JO DISCOVERY REPORTS 



The following observations were made : 



Total length of larva. 



Telson : (a) Number of terminal spines. 



(b) Number of postero-lateral spines. 

 Number of spines on the telson foreshadowed for the succeeding stage. 

 Antenna. Number of segments in inner ramus. 

 Mandibular palp : {a) Number of segments. 



(b) Number of spines on the terminal segment. 

 I St thoracic appendage. Number of segments in the endopod. 

 Presence or absence of a spine on the rostrum. 



In addition to the evidence from these observations, the records of larvae from the 

 70-cm. net samples generally, and from the oblique and horizontal net samples from the 

 circumpolar cruise have been utilized. 



Before proceeding with the discussion of this evidence it is proposed to consider the 

 term "Cyrtopia" applied to euphausian larvae. As stated above (p. 32) the name was 

 applied by Dana to distinguish a schizopodous genus which was afterwards shown by 

 Claus to be a stage in the development of Euphausiidae. Later workers have adopted 

 this division of larval development, and recognize the Cyrtopia by the alteration in form 

 and function of the antenna of earlier stages. Rustad states (1930, pp. 68 et seq.) : " The 

 Furcilia stages are characterized by having the antennae retained as swimming ap- 

 pendages whereas in the Cyrtopia stages they are directed forwards and evidently have 

 no importance for swimming. In preserved material, however, where the antennae as 

 a rule are directed forwards in late Furcilia stages one must have recourse to morpho- 

 logical characteristics and concerning this Lebour writes (1925, p. 816) 'The best way 

 to distinguish a late Furcilia from an early Cyrtopia is by the flagellum of the antenna 

 which is unjointed in the Furcilia and jointed in the Cyrtopia'. " 



Considering the indefinite nature of later euphausian development which has already 

 been demonstrated and anticipating the conclusions derived from the sample of larvae 

 from St. 374, it seems that the recognition of the Cyrtopia places too much stress on the 

 alteration of form and function of one particular appendage. The larvae designated 

 Cyrtopia are not recognizable from Furcilia by any sudden increase in size as they change 

 from the old to the altered form. The change in the antenna does not necessarily coincide 

 with equally significant changes in the form of other appendages, for example, the 

 mandibular palp and the ist thoracic appendage. It varies also in relation to the de- 

 velopment of the telson: for the number of terminal telson spines in the first Cyrtopia 

 of various species of euphausians is given as : 



Meganyctiphanes norvegica\ ^^^^.^^^ ^^.^^^^ 



Thysanoessa raschn J 



Eiiphausia krohnii 5 ,, 



Nyctiphanes couchii 3 „ 



Nematoscelis tnicrops ] 



Thysaiiopoda aequalis J 



It is apparent, therefore, that the change in the antenna is not of the significance 

 formerly attributed to it, and the altered form does not merit the distinction of a division 



