Flatworms are flat because they have no body cavity, no circulatory system, and no lungs. These invertebrates get their oxygen and nutrients by diffusion and are considered bilaterally symmetrical, which means that their left side is a mirror image of their right side. While all of those things are interesting, they aren’t the most fascinating thing about flatworms. Their kookiest biological fact? The majority of them are hermaphroditic and have two barbed penises. These penises “fence” each other until one finally stabs the epidermis and hypodermically inseminates itself. So they don’t have a spine or lungs, but they have two penises? Interesting choice, evolution. Photo cred: Bunaken Hans #science #marinebiology #evolution #wtfevolution













