The cyclopsian nature of the copepod is a deep mystery of evolution. Even flatworms have two eyes. (I used to be rather distainful of the acoels, until I met marine platyhelminthes--some are more spectacular than nudibranchs.) Photosensors came before bilateral symmetry? Who was the first crustacean? I can only assume that whoever he was, he had two eyes, and the one-eyed character of copepods is a derived one. Even flatfish don't lose their lower eye, but bring it around to the other side.
I've no time to research this at the moment, but soon.