in. 10 LARVAL ANCESTRY OF CHORDATES 77 



violates no established morphological principles and certainly enables 

 us to see how a ciliated auricularia-like larva could be converted by 

 progressive stages into a fish-like creature with muscular locomotion, 

 while the adults, at first sedentary, substituted gill-slits and endostyle 

 for the original lophophore. The alternative is to suppose that the 

 ascidian tadpole arose as a purely tunicate development, providing 

 sufficient receptor and muscular organs to allow for the finding of 

 suitable sites on the bottom (Berrill, 1955). 



We may plausibly regard the adult tunicate organization as directly 

 derived from that of sessile lophophore-feeding creatures, and the 

 larval organization as descended from an echinoderm-like larva. 

 There is no need, on this view, to regard the sessile adult tunicate 

 as a 'degenerate' chordate. The problem that remains is in fact not 

 'How have sea-squirts been formed from vertebrates ?' but 'How have 

 vertebrates eliminated the sea-squirt stage from their life-history?' 

 It is wholly reasonable to consider that this has been accomplished by 

 paedomorphosis. Advance of the time of development of the gonads 

 relative to that of the soma is well known to occur in certain special 

 cases such as the axolotl. The example of the Appendicularia shows 

 that a similar process can happen among tunicates! Various workers 

 have stressed the differences between the ascidian tadpole and the 

 adult appendicularian, in attempts to show that the two are not 

 comparable. But the differences, though considerable, are superficial : 

 the similarity of organization is profound. Any sensible biologist with 

 an understanding of the way in which the characteristic forms of 

 animals arise by change in the rate and degree of development of 

 features can see how the Appendicularia may represent modified 

 ascidian larvae. 



The appendicularians do, indeed, carry certain characters of the 

 'adult' sea-squirt, in particular they have gill-slits, though of simple 

 form. Nothing is more likely, however, than that some features of the 

 sessile adult would be adumbrated in its larva and capable of fuller 

 development therein if advantageous. Larva and adult, it must be 

 remembered, possess the same genotype; the remarkable feature in all 

 animals with metamorphosis is the difference between the two stages, 

 not the similarity. Any characteristic may appear at either larval or 

 adult stage or be transferred by evolutionary selection from one to the 

 other. There is no serious objection to the view that the early adult 

 free-swimming chordates arose by paedomorphosis of some tunicate- 

 like metamorphosing form. If the creatures abandoned the habit 

 of fixation it would be possible for characters previously present 



