252 The Animal Mind 



was found by Hesse to give no further response to sudden 

 shadows when the stimulus was frequently repeated (321). 

 Hargitt (285) reports the same of tube-dwelling annelids. 

 Von Uexkiill reports that the sea-urchin Centrostephanus 

 longispinus ceased to respond to shadows after three suc- 

 cessive stimulations (736). Nagel observed that certain 

 eyeless mollusks which react to sudden darkening very 

 quickly get used to the stimulus and cease to respond; 

 often after one reaction they decline to react for several 

 hours. 1 The mollusks that responded to sudden bright- 

 ening rather than to shadows, that were in Nagel's phrase 

 photoptic rather than skioptic, took longer to become ac- 

 customed to repeated stimulation, but did so by gradually 

 weakening their reaction (520). A web-making spider 

 that was found by the Peckhams to drop from its web at 

 the sound of a large tuning fork declined to disturb itself 

 after the stimulus had been repeated from five to seven times 

 (570). Ants " become used" to the ultra-violet rays which 

 they ordinarily avoid (220). The responses of dragon fly 

 nymphs to light are less marked as the stimulus is re- 

 peated (636), and the same is true of mosquito larvae 



(338). 



Where such an effect as this is temporary, the most 

 obviously suggested cause for it is fatigue. In our own 

 experience this word is used chiefly with reference to motor 

 processes ; we perceive a certain signal, but are too fatigued 

 to respond. On the sensory side, when a repeated or con- 

 tinued stimulus is no longer perceived, we call the phenom- 



1 The opposite phenomenon is reported by Rawitz of the mollusk Pecten, 

 whose response to a shadow was the shutting of its shell. Repeated or long- 

 continued shadowing, instead of doing away with the reaction, caused the 

 animal to remain with closed shell for a long time ; an intensification of the 

 reaction which suggests the effect of summation of stimuli (628). We may 

 infer that the stimulus in such a case is injurious. 



