Where is Polonius? At supper! I suppose biologists have a name for the food-chain progress for a king going a’coursing from grave to worm to fish to beggar to dung to worm…

Claudius hints at his plan in line 55, and makes them clear in lines 67-77: the sealed letters that Hamlet will deliver are secretly his own death warrant. But Hamlet “sees a cherub that sees” Claudius’ purpose (line 56).