A slug was a gastropod mollusc which was found on Earth (TV: The Ark in Space) and could be dehydrated by salt.
Luke Smith suggested that if the Slitheen were made mostly of water, salt would dehydrate them like slugs. However, it was soon realised that the Slitheen were calcium-based and allergic to vinegar. (TV: Revenge of the Slitheen)
The Builders resembled eight-inch slugs. (PROSE: Primitives)