The Sontila were natives of the planet Sonti. They had the ability to shed their shells and take up another, better shell that was dropped by another. The Sixth Doctor compared this to the ability of the Hermit crab. They used table salt as their currency.
When the Sixth Doctor arrived on Sonti, a young Sontila took his coat as a shell by mistake. The Doctor managed to bribe him to get it back. (ST: The Doctor's Coat)