44 



EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS. 



Transverse Cords. — Numerous transverse, or circular bands intersect the 

 longitudinal ones, and lay the foundations for the transverse commissures, and 

 for the segmental peripheral nerves. The latter usually lead by smaller branches 

 into a subdermal plexus, from which the nerve ends are distributed to their re- 

 spective terminals. 



The ventral cords and the middle cord are confined to that surface of the 

 embryo that is the first to develop. Their position during the early stages is the 

 same in all segmented animals, and their presence definitely locates the primitive 

 oral, or neural surface of the body. 



Fig. 36. — Brain and nerve cord of a young Limulus in the second larval stage. A, Haemal surface; B, neural surface. 

 Fig. 37. — Sections of same. A, Through the optic ganglia and olfactory organs; B, through the middle of the 

 hemispheres and the posterior part of the forebrain; O, through the cheliceral ganglia; E, through the suprasto- 

 modeal commissure and the lateral stomodsal ganglia. 



The longitudinal cords serve to conduct nervous impulses in a longitudinal 

 direction; in them are located the great majority of the nerve cells. The trans- 

 verse bands serve to conduct nervous impulses in a centripetal or centrifugal 

 direction. The comparatively few nerve cells that belong in them, as a rule, lie 

 near their central or peripheral terminals. 



