752 



AUSTRALIAN CRUSTACEAN FAUNA, 



External sexual characteristics.— One of the first character- 

 istics that is calculated to strike the casual observer in 

 connection with the brachyurous Crustacea is the difference in 

 shape exhibited by the pleons of the different sexes. In most 

 cases this is a sure guide to the sex, ])ut not in all ; as there are 



some, certain burrowing crabs, in which the 

 structure of the pleon in both sexes is 

 approximately the same. Here the only 

 sure guides are the appendages, which lie 

 concealed by the j^leon (excepting a few 

 species where the eyes of male and female 

 exhibit some difference in shape). 



The genus Neptunus, in common with 

 some other genera, is characterised by three 

 distinct types of pleon (Fig. 2). 



The pleon of the male (Fig. 2, A), which is 

 the narrowest, is composed of only five mov- 

 able segments; the thii'd being really formed 

 by the coalescence of the third, fourth, 

 and fifth normal segments; the sutures — 

 which in the diagram are denoted by dotted 

 lines — being distinctly visible. This would 

 make up seven somites (the usual number 

 for the crustacean abdomen). The sternal 

 aspect of the male pleon i^ membranous, 

 excepting the first segment, which is partly 

 calcified so as to give support to the first 

 pair of copulatory appendages. 



There are two pairs of appendages 

 attached respectively to the first and second 

 somites of the pleon. Those chiefly concerned 

 in reproduction, the first pair (Fig. 3, C) are divided each into two 

 joints; the first, a thick, rather short basal portion, about one- 



Fig. 2 — Neptunus pelagicus. — Pleons. 

 A, Male ; B, Sterile Female ; C, Female. 



