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PACIFIC SCIENCE, Vol. IV, July, 1950 
inner faces of the limbs by numerous capil- 
lary ducts which enter a larger central canal 
lying between the inner faces of the limbs 
of the U-tube. This canal is bounded dor- 
sally and ventrally by concave membranes 
and has only one opening, which is at the 
anus. The whole luminous gland from a 
specimen 145 mm. long measured 14 mm., 
and from a fish 110 mm. measured 13 mm. 
External Opening — In the gland, bacteria 
occur in the film of material which lies on 
the inner surface of the epithelial cells. These 
bacteria are present in clusters or else are 
attached to the surface of the gland cells. 
They pass from there into the duct or cavity 
of the U-shaped gland via the gland ducts. 
Here they remain until they pass through one 
of the numerous capillaries or pores into the 
larger central canal which opens in front of 
the anus. This is the only external opening 
of this canal. A luminous organ with an 
opening of this type is known as an open 
type of luminous organ. 
Reflector — The reflector is a white, opales- 
cent, opaque membrane lying above the lu- 
minous duct and extending from the isthmus 
to the end of the caudal peduncle. It lies 
rather low ventrally, and separates the lower 
ventral or keel muscles from the upper lat- 
eral and dorsal muscles. It passes below the 
pericardial and perivisceral cavities; under 
the latter it is depressed and forced down- 
ward into a concavity. Elsewhere it lies flat. 
Lens Structure — The lower ventral or keel 
muscles below this membrane are lenticular 
and modified to transmit light. They are 
translucent but cloudy. They lie longitudi- 
nally and in pairs, i.e., one pair on each side. 
The first pair extends, one on each side, from 
the isthmus to the anus. The second pair lies, 
one on each side, from the anus to the end 
of the caudal peduncle. As they lie extended 
they may touch each other in places, but 
where they are not in contact they are sepa- 
rated by non-translucent muscle of ordinary 
structure. The luminous gland passes through 
these muscles, and each limb passes back- 
ward, one on each side, the bend being in the 
anterior part of the body of the fish, and the 
closed ends of the limbs being directed back- 
ward in the direction of the anus. When I 
made complete cross sections of fresh fish, I 
noticed that where I had cut through the 
limbs of the organ, the luminosity was bril- 
liant; it was only moderate and diffused in 
the muscles which were cut. The light was 
screened by the reflecting membrane above, 
but was reflected downward and laterally 
through the skin of the fish. Where I cut a 
cross section of a fish without cutting the 
limbs of the luminous gland, the muscles sur- 
rounding the limbs appeared to be luminous, 
suggesting that the muscles themselves were 
luminescent. 
Light Control Mechanism ■ — This luminous 
organ has in itself no regulating mechanism 
for controlling the display of light. On the 
other hand, the ventral area and lower lateral 
area below the reflecting membrane are fur- 
nished with a great number of chromato- 
phores in the surface of the skin. Above the 
membrane this particular kind of chromato- 
phore disappears and is replaced by another 
kind. The former are the chromatophores 
which at one time were considered to be the 
actual luminous organs, but by their contrac- 
tion and expansion they probably control the 
amount of light emitted. 
Luminous Organs of A. japonicum Type II 
(Fig. 2) 
The luminous gland is situated in the ven- 
tral muscle and is long compared with that 
of Type I. It extends from the isthmus to 
about the 5 th ray of the anal fin, where it 
bends with a short loop on each side of the 
fin. These loops are directed obliquely down- 
ward, and continue in a reverse direction for 
a short distance where the bend is continued 
to the center. In this way the two limbs form 
a kind of hooked loop, such as would be 
