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 
