88 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



worms, the different segments become almost wholly 

 independent, and indeed almost like separate animals. 



In going up the scale, on the contrary, we find oppo- 

 site changes of two kinds, viz., centralization and cephali- 

 zation. Centralization reaches its highest degree in crabs 

 and spiders, where all the ganglia except the cephalic 

 are consolidated into one in the center of the cephalo- 

 thorax, and therefore in the vicinity of the stomach and 





A/.N 



A/ 



V.:> 



J — -V.V.-.Jl*.."--— 



.--.-.:-.-yl ^_ 



<^r--^3- 



Fig. 60. — Nervous system of a crab. 



\^=v 



Fig. 6i. — Diagram of nerv- 

 ous system of a beetle. 



locomotive organs (Fig. 60). The cephalization reaches 

 its highest degree in the high insects, as bees, flies, beetles,' 

 etc. (Fig. 61). 



Embryonic History of the Nervous System. — The same 

 changes are gone through in the embryonic history of 

 one of the higher insects, such as the butterfly. In the 

 caterpillar all the ganglia are separate, one to each seg- 

 ment. In the chrysalis the cephalic ganglion increases 

 in size, and three or four of the anterior body ganglia 

 are drawing closer together. In the butterfly the cephalic 

 ganglion is still larger, and four of the ventral ganglia, 

 together with the oesophageal, have united into a great 

 thoracic ganglion to control the powerful muscles moving 



