J. A. C. NICOL 305 



flashes, with intervals of 0.1-1 second, and flashing continues for a 

 minute or more (Fig. 5). Each flash is very brief, having a latent 

 period of some 19 milliseconds, and lasting 83 milliseconds; and 

 fusion and summation attend rapid stimulation, above 8 per second 

 (Bonhomme, 1942; Nicol, 1953). 



Repetitive flashing is regulated by the peripheral elytral ganglion 

 and fails to occur in its absence, a single shock then inducing a single 

 flash. Of particular interest is the fact that peripheral facilitation 

 is well displayed. The first few responses show a progressive augmen- 

 tation of maximal intensity, and the increment in intensity between 

 successive impulses is inversely related to the stimulation interval 

 (Fig. 5B). There is thus evidence for accumulation and decay of 

 facilitator (Nicol, 1954a). 



H 



rn-nn^nnrTTTTirrTT 



1 1 1 1 ■ 1 r-l 1 i-l 1 r i-l 1 1 1 



Fig. 5. Luminescent responses from single elytra of Acholbe astericola (polynoid 

 worm). A, prolonged flashing following a single electrical stimulus. This is a 

 continuous record. B, single flashes induced by repetitive stimulation at a 

 slow rate (42 per minute). Note facilitation. Time scale of these records, 

 1 per second (Nicol, 1953, 1954). 



Luminescence is normally evoked reflexly by tactile stimulation: 

 afferent impulses enter the central nervous system and are relayed 

 peripherally in efferent pathways to the elytra. Excitation is also 

 transmitted up and down the nerve cord and causes adjacent segments 

 to flash. Another notable featiue of the response is that it is provoked 

 by autotomy of the scales. The elytrophore possesses a visible breaking 

 plane, and when a scale is cast off under tactile stimulation, it begins 

 to flash rhythmically. This is occasioned either by the discharge of 

 impulses from the central nervous system at the same time as autotomy 

 is induced, or by rupture and stimulation of nerve fibers during 

 autotomy. In any event the resultant impulses excite the elytral gan- 

 glion, provoking rhythmic discharge and flashing. 



