12 THE VOYAGE OF H.M.S. CHALLENGEK. 



terminal branches, as two triangular, pointed processes, having between them the anal 

 orifice (see PI. II. fig. 10). 



The carapace (see PI. I. fig. 1 ; PI. II. fig. 1) is rather large, covering the greater part 

 of the body, including the whole cephalic part, the trunk, and part of the pleon. It is 

 highly compressed, the free lateral parts extending in the form of two valves perpen- 

 dicularly along the sides, so as wholly to cover the oral parts, and to include between 

 them the basal parts of the two pairs of antennae, as also the greater part of the branchial 

 legs. Above, the two valves pass immediately into each other by an even curve, without 

 any intervening hinge. As seen laterally (PI. I. fig. 1), the carapace exhibits a somewhat 

 oval form, with the dorsal line slightly arched, the anterior edges strongly curved, and 

 joining the inferior without any intervening angle. Posteriorly, each valve forms an 

 obtusely rounded lobe, advancing to a more or less extent over the side of the pleon, 

 without however covering the dorsal surface of that division. These lobes are separated 

 above by a deep emargination, at the bottom of which a small and narrow incision is 

 se3n (see PI. II. fig. l). Owing to this emargination, the posterior edges of the 

 carapace appear in a lateral aspect (PL I. fig. l) obliquely truncate, and joining the dorsal 

 line at an obtuse angle. The so-called rostrum (PL I. fig. 1, R; fig. 2) forms an oblong 

 tongue-shaj)ed plate, movably articulated to the carapace in front, above the insertion of 

 the eyes. It is slightly arcuate, with the upper side convex, the lower concave, and 

 terminates in a sharp spiniform projection issuing from a slight longitudinal eleva- 

 tion running along the concave side of the rostrum. As seen from above or below (fig. 2) 

 this plate appears broadest near the base and tapers slightly towards the end, whereas 

 in Nebalia it is more regularly oblong — oval in form. AVhen lowered and applied against 

 the anterior part of the carapace, it almost reaches to the inferior edges, thus, as it were, 

 closing the anterior aperture of the carapace as an operculum. The carapace is connected 

 with the body dorsally along a rather restricted space, extending from the base of the 

 rostral plate to about the middle of its length. The limits of this space are faintly 

 traced both in the lateral and dorsal aspect of the animal (see PL I. fig. 1 ; PL II. fig. 1), 

 and according to its position, this space may properly represent the gastric region in the 

 higher Crustacea. Throughout this limited dorsal area the carapace forms the immediate 

 body-wall, whereas elsewhere it only loosely covers the body, which admits of being 

 moved to a certain extent within it. On each side of the anterior part of the carapace, 

 immediately beneath the above-named region, and at a short distance behind the man- 

 dibles, an assemblage of lucid spots, forming together a well-defined, rounded, oval area 

 (PL 1, fig. 1, Cm) may easily be distinguished. This area is produced by the insertion 

 of the strong adductor muscle, by the aid of which the valves of the carapace admit of 

 being approached to each other to a certain extent. Both in form and position this 

 muscle entirely agrees with the strong adductor muscle of the shell met with in the 

 bivalved Phyllopoda, thus giving the carapace an evident phyllopodous character. As 



