NILS ROSÉN, STUDIES ON THE PLECTOGNATHS. 21 



discussed in the chapter on the skeleton. Even the long 

 unpaired posterior cardinal vein is perhaps worth some re- 

 marks, but this is closeJy allied to questions about the deve- 

 lopment and movement of the kidneys. Before ending this 

 treatise I shall once more point out, that the ideas upon the 

 phylogenetical evolution of the Plectognaths here put forth 

 based as they are solely on the blood vascular system may 

 be due to alterations caused by the studies of other systems. 

 They only express the manner in which this evolution most 

 naturally seems to have taken place when we have to explain 

 the variations shown by this system within the group. 



