There are several references to races from earlier "Doctor Who" episodes. At one point, the Doctor uses the title "Sir Doctor of TARDIS," which had been awarded to him by Queen Victoria in "Tooth and Claw" (2006). The Carrionites' contribution to "Love's Labour's Won" includes a reference to "Dravidian shores"; a "Dravidians starship" is mentioned in "The Brain of Morbius" (1976). Lilith refers to the Eternals, a race introduced in the original series serial "Enlightenment" (1983). In addition, the Doctor finds a skull in Shakespeare's prop store that reminds him of the Sycorax race from "The Christmas Invasion" (2005); when the Doctor mentions the name "Sycorax" to Shakespeare, Shakespeare says that he will use the name (the joke is that the name in fact derives from Caliban's mother in Shakespeare's play "The Tempest".)