Chordate
According to a long-standing theory, the origin of chordates may be found in transformed larvae of sea-squirts (tunicates). Adult tunicates are sessile, but their larvae are motile, and have some features found in early vertebrates.  The process of paedomorphosis, where juvenile features are retained in the adult, is the proposed mechanism. Genome analysis shows that tunicates are the closest living relatives of vertebrates.