LUMINESCENCE 563 



PHYSIOLOGY OF LUMINESCENCE 



Most luminescent animals seem to have some method of controlling the 

 appearance of their light. When this is continuous it usually indicates 

 bacterial origin, but even bacterial light is subject to some degree of control 

 in various animals. The methods utilized by animals for regulating the 

 appearance of light can be classified into categories as follows. In one type 

 the photogenic cells or a reservoir can be squeezed by neighbouring muscle 

 fibres and the secretion poured forth to the exterior. Control in this case is 

 indirect and is a typical neuromuscular phenomenon. Again, the gland 

 cells themselves may be directly innervated and discharge their contents 

 on excitation. The control of luminescence in this case becomes one of 

 nervous regulation of glandular secretion. When luminescence is intracel- 

 lular, excitation evokes cellular changes leading to the activation of photo- 

 genic material. Finally, the emission of light may be controlled by rotating 

 the light-organ, or by the use of screens or shutters (49). 



Among protozoa the dinoflagellates and radiolarians display intra- 

 cellular luminescence and flash only on stimulation. In Noctiluca a wave 

 of luminescence passes over the cell, and this indicates the passage of an 

 excitatory phase, perhaps resulting from the progressive depolarization of 

 the cell membrane. The normal method of excitation in this animal is 

 by mechanical stimulation — for example, by wave agitation — but osmotic, 

 thermal and electrical stimuli are also effective (27). A brief mechanical 

 or electrical shock evokes a quick flash lasting some 100 msec. The latent 

 period is very short, about 10 msec. With repetitive electrical stimulation, 

 fusion of separate flashes occurs at frequencies around 30/sec. Following 

 bursts of high-frequency stimulation there is usually a prolonged after- 

 glow, lasting up to 0-75 sec. When subjected to repeated mechanical or 

 electrical stimulation, the flashes of Noctiluca soon decline in intensity 

 owing to onset of fatigue. Recovery takes place after a minute or so, when 

 flash-intensity is restored to its initial level. Under electrical stimulation 

 consecutive flashes show facilitatory increment, at least until fatigue sets 

 in. Such facilitatory increment occurs at intervals up to 1*4 sec and is more 

 pronounced with shorter periods between shocks. 



Nothing is known about the regulation of luminescence in sponges. 

 In higher metazoa control of light-production is exercised by the nervous 

 system. 



Stimulation 



A wealth of older accounts refers to various kinds of stimuli — mechan- 

 ical, electrical and chemical — that evoke a response. Most of these have 

 been used haphazardly, without adequate control, and the observations 

 yield few quantitative data that afford any insight into the mechanics of 

 the processes involved in excitation. 



A hydromedusan such as Aequorea has light-organs distributed around 

 the periphery of the umbrella. Localized stimulation evokes light in the 



