J. A. C. NICOL 



313 



stimulus produces a brighter glow than a weak one, as the result of 

 stronger tactile excitation producing more nerve impulses. The mode 

 of luminescence in Tijrosoma is not known with certainty, and the 

 problem is still obscured by Pierantoni's claim that light is due to 

 intracellular symbiotic bacteria. 



In many animals, where the response is intracellular, luminescence 

 takes the form of a short flash or series of flashes, and the intensity 

 is often governed by facilitation, e.g., Renilla, Beroe, Polynoe. In 



4 



TIME SEC. 



Fig. 10. Facilitation-decay cur\'es for Renilla (A) and Polijnde (B). In A, 

 Renilla, the specimen was stimulated with paired shocks, separated by inter- 

 vals ranging from 0.25 to 4 seconds. The first response appeared on tlie 

 second shock, and ordinates represent the intensity of response at different 

 stimulation intervals. B, Polynoe, shows increment of second o\'er first re- 

 sponse to paired shocks, separated by intervals ranging from 1 to 120 

 seconds (from Nicol, 1954). 



Renilla and polynoids at least, facilitation occurs peripherally. It 

 manifests itself by progressive increase in intensity of successive 

 flashes and depends in some way upon arrival of successive impulses 

 at the photocytes. It is generally assumed that augmentation of 

 response is owing to a banking up of facilitator faster than it can be 

 removed (Fig. 10). This effect is observed under normal modes of 

 stimulation (tactile), as well as in response to electrical shocks, and, 

 therefore, is probably a factor regulating the intensity of response to 

 environmental agencies. 



