Source : Webster's Revised Unabridged Dictionary (1913)

Sea mouse \Sea" mouse`\ (Zo["o]l.)
   (a) A dorsibranchiate annelid, belonging to {Aphrodite} and
       allied genera, having long, slender, hairlike set[ae] on
       the sides.
   (b) The dunlin.

Source : WordNet®

sea mouse
     n : any of several large worms having a broad flattened body
         with a mat of coarse hairs covering the back