GENERAL BIOLOGY 675 



single row of luminous cells in the usual epithelium, and a 

 lens formed by the cuticula or chitinous layer of the epidermis. 

 In squids and fishes the organs are very complicated, as we 

 shall presently see. 



The object of the " Michael Sars " Expedition being mainly 

 the investigation of the distribution of animals, the examination 

 of the collections has necessarily been limited to the determina- 

 tion of the species, and my contributions to this fascinating 

 section of the science of marine life will largely consist in dis- 

 cussing the distribution of animals possessing light- organs, 

 which occur in salt water only, for no luminous animals are 

 known from fresh water and no phosphorescence occurs there. 



Glandular, clearly defined, and localised light-organs are Light-organs 

 found mainly in pelagic animals. Among bottom animals principally 

 from the coast banks luminosity is exceed- in pelagic 



ingly rare, but on the other hand, many ' — ammas. 



bottom animals have been brought up 

 from the abyssal region in a luminous 

 condition, and have continued to emit 

 light when placed in dark surroundings ^ 

 on board (see Fig. 70, p. 88, representing 

 a luminous umbellularian). No special 

 luminous structure has been found in fi^ 4^2 



these cases, the luminosity being attached Light-organ of sergestes chai- 

 to the surface epithelium. As regards irl";„«:ie'rrl„le: 

 fishes, Giinther has drawn attention to lens ; </, glandular ceiis ; 

 the fact that many deep-sea forms secrete HaLen,°from' st'euer. ) 

 a large amount of slime. The heads of 



many deep-sea Macruridae exhibit certain pits and channels, 

 which produce great quantities of slime. This slime is supposed 

 to be luminous, and to perform the function of ordinary glan- 

 dular light-organs, which last are found only in a few fishes sup- 

 posed to live along the bottom, for instance, sharks (Spinacidse, 

 Spinax niger), and even in these they occur only as isolated 

 organs, not in such numbers as in the genuine luminous fishes. 



Among the pelagic fishes of the coast banks no species is 

 known to possess light-organs ; neither the herrings nor the 

 mackerels have any representatives with light- organs. As 

 shown in Chapter IX. there is not a single independent pelagic 

 fish-species in the northern boreal waters, and as a consequence 

 no boreal pelagic fish-species possesses light-organs.^ A minute 

 examination of the lower forms has never been made, and at 



^ I regard the Scopelidse in the Norwegian Sea as visitors, and not as true boreal forms. 



