72. TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
neighbouring objects; and, at the same time, the region 
of the body between the point of fixation and the mouth 
(branchial aperture) increases rapidly in extent so as to 
cause the body of the Ascidian to- rotate through about 
180°, and so carry the branchial siphon to the opposite 
end from the area of attachment (see figs. 10, 11, 12, and 
tSvon (PAW). 
Finally, the gonads and their ducts form in the meso- 
derm between stomach and intestine, and so bring us to 
the sedentary degenerate fixed adult Ascidian with little 
or no trace of the Chordate characteristics so narked in 
the earlier larval stage (compare figs. 13 and 9). The free- 
swimming tailed larva shows the Ascidian at the highest 
level of its organisation, and is the stage that indicates the 
genetic relationship of the Tunicata with the Vertebrata. 
In some Ascidians with more food-yolk in the egg, or in 
which the development takes place within the body of the 
parent, the life-history as given above is more or less — 
modified and abbreviated, and in some few forms the 
tailed larval stage is missing. 
The remarkable life-history of the typical Ascidian, of 
which the outlines are given above, is of importance from 
two points of view :— 
ist. Itis an excellent example of degeneration. The 
free-swimming larva is a more highly developed animal 
than the adult Ascidian. The larva is, as we have seen, 
comparable with a larval fish or a young tadpole, and so is 
a chordate animal showing evident relationship to the 
Vertebrata; while the adult is in its structure non-chordate, 
and may be regarded as being on a level with some of the 
worms, or with the lower Mollusca, in its organisation— 
although of an entirely different type. 
