THE SIGNIFICANCE OF TROPISMS 119 



the larvae should be placed only in a very shallow depth of sea-water. 



Even in a strong light which is not so rich in ultra-violet rays as the 

 light of the mercury lamp, it is sometimes possible to cause positively 

 heliotropic animals to become negatively heliotropic. This is the case, 

 for instance, with the larvae of Polygordius. But it would be wrong in 

 this case to speak of an adaptation of the animal to a certain light 

 intensity. In my opinion it is merely a case where a metabolic product 

 either alters the photochemical action or so influences the central 

 nervous system that even the excitation of the retina by the light 

 weakens the tonus of the muscles, instead of strengthening it. 



Some of the other mistakes have perhaps also arisen because the 

 writers worked with complicated experimental conditions instead of 

 with simple ones, for instance, because they used a hollow prism filled 

 with ink in order to produce a gradual decrease in the light intensity. 

 In the semi-darkness thus produced, the intensity of light often remains 

 beneath or near the threshold of stimulation, and the writers fall 

 victims to that class of errors which we have already pointed out in 

 speaking of the influence of lesser intensities of light. 



VIII 



Heliotropic phenomena are determined by the relative rates of 

 chemical reactions occurring simultaneously in symmetrical surface ele- 

 ments of an animal. There is a second class of phenomena which is 

 determined by a sudden change in the rate of chemical reactions in the 

 same surface elements. Reactions to sudden change of light intensity 

 are shown most clearly in marine tube worms, whose gills are exposed to 

 light. If the light intensity in the aquarium is suddenly diminished the 

 worms withdraw quickly into their tubes. A sudden increase of light 

 intensity has no such effect. With other forms, for instance, with 

 planarians, a sudden decrease of the intensity of the light causes a 

 decrease in movement. Such animals gather chiefly in parts of the 

 space where the light intensity is relatively small. I have designated 

 such reactions as the expression of sensitiveness to change in intensity 

 of a stimulus (Unterschiedsempfindlichkeit), in order to distinguish 

 them from tropisms. 17 



It is hardly necessary to point out here that the effects of rapid 

 changes in intensity, when they are very marked, can easily complicate 

 and entirely obscure the heliotropic phenomena. In Hypotricha and 

 other infusoria this sense of difference is very pronounced in response to 

 sudden touch or sudden alteration of the chemical medium, and like 



"Loeb, "Uber die Umwandlung positiv heliotropischer Tiere usw," 

 Tflilgers Archiv, 1893. See also the recent investigations of Georg Bohn, "La 

 naissanee de 1 'intelligence, " Paris, 1909; "Les essais et les erreurs chez les 

 etioles de mer," Bull. Inst. gen. psychol., 1907; "Intervention des reactions 

 oscillatoires dans les tropismes," Ass. franc, d. Sciences, 1907. 



