What happens to cephalochordate larvae before they become juveniles?

Before becoming juveniles, cephalochordate larvae undergo a period of growth and development while spending much of their time feeding in open water, though they can also be found on the substrate.1

These larvae are structurally similar to adults but are smaller, simpler, and peculiarly asymmetrical, characterized by having fewer gill slits and lacking an atrium. During this developmental phase, the gill slits on one side of the larva develop first. Once they have sufficiently grown and developed, the larvae undergo metamorphosis to transform into the adult form, ultimately completing their life history within the substrate.1