Oh, barnacles! As if being eternally glued to a submerged rock was not hard enough, mating seems far from easy for these animals, since they do not leave their shells. Fortunately for some barnacles, they physically copulate with each other, while others reproduce by picking up the males’ sperms when it is released into the water. These underwater creatures have some of the longest penises which stretch eight times as long as their bodies. The testes are located towards the back of the head and extend into the thorax, while the ovaries are in the base to facilitate the genetic transfer between remote barnacles.