THE ORIGIN OF THE NERVOUS SYSTEM 



141 



if any, influence on the other side. Moreover, the fact of intimate local 

 relations between nerve and muscle, as seen in the anatomy of these 

 animals, supports the idea of neuromuscular independence instead of 

 centralized relations. This is well exemplified in the reactions of the 

 tentacles. If a tentacle of Metridium is stimulated by food, it turns 

 and twists irregularly and then points toward the mouth. If the same 

 tentacle is cut off and held filled with water so that its original relations 

 in the animal as a whole can be kept in mind, it will be found to react to 

 food as it formerly did, in that it will finally turn toward that side which 

 was originally next the mouth. 

 Hence we may conclude that 

 the tentacle has within itself 

 all that is necessary by way of 

 neuromuscular mechanism for 

 its characteristic reactions and 

 is not dependent for these on 

 such other parts of the sea-ane- 

 mone as have been regarded as 

 central nervous organs. Phys- 

 iologically as well as anatomic- 

 ally the sea-anemone seems to 

 possess a diffuse rather than a 

 centralized nervous system, and 

 its neuromuscular mechanism 

 consists of receptors and effect- 

 ors connected by a nervous net 

 which is composed partly of the 

 nervous processes of the receptor 

 ;ells and partly of similar proc- 

 esses from ganglion cells. 



The type of neuromuscular 

 mechanism found in the sea-ane- 

 mones probably also recurs in 

 the digestive tube of vertebrates. 

 This view is supported not only 

 by the action of the intestine, 



but also by its structure (Fig. fig. 3. Longitudinal Section of the in- 

 3). Omitting for the moment te stinal wall of a vertebrate, showing the 



,i , 1 , ,, nervous and muscular constituents ; ap, Auer- 



tne OUter serous layer and. the bach's plexus; cm, circular muscles; Im, longi- 



inner muCOUS layer of the intes- tudina 1 muscles ; m, mucous layer ; mp, Meiss- 



i-- in. >• . t t.,1 ner ' s plexus; 8, serous layer. 



tine, both of which have little or 



nothing directly to do with its neuromuscular mechanism, there are left 

 the outer or longitudinal muscular layer, followed internally by a 

 nervous layer, Auerbach's plexus, which in turn is followed by the cir- 



