DR DOHRN'S HYPOTHESIS. 389 



Professor Gegenbaur 1 looks upon the central nervous system of 

 Vertebrates as equivalent to the superior cesophageal ganglia 

 of Annelids and Arthropods only, while Professors Leydig 2 and 

 Semper 3 and Dr Dohrn 4 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 an- 

 cestors, 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 ex- 

 isting 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 cesophageal ganglia. Dr Dohrn 5 , 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 de- 

 scended, 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 introduction 

 of nutriment into the alimentary canal, in fact as supplementary 

 mouths as well as for respiration. Eventually the old mouth 

 and throat atrophied, and one pair of coalesced gill-slits came 

 to serve as the sole mouth. Thus it came about that on the 

 disappearance of that portion of the alimentary canal, which 

 penetrated the cesophageal nervous ring, the latter structure 

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



1 Grundriss d. vergleichenden Anat. p. 264. 



2 Ban des thierischen Korpers. 



3 Stammesverwandschaft d. Wirbelthiere u. Wirbellosen and Die Venvandschafts- 

 beziehungen 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 very suggestive ; some of the conclusions with reference 

 to the Nervous System appear to me however to be directly opposed to the observa- 

 tions on Spinal Nerves above recorded. 



4 Ursprung d. Wirbelthiere u. Princip des Functionsivechsels. 

 6 Loc. tit. 



