CHEMICAL STRUCTURE AND METABOLISM 



101 



but do possess a notochord in some stage of development (the protochor- 

 dates). Of this latter group the tunicates were mentioned in the preced- 

 ing chapter (pp. 73-75). Also included among the protochordates are the 

 lancelet, Amphioxiis, and the somewhat wormlike group typified by 

 Balanoglossus (Fig. 5.5). Each of these three groups has at one time or 



cilicil'ed boinol 



BALANOGLOSSUS- TORNARl A LARVA 



BALANOGLOSSUS 



AMPHIOXUS 



FIG. 5.5. Two protochordates: Amph'ioxus end Balanoglossus, together with the tornario 

 larva of the latter. 



another been proposed as possible ancestors of vertebrates. But if one of 

 them is ancestral, from what group of invertebrates did it arise in its 

 turn? Phylum Chordata is singularly isolated among the phyla; none of 

 the other phyla possess structural features which approach those of chor- 

 dates at all closely. Consequently, the evolutionary origin of vertebrates 

 is a much debated point. One clue is furnished by the fact that Balanoglos- 

 sus passes through a most unchordatelike larval stage (Fig. 5.5). Its larva 

 resembles nothing encountered elsewhere in Phylum Chordata, but it does 



