14 THE NATURE OF ANIMAL LIGHT 



bacteria occur which are responsible for the light of these 

 animals, and he claims in the case of cephalopods and 

 Pyrosoma to have been able to isolate these in pure culture 

 on artificial culture media. In the firefly they can be seen 

 but not grown and, in luminous animals where no visible 

 bacteria-like structures are apparent he believes we are 

 dealing with ultra-microscopic luminous bacteria similar 

 to the pathogenic forms suspected in filterable viruses. 

 While the assumption of ultra-microscopic organisms 

 makes the refutation of Pierantoni's views a somewhat 

 hazardous task, no one can deny that even an ultra-micro- 

 scopic organism will be killed by boiling with 20 per cent, 

 (by wt.) HCl for 6 hours. As we shall see, the luminous 

 material of Cypridina, an ostracod crustacean, can with- 

 stand such prolonged boiling with strong acid. The light 

 of one animal at least, and I believe many others ifclso, 

 cannot be due to any sort of symbiotic organism. 



Apart from these cases where light is actually pro- 

 duced but is not primary, not produced by the animal 

 itself, there are many forms whose surface is so consti- 

 tuted as to produce interference colors. This is true in 

 many cases among the birds and butterflies whose feathers 

 and scales are iridescent. Some of these have been erro- 

 neously described as luminous. Perhaps the best known 

 case among aquatic animals is Sapphirina, a marine cope- 

 pod living at the surface of the sea, and especially likely 

 to be collected with other luminous forms. Its cuticle 

 is so ruled with fine lines as to diffract the light and flash 

 on moving much as a fire opal. Needless to say no trace 

 of light is given off from this animal in a totally dark room. 



It has often been supposed that the eye of a cat or 

 of other animals i« luminous. The eyes of a rtioth, also, 



