GENERAL BIOLOGY 



677 



to correspond with different depths, and that the light-organs are 

 not peculiar to the deepest and darkest water-layers. Previously 

 this belief was generally adopted because the light -organs 

 were looked upon as a means of illuminating the dark abyssal 

 region. Brauer indicates that of the six species of Cyclothone 

 five are black and live in deep water, while one species 

 (C signata) is grey, lives in much shallower water, and has by 

 far the largest light-organs (see Fig. 493, showing the small light- 

 organs of the dark forms and the large ones of C. signata). Of 

 the Scopelidae, the surface forms of the genus Mydophum (s.s.) 

 possess the largest light -organs, 

 while the sub-genus Lampanydus, 

 taken in closing- net hauls by the 

 "Valdivia" between 800 and 600 

 metres, has very small light-organs. 

 If now we consider the captures 

 of the "Michael Sars," and the 

 vertical distribution of the fishes 

 previously described, we see that 

 our experience confirms Brauer's 

 views. Cyclothone mi c to don with 

 small light-organs was found much 

 deeper than C. signata (see Plate 

 I., showing these two forms, the 

 difference between their light- 

 organs being easily observed). Of 

 special interest is Fig. 490, showing 

 the vertical distribution of five 

 black fish-species, two of which 



iGastrOStomUS bairdii^ and CyeUia Go?iosto?na rhodadenia, GWh. Photophore 

 , \ 1 1 • 1 , from upper lateral series (\^-). 



atrunt) have no light - organs ; 



Gonostojna grande has very small light-organs, while those of 

 Gonostoma rhodadenia and Photostoviias guernei are large (see 

 Plate II., showing the two species of Gonostoma, Fig. 67, a, p. 

 86, representing Photostomias giternei, and Fig. 494, showing 

 a light -organ of Gonostoma rhodadenia magnified). Besides 

 these we found in our deepest hauls many forms without light- 

 organs, for instance, species belonging to the genera Aceratias, 

 Melampha es, Cetom im us. 



Light-organs are, therefore, specially characteristic of fishes 

 belonging to the upper 500 metres in warm oceanic waters. 



^ On the tip of the tail this species is provided with an organ, the function of which is un- 

 known ; it has been regarded as a light-organ, but this does not alter our view. 



Fig. 494. 



