434 GENERAL PRINCIPLES 



classes of animals from the Protozoa to Fishes, and also among 

 fungi. Decaying vegetable and animal matter is often lumi- 

 nous as a 'result of the action of bacteria. The mycelium of 

 other fungi also yields light at times. In these cases the light 

 is apparently a by-product, which is not to be regarded as 

 having any adaptive function. 



870. The light-producing power is especially common 

 among marine invertebrates and Fishes. Phosphorescence 

 occurs among the Protozoa, Coelenterates, Worms, the smaller 

 Crustaceae, Bryozoa, Rotifers, free-swimming Ascidians, Fishes 

 and Insects. In some cases the light is produced at certain 

 points within the protoplasm of the cell. In others the slime 

 secreted by glands in the skin is luminous. And again there 

 are special organs which may be simple or complex in structure, 

 but which bear evidence of having been developed from groups 

 of glands, though there is no duct and no external secretion. 

 The more highly developed photogenic organs have a structure 

 resembling that of an eye with a layer of pigment at the back 

 and a lens in front. In the lower animals the light is only 

 emitted when the animal is stimulated, and the purpose of it is 

 unknown. In the higher forms, however, the organ is well 

 supplied with nerve elements and is under voluntary nervous 

 control. 



871. Among Fishes the photogenic organs are especially 

 common and well developed in deep sea species. Some " an- 

 glers" carry a lantern at the tip of the long anterior dorsal 

 spine. This lantern is suspended directly above the mouth 

 of the animal and is regarded as a bait by which the angler 

 attracts his prey. In other cases the photogenic organs may 

 occur on almost any part of the body. 



872. The common "fire fly" is a beetle belonging to the 

 family, "Lampyridae," which contains many luminescent 

 species. The eggs, larvae, male and female, may all be lumi- 

 nescent. In some species the female is wingless, but has an 



