Why do appendicularians retain a notochord and tail as adults?

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Bright blue sea squirt Rhopalaea crassa with yellow-edged openings grows on a coral reef among various corals in the Philippines. — tunicate Blue spot sea squirt (*Clavelina moluccensis*), a colonial tunicate.

Appendicularians (also known as larvaceans) retain a notochord and tail as adults because they undergo a developmental process known as paedomorphosis, which is the retention of juvenile features in the adult stage.1

In most other tunicates, chordate features such as the notochord and tail are present in the tadpole-like larva but disappear during metamorphosis as the animal matures into a sessile, bottom-dwelling adult. Appendicularians, however, reach sexual maturity before this metamorphosis occurs. Consequently, the adult appendicularian maintains the tadpole-like body structure of its larval stage, including the tail and notochord, which it uses to propel itself and its gelatinous "house" through the open sea.21345