OHAPTEE III 



WORMS TO VEBTEBBATES : SKELETON AND HEAD 



In tracing tlie genealogy of any American family it 

 is often difficult or impossible to say whether a certain 

 branch is descended from John Oldworthy or his 

 cousin or second cousin. In the latter cases to find 

 the common ancestor we must go back to the grand- 

 father or great-grandfather. The same difficulty, but 

 greatly enhanced, meets us when we try to make a 

 genealogical tree of the animal kingdom. Thus it 

 seems altogether probable that all higher forms are 

 descended from an ancestor of the same general struct- 

 ure and grade of organization as the turbellaria, al- 

 though probably free swimming, and hence with some- 

 what diiferent form and development, especially of the 

 muscular system. It seems to me altogether probable 

 that all, except possibly MoUusca, are descended from 

 a common ancestor closely resembling the schematic 

 worm last described. Some would, however, maintain 

 that they diverged rather earlier than even the turbel- 

 laria ; others after the schematic worm, if such ever 

 existed. As far as our argument is concerned it makes 

 little difference which of these views we adopt. 



From our turbellaria, or possibly from some even 

 more primitive ancestor, many lines diverged. And 

 this was to be expected. The coelenterata, as we saw in 

 hydra, had developed rude digestive and reproduc- 



