224 
PACIFIC SCIENCE, Vol. IV, July, 1950 
have, in fact, a remarkably efficient para- 
bolic reflector which reflects the emitted light 
backward even as far as the muscles of the 
caudal peduncle. 
The ventral light spot directs its light 
downward, and, because of the cloudy trans- 
lucent muscles of the thorax and abdomen, 
a diffused luminescence is seen in this area. 
Because of the presence of a white opaque 
membrane extending dorsally from the up- 
per borders of the abdominal muscles of both 
sides, no light reaches the dorsal part of the 
body. 
The bean-shaped areas are furnished with 
chromatophores, and it is perhaps due to 
their expansion that the light can be shut 
off, and by their contraction displayed. Pos- 
sibly the white membrane which covers the 
ring can be contracted to cover the light 
spots, but this I was unable to confirm. Nor- 
mally it is expanded to uncover them. 
The complete luminous organ therefore 
consists of three components, possibly four: 
(1) the luminous ring gland; (2) the re- 
flector; and (3) the lenses; with possibly 
( 4 ) , a shutter for the light spots which may 
be opened or closed either by the chroma- 
tophores or by the white covering membrane 
of the ring working independently or simul- 
taneously with the chromatophores. In re- 
cently dead specimens these spots are closed 
by the membrane, but the gland material is 
still luminous. 
As previously stated this fish differs from 
other luminous fish by its unusually large 
luminous area, utilizing in fact half the mus- 
cles of its extremely complicated body struc- 
ture. The luminous glands are situated in 
the body, and may be considered to be of an 
indirect or reflecting type, functioning by re- 
flected light in the same way as described 
for Acropoma japonicum; but Gazza minuta 
is not only provided with a membrane and 
chromatophores for displaying its lumines- 
cence, it has a far more complex luminous 
organ than Acropoma japonicum. 
Fig. 5. Diagrammatic figure of the luminous 
organ of Secutor insidiator : phot, luminous gland; 
refli, refk, reflectors; lensi, lens 2 , lenses; vise, inter- 
nal organs. 
Secutor spp.: Secutor insidiator (Fig. 5) 
and S. ruconius are luminous Leiognathidae, 
similar to Gazza minuta, but possessing a rel- 
atively larger luminous gland than does G. 
minuta. It is yellow in fresh material instead 
of white as in G. minuta. 
The reflector is very efficiently arranged 
and is lined with a bright glossy surface. 
The ventral muscles are cloudy and trans- 
lucent. The ventral cavity walls are ex- 
tremely thin and, because of this, its lumi- 
nosity is greater than that of any of the other 
members of the family Leiognathidae. 
Leiognathus spp.: There are many species 
of Leiognathus, varying in length from 50 
mm. to 200 mm. With the exception of the 
male L. rivulatum they have comparatively 
small luminous glands. Even when com- 
pared with Gazza and Secutor, this has a com- 
paratively small luminous body and thick 
ventral cavity walls, which are only moder- 
ately translucent, thus diminishing the bright- 
ness of the luminosity. 
The species L. equulus, L. fasciatus, L. 
splendens, L. lineolatus, L. hindus, L. her his, 
L. argenteum, L. dussumieri, L. stercorarius, 
and L. daura are also only moderately lumi- 
nous. There is an interesting feature of the 
Japanese species, L. rivulatum, viz.: the lumi- 
nous gland of the male is either very large 
or else very small. I am of the opinion, 
though by no means certain, that this differ- 
