348 W. von BUDDENBROCK 



the movement in a straight line toward or away from the light 

 involves a symmetrical muscular movement of both symmetrical 

 body halves. Now the side wise movement of the crab, although 

 it is a movement in a straight line away from the light, is caused 

 by an asymmetrical cooperation of both sides, since, for example, 

 in moving to the right, the legs of the left side push the body 

 so that the extensors are in activity, while the legs of the right 

 side move with the flexors doing the work. This is not explic- 

 able by the theory and can only be understood as a purposeful 

 and elaborate corporate action of the animal. The disagree- 

 ment with the theory could scarcely be greater. The assump- 

 tion is also untenable that the action of energy upon a sym- 

 metrical form is here concealed by some hypothetical second 

 factor. For since the light, if the action takes place according 

 to Loeb's scheme, must cause a turning of the crab, then this 

 second factor, which inhibits the turning, must, when left to 

 itself, cause a turning toward the opposite side. This would 

 mean then that the crab, when not stimulated by light, would 

 move continually in a circle, an evident absurdity. 



A second example of a tropism which occurs in a manner 

 that contradicts the theory, in spite of the presence of all the 

 premised conditions, is a peculiar phenomenon which may be 

 observed in the starfish, and in a crustacean, and which I may 

 call changeable heliotropism. 



If a starfish is brought into a uniformly lighted field in which 

 is a spot of light of different intensity, such as a deep shadow 

 or a brighter light, in both cases the animal creeps toward this 

 spot. 8 



Among the Crustacea I have observed an analogous phenom- 

 enon in Hemimysis lamornae. This animal swims constantly 

 back and forth in the aquarium, as far as the space permits, 

 but always in a perfectly definite direction, forward toward the 

 light coming from the window, backward away from it. This 

 crustacean therefore changes the character of its heliotropism 

 with each turn that it makes at the glass wall of the aquarium. 



That one and the same animal should show both positive and 



negative heliotropism is in itself nothing remarkable and in nowise 



contradicts the theory, it is moreover of frequent occurrence. 



8 Plessner, H. Untersuchungen iiber die Phvsiologie der Seesterne. Zool. 

 Jahrb., Abt. f. allg. Zool, 33, 1913. 



