PHOTOGENIC MARINE ANIMALS. 19 



presenting at one time a rod- shape, at others a spirillum, whilst 

 mixed forms are not infrequent. They are readily developed in 

 ordinary peptone beef-broth gelatine, and no medium is favour- 

 able to their growth that does not contain soluble chlorides, or 

 solutions resembling sea- water, e. g. sodic chloride, 26*5 grammes ; 

 potassic chloride, *75 grammes; magnesia chloride, 3*25 grammes. 

 They develop rapidly on the recently dead fishes, and cause them 

 to be luminous. They take no part in the decomposition of 

 fishes, and appear to be non-pathogenic, except in the instance 

 already referred to, viz. Giard's Sandhopper. A supply of oxygen 

 is necessary for the exhibition of their phosphorescence, and 

 when grown in an atmosphere of oxygen the light is brilliant. 

 When the production of light is hindered the amount of oxygen 

 absorbed is less. All agents which affect the life of the organism 

 also affect the production of light. The phosphorescent principle 

 does not pass through a Berkefield filter. 



In conclusion, few subjects are more striking than this light- 

 producing property of animals, and hence such forms on land 

 have always received much attention. Some have even been 

 used by man for purposes of ornament, or, as the French 

 authors call it, " charming caprice " — for instance, the fireflies 

 in the dark hair of the Mexican ladies, occasionally as miniature 

 lamps, or as a lure in fishing ; whilst the Baya-bird of India 

 fixes them (Lampyris) to mud to illuminate its nest — it may be 

 for purposes of warning and protection. 



As regards the photogenic marine forms no special utility 

 has been found, though it is long since the Abbe Dicquemare 

 proposed to utilize certain species for determining the position 

 where they were found ; and more recently Decharme thought 

 Noctiluca might be made available for prognosticating storms. 



Moreover, the observer is struck by the simplicity of the 

 light-producing mechanism, and by the absence of heat. Thus 

 the light of a firefly, or a Pholas, has no sensible heat, whereas 

 a temperature approaching 2000° Fahr. would be necessary to 

 make it by the usual processes, except the Geissler tube. So 

 impressed were Prof. Langley and Mr. Vesy* with this feature 



* "On the Cheapest Forms of Light," Araeric. Journ. Sc, 3rd ser. xi. 

 No. 236. August, 1890. Quoted also by Watase in one of his able lectures 

 on the Physical Basis of Animal Phosphorescence. Boston, 1896. 



c 2 



