DEVELOPMENT AND GROWTH 333 



writing of a furcilia of Nematoscelis megalops G. O. Sars, 1885, which bears one terminal spine, states 

 ' I have called this the sixth furcilia, however, rather than the fifth. If it were the fifth the number of 

 terminal spines would have to be reduced from five to one in one stage. This sequence has never been 

 reported in any other life history and it seems justifiable to suspect that there is at least one stage with 

 three terminal spines on the telson'. John (1936, p. 257) does record two larvae of E. vallentini in 

 which seven spines are followed by three but this sequence does not occur in the majority of the 

 larvae. Lebour (1926, p. 786), on the other hand, shows that this is the usual developmental sequence 

 in Nematoscelis microps G. O. Sars, 1883 from the Mediterranean and it is confirmed by Gurney 

 (1947, p. 60) from his Bermuda material. 



In view of this and of the results obtained in the case of E. triacantha it is possible that when the 

 larval development of more species is known it will be found that the omission of some of the later 

 furcilia stages is more common than has previously been thought. 



It is not intended to suggest that this necessarily applies in the case of Nematoscelis megalops but, 

 as both Boden, who had, however, only a few late furcilia, and Frost (1935) failed to find larvae with 

 three terminal spines, it is a point that will bear further consideration when more material is available. 



Spawning 



Both the time and the place of spawning can only be inferred from the presence of young larvae since 

 neither eggs which can be identified as those of E. triacantha, nor gravid females have been found in 

 the plankton. The earliest date at which first calyptopes have been found is 23 September and the 

 latest 16 December. In order to obtain some indication of the time of spawning it is necessary to 

 know the period required for development from the egg to the first calyptopis. The only estimate 

 available is that from Lebour's (1924, p. 408) laboratory experiments on Nyctiphanes couchii (Bell), 

 1853, which gave a period of 8-9 days for development from nauplii free in the egg-sacs to the first 

 calyptopes. If the assumption is made that a similar period is required in E. triacantha, spawning 

 could be expected to occur from mid-September to early December, although it must occasionally 

 take place later than this, for a few third calyptopes (a total of eighteen from five stations) have been 

 taken in January and early February. However, the maximum number of calyptopes are found in late 

 October and November and the main spawning almost certainly takes place during these months. 



It is probably reasonable to assume that spawning takes place in the region of maximum abundance 

 of larvae, that is in a belt 200 miles wide south of the Antarctic convergence. There is, however, no 

 evidence regarding the depth at which it occurs. 



Adolescents and adults 

 Prior to the development of the external reproductive organs, the petasma in the male and the thelycum 

 in the female, the only external character distinguishing the sexes in E. triacantha is the presence of 

 setae on the coxopodite of the sixth thoracic limb of the male and their absence in the female. Thus 

 E. triacantha differs from E. siiperba in which the male has a narrower and more shallow carapace 

 than the female and in which there are also differences in the shape of the rostrum and of the spine 

 at the anterior lateral corner of the carapace (Bargmann, 1937, p. 328). However, the sex of nearly 

 all post-larval E. triacantha can be determined by examination of the reproductive system. 



The development of the reproductive systems follows very closely the patterns described by Barg- 

 mann (1937) for E. superba and it is not proposed to describe it in detail here. In the male the only 

 differences appear to be the number of lobes of the testis — twelve in E. triacantha as compared with 

 fifteen in E. superba— and, in E. triacantha, the lesser degree of coiling of the vasa deferentia between 

 the anterior and posterior flexures. 



