THE ANCESTRY OF THE VERTEBRATES 



629 



meant is that annelids retain certain traits which more or less closely 

 match those which the ancestors of vertebrates are thought to have 



COELENTERATE 



FLATWORM 



BLASTULAE 

 Fig. 520. — 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 flatworms can be said to be ancestral to chordates. 

 (Redrawn after H. E. Ziegler.) 



possessed. Since, then, a group treated as ancestral may resemble an 

 ancestor in one respect, but depart widely from the ancestral condition 

 in others, the conventional family tree may be misleading, unless it is 



