GENERAL BIOLOGY 679 



species which live in intermediate depths, and are now and 

 again, like the ScopeHdae, captured at the surface (see p. 649). 



The function and importance of the h'ght-organs in the hfe Function of 

 of animals have been subjects of controversy in the world of i^ght-organs. 

 science. The production of light has been explained as a 

 simple consequence of metabolism, and it has been supposed 

 that the light itself serves no purpose. Comparisons have 

 been drawn between the accumulation of mucous substance 

 and the mucous secretion of the light - organs, and it has 

 been pointed out that these organs occur particularly in 

 pelagic animals, which in order to float in the water are 

 supposed to need the mucus for the purpose of reducing 

 their specific gravity. Brandt, who has studied the adapta- 

 tions of animals to pelagic life, is perhaps right in supposing 

 that metabolic factors have played a part in the history of 

 the development of light-organs, but a closer scrutiny of the 

 structure of these organs, and particularly the discovery of 

 reflectors and lenses, seem to place it beyond doubt that the 

 light-organs serve the function of projecting light in definite 

 directions. This is the function for which the higher animals 

 use their light-organs, but for what purpose do they project 

 light ? Is it in order to illuminate the surrounding water, to 

 avoid foes, or to recognise their own kind } These questions 

 are not easy to answer with any certainty. At all events the 

 answers would probably tend to show that the many different 

 kinds of light-organs serve different purposes. For instance, 

 the large light-organs carried on the tentacles of the Ceratiidse 

 are probably used for other purposes than the smaller organs 

 found in Vinciguerria on the side of the body. 



Brauer has examined the position of light-organs in relation Light-organs 

 to body segments in different species, and has found them chaSfrs. 

 to be arranged in exactly the same manner in all individuals 

 belonging to the same species, and consequently the number 

 and position of the light-organs are specific characters. He 

 advocates the idea that in the ocean the light-organs replace the 

 specific colour-markings of terrestrial animals. 



Is it possible to explain the peculiar geographical distribution 

 of luminous animals, for instance, fishes ? The fact that light- 

 organs are found only in marine animals has been explained by 

 supposing the salt to be necessary for the production of light. 

 Experiments have shown that luminous bacteria develop and 

 emit light only when sodium chloride or calcium chloride is 

 present. As regards those organisms which secrete a slime 



