282 



THE RISE OF ANIMAL LIFE 



hypothe+icoil 

 larvae 



Cchinoderm 

 line 



s+arfisb 



acorn worm 



Fig. 13-4. A study of the larval stages of the acorn worm and echinoderms has lent support to the idea 



that both came from a common ancestor. 



glance it thus seems plausible to conclude 

 that some annelid forerunner might have 

 given rise to the vertebrates. There are, 

 however, a great many anatomical features 

 in the annelid that are impossible to corre- 

 late even with the acorn worm. For in- 

 stance, the annelid has a ventral instead of 

 a dorsal nerve cord, and it has no gill slits, 

 and no notochord. In addition there is noth- 



ing in the embryology that would lead one 

 to believe they are related. Embryological 

 development repeats the history of the race 

 and thus indicates similarities of even dis- 

 tantly related forms. 



Some biologists look with favor on the 

 theory that the chordate ancestor stemmed 

 from the same stalk that gave rise to the 

 echinoderms. Remarkable similarities have 



ancestral 



two- layered 



an'imais 



I 

 echinoderms- chordate line ' 



I ,^ I ^cinnelid - arthropod - mollusk 

 /7\\ \ / .rr7T>. line 



s 



-^ ^^ / / fla+worm line \ 



\ 



\ 



H 



w 



Fig. 13-5. The coelenterates probably gave rise to the echinoderm-chordate line, the flatworm line, and the annelid- 



arthropod-mollusk line. 



