476 



COMPARATIVE ANATOMY 



been asserted that the ventral nerve cords are larger in flatworms which 

 crawl, while the dorsal cords are larger in free-swimming types. 



As we pass from the flatworms to higher groups, two contrasting trends 

 are noticeable. In annelids and arthropods the nerve cords become 

 markedly metameric and are non-tubular, while that of chordates is 

 tubular and primarily non-metameric, as in Protochordates. 



NERVE CORD 



FLATWORM 



Fig. 393. — A diagram illustrating the hypothetical evolution of the nerve-net of a 

 Hydra into the sub-unibrellar nerve-ring of a medusa on the one hand and into the 

 paired nerve cords of flatworms and annelids on the other. The nerve cords are formed, 

 it is assumed, by the concentration of the fibrillar and cellular elements of the nerve-net. 

 (Redrawn after Kahn's "Das Leben Des Menschen," W. Keller & Co.) 



The nerve cord of the primitive annelids consists of a chain of paired 

 ganglia linked together both by longitudinal connectives and by trans- 

 verse commissures. The longitudinal connectives pass around the eso- 

 phagus to connect the supra- and sub-esophageal ganglia. In the higher 

 annelids the paired ganglia tend to unite in the mid-ventral line and to 

 lose the primitive rope-ladder arrangement. Concentration and fusion 

 greatly reduce the number of ganglia, especially in arthropods. (Fig. 394) 



These profound changes in the form of the nervous system of articu- 

 lates are accompanied by histological and physiological differences. Most 

 of the nerve cells become definitely polarized to transmit impulses in one 

 direction only, either towards or away from the central nerve cord. Thus 



