180 FISHES CHAP. 
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 Pachystomias 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 
Fic. 98.—Pachystomias microdon. Section of modified dermal scale. The 
te guotior eporbial org Imam" axial part of the organ is 
like diaphragm ; /, lens-like body ; p.s, pig- occupied by a number of 
op TART cdc Poe radial - disposed 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 
