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



Scopelus henoiti, Pachystomias microdon, Opostomias barhatum, and Malacosteus 

 indicus. 



Leydig ^ mentions organs which, belong to this group in several species of Scopelus 

 besides Scopelus henoiti. 



h. Structure. 



(1) Genercd. 



The organs are glandular and generally surrounded by an exceedingly thick light- 

 reflecting spicule-layer, but the pigment layer is often not very highly developed. 



There is a tangential lobe and an extension reaching down into the interior of the 

 orbital fossa as in Opostomias micnpmis. The latter is generally larger than the former, 

 spherical in shape and connected ■^vdth the tangential portion by a comparatively narrow 

 neck. On the surface which looks outwards, vertical striations are found, which in fine 

 sections can be resolved under high powers into elements similar to those of Opos- 

 tomias micripnus above described. The distinction between the interior glandular and 

 external epithelial portion is, however, not clearly marked, and the whole structure is 

 much less regular than in that fish. 



"o 



(2) Special description of the suborbital organs in Pachystomias microdon. 



a. General appearance. — Seen from the outside the suborbital organs in this fish 

 appear as two very conspicuous white masses below the eye (PL LXXI. fig. 25). The 

 anterior one, which lies below and in front of the eye, is oval, and its upper margin 

 is slightly concave. The anterior end extends forwards and slightly upwards. It is 

 3 mm. long and 1'5 mm. wdde. The posterior organ is sausage-shaped, being 1"5 mm. 

 wide and 17 mm. long. Its anterior end lies just below the posterior end of the smaller 

 anterior organ (PI. LXXI. fig. 25, b), and it extends backwards and slightly downwards. 

 The posterior end is slightly turned up. 



On dissection it appears that the orbital cavity in the skeleton of the head is greatly 

 extended downwards and backwards (PI. LXXI. fig. 26), and in the lower portion of this 

 extended orbital cavity the two phosphorescent organs are situated. The nerves which 

 supply them come from the angle between the anterior and internal walls of the large 

 cavity (PI. LXXI. fig. 26), and to each of the organs an extensive spherical mass appears 

 to be attached internally, which, of course, is not visible from the surface before dissection. 



^. The smaller anterior organ. — The non-transparent and soft membrane which 

 covers the portion of the orbital cavity below the eye, is perforated so as to allow 



' Leydig, Die augeniiluiliclien Organe der Fisclie. 



