THE ORIGIN AND EVOLUTION OF THE VERTEBRATES 85 



Balanoglossus and the bilateral ancestors from which the radially- 

 symmetrical Echinoderms are probably descended." 



The view is taken by several authors that the Enteropneusta and 

 the echinoderms were derived from a common ancestral stock, which 

 is now copied in a simplified form by the larvae of both groups. " The 

 common characters of all the larvae are," says Wilder, " bilaterality, 



FIG. 46. Comparison of Tornaria larva with larval echinoderms. Main 

 ciliated bands in black, lesser systems cross-lined. Upper row ventral aspect; 

 lower row right lateral aspect. A. A ', Tornaria; B. B ', Auricularia (sea cucum- 

 ber); C. C ', Bipennaria (star-fish). (From Lull, after Wilder.) 



transparency, locomotion by bands of cilia, and pelagic life." A com- 

 mon ancestor having these characters may well haye existed. "The 

 lineal descendents of this hypothetical ancestor chose two paths, the 

 one leading to the Echinodermata, the other to Balanoglossus, the 

 Tunicata, Amphioxus, and eventually the Vertebrate." This view 

 makes the Enteropneusta ancestral to the tunicates and the tunicates 

 ancestral to Amphioxus. In a previous discussion of the Amphioxus 

 theory we have dealt with the tunicates as degenerate derivatives of 

 Amphioxus-like ancestors; just the reverse of the opinion expressed 

 by Wilder. It should also be said that, although Amphioxus is ob- 

 viously not at the very bottom of the vertebrate ancestral trunk, 



