LATER LARVAL DEVELOPMENT 253 



THE LATER LARVAL DEVELOPMENT OF FIVE SPECIES 



LARVAL DEVELOPMENT 



Eraser (1936) has just published in this volume an account of his examination of the 

 larval stages of Eiiphaiisia superba, more detailed and based upon far larger numbers 

 of specimens than any previous account of the development of a euphausiid. He dis- 

 cards the division of the later larval stages of the Euphausiidae into Furcilia and 

 Cyrtopia. The division was based upon the change in form and function of the antenna 

 from a swimming to a non-swimming organ : in the Furcilia the limb was directed out- 

 wards, and the endopod and exopod were similar to one another in size and shape ; in 

 the Cyrtopia the endopod was segmented and the exopod scale-like, the whole pointing 

 forwards. Eraser points out that this change in form is not of the same magnitude as 

 those which distinguish other stages (for example, the Calyptopis and Furcilia), and he 

 shows that it is one of several changes of, seemingly, as great a magnitude and sig- 

 nificance which take place at about the same time, but not necessarily at exactly the 

 same time. He has called all those stages, which were before called Furcilia and 

 Cyrtopia, Furcilia stages, and in this I follow him. 



Brook and Hoyle (1888, p. 419) recognized, by the comparative development of the 

 pleopods, eleven Furcilia stages — using the term in its old sense — in one species, though 

 Lebour (1925, pp. 811-12) appears to imply that that one species might have been a 

 mixture of two {Thysanoessa) species. Lebour in her General Survey of Larval Eii- 

 phansiids, etc. (1926 i, pp. 523-4) gave (i) a list of the kinds of euphausiid Furcilia, 

 fourteen in number, which had been found at that time, and (ii) a list of the species of 

 which larvae had been found, showing, where those larvae included the Furcilia, what 

 Furcilia stages they were. The largest number of stages out of the fourteen "possible" 

 ones to be found in one species was eleven — in both Nyctiphanes coiichii and Mega- 

 nyctiphanes norvegica. She stated (p. 525) that "the evidence is in favour of a certain 

 order of development of the pleopods" (i.e. a certain succession of Furcilia stages) "in 

 the various genera"; and that these "distinctive stages" (of a genus) "nearly always 

 seem to occur as though certain stages were dominant ". " There is distinct evidence of 

 the jumping of stages in some species." In a later paper she gives examples of dominant 

 Furcilia stages in two species, Nematoscelis microps and Eiiphaiisia krohnii (Lebour, 

 1926 c, pp. 766, 770), and a description of what is undoubtedly the succession of 

 dominant stages (though she does not call them that) in a third species, Thysanopoda 

 aequalis (p. 768). Macdonald (1927), the first worker to publish a paper with quanti- 

 tative results on the larval stages of a euphausiid, showed that of 302 specimens, in- 

 cluding eleven Furcilia stages, of Meganyctiphanes norvegica nearly 20 per cent were of 

 one stage and over 26 per cent of another, higher, stage ; the remaining 54 per cent being 

 made up of the totals of the other nine Furcilia stages. The two stages which occurred in 

 far greater numbers than any of the others he described as "tending to be dominant". 



