ii] HABITS OF CONVOLUTA 41 



scribe the response as tropistic. If the plant or 

 animal is not fixed but free, it responds by moving 

 in a definite direction and the response is described 

 as tactic. Inasmuch as the end of either reaction 

 is the achievement of a definite orientation and inas- 

 much also as the fixed plant, not only curves till it 

 assumes a definite position, but also, having done so, 

 moves by growth in the direction to which its curva- 

 ture has brought it, we may use the term tropistic to 

 describe the reactions of both fixed and free organisms 

 to directive stimuli. 



We have now to consider the tropisms of our 

 plant-animals. 



Brought into the laboratory and placed in sea- 

 water in a glass vessel near the window, C. ros- 

 coffensis behaves precisely like the leaf of the 

 geranium in the cottage window. Each animal turns 

 to the light, moves toward it and finally exposes 

 the surface of its body athwart the line of light. 

 Within a minute or two the reaction is completed. 

 Swiftly and, as it would seem, inevitably the animals 

 assemble on the side of the vessel toward the 

 light, and form a green scum on the surface of 

 the water. If the vessel is turned round, the 

 animals release their holds and, either falling like 

 a precipitate to the bottom or edging round the 

 side of the vessel, arrive once again at the water's 

 edge on the side of the vessel directed toward the 



