346 



CHOEDATE ANATOMY 



In the simple flatworm Planocera, there is a single pair of nerve cords. 

 The number increases in other forms, and the cords may be dorsal and 

 ventral as well as lateral. This fact is important in its bearing upon the 

 development of the nervous systems of higher animals which, in general, 

 are assumed to have evolved from flatworm-like ancestors. For the 

 presence of both dorsal and ventral nerve cords in flatworms makes it 

 possible to derive annelids and arthropods from flatworms in which the 

 ventral cords become the dominant nervous centers, and to derive verte- 



NERVE CORD 



FLATWORM 



Fig. 307. — A diagram illustrating the hypothetical evolution of the nerve-net of a 

 Hydra into the subumbrellar 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 Fritz Kahn, " Der Mensch," Albert Miiller, Zurich.) 



brates from flatworms in which the dorsal cords become predominant. 

 The dorsal nerve cord of vertebrates, therefore, need not have been derived 

 from the ventral cord of annelids by the inversion of the worm. It has 

 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 j)rimarily non-metameric, as in protochordates. 



