264 DISCOVERY REPORTS 



The different paths that development takes in the species for which it is known are 

 shown in Table IV. Other forms of Furcilia than those shown as stages occur in most 

 of the species ; it may be that in those I have interpreted here in terms of our Furcilia 

 stages from the writings of others they occur in numbers large enough, if they were 

 known, to make necessary some less simple interpretation of the path of development. 

 It may be found that there are two instars where it appears that there is only one— 

 as Eraser found that the Furcilia of Euphaiisia superba with five setose pleopods and 

 seven terminal spines are composed of two stages. But the path of development shown 

 for each species, even if it conceals a step by showing two steps as one, and even though 

 some individuals develop along other paths, is the one followed by most of the individuals 

 of the species. 



Eiiphausia triacantha, E. longirostris and E. spinifera are closely allied species forming 

 with E. hanseni a natural group within the genus. E. spinifera lives in the sub-Tropical 

 Zone, E. longirostris in the sub-Antarctic, and E. triacantha in the higher (the colder) 

 sub-Antarctic and the Antarctic Zones. It is interesting to note that E. triacantha has 

 a different path of Furcilia development to its allies. 



Descriptions of the larval development of five species of Euphatisia follow. All the 

 measurements are of material preserved in formalin. I have followed English writers in 

 calling the first thoracic limb by that name, and those that follow, the second to eighth. 

 Scandinavian writers have called the first limb the maxilliped, and those following, the 

 first to seventh thoracic limbs. 



Development of Euphausia frigida 



Rustad (1930) described most of the larval forms of E. frigida, none of which had been 

 recognized before. He had large numbers (no) of the three Calyptopis; smaller num- 

 bers (21) of the earlier Furcilia, my stages I-III; and small numbers (12 only) of the 

 later Furcilia, his Cyrtopia— my FurciUa stages V and VI. Ruud (1932, p. 52, table 13) 

 recorded a smaller collection of larvae, richer like Rustad's in Calyptopis than in later 

 stages. Rustad's Furcilia, especially the later, were not numerous enough for him to 

 recognize the steps by which development proceeds, but in a later paper (1934, p. 15) 

 he showed that the seventy-five earlier Furcilia of a larger collection fell into three 

 groups — my Furcilia stages I, II and III. 



The larval forms and the numbers of each that I found are as follows : 



Calyptopis: Second ... ■•• ••• ••• ••■ ■•• ■•• ^3 



Third 50 



Furcilia: 



With 3 pairs of non-setose pleopods ... ••• •■■ ••• i 



Stage I. With 4 pairs of non-setose pleopods 123 



With 5 pairs of non-setose pleopods ... ... ... ••• ... ••• i 



Stage II. With 4 pairs of setose, I pair of non-setose pleopods 194 



