46 



DEGENERATION. 



the brain) disappears. The number of gill-sHts 

 increases as the animal grows in size and its outer skin 

 becomes tough and leather-Hke. 



GRAIN 



NOTOCHORO 



TAIU 



TiG. 19.— Degeneration of Ascidian Tadpole to form the adult. The black pieces 

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



MOUTH 



TAIL 



Fig. 20 —Very young Ascidian with only two gill-'^lits. CoTipare with Fig 15 ; which 

 is, however, seen from the other side, so that left there corresponds to right here. 



