ON THE ANCESTRAL FORM OF THE OHORDATA. 363" 



The brilliant researches of Lang on Gunda segmentata, 

 and of Hatschek on the ' Development of the Amphioxus,' must 

 here in the first instance guide us ; and anybody having care- 

 fully perused those important contributions, and having com- 

 pared them with each other, must have been struck by the 

 great probability of the view advocated by Lang, that the ali- 

 mentary diverticula of these Platyelminihs are ihe fore-runners 

 of the arrangement of the ccelom in the higher enterocoelous 

 worms, and that through this link a glimpse is gained at the 

 road along which Annelids may have developed out of an an- 

 cestral Platyelminth stock.^ 



On the other hand, the stages in the development of Amphi- 

 oxus, where a double set of lateral diverticula of the archenteron 

 is present (fig. 12), which ultimately become converted into the 

 mesoblastic somites, appear to be of very great importance, 

 in so far as they render it highly probable that in the an- 

 cestry of vertebrates certain forms with metamerically placed 

 alimentary cteca must have obtained, of which the larval stage 

 of Amphioxus is the reminiscence. In the remaining verte- 

 brates the primitive alimentary diverticula giving rise to the 

 ccelom are reduced to two. This appears to be an ulterior 

 simplification. An attempt to explain this simplification, and 

 to bring the process of the formation of the ccelom in Amphi- 

 oxus under the same head with that in the other vertebrates, 

 has at the present moment not yet been made. It is, however, 

 sure to be made some day by the leading authorities on the 

 subject. For the present it may suffice to point out that the 

 ulterior development of the mesoblastic somites in the bulk 

 of the vertebrates re-establishes the homology with the more 

 primitive processes in Amphioxus. 



For us this larval stage of Amphioxus is all the more inter- 

 esting, because it must lead up to Platyelminths, correspond- 

 ing with Gunda in the presence of alimentary cseca, metame- 



' It must here be noticed that Lang has only very lately (' Biologisches 

 Centralblatt,' May, 1S83) emitted serious doubts concerning his own proposi- 

 tions. It remains to be seen whether future investigations will not tend to 

 confirm his original suggestive hypothesis rather than these doubts. 



