180 The Animal Mind 



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 dryness 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 mollusks, they display negative 

 phototropism and seek shelter in rock cavities ; while as soon 

 as they are again exposed to the air, their phototropism be- 

 comes 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 (275). 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 oscil- 

 lations 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 

 prolonged and intense desiccation, habitually move following 

 the direction of the luminous field in the negative sense ; the 

 Littorinas from low levels, which undergo only short and 

 slight desiccation, move, habitually, following the direction 

 of the luminous field in the positive sense." The former 

 become positively phototropic at the time of highest water, 

 the latter negatively phototropic at the time of low water. 

 In all cases, the tendency is for the animals to become nega- 

 tive at low-water time. The attraction of the dark screens 



