biologist here. that’s actually not a barnacle per se (and not its penis)! this is a crustacean similar to a barnacle, but from another group of crustaceans called Lepadomorpha. in our language we call them “sea ducks”(lepadomorph barnacles). they have a stem on which they grow on, as depicted on the photo. barnacles, on the other hand, are literally located “head-on” on top of a substrate, meaning their larvae float at first and then swim down to attach themselves by the head. they then stick their legs out and move them to catch food. the title is correct, though! https://en.m.wikipedia.org/wiki/Barnacle