XXVII. THE NERVOUS SYSTEM IN THE 

 LOWER ANIMALS.' 



Nervous tissue, wherever it exists, is in the form of 

 nerve cells and fibers, as in man. Frequenth'' the nerve 

 cells are collected in masses which are called ganglia, and 



Fig. 198 — A nerve ganglion. The round bodies are nucleated nerve cells; the 

 fibers are nerves. 



in this chapter the use of the word ganglion may be taken 

 to mean a collection of nerve cells. The fibers are always 

 developed as processes of the nerve cells. 



Protozoans and coelenterates. — The one-celled animals can 

 evidently have no true nervous system. Their protoplasm, 

 however, is sensitive to stimuli and conducts impulses. 

 Such a condition is interesting mainly as showing the rela- 

 tion of ordinary protoplasm to the specialized protoplasm 

 of the nerve cell and as indicating the origin of the ner- 

 vous system. The first appearance of nerve cells and 



' See Footnote, p. 146, Chapter X. 

 429 



