ORIGIN OF THE VERTEBRATE NERVOUS SYSTEM. 393 



surface, which, though so far unsupported by any firm basis of 

 observed facts, nevertheless appears to me worth suggesting. It 

 assumes that Vertebrates are descended not through the present 

 line of segmented Vermes, but through some other line which 

 has now, so far as is known, completely vanished. This line 

 must be supposed to have originated from the same unsegmented 

 Vermes as the present segmented Annelids. They therefore 

 acquired fundamentally similar segmental and other Annelidan 

 organs. 



The difference between the two branches of the Vermes lay 

 in the nervous system. The unsegmented ancestors of the 

 present Annelids seem to have had a pair of super-cesophageal 

 ganglia, from which two main nervous stems extended back- 

 wards, one on each side of the body. Such a nervous system in 

 fact as is possessed by existing Nemertines or Turbellarians 1 . 

 As the Vermes became segmented and formed the Annelids, 

 these side nerves seem to have developed ganglia, corresponding 

 in number with the segments, and finally, approximating on the 

 ventral surface, to have formed the ventral cord 2 . 



The other branch of Vermes which I suppose to have been 

 the ancestors of Vertebrates started from the same stock as 

 existing Annelids, but I conceive the lateral nerve-cords, instead 

 of approximating ventrally, to have done so dorsally, and thus a 

 dorsal cord to have become formed analogous to the ventral cord 

 of living Annelids, only without an cesophageal nerve-ring 3 . 



not bring us very much nearer to a solution of the vertebrate-annelid mouth question, 

 but merely substitutes one difficulty for another ; and does not appear to me so satis- 

 factory as the hypothesis suggested in the text. 



At the same time Professor Semper's hypothesis suggests an explanation of that 

 curious organ the Nemertine proboscis. If the order of changes suggested by him 

 were altered it might be possible to suppose that there never was more than one 

 mouth for all Vermes, but that the proboscis in Nemertines gradually split itself off 

 from the oesophagus to which it originally belonged, and became quite free and pro- 

 vided with a separate opening and perhaps carried with it the so-called vagus of 

 Professors Semper and Leydig. 



1 It is not of course to be supposed that the primitive nervous system was pierced 

 by a proboscis like that of the Nemertines. 



2 This is Gegenbaur's view of the development of the ventral cord, and I regard 

 it in the meantime as the most probable view which has been suggested. 



3 A dorsal instead of a ventral approximation of the lateral nerve-cords would be 

 possible in the descendants of such living segmented Vermes as Saccocirrus and Poly- 

 gordius. 



B. 26 



