Spatially Determined Reactions 179 



from the light, may become positive. In course of time such 

 a positive animal will become negative of its own accord, so 

 to speak, without further mechanical stimulation, but such 

 stimulation, if applied, makes it negative at once (405). 



Similar experiments upon Daphnia and Cypris gave results 

 of the same general character. The strong positive tendency 

 of the former may, by several times taking the animal up in a 

 pipette, be made very temporarily negative; the opposite 

 effect could not be well tested because of the difficulty of pre- 

 serving the negative state long enough to experiment on it. 

 In the case of Cypris, an animal temporarily negative could be 

 made positive by picking it up, but the positive phase could 

 not be similarly reversed. No other sudden stimulus pro- 

 duces the effect which is thus induced by mechanical contact 

 (449). And no possible analogy from our own experience 

 suggests itself ; the phenomenon remains a mystery. 



The effect of contact was observed by Holmes in the ter- 

 restrial amphipod Orchestia agilis. The most permanent 

 phase of these animals is positive, although they are at rest 

 under seaweed on the beach by day. But when they are 

 thrown into the water, they become strongly negative, no 

 matter what the intensity of the light ; and to a considerable 

 extent this effect is independent of the temperature (181). In 

 the case of the copepod Labidocera astiva, being picked up in 

 a pipette will make the females, ordinarily positive, negative 

 for a time. The males are normally slightly negative, but 

 picking them up, instead of reversing this tendency, increases 

 it (304). The strong positive phototropism of the "water 

 scorpion" Ranatra, an hemipterous insect, may be made nega- 

 tive by handling, and especially by dipping in water (186). 



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 



