314 PHYSIOLOGICAL CONTROL IN ANIMALS 



Response duration 



Here we are concerned with two distinct phenomena, viz., duration 

 of a single flash or glow and total duration of repetitive flashing. 



In those animals in which a response or glow is elicited by each 

 pulse, lengthening of response duration is achieved by prolonged 

 stimulation. Thus, in Chaetopterus, which secretes a luminous ma- 

 terial, continued electrical stimulation, at frequencies high enough 

 to produce fusion of separate responses, also results in a longer, more 

 durable glow (Fig. 8). This is a consequence both of summation in 

 a contractile mechanism, responsible for expressing secretion, and of 

 an accumulation of photogenic material at a rate faster than it can 

 be consumed. 



The second method concerns regulation of repetitive flashing. 

 Continued flashing once stimulation has ceased, or the evocation of 

 many flashes by a single stimulus, has been observed in Renilla, 

 Beroe, and generally in luminescent polynoids (Figs. 1, 4, and 5). A 

 strong tactile stimulus, or protracted electrical stimulation often sends 

 Renilla into a hyperexcitatory state in which it continues to flash 

 repeatedly, sometimes for long periods. This state of maintained flash- 

 ing is an external manifestation of rhythmic discharge in the nerve net, 

 and it would appear that nerve cells in Renilla can be charged up to 

 high levels of excitability, when they pass into a rhythmic oscillatory 

 state, expressing itself in periodic discharge across the nerve net. In 

 polynoids repetitive flashing is the characteristic mode of response, 

 and even a single electrical stimulus sets the elytram flashing. In these 

 animals the response is regulated by a peripheral elytral ganglion, 

 which passes into some form of oscillatory condition when excited 

 and discharges with great regularity for many seconds. In these 

 instances, which I have mentioned, the response can be greatly pro- 

 longed beyond the original stimulus by maintained excitatory states 

 engendered in the nervous system, be it nerve net or ganglion (Nicol, 

 1953, 1954a; Buck, 1953, 1955). 



Spatial Distribution 



In pennatulids, possessing a nerve net, each luminescent response 

 sweeps over the entire surface of the animal. There are many instances 



