Skin

Toby, possessed by the Beast, had markings on his skin. (TV: The Impossible Planet)

Amy, with marks on her skin to remind herself she had met the Silence. (TV: Day of the Moon)

The skin was the outer covering of animals.

The Foamasi (TV: The Leisure Hive, PROSE: Placebo Effect) and the Slitheen (TV: Aliens of London, World War Three) used skin suits to disguise themselves as humans.

By the time the Ninth Doctor and Rose Tyler met Lady Cassandra O'Brien.Δ17, she was little more than a piece of skin stretched onto a frame with eyes and a mouth, connected to a brain. (TV: The End of the World)

When Toby Zed was possessed by the Beast, his eyes were red and he had markings on his skin. (TV: The Impossible Planet / The Satan Pit)

Jenny was created in a progenation machine from a skin sample taken from the Tenth Doctor. (TV: The Doctor's Daughter)

One way that individuals reminded themselves they had encountered the Silence was to make tally marks on their own skin. (TV: Day of the Moon)

When Mels regenerated into River Song, her skin colour changed. (TV: Let's Kill Hitler)