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



The really phosphorescent apparatus is to be sought in the cells which occupy the 

 cavity of the cup, and which are evidently connected with the ganglion-cells by means 

 of the radial fibres issuing from the disc. 



The structure is intermediate lietween the simple form in the simple ocellar 

 phosphorescent organs and tlie more complicated structure of the composite organs with 

 special reflector, to be described below. 



The fish can at its option incite the organ, which under ordinary circumstances 

 is non-luminous, to phosphorescence. The voluntary impulse is transmitted by the 

 thick nerve of Leydig to the disc of ganglion cells, which excite the phosphorescent cells 

 in the cup to action by means of the radial nerve-fibres. 



The phosphorescence of the latter requires, however, the secretion of the gland in the 

 sphere as fuel, in a manner similar to that which has been described in the simple organs. 



The secretion passes through the disc and the radial fibres to its destination, where 

 it is consumed to produce light. 



e. Development. 



Although no transitional forms have been observed by mc, I do not doubt that these 

 composite phosphorescent organs have been developed from the simple ones by continued 

 diS'erentiation of the parts. During this progress of development the shape, which is 

 very variable in the simple organs, has been determined. Their number has been 

 reduced and likewise determined, together with their position. From a great number of 

 undifferentiated organs scattered irregularly over the surface of the body, a small 

 number of more highly diflorentiated organs, with a definite position, has been 

 evolved. 



4. Composite ocellar organs, with special reflector. 



a. Distnbutton. 



These most highly differentiated phosphorescent organs have been found by me in 

 Argyropelecvs Jiemigymnns, Sternoptyx diaphana, and Scojielns benoiti, whilst Ussow ' 

 has seen them in Argyropeleciis hemigymmis and designates them as " driisenahnliche 

 Organe." Leydig,^ who designates them as " glasperlenartige Organe," has found them 

 in Gonostoma denudatum, Argyropelecus hemigymmis, Scopelus rissoi, Scopelus htim- 

 holdtii, Scopelus henoiti, Scojyehis honapartii , Scojyelus rafinesquii, and Scojiehis 

 laetojjoclampus. 



The number of these organs and their distribution on the body are very regular, and 



' M. Ussow, loc. cit., \>. 108. ' M. Ussow, loc. cit. 



