Barnacle

Barnacles were a group of marine animals found on Earth. The cirripedes encountered by the Second Doctor on an asteroid in 7691 were also barnacles, but evolved by parallel evolution. (AUDIO: The Outliers)

Space barnacles could be found on the outsides of spacecraft travelling through space. (PROSE: The Hungry Night) One kind of space barnacle, the electromagnetic barnacle, was a much more complex and sentient creature. During her time as a tour guide on Gendar, Maxie Masters recalled giving guided tours of the Babbling Bibliothèque to various alien tourists, including electromagnetic barnacles. (PROSE: Out of the Box)

In the Land of Fiction, Rob Roy MacGregor returned the Doctor's TARDIS to him once when it had been lost underwater, and had been recovered covered in barnacles. (AUDIO: Legend of the Cybermen)

Donna Noble once compared an alien tourist to a "walking" barnacle, with tentacles. (AUDIO: Death's Deal)