Oh, it gets better. Barnacles aren't just descended from mobile leg having normal animals, they ARE mobile leg having normal animals! A barnacle larva looks like this:
Waldo Nell, flickr
... and do they ever swim around. Quickly. Effectively. Tirelessly. It's only at barnacle puberty that they stick themselves upside down to a convenient rock and scramble their body plan through metamorphosis. The feeding tendrils (cirri) on adult barnacles are actually the remaining legs! See?
Matt Lau, Point Reyes National Park
Yeah...
There is in fact a group of barnacles, called the Rhizocephala, that do the ungodly horrorparasite thing. Instead of finding a rock or section of timber, that normal looking larva finds the skin of a host. Instead of turning upside down and growing a shell, it expels a tiny, featureless glob of cells that burrows into the host and begins to grow. And grow. And grow. The resulting animal has no symmetry, no segments, no organs except ovaries. It is a filigreed tangle of thirsty roots, weaving into the host's body, stealing its shape and drinking its life.
Conventional crustaceans, like this hermit crab here!!
Worst family dinner ever!
Yeah if you sequenced the genomes of these entwined bodies you would register kin.
There are barnacle relatives that we've only ever seen as larvae because they're parasites and we can't find the host. Hundreds of years of careful documentation and we do not know what they turn into. What a lovely, terrifying mystery!
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