Luminous Organs of Fish — HANEDA 
225 
ence in size is due to age. The luminous 
gland of the female is smaller and the lumi- 
nosity of the female is therefore very weak 
compared with that of the male. 
The luminous gland of L. rivulatum dif- 
fers from that of all other Leiognathidae in 
that it does not surround the oesophagus 
completely, but occurs as an external swell- 
ing on the dorsal part only. 
L. elongatus also possesses a very small 
luminous gland with a poor reflector; in con- 
sequence its luminosity is feeble. 
Fig. 6 . a, Leiognathus equulus ( Forsk. ) ; b, Secutor insidiator (Bloch); c, Gazza minuta (Bloch). 
The luminous organs are not seen externally, so that the fish look like common non-luminous fishes, but 
the lower half of these fish bodies, namely, the parts below the black lines, are a lightning-like bluish- 
white color. 
