THE INFLUENCE OF LIGHT ON ORGANS 433 



We must therefore conclude that not all the rays of diffuse 

 daylight influence the formation of polyps equally, but that 

 only the more refrangible (blue) rays of the visible spectrum 

 favor the development of polyps, while the less refrangible 

 (red) rays act as darkness a fact similar to that established 

 for heliotropism. 



3. We may finally raise the question whether or not light 

 influences the development of Planula larvae. My studies in 

 this direction are not yet completed, but I found that in 

 isolated cases, in spite of an exposure to darkness for several 

 weeks, during which time no new polyps were formed, the 

 development of the larvae progressed as under normal condi- 

 tions; at least they were normal and not retarded in their 

 development. I was able to make some experiments with 

 the larvse themselves. These larvae are pear-shaped, and are 

 able to move forward very slowly by means of cilia. They 

 are, as I have already shown, energetically positively helio- 

 tropic. The blue rays are in these experiments more effective 

 than the red. About forty-eight hours after they begin to 

 swim, the larvae attach themselves to some solid base, and 

 within the next twelve hours their pointed ends begin to 

 grow and to form polyps, while their blunt ends form roots. 

 When lighted from one side only the whole organism bends 

 toward the light. The question therefore arose whether the 

 Planula larva can develop a polyp in the dark. This is the 

 case when a larva has developed in the presence of light. 

 When brought into the dark such a larva develops a polyp 

 within twelve to twenty-four hours. In this, however, we need 

 not see any contradiction to the other experiments, as in 

 these the development of the polyps required three to four 

 days. It would be interesting to determine whether Planula 

 larvae, if their whole development occurs in the dark, can 

 develop polyps in the absence of light. 



In conclusion I wish to mention that the polyp-bearing 



