs of non-setose pleopods, is 

 followed by one having three pairs setose and two pairs non-setose. 



In a very recent paper by Frost (1935) there is still further evidence of a succession of 

 early Furcilia stages in which non-setose pleopods become setose in the next ensuing 

 moult. The pleopod arrangement in Nematoscelis megalops is: Two pairs non-setose — 

 two pairs setose, three pairs non-setose — all five pairs setose. In Stylocheiron longicorne 



