The Spanish slug, also known by its scientific name Arion vulgaris, more commonly known in English-speaking countries under the incorrectly applied Latin name Arion lusitanicus, is a species of air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs. They can lay up to 400 eggs in a single summer. These eggs often hatch within just 3.5–5 weeks. This is a highly invasive slug species. It is often considered a pest, not only in areas where it has been accidentally introduced, but even in places where it is indigenous.