The barnacle is a type of arthropod that is related to crabs and lobsters, yet usually much smaller. They are exclusively marine dwelling animals and tend to live in shallow waters.
However, while most people have seen barnacles at the beach, they probably never knew that these animals are famous for their penis size! Barnacles are suspension feeders so movement is tough.
Because of this, reproduction is even more difficult. Most barnacles are hermaphroditic, which means they switch from male to female throughout their life, yet when they are male the organisms cannot leave their shell to mate. Thus, over time they developed penises forty times larger than their entire body to reach the female!