REPORT ON THE ISOPODA. 15 



of tlio thoracic segments undergo a retrograde development and almost disappear, and the 

 animal does not appear to take any nourishment ; in all the mature females that I have 

 examined the young appear to be actually contained within the body of the mother, the 

 alimentary canal is pressed up against the dorsal surface of the body, and its cavity is 

 reduced to a minimum ; a delicate chitinous membrane is all that remains of the 

 thoracic sterna. 



Secondary Sexual Characters. — The males of SeroUs also diifer from the females 

 in a number of secondary sexual characters, which may be divided into two groups — 

 (l) those which are common to all the species of the genus, (2) certain other characters 

 which are confined to one or more species. 



1. As a general rule the males of Serolis are somewhat larger than the females ; 

 this is certainly the case with Serolis bromlei/ana and Seivlis necera, in which species 

 the males are not only longer but also broader, owing chiefly to the greater development 

 of the spine-like epimera so characteristic of these two species. In Serolis cornuta, 

 Serolis schijthei, and Serolis latifrons the difierence in length l)etween the two sexes does 

 not appear to be great, but the males are considerably broader than the females; especially 

 is this so in Serolis schytliei, where the proportion between length and breadth is almost 

 reversed in the two sexes ; in the male the breadth is greater than the length, in the 

 female, on the contrary, the length is greater than the breadth, and the difierence between 

 these two dimensions is very near to being the same in both sexes. In Serolis antarctica 

 and Serolis gracilis the males have a greater antero-j)osterior as well as transverse 

 diameter than the females. 



Among the Australian species, on the contrary, the females appear to be larger than 

 the males, but since the number of specimens in each case was extremely small, it is 

 impossible to speak with certainty. Of Serolis aiist7-aliensis two out of the three speci- 

 mens are males and smaller than the single female specimen ; of Serolis tiiberculata the 

 Challenger obtained two specimens, one of each sex, and here again the male is the 

 smaller ; the male specimen also which has been described l)y Grube ^ is of aliout the 

 same size as the male which I have examined ; finally, in the only other Australian species, 

 Serolis j^nlJida, of which more than a single example was obtained by the Challenger, 

 the female is larger than the male. 



In the majority of species of Serolis tliere is a difference in the length of the epimera 

 in the two sexes, and this difference is most .strikingly shown in Serolis (jracilis; in the 

 males of this species the epimera gradually increase up to the sixth, the last thoracic 

 epimera, which are enormously elongated and extend beyond the termination of the 

 caudal shield for a considerable distance. 



In the female the epimera are very much reduced in size ; the sixth pair, instead of 

 being prolonged beyond the caudal shield, do not reach as far as its extremity. 



' Lor. cit., p. 227. 



