A slug is a terrestrial gastropod mollusk, essentially a snail that has lost its large external shell through evolution. Slugs are soft-bodied invertebrates belonging to the phylum Mollusca. They do not possess hair, fur, or any keratin-based external filaments typical of mammals. Their unique biological structure relies on different mechanisms for protection, movement, and survival.
External Structure of a Slug
The outer covering of a slug is a soft, permeable integument highly susceptible to drying out. This external layer is an epidermis composed of a single layer of specialized cells, unlike the multi-layered, keratinized skin of animals that grow hair. Since the slug’s body is primarily made up of water, it requires a specific strategy for managing moisture loss.
A distinct anatomical feature is the mantle, a saddle-shaped, fleshy lobe located behind the head. This structure is a fold of the body wall that covers and protects several internal organs, including the respiratory cavity. The mantle contains the pneumostome, a visible opening that serves as the slug’s breathing pore, leading to a single lung.
The mantle often contains a vestigial, internalized shell remnant, a calcium-rich structure indicating evolutionary descent from shelled snails. This remnant stores calcium salts rather than functioning as an external defense mechanism. Lacking a shell or hair, the slug’s body is smooth and soft, relying on internal mechanisms and external secretions to maintain its form and function.
The Role of Mucus and Hydration
Since the slug’s external structure is highly permeable, it must constantly produce mucus, or slime, to prevent desiccation (fatal loss of body moisture). This ecto-secretion is produced outside the body and consists of 91% to 99% water combined with high-molecular-weight organic polymers like glycoproteins and mucins. This gel-like substance forms a protective, hygroscopic layer that seals the slug’s surface against dry air.
The mucus serves a dual function: hydration and locomotion. Slugs move by generating rhythmic waves of muscular contraction along the underside of their foot, a process called adhesive locomotion. A thin layer of pedal mucus is laid down beneath the foot to act as a lubricant, reducing friction while providing adhesion necessary to climb vertical surfaces.
The secreted slime is also used for chemical defense and communication, making the slug slippery and unpalatable to predators. This versatile, water-based gel provides the protective and insulating properties that hair or scales offer other animals. The mucus can change its physical properties from a liquid to a gel under pressure, allowing the slug to navigate complex terrain while keeping its soft body protected and hydrated.