i8o 



FISHES 



50 to 100, are arranged in transverse bands on the sides of 

 Opostomias micripnus, and appear as small white spots on the 

 otherwise black skin of this Fish. 



Each organ has the shape of a biconvex lens, sunk to about 

 half its thickness in the skin. The inner half is formed of 

 radially-arranged gland tubes filled with small granular cells, 

 and converging towards the centre of the organ. Into the con- 

 nective-tissue walls of the tubes extend blood-vessels and nerves. 

 External to the gland tubes there is a layer of long slender cells 

 arranged perpendicularly to the surface, and more externally 

 still a layer of ganglion cells. There is evidence that these 

 organs multiply by division. Such simple phosphorescent organs 

 as these differ little from the groups of epidermic gland cells, 

 which probably formed the evolutionary starting-point in the 

 development of these singular structures. 



A much more complex type of luminous organ is to be found 

 in the suborbital organs of Pacliystomias microdon, of which 

 there are two on each side, appearing as conspicuous white 

 masses, one in front of the other, and situated just below the 

 eye. The more anterior of the two organs is somewhat pouch- 

 shaped in section, its walls 

 consisting of several con- 

 centric layers (Fig. 98). 

 Externally there is a layer 

 of black pigment, within 

 which is a stratum of 

 irregular gland tubes. More 

 internally still there is a 

 thick layer of light-reflect- 

 ing spicules, probably de- 

 rived from an inverted and 

 Section of modified dermal scale. The 



the anterior suborbital organ, g, Irregular • i , r ^i nro-nn k 



gland tubes ; g\ radial gland tubes ; i, iris- ^^^^^ ^^^^ °^ ^^^ °^S*^ ^^ 



like diaphragm ; I, lens-like body ; p.s, pig- OCCUpied by a number of 

 ment sheath; s, layer of light - reflecting j- i t i i. i_ 



spicules. (After Lendenfeld.) radial -dlSpOSCd strUCtureS, 



probably similar to the 

 gland tubes of the simple organs of Opostomias, and continuous 

 with a lens-like structure which, as it were, closes the expanded 

 mouth of the pouch. The superficial skin which forms the 

 margin of the aperture partially projects over the outer surface 



Fig. 98. — PachystoTnias microdon. 



