34 DISCOVERY REPORTS 



to be sometimes omitted during development in captivity were less frequent m the 

 plankton than the other Furcilia stages ". "On the other hand ", he states, "there were 

 certain stages which predominated in numbers in the plankton." The graph of the fre- 

 quency of distribution of his eleven Furcilia stages for 302 larvae shows a distinctly 

 bimodal curve, having the one maximum at stage 4 (i.e. three pairs of simple pleopods) 

 and the second at stage 9 (three pairs of setose pleopods and two pairs simple). It is also 

 interesting in this paper to note that of twenty-six larvae kept in aquaria more than half 

 moulted irregularly. 



Attention is drawn by Macdonald to further evidence of curtailed larval history pro- 

 vided by another euphausiid, Nemntoscelis microps, writing of which Lebour states 

 (1926 d, p. 766): "The youngest stage found is presumably the second furcilia, mea- 

 suring 2-4 mm. in length and having one pair of simple bud-like pleopods The next 



seen has one pair of pleopods setose and three pairs simple. It is striking that these stages, 

 together with the tenth,i seem to be dominant, as no intermediate stages were found whilst 

 these were abundant. Moreover, the sixth and tenth are the stages described by Sars." 

 Again, on p. 770 Lebour states with reference to Euphmtsia krohiii: " Sars describes 

 the second and seventh furcilia- having one pair of simple pleopods, and one pair setose 

 and four pairs simple respectively. No stages between these two have been found in 

 the Alexandria samples." "The Furcilia with all pleopods setose is presumably the last." 

 On pp. 768 et seq. of this paper on euphausiids from the Mediterranean, Lebour de- 

 scribes the larvae of Thysanopoda aeqtialis, Hansen. " The first furcilia ", she states, " the 

 earliest stage seen, measures 2-6 mm. in length. It has no pleopods. The next stage 

 present, measuring 2-9 mm. in length, is probably the third Furcilia having two pairs of 

 simple pleopods. Then apparently several stages are absent and the next seen, measuring 

 3-5 mm. and probably the seventh Furcilia, has two pairs of setose pleopods and two 

 pairs simple." The following Furcilia stage (text-fig. 2, p. 769) is that in which there are 

 four pairs of setose pleopods and the last pair non-setose. 



Rustad (1934, p. 15) shows that in Euphausia frigida the Furcilia stages 5, 13 and 14 

 of Lebour 's pleopod diagram are the only ones to be found ; these stages are, respectively, 

 forms having four pairs of non-setose pleopods, four pairs setose and one pair non- 

 setose, and all five pairs setose. In the same publication, under Thysanoessa macrura, 

 G. O. Sars, and Th. vicina, Hansen (the two species are not separated), Rustad states 

 that he found numbers i, 6 and 14 of the Furcilia stages of Lebour, representing larvae 

 having respectively no pleopods, five pairs of non-setose, and five pairs setose. 



Lebour suggests the dominance of certain stages in her earlier paper (1926 c) and 

 demonstrates it in the species Nematoscelis microps, Euphausia krohiii and Thysanopoda 

 aeqtialis (1926 J). Macdonald shows that in a sample of Meganyctiphanes norvegica, 

 where eleven different Furcilia forms occur, two stages are dominant, set apart from one 

 another by an interval of several intermediate stages. 



1 The tenth stage referred to is the tenth for this species, not the tenth stage of Lebour's scheme. The 

 pleopod arrangement is four pairs setose and one pair non-setose. 



2 The seventh stage here is the 9th Furcilia stage of Lebour's scheme. 



