The terrestrial slug is a common resident of gardens and damp environments, representing a fascinating example of biological adaptation. As a gastropod mollusk without a protective external shell, its survival relies on the unique composition of its soft body and its ability to produce specialized mucus. Understanding what a slug is made of requires examining its fundamental material composition, its complex anatomy, and the chemistry behind its signature locomotion and defense mechanism: its slime.
Classification and Fundamental Composition
Slugs belong to the Phylum Mollusca and the Class Gastropoda, placing them in the same category as snails and octopuses. Terrestrial slugs are part of the Subclass Pulmonata, characterized by a lung-like cavity for breathing air. This soft-bodied structure is primarily composed of water, averaging 85 to 90% by mass, which necessitates a constantly moist environment to prevent fatal drying.
The body’s shape and rigidity are maintained by a hydrostatic skeleton, a system supported by fluid pressure within the body cavity. Muscles surrounding this fluid-filled volume contract to change the internal pressure, allowing the slug to move and maintain its form. This reliance on internal fluid pressure means the slug’s outer layer, or tegument, must be soft and flexible to facilitate these shape changes.
Detailed External and Internal Structure
The slug’s physical form is a streamlined adaptation of its shelled snail ancestors. The most apparent external feature is the muscular underside, known as the foot, which is responsible for slow, rhythmic locomotion. This continuous movement propels the animal forward and requires the simultaneous secretion of a lubricating mucus layer.
Located on the dorsal side, slightly behind the head, is the saddle-like mantle, a thickened area of flesh that protects the internal organs. In many species, the mantle contains a small, internalized remnant of a shell, which serves as storage for calcium salts. On the right side of the mantle is the pneumostome, a respiratory pore that opens and closes to allow air into the slug’s single lung cavity.
The head has two pairs of retractable tentacles that function as sensory apparatuses. The longer, upper pair are the optical tentacles, which have eyespots for detecting light and dark, and are also used for smelling. The shorter, lower pair are dedicated to tactile sensing and tasting the immediate environment. Internally, the mouth holds the radula, a ribbon-like structure covered in thousands of microscopic, tooth-like projections. The slug uses this rasping organ to scrape and grind food material, such as decaying vegetation.
The Essential Chemistry and Function of Slime
The slime that coats the slug is a hydrogel, a substance that is mostly water but behaves like a solid due to a network of polymers. This mucus is an aqueous solution, containing 91 to 98% water, but its viscosity comes from a high concentration of complex, heavily glycosylated proteins known as mucins. These mucins form a tangled, cross-linked network, giving the slime its unique physical properties.
Slugs produce at least two distinct types of mucus to serve different functions. The pedal mucus, secreted by the foot, is a thin, watery lubricant that reduces friction during movement and protects the sensitive foot tissue. Conversely, the slug can secrete a much thicker, highly adhesive mucus for defensive purposes, making the slug difficult for a predator to grasp.
The slime’s ability to change its physical state makes it a non-Newtonian fluid, meaning its viscosity changes in response to stress. This property allows the slug to use the mucus as a strong adhesive to climb vertical surfaces or flow smoothly for horizontal travel. The slime is also hygroscopic, absorbing moisture from the air to help the slug maintain its high body water content. The trails also contain pheromones, which other slugs can follow for navigation or locating a mate.