2o6 The Animal Mind 



longed 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 

 represents that of the dark surface of the rocks (80). 

 Similar oscillations corresponding to the periodicity of the 

 tides were observed in the annelid Hedista diver sicolor (80), 

 in the sea-anemone Actinia equina (65), and in the hermit 

 crab (192, 194). 



It is probable that such rhythmic changes in the sense 

 of light response are due to the effect of a rhythmically 

 recurring cause, such, for instance, as the mechanical dis- 

 turbance caused when the waters of the rising tide begin 

 to agitate the pool in which the animal dwells, or to the 

 wetness or dryness of the tissues. Bohn has suggested this 

 explanation for the oscillation of Hedista, just mentioned. 

 He supposes that when the annelid is dry, light has the 

 power of exciting muscular movements, that is, a kinetic 

 effect. This means that when the worms have accidentally 

 crept into the shade they come to rest. If one eye has its 

 illumination diminished, there is an inhibition of muscular 

 activity on that side, and consequently a turning in that 

 direction. At the period of high tide, when the muscles 

 are wet, the action of light on the animal is inhibitory and 

 the above phenomena are reversed (80). Heat and dry- 

 ness make terrestrial amphipod crustaceans positive to 

 light; cold and wetness make them negative (106). 



The state of rest or movement is still another factor. The 



