A creature often described as a “snail without a shell” is commonly known as a slug. Both slugs and snails belong to a group of soft-bodied animals called gastropod mollusks. While snails are recognized by their prominent, coiled external shells, slugs have largely or entirely evolved to lack such a protective covering.
What a Slug Looks Like
A slug’s body is elongated, soft, and cylindrical, tapering towards both ends. Its skin has a moist, glistening appearance due to a layer of mucus that covers its entire body. This mucus serves purposes, aiding in locomotion by reducing friction and protecting the slug from dehydration and injury as it moves across surfaces.
The head of a slug is formed, featuring two pairs of retractable tentacles. The longer, upper pair functions for light detection, with light-sensitive eyespots located at their tips. These tentacles also assist with the sense of smell. Below them, a shorter pair of tentacles is used for feeling and tasting the environment.
Behind the head, on the slug’s back, lies a saddle-shaped or shield-like structure called the mantle. This fleshy area is a remnant of the shell-producing organ found in snails. In many slug species, a small, internal, shell plate or vestigial shell is embedded within this mantle, serving as a storage for calcium salts.
The underside of the slug is a broad, muscular structure known as the foot. Rhythmic waves of muscular contractions along this foot, combined with the secreted mucus, allow the slug to glide smoothly, leaving a characteristic slime trail. On the right side of the mantle, a small opening called the pneumostome is visible; this is the slug’s breathing pore, leading to its single lung.
Slugs exhibit a range of colors, including shades of brown, black, gray, and sometimes more vibrant hues like orange or yellow, with some species displaying spots or patterns. Their size varies considerably, from very small species to larger garden slugs that can reach up to 10 inches in length.
How Slugs Differ from Snails
The distinction between slugs and snails is the presence or absence of a prominent external shell. Snails possess a coiled shell large enough for them to fully retract their soft bodies into for protection. Slugs, conversely, lack this large, visible external shell.
Despite appearing “shell-less,” many slugs do have a remnant of a shell. This is a small, reduced internal shell or shell plate embedded within their mantle, which highlights their shared evolutionary lineage with snails. This internal shell primarily functions for calcium storage.
The absence of a large external shell makes slugs more susceptible to desiccation. As a result, slugs prefer moist, damp environments, found under logs, rocks, or in garden foliage, and are most active during cooler, humid conditions or at night. Snails, while still requiring moisture, have better protection against water loss due to their shell.
Without a shell, slugs have developed alternative defense mechanisms. When threatened, they may retract their head and body, produce thick, sticky, or foul-tasting mucus to deter predators. This contrasts with a snail’s primary defense of retreating fully into its hard shell. The lack of a cumbersome shell also allows slugs to access tighter spaces and to burrow more easily.