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



The most important work on this subject has boon published by Leydig,^ who 

 examined the following species : — 



Gonostoma denudatum. 

 IcJi tliyococcus ovatus. 

 Jddhijoccocus pahneinsp. 

 A rgyropelecxis heniigymnus. 

 ScajJelus fissoi. 



Scojiehis Jmmboldtii. 

 Scopelus henoiti. 

 Scopelus bonapartii. 

 Scojyelus rcifinesquii. 

 Scojjehis metojjoclamjjns. 



Leydig divides the organs examined by him and described by previous authors into 

 three gi-oups : — " Augenahnliche Organe," " Glasperlenahnliche Organe," and " Leucht- 

 organe." 



The meaning of these terms as compared with those used by Ussow and myself will 

 be explained below. 



The description given of the microscopic structure of these organs I am able to 

 endorse ; but minute details are not described, and most of the material at Leydig's 

 tlisposal was not in very good condition. 



Leydig considers these organs as " electric or pseudoelcctric." This positive state- 

 ment appears rather vague, but his negative assertions, however, are very decisive. He 

 combats the view held by Ussow {loc. cit.) particularly, that these organs are accessory 

 eyes, and he decides with equal emphasis that they arc not glandular. 



He acknowledges that they may emit light, but he thinks that this is not their 

 primary or principal function. He considers them as pseudoelcctric apparatus which 

 may in some cases emit light. 



From this it appears that the physiological results are not at all in proportion to tli(>. 

 excellence and extent of the facts and observations on the subject hitherto ascertained. 



Emery" has i)ublished a description of the minute structure of the phosphorescent 

 organs of Scopelus. He gives a correct figure, but has failed to observe the cells 

 described below, which appear so very peculiar, and which, being found in all these 

 organs, may be considered as their principal constituents. 



He considers the reflector at the back of the organ to be an inverted scale, and says 

 that the organ is covered by another scale, which has been converted into a lens. He. 

 considers these organs to be phosphorescent. 



I shall now enter on the results of my own studies on this subject, j)ostponing the 

 consideration of their physiological and general results to the end of this Report. 



Altliough these organs appear to be of uniform histological structure in different 

 groups of fishes, they nevertheless present such diflerences in shape, distribution, and 

 origin in the different groups that it appears advantageous to dwell on the different forms 

 separately. 



' F. Leydig, Die augeniilinlichen Orgaiie der Fisclie, ID pis., Bonn (E. Strau!*.-'), 18H1. 

 * E. Emery, Millheil. d. zool. Station zu Ncapcl, lid. v. 



