DEGENERA TION, 



It is clear then that the Ascidians must be admitted 

 to be Vertebrates, and must be classified in that 

 great sub-kingdom or branch of the animal pedigree. 

 The Ascidian tadpole is very unlike its parent the 

 Ascidian, and has to go through a process of 

 degeneratiun in order to arrive at the adult structure. 





Fig. i8. — Ascidian Tadpole with a part only of the tail C. N, nervous system with 

 the enlarged brain in front and the narrow spinal cord behind ;/ ; iV', is placed in 

 the cavity of the brain : O. the single cerebral eye lying in the brain ; a. similarly 

 placed auditory organ ; A", pharynx ; d, intestine ; o. rudiment of the mouth ; 

 c/i, nr tochord or primitive backbone. (Fr>^m Gegenbaur's " Elements of Com- 

 parative Anatomy.") 



The diagrams which are reproduced in Figs. 19 and 

 20, show how this degeneration proceeds. It will be 

 observed, that in somewhat the same manner as the 

 young barnacle, the young Ascidian fixes itself to a 

 stone by its head : then the tail with its notochord 

 and nerve-chord atrophies. The body grows and 

 gradually changes its shape, whilst the cloacal chamber 

 forms. The brain remains quite small and undeve- 

 loped, and the remarkable myelonic eye (the eye in 



