HOMOLOGY. 



27- 



Bohrn *, wliile admitting the homology or equivalency of 

 the supraffisophageal ganglions, suboesophageal ganglions, and 

 under or ventral cords therefrom continued, whether ganglionic 

 or otherwise, in Annelids and Arthropods, with the inyelen- 

 cephalous tract in Vertebrates, notwithstanding the opposite 

 sides of the body which they seem to hold, has recourse to 

 ideal ancestral forms in order to reconcile the differences 

 as to relative position shoAvn by the actual or modern 

 subjects f. 



Whilst the aspect of the body in connexion and communi- 

 cation with the umbilical vesicle was held to be on opposite 

 sides in Vertebrates and Invertebrates, a strong argument 

 was afforded to objectors to any ascent of the lower to the 

 higher division of animals. So the accompHshed Professor of 

 Palseontology, in the ' Museum d'Histoire Naturelle,' Paris, 

 writes : — " II importe toutefois de faire remarquer que I'idee 

 de faire descendre les Vertebres des Invertebres n'est raison- 

 able qu'a la condition de supposer dans les temps anciens 

 des animaux sans vertebres organises autrement que ceux 

 d'aujourdhui ; car il y a des differences profondes entre les 

 Vertebres et les Invertebres. A I'epoque actuelle, les poissons 

 ont des cartilages, des os en phosphate et carbonate de chaux, 

 leur vesicule ombilicale se continue avec le ventre ; tandis que 

 les crustaces ont de la chitine, n'ont pas de cartilages, n'ont 

 pas de phosphate de chaux, et se developpent en ayant la 

 vesicule ombilicale sur le dos " {. 



My contention is that the true grounds for determining 

 the homology in question are not the positions of the body 



* ' Ursprung der Wirbelthiere,' &c. 



t I concur with the remarks by Ealfour (loc. cit. p. 17) on Dohrn's 

 hypothesis, and deem any other objection superfluoiis. 



% ' Les Enchainements du Monde Animal dans les Temps geologiques,' 

 par Albert Gaudry (8vo, 1883), vol. i. p. 249. 



