ON LUMINOUS ORGANS OF FISHES. 17 



The morphological nature of the photogenic body has been 

 more or less erroneously construed by previous authors. Brauee's 

 opinion regarding it appears to me to be the most probable. 



The element which I call the peculiar membrane was found 

 by Brauer in the "schüsseiförmigen Organe" of the subgenera 

 Diaphus and Lampadena, lying directly behind the lens and 

 appearing transversely striated in section. I have also found it in 

 the subgenus 3Jyctophum, in which Brauer ftiiled to find it, lining 

 the ventral half of the orifice and lying over the gelatinous tissue. 



Very often we meet with another peculiar layer composed 

 of hexagonal, iridescent elements, lining the concave surface 

 of the pigment layer. This is visible from the outside with the 

 aid of the microscope, and Leydig has already dealt with it. 

 It doubtlessly serves as a reflector and consists of short and 

 relatively thick hexagonal prisms of spicules arranged obliquely to 

 the surface of the pigment layer. When treated with acids, the 

 spicules are dissolved, leaving behind a part of the gelatinous con- 

 nective tissue that fills up the intervals, thus giving rise, in hori- 

 zontal sections, to an appearance of honey-comb like hexagonal 

 septa and in transverse sections, to that of oblique striations. 



Another form of reflector is composed of bundles of thin 

 spicules running parallel to the surface, a form that has been 

 well know^n to previous observers (PI. I, fig. 9, r). 



The lenticular thickening of the scale associated with the 

 organ is a peculiarity of the Myctophidœ. Well developed lenses 

 were observed in the subgenus Myctophum, while in Diaphus they 

 are but weakly developed. Usually they are more convex on 

 the inside than on the outside, and their thickest part lies op- 

 posite the ventral half of the organ. 



Considerable differences are to be noticed between the luminous 



