DEVELOPMENT OF ELASMOBRANCH FISHES. 1G7 



of Annelids and Arthropods only, while Professors Leydig^ and 

 Semper^ and Dr Dohrn^ compare it with the whole Annelidan 

 nervous system. 



The first of these two views is only possible on the suppo- 

 sition that Vertebrates are descended from unsegmented ances- 

 tors, and even then presents considerable difficulties. If the 

 ancestors of Vertebrates were segmented animals, and several of 

 the recent researches tend to shew that they were, they must 

 almost certainly have possessed a nervous cord like that of exist- 

 ing Annelids. If such were the case, it is almost inconceivable 

 that the greater portion of the nervous system which forms the 

 ventral cord can have become lost, and the system reduced to 

 the superior oesophageal ganglia. Dr Dohrn*, who has specu- 

 lated very profoundly on this matter, has attempted to explain 

 and remove some of the difficulties which arise in comparing 

 the nervous systems of Vertebrates and Annelids. He supposes 

 that the segmented Annelids, from which Vertebrates are 

 descended, were swimming animals. He further supposes that 

 their alimentary canal was pierced by a number of gill-slits, 

 and that the anterior amongst these served for the intro- 

 duction of nutriment into the alimentary canal, in fact as 

 supplementary mouths as well as for respiration. Eventually 

 the old mouth and throat atroj)hied, and one pair of coa- 

 lesced gill-slits came to serve as the sole mouth. Thus it came 

 about that on the disappearance of that portion of the alimen- 

 tary canal, which penetrated the oesophageal nervous ring, the 

 latter structure ceased to be visible as such, and no part of the 

 alimentary canal was any longer enclosed by a commissure of the 

 central nervous system. With the change of mouth Dr Dohrn 

 also supposes that there took place a change, which would for a 

 swimming animal be one of no great difficulty, of the ventral for 



1 Bau des tJiierischen Kdrpers. 



2 Stammesvericandscliaftd.Wirhelthiere u. Wirbellosen and Die Verwandschafts- 

 heziehungen d. gegliederten Thiere. This latter work, for a copy of which I 

 return my best thanks to the author, came into my hands after what follows 

 was written, and I much regret only to have been able to make one or two passing 

 allusions to it. The work is a most important contribution to the questions 

 about to be discussed, and contains a great deal that is veiy suggestive ; some of 

 the conclusions with reference to the Nervous System appear to me however to 

 be directly opposed to the observations on Spinal Nerves above recorded. 



^ Ur.^prung d. Wirbelthiere u. Frincip des Functionswechsels. 

 ^ Loc. cit. 



