J. A. C. NICOL 



311 



Excitation 



Let us consider now positive excitation of luminescent organs and 

 the ways in which the response can be modulated and controlled. 

 The luminescent response is often a triggered response, i.e., a single 

 impulse will set it into operation. This is the case in Pelagia, Beroe, 

 Pohpwe, Chaetopterus, and Vyrosoma, to mention some established 



' 



' * 



Fig. 8. Luminescent responses of Chaetopterus variopedatus, a polychaete which 

 discharges a luminous secretion. A, response to a single electrical shock. 

 Time scale 1 per second. B-F, responses to bursts of electrical shocks at 

 frequencies shown. Time scale, 1 per 10 second (Nicol, 1952). 



examples (Figs. 2, 4, 5, 8, 9). In the nerve net of pennatulids {Renilla, 

 Cavernularia ) on the contrary, facilitation is operative in the initiation 

 of the response, as manifested by the several shocks necessary to 

 produce the first flash (Fig. 1) (Harvey, 1917; Buck, 1953, 1955). 

 There is also the observation that in Porichthys, a burst of impulses 

 is necessary to bring out the response. 



