Aplan

Aplans were the native inhabitants of the planet Alfava Metraxis.

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

Culture and History
According to the Doctor, the Aplans were a relaxed and cheerful species - a fact he attributed to having the constant company of a second head. He also described them 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 had six levels, representing the ascent of the soul in the Aplan religion.

The Aplans were fastidious at recording their social, cultural and political history, and hence the sudden unexplained collapse of their civilization in the 47th century was particularly mysterious. The end of the Aplans was eventually discovered to have been caused by the Weeping Angels, which infested the dark underground Mortaria, 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. (DW: The Time of Angels)

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.