126 THE ELEMENTAEY NERVOUS SYSTEM 



oral disc. They lead to the conclusion that the pedal disc 

 and its immediately adjacent parts contain all the neuro- 

 muscular mechanism that is necessary to creeping; in 

 other words, this function is in no sense dependent upon 

 assumed nervous centers in other parts. 



Thus the movements of the tentacles and of the pedal 

 discs of actinians, parts whose nervous organization is 

 that of the nerve-net, exhibit in a striking way an unusual 

 degree of autonomy, and in this respect these organs are 

 in strong contrast with such parts as the appendages 

 of arthropods or of vertebrates whose activities are de- 

 pendent upon a centralized nervous system and not a 

 nerve-net, and disappear almost completely when they are 

 severed from that part of the body in which the central 

 organ is located. Thus the autonomy conferred by a 

 nerve-net upon a given organ is one of the striking fea- 

 tures of this type of organization as compared with that 

 seen in animals with centralized systems. 



Autonomy is characteristic not only of coelenterate 

 organs in which there are nerve-nets, but also of those 

 parts of the higher animals in which there are similar 

 structural conditions. The pedicellariae, spines, and am- 

 bulacral feet of the echinoderms very probably belong 

 under this head as well as the remarkable cloacal organ 

 of holothurians whose pulsations have recently been 

 studied by Crozier (1916). Autonomous organs with 

 their nerve-nets seem to be less in evidence in worms and 

 in arthropods than in the lower animals. In mollusks the 

 foot is apparently largely autonomous and provided with 

 a nerve-net, and the labial palps of the bivalve Anodonta 

 are in this respect truly remarkable (Cobb, 1918). In 

 vertebrates nerve-nets are abundantly present in con- 

 nection with the circulatory system. The adult verte- 



