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



that part, whereas the three others become suddenly much lower and scarcely at all 

 broader than the caudal segments. These hinder segments of the trunk are moreover 

 very movably connected with each other, flexible membranous spaces being interposed 

 both above and below. Besides a median dorsal keel, also continued on the carapace, 

 these segments exhibit on each side of that keel another running along the dorsal surface 

 (see fig. 2). The epimera of the first segment are somewhat expanded and truncated at 

 the tip ; those of the three posterior segments are narrowly rounded. 



The tail, especially in female specimens (fig. 3), is extremely slender, even somewhat 

 longer than the anterior division of the body, and of a narrow cylindrical form. It has 

 a distinct median dorsal keel running along all the segments, and, besides, the two 

 anterior segments exhibit on each side of the median another subdorsal keel, as on the 

 posterior segments of the trunk. Laterally each of the five anterior caudal segments is 

 provided at the anterior margin with a small knob-like process that fits into a corre- 

 sponding notch in the posterior margin of the preceding segment when the tail is fully 

 extended. As to the relative size of the segments, they slightly increase in length 

 posteriorly to the penultimate, which is much longer than any of the others. The last 

 segment is somewhat shorter, and produced at the end as an obtusely rounded protuberance 

 (see fig. 20). 



The integuments are highly indurated and calcareous, showing under the microscope 

 partly a granular, partly a squamous or densely reticulate structure. 



The colour of the preserved specimens is light brown with darker shadings, especially 

 on the anterior part of the carapace. 



The antennulse (fig. 5, a 1 ) are attached close together beneath the pseudorostral pro- 

 jection, being partly concealed by that prominence, their outer part, however, projecting 

 freely from the anterior notches of the carapace. They consist each of a triarticulate 

 peduncle and two very short flagella. The first joint of the peduncle is rather large and 

 highly indurated ; it is turned obliquely outwards and has the outer edge finely ciliated, 

 the inner, close to the end, is provided with a few short bristles. The two succeeding 

 joints of the peduncle are much smaller, and taken together scarcely as long as the basal. 

 The second joint has at the end on the inner side a bunch of three auditory setae, and 

 exteriorly a small simple bristle ; the last joint is provided with a single auditory seta 

 (see fig. 6). Of the flagella, the one is quite rudimentary, only consisting of a small knob- 

 like joint bearing a very minute auditory seta and two simple bristles. The other flagellum 

 is about as long as the last joint of the peduncle and Particulate, the terminal joint being 

 the smaller, and bearing at the tip two very long and regularly segmented sensory 

 appendages, besides two simple bristles, one of which is rather long. The latter flagellum 

 must undoubtedly be regarded as homologous with the outer flagellum in other Crustacea, 

 since it bears the sensory appendages, but the antennula is generally twisted in such a 

 manner that it in reality lies inside the other (see fig. 5). 



