172 SYMPATHETIC NERYOUS SYSTEM. 



nervous system presupposed in my hypothesis, well accords with 

 what w^e know of the formation of the ventral cord in exist- 

 ing Annelids. 



The supposition of the existence of another branch of seg- 

 mented Vermes is not a very great difficulty. Even at the 

 present day we have possibly more than one branch of Vermes 

 which have independently acquired segmentation, viz. : the 

 Choetopodous Annelids and the Hirudinea. If the latter is an 

 isolated branch, it is especially interesting from having inde- 

 pendently developed a series of segmental organs like those of 

 Choetopodous Annelids, which we must suppose the ancestors of 

 Vertebrates also to have done if they too form an independent 

 branch. 



In addition to the difficulty of imagining a fresh line of 

 segmented Vermes, there is another difficulty to my view, viz. : 

 the fact that in almost all Vermes, the blood flows forwards 

 in the dorsal vessel, and backwards in the ventral vessel. 

 This condition of the circulation very well suits the view of a 

 change of the dorsal for the ventral surfaces, but is opposed to 

 these surfaces being the same for Vertebrates and Vermes. 

 I cannot however regard this point as a very serious difficulty to 

 my view, considering how undefined is the circulation in the 

 unsegmented grouj)s of the Vermes. 



Sympathetic nervous system. 



Between stages K and L there may be seen short branches 

 from the spinal nerves, which take a course towards the median 

 line of the body, and terminate in small irregular cellular 

 masses immediately dorsal to the cardinal veins (PL xvii. 

 fig. 1, sy. g.). These form the first traces that have come under 

 my notice of the sympathetic nervous system. In the youngest 

 of my embryos in which I have detected these it has not been 

 possible for me either definitely to determine the antero- 

 posterior limits of the system, or to make certain whether the 

 terminal masses of cells which form the ganglia are connected 

 by a longitudinal commissure. In a stage slightly younger than 

 L the ganglia are much more definite, the anterior one is situ- 

 ated in the cardiac region close to the end of the intestinal 

 branch of the vagus, and the last of them quite at the posterior 



