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A fully grown red slug, Arion rufus, is 7 to 14 cm long. They have wrinkled surface and are often red-orange with dark tentacles. The respiratory pore is visible on the right side of the slug. For a different angle see this earlier post.

The individual could  actually be a spanish slug or a black slug instead, since they are hard to distinguish certainly without dissection or molecular analysis.

[Source: Wikipedia]