178 The Animal Mind 



emulsion is added to the water. Since they would become 

 so in time anyway, Ostwald thinks the mechanical friction 

 of the sticky liquid simply acts as a " sensibilator " and brings 

 on this positive phase sooner (302). 



Change in_the purity of the water, also sometimes produces 

 change of sign in the response to light. The amphipod 

 Jassa, negative in ordinary sea water, becomes positive in 

 foul sea water (181). The presence of chemicals is an influ- 

 ence probably identical with the one just mentioned. Vari- 

 ous Crustacea have had the direction of their reactions 

 changed by carbonic or other acids, ammonium salts, ether, 

 chloroform, paraldehyd, and alcohol (244). The ultra-violet 

 rays will make positive Balanus larvae temporarily negative 



(245). 



The^ statejof hunger_or satiety in an animal must be reck- 

 oned with: the caterpillars of Porthesia, for example, are 

 decidedly positive when hungry, much less so when fed (236). 

 The slug Limax maximus, ordinarily negative to strong light, 

 is positive to light of any intensity when hungry (135). 



Mechanical stimulation is most striking in its effect on light 

 reactions. Pouchet in 1872 noted that fly larvae after having 

 been shaken fail to display their usual orientation to light 

 (347). The copepod Temora longicornis y usually negative, 

 can be made positive by shaking it (239). Very curious 

 phenomena of a similar nature have been observed in the 

 case of some Entomostraca. Certain individual specimens 

 of the ostracod Cypridopsis appeared to be decidedly positive, 

 others negative. Careful experimental analysis of the condi- 

 tions 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 



