Aplan

 were the native inhabitants of the planet Alfava Metraxis.

Biology
The Aplans were humanoid lifeforms with two heads. The heads apparantly had their own seperate intelligence and personalities as, for in at least one period, heads were allowed to marry each other. (DW: The Time of Angels)

History
The Aplans were described as prolific builders, and their civilisation left notable landmarks on their home planet of Alfava Metraxis. These included the Aplan Mortaria, sometimes known as Mazes of the Dead. These were large subterranean labyrinths with tombs in the walls, adorned with statues of the dead. The Mazes have six levels, to represent the ascent of the soul.

River Song noted that the Aplan civilisation came to a mysterious and abrupt end. This was revealed to have been caused by the Weeping Angels, which infested the dark underground Mazes, well hidden amid the numerous statues. Having hunted the Aplans to extinction, the Angel colony lacked any further energy source and over the centuries, the being energy deprived began to degrade and went into hibernation.

Trivia

 * The Aplans may be a reference to Douglas Adams' two headed character Zaphod Beeblebrox from 'The Hitchhhiker's Guide to the Galaxy'. Adams was also a writer for Doctor Who.