OBSERVATIONS ON HOTARU-IKA WATASENIA SCINTILLANS. 79 
histological differences. The third arms have only about 7 organs arranged 
in a series. 
Each organ in the fresh specimen has a substance of purplish hue in 
the centre ; this substance seems to be that discharging light when the 
animal is living. When the organ is exposed in the air, the purplish hue 
of the subtance changes to greenish blue after a while, and finally resolves 
into a true green (PI. I, fig. 3). The substance is covered by a pigment 
layer of darkish brown or deep purple which has a hole resembling the 
pupil of an eye, through which the substance can easily be seen. The 
light of the substance at night is whiter and less luminous than that of 
the brachial organ. 
Ocular Organ. When the eyelid of the fresh specimen is removed 
and the eyeball exposed, there are seen 5 luminous organs arranged in a 
series along the ventral circumference of the eyeball, the organ on either 
ehd of the series being a little larger than the remaining 3. The colour 
of all these organs is pearly white (PI. I, fig. 5). When the organ is seen 
at night in the living animal, the phosphorescence is not distinguishable 
from that of the minute organ on the body. 
Difference of PhospJioi-csccnce in the Sexes. On examining the preserved 
specimens to discover the difference of the external forms as well as the 
histological structures of their luminous organs as occurring in the male 
and female, none could be discerned. But in the female specimens there 
are one hundred or so more of the minute organs of the mantle than in 
the male. Whether there is any meaning as to sexual selection, it is 
difficult to say, the data concerned being insufficient at present to announce 
any opinion. 
Next, as to the difference of phosphorescence between the sexes in 
their living state, the means of investigation proved to be very difficult. 
At first I repeatedly undertook to keep the animal in an aquarium, but 
no success was attained. The reason for the failure is that first of all, 
the animals are very delicate, and next the aquarium was defective. The 
