Shell-less gastropod

SLUG

This slimy garden dweller leaves a shiny trail behind it and thrives in damp conditions, often hiding under leaves or rocks.

A slug is a type of gastropod, just like snails, but it lacks the hard external shell that snails have. The clue "shell-less gastropod" directly describes this key difference, pointing to the answer. There’s no wordplay here—it’s a straightforward definition that highlights the slug’s most recognizable trait.