Why does a platypus have 2 eyelids?

Platypuses have two eyelids for protection. One is used the same way as humans use theirs, to help moisten the eyeball, but the other one is used for protection specifically when the platypus is hunting underwater. Platypuses close their eyes underwater and the second eyelid helps protect the platypus's eyes from sediments they stir up as they shovel in the mud for invertebrates and crustaceans.