36 



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 

 13 on PI. v.). 



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 marked 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 : — 



1st. It is an excellent example of degeneration. I^he 

 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. 



