428 FISHES 



condensing the light produced in the spherical inner part of the 

 organ. The whole organ is surrounded by a layer of pigment 

 cells within which is a thin reflecting membrane, much thinner 

 than the reflector in the organs of Sternoptyx but probably of 

 the same nature. The smaller simpler organs of Astronest/ies 

 are like the inner portion of the compound organ without the 

 outer cup. 



In many fishes there are luminous organs of larger extent 

 and less regular shape than those above described. These 

 usually consist of glandular tissue similar to that found in the 

 inner parts of the compound definite organs ; the suborbital 

 organ of Astronesthes for example is enclosed internally by a 

 thin light-reflecting membrane and outside this by a pigment 

 layer, but there is no well-developed reflector. In other cases, 

 as in Pachystoviias microdon, the suborbital organs have a com- 

 pound structure like that described in the compound organs of 

 AstronestJies, but the organs are of greater superficial extent. 

 The internal portion is glandular and is surrounded by a thick 

 reflector. The surrounding skin extends somewhat over the 

 margin of the organ, and it is probable that in the living fish 

 the organ can be covered or exposed at the will of the fish, as 

 is known to be the case in the little East Indian fish Anomalous, 

 described above. 



The projecting organs of Xenodermichthys are very different 

 in structure from any yet described, but they are covered by 

 pigment at the sides and not at the ends, and their internal 

 tissue seems to contain gland-cells and refringent club-shaped 

 cells like the more typical luminous organs. 



It is obvious enough that the pigment and reflecting layer 

 surrounding the luminous organs on the internal side are special 

 developments of the ordinary pigment cells and iridocytes of 

 the skin, but the origin of the essential tissue of the organs has 

 not, in the organs hitherto considered, been described in detail ; 

 some Italian investigators, however, state that they have found 

 in certain species of the Stomiatidce that the light-producing 

 tissue is derived, as would be expected, from the epidermis. 

 The most complete account we have of the structure and develop- 

 ment of the various constituents of a phosphorescent organ is 

 that given by the American investigator Greene for the organs 

 of Porichthys, and although these are more rudimentary and, as 



