spatially Determined Reactions 203 



W. Ostwald has called attention to the possibility that 

 "internal friction" between the organism and the medium 

 may affect various tropisms. Freshly caught Daphnias 

 which are negative or indifferent, quickly become positive 

 if gelatine or quince emulsion is added to the water. Since 

 they would become so in time an)n?/ay, Ostwald thinks the 

 mechanical friction of the sticky liquid simply acts as a 

 "sensibilator" and brings on this positive phase sooner 



(532). 



Change in the purity of the water also sometimes produces 

 change of sign in the response to Hght. The amphipod 

 Jassa, negative in ordinary sea water, becomes positive in 

 foul sea water (330). The presence of chemicals is an in- 

 fluence probably identical with the one just mentioned. 

 Various Crustacea have had the direction of their reactions 

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

 chloroform, paraldehyd, and alcohol (430). Acids and 

 salts will reverse the responses of May fly larvae (794) . The 

 ultra-violet rays wiU make positive Balanus larvae tempora- 

 rily negative and have a similar effect on Daphnia (502). 



The state of hunger or satiety in an animal must be reckoned 

 with: the caterpillars of Porthesia, for example, are de- 

 cidedly positive when hungry, much less so when fed (423). 

 The slug Limax maximus, ordinarily negative to strong 

 light, is positive to light of any intensity when hungry 



(236). 



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 (614). The copepod Temora longicornis, usually 

 negative, can be made positive by shaking it (425). Very 

 curious phenomena of a similar nature have been observed 

 in the case of some Entomostraca. Certain individual 



