POST-LARVAL DEVELOPMENT 261 



appendages are fully formed as far as the exopodite of the male seventh thoracic limb is concerned, 

 and also the number of gill-lobes in both sexes. It has, therefore, been thought advisable to omit 

 the term post-larval altogether, and regard the late Cyrtopia to be the direct predecessor of the 

 aduh. 



Lebour, Macdonald, Rustad and Frost described as Cyrtopia stages the wliole series 

 of moults between their last Furciha and the adult ; none of them fixed an easily recog- 

 nized upper Hmit to the Cyrtopia. 



I suggest that the term post-larval should be brought into use again ; it is convenient, 

 for the species described in this paper and for a -number of others, to regard the larval 

 stages as ending at that point where the three pairs of postero-lateral spines of the telson 

 become reduced to two by the disappearance of the middle pair (the " outer long lateral 

 spines" of Lebour, Macdonald and Frost). The preceding pages have shown that up 

 to that point it is easy to recognize successive stages, first by the pleopods, then by the 

 terminal spines of the telson. There is no such easy way of recognizing later stages : they 

 "merge imperceptibly into the adult"; and in doing so they soon acquire some of the 

 characters of the adult and may be recognized by them. For example: Euphaiisia 

 kroJuiii shows in the last stage in which it has three pairs of postero-lateral spines the 

 antennular lappet developing towards its adult and characteristic form ; at a slightly later 

 stage E. vallentini shows the beginnings of the spine on the third abdominal segment ; 

 both E. longirostris and E. spinifera develop the antennular lappets and spines on the 

 fourth and fifth abdominal segments. I have called those forms which follow the last 

 stage to have three pairs of postero-lateral spines on the telson (my last Furcilia stage) 

 post-larval forms, but I have made no attempt to describe the successive moults which 

 take place. 



The last Furcilia in each of my five species has one terminal spine and three pairs of 



postero-lateral spines on the telson. There is such a stage in each of the following 



euphausiids : 



Thysanopoda aequalis (Lebour, 1926 c, p. 770). 



Nyctiphanes coiichii (Lebour, 1925, pi. ii, figs. 7, 8). 



Thysanoessa inermis (Lebour, 1926 a, pi. iv, figs. 9, 10). 



T. raschii (Macdonald, 1928, p. 64). 



T. macrura (Rustad, 1930, fig. 45 a). 



Euphaiisii krohnii (Frost, 1934, fig. 9 B). 



E. superba (Fraser, 1936, p. 97). 



In the development of twelve euphausiid species then, of four genera, the middlemost 

 of the three pairs of postero-lateral spines is not lost until after the number of terminal 

 spines is reduced to one. But in one species it is known to be otherwise: in Mega- 

 nyctiphanes norvegica (Lebour, 1925, p. 826, pi. ix, figs. 8, 9) the middlemost pair of 

 postero-lateral spines is lost while the telson still has three terminal spines. In this 

 species I would suggest that the point at which the number of terminal spines is reduced 

 to one would most suitably be regarded as dividing larval and post-larval forms. It is 

 clear that no general rule, based on characters chosen because they are easy to recognize, 

 can be laid down for separating larvae from adolescents in Euphausiidae. 



