DEVELOPMENTAL PHASES 3Si 



The abdomen now has six segments and a telson. There are no abdominal appendages. The two 

 rami of the lateral uropods are now visible, the outer ramus being somewhat longer than the inner 

 and having its outer margin produced as a strong spine. The remaining armature of the telson is 



unchanged. 



Dominance in the furcilia stages 



For reasons which have been adequately discussed before (Fraser, 1936; John, 1936; Einarsson, 

 1945; Boden, 19506, 1951) the term ' cyrtopia ' for the later larval stages has been discarded, and these 

 stages are now included in the furcilia series. Thus the furcilia stages in this report include all the 

 instars, from the one in which the carapace first withdraws from over the eyes and there are no pleopods 

 at all, to that in which there are five setose pleopods, the second antennal endopod is segmented and 

 there is a single terminal and three pairs of postero-lateral spines on the telson. After one pair of 

 postero-lateral spines has been lost the animal is regarded as 'juvenile ' or post-larval, until it reaches 

 adult size. This is done because the ecdyses in the juvenile condition (apparently 9 or 10) are orderly 

 and there is no evidence of dominance in any of the stages. In addition the animal may be sexually 

 mature before attaining fully the adult form. 



There are twenty-five possible instars in this series. Six are numerically dominant and are regarded as 

 the actual furcilia stages (Fig. 7). This scheme has been followed by most modern workers since it was 

 first suggested by Fraser (1936). Recently, however, Sheard (1953) has proposed an alternative scheme, 

 based mainly on an exhaustive investigation of Nyctiphanes australis, in which the number of furcilia 

 stages is reduced to three and the dominants are disregarded. The stages he proposes are as follows: 



Furcilia stage 1 (F I). Eyes developed and free of the carapace, pleopods absent or present as 

 non-setose rudiments. Terminal spines (of telson) generally seven. Lateral spines (postero-lateral of 

 telson) three in number, the central spine always present. 



Furcilia stage 2 (F II). Some, or all, pleopods setose. Terminal spines sometimes reduced in 

 number, the central spine always present. The pair of long lateral spines unaltered at the base. 



Furcilia stage 3 (F III). All pleopods setose and functional. The pair of long lateral spines are 

 altered at the base. The terminal spines are progressively reduced to one (the median spine). The 

 stage terminates with the end of the phase. 



The outer limits of Sheard's series are actually the same as those proposed herein for N. capensis, 

 or those of N. simplex (Boden, 1951), and his F I and F II stages are the same as the first and second 

 furcilia stages presented here. His F III stage, however, embraces the final four furcilia stages of 

 N. capensis. Sheard's F II and Fill stages are differentiated mainly on the alteration in the shape of 

 the base of the long lateral spines on the telson. This is a somewhat nebulous character, frequently 

 rather difficult to detect, and is not as reliable an indicator as a definite reduction in the numbers of 

 terminal spines on the telson or the sudden change in both form and function of the second antennal 

 endopod. 



Sheard 'lumps' the final furcilia stages into his F III stage and throws considerable doubt on the 

 validity of regarding the dominant instars as actual stages and the remainder as variants, on the grounds 

 that the dominants will change from time to time and from place to place. This concept of the 

 dominance of certain instars he regards as a result of the inadequacy of plankton sampling. His 

 arguments are based almost exclusively on the data he obtained for N. australis and are applied in an 

 inferential manner to the other species whose development he describes. 



It is perfectly true that most plankton sampling is pitifully inadequate, but this cannot be said of 

 Fraser 's (1936) work, in which the concept of the dominants constituting the actual stages was origi- 

 nally expressed. His material was spread adequately through time and space and a very large number 

 of individuals was examined. It is also untrue of Einarsson's (1945) work and most of John's (1936). 



