Slugs and snails are familiar occupants of gardens and damp environments. While they appear distinct, with one possessing a prominent shell and the other lacking it, these creatures are closely related through a common evolutionary path.
Shared Characteristics
Slugs and snails share a fundamental body plan as soft-bodied invertebrates. Both possess a muscular foot, which they use for locomotion by secreting mucus. This mucus helps them glide smoothly and protects their delicate undersides. They also have two pairs of retractable tentacles; the longer pair typically bears eyes, while the shorter pair serves as feelers and taste buds.
These creatures thrive in moist environments, frequently found in gardens, under logs, or after rainfall. Their diets often overlap, with most species being herbivorous, consuming plants, fungi, and decaying organic matter. Some species can also be omnivorous, occasionally eating insects or carrion. Both slugs and snails use a specialized, tongue-like organ called a radula, covered in tiny teeth, to scrape and rasp their food.
Primary Differences
The most apparent distinction between slugs and snails lies in the presence or absence of a large, external shell. Snails carry a coiled shell on their backs, which provides protection from predators and helps prevent desiccation by allowing them to fully retract inside. This shell also limits the types of habitats snails can access, as it can hinder movement in tight spaces.
Slugs, conversely, either completely lack an external shell or possess only a greatly reduced internal shell. The absence of a large shell offers slugs increased flexibility, enabling them to squeeze into narrow crevices and explore habitats inaccessible to their shelled relatives. Without an external shell for moisture retention, slugs rely more heavily on secreting mucus to keep their skin moist and are more vulnerable to drying out.
Their Evolutionary Link
Slugs and snails are both members of the class Gastropoda, part of the larger phylum Mollusca. This classification indicates a shared ancestry. The name “Gastropoda” itself comes from Greek words meaning “stomach” and “foot,” referring to their characteristic body structure where the foot is located beneath the visceral mass.
Over evolutionary time, slugs have developed from shelled snail ancestors. This adaptation involved the reduction or complete loss of the external shell, a process that has occurred independently multiple times. Some slugs still retain a vestigial, internal shell, a remnant of their shelled past. This shell loss allowed slugs to exploit new ecological niches.