284 The Animal Mind 



cessive movement systems is, in our own experience and 

 probably in that of an animal, the diminishing of atten- 

 tion to outside stimuli, and the disappearance of such emo- 

 tional states as uncertainty and the unpleasantness of 

 errors ; a feeling of confidence and security replacing them. 

 There is an alleged case of learning on the part of certain 

 marine animals which, if it exists, probably belongs under 

 the head of the formation of successive systems. This is 

 the acquisition of rhythmic reactions, to stimuli which 

 occur at equal time intervals, and the persistence of such 

 rhythms when the stimuli have ceased to act. Marine 

 snails, sea-anemones, annelid worms, and hermit crabs 

 show changes in the direction of their responses to light 

 and to gravity which correspond with the state of the tides : 

 the sea-anemone, for instance, opens at high tide and shuts 

 at low tide. Now certain French observers, Bohn (80, 90), 

 Pieron (584), and Drzewina (192) report that when the 

 animals are removed to the aquarium, they continue to 

 show fluctuations in their light and gravity reactions at 

 the times of high and low tide, although of course the actual 

 stimuli which the tide gives them, for instance the mechani- 

 cal jarring of the waves entering their pool as the tide rises, 

 are now wholly lacking. No American observer has been 

 able to show such a continuance of the tidal rhythm in 

 animals removed from direct tidal action (256, 293, 509 a, 

 551). The phenomenon suggests analogies from our own 

 experience; for instance, there is the case of "habit hun- 

 ger." We feel hungry at the time of day when we are ac- 

 customed to be fed ; if we do not get food at this time, in 

 half an hour or so the hunger sensations disappear, and 

 we can go quite comfortably without food for some time 

 longer. The hunger sensations are due to movements of 

 the stomach; now these movements were originally in- 



