OBSERVATIONS ON HOTARU-IKA WATASENIA SCINTILI,ANS. 77 
length being 44-54 mm.; this may also be taken as the length of the fully 
mature niale for the same reason as stated above. The mantle is slenderer 
than that of the female and rather "conical in shape, the broadest part 
being in the anterior. The fin is of equal breadth and length, and is 
proportionately smaller than that of the female, the length being only a 
little longer than half the mantle-length (PI. II, fig. 4). 
The hectocotylus is the right ventral arm, and provided with 2 small 
semilunar membranes in the terminal portion, but beyond that it shows no 
special modification (PI. II, fig. 3). 
The pen is distinctly narrower than that of the female, the breadth 
being about 1/7 of the length (PI. II, fig. 2). 
2. Luminous Organs and their Phosphorescence (PI. H, figs. 2-5). 
Prof. Dr. S. Watase was the first to make zoological observations on 
the phosphorescence of the luminous organs of Hotam-ika at Namerikawa, 
and he described it in the Dobutsugaku Zasshi, Tokyo (1905, p. 119), 
and also announced it at the Seventh International Zoological Congress at 
Boston, U. S. A., 1907. The luminous organs are classified here also into 
3 kinds according to Prof. Watase, i. e. (a) brachial organs, (b) minute 
organs scattered on the ventral surface of the whole body, (c) ocular organs. 
Brachial Organ. This is the largest organ, and when I made observa- 
tions in the fishing season, it was much more active in phosphorescence 
than other organs. It is situated at the end of each ventral arm, composed 
of 3 globules arranged in a series. The globules are ovoid in shape and 
nearly squal in size, but the middle one in the series is generally a little 
larger than the others, the dimensions being i .4 mm long and about i mm 
broad. In fresh specimens they show a greenish cobalt colour, and there 
are 2 or 3 layers of large brownish chromatophores covering a part of 
the preceding substance (PI, I, fig. 4). These chromatophores are con- 
stantly contracting and expanding. When they were observed at night 
on the living animals, they were seen to discharge light in all directions 
much brighter than any of Japanese fireflies. The colour of the light is 
