376 
PACIFIC SCIENCE, Vol. V, October, 1951 
ventral area is scaleless and there are two ex- 
ternal apertures. Behind the swelling at the 
posterior end is a reflector. By examining 
fresh material in the dark, two luminous 
points can be seen. 
The combination of a very long canal and 
a compound lens occurs in some species, such 
as Hymenocephalus striatissimus Jordan and 
Gilbert, H. gracilis Gilbert and Hubbs, and 
H. kuronumai Kamohara. In these there is a 
long, thin canal at the end of which is a club- 
shaped luminous gland to which is attached, 
ventrally, a swollen pouch-shaped body. This 
is a lens. Just behind and below it in the skin 
of the abdomen is a swelling which is doubly 
convex. This is a second lens which directs 
the light ventrally to the outside. It is a very 
complex organ. This type of luminous organ 
has already been reported by Vinciguerra 
(1932). 
Fig. 4. Diagram of the luminous organ of Hymen- 
ocephalus kuronumai and its longitudinal section, phot, 
luminous gland; Lest, external lens; Lint, internal lens; 
cond, conduit; an, anus; rect, rectum; op, opening of 
conduit. 
All of these are luminous glands and their 
innermost surfaces are lined with a layer of 
epithelial cells. Within the cavities formed by 
the folds of the glands resting on the inner 
ends of the epithelial cells are other cells j 
which contain capsules in which are masses |j 
of bacteria, variously arranged. Some of these : 
capsules are entire and contain the bacteria, , 
others are ruptured and have allowed the ‘ 
bacteria to escape into the fluid of the gland i 
ducts. Hickling says these masses are gran- ! 
ules of luminous matter and that they exist 
in four states, viz.: (a) as closely packed 
masses filling their capsule; (b) as closely ■ 
packed masses, but only half filling their cap- 
sule; (c) as loosely packed individuals in a I 
ruptured capsule; and (d) as dispersed in- !| 
dividuals free from their now empty capsule. || 
Both Kishitani and I, however, consider |! 
these so-called luminous granules to be j 
masses of luminous bacteria. We have ex- | 
amined a great amount of material from many 
species and it all shows the same character- j 
istics. I 
i 
LUMINOUS PHENOMENA OF THESE ORGANS j 
WHEN SEEN IN DARKNESS ! 
Inasmuch as Lotella phycis and Physiculus 
japonicus live in comparatively shallow water, | 
they can be kept in aquaria where their lu- 
minosity, if visible, could be seen in the dark. 
I have kept many specimens under observa- 
tion but almost all of them displayed no 
luminosity which could be seen externally. 
In August, 1936, while working at the 
Asamushi Marine Biological Station, I had a 
further opportunity to observe these fishes in 
the dark. Among many specimens of Physi- 
culus japonicus in an aquarium tank, I saw one 
hsh with a luminous ventral aperture. This 
aperture could be seen only when the fish 
swam upwards and thus showed its ventral 
surface. It could not be seen when it swam 
downwards from the top of the tank, show- 
ing only its dorsal surface, or when it swam 
horizontally, showing either one of its 
lateral surfaces. 
I could not see any luminous, cloudy dis- 
charge from the opening, as had been sug- 
gested, and this led me to believe that the 
luminosity arose from a true luminous organ. 
