BEHAVIOR IN LOWER METAZOA 235 



it from the body. Small animals coming in contact with the sea urchin 

 are seized by the pedicellariae and held, till they are grasped by the slow- 

 moving tube feet and spines, and by them carried to the mouth and eaten. 

 When the sea urchin is attacked by an enemy, the spines all bend 

 toward the region of attack, presenting a serried array of sharp points to 

 the advancing enemy. In some species this occurs even when a shadow 

 falls upon the animal. The spines present their points to the shaded 

 side, thus arranging for an effective defence in case the animal which 

 has cast the shadow shall advance to an attack. In some sea urchins, 

 poisonous pedicellariae seize an enemy, usually causing a quick retreat. 

 Further, when the animal is severely stimulated from one side, the 

 reflexes of the spines are so arranged as to carry the animal in the op- 

 posite direction. When attacked, the animal is thus effectively defended, 

 while at the same time it flees. 



V. Uexkiill emphasizes the independence of these organs, the defi- 

 nite character of their reflexes, and the definiteness of the interconnections 

 between them. These qualities give the characteristic stamp to the 

 behavior of the sea urchin. According to v. Uexkiill, this animal is 

 a "republic of reflexes." Every reflex is of the same rank, and is in- 

 dependent of the others, save for the definite connections that we have 

 mentioned. There is nothing like a central unity controlling the re- 

 flexes, according to v. Uexkiill. The sea urchin, he holds, is a bundle 

 of independent organs, and it is only through the arrangement of these 

 organs that a seemingly unified action is produced. "It is only by the 

 synchronous course of the different reflexes that there is simulated a 

 unified action, which really does not exist. It is not that the action is 

 unified, but the movements are ordered, i.e. the course of the different 

 reflexes is not the result of a common impulse, but the separate reflex 

 arcs are so constituted and so put together that the simultaneous but 

 independent course of the reflexes in response to an outer stimulus 

 produces a definite general action, just as in animals in which a common 

 centre produces the action" (1899, p. 390). The difference between 

 the behavior of the sea urchin and that of higher animals is concretely 

 expressed by v. Uexkiill in the statement that when a dog runs the 

 animal moves its legs ; when the sea urchin runs the legs (spines) move 

 the animal. 



Yet the fixity of these reactions is by no means absolute, even in the 

 sea urchin. As we shall see in the next section, v. Uexkiill discovered 

 a number of definite laws in accordance with which they change, and 

 there is positive evidence of still other modifying factors not easily 

 formulated. 



In scarcely any other group of lower animals does there appear to 



