Plica semilunaris Better known as your third eyelid, or maybe not, this organ in our bodies is just one of the many parts that we don’t need. If you pull up on each of your eyelids, the plica semilunaris will be exposed. This part of the eye is known to produce “sleep,” or that weird crust you have in your eyes when you wake up. Thank you plica semilunaris. How could we live without waking up with weird stuff in our eyes?