Face spider

Face spiders were a species of spiders with the the faces of babies. They were specially evolved to live at the back of wardrobes, though in the middle of the night they also lived in mattresses. The Doctor checked for face spiders in Kazran Sardick's wardrobe when visiting him. (DW: A Christmas Carol)

Possibly a reference to the Father Ted episode "Good luck, Father Ted" in which Dougal wants to visit the fair to see the Spider Baby which is described as having "the body of a spider, but the mind of a baby!"
 * Considering the subject matter, the Doctor may have been kidding about the existence of face spiders.