BEHAVIOR IN LOWER METAZOA 253 



tion ("variabel und anpassungsfahig," 1900, p. 98). This adaptation, 

 under unusual conditions, of the movements of the spines to the needs of 

 the organism as a whole, seems to remove all difference in principle 

 between the behavior of the sea urchin and that of higher animals. 



Many illustrations of varied physiological states could be given from 

 an analysis of the behavior of the starfish in the righting reaction, and 

 in the various experiments devised by Preyer (see p. 239). 



In the flat worm Planaria the work of Pearl (1903) shows that the be- 

 havior depends largely upon the physiological state. In this animal the 

 following different states determining behavior may be distinguished : 



1. Conditions of hunger and satiety, determining the reactions to 

 food in a regulatory way. 



2. A resting or "sleeping" condition. The animal is often found 

 lying quietly under rocks, the muscles relaxed. In this condition it 

 fails to react to weak stimuli, but strong stimulation induces the negative 

 reaction, followed by continued activity. 



3. The condition of normal, undisturbed activity. The animal now 

 responds to weak stimuli of all sorts by the positive reaction, turning 

 toward the side stimulated, while strong stimuli cause the negative 

 reaction. 



4. A condition of heightened activity, in which the worm makes 

 many "testing" movements with the head, and reacts positively to most 

 stimuli, whether strong or weak. In this condition the planarian makes 

 the appearance of actively seeking something, and of following up any 

 source of stimulation which it finds. 



5. An "excited" condition, produced by stimulating the animal 

 strongly and repeatedly. In this condition the animal moves about vio- 

 lently and reacts negatively to most stimuli to which it reacts at all. 



6. Possibly due to an accentuation of the condition last described is 

 a change of reaction observed by Pearl when one side of the head of an 

 excited specimen is stimulated by repeated blows. At first the animal 

 turns farther and farther away from the side stimulated. Then suddenly 

 it jerks strongly backward, and turns far in a direction opposite its 

 previous turning that is, toward the side stimulated. "The reaction 

 appears as if, after the animal had tried in vain to get away from an un- 

 comfortable stimulus by its ordinary reaction, it finally tries a wild jump 

 in the opposite direction" (Pearl, 1903, p. 580). 



The different physiological conditions are determined largely by the 

 history of the individual worm, so that in this sense its behavior may be 

 said to depend on its experience. The dependence of the reactions on 

 the physiological state is in a given specimen very great, so that two in- 

 dividuals often react in opposite ways to the same stimulus. The same 



