234 BEHAVIOR OF THE LOWER ORGANISMS 



reaction forms is that of the sea urchin, as studied by v. Uexkiill (1897, 

 1897a, 1899, 1900, 1900a). The sea urchin differs from most lower 

 animals in bearing large numbers of motor organs 

 scattered over its entire surface. Most prominent of 

 these are the spines, which are movable, and may be 

 used as legs, or as means of defence. Among the 

 spines are certain peculiar jawlike organs known as 

 pedicellariae (Fig. 133), each borne on a movable stalk. 

 These jaws frequently open and close, seizing foreign 

 objects. The surface of the body between the spines 

 and pedicellariae is covered with cilia. Finally, the 

 body bears five double rows of tube feet, — - fleshy 

 tubular suckers, protruded through rows of holes in 

 the shell. These are important organs of prehension 

 Fig. 133. — One ^ locomotion. All these different sets of organs 



of the pedicellariae _ ° 



from a sea urchin, are interconnected by a network of nerves, one set 

 After v. Uexkiill. iyi n g on the outer surface of the shell, another on the 

 inner surface. These nerves connect with the five radial nerve trunks, 

 which unite to form a ring surrounding the mouth. 



V. Uexkiill finds that each of these organs (omitting the cilia) has 

 a number of definite reactions or reflexes, which it performs in response 

 to definite stimuli. In these reactions each organ may act as an inde- 

 pendent individual. If a piece of the shell bearing but a single spine or 

 pedicellaria is removed, this organ reacts to external stimuli in essen- 

 tially the same way as when connected with the entire animal. These 

 reflexes change with different intensities and qualities of stimuli, and 

 with certain other conditions, and they are different in diverse sorts of 

 pedicellariae. But each reflex has a very definite character. Thus the 

 sea urchin appears to be made up of a colony of almost independent 

 structures. Each of these structures has reactions of such a character 

 that they perform certain functions that are useful in the life economy 

 of the animal. 



Yet these organs are not entirely independent. They are connected 

 by the nervous network in certain definite ways, so that when one of 

 them performs a certain action, others may receive a transmitted stimu- 

 lus, and may perform the same or a differing action. That is, each 

 organ may receive stimuli not only from the outer world, but also, 

 through the nerves, from other parts of the body. These interconnec- 

 tions are of such a character that they cause the various organs to work 

 in harmony, usually assisting to perform certain necessary functions. 



Thus, if debris falls upon the sea urchin, the pedicellariae seize it, 

 break it into bits, and with the aid of the spines and the cilia remove 



