Spatially Determined Reactions 181 



represents that of the dark surface of the rocks (55). Similar 

 oscillations corresponding to the periodicity of the tides were 

 observed in the annelid Hedista diver sicolor (55) and in the 

 sea-anemone Actinia equina (65). 



A further influence upon light reactions which is doubtless 

 involved in the formation of the rhythms just described, has 

 been emphasized by Bohn; namely, the " hydratation " or 

 desiccation the wetness or dryness of the tissues. The 

 oscillations of Hedista just mentioned may be explained by 

 supposing that when the annelid is dry, light has the power 

 of exciting muscular movements. This means that when the 

 worms have accidentally crept into the shade, they stop, giv- 

 ing the effect of negative photopathy. If one eye has its 

 illumination diminished, there is an inhibition of muscular 

 activity on that side, and consequently a turning in that direc- 

 tion. 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. When the Littorinas observed by 

 Bohn are decidedly moist or decidedly dry, black and white 

 screens exert an influence that is proportional to their area; 

 the attractions and repulsions seem irresistible, "the mollusk 

 in the neighborhood of shelter or food continues on its way 

 toward the screen as if drawn by a fatal force, as if it saw and 

 felt nothing." But when the tissues of its body are in an 

 intermediate stage between "hydratation" and desiccation, 

 large screens have no effect upon it ; it reacts to small objects 

 in its neighborhood. "The animal seems, as it were, to dis- 

 engage itself from the influence of external forces, seems no 

 longer to behave like a pure machine : it goes to the stones 

 and seaweed where it may find shelter and nourishment, as 

 if it saw and was conscious of them" (55). 



The state_ofjrest or movement^ still another factor. The 

 "mourning cloak" butterfly, ^Vanessa antiopa, on coming to 



