CHAPTER XII 



CHANGE IN THE SENSE OF HELIOTROPISM 



We have stated tliat while in a positively heliotropic 

 animal a one-sided illumination increases the tension of 

 the muscles which turn the animal toward the source of 

 light, in the negatively heliotropic animal the one-sided 

 illumination must result in the opposite effect, namely, in 

 a diminution of tension in the same muscles. As a conse- 

 quence, the negatively heliotropic animal can turn more 

 easily away from the light than toward the light. 



Groom and Loeb 183 noticed that the larvae of the bar- 

 nacle upon hatching go directly to the light and gather 

 at the light side of a dish, but that sooner or later their 

 positive heliotropism may give way to an equally pro- 

 nounced negative heliotropism. The stronger the light the 

 more rapidly the larvae are transformed into negatively 

 heliotropic organisms. Later the reversibility of the sense 

 of heliotropism was observed and studied in a number 

 of organisms. 291 In a summary of the subject 30 ° (p. 470) 

 the writer pointed out that this reversion was due either 

 to a modification of photochemical processes or to an 

 effect upon the nervous system. That an influence on the 

 nervous system can indeed bring about a change in the 

 sign of the reaction is very strikingly demonstrated in the 

 following observation of A. R. Moore on starfish. 525 



Ordinarily, when a starfish which is moving in an aquarium is 

 touched, it stops immediately and clings tenaciously to the surface of 

 the vessel with its tnbe feet, so that it is impossible to remove the animal 

 without injury to the tube feet. This normal response to sudden contact 

 can be completely reversed by the administration of strychnine, so that 

 when touched the animal loosens its hold on the bottom completely. 

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