324 CONTROL OF BIOLUMINESCENCE 



index of neural activity as is muscular contraction) rather than in the 

 actual linking of the nerve impulse to the control of light production. 

 However, I was happy to see that his records confirm the observation 

 that facilitation occurs at the neurophotocyte junction in Renilla 

 (Buck, 1953), because this suggests a parallel between photogenic 

 control and events occurring at the motor end plate. In this connec- 

 tion, and in apparent opposition to Parker's (1919) generalization 

 that efiFectors have preceded neural regulation in evolution, Dr. Nicol 

 makes the interesting suggestion that the photogenic system may have 

 been able to hook into an already existent and widespread control 

 mechanism, the neuromuscular system. This may make less puzzling 

 at least the existence of diverse but highly developed photogenic 

 control systems throughout the animal kingdom, if not the apparently 

 haphazard development of the photogenic capacity itself (Harvey, 

 1952). 



The presence of two effector systems, muscles and photocytes, 

 dependent on a single conduction system, has necessitated a notable 

 degree of coordination even in so simple an animal as Renilla, in 

 which hght production and zooid retraction may occur together or 

 independently. At the same time, as Dr. Nicol has suggested, the 

 inhibition of luminescence by light, as seen particularly in the more 

 primitive luminous organisms, may have utility in sparing luminous 

 substrate for the hours of darkness. In view of the teleological at- 

 tractiveness of this speculation it would be of interest to know whether 

 the photoinhibition involves, as in Mnemiopsis (Moore, 1924), the 

 control system (and in this I suggest the possibility of direct action 

 on the nerves as well as via light-sensitive receptors), or also, or 

 instead, photochemical destruction of the photogenic substrate as sug- 

 gested by Dr. Harvey's (1921) experiments with BoUna. In the latter 

 instance it would be desirable to show that reversing the photoinac- 

 tivation, or photodegradation, is less demanding, energetically speak- 

 ing, than the synthetic replacement of substrate simply allowed to 

 "burn" irreversibly. 



Dr. Nicol's figure for the response of Pelagia seems to indicate that 

 duration of flash declines as intensity falls off in consecutive flashes. 

 This would be contrary to the findings of Brown and King (1931) on 

 fireflies and to what would be expected in luminous extracts, where 



