152 BIOLOGY: GENERAL AND MEDICAL 



and often fused to make one mass. This chief brain is, 

 however, by no means indispensable to the animal, for 

 it not infrequently suffers the loss of some of the 

 anterior segments with the brain (as when birds seize 

 hold of earth-worms and break them off), but is sub- 

 sequently able to regenerate the lost segments, including 

 the brain. 



This arrangement, a double chain of intercommuni- 

 cating nervous ganglia corresponding in number to the 

 segments of the body and increasing size and importance 

 of the anterior ganglia by which the brain is formed, 

 constitutes the foundation of the central nervous system 

 throughout the remainder of the zoological scale. 



But in transferring our attention from the surface of 



FIG. 56. Diagram to express the fundamental structure of an arthropod, a , 

 antenna; al, alimentary canal; 6, brain; d, dorsal vessel; ex, exoskeleton; I, 

 limb; n, nerve chain; s, subcesophageal ganglion. (After Schmeil.) 



the body, where the nervous tissue first makes its appear- 

 ance in the lowly forms of life, to the skull and spinal 

 canal where it concentrates in the highest forms, the 

 vertebrates, it must not be forgotten that while the 

 improvement in the central nervous system has been 

 in progress, there has been a no less remarkable improve- 

 ment in the peripheral nervous system among whose 

 specializations must be embraced all the organs of the 

 special senses as well as the various nerve endings in 

 muscles and glands. 



When we come to consider this fact, it appears as 

 though the development of the peripheral nervous system 

 and the improvement of the organs of special sense con- 

 tribute largely to the elaborate and complex develop- 



