DEVELOPMENT OF ELASMOBRANCH FISHES. 171 



firm basis of observed facts, nevertheless apjoears 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 Anne- 

 lids. 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-oesophageal 

 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^. 

 As the Vermes became segmented and formed the Annelids, 

 these side nerves seem to have developed ganglia, corresponding 

 in number wdth the segments, and finally, approximating on 

 the ventral surface, to have formed the ventral cord^ 



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 oesophageal nerve-ring ^ 



It appears to me, (if the difficulties of comparing the 

 Annelidan ventral cord with the spinal cord of Vertebrates are 

 found to be insurmountable), that this hypothesis would involve 

 far fewer improbabilities than one which supposes the whole 

 central nervous system of Vertebrates to be homologous with 

 the super-oesophageal ganglia. The mode of formation of a 



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 spht itself off from the oesophagus to which it originally 

 belonged, and became quite free and provided with a separate opening and per- 

 haps 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 develoj^ment of the ventral cord, and I 

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



2 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 Polygordius. 



12—2 



