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



that the two parts enclose an angle of 80°. One portion lies tangentially and occupies 

 the space just below the gray sphincter membrane, and the other extends downwards into 

 the interior of the orbital fossa. The whole organ lies loose in a cavity surrounded by a 

 light-reflecting membrane backed by the usual pigment layer. The wall of this sac or 

 cavity is perforated in one place only — where the nerve and the bloodvessels enter 

 the organ. 



The organ itself consists of two distinct structures ; firstly, meandriform gland-tubes 

 surrounded and divided from each other by fibrous tissue containing nerves and blood- 

 vessels, of which the whole of the inverted and the central portion of the tangential 

 lobe are composed ; secondly, a high cylinder-epithelium on the surface of the tangential 

 lobe. The gland-tubes contain a coating of ordinary gland-cells and their lumen is partly 

 filled with similar cells and partly with a slimy secretion, but they present no striking 

 peculiarity. The cylinder-epithelium on the other hand is a most complex and inter- 

 esting structure. 



Of all the Challenger fish examined by me, this one is the best preserved, so well in 

 fact that the most minute detail in the structure of this organ could be studied in very 

 fine longitudinal sections (PI. LXXII. figs. 40, 41). 



The superficial j^art of the tangential lobe rests on the ordinary fibrous tissue covering 

 the central gland-tubes (PI. LXXII. fig. 41, h), and the nerves contained in this tissue 

 extend upwards into it. It consists of two layers. Below, attached to the floor, we 

 find a layer of ganglion cells (PI. LXXII. fig. 41, c), which are irregularly roundish, 

 highly granular, non-transparent, and measure 0"006 mm. in diameter, their nuclei 

 being spherical, highly stainable and conspicuous. From their upper surface the ganglion 

 cells send forth stout processes, Avhich are in direct connection with club-shaped 

 typical phosphorescent cells found in the upper layer (PI. LXXII. fig. 41, d). 



The upper layer is structurally very similar to the tissue described above from the 

 cup-shaped portion of the composite phosphorescent organs in Scopelus. 



It is chiefly composed of slender, vertical, indifi"erent, transparent, spindle-shaped cells 

 with small nuclei, and between them, rising directly from the ganglion cells below, the 

 typical phosphorescent clavate-cells are situated. These are shorter and stouter than in 

 Scopelus and Xenodermichthys, and are scattered somewhat irregularly, resembling 

 in this respect the corresponding structure in Xenodermichthys. They are O'Ol to 

 0"02 mm. high, the club-shaped end is 0'002 mm. thick. 



The light-refracting vesicle is always single, forming an oval body with a vertical 

 longitudinal axis. It is situated in the club-shaped end of the cell, 0'0024 mm. long 

 and about half as broad. The protoplasm is highly stainable, but singularly enough I did 

 not observe nuclei in these cells. Neither have I observed any pavement epithelium on 

 the surface as in other similar organs. 



