134 A. J. Goldfarh 



days, until about the 13th day, after which no more hydranths 

 are produced. 



When these colonies were removed to a dark chamber, dark 

 enough not to effect photographic plates, the number of hydranths 

 produced in this chamber, and the rate of their development, 

 was not materially altered. Regeneration took place in the dark 

 as well, or almost as well, as in the light. But this statement 

 holds true only during the first cycle^. For, after this period a pro- 

 found change occurred, as a result of which these hydroids did 

 not regenerate a single hydranth so long as they were kept in the 

 dark. This treatment rendered them remarkably responsive to 

 light, so that exposure always induced the regeneration of hy- 

 dranths. But the surprising feature was the intense sensitive- 

 ness which hydroids so treated seemed to possess. Exposure to 

 shaded light of 15 seconds sufficed to stimulate the regeneration 

 of a whole series of hydranths. 



No explanation of this peculiar relation to light was given. It 

 cannot be said that E. ramosum is dependent upon light by virtue 

 of substances produced by symbiotically associated organisms. 

 Nor is it dependent directly upon light for its sustenance in the 

 manner of plants. 



The following experiments were undertaken with the idea of 

 finding what influence light has upon the regeneration of the other 

 two closely associated hydroids, to ascertain whether this extreme 

 sensitiveness to light is common to the other forms, and possibly 

 to determine what significance this fact has in the life history of 

 these plant-like animals^. 



The procedure was practically the same in all the experiments. 

 The hydranths were removed from the selected stalks and the 

 same number were placed in wide shallow dishes containing the 

 same quantity of sea-water. To prevent the rapid accumulation 

 of bacteria, the water was changed at first daily, and in the later 

 experiments, on alternate days. The controls were kept in the 

 shaded light of the laboratory, the others were prepared, in the 



•* Vide Journ. Exp. Zool. Vol. 3, 1905. 



* The experiments were made at the Marine Biological Laboratory, Woods Hole, Mass. For the priv- 

 ilege of working therein I am indebted to the Director, Prof. F. R. Lillie. 



