So THE DARWINIAN THEORY 



they were grouped with the Brachiopoda and 

 Polyzoa, as a subdivision of Mollusca. As soon as 

 their development was worked out, it was found that 

 they were really members of the Vertebrata, inas- 

 much as when young they show a curious resemblance 



Fig. 8. 



Tadpoles of Frog and Ascidian 



(A, Frog ; B, Ascidian). 



b, brain ; d, spinal cord ; e, eye ; i, intestine ; m, mouth ; n, notochord ; 



t, pharynx with gill-slits ; s, spiracle 



to tadpoles — not merely in form and appearance, but 

 in all essential points of structure ; while the mode 

 of formation of their nervous system, skeleton, 

 alimentary canal, and other parts, is exactly that 

 obtaining among other Vertebrates, and entirely un- 

 like that of all Invertebrate groups. (Fig. 8.) 



