NO. 6.] ACCOUNT OF THE SPECIES. 43 



In young, not yet sexually mature males, this pair of legs (fig. 8) are 

 likewise very small, but rather different in structure from those in the female, 

 and more resembling in this respect the natatory legs. As in the latter, each 

 leg is composed of a distinctly biarticulate basal part and 2 unequal rami, 

 the inner uniarticulate, the outer biarticulate. The rami do not, however, 

 carry any natatory setae, and there are also only very slight traces of spines 

 to be detected. Both legs exhibit a soft cellular structure, and are still only 

 very slightly chitinized, indicating that they are as yet imperfectly developed, 

 and there is but little difference between them, whereas in the sexually 

 mature state, as will be shown below, these legs are very unequal. 



Description of the Sexually Mature Male. 

 (PL IX). 



The length of the body does not, in any of the specimens found, exceed 

 4V2 mm. 



The general form (see figs. 1 & 2) is very different in appearance from 

 that of the female, both as regards the anterior and posterior divisions. 



The former is less pronouncedly navicular in form, on account of the 

 absence of the cephalic crest. Seen dorsally (fig. 1), it is abruptly con- 

 tracted anteriorly, with the front obtusely truncated, not, as in the female, 

 sharply pointed (comp. PI. VII, fig. 1); and posteriorly it is but very slightly 

 narrowed. The last segment is extremely small, and more broadly emargin- 

 ated than in the female, with the lateral parts much less produced behind. 



The tail is comparatively larger than in the female, considerably exceeding 

 half the length of the anterior division, and is composed of 5 well defined 

 segments. Of these the 1st is very small and greatly constricted at the base, 

 whereas the 2nd segment is of unusually large size and considerably tumefied, 

 equalling in length the 2 succeeding ones combined. The last, or anal seg- 

 ment, as in the female, is very short. The caudal rami, which in the female 

 are quite immobile, have here a very mobile articulation with the last seg- 

 ment, allowing of their being considerably spread out to each side (see fig. 1). 

 The caudal setae, too, are less fully developed than in the female, though 

 apparently present in the same number; and in all the specimens examined, 

 they were widely divergent. 



