372 YO K. OKADA. 



Monocentris can live in fresh water for a considerable length 

 of time, even for ten hours. Nevertheless, they are quite 

 uneasy under such conditions, as indicated by rapid beats of 

 the fins, stirring movement of the tail, a bubble formation on 

 the skin and, especially, the emission of a continuous bright 

 light. 



Monocentris can live in the sea water w r ithin a range of temper- 

 ature between o C. and 40 C., and light is produced within 

 somewhat wider limits, the maximum temperature being about 

 42 C. and the minimum about 10 C. 



Monocentris can show light at any time of the day and night, 

 but the natural luminescence seems to cease in the day time, 

 for the fish after being removed from daylight to darkness did 

 not show this property at least for ten to fifteen seconds. 



The regeneration experiment of the photogenic organs, which 

 I have performed, may be interesting and is somewhat suggestive 

 in regard to the problem of the animal luminescence in general. 

 The photogenic organs of eight fishes were excised in varying 

 extent. The wound healed very soon and in the course of about 

 tw r o weeks the integument was completely regenerated. The 

 internal changes following these operations were studied by means 

 of serial sections. The result shows that the tubules of the 

 organs, either completely or partially removed, are not regenerated 

 but are replaced by a spongy tissue consisting of exceedingly 

 enlarged blood vessels. If the granules in the organ were 

 bacterial bodies, they might have been expected to increase in 

 amount, because they were subjected to a favorable condition 

 with an abundant supply of blood. But this does not seem to 

 have been the case. There was no increase in the amount 

 of granular matter in the remaining tubules. 



SIMILAR PHOTOGENIC ORGANS OF OTHER FISHES. 



Finally the only photogenic organs which at all approach those 

 which have been considered are to be found in two Malayan fishes, 

 Anomalops katoptron (Bleeker) and Photoblepharon palpebratus 

 (Boddaert) which have been made the subject of memoirs by 

 Steche (1907, p. 85 and 1909, p. 349). A glance at his figures in 

 Pis. XIX-XXI will at once show the fundamental similarity of 



