36 



DEGENERATION : 



perhaps most distinctive and clinching as an evidence 

 of affinity, the myelonic or cerebral eye. 



Now let us convince ourselves that these four 

 features exist not only in the frog's tadpole, as they do 

 in all fishes, reptiles, birds, and beasts, but that they 

 also exist in the Ascidian tadpole, and, it may be 

 added, coexist in no other animals at all. 



The corresponding parts are named in Figs. 1 6 and 

 1 7, in such a way as to render their agreement tolerably 

 clear, whilst in Fig. 18 a more detailed representation 

 of the head of an Ascidian tadpole is given. 



Fig. 18. — Ascidian Tadpole with a part only of the tail C. X, nervous system 

 with the enlarged brain in front and the narrow spinal chord behind {n); N', 

 is placed in the cavity of the brain ; 0, the single cerebral eye lying in the 

 brain ; a, similarly placed auditory organ ; AT, pharynx ; d, intestine ; o, 

 rudiment of the mouth ; ch, notochord or primitive backbone. (From 

 Gegenbaur's Elements of Comparative Anatomy.) 



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 degeneration in 

 order to arrive at the adult structure. The diagrams 

 which are reproduced in Figs. 19 and 20 show how 

 this degeneration proceeds. It will be observed, that 



