STRUCTURE OF LUMINOUS ORGANS C9 



* ^ stimulation ' ' in the same sense in which it is used in 

 connection with nerve or muscle tissue. Bacteria, fungi, 

 and a few fish produce light continuously and independ- 

 ently of stimulation. Its intensity varies only over long 

 periods of time and is dependent on the nature of the 

 nutrient medium or general physiological condition of 

 the organism. All other forms give off no light until they 

 are stimulated. Stimulation may of course come from 

 the inside (nerves) or outside. Only under unfavorable 

 conditions, such as will eventually lead to the destruction 

 of the luminous cells, do these forms give off a continuous 

 light. This has often been spoken of as the ^ ^ death glow, ' ' 

 and is to be compared with rigor in muscle tissue. 



Some of the fish which produce a continuous light pos- 

 sess a movable screen similar to an eyelid which can be 

 drawn across the organ, thus shutting off the light, so that 

 the animal appears to belong to the group which flashes 

 on stimulation. This is true of Photohlepharon, while 

 Anomalops can rotate the light organ itself downward, 

 so as to bring the lighting surface against the body wall 

 and thus cut off the light (Steche, 1909). Other fish 

 (Monocentris) are unable to ^'turn off ^' their light. 



Animals which flash spontaneously on stimulation 

 through nerves from within, possess a very varied rhythm. 

 The different species of fireflies can be distinguished by 

 the character of their flashing* (McDermott, 1910-17; 

 Mast, 1912). Fig. 16 shows the method of flashing of 

 some common eastern North America species. The glow- 

 worm light lasts for many seconds and then dies out. This 

 interval of darkness persists for some minutes and is then 

 followed by another period of glowing. Some fireflies have 

 a light which may be described as partially intermittent. 



