80 THE ELEMENTARY NERVOUS SYSTEM 



structure. The superficial portion of each is epithelial 

 in character (Fig. 19) and contains among its various 

 types of cells a number of sensory cells that terminate 

 superficially in free, bristle-like endings and that branch 

 at their deep ends into delicate fibrils. This epithelial 

 portion may be designated as the first sublayer. The 



fibrils from the deep ends of the sense 

 cells constitute collectively a nervous 

 sheet, the second sublayer, in which may 

 be found not only the deep terminations 

 of the sense cells but also special ele- 

 ments, the so-called ganglion cells, 

 whose branches add to the wealth of fine 

 fibrils from the sense cells. Still deeper 

 than the nervous layer is the muscular 

 or third sublayer composed almost en- 

 tirely of elongated muscle cells closely 

 applied to the supporting lamella or even 

 partly imbedded in it. These three sub- 

 layers can commonly be identified in 

 many parts of the ectoderm and the 

 entoderm. 



According to the Hertwigs, when the sensory cells of 

 a sea-anemone are stimulated, they transmit impulses 

 to the nervous sublayer which in turn excites the muscles 

 to action and thus the animal is brought to respond to an 

 external change. If the stimulated sensory cells are in 

 the ectoderm and the responding muscles are in the ento- 

 derm, it was suppbsed by these investigators that the 

 nervous impulses pass through the ectodermic nervous 

 sublayer over the exterior of the animal to the resophagus, 

 at whose inner end the impulses . are transferred from 

 the ectoderm to the entoderm and thus gain access to 



Fio. 19. Diagram 

 of the ectoderm of a 

 sea-anemone showing 

 the epithelial e, the 

 nervous n, and the 

 muscular m sublayers. 



