548 GENERAL CONSIDERATIONS 



represents not one line of descent but many, the diagrams have repre- 

 sented it as possessed of one main trunk with all of the branches springing 

 from it. The relationship between the various phyla above the coelen- 

 terates is so uncertain that in the present state of our knowledge it seems 

 impossible to present exact continuity here. In interpreting such a 

 diagram the fact should also be borne in mind that no existing type is 

 ancestral to any other existing type but that if the two are related this 

 relationship is through a common ancestor, now extinct. With these 

 facts in mind the accompanying scheme (Fig. 330) is presented. 



