62 . ASCIDIANS CHAP. II 
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. 
a 
