450 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



oxygen through the tracheoles or air tubes to the light cells. Recent 

 studies have undermined this hypothesis, and it now seems likely 

 that nerve fibers supjDly the light cells directly. Nerve impulses con- 

 cerned with flashing have been recorded from the ventral nerve cord, 

 and it has been suggested that acetylcholine, released at the nerve 

 ending, activates tlie light cells by releasing luciferin from an in- 

 hibitory complex. The introduction of this concept brings regulation 

 of luminescence into line with neuromuscular physiology, where 

 acetylcholine has long been accepted as a chemical transmitter between 

 nerve endings and muscle fibers. 



The lanterns of fishes are also under control of the central nervous 

 system. Beebe found that lantern fish (myctophids) would respond 

 by flashing, to the display of his luminous watch dial. Nervous path- 

 ways obviously exist from the eyes through the brain and spinal cord 

 to the light organs scattered along the entire length of the fish. The 

 terminal pathways, from the spinal cord to the periphery, lie in that 

 part of the visceral nervous system known as the sympathetic, which 

 supplies blood vessels, viscera, contractile pigment cells, and light 

 organs. 



Animals containing luminous bacteria also have mechanisms for 

 dimming the light although the bacteria, being independent entities, 

 are not subject to direct control. The light of the bacteria is cut off 

 by rotating the organ out of sight, by drawing a screen over it, or 

 by expanding dark pigment cells over the external surface. All these 

 methods are utilized by fish. In squid the organ containing the 

 bacteria is partially embedded in the ink sac, and the light is cut off 

 by pressing a film of black ink over the outer surface of the light 

 organ. 



ILLUMINATION AND DIURNAL RHYTHMS 



Most animals, when excited, flash at any time of the day or night. 

 Since luminescent light is very faint, it is scarcely perceptible in 

 daylight and can be regarded as biologically useless. Those animals 

 which are nocturnal, or which live in dark places, or in the dark 

 depths of the sea, are not troubled by this misplaced or mistimed 

 behavior. There are some organisms, however, which flash only in 

 darkness or at night, and investigations of dark adaptation and of 

 diurnal rhythm icity have revealed many features of interest. 



In some animals, notably sea pens, comb-jellies (pi. 3, fig. 1), and 

 PyrosoTtiae (a colonial planktonic animal whose name means firebody ; 

 see pi. 4, fig. 1) , the luminescent response is inhibited by light, and it is 

 gradually restored when the animals are placed in darkness. It takes 

 about 30 minutes for a sea pen or comb-jelly, previously exposed to 

 light, to recover its luminescent ability in darkness. Details of the 

 process have been worked out more fully in comb-jellies. Inhibition 



