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 moUusks which react to sudden darkening very 

 quickly get used to the stimulus and cease to respond; 

 often after one reaction they decUne to react for several 

 hours.^ The mollusks that responded to sudden bright- 

 lening 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 soimd of a large tuning fork decHned 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 

 n)Tnphs to light are less marked as the stimulus is re- 

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



(338). 

 I Where such an effect as this is temporary, the most 

 I obviously suggested cause for it is fatigue. In our own 

 I 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- 

 \J;inued stimulus is no longer perceived, we call the phenom- 



^ 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. 



