428 



CHORDATE ANATOMY 



which, there is reason to believe, resemble those of the predecessors of 

 chordates. But this does not demonstrate that their possessors are 



COELENTERATE 



FLATWORM 



BLASTULAE 

 Fig. 374. — A diagram showing hypothetical stages in the ancestry of sponges, 

 coelenterates, flatworms and chordates. Such a series is evidently based chiefly upon 

 embryological evidence. If the diagram represents correctly the early phases of 

 phylogenesis, chordates have split off from other phyla earlier than morphologists 

 generally have assumed and not even coelenterates or flatworms can be said to be 

 ancestral to chordates. (Redrawn after H. E. Ziegler.) 



ancestors of chordates. Consequently the phylogenesis of each organ 

 system must be worked out more or less independently of other organs. 

 To believe that the different phyla have acquired their systems of organs 



