252 BEHAVIOR OF THE LOWER ORGANISMS 



progress of metabolism. Hungry animals react positively to possible 

 food, while satiated ones react negatively to the same stimuli. This 

 most significant relation is, of course, almost universal in organisms; 

 it shows directly the dependence of behavior on the relation of external 

 agents to internal processes. 



V. Uexkiill has made precise studies of certain physiological states 

 and of the factors on which they depend, in the sea urchin and a number 

 of other lower animals. In the sea urchin, some of the pedicellarice 

 will not close in response to a mechanical stimulus, save in case this has 

 been preceded by a chemical stimulus. The latter changes the physio- 

 logical state of the protoplasm (muscle or nerve), so that it now reacts 

 to a stimulus which before would have had no effect. The spines of the 

 sea urchin usually bend toward a spot on the surface of the body that is 

 mechanically stimulated, as by a needle. But if this stimulus has been 

 preceded by the action of a chemical, the spines now reverse the reaction 

 and bend away from the region stimulated. Many such changes in 

 physiological state are brought by v. Uexkiill under the heading of 

 changes in tonus of the muscles or nerves. Steady tension, such as is 

 produced in certain muscles by pressing a spine of the sea urchin to one 

 side, decreases the tonus, so that the muscles are no longer so tense as 

 before. Such muscles react more readily to stimuli than do those of 

 higher tonus. Sudden jarring produces the opposite effect, the muscles 

 pull harder and react less readily than before. Decrease of tonus caused 

 by tension is transmitted in some way to neighboring spines, so that 

 after a certain spine has been pressed to one side, all those about it bend 

 in the same direction and react more readily than before. These changes 

 in physiological state play a large part in determining the behavior of 

 the sea urchin under natural conditions. 



Besides such changes, there are in the sea urchin others that are less 

 easy to formulate, and that have not been analyzed. V. Uexkiill found 

 that the set reflexes of the spines and the changes in tonus mentioned 

 above impose on the sea urchin a behavior that under most conditions 

 seems stereotyped and predictable. This leads the author named to 

 contrast the sea urchin as a "repubhc of reflexes" with higher animals 

 in which the behavior is unified. But the difference is only one of de- 

 gree. If the sea urchin is placed on its back, the usual reflexes and their 

 stereotyped interrelations would not restore the animal to the natural 

 position, but merely cause it to walk forward while lying on its back. 

 As a result, we find a physiological state induced that causes a thorough- 

 going change in the behavior of the spines. They now move in such a 

 way as to turn the sea urchin again on its ventral surface. As v. Uex- 

 kiill says, the behavior of the spines is variable and capable of adapta- 



