Question

How do amphioxus undergo retrogressive metamorphosis???? What changes occur later

Answer 1

I think there is mistake in ur question Amphioxus comes under Cephalochordata and Cephalochordata doesnt go under retrogressive metamorphosis.

Urochordata undergoes retrogressive metamorphosis.
Urochordata are most primitive chordate which are living today.
Changes occur :
1) Notochord is present in the tail of larva but totally absent in adult.
2) Nerve cord also present in larva but absent in adult.
3) Larva are free swimming but adult are sessile.

Only chordate character present in adult is pharyngeal gill slits.

Hope it was helpful:-)