Harper, Reactions of Perichata. 585 



a shrinking movement. The tropic response in the earthworm differs 

 in this respect therefore from that usually found, e. g., in arthro- 

 pods. Indeed, the earthworm is a serviceable form for study in just 

 this point that it shows us the tropic response dissociated usually 

 from shrinking movements which are due to the change produced 

 by the stimulus, while the tropic response itself appears due 

 to continuing tonic effects after the shock of the stimulus has sub- 

 sided. 



Annelids illustrate some general aspects of animal behavior in 

 about the same degree that they afford a recognized type of gener- 

 alized stnicture among segmented animals. It is an advantage that 

 the movements of the earthw^orm may be described directly in terms 

 of muscular activities rather than of the movements of appendages. 

 Only the movements of the minute muscles which control the setae 

 are of a kind which must be studied in a less direct way through 

 their effects. There may be an advantage in studying the muscular 

 basis of movements more directly. If, for example, it is desired to 

 distinguish the shock effects from tonic effects of stimuli upon an 

 animal, such basic differences may be lost sight of if we study only 

 movements of body and appendages rather than their muscular 

 basis. The earthworm with its shock and tonic effects of stimuli 

 usually dissociated and occurring as successive phases of the same 

 reaction, and its occasional exhibition of these effects fused, is a 

 valuable introduction to the study of more specialized behavior in 

 groups like the arthropods. 



Summary. 



1. In the earthworm, since the reactions may be described directly 

 in terms of muscular activities rather than of movements of body 

 and appendages, the shock and tonic or tropic effects of stimuli may 

 be recognized, usually as successive phases of a reaction, sometimes 

 fused. 



2. The tropic reaction appears alone only as the response to a 

 very weak stimulus, since the threshold for tonic effects is lower 

 than for shock (unterschiedsempfindlich) effects. 



3. In all higher forms of responses both shock and tropic effects 



