LARVAL DEVELOPMENT 255 



of the forms represented by the several combinations of pairs of non-setose, of setose 

 and non-setose, and of setose pleopods, is a Furcilia stage. There may be as many as 

 fourteen {E. superba, Fraser, 1936) in one species. 



Fraser's Furcilia stage. Each of the earlier of Eraser's Eurcilia stages is a group of 

 Lebour's stages centred around, and made up for the most part (in terms of the numbers 

 occurring) of one^ of her dominant stages ; there are as many of Eraser's earlier Eurcilia 

 stages in a species as there would be of Lebour's dominant stages. His later stages, 

 those among the forms that were previously called Cyrtopia, are recognized by the 

 number of terminal spines on the telson ; those with five, three and one spines form, 

 respectively, successive stages. 



The results of my work on the larval stages of five species of Euphaiisia, only one of 

 which is mentioned above, confirm Eraser's view of euphausiid development. In the 

 pages that follow the term Eurcilia stage has Eraser's meaning (or something very near 

 it), and I use Eurcilia, as he does, to include both the Eurcilia and the Cyrtopia of earlier 

 writers. 



I have examined large numbers of Eurcilia of E. frigida, E. vallentini and E. triacantha, 

 smaller numbers of those of E. longirostris and E. spinifera. In Table I the forms which 

 occurred in these species and the numbers of each are shown. The numbers of E. longi- 

 rostris include those recorded by Sars and Tattersall (one and twenty respectively, see 

 p. 285) ; the numbers of E. spinifera include six that I have examined from the Deutsche 

 Tiefsee-Expedition material (see p. 294). The forms which, in the various species, are 

 Furcilia stages are distinguished by having the numbers in which they occurred printed 

 in heavy type. 



It is convenient to consider the two groups of forms shown in Table I separately : 



Those recognized by the character and number of the pleopods. The table makes it clear 

 that the course of early Furcilia development in E. frigida, E. vallentini and E. triacantha 

 is more fixed than it is known to be in any other euphausian ; almost all the individuals 

 of a fairly large collection of each species belong to one of three Furcilia stages which 

 are the same in each species ; very few other forms occur (two, one and two respectively 

 of the three species) and specimens of them are rare. In Lebour's terms it might be said 

 that the dominant stages are almost the only ones. 



The numbers of E. longirostris and E. spinifera are few, but it is significant that those 

 few belong, with the exception of one E. spi?iifera, to one of four Furcilia stages which 

 are similar in both species. I believe that if large numbers of the early Furcilia of 

 E. longirostris and E. spinifera were seen, most of them would be found to belong to 

 those four stages. 



1 This definition does not strictly cover E. superba : its dominant earlier Furcilia stages, of which there are 

 two, are each made up of two Furcilia stages in Lebour's sense ; the first is of individuals having four or five 

 pairs of non-setose pleopods and it follows that the second is of individuals having four pairs of setose and 

 one pair of non-setose, or five pairs of setose pleopods. In the examples of Euphausiid development that 

 Fraser quotes from earlier writings, and in the five species that I describe here, each dominant stage is one 

 Furcilia form, one of Lebour's Furcilia stages. 



