366 YO K. OKADA. 



LOCALIZATION AND GENERAL STRUCTURE OF THE 

 PHOTOGENIC ORGANS. 



The photogenic organs of Monocentris consist of two ovoid 

 protuberances lying side by side on each side of the median 

 thickening of the lower jaw, their longer diameter being nearly 

 at right angles to the median plane (Fig. I, gl). In a fish 12 cm. 

 long the protuberances measure about 4 mm. in length and 

 3 mm. in width. Their whole surface being covered with 

 minute dermal papillae of dark brown color, the photogenic 

 protuberances do not stand out clearly above the surrounding 



FIG. i. Ventral view of the lower jaw of Monocentris japoniciis. About X 2. 

 gl, photogenic gland; op, opening of gland. 



surfaces. This fact seems to be the reason why the organs in 

 question have remained unnoticed by most ichthyologists. 



The photogenic organs of Monocentris are glands of an excretory 

 nature, although luminous material does not escape under 

 normal conditions. The glandular nature of the organs can 

 easily be demonstrated even without dissection. Each photo- 

 genic protuberance has a slit-like opening at the anterior end 

 (Fig. i, op) and when pressed under conditions of darkness a 

 luminous fluid is observed to be forced out through this point. 

 The opening is crescentic and its longer diameter is almost 

 parallel to that of the protuberance. 



The interior of the photogenic organs is white and spongy, 

 containing a large quantity of minute crystals. A great 

 number of simple tubules develop around the central tissue. 



