CHAPTER XVIII 



REACTIONS OF MULTICELLULAR ANIMALS IN LIGHT 

 CONSISTING OF WAVES DIFFERING IN LENGTH 



I. Experiments of Wilson on Hydra 



Among the most thorough and rehable experiments on 

 the reactions to Hght of different wave lengths are those 

 of Wilson (1891) on Hydra. Wilson had "a fraternity of 

 Hydras five hundred to a thousand strong all of which 

 had arisen in [a large] aquarium [in a north room] from a 

 group of three or four progenitors" (Footnote, p. 415). 

 The window side of the aquarium was divided into equal 

 areas which were covered with s.trips of red, yellow, green, 

 blue or colorless glass. In some instances there were two 

 areas of each color, one with a single thickness of glass, 

 the other with two, producing two fields of the same color 

 but o*r different intensity. 



The activities of the animals and the changes in posi- 

 tion were studied and recorded for a week, during which 

 the glass strips were frequently rearranged. It was 

 found that the Hydras tend to collect in the colorless 

 region and in the more intensely illuminated regions of 

 each color, i.e., back of the areas covered with only one 

 thickness of glass, but that with reference to the different 

 colors they tend to aggregate in the blue even if the inten- 

 sity in it is much lower than that in any other region. 

 The tabulated results of two series of observations will 

 serve to emphasize this. These results were obtained 

 after rearranging the plates so as to change the color of 

 the different regions. 



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