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



produce a peculiar folding of their integument, increasing in breadth inferiorly and 

 exhibitintr here as it were an imbricate appearance. These peculiar cuticular folds, first 

 noticed by the author when describing the fresh-water variety of Mysis oculata, Fabr. 

 {Mysis relicta, Loven), occur more or less distinctly developed in all Mysidans, and may 

 to some extent compensate for the absence of true gills, these parts being constantly bathed 

 by the current of water flowing beneath the free parts of the carapace, and chiefly 

 produced by the oscillatory movements of the epipodite of the maxilliped that projects 

 within the branchial cavity anteriorly. 



The caudal segments are perfectly cylindrical, without the slightest trace of epimera, 

 and somewhat less in breadth than the anterior part of the carapace. The last segment 

 is much the longest, about twice as long as the preceding. 



The eyes (fig. 13) exhibit nearly the same imperfect structure as in the genus Petaloph- 

 thahnus, lacking, as they do, every trace of pigment and visual elements, and constituting 

 merely simple lamellar or petal oid expansions, mounted in a vertical position on short 

 pedicles. As to form, they differ somewhat from those in Petalophthalmus armiger, being 

 distinctly hollowed on the outer face, or well-nigh calyciform, their aspect being almost 

 as if the true eye-globe or cornea were extirpated, and only the skin of the pedicle left. 

 Anteriorly they form a slight angular projection, being for the rest of a somewhat irregular 

 oval form. 



The antennular peduncle (see figs. 10-12) is comparatively smaller than in the other 

 species of the genus, and has also a more regular cylindrical foi'm, the second joint being 

 simple and not discoidal or cup-shaped. The outer flagellum forms only a slightly 

 defined expansion at the base. 



The antennal scale (see fig. 12) is rather large, almost twice as long as the antennular 

 peduncle, tapering toward the apex, which is obliquely rounded, w'ith the outer corner 

 somewhat projecting and dentiform. 



The mandiljles (fig. 14) exhibit the structure characteristic of the genus, the palp 

 being rather elongate, with its terminal joint narrowly oblong. 



The first pair of maxilhe (fig. 15) are also c_[uite normal in structure and agree as 

 regards all essential features with those organs in the other species of the genus. 



The second pair of maxillas (fig. 16) have the basal part rather large, forming inwards 

 a slightly arched and sharp border, densely beset with delicate curving bristles. There 

 are, as usual, three masticatory lobes densely crowded together at the inner corner of the 

 basal part. The exognath is but of moderate size and elliptical in form, fringed with 

 delicate plumose setae along the outer edge. The terminal joint of the palp is oblong, 

 and has the bristles of the outer edge recurved, as in the other species of the genus. 



The maxillipeds (fig. 17) are not particularly powerful in development, and have the 

 basal part of about the same length as the terminal or palp, projecting inward as a 

 narrow, linguiform masticatory lobe. Of the joints of the palp the antepenultimate 



