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Slug

A slug is a shell-less or nearly shell-less gastropod mollusc, most often referring to terrestrial species.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

A slug is a shell-less or nearly shell-less gastropod mollusc, most often referring to terrestrial species. The term is used for many different gastropod lineages that share the loss or reduction of an external shell rather than for a single natural group. Slugs are typically soft-bodied and are usually associated with moist habitats on land.

Definition

A slug is commonly defined as a gastropod mollusc with no external shell, or with a very reduced internal shell. Britannica notes that the term generally refers to a land snail, while other sources describe it more broadly as a shell-less terrestrial gastropod.

Classification

Slugs belong to the phylum Mollusca and the class Gastropoda. Because the common name is based on body form rather than shared ancestry, the term covers several different groups of gastropods.

Habitat and biology

Most slugs live in damp terrestrial environments, where moisture helps prevent desiccation. Some slug-like gastropods also occur in freshwater and marine habitats.

Relation to snails

Slugs and snails are closely related gastropods, but slugs lack the prominent external shell typical of snails. In some species, the shell persists only as an internal plate or small remnant.

Key facts

  • The word **slug** usually refers to a land gastropod with no visible external shell.
  • Slugs are **molluscs** in the class Gastropoda, not insects or worms.
  • The shell may be absent or reduced to an internal structure in many slug species.
  • The term applies to multiple gastropod lineages and is not a single species or a strictly natural group.
  • Slugs are generally associated with **moist habitats** because their soft bodies lose water easily.
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Canadian context

Slugs are familiar to Canadian gardeners and farmers because several species occur in gardens, farms, and damp natural habitats across Canada.

Frequently asked questions

What is a slug?
A slug is a gastropod mollusc that lacks a visible external shell or has only a reduced internal shell.
Are slugs insects?
No. Slugs are molluscs in the class Gastropoda, which also includes snails.
Do all slugs lack shells?
Not always. Some slugs have a small internal shell or shell remnants rather than a full external shell.
Are slugs a single species?
No. “Slug” is a common name applied to many gastropod species with similar shell reduction, not one species.
Where do slugs usually live?
They are most often found in moist terrestrial habitats, though some related forms live in freshwater or marine environments.

References

  1. Encyclopaedia Britannica — https://www.britannica.com/animal/slug-mollusk
    Supports: Definition of slug as a gastropod with reduced or absent shell; moist terrestrial habitat; general reference to the term
  2. University of Florida — https://mrec.ifas.ufl.edu/lso/entomol/ncstate/slugintr.htm
    Supports: Slugs and snails as molluscs in Gastropoda; internal shell in slugs; related anatomy and biology
  3. TERRO Learning Center — https://www.terro.com/slugs
    Supports: Shell-less or internal-shell description; mucus and general slug characteristics
  4. Twinkl — https://www.twinkl.co.in/teaching-wiki/slug
    Supports: Slugs as multiple families and not all sharing a common ancestor; shell reduction; general dietary notes
  5. University of Wisconsin-Milwaukee — https://uwm.edu/field-station/bug-of-the-week/slug/
    Supports: Slugs as mollusks and gastropods; shell reduction in some species
  6. University of Minnesota Extension — https://extension.umn.edu/yard-and-garden-insects/slugs
    Supports: Slugs described as snails without shells; relation to mollusks