. spatially Determined Reactions 205 



slightly negative, but picking them up, instead of reversing 

 this tendency, increases it (534). The strong positive 

 photo tropism of the "water scorpion" Ranatra, an hem- 

 ipterous insect, may be made negative by handling, and 

 especially by dipping in water (335). 



Periodical changes in the sense of response to light have 

 been observed in animals subjected to periodical changes in 

 environment. The gasteropod mollusk Littorina lives on 

 the rocks of the seacoast in regions where it is covered 

 with water at high tide and exposed to the air at low tide. 

 According to the height at which they are found, some of 

 these animals undergo the alternations of wetness and dry- 

 ness at the ordinary tidal periods, twice a day, while others 

 are reached by the water only at the special high tides 

 occurring every fourteen days. Mitsukuri showed that 

 when the waves of a rising tide cover these moUusks, they 

 display negative phototropism and seek shelter in rock 

 cavities ; while as soon as they are again exposed to the air, 

 their phototropism becomes positive and they emerge in 

 search of food. Further, he found that a Littorina whose 

 phototaxis was negative could be made positive by being 

 subjected to the action of a stream of water for a time (496). 

 Bohn later studied the effects of placing black or white 

 screens near the animals at various angles to their crawling 

 movements, and found that the black screens exerted an 

 attractive influence at certain times, the white screens at 

 others. These changes in the "sense" of the phototropism 

 correspond in time to the oscillations of the tide, even 

 though the animals are studied in the laboratory; they 

 tend gradually to grow less pronounced, however, under 

 such circumstances. Further, the level from which the 

 Littorinas are taken influences the nature of their response 

 to light. Those from high levels, "which undergo pro- 



