36 THE ORIGIN OF MAN 



known as the homocercal type because it is sym- 

 metric and has only two lobes. 



In spite of the fact that fishes have bony skel- 

 etons and that they are known back to the Ordo- 

 vician, their early ancestors are still undiscovered. 

 Even among the vast horde of living forms and 

 their embryos their ancestry has not been deter- 

 mined, nor as yet has the missing link between the 

 invertebrates and the vertebrates been found. 

 The oldest fossil vertebrates are undoubted fishes, 

 seemingly of elasmobranch character and while 

 vertebrates of a lower type may yet be found it 

 seems as if natural conditions are against this 

 possibility, because the earliest vertebrates must 

 have been not only small but also without hard 

 parts. Among the unsegmented worms (nemer- 

 tians) the closest approach to the lowest type of 

 vertebrate ancestors in animals often spoken of 

 as half -vertebrates (Hemichordata) is found. 

 Stilll the structural gap between them is, how- 

 ever, great. The Hemichordata are worm-like 



