the larvæ 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 larvæ 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.
Heliotropic phenomena are determined by the relative rates of chemical reactions occurring simultaneously in symmetrical surface elements 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.
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," Pflügers Archiv, 1893. See also the recent investigations of Georg Bohn, "La naissanee de l'intelligence," Paris, 1909; "Les essais et les erreurs chez les étioles de mer," Bull. Inst. gén. psychol., 1907; "Intervention des réactions oscillatoires dans les tropismes," Ass. franc, d. Sciences, 1907.