204 The Animal Mind 



specimens of the ostracod Cypridopsis appeared to be 

 decidedly positive, others negative. Careful experimental 

 analysis of the conditions revealed the following as the true 

 state of affairs. The animals are predominantly negative. 

 But contact with a mechanical stimulus has the effect of 

 making them positive; thus a negative animal that is 

 picked up in a pipette, or merely comes in contact with the 

 end of the trough in swimming away 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 (718). 



Similar experiments upon Daphnia and Cypris gave results 

 of the same general character. The strong positive ten- 

 dency 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 diffi- 

 culty of preserving the negative state long enough to experi- 

 ment on it. In the case of Cypris, an individual temporarily 

 negative could be made positive by picking it up, but the 

 positive phase could not be similarly reversed. No other 

 sudden stimulus produces the effect which is thus induced 

 by mechanical contact (800). 



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 (330, 

 106). In the case of the copepod Labidocera czstiva, being 

 picked up in a pipette will make the females, ordinarily 

 positive, negative for a time. The males are normal!}/ 



