254 DISCOVERY REPORTS 



Rustad's records (1934, pp. 15, 28-30, 41) of the analyses of seventy-five Furcilia of 

 Euphausiafrigida, and of 708 of Thysmioessa {macrura and vicina), show that in both only 

 three stages occurred ; the stages in Euphausio frigida were different from those in 

 Thysanoessa. 



Fraser (1936, p. 55) shows that in each of the examples of a succession of " dominant " 

 Furcilia stages given above the relationship of a lower stage of the succession to the next 

 higher is the same : the higher rises from the lower by the non-setose pleopods of the 

 lower becoming setose/ and (if the number of pairs of pleopods of the lower is not 

 already the full number, five) by the addition posteriorly of a pair, or of pairs, of non- 

 setose pleopods. (This is illustrated diagrammatically for my own species in Fig. 34.) 

 He examined enormous numbers (about 3000) of Furcilia (still using the term in its 

 older, more restricted sense) of Eiiphausia superba, and although there were among them 

 fourteen dift'erent stages,^ two groups, each of two successive stages, occurred in far 

 greater numbers than any of the others ; these were in other words the dominant stages, 

 and the relationship between them was that given in the previous sentence. 



It appears, then, that in euphausiids in which the Furcilia development is best known 

 a few only of the stages that are found within each species occur numerously, and they 

 have an obvious relationship to one another as the members of a series : they are suc- 

 cessive instars; in other words the majority of the individuals of a species follow, as 

 Furcilia larvae, the same course in development — and a shorter course than was pre- 

 viously supposed. The numerously occurring stages have been known hitherto, where 

 they have been recognized, as dominant stages : Fraser suggests that in each species they 

 "should be regarded as the actual stages of that part of the developmental history", 

 and the other stages as variants. 



The earlier workers had not recognized among the Cyrtopia any dominant stages such 

 as they saw in the Furcilia, but Fraser found that in E. superba "in the succession of 

 moults which follow on the larvae having five setose pairs of pleopods ... the ecdyses 

 generally coincide with a reduction in the number of terminal spines on the telson from 

 7 to 5, 5 to 3 and 3 to i, but that. . .by no means all the larvae conform to this scheme". 

 Since most of them do he regards those with five, three and one terminal spines on the 

 telson in the same way as the " dominant" Furcilia stages, that is, as actual stages of the 

 developmental history, and those with other numbers as abnormalities. He calls them 

 all, as I have said, Furcilia and not Cyrtopia. 



In these ways he gives a new use to the term " Furcilia stage": 



Lebour's Furcilia stage is any one of the various Furcilia (in the old sense, i.e. with 

 unsegmented antennal endopod) which occur : the form with no pleopods and each one 



1 Fraser examined 262 Euphauda superba Furcilia having non-setose pleopods and in all of them those 

 pleopods " had within the integument the rudiments of setae ", so that when they moulted they must become 

 setose. Macdonald (1927, p. 789), however, saw Meganyctiphanes norvegica Furcilia with non-setose pleo- 

 pods moult and some of the pleopods remain non-setose. 



2 Although this number of stages agrees with Lebour's number of 14 " possible (or known) stages" they 

 are not identical : Fraser did not find three of the stages given by Lebour but he found three stages that were 

 not known before. The number of known stages thus becomes seventeen. 



