TUNICATA. 89 



and growth proceeds in such a manner that the mouth is pushed 

 round to a position opposite to the fixed area, and gradually the 

 animal becomes the adult ascidian. 



This wonderful metamorphosis presents a striking example of 

 DEGENERATION resulting from the adoption of a fixed mode of life. 

 The active free-swimming larva with its brain, eye, hearing organ, 

 and muscular tail becomes transformed into a comparatively inert sac. 



The tadpole of an Ascidian resembles that of a frog (Figs. 8, 9), 

 not merely superficially, but also in its general structure and mode 

 of development. The Tunicata are now generally regarded as a 



Fi?. 8. 



EYE MOUTH 



ASCIDIAN 



Tadpole of Frog and Ascidian. Surface view. 

 (Lankester's ' Degeneration.') 



Fig. 9. 



SPINAL CHORD 



NOTOCHORD 



POSITION OE SPIRACLC gilCsLITSV. 

 SPINAL'CHORD / IBBMN MOUTH 



Tadpole of Frog and Ascidian. Diagram representing the chief internal 

 organs. (Lankester's ' Degeneration. A chapter in Darwinism.') 



degenerate offshoot from the ancestral stock of the Vertebrata, in 

 that the larva possesses a skeletal rod (rudimentary backbone) 

 separating the dorsally situated nerve-tube (cerebro-spinal axis) from 

 the ventrally situated intestinal tube, the existence of the cerebral 



