Maybe they're like starfish, which mature from bilaterally symmetrical larvae into many-limbed adults?

Note how the adult starfish body starts out like an organ in the larva and then grows as the rest of the larva is absorbed.

Fun fact: vertebrates are descended from echinoderms - our ancestors were larvae that acquired the ability to reproduce without developing into adults. So if you're feeling like something is missing from your life but you don't know exactly what it is, consider the possibility that you subconsciously long to transform into your true adult form, which would look something like this:

Oh to be a sessile filter feeder!