6 TEE ZOOLOGIST. 



diffused silvery phosphorescence so familiar to voyagers. De 

 Quatrefages attributes the emission of the clear bluish light of 

 this species in quiet water, or the white light with greenish or 

 bluish touches in broken water, to any physical agent which 

 produces contraction, the large number of minute scintillations 

 arising from the rupture and rapid contraction of the proto- 

 plasmic filaments in the interior. 



In this connection Watase has recently made some interesting 

 remarks on the relationship of protoplasmic contractility and 

 phosphorescence. The physicists have shown that heat and 

 light are simply variations of the same radiant energy. The 

 heat-producing particles and the light-producing particles, 

 objectively considered, may not be very different from each 

 other. " They may be variations of similar chemical sub- 

 stances, as the resulting energies, the products of their oxida- 

 tion, are the variations of the same radiant energy. The 

 stimuli, therefore, which induce combustion of the thermogenic 

 molecules may also be presumed to incite combustion of the 

 photogenic molecules. The luminosity is due to the metabolism 

 of the definite tissue-cells and the subsequent oxidation of the 

 metabolic product, resulting in the emission of light." The 

 luminous tissue gives out carbon dioxide. 



In the open sea the naturalists of the ' Challenger ' found 

 Pyrocystis,* a form closely allied to Noctiluca, the light from 

 which is stated to proceed from the nucleus. Sir Wyville 

 Thomson observed that when shaken in a glass it gave out the 

 uniform soft light of an illuminated ground-glass globe. 



Several authors have mentioned phosphorescent sponges, 

 but there is considerable dubiety. Parasitic luminous forms 

 are numerous in sponges, and in some cases misinterpretation 

 may have occurred. 



In no group has phosphorescence been longer known, is more 

 general in its distribution, or more beautiful in its manifestations 

 than in the Ccelenterates, comprising Zoophytes, Jelly-fishes, 

 Sea-pens, and Sea-fans. On our own shores the tidal region, 

 the laminarian zone, the coralline, and deep sea areas are equally 

 the home of luminous representatives. 



The Hydroids (or Zoophytes, as they are often called) are 

 * As described by Sir John Murray. 



