x1 is a surrounding cover/curtain that surrounds the eye structure of x2
This can include eye stalks (if the eye can be retracted into/enveloped by the surrounding tissue), etc.; it might include eye lids, although it is arguable that they do not really "surround" the eye. A purpose of the curtain must be to cover the eye, not just encase it. Essentially, this must be a circular/ringlike/sphincter eyelid.