A CHAPTER m DARWINISM 



37 



in somewhat the same manner as the young barnacle, 

 the young Ascidian fixes itself to a stone by its head ; 



NOTOCHOPD 



Fig. 19.— Degeneration of Ascidian Tadpole to form the adult. Tlie black pieces 

 represent the rock or stone to which the Tadpole has fixed its head. 



TAIL 



Fig. 20. — Very young Ascidian with only two gill-slits. Compare with Fig. 15 ; 

 which is, however, seen from the other side, so that left there corresponds to 

 right here. 



then the tail with its notochord and nerve-chord 

 atrophies. The body grows and gradually changes its 



