PHYSIOLOGICAL 427 



due to some peculiarity in the metabolism; others may be extrinsic 

 and due to clusters of symbiotic bacteria. 



Newton Harvey has made a detailed study of two East Indian 

 fishes already mentioned, Anomalops and Photoblepharon. They 

 are common off the Banda Islands and they have some peculiarities. 

 Thus the large luminous organs give out light continually, night 

 and day, and without stimulation .» The investigator could not 

 demonstrate luciferin and lucif erase, but under the microscope he 

 found in the luminous organs innumerable mobile bacteria, com- 

 parable to those that make dead fishes "shine in the dark". When 

 the organ was dried and re-moistened, it gave only a faint light; 

 which is also true of luminous bacteria. The light was extinguished 

 without a preliminary flash when fresh water was added, as is also 

 true of luminous bacteria. Moreover, poisons that put out the light 

 of luminous bacteria had a similar effect on the light-organs of 

 these two Banda fishes. So the suspicion grew into a hypothesis: 

 that the luminous organs in these animals were incubators for the 

 growth and nourishment of luminous bacteria living in partnership 

 or symbiosis with the animals. 



It may be asked whether it was proved that the bacteria observed 

 were themselves luminescent; and the answer is in the negative. 

 Yet this is not necessarily fatal to the theory. When the bacteria 

 were isolated and made to grow by themselves in a jelly culture, 

 they gave forth no light. This may mean that the theory is wrong, 

 and that the light is produced by the metabolism of living cells 

 in the fish. Yet it may only mean that the bacteria do not light 

 up except in certain surroundings and with certain food. Further 

 experiments are necessary. 



We have already referred to the extraordinary Fire-flame or 

 Pyrosome, a tubular colony of free-swimming pelagic Tunicates, 

 briUiantly "phosphorescent" with greenish-blue light or with 

 changing colours. A common size of colony is the length of one's 

 hand; but it may grow to the length of one's arm, and one of this 

 size will light up a dark room so that the furniture can be seen. 

 The light is discontinuous, unlike that of the Banda fishes; and 

 another difference is that the Pyrosome 's light seems to require 

 a stimulus, such as a collision with another animal or a splash 

 from a wave. When one is kept in a quiet aquarium, it is brilliant 

 for a while, and then the light fails. These three facts seem quite 

 against the bacterial theory of luminescence. But the question is 

 not so readily answered. When we look into the structure of a 

 Pyrosome, we see a tubular colony of thousands of individuals, 

 each of them with two luminous organs like little jewels. The cells 

 of these spots include minute corpuscles like rodlets or horseshoes 

 in shape, very suggestive of some forms of bacteria. The difficulty 

 is to decide between the two interpretations, the one regarding 



