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



Externally the organ in its dorsal portion is, as mentioned above, covered by a thickly 

 pigmented shell, which possesses a crest, projecting inwards and dividing the contents of 

 the organ into a proximal and distal portion. In this respect this organ appears as a 

 transition between the simple and composite ocellar organs. The side which touches the 

 fish below the base of attachment likewise possesses a thick layer of pigment (PI. 

 LXXIII. fig. 50, g). In the lower end a few superficially situated patches of pigment are 

 found (PI. LXXIII. fig. 50, k). The proximal portion, which corresponds to the sac- 

 shaped part of the composite ocellar organs, is occupied by a mass, which is readily stained, 

 and which contains numerous conspicuous nuclei. The distal portion is filled with 

 elongated cells, which extend parallel to the long axis of the organ, and accordingly point 

 downwards ; these cells are very similar to those of the cup-shaped portion of the 

 composite ocellar organs described above. We find, namely, slender spindle-shaped cells 

 (PI. LXXIII. fig. 51), and the typical phosphorescent clavate cells (PI. LXXIII. figs. 52, 

 53, 50, i), the club-shaped ends of which are situated distally. They do not form one 

 single regular layer, but are scattered throughout the whole of the distal portion of the 

 organ. The vesicle which reflects the light sometimes appears to be divided into two 

 portions (PI. LXXIII. fig. 53), but generally it is simple and oval, like the vesicles 

 described above. 



c. Innervation. 



A very stout nerve enters the organ at the base of attachment and extends into the 

 proximal portion (PI. LXXIII. fig. 50, f, h). A direct connection of this nerve with 

 the typical clavate cells is highly probable. 



d. Function. 



Within this organ no glandular portion can be discerned, but it seems not improbable 

 that the secretion of the slime-canal system may pass into the organ. There can be no 

 doubt that the typical clavate phosj)horescent cells are the light-producing elements of 

 the organ. 



e. Development. 



It appears that these organs are developed in a similar manner to those sunk in the 

 body, which have been described above. The peculiar structures of the same fish, 

 which are described below, may throw some light on the origin of these organs from the 

 slime-canal system. 



f. Doubtful structures in the skin. 



On the sides of the body, in close proximity to the organs described above, we find 

 thickenings of the fibrous dermal layer, which lies outside the thick pigment-stratum 



