62 ASCIDIANS 



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. Some 

 exceptional cases of development will be noted below under the 

 groups to which they belong. 



The remarkable life-history of the typical Ascidian, of which 

 the outlines are given above, is of importance from two points of 

 view : — 



1. It is 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 is thus a Chordate animal 

 showing evident relationship to the Vertebrata ; while the adult 

 is in its structure non-Chordate, and is on a level with some of the 

 worms, or with the lower Mollusca, in its organisation, although of 

 an entirely different type. 



2. It shows us the true position of the Ascidians (Tunicata) 

 in the animal series. If we knew only the adult forms we 

 might regard them as being an aberrant group of Worms, or 

 possibly as occupying a position between worms and the lower 

 Mollusca, or we might place them as an independent group ; but 

 we should certainly have to class them as Invertebrate animals. 

 But when we know the whole life-history, and consider it in the 

 light of " recapitulation " and " evolutionary '' views we recognise 

 that the Ascidians are evidently related to the Vertebrata, and 

 were at one time free-swimming Chordata occupying a position 

 somewhere below the lowest Fishes. 



