The Spanish slug Arion vulgaris (still often incorrectly referred to as Arion lusitanicus) is a large land slug with a body length of up to 12 cm. It is brown in color. This is a highly invasive species and is considered to be the worst European agricultural pest. It belongs to the family Arionidae, the roundback slugs. Species in this family of slugs have the respiratory opening or pneumostome positioned just anterior to the mid-point of the mantle. The pneumostome is visible in this image as a circular opening behind the head on the right-hand side of the slug. The shell of the Spanish slug is completely reduced, existing only as internal microscopic calcareous granules.