ZOOLOGY. 



ties in (5) some form of related cells: muscular, glandular, 

 or nervous. It is readily apparent how increase of volume and 

 differentiation of the other parts will make necessary a more 

 complicated nervous system. The special arrangement of these 

 parts of a complete system differs very much in various animal 

 groups, yet it may be said that there is a progressive accumula- 

 tion of the central nervous matter at the anterior end of the 

 body as we ascend the scale of animal life. When this con- 

 centration is well advanced the mass of nervous matter is 

 called the central nervous system which always includes the 

 brain. The nerves passing to and from the central part and 

 their endings, taken collectively, are described as the peripheral 

 nervous system. 



104. Arrangement of the Central Nervous System. The 

 ganglion cells composing the nervous system may be so scat- 

 tered through the superficial layers as scarcely to deserve the 

 name " central " (Hydra). The nerve cells may be arranged 



FIG. 35. 



FIG. 35. Diagram showing arrangement of the nervous matter in Starfish, c, gangli- 



onated ring about the mouth; o, mouth; r.n., radial nerve in each arm. 



FIG. 36. The nervous system of the Clam, from the dorsal aspect, a, anterior; 



o, mouth; e.g., cerebral ganglia (brain); p.g, pedal ganglia; v.g., visceral ganglia. 



Questions on Fig. 35. Describe in your own terms the way in 

 which the principal nerve elements are arranged in the starfish. Compare 

 it with those which follow. In what respects similar? In what unlike 

 them? 



