38 DISCOVERY REPORTS 



Anatomical examination of these larvae shows that the progress of development can- 

 not be through a series in which at first non-setose pleopods are successively added, and 

 then by these becoming setose until five setose pleopods result. On the contrary practi- 

 cally every one of the larvae having non-setose pleopods has within the integument the 

 rudiments of setae, so that larvae having two pairs of non-setose pleopods would on 

 moulting become forms having two pairs of setose pleopods and presumably one, two, 

 or three pairs of non-setose pleopods. Larvae with three pairs of non-setose pleopods on 

 moulting become larvae having three pairs of setose pleopods and one or two pairs non- 

 setose, and so on. Thus one would expect to find that the relative abundance of the dif- 

 ferent kinds of non-setose larvae would be reflected in the setose section and this is what 

 actually happens, for scarce forms such as those with two or three pairs of non-setose 

 pleopods are represented by equally scarce setose forms with two or three pairs setose 

 and one or two pairs non-setose. Likewise those with four or five pairs of non-setose 

 pleopods, which occur abundantly, are followed by equally abundant forms having four 

 pairs setose and one pair non-setose or all five pairs setose. 



The suggestion that the larvae moult from forms with non-setose pleopods directly 

 into forms in which those pleopods are setose is supported by the frequency distribution 

 of larvae at stations where a large number occur in the samples. Table XII below shows 

 the larvae from such stations classified according to the condition of the pleopods. 

 The Calyptopis have been included to show that by far the greater number of larvae 

 moult from the third Calyptopis into such as have four or five pairs of non-setose 

 pleopods, and then into larvae having five pairs of setose pleopods or four pairs setose 

 and one pair non-setose. 



It is fair to conclude then that in Euphausia superba the progress of Furcilia develop- 

 ment is from forms in which four or five non-setose pleopods become directly changed 

 on ecdysis into larvae recognizable by having the four or five pairs of non-setose pleo- 

 pods changed into setose pleopods, and in the former alternative by the addition of one 

 pair of non-setose pleopods. Smaller numbers of non-setose pleopods should be re- 

 garded merely as variations of the dominant numbers and, likewise, other combinations 

 of setose and non-setose pleopods should be regarded as infrequently occurring variants 

 of the later dominant form. Thus up to the point when the Furcilia has five pairs of 

 setose pleopods only two Furcilia stages of E. superba are recognizable. 



INTERPRETATION OF DOMINANT STAGES 



Comparing what happens in E. superba with what has been observed in Nemato- 

 scelis microps, Euphausia kroJmii, E. frigida, Thysanopoda aequalis, Meganyctiphanes 

 norvegica, Thysanoessa macrura and Th. vicina, I would suggest that the stages recog- 

 nized as dominant in these species should be regarded as actual stages of that part of 

 the developmental history. Thus in Nematoscelis tnicrops the stages referred to as 

 2nd, 6th and loth, based on the successive addition of pleopods and by these becoming 

 setose, should be regarded as stages i, 2 and 3, and so with the other species mentioned 

 above, except Meganyctiphanes, in which conditions are somewhat different. In this 



