REPORT ON THE DEEP-SEA FISHES. 271 



of the organs which occupy the floors of the orbital (?) cavities (fig. 1, a) lie so deep as to 

 be almost or quite invisible from the surface. It is only in the regions where the phos- 

 phorescent organs lie very near the skull-roof, that their component hexagonal bodies 

 can be seen with a microscope in the uninjured condition of the fish. 



The phosphorescent organ is of identical structure throughout its extent, and no 

 difi"erence in its composition could be detected in its various regions. When it is viewed 

 from above it is seen to be marked over its entire upper surface with a series of 

 hexagonal areas of about 0'04 mm. in diameter. When its surface is viewed by reflected 

 light, the appearance is of a number of glistening, white, isolated, short columns, 

 hexagonal in outline, standing up in relief from its basal membrane (PI. LXVIII. fig. 3). 

 The organ forms a continuous membranous expanse about 0'04 mm. in thickness, excluding 

 the connective tissue and vascular layer. It is composed of the hexagonal columnar masses 

 arranged with considerable regularity in rows, and resting inferiorly on a pigmented con- 

 nective tissue layer. Each hexagonal column is composed of a number (about thirty or forty) 

 of transparent rods disposed side by side at right angles to the outer surface of the organ, 

 and with their bases applied against the concave surface of a large hexagonal pigment-cell, 

 one of which forms the base of each hexagonal column. The basal pigment-ceUs (figs. 

 10, 14) are hexagonal in outline and are cup-like, concavo-convex in form and of the same 

 breadth as the hexagonal columns. The under surface of the phosphorescent organ is thus 

 covered with a series of hexagonal areas each presenting a rounded convex prominence 

 (fif. 5, c). The outer extremity of each rod is surmounted by a hexagonal nucleated 

 cell (PL LXVIII. figs. 5, 6, 7). 



In stained sections (fig. 5) each phosphorescent organ is seen to be composed of 

 three strata ; the superficial layer of cells (e) deeply stained, the layer of rods very little 

 stained and the layer of pigmented-cells (c) conspicuous by its dark bro^Ti colour. 



The superficial layer is composed of cells all alike, of hexagonal outline, each with a 

 large nucleus filled with fine granules. The protoplasm of the cells also contains 

 granules. These cells are present in exactly the same number as the rods, and each when 

 m situ forms the summit of the rod beneath it. The cells of this layer are closely 

 connected together and the layer composed of them is frequently found in microscopic 

 sections separated as a continuous sheet from large areas of the organ, leaving the ends 

 of the rods exposed. 



The rods (figs. 6, 7) are more or less regularly hexagonal in section apparently 

 perfectly clear, hyaline and transparent in structure. They appear to taper slightly 

 towards their lower extremities where they rest on the concave surfaces of the 

 hexagonal pigment-cells. In some few preparations I have seen a highly stained small 

 oval nucleus present in the lower extremities of certain rods, but have failed to detect 

 such in the large majority of instances (fig. 8). 



The basal hexagonal pigment-cells have each a well-marked nucleus. Their substance 



