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 
Fig. 8. 
ASCIDIAN 
Tadpole of Frog and Ascidian. Surface view. 
(Lankester's ' Degeneration.') 
CILL SLITS 
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 Yertebrata, 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 
