NEUROMUSCULAR MECHANISM IN METRIDIUM 455 



3. These four types of organization may be regarded as repre- 

 senting developmental steps in the evolution of the neuro- 

 muscular mechanism of the higher animals. 



4. The nervous system is not limited to the ectoderm and 

 to the entoderm but in certain regions it penetrates the sup- 

 porting lamella and thus connects one layer of the body with the 

 other; the supporting lamella thus comes to contain nervous 

 elements. 



