THE ORIGIN OF VERTEBRATED ANIMALS 191 



In their development, however, we find that they 

 pass through the stage of a free-swimming larva, like 

 a tadpole, which possesses a swimming tail, a nerve 

 cord, a notochord, and an eye and ear. (See . 

 Fig. 8.) This larva after a time becomes fixed ; 

 the tail shrivels up and is absorbed ; the nervous 

 system becomes reduced to a single ganglion ; the 

 ear and eye become aborted, and the pharynx 

 enlarges. In fact, Ascidians are degenerate animals, 

 which in their larval stage stand at about the same 

 level as Amphioxus — if anything somewhat higher. 

 Some Ascidians, such as Appendicularia, never get 

 beyond the larval stage, and are found as minute 

 free-swimming pelagic animals, with only one pair 

 of gill clefts. 



Concerning the zoological position of Ascidians, it 

 is accepted by every one that they are Vertebrates, 

 which is proved by their development. Mistakes 

 have, however, been made in assuming that they 

 must be ancestral, and this mistake was made by 

 Darwin himself. If ancestral, they should stop at 

 what is a transitional stage in the development of 

 higher animals, but they do not do this. The 

 resemblance is not between the adult Ascidian and 

 the embryo Vertebrate, but between the embryo 

 Ascidian and the embryo Vertebrate. This in- 

 dicates descent from some common ancestral stock, 

 but not the descent of one group directly from the 

 other. 



Amphioxus and Appendicularia are the simplest 

 r existing forms with the characteristic vertebrate 

 ^structure, and represent the nearest approach yet 



