TRANSFORMATION OF HELIOTROPIO ANIMALS 289 



2. There are photokinetic animals which react more 

 rapidly to changes in the intensity of the light than do 

 Planarians. I noticed this form of reaction at Naples in 

 certain Annelids living in tubes; for example, Serpula 

 uncinata. The gills of the animals are often exposed to the 

 light. When the hand is moved between the animals and 

 the source of light, they quickly draw back into their tubes 

 as soon as they are struck by the shadow. In order to see 

 whether positive and negative changes in the intensity of 

 the light had the same effect, I made the following experi- 

 ment: A glass aquarium which was closed by a glass cover 

 was set upon an isolated table about 2 m. distant from the 

 window. When I closed the shutters rapidly, the worms 

 quickly withdrew into their tubes, much as does a snail when 

 touched suddenly. The shutters did not close absolutely, 

 and it was always light enough in the room to observe the 

 animals. After some time the animals would again stretch 

 out their gills. When I now suddenly opened the shutters 

 quickly, the animals did not react. Even when the animals 

 had withdrawn into their tubes, an increase in the intensity 

 of the light did not again bring them out. It is therefore 

 only the decrease in the intensity of the light which acts as 

 a stimulus upon the animals. One notices, however, that 

 these reactions cannot always be relied upon. Andrews has 

 noticed such reactions also in Annelids whose gills are free 

 from eyes or eye-like organs. 1 



VI. ON SOME PHYSIOLOGICAL CONDITIONS WHICH DETER- 

 MINE THE DEPTH-DISTRIBUTION AND DEPTH-MIGRATION 

 OF MARINE ANIMALS 



1. Investigations concerning the depth-distribution of 

 marine animals seem to show that we meet with a consider- 

 able amount of animal life only in two regions of the sea 



IE. A. ANDREWS, Journal of Morphology, Vol. V (1891). 



