I 
THE COLOURS OF ORGANISMS 
i9 
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 , 
Phyllirhoe, 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 
