4 SURVEY OF LUMINOUS ORGANISMS 



continuous and independent of stimulation. The luminescence intensity 

 varies only with environmental changes, such as temperature, pH, and 

 salt content of the medium. Among animals, no light appears until the 

 luminous region is excited in some way — by nerves or directly by 

 mechanical, electrical, and chemical stimulation. The resulting lumi- 

 nescence is momentary or of short duration. In a few fish (Poriclithys, 

 Echiostoma, Maiirolictis, Argyropelecus) light emission of photophores 

 is hormone (epinephrin) controlled and the light lasts a longer time 

 than in the case of direct nerve stimulation. 



No bacteria or fungi are known, which can be stimulated to lumi- 

 nescence, but a few animals, whose light is not due to luminous 

 bacteria, can luminesce continuously. These exceptions to the general 

 rule are found in the diplopod millipede Ltiminodesmtis (Davenport, 

 Wootton, and Gushing, 1952), and in various stages of firefly develop- 

 ment. The firefly egg and the pupa are continuously luminous, as are 

 adult cream-colored wingless females of the related beetle, Phengodes. 

 The luminous organs of the females may remain brightly luminous 

 for days, with no change in intensity. Lwninodesmus is continuously 

 luminous, with no voluntary control, and luminous bacteria appear 

 to be absent, at least they have not been demonstrated. 



Light emission of both larval and adult luminous organs of the 

 firefly is controlled by nerves, but during the pupal stage, the larval 

 organ ceases to function and the adult lantern of the firefly develops 

 as a wholly new structure, in a new position on the abdominal seg- 

 ments. It is completely reconstituted by luminous wandering cells ( not 

 subject to nerve stimulation ) whose light shines through the chitinous 

 integument. 



Protozoa respond directly to stimulation, usually to mechanical 

 disturbance resulting in the "phosphorescence of the sea," but meta- 

 zoan* luminous cells are supplied by nerves, and reflex luminescence 



All other reported cases of luminescence of plants are due to reflection or 

 in the case of marine algae, due to luminous organisms growing on the sea- 

 weed. Luminous dinoflagellates or peridineae are regarded as animals for 

 this discussion because they luminesce only on stimulation. 



" Among sponges no electrical excitation of luminescence could be elicited, 

 but response occurred on rubbing the sponge (Harvey, 1921). Sponges do 

 not possess nerves. 



