VEKTEBitATB SYSTEMS 



433 



This brain ganglion and cord constitute a central system 

 in distinction to the fibers which arise from the small 

 ganglia and ^\-hich correspond to our peripheral system. 



Arthropods. — In the lower arthropods, such as the 

 lo^^■er forms of crustaceans, and the wormlike larva; of 

 insects, the nervous system is practically 

 identical with that of the earthworm. In 

 the higher arthi-opods such as the higher 

 crustaceans and insects, the plan is the same, 

 but many of the ganglia are fused together 

 in the head and thorax making this end of 

 the system much more brainlike in structure. 

 This head mass also gives nerves ^vhich 

 supply the eyes and antennae much as our 

 cranial nerves are given off from the brain. 

 The ventral cord shows fewer ganglia, and 

 each ganglion furnishes motor and sensory 

 fibers to larger areas than in the worms. 



In short, while this system is similar in 

 composition to that of the worms the ten- 

 dency is to centralize control in the head. 



Vertebrate systems. — In the vertebrates 

 the most remarkable changes are the develop- 

 ment of the true brain as an organ of mental 

 operations, and the transfer of the whole 

 system to the dorsal side of the body where 

 it is protected in the skeletal structures. 

 The essential features of the systems found in' the 

 different forms of vertebrates are identical, the variation 

 being in the direction of complexity. Figures 203 and 

 204 illustrate the manner in which the complex brain 

 of man has been developed, and the relative stages in 



EDDY. PHYS. — 28 



3 



Fig. 202 — Ner- 

 vous system 

 of a cater- 

 pillar. 



