I THE COLOURS OF ORGANISMS 19 



luminous organs. Many of the deep-sea fishes are 

 very markedly phosphorescent ; the phosphorescence 

 may be limited to spots on various parts of the body 

 surface, or there may be special luminous organs on 

 the head, while in many cases there are delicate 

 tactile processes phosphorescent at the tip ; in some 

 cases the fins are themselves luminous. 



As to the details of the mechanism of phosphor- 

 escence, Prof. Panceri of Naples made some interest- 

 ing observations more than twenty years ago. He 

 studied many marine animals, especially Pyrosoma, 

 Phyllirho'e, Pholas, and others. His papers display 

 a combination of careful observation and acute 

 deduction which make them models of scientific in- 

 vestigation. By the employment of very ingenious 

 methods he demonstrated the fact that in Pyrosoma 

 the luminosity is due to two cell-clusters in each 

 ascidiozooid. As each colony contains thousands of 

 individuals, the number of luminous spots is enormous 

 even in a relatively small colony. The cell-clusters 

 lie " on each side of the anterior end of the branchial 

 sac," not far from the nerve ganglion, and consist of 

 spherical glandular cells. They contain a substance 

 soluble in ether, probably of fatty nature, which 

 apparently becomes luminous when oxidised. In 

 ordinary circumstances the light is only excited by 

 a mechanical stimulus, and then is first aroused at 

 the stimulated point and spreads more slowly 

 throughout the colony. These " luminous currents " 

 are not nearly so rapid as in some of the Coelentera, 

 Pennatula for example, and to this is probably due 

 the fact that when Moseley wrote his name on a 

 large Pyrosoma it stood out in " letters of fire," which 



