158 MODIFICATIONS OF BEHAVIOR 



Among higher invertebrates tidal rhythms have been 

 observed by Drzewina in the hermit crab Clibanarius mis- 

 anthropus. Specimens were collected at various times and 

 kept in aquaria one end of which was shaded. Nearly all 

 of the individuals manifested a positive phototaxis when the 

 tide was high, and became negative when the tide was low. 

 These regular changes persisted in the lot which was kept 

 longest, for three weeks. 



We have in these periodic variations of behavior habits 

 of action in relation to different influences of the environ- 

 ment which have been acquired by the experience of the 

 organism. These habits have probably not been acquired 

 through intelligence in most cases and certainly not in the case 

 of the diatoms, and it does not seem improbable that all of 

 them may be dependent upon some general modifications 

 of the organism as a whole rather than upon merely the 

 mechanism of response to stimuli. 



HABITS VARIOUSLY CAUSED 



Habits may be due to a variety of causes. They may 

 result from the repeated displacements of given structures. 

 They may depend upon artificially induced organic rhythms. 

 They may arise as the outcome of intelligently formed acts. 

 What closely simulates a temporary habit may be the physi- 

 ological effect of the summation of stimuli. An action 

 system put in operation one or more times by a given stimu- 

 lus may, up to a certain point, become increasingly respon- 

 sive to that stimulus. There is an increase of the tonus of 

 the parts concerned. If in righting itself a starfish comes 

 to employ a particular ray, that ray as a result of its exercise 

 may respond with increased readiness. Whether the tem- 

 porary habits formed in the starfish studied by Jennings 

 were due in part at least to an increase of tonus as the result 



