20 THE ELEMENTARY NERVOUS SYSTEM 



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real question that must be confronted concerns the inter- 

 relations within the neuromuscular mechanism rather 

 than those that are simply within the nervous system 

 itself. 



The type of neuromuscular mechanism described in 

 the preceding paragraphs in which a group of receptors 

 is connected with a well centralized ad- 

 justor that in turn controls a complex sys- 

 tem of effectors, is found only in the more 

 differentiated animals. Certainly in the 

 simple forms, like the jellyfishes, corals, 

 sea-anemones and so forth, only the slight- 

 est evidence of this type of nervous organ- 

 ization can be discovered. Nevertheless, 

 these animals possess a neuromuscular 

 mechanism, but on so simple a plan that in- 

 vestigators have. long been inclined to re- 

 gard it as representing the first step in the 

 differentiation of neuromuscular organs. 

 This plan of structure is well represented 

 in the sea-anemones. Each of the two lay- 

 ers of cells that make up the living sub- 

 stance of the sea-anemone's body consists 

 ordinarily of three sublayers ; a superficial or epithelial 

 layer, a middle or nervous layer, and a deep or muscular 

 layer (Fig. 3). The epithelial layer contains, besides 

 many other kinds of cells, large numbers of sensory cells 

 which terminate peripherally in bristle-like receptive 

 ends and centrally in fine nervous branches. These fine 

 branches constitute collectively the middle or nervous 

 layer in which occasionally large branching cells, the so- 

 called ganglionic cells, occur. Immediately under the 

 nervous layer is the deep layer of elongated muscle-cells. 



Fio. 3. E c t o- 

 dermio layer from 

 the tentacle of a 

 sea-anemone show- 

 ing the three sub- 

 layers, epithelial , 

 nervous n, and 

 muscular m. 



