18 THE VOYAGE OF H.M.S. CHALLENGER. 



oblongo-quadi-angular in form, and in the three posterior pairs having the outer edge 



reguharly serrate. Of the branches the outer one is the shorter, and uniarticulate with the 



outer edge, and the apex strongly spinose, the inner edge being bordered by a regular row 



of ciliated setae, two or three of which issue from the apex inside the terminal spines. 



The inner branch is somewhat more slender and distinctly biarticulate, the proximal jomt 



very short, the distal elongate and slightly tapering to the apex, which juts out as a 



short dentiform projection and, inside the latter, bears a strong spine. Along both edges 



of this joint a dense series of ciliated setse increasing successively in length toward the 



tip is affixed, and from the inner side of the proximal joint a linguiform lateral lobe 



(fig. 6a) arises, armed at the tip with three strongly chitinised hooks (fig. 6h). This 



lobe meets the one on the other side, and by the aid of the above-mentioned hooks both 



plcopods become thus, as it were, clasped together, so as only to admit of a quite 



simultaneous movement. In comparing the four pairs of pleopoda with each other, we 



find some slight differences in their armature and in the relative length of the branches. 



Thus, in the first pair (fig. 5) the outer edge of the basal part is quite smooth, without any 



trace of the regular serrations occurring in the other three pairs. Moreover, the outer 



branch in this pair is considerably shorter as compared with the inner, and the spines of the 



outer edge are much more numerous and also arranged in a different manner, being only 



in the distal part disposed in fascicles, whereas those of the proximal part form a dense 



comb-like series. In all the other pairs (figs. 6, 7) the spines of the outer branch are 



arranged in five distinct fascicles, two in each fascicle, besides a small seta, and to the 



obliquely truncate apex three somewhat stronger spines increasing successively in size are 



affixed. The last pair of pleopoda (fig. 7) are somewhat smaller than the preceding 



pairs, and have the inner branch only very slightly longer than the outer, and the spines 



of the latter branch are also considerably smaller. 



The two pairs of rudimentary caudal limbs (PL I. fig. 1, cp^, cp~; PL 11. figs. 8, 9) 

 succeeding the true pleopoda are both of a very simple structure, forming narrow 

 uniarticulate plates, scarcely at all movable. They are setose at the inner edge and 

 armed at the obliquely truncate apex with three spines. Those of the first pair (fig. 8) 

 are somewhat larger and more densely beset wdth bristles than the posterior pair (fig. 9), 

 with the apical spines successively increasing in length, whereas on the latter pair the 

 middle spine is the longest. 



The caudal rami (PL I. fig. 1, vr ; PL II. fig. 10) have the form of two more or less 

 diverging narrow plates, movably articulated to the end of the last segment, and about 

 as long as the two posteiior caudal segments taken together. They are minutely 

 spinous along the outer edge, and along the inner they are provided with a row of 

 short ciliated bristles, besides a scries of minute spines. From the obliquely truncate 

 apex a dense fascicle of very long and slender setaa arises, which, however, were mostly 

 broken in the specimens examined. 



