Infection with luminous bacteria is especially liable to occur in any dead marine animal. The flesh is an excellent culture medium. I have seen non-luminous species of squid, recently killed, covered with minute growing colonies, quite evenly spaced, so as to closely resemble luminous species whose light is restricted to scattered light organs over the surface of the body. Indeed Pierantoni (1918) has carried this idea to extremes. He believes that in the luminous organs of fireflies, cephalopods and _Pyrosoma_, luminous symbiotic 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-microscopic 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 withstand such prolonged boiling with strong acid. The light of one animal at least, and I believe many others also, cannot be due to any sort of symbiotic organism. Apart from these cases where light is actually produced but is not primary, not produced by the animal itself, there are many forms whose surface is so constituted 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 erroneously described as luminous. Perhaps the best known case among aquatic animals is _Sapphirina_, a marine copepod 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.