LUMINESCENCE 



567 



Direct Nervous Control 



Certain forms have already been mentioned in which a luminescent 

 slime is discharged into the water as the result of glandular secretion. On 

 reaching the exterior, or perhaps even within the gland itself as soon as the 

 secretory process is initiated, the photogenic material undergoes oxidation 

 and luminesces. The term 'slime' refers to the superficial appearance of the 

 luminescent material only, and microscopic examination shows that the 

 light originates in a multitude of scintillating spots corresponding to 

 granules of photogenic material. In these forms it is observed that the 



2.000 



4,000 6,000 8,000 



Light Intensity (metre- candles) 



Fig. 13.24. Photic Inhibition of Luminescence in the Ctenophore Beroe ovata 



The curve relates intensity of illumination to mean exposure times necessary to inhibit 

 the production of light under mechanical stimulation. (From Heymans and Moore, 

 1925.) 



glandular light-cells contain similar granules, which slowly break down 

 when discharged into the sea water. Often, but not always, the luminescent 

 material is suspended in mucus, for example in Pelagia and Chaetopterus. 

 In the latter animal the bountiful luminous secretion from the aliform 

 notopodia is accompanied by mucus and is dispersed in the surrounding 

 sea water by ciliary action. The problem of how the photogenic cell, when 

 excited, discharges its contents, is one common to all glandular cells and 

 involves a consideration of intracellular dynamics. 



In luminescent coelenterates excitation usually is transmitted as a 

 wave over the surface of the animal. The nervous system in this group is 

 organized on the basis of a nerve-net, and transmission associated with 

 luminescence shows some features common to those involved in muscular 



