TRANSFORMATION OF HELIOTROPIO ANIMALS 271 
tension of the muscles turning the animal to the right 
and to the left is then the same. As soon, however, as 
the rays of light SS, strike the right side, the tension of 
the muscles which turn the animal toward the light side 
either becomes (1) greater or (2) less; and this difference in 
the tension of the symmetrically situated 
muscles will in either case be greater at 
the more irritable, oral end « of the animal 
than at the lessirritable, aboralend b. In 
the former case the animal will be forced 
to assume the position ba,, and, further 
more, under the same conditions, to bring 
its median plane into the direction of the 
rays of light; it is positively heliotropic. 
In the latter case it will be forced to assume 
the position ba,; it is negatively heliotropic. FIG. 65 
As soon as the plane of symmetry coincides with the direction 
of the rays of light, symmetrically situated points on the 
body of the animal are struck at the same angle by equally 
strong rays of light, and the animal can then no longer be 
driven either to the right or to the left by the light, and 
consequently continues to move in the direction of the rays 
of light. Assoon, however, as the animal is again disturbed 
in its movements in this direction, through some other 
external or internal stimulus, symmetrically situated points of 
the animal are again stimulated unequally by the light. In 
consequence there is a corresponding change in the tension 
of the symmetrical muscles, and as a result of this the 
animal is again brought into its proper orientation. 
I wish, however, particularly to emphasize the fact that 
the progressive movement of heliotropic animals in the 
direction of the rays of light is a fact which can be directly 
observed and demonstrated, and is not a mere hypothesis. 
The question further arises whether facts are indeed at 
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Heeccecaciccedoeuncceesee G 
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