ON THE PHOTOGENIC ORGAN OF THE KNIGHT-FISH. 37! 



to form veins which enter into the connective-tissue trabeculae 

 of the medullary part of the organ. 



The tissue of the central part, and to some extent that which 

 encloses the proximal part of the organ, is always loaded with a 

 large quantity of very minute crystals which appear chalky 

 white by reflected light and brown by transmitted light (comp. 

 Figs. 6 and 7, pi. I.). 



Harvey states that the luminescence of Monocentris is con- 

 tinuous. The fish can produce light both day and night. But 

 the emission of light seems to be controlled to some extent by 

 internal stimuli. The operating mechanism has not, however, 

 been completely ascertained. No special muscle fibers have been 

 found in the organ. Nevertheless it may not be unjustifiable to 

 suppose that the thin stratum of more or less elastic fibers 

 surrounding the outer surface of the tubular region exerts a 

 contraction by which the spontaneous light is produced. Other- 

 wise it is quite difficult to understand the spontaneous lumi- 

 nescence occasionally produced by the fish. This stratum is not 

 clearly differentiated from the ordinary dermal tissue. 



I have not been able to detect any nerves entering the gland 



proper. 



OBSERVATIONS ON THE LIVING FISH. 



In connection with this morphological study on the photo- 

 genic organs of Monocentris I have made some biological observa- 

 tions on the living fish. The emission of light was seen in dark- 

 ness under certain conditions. It could easily be produced by 

 agitating the water in which the fish was kept or by adding to it 

 a few drops of ammonia, but very often the luminescence was 

 spontaneous. Luminescence is continuous for even several 

 hours. By rubbing the luminous protuberances with a piece of 

 stick or by scraping them with a knife edge, the luminosity 

 could partially be transferred to the surface of the stick or knife, 

 remaining visible there for several seconds. Although it will 

 be seen that in this case we are dealing with a substance excreted 

 by the organs, it should not be considered that the luminescence 

 is ordinarily due to excreted matter the "external lumi- 

 nescence" because no luminous material was seen excreted into 

 the water by a living fish; the luminescence is "extracellular* 

 but "intraglanclular." 



