208 The Animal Mind 



One form of such modification is found where the stimulus 

 is of moderate intensity and not harmful to the animal. 

 The Ciliata Vorticella and Stentor, which spend a part of their 

 time attached to solids by a contractile stem, contract at the 

 first application of a moderately intense mechanical stimulus, 

 but fail to react at all when the stimulus is several times 

 repeated (203). Hydra responds to mechanical stimulation 

 by contraction, but gets used to the process when repeated 

 and gives no further reaction (418). The sea-anemone 

 Aiptasia reacts by a sharp contraction to a drop of water 

 falling on it ; later it ceases its response to this stimulus. If 

 exposed to light, it contracts and remains in this state for some 

 hours, but afterwards expands again (207). The annelid 

 Bispira voluticornis was found by Hesse to give no further 

 response to sudden shadows when the stimulus was frequently 

 repeated (173). Von Uexkiill reports that the sea urchin 

 Centrostephanus longispinus ceased to respond to shadows 

 after three successive stimulations (410). 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 brightening rather 

 than to shadows, that were in NageFs phrase photoptic rather 

 than skioptic, took longer to become accustomed to repeated 

 stimulation, but did so by gradually weakening their reaction 

 (290). A web-making spider that was found by the Peck- 

 hams to drop from its web at the sound of a large tuning 

 fork declined to disturb itself after the stimulus had been 



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 (360). We may 

 infer that the stimulus in such a case is injurious. 



