THE HIGHER ORGANISMS 155 



The specialized nerve cells become more definitely 

 localized as we ascend among the lower types of animals, 

 and early concentrates in a more or less centralized 

 situation. 



In the unsegmented worms such an aggregation of 

 nerve cells at the anterior end seems to mark the begin- 

 ning of the appearance of a "brain." From it two lateral 

 nerve trunks extend to the tail, becoming smaller as they 

 recede from the "brain," apparently through the loss of 

 fibers that are given off to the muscles in due course. 



In the segmented worms, in addition to the "brain," 

 there is a pair of nervous ganglia for each segment, con- 

 necting with the "brain," with one another, and with those 

 of adjoining segments by delicate bundles of fibres. In 



FIG. 57. Diagram to express the fundamental structure of an arthropod, a, 

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

 limb; n, nerve chain; s, suboesophageal ganglion. (After Schmeil.} 



such creatures each segment may be said to possess its 

 own "brains," though the anterior pair of ganglia often 

 pass into one mass and constitute the "chief brain." The 

 latter is, however, by no means indispensable and can be 

 regenerated should it be lost, which it probably frequently 

 is in earthworms, whose heads are pulled off by birds. 



This general arrangement of a double chain of com- 

 municating and intercommunicating ganglia, running from 

 end to end of the body, and increasing in complexity and 

 importance anteriorly when the greatest number of 

 special senses connect with it, and elsewhere in proportion 

 to the position and importance of the limbs or appendages, 

 constitutes the foundation of the central nervous system 

 which undergoes progressive development with increasing 



