270 THE VOYAGE OF H.M.S. CHALLENGEE. 



skull, which is extremely thin and completely transparent in the region lying over them. 

 This upper wall consists of membrane and membrane bone, but I was unable to ascertain 

 the exact extension of the latter, since I had no macerated specimen to refer to and 

 was obliged to decalcify the one used for sections. Where the bony matter is thinnest it 

 appears as a more or less open network of very fine spicules embedded in the membrane. 

 The margins of the phosphorescent organs are extremely clearly defined, because they bear 

 at their edge a special band of dark pigment (PL LXVIL). 



The organs rest in a pair of elongate cavities, one on either side of skull. These 

 cavities are separated from one another by a longitudinal median septum, which in 

 their anterior region is composed of a vertical plate of cartilage continuous with a 

 horizontal plate of cartilage which forms the roof of the mouth cavity. The floors of the 

 cavities are nearly flat, but owing to the rising in height of the septum and roof of the 

 skull posteriorly, the cavities there become deeper, and are deepest in the region indicated 

 by the letter a (PI. LXVIII. fig. 1) immediately on either side of the base of the median 

 septum. Probably these regions represent the former position of the orbital cavities, and 

 the septum the interorbital septum, although all trace of eyes has disappeared. The 

 skull-roof lying immediately over these areas forms a pair of convex cornea-like 

 prominences (PI. LXVIII. fig. 1, a). Beyond these anteriorly and laterally it is 

 flattened, and shows concentrically disposed striae {Jhicl., fig. 1, b). On either side 

 of the median septum the skull-wall is traversed by a long closed canal (fig. 1, J"). 

 These canals are seen in section in fig. 2, and contain organs which in spirit are 

 opaque, white, apparently mucous canals, and also transmit a nerve on each side to 

 the nasal capsule, no doubt the nasal branch of the fifth nerve. The canals are con- 

 tinued outwards obliquely from a point coinciding nearly with the commencement of 

 the cranial cavity and brain, and thus mark off" a posterior region ((/) of the phosphorescent 

 organs. 



The phosphorescent organs form membranous structures about 0"04 mm. in thick- 

 ness, which are stretched over the entire floors of the two cavities described. They 

 probably, in the recent state of the animal, lie close to the floors of the cavities. They 

 rise vertically up the sides of the median septum as they extend inwards towards the 

 middle line on either hand, and thus present their free edges to view on either side of the 

 septum when viewed through the skull-roof from above. Their edges are similarly 

 turned up to the surface all along their margins, and lie all round immediately in 

 contact with the transparent skull-roof (PI. LXVIII. fig. 2, W. V.). 



The view of the phosphorescent organs is completely obstructed by the bony canals 

 (fig. 1, T) and thus on either side of the median septum there appear to be isolated strips of 

 the organ. The posterior areas of the phosphorescent organs (fig. 1, d) appear somewhat 

 isolated as they lie immediately above the roof of the cranial region of the skull, and 

 thus very near to the surface. In contrast with them the immediately adjoining regions 



