20 CHILDHOOD OF ANIMALS 



did not know its history, we should find very difficult to associate 

 with backboned animals. How far the larva of the sea-squirt 

 shows a repetition of the structure of its ancestors, or how far its 



Fig, 9. Metamorphosis of an Ascidian. The upper figure shows the tadpole 

 adhering to a flat surface ; the lower figure shows the young Ascidian 

 similarly but permanently attached. {A fter Lankestek, Kowalevsky 

 audHERDMAN; much magnified.) 



shape and its organs have been formed and adapted for the purposes 

 of its own life, can only be guessed, and different zoologists have 

 made very different guesses. The most usual interpretation is that 

 the larva is in the main ancestral, and that the degradation of the 

 adult is pure degeneration. The sea-squirts are taken to be humble 

 relations of the vertebrates which became degenerate because they 



