Spatially Determined Reactions 177 



positive. A sudden change in light intensity, either brighten- 

 ing or darkening, has the effect of making positive Daphnias 

 temporarily negative (302). 



Temperature changes influence response to light. The ob- 

 vious suggestion here would be that since increased tempera- 

 ture often accompanies increased intensity of light, animals 

 that are positively phototropic only up to a certain degree of 

 illumination ought to become negative when the temperature 

 is decidedly raised. This, however, is by no means always 

 the effect produced by increased temperature. Strasburger's 

 swarm spores became positive in higher temperatures, nega- 

 tive in lowered ones (392). Orchestia agilis, which we have 

 just seen becomes temporarily negative on being brought from 

 strong into weak light, may be made positive again if the water 

 is slightly warmed. When the same animal is dropped into 

 water, it becomes strongly negative, but it will show a positive 

 response if the water is heated almost to a fatal point (181). 

 On the other hand, the copepods and annelid larvae studied by 

 Loeb were made negative by increased, positive by lowered, 

 temperature. Other crustaceans, e.g. Daphnia (457), had 

 their responses to light unaffected by a fairly wide range 

 of temperature changes. 



Increasing or decreasing the-devmty ^Jhe_juiaier will also 

 affect phototropism. In some copepods diluting the water 

 produced negative responses to light, while increasing its den- 

 sity brought about those of the opposite sign (239). Diluting 

 the water produced negative phototaxis in the larvae of 

 Palaemonetes (257). Parker failed to find any such effect in 

 the case of the copepods studied by him (304). W. Ostwald 

 has called attention to the possibility that "internal friction'' 

 between the organism and the medium may affect various 

 tropisms. Freshly caught Daphnias, which are negative 

 or indifferent, quickly become positive if gelatine or quince 



