Vol. L May, 1926 No. 5 



BIOLOGICAL BULLETIN 





ON THE PHOTOGENIC ORGAN OF THE KNIGHT-FISH 

 (MONOCENTRIS JAPONICUS (HOUTTUYN)). 



Y6 K. OKADA, 

 NABA, HOGO-KEN (JAPAN). 



It was Stead (1906, p. 89) who first described the luminescence 

 of the knight-fish (Monocentris gloria-maris) . "On each side of 

 the head near the mouth are peculiar luminous discs, which are 

 probably of service to the fish in assisting it to obtain its food." 

 Unfortunately this interesting observation has escaped attention 

 up to this date. 



Monocentris japonicus, another well-known species of the 

 knight-fish, is a rather common animal along the coast of Japan. 

 This fish also has the same property of light production as the 

 Australian form. Light is produced by a pair of glands situated 

 just under the lower jaw of the fish. Their function and structure 

 were described by Yoshizawa (1916, p. 411) in Dobutsu-Gaku- 

 Zashi, a Japanese zoological periodical. His brief description 

 was illustrated with figures, but several points remain un- 

 explained. According to Yoshizawa the photogenic organs of 

 Monocentris have the construction of photospheres, composed of 

 large glandular cells with nuclei of a considerable size, but with- 

 out accessory structures such as the reflector. He seems not to 

 have been aware of the truly glandular nature of the organs. 



In 1917 Harvey came to Japan and made at the Misaki 

 Biological Station some observations upon the interesting 

 luminous shore-fish. These observations have been recorded in 

 several of his papers. The photogenic property of Monocentris 

 thus has come to be generally known as a fact. 



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