SYSTEMATIC REPORT 137 



Both these workers realized that there were considerable sexual differences present in this species. 

 Zimmer (1914, p. 398) says that the antennal scale is variable in its size, that it is smaller in females 

 than in males of the same size and that it varies also in comparison with the spine at the outer distal 

 margin of the sympod. From the Discovery material I am able to trace the gradual development and 

 increase in size of the antennal scale in growing males. In the young of both sexes the scale appears 

 in the form of a small papilla, shorter than the spine arising from the sympod at its base and armed 

 only with a minute bristle at its apex. In fully adult females the scale appears very much as figured 

 by Ortmann (1893, pi. I, fig. 8c), as a small styliform outgrowth tipped with two bristles. The largest 

 males in the Discovery collection have a small well-developed scale of the normal leaf-like form, 

 extending almost to the distal margin of the second segment of the peduncle and armed around the 

 apex with 7-9 long plumose setae (Fig. 31 A). The proportions of the abdomen and tail-fan show very 

 marked changes as growth proceeds. In young forms the abdomen is very short and is bent forward 

 beneath the flattened cephalothorax so that, with the very large eyes and the laterally spread thoracic 

 appendages, the animals look rather like megalopa larvae. As growth proceeds, the abdomen tends 

 to become straightened out and longer, but the most noteworthy change takes place in the uropods. 

 In small animals they are short and very little longer than the telson with the exopods shorter than the 

 endopods. Gradually the uropods lengthen and the rami become more equal in length until in adult 

 females they are fully twice as long as the telson with the rami of equal length. In fully grown males, 

 the uropods are nearly three times as long as the telson and the exopods are markedly longer than the 

 endopods. 



Considerable variation occurs in the armature of the telson, but this does not appear to bear the 

 same direct relation to size as do the other variable characters. Though the general tendency with 

 growth is towards an increase in the number of spines arming the apex and distal portion of the lateral 

 margins, quite large specimens may have no more spines than small ones. Usually there are three 

 spines on the apex and just one smaller spine on one of the lateral margins (Fig. 31 G). The statocyst 

 is very large in young animals, but does not increase in size with the growth of the endopods, so that 

 a feature which is very noticeable in juveniles is no longer apparent in adults. 



The male pleopods are particularly robust and project beyond the lateral margins of the abdominal 

 somites, so that the abdomen appears much more robust than it actually is. I have not seen a male with 

 the abdomen flexed under the body as is so regularly the case in the female and the outstretched pleon 

 with the longer uropods and the spreading pleopods give the males (Fig. 31G) a vastly different 

 appearance from females of the same length. The marsupial pouch is surprisingly large in ovigerous 

 females and extends backward almost to the telson, thus further increasing the difference in general 

 form between the two sexes. 



The first thoracic appendage is essentially as figured by Zimmer ( 1 9 1 4, pi. xxiv, fig. 30) in the general 

 form of the appendage, but I found in my larger male specimens that the vestige of the exopod is 

 longer and is tipped with one or two short bristles (Fig. 3 1 C). Zimmer did not figure an endite on the 

 basis at the point of origin of the endopod, but there is a well-developed one in my specimens. The 

 spines at the apex of the dactylus and the nail are armed along one side with long spinules (Fig. 31 D). 

 In the endopods of the third to the eighth thoracic appendages the carpus is considerably longer 

 than the propodus and is divided from it by an oblique articulation. The propodus is secondarily 

 divided into two almost equal sub-segments. The genital organ on the eighth thoracic appendage is 

 barrel-shaped and armed distally with a strong bristle (Fig. 31E, F). 



One further point of sexual difference can be seen in the finger-like outgrowth from the inner side 

 of the eyestalk. In the female this is very small and difficult to see, but in large males it is much 

 longer and extends almost to the distal margin of the cornea (Fig. 31, A). 



18 D 



